CN112209355A - Method for preparing ultrathin two-dimensional doped carbon nitride nanosheet through supercritical stripping and application of ultrathin two-dimensional doped carbon nitride nanosheet in efficient photocatalytic water decomposition - Google Patents
Method for preparing ultrathin two-dimensional doped carbon nitride nanosheet through supercritical stripping and application of ultrathin two-dimensional doped carbon nitride nanosheet in efficient photocatalytic water decomposition Download PDFInfo
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Abstract
The invention discloses a method for preparing an ultrathin two-dimensional doped carbon nitride nanosheet through supercritical stripping and application of the ultrathin two-dimensional doped carbon nitride nanosheet in efficient photocatalytic water decomposition. The invention adopts a novel effective supercritical stripping re-doping method to modify the traditional massive carbon nitride or phosphorized carbon nitride. The stripped and doped carbon nitride or the phosphated carbon nitride can expose more active sites, so that more channels are provided for electron transmission, and the photocatalytic activity of the carbon nitride is effectively improved. The compressed carbon dioxide gas adopted by the invention is green and environment-friendly, and only simple exhaust operation is needed after stripping is finished, and no additional treatment is needed. The invention can not only complete stripping in a small reaction kettle, but also carry out batch supercritical stripping in a medium-sized reaction vessel, and has the advantage of large-scale practical application.
Description
Technical Field
The invention belongs to the technical field of green photocatalysis, and particularly relates to a method for preparing an ultrathin two-dimensional doped carbon nitride nanosheet through supercritical stripping and application of the ultrathin two-dimensional doped carbon nitride nanosheet in efficient photocatalytic water decomposition.
Background
Graphitized carbon nitride (g-C)3N4) As a two-dimensional semiconductor material, the photocatalyst has a proper band gap, earth resource abundance, good thermal stability, a proper energy band structure and an adjustable molecular structure, and is a novel photocatalyst for generating hydrogen/oxygen by cracking water in the presence of visible light and a sacrificial reagent. To increase g-C3N4A number of strategies have been developed including the introduction of heteroatoms, the control of morphology, the construction of heterostructures, and the design of nanostructures, among others. Wherein the nanostructure is designed to effectively increase g-C3N4One of the most effective strategies for photocatalytic performance is by increasing the specific surface area and facilitating the transfer of photogenerated charge carriers. However, the existing methods have the defects of complicated experimental post-treatment steps, environmental unfriendliness and the like, are not beneficial to large-scale application, and urgently need a simpler and more economic method.
Disclosure of Invention
The invention aims to provide a method for preparing ultrathin two-dimensional doped carbon nitride nanosheets by supercritical stripping and application of the ultrathin two-dimensional doped carbon nitride nanosheets in efficient photocatalytic water decomposition.
The method for preparing the ultrathin two-dimensional carbon nitride or carbon nitride phosphide nanosheet by supercritical stripping comprises the following steps: compressing carbon dioxide to reach a supercritical state in a high-pressure closed reaction kettle, so that carbon dioxide molecules are embedded into a two-dimensional carbon nitride or phosphorized carbon nitride material in a liquid molecule form for supercritical stripping, and the carbon nitride or phosphorized carbon nitride material is stripped into a lamellar layer; then ultrasonic-assisted stripping is carried out, and the lamellar carbon nitride or carbon phosphide nitride is further stripped into ultrathin two-dimensional carbon nitride or carbon phosphide nanosheets.
The method for preparing the ultrathin two-dimensional doped carbon nitride or carbon phosphide nanosheet by supercritical stripping comprises the following steps: and (3) carrying out monatomic doping on the ultrathin two-dimensional carbon nitride or phosphatized carbon nitride nanosheet prepared by the method.
The ultrasonic-assisted stripping is carried out in one or a mixture of several solvents of water, absolute ethyl alcohol and isopropanol for 10-60min by ultrasonic treatment.
The doping method comprises the following steps: dissolving ultrathin two-dimensional carbon nitride or carbon phosphide nanosheets and cobalt salt in an acetonitrile solvent, performing ultrasonic treatment for 2-10min to fully disperse a sample in acetonitrile, then dropwise adding triethylamine under uniform stirring, fully stirring the mixed solution for 0.5-2h, then transferring the mixed solution into a microwave tube, performing microwave reaction for 1-4h, centrifugally collecting products, washing with trichloromethane, anhydrous methanol and acetonitrile respectively, and drying to obtain ultrathin two-dimensional carbon nitride or carbon phosphide doped nanosheets.
The prepared ultrathin two-dimensional carbon nitride or carbon nitride phosphide nanosheet is used as a photocatalyst to be applied to a reaction of photocatalytic water decomposition.
The prepared ultrathin two-dimensional doped carbon nitride or carbon nitride phosphide nanosheet is used as a photocatalyst to be applied to a reaction of photocatalytic water decomposition.
The reaction conditions of the photocatalytic water decomposition are as follows: in a photocatalytic hydrogen production device, a photocatalyst, distilled water, triethanolamine and H are added2PtCl6Mixing, ultrasonic dispersing, discharging air with nitrogen, sealing, and photocatalytic water decomposing reaction under irradiation of visible light.
The invention adopts a novel effective stripping and re-doping method to modify the traditional massive carbon nitride or phosphated carbon nitride. The stripped and doped carbon nitride or the phosphated carbon nitride can expose more active sites, so that more channels are provided for electron transmission, and the photocatalytic activity of the carbon nitride is effectively improved. The compressed carbon dioxide gas adopted by the invention is green and environment-friendly, and only simple exhaust operation is needed after stripping is finished, and no additional treatment is needed. The invention can not only complete stripping in a small reaction kettle, but also carry out batch supercritical stripping in a medium-sized reaction vessel, and has the advantage of large-scale practical application.
Drawings
Fig. 1 is a TEM image of bulk carbon nitride and exfoliated carbon nitride nanoplates; a1 and a2 are bulk carbon nitride, b1 and b2 are 8MPa stripped carbon nitride nanosheets in example 1; c1 and c2 represent the carbon nitride nanosheets stripped at 30MPa in example 2. It can be clearly seen from the figure that after the supercritical carbon dioxide stripping, the sample is more transparent and the layered morphology is more obvious, and the sample is obviously curled after the supercritical stripping, which shows that the supercritical carbon dioxide has a good stripping effect on the carbon nitride.
FIG. 2 is a TEM image of bulk carbon nitride, phosphated carbon nitride and stripped phosphated carbon nitride nanosheets; a1 and a2 are bulk carbon nitride, e1 and e2 are phosphated carbon nitride, and f1 and f2 are the phosphated carbon nitride nanosheets stripped in example 3. The figure shows that the carbon nitride after phosphating is slightly bright compared with the traditional massive carbon nitride, and the phosphated carbon nitride nanosheet after stripping is more bright and has more obvious layer.
Fig. 3 is an AFM image and AFM data of bulk carbon nitride (g1, g2) and ultra-thin two-dimensional carbon nitride nanosheets (h1, h2) subjected to supercritical exfoliation in example 1. As can be seen from the figure, the average thickness of the traditional massive carbon nitride is-70 nm, and the average thickness of the ultrathin two-dimensional carbon nitride nanosheet obtained through supercritical stripping is-0.8 nm, which is enough to prove the stripping effect of the supercritical carbon nitride.
Fig. 4 is a bar graph of photocatalytic hydrogen production rate of bulk carbon nitride, 8MPa stripped doped carbon nitride nanosheets in example 1, and 30MPa stripped doped carbon nitride nanosheets in example 2. It can be seen from the figure that the photocatalytic hydrogen production rate of the carbon nitride nanosheets subjected to supercritical stripping doping is obviously improved, and in comparison, the photocatalytic hydrogen production rate of the carbon nitride nanosheets subjected to 30MPa stripping doping is higher than that of the carbon nitride nanosheets subjected to 8MPa stripping doping, which illustrates the positive effect of the high-pressure supercritical stripping on the photocatalytic hydrogen production of the carbon nitride.
Fig. 5 is a bar graph of photocatalytic hydrogen production rates for bulk carbon nitride, phosphated carbon nitride, stripped phosphated carbon nitride nanoplates from example 3, and stripped doped phosphated carbon nitride nanoplates from example 3. As can be seen from the figure, after the carbon nitride is subjected to phosphorization, phosphorization stripping and phosphorization stripping doping, the photocatalytic hydrogen production performance of the carbon nitride is greatly improved, and the phosphorization, stripping and doping all play a positive role in the photocatalytic hydrogen production effect of the carbon nitride.
Detailed Description
Example 1: low pressure (8MPa) supercritical stripping, doping, photocatalysis
Preparing carbon nitride: the method comprises the steps of taking 5.0g of melamine by adopting a traditional thermal polymerization mode, fully grinding, placing in a crucible with a cover, heating to 600 ℃ in a tube furnace at the speed of 5 ℃/min, keeping for 120min, and collecting a light yellow product, namely blocky carbon nitride after annealing.
Stripping of carbon nitride: 0.2g of carbon nitride was taken and placed in a 50ml small-sized high-pressure closed reaction vessel to ensure the sealing property of the apparatus. And introducing the compressed carbon dioxide into the reaction kettle, and performing air exhaust operation. And then setting the rotating speed of 1200rad/s, and continuously introducing carbon dioxide into the reaction kettle to 8Mpa, wherein the carbon dioxide reaches a supercritical state. In the whole process, the carbon nitride powder is gradually filled in the reaction kettle, and the compressed carbon dioxide is converted into a liquid state from a gaseous state along with the gradual increase of the pressure. And after 48h of supercritical stripping, closing stirring, opening an exhaust valve, slowly releasing compressed carbon dioxide, collecting a product, placing the stripped carbon nitride in a mixed solvent of deionized water and absolute ethyl alcohol, ultrasonically stripping for 30min, standing for 2h, centrifuging, collecting supernatant, and drying to obtain the ultrathin two-dimensional carbon nitride nanosheet.
Doping carbon nitride: dissolving 0.1g of ultrathin two-dimensional carbon nitride nanosheet and 0.012g of cobalt chloride hexahydrate in 7.5ml of acetonitrile solvent, performing ultrasonic treatment for 5min to fully disperse the sample in acetonitrile, then dropwise adding 65 mu l of triethylamine under uniform stirring, fully stirring the mixed solution for 1h, transferring the obtained mixed solution into a microwave tube, and performing microwave reaction for 2 h. And centrifuging to collect a product, washing the product with chloroform, absolute methanol and acetonitrile respectively, and drying the product at 80 ℃ overnight to obtain the ultrathin two-dimensional doped carbon nitride nanosheet.
And (3) photocatalysis: carrying out a photocatalytic hydrogen evolution experiment in a photocatalytic hydrogen production system; 20mg of blocky carbon nitride and ultrathin two-dimensional doped carbon nitride nanosheets are respectively dispersed in a solvent containing 40ml of distilled water, 10ml of triethanolamine and H2PtCl6·6H2In a mixed solution of O, H2PtCl6·6H2Adding O in an amount of 3 wt% of the blocky carbon nitride or ultrathin two-dimensional doped carbon nitride nanosheet, ultrasonically dispersing for 30min, introducing nitrogen and exhausting air for 30min, sealing the device, irradiating under a 300w xenon lamp with a 420nm optical filter, sampling every one hour, and analyzing evolution of hydrogen by using an online gas chromatograph.
Example 2: high pressure (30MPa) supercritical stripping, doping, photocatalysis
Preparing carbon nitride: the method comprises the steps of taking 5.0g of melamine by adopting a traditional thermal polymerization mode, fully grinding, placing in a crucible with a cover, heating to 600 ℃ in a tube furnace at the speed of 5 ℃/min, keeping for 120min, and collecting a light yellow product, namely blocky carbon nitride after annealing.
Stripping of carbon nitride: batch and high pressure stripping operations are achieved in a medium scale closed reactor. 0.2g of carbon nitride was placed in a closed reaction vessel and the sealing of the apparatus was ensured. Introducing compressed carbon dioxide into the reactor, exhausting air, and introducing carbon dioxide into the reactor to 30Mpa until the carbon dioxide reaches supercritical state. And after 48h of supercritical stripping, opening an exhaust valve, slowly releasing compressed carbon dioxide, collecting a product, placing the stripped carbon nitride in a mixed solvent of deionized water and absolute ethyl alcohol, ultrasonically assisting to strip for 30min, standing for 2h, centrifuging, collecting supernatant, and drying to obtain the ultrathin two-dimensional carbon nitride nanosheet.
Doping carbon nitride: dissolving 0.1g of ultrathin two-dimensional carbon nitride nanosheet and 0.012g of cobalt chloride hexahydrate in 7.5ml of acetonitrile solvent, performing ultrasonic treatment for 5min to fully disperse the sample in acetonitrile, then dropwise adding 65 mu l of triethylamine under uniform stirring, fully stirring the mixed solution for 1h, transferring the obtained mixed solution into a microwave tube, and performing microwave reaction for 2 h. And centrifuging to collect a product, washing the product with chloroform, absolute methanol and acetonitrile respectively, and drying the product at 80 ℃ overnight to obtain the ultrathin two-dimensional doped carbon nitride nanosheet.
And (3) photocatalysis: carrying out a photocatalytic hydrogen evolution experiment in a photocatalytic hydrogen production system; 20mg of blocky carbon nitride and ultrathin two-dimensional doped carbon nitride nanosheets are respectively dispersed in a solvent containing 40ml of distilled water, 10ml of triethanolamine and H2PtCl6·6H2In a mixed solution of O, H2PtCl6·6H2Adding O in an amount of 3 wt% of the blocky carbon nitride or ultrathin two-dimensional doped carbon nitride nanosheet, ultrasonically dispersing for 30min, introducing nitrogen and exhausting air for 30min, sealing the device, irradiating under a 300w xenon lamp with a 420nm optical filter, sampling every one hour, and analyzing evolution of hydrogen by using an online gas chromatograph.
Example 3: phosphorizing carbon nitride, supercritical stripping, doping, and photocatalysis
Preparation of phosphated carbon nitride: mixing 2.5g of melamine and 2.5g of urea by adopting a traditional thermal polymerization mode, fully grinding, placing in a crucible with a cover, heating to 600 ℃ in a tube furnace at the speed of 5 ℃/min, keeping for 120min, and collecting a light yellow product, namely blocky carbon nitride after annealing;
placing carbon nitride and sodium hypophosphite with the mass ratio of 5:1 into a mortar, physically and uniformly mixing, dripping absolute ethyl alcohol to soak the mixture, fully grinding, drying the mixture, placing the dried mixture into a crucible with a cover, heating to 400 ℃ at the speed of 5 ℃/min in a tubular furnace, keeping the temperature for 120min, and collecting dark yellow products after annealing to obtain the phosphorized carbon nitride.
Stripping of phosphated carbon nitride: batch and high pressure stripping operations are achieved in a medium scale closed reactor. 0.2g of carbon phosphide nitride powder was placed in a closed reaction vessel and the sealing of the apparatus was ensured. Introducing compressed carbon dioxide into the reactor, performing air exhaust operation, and then continuously introducing the carbon dioxide into the reactor to 30MPa, wherein the carbon dioxide reaches a supercritical state. And after 48h of supercritical stripping, opening an exhaust valve, slowly releasing compressed carbon dioxide, collecting a product, placing the stripped carbon phosphide nitride in a mixed solvent of deionized water and absolute ethyl alcohol, ultrasonically stripping for 30min, standing for 2h, centrifuging, collecting a supernatant, and drying to obtain the ultrathin two-dimensional carbon phosphide nanosheet.
Doping of phosphorized carbon nitride: dissolving 0.1g of ultrathin two-dimensional carbon nitride phosphide nanosheet and 0.012g of cobalt chloride hexahydrate in 7.5ml of acetonitrile solvent, carrying out ultrasonic treatment for 5min to fully disperse the sample in acetonitrile, then dropwise adding 65 mu l of triethylamine under uniform stirring, fully stirring the mixed solution for 1h, transferring the obtained mixed solution into a microwave tube, and carrying out microwave reaction for 2 h. And centrifuging to collect a product, washing the product with chloroform, absolute methanol and acetonitrile respectively, and drying the product at 80 ℃ overnight to obtain the ultrathin two-dimensional doped phosphated carbon nitride nanosheet.
And (3) photocatalysis: photocatalytic hydrogen evolution experiments were performed in a photocatalytic hydrogen production system. 20mg of blocky carbon nitride, carbon phosphide, ultrathin two-dimensional carbon phosphide nanosheet and ultrathin two-dimensional doped carbon phosphide nanosheet are respectively dispersed in a solvent containing 40ml of distilled water, 10ml of triethanolamine and H2PtCl6·6H2In a mixed solution of O, H2PtCl6·6H2Adding 3 wt% of O into the carbon nitride, carbon nitride phosphide, ultrathin two-dimensional carbon nitride phosphide nanosheet or ultrathin two-dimensional doped carbon nitride phosphide nanosheet, ultrasonically dispersing for 30min, then introducing nitrogen and exhausting air for 30min, sealing the device, placing the device under a 300w xenon lamp with a 420nm optical filter for irradiation, sampling every hour, and analyzing evolution of hydrogen by using an online gas chromatograph.
Claims (7)
1. A method for preparing ultrathin two-dimensional carbon nitride or carbon nitride phosphide nanosheets by supercritical stripping is characterized by comprising the following specific steps: compressing carbon dioxide to reach a supercritical state in a high-pressure closed reaction kettle, so that carbon dioxide molecules are embedded into a two-dimensional carbon nitride or phosphorized carbon nitride material in a liquid molecule form for supercritical stripping, and the carbon nitride or phosphorized carbon nitride material is stripped into a lamellar layer; then ultrasonic-assisted stripping is carried out, and the lamellar carbon nitride or carbon phosphide nitride is further stripped into ultrathin two-dimensional carbon nitride or carbon phosphide nanosheets.
2. The method for preparing ultrathin two-dimensional carbon nitride or phosphated carbon nitride nanosheets by supercritical exfoliation according to claim 1, wherein the ultrasonic-assisted exfoliation is carried out in a solvent which is one or a mixture of water, absolute ethyl alcohol and isopropyl alcohol for 10-60min by ultrasound.
3. A method for preparing ultrathin two-dimensional doped carbon nitride or phosphated carbon nitride nanosheets by supercritical stripping is characterized by comprising the following steps: the ultrathin two-dimensional carbon nitride or carbon phosphide nanosheet prepared according to claim 1, being doped monoatomic.
4. The supercritical stripping method for preparing ultrathin two-dimensional doped carbon nitride or carbon phosphide nanosheets according to claim 3, wherein the doping method comprises: dissolving ultrathin two-dimensional carbon nitride or carbon phosphide nanosheets and cobalt salt in an acetonitrile solvent, performing ultrasonic treatment for 2-10min to fully disperse a sample in acetonitrile, then dropwise adding triethylamine under uniform stirring, fully stirring the mixed solution for 0.5-2h, then transferring the mixed solution into a microwave tube, performing microwave reaction for 1-4h, centrifugally collecting products, washing with trichloromethane, anhydrous methanol and acetonitrile respectively, and drying to obtain ultrathin two-dimensional carbon nitride or carbon phosphide doped nanosheets.
5. The application of the ultrathin two-dimensional carbon nitride or phosphated carbon nitride nanosheet prepared according to the method of claim 1 as a photocatalyst in a reaction of photocatalytic decomposition of water.
6. The application of the ultrathin two-dimensional doped carbon nitride or phosphated carbon nitride nanosheet prepared according to the method of claim 3 as a photocatalyst in a photocatalytic water splitting reaction.
7. The use according to claim 5 or 6, wherein the conditions of the reaction for photocatalytic water splitting are: in a photocatalytic hydrogen production device, a photocatalyst, distilled water, triethanolamine and H are added2PtCl6Mixing, ultrasonic dispersing, discharging air with nitrogen, sealing, and photocatalytic water decomposing reaction under irradiation of visible light.
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