Graphene oxide composite separation membrane and preparation method thereof

Abstract

The invention discloses a graphene oxide composite separation membrane and a preparation method. The graphene oxide composite separation membrane comprises a plurality of layers of graphene oxide sheets which are staggered and stacked, and further comprises carbon nanotubes embedded among the graphene oxide sheets; the graphene oxide sheets are cross-linked through a cross-linking agent; the graphene oxide sheet is divided into an intercalation interval and a non-intercalation interval by the carbon nano tube; the diameter of the carbon nano tube is greater than the interlayer spacing between adjacent non-intercalated intervals; a permeation path of the graphene oxide composite separation membrane comprises four channels, and the first channel comprises a channel between adjacent non-intercalated interval sheets; the channel II comprises intrinsic defects, holes and channels formed by sheet layer edges of the graphene oxide sheets; and the channel I is divided into two channels IV by the carbon nano tube, and a channel III with molecular-level length selectivity is arranged at the separation part. According to the invention, the permeability of the graphene oxide composite separation membrane is obviously improved, the selectivity of the graphene oxide composite separation membrane is effectively enhanced, and the performance restrictions of permeability and selectivity of the traditional separation membrane are overcome.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, and particularly to a graphene oxide composite separation membrane combining polydopamine-crosslinked graphene oxide nanosheets and multi-walled carbon nanotube intercalation structures and a preparation method thereof. Background Art

[0002] Molecular sieving is a technique for separating different-sized molecules by utilizing the pore structure or channels of materials. Its core principle is based on the size difference of molecules, and by selectively allowing small molecules to pass through while blocking large molecules, the separation of different molecules in a mixture is achieved. Graphene oxide separation membranes are ideal molecular sieving materials formed by the close stacking of graphene oxide nanosheets. The performance evaluation system of graphene oxide separation membranes is mainly based on three core indicators: permeability, selectivity, and stability, and these indicators jointly determine the effectiveness of the separation membrane in practical applications.

[0003] The structural stability of graphene oxide separation membranes is restricted by their inherent properties, mainly manifested as the dynamic hydrogen bond network and the electrostatic interaction between oxygen functional groups. These interactions lead to membrane structure instability through pathways such as hydration-driven interlayer swelling, conformational rearrangement of nanosheets, and progressive interlayer delamination under operating stress. To solve this problem, the prior art uses polydopamine as a crosslinking agent, and by carrying out a nucleophilic reaction between the amino group of polydopamine and the oxygen-containing functional groups of graphene oxide nanosheets to form strong interfacial bonds, the binding strength between adjacent graphene oxide nanosheet layers is significantly enhanced, and the precise regulation of the interlayer spacing of graphene oxide nanosheets is also achieved by controlling the crosslinking agent density.

[0004] The defect of the prior art is that it is difficult to simultaneously meet the requirements of high permeability and excellent selectivity. The reason for this defect is that in order to achieve efficient molecular transport, graphene oxide separation membranes need to have larger interlayer spacing channels to reduce the resistance when molecules pass through. However, larger interlayer spacing channels will seriously reduce selectivity. Summary of the Invention

[0005] In view of the above-mentioned defects of the prior art, the present invention provides a graphene oxide composite separation membrane and a preparation method thereof, which not only significantly improve the permeation performance of the graphene oxide composite separation membrane, but also effectively enhance the selectivity of the graphene oxide composite separation membrane, thereby overcoming the performance constraints of permeability and selectivity of traditional separation membranes.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] In a first aspect, a graphene oxide composite separation membrane includes multiple layers of alternately stacked graphene oxide sheets and also includes carbon nanotubes embedded between the graphene oxide sheets;

[0008] The graphene oxide sheets are crosslinked by a crosslinking agent;

[0009] The graphene oxide sheets are divided into an intercalated region and a non-intercalated region by the carbon nanotubes;

[0010] The diameter of the carbon nanotubes is larger than the interlayer spacing of the graphene oxide sheets in the non-intercalated region;

[0011] The permeation path of the graphene oxide composite separation membrane includes Channel 1, Channel 2, Channel 3, and Channel 4;

[0012] Channel 1 includes the channels between adjacent graphene oxide sheets in the non-intercalated region;

[0013] Channel 2 includes the channels formed by the intrinsic defects, holes, and sheet edges of the graphene oxide sheets;

[0014] Channel 1 is divided into two Channel 4 by the carbon nanotubes, and the dividing point is Channel 3.

[0015] Preferably, the length of Channel 4 is at the molecular level.

[0016] Preferably, in Channel 4, the interfacial spacing between the graphene oxide sheets and the carbon nanotubes is smaller than the interlayer spacing of the graphene oxide sheets in the non-intercalated region.

[0017] Preferably, both the length and width of the graphene oxide sheets are 1 - 500 microns.

[0018] Preferably, the length of the carbon nanotubes is 0.5 - 50 microns, and the diameter is 3 - 50 nanometers.

[0019] Preferably, the interlayer spacing between adjacent graphene oxide sheets in the non-intercalated region is 5 - 15 angstroms.

[0020] Preferably, the crosslinking agent is polydopamine.

[0021] Preferably, the carbon nanotubes are multi-walled carbon nanotubes.

[0022] In a second aspect, a method for preparing a graphene oxide composite separation membrane includes the following steps:

[0023] Step S1: Mix a graphene oxide dispersion and a carbon nanotube dispersion to form a mixed dispersion;

[0024] Step S2: Immerse a base membrane in a crosslinking solution for surface modification to obtain a modified base membrane;

[0025] Step S3: Using the modified base membrane as a substrate, filter the mixed dispersion through a vacuum filtration device.

[0026] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0027] 1. It is precisely because multi-walled carbon nanotubes are used for intercalation to generate the channel III1 with high aspect ratio nano-pores, which effectively enhances the permeation performance of the graphene oxide composite separation membrane. And due to the cross-linking effect, the layer spacing in the non-intercalated area is stabilized, and its size-selective function is not affected by the intercalation of multi-walled carbon nanotubes, so that the stability and permeability of the graphene oxide composite separation membrane are effectively balanced.

[0028] 2. Since the graphene oxide sheets are cross-linked by a cross-linking agent, it not only significantly enhances the structural stability of the membrane, but also precisely regulates the interlayer spacing through the swelling effect induced by the cross-linking molecules at specific binding sites. This structural regulation not only optimizes the separation performance of the graphene oxide composite separation membrane, but also further improves its mechanical strength and long-term stability, providing an important structural basis for the design of high-performance separation membranes.

[0029] 3. Through the synergistic effect of polydopamine cross-linking and multi-walled carbon nanotube intercalation, an additional channel IVI2 is formed in the interfacial gap between the multi-walled carbon nanotubes and the graphene oxide sheets. The channel IVI2 is a molecular-level length selective barrier, which further improves the selectivity of the graphene oxide composite separation membrane, thus overcoming the performance limitation between permeability and selectivity of traditional membrane materials. This innovative structural design not only significantly improves the separation efficiency of the graphene oxide composite separation membrane, but also provides a multi-functional framework for optimizing the performance of advanced filtration materials. By fully exploiting the synergistic effect of these structural features, the composite membrane achieves a double breakthrough in selectivity and permeability, thus laying a solid foundation for the transformative development of next-generation applications such as molecular separation and water purification. Description of the Drawings

[0030] Figure 1 is a schematic diagram of the microscopic level effect of the graphene oxide separation membrane of Example 1;

[0031] Figure 2 is a schematic diagram of the cross-linking of polydopamine molecules and graphene oxide sheets of Example 1;

[0032] Figure 3 is the SEM image of the polydopamine cross-linked graphene oxide membrane inserted with 7wt% multi-walled carbon nanotubes of Example 1;

[0033] Figure 4 is the comparative diagram of the wide-scan XPS spectra of the three membranes of Example 1;

[0034] Figure 5 is the high-resolution narrow-scan XPS spectrum of the uncross-linked graphene oxide separation membrane of Example 1;

[0035] Figure 6It is the high-resolution narrow-scan XPS spectrum of the polydopamine-crosslinked graphene oxide separation membrane of Example 1;

[0036] Figure 7 It is the high-resolution narrow-scan XPS spectrum of the polydopamine-crosslinked graphene oxide membrane inserted with multi-walled carbon nanotubes of Example 1;

[0037] Figure 8 It is the Fourier transform infrared spectrum of the polydopamine-crosslinked graphene oxide membrane inserted with multi-walled carbon nanotubes of Example 1;

[0038] Figure 9 It is the schematic diagram of the molecular sieve separation path of Example 1;

[0039] Figure 10 It is the representative surface SEM image of the graphene oxide separation membrane of Example 1;

[0040] Figure 11 It is the SEM image of the polydopamine-crosslinked graphene oxide membrane without inserted multi-walled carbon nanotubes of Example 1;

[0041] Figure 12 It is the graph of permeability vs. graphene oxide content of the polydopamine-crosslinked graphene oxide membrane without inserted multi-walled carbon nanotubes of Example 1;

[0042] Figure 13 It is the curve of molecular cut-off vs. graphene oxide content of the polydopamine-crosslinked graphene oxide membrane without inserted multi-walled carbon nanotubes under different solutes of Example 1;

[0043] Figure 14 It is the graph of permeability vs. content of inserted multi-walled carbon nanotubes of the polydopamine-crosslinked graphene oxide membrane with fixed graphene oxide content of Example 1;

[0044] Figure 15 It is the curve of molecular cut-off vs. content of inserted multi-walled carbon nanotubes of the polydopamine-crosslinked graphene oxide membrane with fixed graphene oxide content under different solutes of Example 1;

[0045] Figure 16 It is the macroscopic optical image of the graphene oxide separation membrane of Example 1;

[0046] Figure 17 It is the process flow chart of Example 2;

[0047] Figure 18 It is the schematic diagram of the material system of Example 2.

[0048] Wherein:

[0049] N1, Channel 1; N2, Channel 2; I1, Channel 3; I2, Channel 4. Detailed implementation manners

[0050] In order to make the technical means, creative features, achieved purposes and effects of the invention easy to understand, the present invention will be further described below in conjunction with specific drawings. However, the present invention is not limited to the following implemented cases.

[0051] It should be noted that the structures, ratios, sizes, etc. shown in the drawings of this specification are only used to cooperate with the content disclosed in the specification for those skilled in this technology to understand and read, and are not used to limit the implementation conditions of the present invention. Therefore, they do not have substantial technical significance. Any modification of the structure, change of the proportional relationship or adjustment of the size, without affecting the effects that the present invention can produce and the purposes that can be achieved, should still fall within the scope covered by the technical content disclosed in the present invention.

[0052] Example 1:

[0053] As Figure 1 shown, a graphene oxide composite separation membrane contains multiple layers of interleaved and stacked graphene oxide sheets (hereinafter simply referred to as "nanosheets"), whose surfaces are rich in various exposed oxygen-containing functional groups, including epoxy groups (C-O-C), hydroxyl groups (-OH), carboxyl groups (-COOH), etc. These functional groups act as molecular spacers between adjacent nanosheets, forming an interlayer nano-capillary percolation network between the nanosheets. These networks are interconnected through the intrinsic defects, pores and sheet edges of the nanosheets, constituting continuous permeation channels and providing an efficient path for molecular transport.

[0054] Regarding the stability of the graphene oxide composite separation membrane. The nanosheets are crosslinked by a crosslinking agent. In this embodiment, the crosslinking agent is polydopamine. As Figure 2 shown, the polydopamine (PDA) molecular chains act as molecular bridges between adjacent graphene oxide nanosheets (GONs) through the formation of covalent bonds, where the amino groups of PDA chemically react with the oxygen-containing functional groups on the surface of GON to form C-N covalent bonds. This crosslinking mechanism not only significantly enhances the stability of the membrane by forming strong covalent bonds between GONs and PDA chains, but also precisely regulates the interlayer spacing, thereby optimizing the structural integrity and separation performance of the membrane.

[0055] The graphene oxide composite separation membrane also contains carbon nanotubes embedded between the nanosheets. In this embodiment, the carbon nanotubes are preferably multi-walled carbon nanotubes (MWCNT). The strategic embedding of the multi-walled carbon nanotubes induces local expansion within the interlayer structure of the nanosheets. The nanosheets are divided into intercalated intervals and non-intercalated intervals by the multi-walled carbon nanotubes, thereby dividing the interlayer percolation network into two different regions: the intercalated region and the non-intercalated region. Optionally, the length of the multi-walled carbon nanotubes is 0.5 - 50 microns. Figure 3The structural characteristics of a polydopamine-crosslinked graphene oxide membrane (PDA-c-GOM) inserted with 7 wt% multi-walled carbon nanotubes are shown. Through high-resolution surface and cross-sectional SEM analysis, the presence of multi-walled carbon nanotubes with a high aspect ratio can be clearly observed. Their diameter is approximately 10 nanometers, and their length reaches the micron scale, and they are strategically embedded between adjacent nanosheets.

[0056] To comprehensively study the chemical bond transformation caused by polydopamine crosslinking and multi-walled carbon nanotube insertion, the present invention performed X-ray photoelectron spectroscopy (XPS) analysis on three membrane samples: the original graphene oxide composite separation membrane (GOM), the polydopamine-crosslinked graphene oxide membrane without multi-walled carbon nanotube insertion (PDA-c-GOM), and the polydopamine-crosslinked graphene oxide membrane inserted with 7 wt% multi-walled carbon nanotubes (PDA-c-GOM / MWCNT). Figure 4 The comparative wide-scan XPS spectra of the three membranes are shown, in which the characteristic photoelectron emission peaks corresponding to C1s (~284 eV), O1s (~530 eV), and N1s (~400 eV) are clearly visible. It is worth noting that the presence of the N1s characteristic peak in the spectrum of the original graphene oxide composite separation membrane is due to the nitrogen-containing residues introduced during the oxidation of graphene in a mixed acid containing nitric acid.

[0057] The high-resolution C1s spectra were analyzed in detail using ThermoAvantage software and calibrated with a C-C bond reference peak at 284.8 eV to deeply analyze the evolution process of carbon-related chemical bonds. As Figure 5 shown, the original graphene oxide composite separation membrane shows three distinct deconvoluted peaks: the characteristic C-C bond at 284.8 eV, the C-O-C bond corresponding to the epoxy group at 286.2 eV, and the C=O bond associated with the carboxyl group (-COOH) at 287.4 eV.

[0058] As Figure 6As shown, the profound impact of polydopamine crosslinking is clearly demonstrated in the narrow-scan XPS spectra of polydopamine-crosslinked graphene oxide membranes without inserted multi-walled carbon nanotubes, specifically manifested as three significant chemical shifts: (1) C-O-C bond shift: The binding energy increases from 286.2 eV to 286.5 eV, reflecting the decrease in electron density due to the formation of covalent bonds between polydopamine and the nanosheets, indicating that epoxy groups are involved in the crosslinking reaction. (2) Carboxyl group transformation: The characteristic C=O bond peak at 287.4 eV disappears and is replaced by a weakened O-C=O peak at 288.3 eV, indicating that a large number of carboxyl groups are consumed through chemical reactions with polydopamine, further confirming the reactivity of carboxyl groups in the crosslinking reaction. (3) C-N bond formation: The newly emerged peak at 285.3 eV provides direct evidence for the formation of covalent bonds between the amino groups of polydopamine and the oxygen-containing functional groups of the nanosheets. This transformation occurs through two main reaction pathways: the condensation reaction between amino and hydroxyl groups: amino (-NH2) + hydroxyl (-OH) → C-N + H2O, and the nucleophilic addition reaction between amino and epoxy groups.

[0059] As Figure 7 shown, the high-resolution narrow-scan XPS spectra of polydopamine-crosslinked graphene oxide membranes inserted with multi-walled carbon nanotubes show remarkable consistency with the membranes without inserted multi-walled carbon nanotubes in terms of peak position, spectral profile, and relative intensity. This striking similarity in chemical characteristics provides strong evidence that the insertion of multi-walled carbon nanotubes is achieved through physical embedding rather than chemical modification. The retention of the same chemical bond pattern further confirms that the insertion of multi-walled carbon nanotubes does not disrupt the covalent crosslinking structure established between polydopamine and the nanosheets, but only introduces physical modifications to the structural framework of the membrane.

[0060] As Figure 8 shown, the chemical bond transformations caused by polydopamine crosslinking and multi-walled carbon nanotube insertion are further confirmed by Fourier transform infrared spectroscopy (FTIR) analysis (transmittance). The spectral data reveals four key observations, which not only verify the XPS findings but also provide supplementary information: (1) C-C bond consistency: The C-C bond peak at 1577 cm- 1 is consistently present in all membrane samples, confirming that the basic skeletal structure of the nanosheets is maintained during the modification process. (2) Hydroxyl group reduction: The broad hydroxyl peak at 3394 cm- 1 is significantly weakened in both polydopamine-crosslinked membranes, providing direct evidence for the participation of hydroxyl groups in the crosslinking reaction. (3) Carboxyl / epoxy group consumption: 1487 cm- 1 (carboxyl group) and 1240 cm- 1The characteristic peak at the (epoxy group) shows a significant intensity decrease in the polydopamine-crosslinked graphene oxide film. These spectral changes are in complete agreement with the carboxyl group transformation (C=O → O-C=O) and epoxy group consumption observed by XPS, further confirming that these functional groups actively participate in the formation of covalent bonds with polydopamine. (4) Polydopamine crosslinking characteristic signals: The newly emerging vibration modes at 2920 cm -1 and 2848 cm -1 in the polydopamine-crosslinked graphene oxide film s, corresponding to the symmetric and asymmetric -CH2 stretching vibrations in the polydopamine molecular chain, are unique fingerprints of successful polydopamine crosslinking. In addition, the appearance of the characteristic peak at 1407 cm -1 in the polydopamine-crosslinked graphene oxide film, attributed to C-N stretching vibration, further confirms the formation of covalent bonds between polydopamine and the nanosheets. It should be noted that a weak C-N signal was also observed in the spectrum of the original graphene oxide composite separation membrane, which originated from the nitrogen-containing residues introduced during the oxidation of graphene in a mixed acid containing nitric acid.

[0061] The permeation paths of the graphene oxide composite separation membrane include channel one N1, channel two N2, channel three I1, and channel four I2. Channel one N1 is the channel between adjacent un-intercalated sheets, channel two N2 contains the channel between the edge of the un-intercalated sheet and its adjacent intercalated sheet, channel one N1 is separated into two channel fours I2 by carbon nanotubes, and the separation point is channel three I1.

[0062] Regarding the permeability of the graphene oxide composite separation membrane. The overall permeation performance of the graphene oxide composite separation membrane is jointly affected by all four types of channels, among which channel two N2 and channel three I1 have a significant promoting effect on permeability.

[0063] As Figure 9 shown, in the polydopamine-crosslinked graphene oxide film intercalated with multi-walled carbon nanotubes, water molecules permeate through well-defined transport paths. These paths can be clearly divided into two structurally distinct regions: the non-intercalated region and the multi-walled carbon nanotube intercalated region. In the non-intercalated region, the transport of water molecules is mainly achieved through the following two types of channels: channel one N1, a two-dimensional low-friction nano-capillary percolation network formed by the close stacking of adjacent nanosheets, and channel two N2, the structural defects, pores, and edge regions of the nanosheets. In the intercalated region, the transport of water molecules is mainly achieved through two types of channels, channel three I1 and channel four I2.

[0064] Through high-resolution SEM imaging technology, the structural characteristics of the graphene oxide composite separation membrane can be deeply analyzed. Figure 10Shows a representative low-magnification scanning electron microscope (SEM) image of the surface of the graphene oxide composite separation membrane, clearly revealing the stacked structural characteristics of graphene oxide nanosheets. In the image, the lateral size of the nanosheets is in the micron scale, and their edges play a crucial role in the nanoscale permeation paths within the membrane structure, providing an efficient channel for molecular transport. As Figure 11 shown, the pristine polydopamine-crosslinked graphene oxide membrane without inserted multi-walled carbon nanotubes exhibits a distinct layered structure with a measured thickness of 120.9 μm. The surface SEM images ( Figure 3 and Figure 11 ) also show the intrinsic defects and pores of the nanosheets, which, together with the nanosheet edges, contribute to the formation of permeation paths. Notably, the intrinsic defects, pores, and the structure with nanosheet edge sizes ranging from dozens to hundreds of nanometers in Channel II N2 do not affect the molecular selectivity of the membrane because their sizes are much larger than the angstrom-scale layer spacing of Channel I N1. Quantitative analysis shows that the successful insertion of multi-walled carbon nanotubes leads to a significant increase in membrane thickness, from 120.9 μm of the pristine membrane without inserted multi-walled carbon nanotubes to 135 μm after the insertion of multi-walled carbon nanotubes, with a swelling rate of 11.7% in the vertical direction.

[0065] Figure 12 Shows the permeability curves of polydopamine-crosslinked graphene oxide membranes without inserted multi-walled carbon nanotubes with varying graphene oxide contents from 0.5 mg to 1.5 mg. The experimental results show that the pure water permeability reaches a maximum of 31.62 Lm- 2 h- 1 bar- 1 at the lowest graphene oxide content (0.5 mg) and maintains the best performance before the graphene oxide loading reaches 1.0 mg. However, as the graphene oxide content further increases to 1.5 mg, the pure water permeability significantly decreases to 18.25 L m- 2 h- 1 bar- 1 . Similarly, although the absolute values decrease, the permeabilities of salt and dye solutions also show the same trend.

[0066] When there are no multi-walled carbon nanotubes embedded, the selectivity of the graphene oxide composite separation membrane is mainly determined by the precise layer spacing of Channel I N1. In this example, the surface of the nanosheets has protruding oxidized functional groups, maintaining the layer spacing of Channel I N1 within the range, allowing water molecules to transport in Channel I N1. Figure 13The shown rejection performance demonstrates the unique behavior of different solutes. The dye rejection is positively correlated with the graphene oxide content, with the rejection rate of Congo red increasing from 71% to 82% and that of methylene blue increasing from 68% to 76%. In contrast, the salt rejection remains relatively constant within the tested graphene oxide content range, with the rejection rates of NaCl and MgCl2 being approximately 10% and 9% respectively. This differential behavior highlights the size selectivity of the membrane separation mechanism. As the graphene oxide content (membrane thickness) increases, the rejection rate of larger-sized dye molecules (Congo red: ~1.3 nm; methylene blue: ~0.8 nm) increases significantly, while the rejection rate of smaller-sized hydrated salt ions (Na + : ~0.36 nm; Mg 2+ : ~0.43 nm) shows no obvious change.

[0067] The embedding of multi-walled carbon nanotubes forms two different types of transport channels: Channel III I1, locally expanded nanopores, and Channel IV I2, the interfacial gap between multi-walled carbon nanotubes and nanosheets. Channel III I1 in the intercalation region is an extended nanopore with a high aspect ratio formed by the spatial confinement effect of multi-walled carbon nanotubes. These nanopores significantly enhance the molecular transport driving force, thus greatly improving the overall permeability of the membrane. At a low multi-walled carbon nanotube intercalation level, the introduction of high-aspect-ratio nanopores in Channel III I1 significantly changes the pore size distribution of the membrane, shifting it towards larger sizes, thereby greatly enhancing the permeability of the membrane.

[0068] As Figure 14 shown. With the increase in the multi-walled carbon nanotube loading, the pure water permeability increases significantly, reaching an optimum value of 48.6 L m- 2 h- 1 bar- 1 at 7 wt% intercalation. Compared with the membrane without multi-walled carbon nanotube intercalation (32.4 L m- 2 h- 1 bar- 1 ), this value is significantly increased by 50%. However, beyond this optimum threshold, further increasing the multi-walled carbon nanotube content leads to a gradual decrease in permeability, indicating that the membrane structure starts to densify at higher intercalation levels. The permeabilities of NaCl and Congo red solutions also show the same trend, but with lower absolute values.

[0069] Regarding selectivity, the nanopore width (dimension parallel to the bottom surface of the multi-walled carbon nanotube) of channel three I1 is 10 nanometers, and the length extends to several microns. This size feature makes it have no significant effect on the selectivity of molecular separation. The size screening function of channel one N1 is not affected by the insertion of multi-walled carbon nanotubes. The diameter of the multi-walled carbon nanotube is much smaller than the lateral dimension of the nanosheet. Optionally, the lateral dimension of the nanosheet is 1-500 microns, and the diameter of the multi-walled carbon nanotube is 3-50 nanometers. Since the diameter of the multi-walled carbon nanotube is much smaller than the length of the nanosheet parallel to the bottom surface of the carbon nanotube (i.e., the lateral dimension), the structural integrity of the non-intercalated region is maintained. The interlayer spacing of the graphene oxide nanosheet is stabilized by covalent cross-linking of polydopamine and is not changed by the embedding of multi-walled carbon nanotubes. This structural integrity ensures that the size screening function of the interlayer spacing of channel one N1 is retained, which is crucial for achieving efficient selective molecular separation.

[0070] In channel four I2, the oxygen-containing functional groups exposed on the surface of the nanosheets act as molecular spacers, precisely regulating the interfacial spacing between the multi-walled carbon nanotubes and the nanosheets, maintaining it at about half the interlayer spacing of the nanosheets. In channel four I2, the interlayer spacing between the graphene oxide sheets and the multi-walled carbon nanotubes is smaller than the interlayer spacing between adjacent unintercalated sheets. Due to the circular cross-sectional geometric characteristics of the multi-walled carbon nanotubes, the interfacial gap formed presents a unique Venturi morphology and has a molecular length scale. This nano-confined structure constructs an additional molecular-level selective barrier that can effectively block the transmission of large molecules (such as dyes) while exerting extremely low resistance to the permeation of small ions (such as salt ions) without significantly affecting the overall permeability of the membrane. Although the gap between the multi-walled carbon nanotubes and the graphene oxide nanosheets forms a Venturi shape, allowing water molecules to flow faster, the length of this gap is very short and will not hinder the passage of water molecules, so it will not affect the overall permeability of the membrane. On the contrary, this design can also help improve the permeability of the membrane, allowing water molecules to pass more easily while maintaining selective blocking of large molecules.

[0071] Figure 15 The retention performance shown reveals a significant improvement in molecular sieving capability through MWCNT insertion. At only 1 wt% MWCNT loading, the membrane achieves significant selectivity enhancement: the dye rejection of Congo red soars from 71% to 95%, and the rejection of methylene blue (MB) increases from 68% to 91%. However, at the same MWCNT insertion level, the salt rejection of NaCl decreases from 10% to 3%, and the salt rejection of MgCl2 decreases from 9% to 4%. Beyond the 1 wt% threshold, both dye and salt rejections level off, indicating that the membrane achieves optimal selective separation performance at lower MWCNT loadings.

[0072] Figure 16Shows a macroscopic optical image of a polydopamine cross-linked graphene oxide membrane inserted with multi-walled carbon nanotubes, showing a uniform 4-inch diameter structure.

[0073] The intercalation of multi-walled carbon nanotubes effectively solves the trade-off problem between permeability and selectivity that has long plagued the membrane separation field through a triple-structure mechanism: (1) stabilizing the layer spacing in the non-intercalated region through polydopamine cross-linking to ensure its size-selective function; (2) generating locally expanded high-aspect-ratio nanopores in the intercalated region through multi-walled carbon nanotube intercalation to significantly improve the permeation performance of the membrane; (3) and forming an additional molecular-scale length-selective barrier through the interfacial gap between multi-walled carbon nanotubes and nanosheets to further enhance the separation selectivity. This innovative structural design not only significantly improves the separation efficiency of the composite membrane but also provides a multi-functional framework for optimizing the performance of advanced filtration materials. By fully leveraging the synergistic effects of these structural features, the composite membrane achieves a dual breakthrough in selectivity and permeability, thus laying a solid foundation for the transformative development of next-generation applications such as molecular separation and water purification.

[0074] Example 2:

[0075] As Figure 17 、 Figure 18 shown, the preparation process of the graphene oxide composite separation membrane mainly includes three key steps: preparation of the mixed dispersion, modification of the substrate membrane, and assembly of the composite membrane.

[0076] Step S1, preparation of the mixed dispersion.

[0077] First, graphene is prepared by microwave-induced exfoliation method, and then the exfoliated product is subjected to mixed acid chemical oxidation treatment to prepare a graphene oxide (GO) dispersion. To prepare the GO dispersion, 5 mg of GO powder is dispersed in 50 mL of deionized water. It is treated with a 300 W probe sonicator (Sonics&Materials, VCX500 sonicator) for 5 minutes to break up aggregates, and then treated with a water bath sonicator (Branson, CPX1800H-C ultrasonic cleaner) for 25 minutes to obtain a uniform dispersion system.

[0078] To prepare a multi-walled carbon nanotube (MWCNT) dispersion, first, 1 mg of MWCNT powder was mixed with 100 mg of deionized water, and 3 mg of sodium dodecylbenzenesulfonate (SDBS) was added as a surfactant to enhance the stability of the dispersion system. Subsequently, the mixture was treated under probe sonication at 400 W for 2 minutes to break up the MWCNT aggregates; then, bath sonication for 25 minutes was carried out to further ensure uniform dispersion. The prepared GO dispersion and MWCNT dispersion were mixed according to a preset weight ratio and subjected to bath sonication for 30 minutes to finally obtain a uniform GO / MWCNT dispersion mixture. As a control experiment, a pure GO dispersion was used to prepare the control membrane material.

[0079] Step S2: Substrate modification.

[0080] In this embodiment, a polyethersulfone membrane (PES) was used as the substrate, and a polydopamine solution (PDA) was used as the cross-linking solution. First, a PDA-functionalized solution of the polyethersulfone membrane was prepared, which was composed of 2.5 mg of polyethyleneimine (PEI) and 50 mg of dopamine hydrochloride (DA-HCl) dissolved in 50 mL of Tris-HCl buffer (pH 8.5). In an alkaline Tris-HCl environment, dopamine undergoes a self-polymerization reaction, and at the same time, PEI acts as a stabilizer to effectively inhibit the excessive aggregation of PDA. After the mixed solution was polymerized at room temperature for 15 minutes, a PES with a diameter of 110 mm was immersed in the resulting viscous PDA solution and allowed to stand for 15 minutes to achieve functional modification of the substrate surface. Subsequently, the modified membrane was rinsed with deionized water to remove unbound PDA residues.

[0081] Step S3: Assembly of the composite membrane.

[0082] Using the PDA-functionalized PES membrane as the substrate, a vacuum filtration device (Jinteng T-50 filtration system) was used to precisely filter the GO / MWCNT dispersion mixture. By adjusting the dispersion concentration and filtration volume, precise control of the thickness of the graphene oxide composite separation membrane was achieved. After filtration, the obtained graphene oxide composite separation membrane was thermally annealed at 50 °C for 30 minutes to complete the final preparation.

Claims

1. A graphene oxide composite separation membrane, comprising multiple layers of cross-stacked graphene oxide sheets, characterized in that: It further comprises carbon nanotubes embedded between the graphene oxide sheets; The graphene oxide sheets are crosslinked by a crosslinking agent; The graphene oxide sheets are divided into intercalation intervals and non-intercalation intervals by the carbon nanotubes; The diameter of the carbon nanotubes is greater than the layer spacing of the graphene oxide sheets in the non-intercalation interval; The permeation path of the graphene oxide composite separation membrane comprises channel one N1, channel two N2, channel three I1 and channel four I2; Channel one N1 comprises the channels between adjacent graphene oxide sheets in the non-intercalation interval; Channel two N2 comprises the channels formed by the intrinsic defects, holes and sheet edges of the graphene oxide sheets; Channel one N1 is divided into two channel four I2 by the carbon nanotubes, and the dividing point is channel three I1.

2. The graphene oxide composite separation membrane according to claim 1, wherein: The length of channel four I2 is at the molecular level.

3. The graphene oxide composite separation membrane according to claim 1, characterized in that: In channel four I2, the interfacial spacing between the graphene oxide sheet and the carbon nanotube is less than the layer spacing of the graphene oxide sheets in the non-intercalation interval.

4. The graphene oxide composite separation membrane according to claim 1, characterized in that: The length and width of the graphene oxide sheets are both 1-500 microns.

5. The graphene oxide composite separation membrane according to claim 4, wherein: The carbon nanotubes have a length of 0.5-50 microns and a diameter of 3-50 nanometers.

6. The graphene oxide composite separation membrane according to claim 5, characterized in that: The layer spacing between adjacent graphene oxide sheets in the non-intercalation interval is 5-15 angstroms.

7. The graphene oxide composite separation membrane according to any one of claims 1-6, characterized in that: The crosslinking agent is polydopamine.

8. The graphene oxide composite separation membrane according to claim 7, wherein: The carbon nanotubes are multi-walled carbon nanotubes.

9. The preparation method of a graphene oxide composite separation membrane according to claim 1, wherein, Comprising the following steps: Step S1: Mix the graphene oxide dispersion and the carbon nanotube dispersion into a mixed dispersion; Step S2: Immerse the base membrane in the crosslinking solution for surface modification to obtain a modified base membrane; Step S3: Using the modified base membrane as a substrate, filter the mixed dispersion through a vacuum filtration device.

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