Mxene-based photothermal composite foam material and preparation method and application thereof

Abstract

The application provides a MXene-based photothermal composite foam material and a preparation method and application thereof, and the preparation method comprises the following steps: mixing MXene nanosheets, polyvinyl alcohol, a crosslinking agent, a foam stabilizer, an acid catalyst and water at-5-10 DEG C to obtain a precursor solution; performing high-speed stirring on the precursor solution at a rotating speed of greater than or equal to 10000 rpm, and placing the precursor solution in a condition of 20-50 DEG C to perform a crosslinking reaction, so as to obtain a crosslinked composite foam material crude product; and performing dialysis treatment on the crosslinked composite foam material crude product in water to obtain a composite foam material. The composite foam material has good compressibility, a relatively light specific gravity, good water absorption, a photothermal effect and the like, and can be used for seawater desalination and the like.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of composite material preparation, and particularly relates to a MXene-based photothermal composite foam material and a preparation method and application thereof. BACKGROUND

[0002] MXene is a two-dimensional material with adjustable components, which has high wide-spectrum absorption characteristics in the entire solar spectrum (280-2500 nm) and is an ideal solar spectrum material, and is expected to be applied to the field of solar photothermal seawater desalination. There are many studies on MXene-based solar photothermal seawater desalination, but the evaporation rate is still low, and the main reasons are as follows: 1) MXene has low chemical stability in water, and MXene will degrade, the color will become gray or even white during long-term storage; 2) caused by the limitation of the structure of the MXene-based seawater desalination device. Therefore, it is necessary to provide a MXene-based photothermal composite material which can be used for efficient seawater desalination. SUMMARY

[0003] Therefore, the present application provides a MXene-based photothermal composite foam material and a preparation method thereof, which can efficiently use MXene for seawater desalination.

[0004] In a first aspect, the present application provides a preparation method of a MXene-based photothermal composite foam material, which comprises the following steps:

[0005] (1) mixing MXene nanosheets, polyvinyl alcohol (PVA), a crosslinking agent, a foam stabilizer, an acid catalyst and water at -5-10°C to obtain a precursor solution;

[0006] (2) performing ultrahigh-speed stirring on the precursor solution at a rotation speed greater than or equal to 10,000 rpm, and crosslinking the precursor solution at 20-50°C to obtain a crude crosslinked composite foam material; and then performing dialysis treatment on the crude crosslinked composite foam material in water to obtain a MXene-based photothermal composite foam material.

[0007] In the present application, the chemical formula of MXene is M n+1 X n T x , wherein M is a transition metal element, X is a carbon element and / or a nitrogen element, n is an integer of 1-3, T x represents a surface group, and T can be selected from at least one of O 2- , OH - , F - and NH4 + . Further, the MXene material can be specifically Ti3C2T x , Ti2CT x , (Ti0.5 Nb 0.5 )2CT x , (V 0.5 , Cr 0.5 )3C2T x , Ta4C3T x , V2CT x , Nb2CT x , Nb4C3T x , (Nb 0.8 , Ti 0.2 )4C3T x , (Nb 0.8 , Zr 0.2 )4C3T x , Ti3CNT x , Mo2TiC2T x , Mo2Ti2C3T x and Cr2TiC2T x The MXene nanoplatelets described above are two-dimensional layered materials with a thickness of nanometers, for example, the thickness can be 3-20 nm. Optionally, the lateral size of the MXene nanoplatelets is 100-8000 nm. The lateral size refers to the length or width of the MXene nanoplatelets.

[0008] The MXene nanoplatelets used in the present application can be commercially available materials or prepared by the applicant. The present application is not limited to the preparation method of the MXene nanoplatelets used, which can be obtained by acid etching, high-temperature etching or gas-phase etching of the A element (such as Al element) in the precursor material MAX (M represents a transition metal element, A is usually Al or Si, X includes C, N or C and N elements).

[0009] For example, the MXene nanoplatelets can be prepared in the following manner: mixing the bulk MXene precursor material MAX, etchant and water, then reacting, centrifuging the obtained reaction mixture to wash the acid, adding water and performing low-temperature water bath ultrasonic treatment, then performing low-speed centrifugation on the ultrasonic liquid, collecting the supernatant, performing high-speed centrifugation on the obtained supernatant, collecting the precipitate, and the obtained precipitate is the MXene nanoplatelets. The etchant can include but is not limited to hydrofluoric acid, hydrochloric acid / lithium fluoride, hydrofluoric acid / hydrogen peroxide, etc. Optionally, the low-temperature water bath ultrasonic treatment can be performed at a temperature not exceeding 10°C, preferably 0-10°C. The power of the water bath ultrasonic treatment can be 300-450 W and the time can be 3-24 h. The low-speed centrifugation of the solution obtained after water bath ultrasonic treatment can be performed at a speed of 2000-4000 rpm. The high-speed centrifugation can be performed at a speed of 10000-15000 rpm.

[0010] In step (1), the cross-linking agent in the precursor solution can be selected from one or more of dialdehydes (such as glyoxal, glutaraldehyde), paraformaldehyde, urea-formaldehyde resin, melamine-formaldehyde resin, boron compounds (such as boric acid, sodium borate, zinc borate), maleic anhydride, etc., and glutaraldehyde is preferred. The acid catalyst can be one or more of hydrochloric acid, sulfuric acid, acetic acid, etc., and hydrochloric acid is preferred, which has better catalytic effect. The foam stabilizer can include one or more of alkyl glycoside, cationic surfactant, anionic surfactant, and non-ionic surfactant, and the effect of non-ionic surfactant Pluronic F-127 is better.

[0011] Optionally, in step (2), the mass ratio of polyvinyl alcohol to MXene nanosheet to cross-linking agent is 1:(0.002-0.05):(0.1-0.5). This helps to form a composite foam material with a suitable cross-linking degree subsequently. Optionally, the mass ratio of cross-linking agent to acid catalyst is (0.1-0.5):(0.1-0.5). An appropriate amount of acid catalyst can ensure that the prepared precursor solution has a suitable cross-linking speed and uniform cross-linking strength at a certain temperature. In addition, the mass ratio of polyvinyl alcohol to foam stabilizer is 1:(0.025-0.2). An appropriate amount of foam stabilizer helps the resulting composite foam material to have good structural stability and stable compression-rebound performance.

[0012] In preparing the precursor solution, the form of each raw material, the specific mixing method, etc. are not limited. For example, an aqueous solution of polyvinyl alcohol and a cross-linking agent solution can be prepared first, and then they are mixed with MXene nanosheet, acid catalyst, and foam stabilizer; or MXene nanosheet can be added to the aqueous solution of polyvinyl alcohol first, then cross-linking agent and foam stabilizer are added, and finally acid catalyst is added, etc. In some embodiments of the present application, the precursor solution is obtained by mixing the poly-ion liquid grafted MXene nanosheet with an aqueous solution of polyvinyl alcohol, a cross-linking agent solution, an acid catalyst, and a foam stabilizer at -5-10°C. Optionally, the mass percentage of polyvinyl alcohol in the aqueous solution of polyvinyl alcohol can be 5-20wt%, for example, 10%. In an embodiment, the mass percentage of cross-linking agent in the cross-linking agent solution is 50wt%. In some embodiments, the amount of poly-ion liquid grafted MXene nanosheet can be 10-50mg, preferably 20-40mg, for example, 35mg. When the foam stabilizer is F-127, its amount can be 10-80mg, for example, 20mg.

[0013] In some embodiments of the present application, before the precursor solution is prepared, the MXene nanosheet surface is grafted with a polyionic liquid, specifically comprising: mixing the MXene nanosheet with a cationic ionic liquid monomer containing a carbon-carbon double bond and water, and performing gamma ray irradiation under anaerobic conditions to graft the polymer of the ionic liquid monomer on the surface of the MXene nanosheet, centrifuging the resulting reaction solution, and collecting the precipitate, the resulting precipitate comprising polyionic liquid grafted MXene nanosheet. It should be noted that when the MXene nanosheet in the precursor solution is specifically a polyionic liquid grafted MXene nanosheet, the mass ratio of the polyionic liquid grafted MXene nanosheet to PVA polyvinyl alcohol still satisfies (0.002-0.05):1.

[0014] In the preparation of the polyionic liquid grafted MXene nanosheet in step (1), gamma rays are used for irradiation, wherein gamma rays have high energy, strong penetration, and good irradiation uniformity, and when they interact with common solvents (such as water, alcohol, etc.), they can generate abundant and uniformly distributed free radicals or electron-rich species, which can act as initiators for unsaturated monomers (here referring to the cationic ionic liquid monomer containing a carbon-carbon double bond). Secondly, the MXene nanosheet has an electron-rich system, and under high-energy gamma ray irradiation, the surface of the MXene nanosheet can undergo bond cleavage of C-OH, C-F, Ti-F, etc., forming free radicals, which can act as initiators to initiate the polymerization of the ionic liquid monomer, and the polymerization product (referred to as "polyionic liquid") is grafted on the surface of the MXene nanosheet.

[0015] After the MXene nanosheet surface is grafted with a polyionic liquid, the surface hydrophobicity is enhanced, which can reduce the contact between the MXene nanosheet and oxygen, and the stability of the MXene nanosheet is greatly improved. In addition, the grafting of the polyionic liquid is equivalent to forming a certain coating on the surface of the MXene nanosheet, which also reduces the degradation of the MXene nanosheet and is conducive to the stable exertion of its photothermal effect.

[0016] In the embodiments of the present application, the cation of the ionic liquid monomer can be a sulfonium salt cation (containing S), a quaternary phosphonium salt cation (containing P), and an imidazole cation, a pyridine cation, a pyrrole cation, a piperidine cation, and a quaternary ammonium salt cation (the latter four all contain N), and the cation all has a carbon-carbon unsaturated double bond to enable polymerization under gamma ray irradiation. Taking the imidazole cation, the pyridine cation, the pyrrole cation, and the piperidine cation as examples, the imidazole ring, the pyridine ring, the pyrrole, and the piperidine ring all have a substituent containing a carbon-carbon double bond.

[0017] In some embodiments of the present application, the cationic carbon-carbon double bond containing ionic liquid monomer can be an allyl type imidazole ionic liquid, the cation of which is an allyl type imidazole cation, and the imidazole ring has at least one allyl type substituent. Specifically, the allyl type imidazole ionic liquid can include one or more of 1-methyl-3-allylimidazole tetrafluoroborate, 1-methyl-3-allylimidazole hexafluorophosphate, 1-aminopropyl-3-allylimidazole tetrafluoroborate, 1-aminopropyl-3-allylimidazole hexafluorophosphate, and 1-methyl-3-allylimidazole bromide, etc. In some embodiments, the allyl type imidazole ionic liquid includes one or both of 1-methyl-3-allylimidazole tetrafluoroborate and 1-methyl-3-allylimidazole hexafluorophosphate, which have better solubility in water.

[0018] In step (1), when preparing the poly ionic liquid grafted MXene nanosheet, the mass ratio of the MXene nanosheet to the ionic liquid monomer to water is 100:(0.5-10):500. This ratio can ensure that the MXene nanosheet is grafted with an appropriate amount of poly ionic liquid, so that the dispersion and stability of the MXene nanosheet are ensured, and the overall light-heat effect of the poly ionic liquid grafted MXene nanosheet is not too low. In some embodiments, the mass ratio is 100:(2-8):500. Alternatively, in the poly ionic liquid grafted MXene nanosheet, the grafting rate of the poly ionic liquid is 0.5-20%.

[0019] In the process of γ-ray irradiation, the irradiation dose rate of the γ-ray used can be 10-100 kGy, and the irradiation time can be 10-120 minutes. Preferably, the irradiation dose rate of the γ-ray can be 20-80 kGy, and the irradiation time can be 20-80 minutes. In some specific embodiments, the irradiation dose rate of the γ-ray is 50 kGy, and the irradiation time is 60 minutes.

[0020] Alternatively, before the γ-ray irradiation, the mixed solution of the MXene nanosheet, the ionic liquid monomer, and water can be subjected to oxygen removal treatment (such as argon bubbling, etc.), so as to prevent the dissolved oxygen in the mixed solution from causing degradation of the MXene nanosheet.

[0021] In the process of γ-ray irradiation, the irradiation dose rate of the γ-ray used can be 10-100 kGy, and the irradiation time can be 10-120 minutes. Preferably, the irradiation dose rate of the γ-ray can be 20-80 kGy, and the irradiation time can be 20-80 minutes. In some specific embodiments, the irradiation dose rate of the γ-ray is 50 kGy, and the irradiation time is 60 minutes.

[0022] In some embodiments of the present application, in step (2), the stirring speed of the ultrahigh-speed stirring can be 20,000-40,000 rpm, for example, 25,000 rpm, 30,000 rpm, 35,000 rpm, etc. Wherein, the ultrahigh-speed stirring of the precursor solution in step (1) is conducive to realizing foaming, especially conducive to PVA forming abundant foam, plus the presence of the foam stabilizer, which can make the obtained foam fine and uniform, while ensuring the short-term stability of the foam. At a suitable temperature (such as 20-50°C mentioned in step (3)), the PVA foam can react with the crosslinking agent to form a MXene-based composite foam material with certain mechanical strength and abundant pores, and the polyionic liquid grafted MXene nanosheet is wrapped therein. Wherein, the coating of the composite foam helps to further reduce the agglomeration of the MXene nanosheet dispersed therein, significantly improving the service life of the MXene nanosheet and the efficient play of its photothermal effect.

[0023] Therefore, the above-obtained MXene-based photothermal composite foam material has a rich porous structure, is light in quality, can float on the sea surface, and exhibits excellent water absorption, and can still float on the sea surface after absorbing water, and can realize seawater desalination by means of the stable photothermal effect of the MXene nanosheet; and the composite foam material also has excellent compressibility and good elasticity, which is convenient for cleaning to realize repeated use.

[0024] Optionally, in step (2), the crosslinking time is 2-20 min, 5-15 min, for example, 10 min. In the present application, after obtaining the crosslinked composite foam material crude product, the purpose of dialysis in water is to remove excess raw materials such as crosslinking agent, acid catalyst, etc. Optionally, the dialysis can be carried out at a temperature of 4-10°C, and the dialysis time can be 48-72 h.

[0025] The preparation method of the MXene-based photothermal composite foam material provided by the present application uses MXene nanosheet as a photothermal reagent and polyvinyl alcohol-based foam as a carrier for dispersing the photothermal reagent, and the prepared MXene-based photothermal composite foam material is a new product, which has good water absorption, light quality, obvious photothermal effect, good compressibility and many other advantages, and can be expected to be applied in the field of solar photothermal seawater desalination. The above preparation method has the advantages of simple process, easy operation, low cost, high yield and easy industrialization production.

[0026] It should be noted that although the preparation method of the photothermal composite foam material provided by the present application is exemplified by using a foam system containing polyvinyl alcohol and a crosslinking agent, it should be understood that other foam systems, such as a system formed by cellulose, polysaccharides or derivatives thereof and a crosslinking agent (such as epichlorohydrin), can also be used to load MXene nanosheet.

[0027] The embodiment of the present application also provides a MXene-based photothermal composite foam material, comprising a porous foam substrate with a porous structure, wherein MXene nanosheets are loaded in the porous foam substrate, and the porous foam substrate has a skeleton structure formed by cross-linking of polyvinyl alcohol, a cross-linking agent and a foam stabilizer.

[0028] The MXene-based photothermal composite foam material has a rich porous structure, is light in weight, can float on the sea surface, has excellent water absorption, and can still float on the sea surface after water absorption. Under the irradiation of sunlight, the MXene nanosheets loaded in the composite foam material exhibit high-efficiency photothermal effect, so that the temperature of the composite foam material is increased, and the seawater adsorbed therein is evaporated. Fresh water evaporated from the composite foam can be collected, and the composite foam material can absorb seawater to fill the free water space due to its porous performance, so that continuous seawater absorption and evaporation can be realized, and fresh water evaporated from the composite foam can be collected. In addition, the composite foam material with a porous structure has excellent compressibility and resilience, and can be easily cleaned and reused like a "sponge".

[0029] It can be seen that the MXene-based photothermal composite foam material enriches the product form of MXene nanosheets, the structure stability of the MXene nanosheets grafted with polyionic liquid is high in the composite foam system, the photothermal effect can be fully exerted, and seawater desalination can be realized by means of the good water absorption and high mechanical lightness of the porous foam substrate, thereby expanding the practical application range of MXene materials.

[0030] In the embodiment of the present application, the porosity of the porous foam substrate is greater than or equal to 90%, for example, greater than or equal to 92%, or greater than or equal to 94%. Higher porosity is beneficial to the absorption of seawater and sunlight by the composite foam material, and is conducive to lightening.

[0031] In the embodiment of the present application, the MXene-based photothermal composite foam material has a porous structure on the surface and inside. The porous structure on the surface is a three-dimensional network structure formed by multi-level pores.

[0032] In the embodiments of the present application, the porous foam substrate has a plurality of first holes, and the hole walls of the first holes have a plurality of second holes. In this way, the composite foam material has a special structure of "hole-in-hole", which helps to improve the porosity of the composite foam material, and at the same time endows the composite foam material with good compression-rebound performance and suitable mechanical strength. Understandably, the pore size of the first holes is larger than that of the second holes. Optionally, the pore size of the first holes is in the range of 0.20mm-0.88mm; and the pore size of the second holes is in the range of 0.042mm-0.20mm. The "first holes" can be referred to as "large holes", and the "second holes" can be referred to as "mesopores". In some embodiments, the hole walls of the second holes further have a plurality of third holes. Among them, the hole wall surface of the third holes is in a cell-like structure. The "third holes" are referred to as "small holes".

[0033] In the embodiments of the present application, the outer surface of the MXene-based photothermal composite foam material is rough. The surface of the hydrogel product formed by cross-linking the aforementioned precursor solution without high-speed stirring is smooth, and therefore the MXene-based photothermal composite foam material provided in the embodiments of the present application can have better performance in absorbing sunlight. Among them, the absorption rate of the MXene-based photothermal composite foam material to full-waveband sunlight of 250nm-2500nm is greater than or equal to 80%. In some embodiments, the absorption rate to full-waveband sunlight is greater than or equal to 90%, or even greater than or equal to 95%.

[0034] In the embodiments of the present application, the contact angle of the outer surface of the MXene-based photothermal composite foam material to water is 0°. This indicates that the composite foam material has superhydrophilicity and can have a high water absorption rate. Among them, the water absorption rate of the MXene-based photothermal composite foam material in 10 seconds is 1690%-2255%. That is, the water absorption rate is 169-225.5%˙s -1 In addition, the photothermal composite foam material can basically reach water absorption equilibrium within 20 seconds.

[0035] In some embodiments of the present application, the MXene nanosheets are loaded in the skeleton structure of the porous foam substrate, and further, the MXene nanosheets can be coated by the porous foam substrate. In this way, the dispersibility of the MXene nanosheets can be further improved.

[0036] In the embodiments of the present application, the mass fraction of the MXene nanosheets in the MXene-based photothermal composite foam material can be 0.177%-2.86%. A lower mass fraction of grafted MXene nanosheets can ensure good photothermal effect of the photothermal composite foam material. In some embodiments, the mass of the MXene nanosheets is (0.002-0.05) times the mass of the polyvinyl alcohol.

[0037] In some embodiments of the present application, the surface of the MXene nanosheet is grafted with a polyionic liquid, i.e., the MXene nanosheet is specifically a polyionic liquid grafted MXene nanosheet. The polyionic liquid includes a polymer of a cationic carbon-carbon double bond containing ionic liquid monomer. After the surface of the MXene nanosheet is grafted with the polyionic liquid, the surface hydrophobicity of the MXene nanosheet is enhanced, the probability of degradation by oxygen and the like is reduced, and the stability is greatly improved. By loading the polyionic liquid grafted MXene nanosheet in the above porous foam matrix, the degradation probability of the MXene nanosheet can be further reduced and the dispersion uniformity thereof can be improved, and the service life and the efficient exertion of the photothermal effect thereof can be significantly improved.

[0038] In the embodiments of the present application, in the polyionic liquid grafted MXene nanosheet, the grafted polyionic liquid forms a coating layer on the surface of the MXene nanosheet. The presence of the polyionic liquid coating layer can prevent the agglomeration between the MXene nanosheets, improve the dispersion thereof, and also inhibit the degradation of the MXene nanosheet, so that the photothermal effect thereof can be more persistent and efficient.

[0039] The embodiments of the present application also provide applications of the above-mentioned MXene-based photothermal composite foam material in the preparation of seawater desalination materials, elastic materials, photothermal materials, and absorbing materials.

[0040] In particular, the above-mentioned MXene-based photothermal composite foam material has many advantages such as high stability, compressibility, obvious photothermal effect, good water absorption, and the like, and is particularly suitable for use in the field of solar photothermal seawater desalination. Of course, the application of the composite foam material is not limited to the above-mentioned field of seawater desalination, and the composite foam material can also be used in the field of water treatment, the field of photothermal, and the like. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 The photos of the foam systems obtained after high-speed stirring treatment of Example 5 and Comparative Example 3 are summarized. (a) is the foam of Comparative Example 3; (b) is the foam of Example 5; (c) is the foam of Comparative Example 3; (d) and (e) are the foams of Example 5.

[0042] Figure 2 The photos of the preparation processes of Example 5 and each comparative example are summarized, wherein, Figure 2 The photos of the precursor solutions of Example 5 and each comparative example are summarized in subgraph a from left to right, which are the precursor solutions before crosslinking of Comparative Example 1, Comparative Example 2, Comparative Example 3 and Example 5; the photos of the self-floating conditions of the crosslinked samples of Comparative Example 1, Comparative Example 2, Comparative Example 3 and Example 5 in water are summarized in subgraph b from left to right; the photos of the wet samples after dialysis of Comparative Example 1, Comparative Example 2, Comparative Example 3 and Example 5 are summarized in subgraph c from left to right; Figure 2Figures d, e, and f show photographs of the top surface, bottom surface, and overall cylindrical product, respectively, of the product of Example 5 in a dry state.

[0043] Figure 3 Polarized light microscope photographs of the precursor solution after high speed stirring of Comparative Example 2 (a), the precursor solution after high speed stirring of Comparative Example 3 (b), the precursor solution after high speed stirring of Example 4 (c), and the precursor solution after high speed stirring of Example 5 (d) are summarized.

[0044] Figure 4 Scanning electron microscope photographs of the final product of Comparative Example 1 (a), the final product of Comparative Example 2 (b), the final product of Comparative Example 3 (c), and the final product of Example 5 (d) are summarized.

[0045] Figure 5 Scanning electron microscope photographs of the “pore-in-pore” structure of the final product of Example 5 (a, top four photographs) and the surface of the final product of Example 5 (b, bottom four photographs) are summarized.

[0046] Figure 6 Water absorption properties of the final products of Examples 2-5 and Comparative Example 1 are summarized.

[0047] Figure 7 Solar light absorption properties of the final products of Examples 2-5 are summarized.

[0048] Figure 8 Compressible recovery curves of the final products of Examples 2-5 at different strains are summarized. DETAILED DESCRIPTION

[0049] The examples of the present application are further illustrated in the following examples. The examples of the present application are not limited to the following specific examples. Changes and modifications can be made within the scope of the appended claims. Unless otherwise specified, the starting materials and other chemicals used in the examples of the present application are commercially available.

[0050] Example 1

[0051] A method for preparing a MXene-based photothermal composite foam material, comprising the following steps:

[0052] (1) Preparation of MXene material: Ti3AlC2 powder was slowly added to a mixed solution of lithium fluoride (LiF) and concentrated hydrochloric acid (HCl) which had been prepared in advance, and an etching reaction was carried out at a temperature of 30°C for 24 hours. The obtained reaction solution was centrifuged and washed, and the solid precipitate was collected until the centrifugate was neutral or close to neutral. Then the solid precipitate obtained by centrifugation was subjected to low-temperature water bath ultrasonic treatment (temperature not exceeding 4°C, ultrasonic power 400W, time 12h) in deionized water. After the water bath ultrasonic treatment, the obtained solution was first subjected to low-speed centrifugation at 2000 revolutions per minute, and the supernatant was collected. Then the supernatant was subjected to high-speed centrifugation at 12000 revolutions per minute, and the obtained precipitate was collected as MXene nanosheets (specifically Ti3C2T x nanosheets).

[0053] (2) 1 mg of the above MXene nanosheets was mixed with a 10wt% aqueous solution of polyvinyl alcohol (PVA) (amount 4g), a 50wt% aqueous solution of glutaraldehyde (glutaraldehyde mass 113.75mg), 20μL of a commercially available concentrated hydrochloric acid solution (concentration 12mol / L), and 20mg of a foam stabilizer (specifically F-127) at -5-10°C to obtain a precursor solution.

[0054] (3) The above precursor solution was subjected to ultrahigh-speed stirring at 35,000 revolutions per minute, and the precursor solution was allowed to undergo crosslinking reaction at 30°C to obtain a crosslinked composite foam crude product. Then the composite foam crude product was subjected to dialysis treatment in deionized water to obtain a MXene-based photothermal composite foam material.

[0055] Example 2

[0056] The difference between Example 2 and Example 1 is that the amount of MXene nanosheets is 1.375mg. That is, the mass ratio of MXene nanosheets to PVA is 0.003434:1.

[0057] Example 3

[0058] The difference between Example 3 and Example 1 is that the amount of MXene nanosheets is 2.75mg. That is, the mass ratio of MXene nanosheets to PVA is 0.006875:1.

[0059] Example 4

[0060] The difference between Example 4 and Example 1 is that the amount of MXene nanosheets is 5.5mg. That is, the mass ratio of MXene nanosheets to PVA is 0.01375:1.

[0061] Example 5

[0062] Example 5 differs from Example 1 in that the amount of MXene nanosheets is 10 mg. That is, the mass ratio of MXene nanosheets to PVA is 0.025:1.

[0063] Example 6

[0064] Example 6 differs from Example 1 in that the amount of MXene nanosheets is 50 mg. That is, the mass ratio of MXene nanosheets to PVA is 0.125:1.

[0065] Example 7

[0066] Example 7 differs from Example 5 in that the MXene nanosheets are further grafted with a polyionic liquid before being mixed with the aqueous PVA solution, and the grafting process comprises the following steps:

[0067] The MXene nanosheets, the ionic liquid (specifically, 1-methyl-3-allyl imidazole hexafluoroborate), and water are mixed in a mass ratio of 100:5:500, and placed in a customized irradiation tube. Argon gas is bubbled to remove oxygen for 40 minutes, and then the mixed solution is subjected to γ-ray irradiation from a cobalt source at a dose rate of 50 kGy for 60 minutes to graft the polymer of the ionic liquid monomer onto the surface of the MXene nanosheets. Subsequently, the obtained reaction solution is subjected to high-speed centrifugation at a speed of 12,000 revolutions per minute, and sequentially washed with methanol, ethanol, and deionized water. The obtained precipitate is the MXene nanosheets grafted with the polyionic liquid.

[0068] Example 8

[0069] Example 8 differs from Example 7 in that the mass ratio of MXene nanosheets to ionic liquid monomers in the MXene nanosheets grafted with the polyionic liquid is 100:2.

[0070] Example 9

[0071] Example 9 differs from Example 7 in that the mass ratio of MXene nanosheets to ionic liquid monomers in the MXene nanosheets grafted with the polyionic liquid is 100:10.

[0072] To highlight the beneficial effects of the present application, the following comparative examples are provided.

[0073] Comparative Example 1

[0074] A PVA hydrogel and aerogel, the preparation process of which is different from that of Example 1 in that the PVA aqueous solution, the glutaraldehyde aqueous solution and the concentrated hydrochloric acid are directly mixed at low temperature to obtain a precursor solution; then the precursor solution is stirred at 500 revolutions per minute and cross-linked at 30°C to obtain a cross-linked hydrogel crude product; then the hydrogel crude product is dialyzed in deionized water for purification, and freeze-dried to obtain a PVA aerogel.

[0075] Comparative Example 2

[0076] A PVA foam material, the preparation process of which is different from that of Example 1 in that the PVA aqueous solution, the glutaraldehyde aqueous solution and the concentrated hydrochloric acid are directly mixed at low temperature to obtain a precursor solution which is subjected to high-speed stirring and cross-linking reaction at 30°C, and then dialyzed in deionized water.

[0077] Comparative Example 3

[0078] A PVA / foam stabilizer-containing foam material, the preparation process of which is different from that of Example 1 in that the precursor solution of step (2) does not contain MXene nanosheets. That is, the precursor solution of Comparative Example 3 is obtained by mixing a PVA aqueous solution (the amount of PVA is 400 mg) with a glutaraldehyde aqueous solution (the mass of glutaraldehyde is 113.75 mg), a commercially available concentrated hydrochloric acid (the molar amount of HCl is 0.24 mmol) and 20 mg of F-127 at -5-10°C.

[0079] Figure 1 The photos of the foam systems obtained after high-speed stirring of Example 5 and Comparative Example 3 are summarized, wherein (a), (c) are the foams of Comparative Example 3; (b), (d), (e) are the foams of Example 5. It can be known from the photos that the PVA foam hydrogel system (Comparative Example 3, Figure 1 It can be known that the PVA foam hydrogel system (Comparative Example 3, Figure 1 (a)) and the PVA / F-127 / MXene foam hydrogel system (Example 5, Figure 1 (b)) are in a completely foaming state after high-speed stirring, and the foams are rich and delicate, which is helpful for the subsequent cross-linking reaction. It can be imagined that the ordinary PVA hydrogel precursor solution (Comparative Example 1) without high-speed stirring is still in a solution type flow state after ordinary stirring (such as 500 revolutions per minute). In addition, the PVA foam hydrogel system (Comparative Example 3, Figure 1 (c)) and the PVA / F-127 / MXene foam hydrogel system (Example 5, Figure 1(d) and (e) exhibit a certain viscosity, showing low flowability, which is beneficial for their adhesion or film formation on common substrates. It is conceivable that the ordinary PVA hydrogel precursor solution (Comparative Example 1) without high-speed stirring has high flowability and is not easy to adhere to relatively flat or sloping substrates.

[0080] Figure 2 The preparation process photos of Example 5 and each comparative example are summarized, among which, Figure 2 The middle image (a) summarizes photographs of the precursor solutions of Example 5 and each comparative example, from left to right: the precursor solutions of Comparative Example 1, Comparative Example 2, Comparative Example 3, and Example 5 before crosslinking; the middle image (b) summarizes photographs of the self-floating state of the crosslinked samples of Comparative Example 1, Comparative Example 2, Comparative Example 3, and Example 5 in water, from left to right; the middle image (c) summarizes photographs of the wetted samples of Comparative Example 1, Comparative Example 2, Comparative Example 3, and Example 5 after dialysis, from left to right. Figure 2 The small figures d, e, and f show photographs of the upper surface, lower surface, and overall cylindrical product of Example 5 in a dry state, respectively.

[0081] Depend on Figure 2 As shown in the small figure (a), the precursor solution corresponding to the preparation of ordinary pure PVA hydrogel (Comparative Example 1) is colorless and transparent after low-speed stirring. After ultra-high-speed stirring, the precursor solution exhibits a foaming emulsified state (Comparative Example 2), with a significant volume expansion, mainly due to the introduction of a large number of air bubbles into the system by high-speed stirring. Adding a foam stabilizer to the precursor solution of Comparative Example 2 (Comparative Example 3) further increases the volume of the precursor solution, indicating a further increase in the air bubble content and improved bubble stability. Further addition of MXene nanosheets to the precursor solution (Example 5) did not affect the foaming state of PVA in the precursor solution; the composite system still exhibited a highly foamed state. Figure 2 As shown in the small figure (b), the precursor solutions of each comparative example and embodiment, after undergoing the crosslinking reaction, yielded products with shapes matching those of the solutions before the crosslinking reaction. Importantly, the samples of Comparative Example 2, Comparative Example 3, and Example 5 exhibited a "self-floating" state in water, while Comparative Example 1 sank. This indicates that the sample obtained through high-speed stirring has a lower bulk density and higher porosity, while the hydrogel sample obtained through ordinary stirring has a higher density and lower porosity, thus failing to float in water. Figure 2As shown in the small figure, the obtained ordinary PVA hydrogel (Comparative Example 1) is relatively hard and has a certain gloss, indicating that its surface roughness is low. In contrast, the samples obtained after high-speed rotation (Comparative Examples 2, 3, and 5) are softer, exhibit compressibility and resilience, and have no metallic luster, indicating that they have a certain degree of surface roughness. Furthermore, the top and bottom surfaces of the final product of Example 5 are rough, which has the advantage of increased light absorption compared to the smooth surface of the conventional hydrogel in Comparative Example 1. Additionally, after drying, the overall product of Example 5 shows obvious pores, contributing to its lower density, which is beneficial for absorbing seawater and sunlight.

[0082] Figure 3 Polarizing microscope images of the precursor solution after high-speed stirring in Comparative Example 2 (small image a), the precursor solution after high-speed stirring in Comparative Example 3 (small image b), the precursor solution after high-speed stirring in Example 4 (small image c), and the precursor solution after high-speed stirring in Example 5 (small image d) are summarized.

[0083] from Figure 3 It can be seen that when the precursor solution of ordinary pure PVA hydrogel is stirred at ultra-high speed, a large number of bubbles are introduced into the system, and these bubbles vary in size considerably. Figure 3 (Figure a). After further introducing a foam stabilizer into the PVA precursor solution system, the bubbles became further refined, and the size difference between the bubbles significantly decreased. Figure 3 (See small figure in b). Upon further penetration into the MXene nanosheets, the size difference of the bubbles did not change significantly, but the bubble walls thickened considerably, indicating that the MXene nanosheets were mainly distributed at the bubble edges. Figure 3 (See small image in middle c). Further increasing the relative mass ratio of MXene nanosheets to PVA resulted in an increase in the number of bubbles, a significant decrease in their size, and a further increase in their wall thickness. Figure 3 (Middle d small image).

[0084] Figure 4 Scanning electron microscope (SEM) images of the final products of Comparative Example 1 (small image a), Comparative Example 2 (small image b), Comparative Example 3 (small image c), and Example 5 (small image d) are compiled. Figure 4It can be seen that Comparative Example 1 yielded a pure PVA hydrogel product, whose surface lacks effective pores, making it unable to effectively achieve rapid water transport. When the PVA precursor solution was stirred at ultra-high speed, a large number of air bubbles were introduced into the system. These bubbles resulted in numerous irregularly shaped pores in the final foam product (see small figure b). Further introduction of a foam stabilizer into the PVA-containing precursor liquid system further refined these pores, forming a typical "pore-within-pore" structure (see small figure c), i.e., small pores exist on the pore walls. Upon further introduction of MXene nanosheets (see small figure d), the resulting composite foam material still exhibited abundant pores on the pore walls, and the number of small pores on the macropore walls increased. These results indicate that the MXene-based composite foam material of this application possesses a rich pore structure and low density, which enables it to exhibit excellent compressibility.

[0085] Figure 5 The "hole-in-hole" structure of the product of Example 5 and scanning electron microscope images of the surface of the product of Example 5 are summarized; among them, Figure 5 The four diagrams corresponding to 'a' in the middle Figure 4 A progressively larger view of the pink solid-lined area within the smaller image (image d). Figure 5 The four diagrams in b correspond to Figure 4 A progressively enlarged view of the orange dashed box area in the small image.

[0086] Depend on Figure 5 As can be seen from Figure a, in Figure 4 Within the pink solid-line frame in section d, the composite foam material of Example 5 exhibits numerous mesopores on its macropore inner walls, and further, numerous micropores on the mesopore inner walls, with the walls of these micropores formed by a cellular structure. Figure 5 As can be seen from Figure b, in Figure 4 Within the orange dashed box area in section d, the surface of the composite foam material in Example 5 also exhibits a three-dimensional network structure composed of hierarchical pores. These results demonstrate that the PVA / foam stabilizer / MXene composite foam material of this application possesses a typical and unique "pore-in-pore" structure, distinct from ordinary PVA hydrogels.

[0087] Figure 6 The water absorption (transportation) characteristics of the final products from Examples 2-5 and Comparative Example 1 of this application are summarized. Figure 6It can be seen that although the ordinary PVA hydrogel product (Comparative Example 1) has a certain porous structure, its water absorption rate is 0 in the extremely short time of 90 seconds; that is, the ordinary PVA hydrogel (Comparative Example 1) cannot absorb water at all in such a short time. However, the products of Examples 2-5 of this application all exhibit extremely fast absorption characteristics. For example, within 10 seconds, the water absorption rates of the products of Examples 2-5 can reach approximately 1700%, 1750%, 2200%, and 2250%, respectively, and the water absorption rates of all examples reach equilibrium after at most 20 seconds. Wherein, water absorption rate = (mass after water absorption - mass before water absorption) / mass of sample before water absorption. The above results indicate that the composite foam material prepared in the examples of this application has excellent water transport characteristics, which is very beneficial for absorbing seawater and continuously supplying seawater.

[0088] Figure 7 The solar absorption characteristics of the final products from Embodiments 2-5 of this application are summarized. Figure 7 It can be seen that the final products of Examples 2-5 exhibit solar absorption behavior dependent on MXene concentration. For example, at a lower MXene content (Example 2), the sample's absorption rate is higher than 80% across the entire solar absorption range; at a higher MXene content (Example 5), the sample's absorption rate is higher than 95% across the entire solar absorption range. The high solar absorption rate of the products implemented in this application is highly beneficial for efficient photothermal conversion, facilitating seawater desalination.

[0089] Figure 8 The compressibility recovery curves of the final products from Embodiments 2-5 of this application under different strains are summarized. From Figure 8 It can be seen that the final products of Examples 2-5 of this application exhibit excellent compressibility and resilience. For example, at a compression deformation of 10%, the products of Examples 2-5 can achieve 100% compression recovery, and when the compression deformation is further increased to 30%, 50%, 70%, and 90%, the products of Examples 2-5 can also achieve 100% compression recovery. These results demonstrate that the composite foam material prepared in the embodiments of this application has excellent mechanical compression behavior, which is highly beneficial for repeated use, cleaning, transportation, and storage in practical applications.

[0090] Furthermore, this application also tested the compressibility recovery curve of the final product of Example 5 after 5000 cycles of cyclic compression at 70% compressive deformation. The results showed that the PVA / foam stabilizer / MXene composite foam material of Example 5 could withstand 5000 cycles of cyclic compression at 70% compressive deformation without damage, demonstrating its robust stability, which is extremely beneficial for long-term use in practical applications.

[0091] The MXene-based photothermal composite foam materials prepared in the embodiments of the present application and the products prepared in the comparative examples were subjected to each test in Table 1 below, and the results are summarized in Table 1 below.

[0092] Table 1

[0093]

[0094]

[0095] In Table 1, the test conditions of the mechanical recovery data are as follows: equipment: Instron 9566, pressure sensor: 500N; test fixture: compression type fixture; test conditions: room temperature; compression speed: 1 mm / min; detection mode: compression recovery mode.

[0096] The test method of water absorption rate in 10s in Table 1 is as follows: the sample (cylindrical, about 5-6 cm high) of each example or comparative example was first subjected to freeze-drying to achieve a completely dry state, and the sample mass was recorded. Then the bottom was respectively immersed in seawater (the immersion depth was 0.5-1 cm), and stayed for 10s, and the mass of each example product after water absorption was recorded. The difference between the mass of the sample after water absorption and the mass of the sample before water absorption was taken as the ratio of the mass of the sample before water absorption, as a measure of water absorption.

[0097] The test method of seawater evaporation rate in Table 1 is as follows: the sample of each example or comparative example was respectively placed into a container containing seawater, and the initial weight of the whole was weighed. The container was irradiated with sunlight for a period of time, and the mass reduction of the whole (i.e. the mass of the evaporated seawater) was taken as the ratio of the surface area of the sample that could receive sunlight and the irradiation time, as the seawater evaporation rate of each sample.

[0098] The test method of energy efficiency of seawater evaporation in Table 1 is as follows: each sample of the embodiments or comparative examples was respectively embedded in a black foam body, and the whole was placed into a container containing seawater, and one times of sunlight was applied from above. The ratio of the energy required by each sample to evaporate seawater to the energy provided by one times of sunlight was taken as the energy efficiency.

[0099] It can be known from Table 1 that the PVA hydrogel product prepared by ordinary low-speed stirring (Comparative Example 1) has a large density and cannot float in water, and has a low porosity and small water absorption property, thereby having a low evaporation rate and a low energy efficiency, and further not having a compression recovery property. The products of Comparative Examples 2-3 and each of the examples prepared by high-speed stirring of the precursor solution can float in water, have a high porosity, and have a good compression recovery property, but the products of Comparative Examples 2-3 have a low seawater evaporation rate, a low energy efficiency of seawater evaporation, and a low water absorption rate, which may be due to the introduction of the MXene material as described above, which increases the number of pores of the product.

[0100] The composite foam material of the embodiments of the present application not only has a compressible and strong mechanical property, but also has a light weight, a high porosity, a product surface with pores, a fast water absorption rate, a rough product surface, a high absorption efficiency of sunlight, and a high energy efficiency of seawater evaporation. In the process of receiving solar radiation, the surface water is evaporated due to the photothermal effect of the MXene, and the fast water absorption / supply rate can ensure the timely water supply of the surface of the evaporative material, thereby increasing the seawater evaporation rate and the energy efficiency. In addition, it can be known from the comparison of Examples 1-6 that when the mass ratio of the MXene material to PVA is increased while the other preparation conditions remain unchanged, the seawater evaporation rate and the energy efficiency of the product are gradually increased, but too much MXene material can also cause a certain decrease in the porosity of the product. It can be known from the comparison of Example 5 and Examples 7-9 that when the MXene material is grafted with a polyionic liquid, the photothermal effect of the obtained product is better, and the seawater evaporation rate and the energy efficiency are higher, which may be due to the more uniform dispersion of the MXene material in the system.

[0101] The above only discloses exemplary embodiments of the present application, and it should be noted that those skilled in the art can make several improvements and refinements without departing from the principles of the embodiments of the present application, and these improvements and refinements are also considered within the protection scope of the embodiments of the present application.

Claims

1. A method for preparing an MXene-based photothermal composite foam material, characterized in that, Includes the following steps: (1) Mix MXene nanosheets with polyvinyl alcohol, crosslinking agent, foam stabilizer, acid catalyst and water at -5~10℃ to obtain precursor solution; (2) The precursor solution is stirred at an ultra-high speed of 10,000 rpm and cross-linked at 20-50°C to obtain a crude cross-linked composite foam material; then the crude composite foam material is dialyzed in water to obtain MXene-based photothermal composite foam material.

2. The preparation method according to claim 1, characterized in that, The mass ratio of polyvinyl alcohol to MXene nanosheets is 1:(0.002~0.05); the mass ratio of polyvinyl alcohol to crosslinking agent is 1:(0.1~0.5); and the mass ratio of polyvinyl alcohol to foam stabilizer is 1:(0.025~0.2).

3. The preparation method according to claim 1, characterized in that, The crosslinking agent is selected from one or more of dialdehyde, paraformaldehyde, urea-formaldehyde resin, melamine-formaldehyde resin, boride, and maleic anhydride; the acid catalyst includes one or more of hydrochloric acid, sulfuric acid, and acetic acid; and the foam stabilizer includes one or more of cationic surfactants, anionic surfactants, and nonionic surfactants.

4. The preparation method according to claim 3, characterized in that, The nonionic surfactant includes one or more of alkyl glycosides and block polyether F-127.

5. The preparation method according to any one of claims 1-4, characterized in that, Before preparing the precursor solution, the method further includes grafting a polyionic liquid onto the surface of the MXene nanosheets, specifically including: MXene nanosheets were mixed with a cationic ionic liquid monomer containing carbon-carbon double bonds and water, and then subjected to γ-ray irradiation under anaerobic conditions to graft the polymer of the ionic liquid monomer onto the surface of the MXene nanosheets. The resulting reaction solution was centrifuged, and the precipitate was collected. The precipitate contained MXene nanosheets grafted with the polyionic liquid.

6. The preparation method according to claim 5, characterized in that, The mass ratio of the MXene nanosheets to the ionic liquid monomer and water is 100:(1~10):

500.

7. An MXene-based photothermal composite foam material, characterized in that, It is prepared by the preparation method described in any one of claims 1-6.

8. The MXene-based photothermal composite foam material as described in claim 7, characterized in that, The MXene-based photothermal composite foam material includes a porous foam matrix and MXene nanosheets loaded in the porous foam matrix. The porous foam matrix has a skeleton structure formed by crosslinking polyvinyl alcohol, a crosslinking agent, and a foam stabilizer. The MXene-based photothermal composite foam material has a plurality of first pores, and the pore walls of the first pores have a plurality of second pores.

9. The MXene-based photothermal composite foam material as described in claim 8, characterized in that, The second hole has multiple third holes in its wall; the surface of the wall of the third hole has a cellular structure.

10. The MXene-based photothermal composite foam material as described in claim 8, characterized in that, The MXene-based photothermal composite foam material has an absorption rate of 80% or greater for the entire wavelength range of sunlight from 250nm to 2500nm.

11. The MXene-based photothermal composite foam material as described in claim 8, characterized in that, The contact angle between the outer surface of the MXene-based photothermal composite foam material and water is 0°.

12. The MXene-based photothermal composite foam material according to any one of claims 8-11, characterized in that, The surface of the MXene nanosheets is grafted with a polyionic liquid, which comprises a polymer of a cationic liquid monomer containing a carbon-carbon double bond.

13. The application of the MXene-based photothermal composite foam material as described in any one of claims 7-12 in the preparation of seawater desalination materials, elastic materials, and photothermal materials.

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