CN117299172B - A carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst and its preparation method and application - Google Patents

Abstract

The present invention discloses a carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst and its preparation method and application. Sodium molybdate, thiourea and carbon nitride in a certain ratio are dissolved in water to form a mixed solution, the pH value of the solution is adjusted to 0.1-2.0, the solution is stirred evenly, and the mixture is reacted at 180-220°C for 20-30h. After cooling, washing and drying, a carbon nitride/molybdenum disulfide heterojunction catalyst is obtained. The carbon nitride/molybdenum disulfide heterojunction can greatly improve the efficiency of removing hexavalent chromium in water under piezoelectric photocatalytic conditions. In addition, the catalyst of the present invention can be recycled and easily recovered after the reaction, and will not cause pollution to the water environment.

Description

A carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst and its preparation method and application

Technical Field

The invention relates to the field of environmental pollution control, and in particular to a carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst and a preparation method and application thereof.

Background technique

As an environmentally friendly and efficient technology, photocatalysis has a global prospect in environmental treatment and energy exchange, which has aroused great interest among researchers. Photocatalysts with ideal performance are the key to the promotion and application of photocatalysis. However, traditional single-component photocatalysts have low sunlight utilization, high recombination rate of photoelectrons and holes, and low photocatalytic activity, which greatly limits the practical application of photocatalysis. Although current modification methods such as doping, dye sensitization, noble metal deposition, and heterostructure construction can promote the separation of photocarriers, most photocarriers are prone to recombination before photocatalytic reaction, which greatly limits the improvement of photocatalytic performance. Therefore, how to promote the separation of photoelectrons and holes and inhibit the secondary recombination of photoelectrons and holes is a core issue in the process of removing environmental pollutants. Among the above modification methods, heterojunction structure, as the most widely studied technology, has stable performance and outstanding practical application prospects in improving photocatalytic performance, and has aroused great interest among researchers. However, the structural defects of the heterojunction interface easily induce the formation of photogenerated charge carrier recombination centers, which reduces the charge carrier separation efficiency to a certain extent. It is worth noting that the built-in electric field formed by the polarization of piezoelectric/ferroelectric materials serves as the intrinsic driving force for the separation of photogenerated electrons and holes, which can improve the separation efficiency of carriers and enhance the photocatalytic activity. In addition, under the action of electrostatic screening, the built-in electric field is very easy to exit, resulting in the saturation of free carriers, thereby reducing the separation efficiency of polarization on carriers. Therefore, driven by mechanical force, the continuous enhancement of the built-in electric field formed by the heterojunction composed of piezoelectric photocatalysts will greatly promote the separation of photogenerated carriers.

Molybdenum disulfide is a common piezoelectric catalytic material. By applying mechanical stress (such as ultrasound, stirring, wind, water flow, friction and extrusion, etc.) to it, polarization occurs inside the molybdenum disulfide, generating electrons and holes.

Summary of the invention

In order to overcome the above-mentioned shortcomings and deficiencies of the existing photocatalytic technology, the purpose of the present invention is to provide a carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst and its preparation method and application.

The present invention aims at the piezoelectric properties of molybdenum disulfide, and uses the common photocatalyst carbon nitride to modify molybdenum disulfide to obtain a carbon nitride/molybdenum disulfide heterojunction. Under ultrasonic conditions, the photogenerated electrons and holes generated by light are effectively separated to directly or indirectly reduce and remove highly toxic hexavalent chromium, thereby improving the removal efficiency of hexavalent chromium.

The technical solution of the present invention is as follows:

A method for preparing a carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst comprises the following steps:

Sodium molybdate, thiourea and carbon nitride are dissolved in water to form a mixed solution, the pH value of the solution is adjusted to 0.1-2.0, and the solution is stirred evenly, and then reacted at 180-220° C. for 20-30 hours. After cooling, washing and drying, a carbon nitride/molybdenum disulfide heterojunction catalyst is obtained; the molar ratio of the sodium molybdate, thiourea and carbon nitride is (3-15):(40-80):(0.5-3).

Preferably, the molar ratio of the sodium molybdate, thiourea and carbon nitride is 10±5:50±10:1; and the concentration of the sodium molybdate in the solution is 0.01-0.03 mol/L.

Preferably, the solution pH value is 0.5-1.0; the stirring time is 60±30 minutes; the reaction temperature is 200±10°C; the reaction time is 24±2h; the drying temperature is 60±20°C; and the drying time is 12±6h.

Preferably, the method for preparing the carbonized nitrogen is as follows: dissolving urea in water, adjusting the pH to 4-5, drying at 60±20° C. for 12±6 hours, heating to 530-570° C. for 2-3 hours, and cooling to obtain carbon nitride.

Preferably, the concentration of urea in water is 0.5-1 g/mL; and the heating rate of the heating process is 5-15° C./min.

The carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst prepared by the above method.

Application of the above carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst in removing hexavalent chromium.

Preferably, the application comprises the following steps:

1) adding a carbon nitride/molybdenum disulfide heterojunction catalyst to the hexavalent chromium-containing water to be treated and stirring; the concentration of the carbon nitride/molybdenum disulfide heterojunction catalyst in the water is 0.3 to 1.5 g/L;

2) subjecting the water to be treated obtained in step 1) to ultrasonic and light treatment; the power of the ultrasonic treatment is 50-150W; the intensity of the light treatment is 100-300mW/cm ² .

Preferably, the concentration of the carbon nitride/molybdenum disulfide heterojunction catalyst in water is 0.5-1.0 g/L; the power of the ultrasonic treatment is 90-120 W; and the intensity of the light treatment is 175-225 mW/cm ² .

Preferably, the ultrasonic treatment uses an ultrasonic cleaning machine; the light source of the light treatment is a xenon lamp or a sun lamp.

The principle of the present invention is that, based on the excellent piezoelectric photocatalytic performance of carbon nitride/molybdenum disulfide, the built-in electric field formed by piezoelectric polarization can make the photogenerated carriers move in a directional manner, promote the separation of photogenerated electron-hole pairs to reduce hexavalent chromium, and achieve the purpose of removing highly toxic hexavalent chromium.

Compared with the prior art, the present invention has the following advantages and beneficial effects:

The carbon nitride/molybdenum disulfide catalyst prepared by the present invention has obvious piezoelectric photocatalytic performance, good stability and excellent catalytic performance, can greatly improve the separation efficiency of photogenerated electron-hole pairs, and the catalyst can greatly improve the efficiency of removing hexavalent chromium wastewater in water under piezoelectric photocatalytic conditions. In addition, the catalyst of the present invention can be recycled and easily recovered after the reaction, and will not cause pollution to the water environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a TEM image and an XRD image of carbon nitride/molybdenum disulfide prepared in Example 1 of the present invention.

Figure 2A shows the performance of carbon nitride/molybdenum disulfide heterojunction in removing hexavalent chromium under visible light irradiation in the presence of different proportions of carbon nitride in Example 2 of the present invention; Figure 2B shows the performance of carbon nitride/molybdenum disulfide heterojunction in removing hexavalent chromium under ultrasonic vibration in the presence of different proportions of carbon nitride in Example 2 of the present invention.

FIG. 3 is a diagram showing the effect of removing hexavalent chromium by piezoelectric photocatalysis of carbon nitride/molybdenum disulfide heterojunction with different concentrations in Example 3 of the present invention.

FIG. 4 is a diagram showing the effect of piezoelectric photocatalysis on the removal of hexavalent chromium by different samples in Example 4 of the present invention.

FIG5 is a diagram showing the effect of carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalysis on removal of hexavalent chromium at different ultrasonic powers in Example 5 of the present invention.

FIG6 is a diagram showing the effect of carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalytic removal of hexavalent chromium under different light intensities in Example 6 of the present invention.

FIG. 7 is a diagram showing the effect of piezoelectric photocatalytic removal of hexavalent chromium by carbon nitride/molybdenum disulfide in real wastewater in Example 7 of the present invention.

FIG8 is a test graph of the cycle performance of carbon nitride/molybdenum disulfide piezoelectric photocatalytic removal of hexavalent chromium in Example 8 of the present invention.

Detailed ways

The present invention is further described in detail below in conjunction with embodiments and drawings, but the embodiments of the present invention are not limited thereto.

Unless otherwise specified, the reagents used in the examples can be purchased from the market.

Example 1

This example provides a method for preparing a carbon nitride/molybdenum disulfide heterojunction catalyst, and the specific steps are as follows:

1) First, carbon nitride nanomaterials were prepared using urea as raw material using reference (DOI: 10.1016/j.apcatb.2016.10.012). The specific steps were as follows: 10 g of urea was dissolved in 15 mL of deionized water, and then the pH was adjusted to 4-5 with hydrochloric acid. The urea solution was then dried at 60 °C for 12 hours. The sample was then transferred to a crucible with a lid. Finally, the crucible was heated to 550 °C at a heating rate of 10 °C/min in a muffle furnace and maintained at this temperature for 2 hours. After cooling, a carbon nitride (C ₃ N ₄ ) sample was obtained.

2) Weigh 0.242g sodium molybdate dihydrate, 0.381g thiourea and 0.009g carbon nitride respectively and dissolve them in 80mL deionized water, then adjust the pH of the solution to 0.5 with 1M hydrochloric acid solution, stir for 60 minutes, transfer the solution to a reactor, and react at 200°C for 24 hours. After the reactor is cooled to room temperature, wash the reactants with water and ethanol three times each, and dry them at 60°C for 12 hours to finally obtain the product carbon nitride/molybdenum disulfide heterojunction.

The TEM image and XRD image of the carbon nitride/molybdenum disulfide heterojunction catalyst prepared in the embodiment of the present invention are shown in Figure 1. It is proved that the method described in the embodiment successfully prepared the carbon nitride/molybdenum disulfide heterojunction.

Example 2

The difference between this embodiment and embodiment 1 is that: without changing the dosage of sodium molybdate and thiourea, the dosage of carbon nitride is changed to 0.0045, 0.009, 0.018, and 0.027 g, respectively, to prepare carbon nitride/molybdenum disulfide heterojunctions with a molybdenum atomic substance content of 5%, 10%, 20%, and 30%, respectively, and hexavalent chromium is selected as a representative of pollutants, and an experiment is carried out on the piezoelectric photocatalytic removal of hexavalent chromium by carbon nitride/molybdenum disulfide heterojunction.

The experimental steps are as follows:

Prepare 40 mL of 50 mg/L hexavalent chromium solution in a beaker. Weigh 20 mg of carbon nitride/molybdenum disulfide heterojunction powder and add it to the hexavalent chromium solution, stirring until the adsorption of the hexavalent chromium solution reaches equilibrium. Place the mixed solution in an ultrasonic cleaner and start the reaction with ultrasound. The concentration of carbon nitride/molybdenum disulfide heterojunction is 0.5 g/L.

The performance of the carbon nitride/molybdenum disulfide heterojunction in removing hexavalent chromium under visible light irradiation and ultrasonic vibration in the presence of different proportions of carbon nitride in the examples of the present invention is shown in Figure 2. For the carbon nitride/molybdenum disulfide heterojunction with 100% and 0 carbon nitride content, the removal rates of hexavalent chromium are 32.2% and 41.3% and 9.27% and 44.3% respectively after 20 minutes of photocatalytic and piezoelectric catalytic reactions. When carbon nitride is compounded with molybdenum disulfide, as the composite content of carbon nitride gradually increases, the removal efficiency of hexavalent chromium by photocatalysis and piezoelectric catalysis shows a phenomenon of first increasing and then decreasing. When the content of carbon nitride in the amount of molybdenum atomic substance is 10%, the removal efficiency of hexavalent chromium by photocatalysis and piezoelectric catalysis reaches the maximum. This result verifies that the preferred ratio of the amount of sodium molybdate, thiourea and carbon nitride for preparing the carbon nitride/molybdenum disulfide heterojunction in the present invention is: 10:50:1.

Example 3

This example uses the carbon nitride/molybdenum disulfide heterojunction prepared in Example 1, selects hexavalent chromium as a representative of pollutants, and conducts experiments on piezoelectric photocatalytic removal of hexavalent chromium by carbon nitride/molybdenum disulfide heterojunctions with different concentrations.

The experimental steps are as follows:

Prepare 40 mL of 50 mg/L hexavalent chromium solution in a beaker. Weigh 4, 10, 20 and 40 mg of carbon nitride/molybdenum disulfide heterojunction powder respectively and add them to the hexavalent chromium solution, stirring until the adsorption of the hexavalent chromium solution reaches equilibrium. Place the mixed solution in an ultrasonic cleaner to start the reaction by ultrasound.

The effect diagram of the piezoelectric photocatalytic removal of hexavalent chromium by carbon nitride/molybdenum disulfide heterojunction with different concentrations in the embodiment of the present invention is shown in Figure 3. It can be seen from the figure that with the continuous increase of the concentration of carbon nitride/molybdenum disulfide heterojunction, the removal efficiency of hexavalent chromium is also continuously improved. When the heterojunction concentration reaches 0.5g/L, the removal efficiency of hexavalent chromium is 99.8%, and when the heterojunction concentration reaches 1g/L, the removal efficiency of hexavalent chromium is 100%. Considering the economic benefits in practical applications, this result verifies that the preferred concentration of carbon nitride/molybdenum disulfide heterojunction for removing hexavalent chromium in the content of the present invention is 0.5g/L.

Example 4

This example uses the carbon nitride/molybdenum disulfide heterojunction prepared in Example 1, selects hexavalent chromium as a representative of pollutants, and conducts experiments on the piezoelectric photocatalytic removal of hexavalent chromium by the carbon nitride/molybdenum disulfide heterojunction at different times.

The experimental steps are as follows:

Prepare 40 mL of 50 mg/L hexavalent chromium solution in a beaker. Weigh 20 mg of carbon nitride/molybdenum disulfide heterojunction powder and add it to the hexavalent chromium solution, stirring until the adsorption of the hexavalent chromium solution reaches equilibrium. Place the mixed solution in an ultrasonic cleaning machine and start the reaction by ultrasound. The concentration of carbon nitride/molybdenum disulfide heterojunction is 0.5 g/L.

The effect of carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalysis on the removal of hexavalent chromium in the example of the present invention is shown in Figure 4. It can be seen from the figure that the carbon nitride/molybdenum disulfide heterojunction has almost no adsorption of hexavalent chromium, and the adsorption is less than 5% within 30 minutes. No hexavalent chromium was reduced within 20 minutes by ultrasound, light irradiation and piezoelectric photocatalysis alone. Carbon nitride and molybdenum disulfide can remove 37.5% and 62.7% of hexavalent chromium respectively within 20 minutes by piezoelectric alone. In contrast, the carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalysis shows excellent effect on the reduction and removal of hexavalent chromium, and the removal rate within 20 minutes reaches 99.8%. The results show that the carbon nitride/molybdenum disulfide heterojunction has excellent carrier separation efficiency and can significantly improve the reduction and removal efficiency of hexavalent chromium.

Example 5

This example uses the carbon nitride/molybdenum disulfide heterojunction prepared in Example 1, selects hexavalent chromium as a representative of pollutants, and conducts experiments on the piezoelectric photocatalytic removal of hexavalent chromium by the carbon nitride/molybdenum disulfide heterojunction under different ultrasonic powers.

The experimental steps are as follows:

Prepare 40 mL of 50 mg/L hexavalent chromium solution in a beaker. Weigh 20 mg of carbon nitride/molybdenum disulfide heterojunction powder and add it to the hexavalent chromium solution, stirring until the adsorption of the hexavalent chromium solution reaches equilibrium. Place the mixed solution in an ultrasonic cleaner, and adjust the power of the ultrasonic cleaner to 25, 50, 75 and 100 W to start the reaction. Among them, the concentration of carbon nitride/molybdenum disulfide heterojunction is 0.5 g/L.

The effect of removing hexavalent chromium by piezoelectric photocatalysis of carbon nitride/molybdenum disulfide heterojunction under different ultrasonic powers in the examples of the present invention is shown in Figure 5. As the ultrasonic power continues to increase, the performance of removing hexavalent chromium by piezoelectric photocatalysis of carbon nitride/molybdenum disulfide heterojunction is also continuously improved. When the ultrasonic power increases to 100W, the removal efficiency of hexavalent chromium is close to 100%, so it is verified that the preferred power of the ultrasonic cleaning machine for removing hexavalent chromium by carbon nitride/molybdenum disulfide heterojunction in the content of the present invention is 100W.

Example 6

This example uses the carbon nitride/molybdenum disulfide heterojunction prepared in Example 1, selects hexavalent chromium as a representative of pollutants, and conducts experiments on the piezoelectric photocatalytic removal of hexavalent chromium by the carbon nitride/molybdenum disulfide heterojunction under different light intensities.

The experimental steps are as follows:

Prepare 40 mL of 50 mg/L hexavalent chromium solution in a beaker. Weigh 20 mg of carbon nitride/molybdenum disulfide heterojunction powder and add it to the hexavalent chromium solution, stirring until the adsorption of the hexavalent chromium solution reaches equilibrium. Place the mixed solution in an ultrasonic cleaner and adjust the light intensity to 50, 100, 150 and 200 mW/ ^cm2 respectively to start the reaction. Among them, the concentration of carbon nitride/molybdenum disulfide heterojunction is 0.5 g/L.

The effect of removing hexavalent chromium by carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalysis under different light intensities in the examples of the present invention is shown in FIG6 . As the light intensity continues to increase, the performance of removing hexavalent chromium by carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalysis continues to improve. When the light intensity increases to 200 mW/cm ² , the removal efficiency of hexavalent chromium is close to 100%. Therefore, it is verified that the preferred light intensity for removing hexavalent chromium by carbon nitride/molybdenum disulfide heterojunction in the content of the present invention is 200 mW/cm ² .

Example 7

This example uses the carbon nitride/molybdenum disulfide heterojunction prepared in Example 1, selects hexavalent chromium as a representative of pollutants, and conducts an experiment on removing hexavalent chromium in real water by piezoelectric photocatalysis using the carbon nitride/molybdenum disulfide heterojunction.

The experimental steps are as follows:

Prepare 40 mL of 50 mg/L hexavalent chromium solution in 5 beakers using rainwater, soil digestion solution and deionized water. Weigh 5 portions of 20 mg carbon nitride/molybdenum disulfide heterojunction powder and add them to the hexavalent chromium solution. Stir until the adsorption of hexavalent chromium reaches equilibrium and start the reaction. The concentration of carbon nitride/molybdenum disulfide heterojunction is 0.5 g/L.

Figure 7 is a diagram showing the effect of piezoelectric photocatalysis of carbon nitride/molybdenum disulfide heterojunction in real wastewater in an embodiment of the present invention to remove hexavalent chromium. Experiments show that hexavalent chromium can be effectively removed from soil, rainwater and deionized water in 20 minutes. This example shows that carbon nitride/molybdenum disulfide heterojunction can efficiently remove hexavalent chromium by piezoelectric photocatalysis in different water environments, which provides a feasible solution for industrial treatment of hexavalent chromium pollution.

Example 8

This example uses the carbon nitride/molybdenum disulfide heterojunction prepared in Example 1, selects hexavalent chromium as a representative of pollutants, and conducts an experiment on catalyst recycling.

The experimental steps are as follows:

Prepare 40mL of 50mg/L hexavalent chromium solution in a beaker. Weigh 20mg of carbon nitride/molybdenum disulfide heterojunction powder and add it to the hexavalent chromium solution, stirring to make the adsorption of the hexavalent chromium solution reach equilibrium. Place the mixed solution in an ultrasonic cleaner and treat it under 100W ultrasound and 200mW/ ^cm2 light for 20min. After the reaction is completed, the remaining solution is centrifuged in a high-speed centrifuge for 15min at a speed of 12,000 rpm, the catalyst is recovered, and the catalyst is added to a 40mL beaker of 50mg/L hexavalent chromium solution, and the above hexavalent chromium removal steps are repeated, and this is repeated six times.

Figure 8 is a diagram showing the effect of recycling the catalyst in the piezoelectric photocatalytic removal of hexavalent chromium by carbon nitride/molybdenum disulfide heterojunction in an embodiment of the present invention. The experiment shows that after the carbon nitride/molybdenum disulfide heterojunction has been recycled for six times, the removal efficiency of hexavalent chromium in 20 minutes has dropped from 99.8% to 94.9%, and the performance has only decreased by about 5%. This example shows that the piezoelectric photocatalytic removal of hexavalent chromium by carbon nitride/molybdenum disulfide heterojunction has recyclability and stability in properties in practical applications.

The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims (8)

1. The application of the carbon nitride/molybdenum disulfide heterojunction piezoelectric photocatalyst in removing hexavalent chromium in water is characterized in that the preparation method of the catalyst comprises the following steps:

Dissolving sodium molybdate, thiourea and carbon nitride in water to form a mixed solution, regulating the pH value of the solution to 0.1-2.0, uniformly stirring, reacting for 20-30 h at 180-220 ℃, cooling, washing and drying to obtain a carbon nitride/molybdenum disulfide heterojunction catalyst; the ratio of the amounts of sodium molybdate, thiourea and carbon nitride is 10+/-5:50+/-10:1.

2. The use according to claim 1, characterized in that the concentration of sodium molybdate in solution is between 0.01 and 0.03mol/L.

3. The use according to claim 2, wherein the pH of the solution is between 0.5 and 1.0; the stirring time is 60+/-30 minutes; the reaction temperature is 200+/-10 ℃; the reaction time is 24+/-2 hours; the drying temperature is 60+/-20 ℃; the drying time is 12+/-6 hours.

4. Use according to claim 1 or 2 or 3, characterized in that the preparation method of the nitrogen carbide is: dissolving urea in water, regulating pH to 4-5, drying at 60+ -20 deg.C for 12+ -6 hr, heating to 530-570 deg.C, maintaining for 2-3 hr, and cooling to obtain carbon nitride.

5. The use according to claim 4, characterized in that the concentration of urea in water is between 0.5 and 1g/mL; the heating rate of the heating process is 5-15 ℃/min.

6. The use according to claim 5, characterized by the steps of:

1) Adding a carbon nitride/molybdenum disulfide heterojunction catalyst into hexavalent chromium-containing water to be treated, and stirring; the concentration of the carbon nitride/molybdenum disulfide heterojunction catalyst in water is 0.3-1.5 g/L;

2) Carrying out ultrasonic and illumination treatment on the water to be treated obtained in the step 1); the power of the ultrasonic treatment is 50-150W; the intensity of the light treatment is 100-300 mW/cm ².

7. The use according to claim 6, wherein the concentration of the carbon nitride/molybdenum disulfide heterojunction catalyst in water is 0.5-1.0 g/L; the power of the ultrasonic treatment is 90-120W; the intensity of the light treatment is 175-225 mW/cm ².

8. The use according to claim 7, wherein the ultrasonic treatment uses an ultrasonic cleaner; the light source for the illumination treatment is a xenon lamp or a solar lamp.