Microporous hydrophobic reduced graphene oxide membrane with thermoplasmonic nanoparticles

Abstract

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Description

TECHNICAL FIELDThe present disclosure relates to a membrane distillation technology. More specifically, thepresent disclosure relates to a microporous hydrophobic membrane comprising a reducedgraphene oxide (rGO) membrane integrated with a thermo-plasmonic nanoparticles (TNPs).The disclosure further relates to a method for preparing the microporous hydrophobicmembrane, which includes developing of reduced graphene oxide membrane, developing thethermo-plasmonic nanoparticles (TNPs), and subsequent integration of thermo-plasmonicnanoparticles onto the surface of the reduced graphene oxide membrane to obtain amicroporous hydrophobic membrane.BACKGROUNDThe growing human population and industrialization heighten concerns over drinking watersecurity. Membrane distillation technology offers exceptional salt reinjection capabilities,making it highly effective for desalination processes. Membrane distillation exhibits excellentscalability regarding material aspects, enabling customization to suit specific applications.However, as it stands today, membrane distillation is a promising solution, but its currentlimitations include the high cost of microporous membranes, significant energy demands forheating, temperature polarization, membrane fouling, and wetting.Membrane distillation also faces several significant challenges that hinder its efficiency andcost-effectiveness. The production of microporous hydrophobic polymeric membranes, suchas PVDF and PTFE, is expensive. Additionally, heating the feed solution is an energy-intensiveprocess, further increasing operational costs. The system is also susceptible to temperaturepolarization, which can negatively impact efficiency by creating unfavorable temperaturegradients. Lastly, membrane surfaces are prone to biofouling from scaling and wetting, leadingto reduced performance over time. In conclusion, it can be stated that. High cost of microporousmembranes, the high energy required to heat the feed solution, susceptibility to temperaturepolarization, and membrane fouling due to surface scaling and membrane-wetting all presentcontemporary challenges that require research attention.The patent application US20170333848A1, discusses membranes enhanced withfunctionalized carbon nanotubes, nanodiamonds, and graphene oxide, which are immobilizedwithin or on the membrane structure. It encompasses various forms of membrane distillation,including air gap, direct contact, vacuum, and sweeping gas methods, allowing the membranesto be utilized in different applications. The nanocarbon materials can be tailored throughfunctionalization to modify their chemical properties, which can enhance solute interactionsand adjust hydrophilicity. The membranes include their ability to operate at lower temperatures(60-90°C) compared to traditional distillation, making them suitable for low-temperature heatsources like waste heat and solar energy. The document details several embodiments, includingbilayer membranes, where different nanocarbons are immobilized in the membrane pores,potentially incorporating various functional groups to optimize performance.The patent utility application CN213977036U, discloses a seawater desalination device thatfeatures a superhydrophobic membrane housed within a glass structure. This device includes aglass shell with an open-ended casing column and a light and heat conversion layer on its topsurface. Inside, a glass inner panel creates a cavity for efficient seawater treatment. Thesuperhydrophobic distillation membrane enhances desalination efficiency by improving watervapor interactions and minimizing heat loss, and a light and heat conversion layer made froma carbon-based composite like graphene or carbon nanotubes, which absorbs solar energy forthe distillation process.Conventionally known membrane distillation technology faces several challenges, includingthe high cost of microporous membranes, significant energy requirements for heating,temperature polarization, membrane fouling, and pore wetting. To address these limitations,the present disclosure, reduced graphene oxide (rGO) emerges as a promising membranematerial. Reduced graphene oxide (rGO) is a material synthesized from graphene oxide (GO)through a process that removes oxygen-containing groups. The present disclosure hereindiscusses the use of thermo-plasmonic nanoparticles (TNPs) in combination with hydrophobicreduced graphene oxide (rGO) membranes. The technology aims to overcome the challengesfaced by existing membrane distillation technology.SUMMARY OF THE INVENTION:Accordingly, the present disclosure provides a microporous hydrophobic reduced grapheneoxide (rGO) membrane integrated with thermo-plasmonic nanoparticles (TNPs) for use in themembrane distillation technology.In another aspect, the present disclosure provides a method for preparing a microporoushydrophobic reduced graphene oxide (rGO) membrane integrated with thermo-plasmonicnanoparticles (TNPs), the method comprises:a) developing the thermo-plasmonic nanoparticles (TNPs);b) developing the porous rGO membrane;c) integrating the thermo-plasmonic nanoparticles onto the surface of the rGO membrane toobtain a microporous hydrophobic rGO membrane; andd) treating the membrane obtained in step (c), by freeze drying and annealing under an inertatmosphere.The microporous hydrophobic reduced graphene oxide membrane with thermo-plasmonicnanoparticles aims to improve energy efficiency, reduce fouling, and optimize the distillationprocess for water purification applications.BRIEF DESCRIPTION OF DRAWINGSThe accompanying drawings, which are incorporated in and constitute a part of this invention,illustrate exemplary embodiments and together with the description, serve to explain thedisclosed principles:Figure 1 represents a schematic representation of the fabrication of TNP@rGO membrane.Figure 2 represents a schematic representation of the process involved in membrane distillationusing TNP@rGO hydrophobic membrane.Figure 3 represents a schematic representation of the Air Gap Membrane Distillation (AGMD)unit.Figure 4 illustrates a UV-Visible spectra of Graphene Oxide.Figure 5 represents nine samples made as part of(design of experiments (DOE)study.Figure 6 illustrates an UV-Vis spectra of GO done using taguchi design of experiments.Figure 7 illustrates a XRD pattern of GO, rGO and Graphite.Figure 8 illustrates a SEM image of prepared GO from 100 mesh size synthetic graphite.Figure 9 illustrates a PVDF membrane shows reduction of contact angle from 118° to 80° afterexposure to 3 hours of solar radiation.Figure 10 illustrates a change in contact angle of PVDF membrane on exposure to solarradiation.Figure 1 SEM Image showing the surface morphology and gold nanoparticles coated over thesurface of the membrane at various magnification.Figure 2 SEM image showing the gold nanoparticle deposited over the rGO membrane. Theanalysis shows the gold nanoparticle size to be around 60 nm.Figure 3 SEM image showing the location used for EDAX analysis.Figure 4 EDAX spectrum showing the presence of Au as well as C and O.Figure 5 illustrates a SEM image showing the cut cross section of the rGO membrane.Figure 6 SEM image of the cut cross section of the membrane; the thickness of the membraneis as shown in the image.Figure 7 the contact angle measurements: (a) 37° for GO membrane; (b) 54° after one hour, (c)73° after two hours and 113° after three hours.Figure 18 illustrates a rGO membrane coated with thermoplasmonic gold nanoparticle: (a) and(b) are thermal imaging before solar irradiation, (c) Thermal image showing attainedtemperature of 69.8 °C after 2 hours.Figure 8 rGO membrane without the coating of gold nanoparticle. (a) and (b) are thermalimaging before solar irradiation, (c) Thermal image showing attained temperature of 44.9 °Cafter 2 hours.Figure 20 illustrates TEM images of KOH-activated reduced graphene oxide (rGO).Figure 9 histogram showing pore size distribution of KOH activated rGO.Figure 10n exploded view of an Air-Gap Membrane Distillation (AGMD) module for testingmembrane performance.Figure 11n complete view of the AGMD module.DETAILED DESCRIPTIONIt is to be understood that the present disclosure is not limited in its application to the detailsof microporous hydrophobic membrane and process set forth in the following description. Thepresent disclosure is capable of other embodiments and of being practiced or of being carriedout in various ways. Also, it is to be understood that the phraseology and terminology usedherein is for the purpose of description and should not be regarded as limiting.The use of "including", "comprising" or "having" and variations thereof herein is meant toencompass the items listed thereafter and equivalents thereof as well as additional items. Before disclosure is described in greater detail, it is to be understood that the present disclosureis not limited to particular embodiments described, as such may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting disclosure.Where a range of values is provided, it is understood that each intervening value, to the tenthof the unit of the lower limit unless the context clearly dictates otherwise, between the upperand lower limit of that range and any other stated or intervening value in that stated range, isencompassed within the methods. The upper and lower limits of these smaller ranges mayindependently be included in the smaller ranges and are also encompassed within the methods,subject to any specifically excluded limit in the stated range. Where the stated range includesone or both of the limits, ranges excluding either or both of those included limits are alsoincluded in the methods.Certain ranges are presented herein with numerical values being preceded by the term "about."The term "about" is used herein to provide literal support for the exact number that it precedes,as well as a number that is near to or approximately the number that the term precedes. Indetermining whether a number is near to or approximately a specifically recited number, thenear or approximating unrecited number may be a number which, in the context in which it ispresented, provides the substantial equivalent of the specifically recited number. In anembodiment, "about" can mean within one or more standard deviations, or within ± 30%, 25%,20%, 15%, 10% or 5% of the stated value.Unless defined otherwise, all technical and scientific terms used herein have the same meaningas commonly understood by those of ordinary skill in the art to which the invention belongs.Although any methods and materials similar or equivalent to those described herein can beused in the practice or testing of the present invention, preferred methods and materials aredescribed.All publications and patents cited in this specification are herein incorporated by reference asif each individual publication or patent were specifically and individually indicated to beincorporated by reference and are incorporated herein by reference to disclose and describe themethods and / or materials in connection with which the publications are cited. The citation ofany publication is for its disclosure prior to the filing date and should not be construed as anadmission that the present disclosure is not entitled to antedate such publication by virtue ofprior disclosure. Further, the dates of publication provided may be different from the actualpublication dates which may need to be independently confirmed.The articles "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one)of the grammatical object of the article. By way of example, "an element" means one elementor more than one element.Other objects, features, and aspects of the present invention are disclosed in or are obviousfrom the following detailed description. It is understood by one of the ordinary skills in the artthat the present discussion is a description of exemplary embodiments only and is not to beconstrued as limiting the broader aspects of the present invention.The present disclosure relates to a microporous hydrophobic membrane comprising:- a hydrophobic microporous reduced graphene oxide membrane; and- a thermo-plasmonic nanoparticles;wherein the thermo-plasmonic nanoparticles (TNPs) are integrated onto the surface of thehydrophobic rGO membrane.The thermo-plasmonic nanoparticles (TNPs) are tiny particles that can absorb sunlight andconvert it into heat. In certain embodiments, the thermo-plasmonic nanoparticles (TNPs) areselected from gold (Au NPs), silver (Ag NPs), copper (Cu NPs), platinum (Pt NPs), carbonnanotubes (CNTs), graphene oxide (GO) or aluminium nanoparticles (Al NPs).Reduced graphene oxide (rGO) is a material synthesized from graphene oxide (GO) through aprocess that removes oxygen-containing groups.The hydrophobic reduced graphene oxide (rGO) membrane incorporated with thermoplasmonic nanoparticles (TNPs) enhances solar energy-driven membrane distillationprocesses. The selection of TNPs is based on choosing metals or metal nanostructures (such as Al, Au,Ag, Cu) with suitable surface passivation to optimize their efficiency for solar membranedistillation and to offer corrosion resistance and transparency, while the core offers sufficientextinction loss and heat generation from the irradiance. The choice of materials that showsufficient absorption and extinction coefficients across the solar spectrum will be selected,allowing efficient solar energy capture for vapor production. Only those compositionsrequiring earth-abundant elements will be chosen, thus simultaneously enabling high scienceand useful technology.In a specific embodiment, the thermo-plasmonic nanoparticles (TNPs) is gold nanoparticlesbut not limited thereto.In certain embodiments, the thermo-plasmonic nanoparticles (TNPs) have an average size inthe range of 20 nm to 100 nm.In certain embodiment, the hydrophobic microporous reduced graphene oxide membrane haswater contact angle greater than 110 o. In a specific embodiment, the hydrophobic microporousreduced graphene oxide membrane has water contact angle in the range of 100-120°.In certain embodiment, the microporous hydrophobic membrane, exhibits a pore size of about0.1-1 μm, and a water-contact angle a minimum of 110º.In an embodiment, the present disclosure relates to a method for preparing the microporoushydrophobic membrane, the method comprises:a. developing thermo-plasmonic nanoparticles (TNPs);b. developing a porous rGO membrane;c. integrating the thermo-plasmonic nanoparticles into the rGO membrane onto thesurface of the porous rGO membrane to obtain a microporous hydrophobic membrane;andd. treating the membrane obtained in step (c), by freeze drying and annealing under aninert atmosphere.In a certain embodiment, thermoplasmonic nanoparticles with suitable surface passivation aredeveloped to achieve both high solar absorption efficiency and corrosion resistance. The TNPsynthesis is carried out through a solvothermal process, which enables controlled nucleationand growth of nanoparticles with desired size distribution, morphology, and optical resonancebehavior.In certain embodiments, the rGO membrane is developed using either of the two scalablefabrication routes as described below:a) Vacuum Filtration of KOH-Activated rGO: This straightforward approach will be usedfor large-scale fabrication of rGO membranes. In this method, vacuum suction isapplied to induce compression, which helps in aligning and stacking the rGO sheetsperpendicular to the flow direction, creating a porous structure that is optimal forfiltration and vapor transport.b) Hydrothermal Growth of 3D Cross-Linked Porous rGO Structure: In this method, ahydrothermal process will be used to grow a 3D cross-linked rGO structure on a stablebase substrate, such as carbon cloth or stainless-steel mesh, at temperatures rangingfrom 150°C to 200°C. This method allows the formation of a stable, highly porous rGOstructure with enhanced mechanical and chemical properties.The hydrophobic reduced graphene oxide (rGO) membranes are specially designed membranesthat repel water, allowing only water vapor to pass through while keeping liquids and othersubstances out.In certain embodiments, the vacuum filtration being conducted under a vacuum pressure of 30kPa abs to 40 kPa abs for a duration of 0.5 hours to 1 hours to obtain a membrane thickness of100 to 200 μm.In certain embodiment, the porous reduced graphene oxide (rGO) membrane is alternativelydeveloped by hydrothermal growth on a substrate selected from carbon cloth or stainless-steelmesh at a temperature ranging from 150°C to 200 °C for 6 hours to 12 hours.In a certain embodiment the total thickness of the porous reduced graphene oxide (rGO)membrane with nylon substrate is 100 μm to 200 μm. The developed rGO membrane has athickness in range of 20-100 μm.In certain embodiments, the carbon-based membranes show good chemical resistance andthermal stability and can be easily scaled up. The rGO membrane is designed to exhibit highmechanical robustness, chemical stability, and water-repellent properties, enabling efficientvapor transport and long-term anti-fouling performance when integrated withthermo-plasmonic nanoparticles (TNPs).In certain embodiments, hydrothermal growth is preferred because it has a higher likelihood ofproducing membranes with high porosity. Greater porosity results in improved vapor flux. Theefficiency of distillation is directly related to the porosity of the membrane. Additionally, a 3Dassembled membrane exhibits increased surface roughness, which contributes to a highercontact angle and enhanced hydrophobic behaviour.In certain embodiments, the membrane is preferably developed using a vacuum filtrationprocess, which enables uniform deposition and controlled thickness of the active layer. Whilehydrothermal treatment may optionally be employed to induce nanoporous growth and networkformation within the reduced graphene oxide (rGO) structure, the final membrane assemblyand consolidation are achieved through vacuum filtration. This approach ensures improvedmechanical integrity, tunable porosity, and consistent layer morphology. The vacuum filtrationmethod further allows precise control over membrane parameters such as thickness and poredistribution, thereby enhancing the reproducibility and scalability of the fabrication process.Consequently, although hydrothermal treatment can serve as an auxiliary step for modifyingthe microstructure of rGO, the overall membrane fabrication is preferably accomplished viavacuum filtration to ensure optimal performance characteristics suitable for vapor transport anddistillation applications.In a certain embodiment, the integration of thermo-plasmonic nanoparticles (TNPs) into thereduced graphene oxide (rGO) matrix performed to obtain a homogeneous TNP-rGOcomposite suitable for subsequent membrane fabrication.is achieved through either an in-situor an ex-situ approach as described below:i. In-situ Synthesis: The TNPs are incorporated into the rGO membrane during thesolvothermal process. This involves synthesizing nanoparticles in a high-temperature,high-pressure solvent environment, allowing for controlled nanoparticle size, shape,and composition; wherein the steps comprise of:- selecting the solvent based on the solubility of the precursor materials and therequired reaction conditions like high boiling point;- selecting the precursor materials, such as inorganic salts or metal complexes,based on the desired TNP composition;- mixing the precursor materials in the solvent within a sealed reaction vessel tomaintain pressure and prevent evaporation;- heating the reaction mixture above the solvent's boiling point while maintaininghigh pressure to ensure that the solvent stays in liquid form;- the reaction proceeds for a specific duration of about 2 hours, allowing thenanoparticles to nucleate and grow on the surface of the rGO;Thus, the solvothermal system, TNPs will be uniformly in situ grown on the plane of rGOmembrane, providing enhanced structural stability in water. The resulting dispersed medium isused to fabricate the membrane using any of the above-mentioned membrane fabricationmethods. Furthermore, the incorporated rigid TNPs, along with the formed TNPs-rGObonding, will provide the preferred membrane with sufficient stability for surviving thepressure and flow of fluid for real-world applications.ii. Ex-situ Synthesis: The KOH-activated rGO nanosheets and TNPs are synthesizedseparately using already known chemical-reduction or physical-deposition methods inthe art. These are then mixed in water and uniformly dispersed using ultrasonication.The resulting dispersion is used to fabricate the microporous hydrophobic reducedgraphene oxide membrane with thermo-plasmonic nanoparticles obtained using theaforementioned fabrication methods.In certain embodiment, the integration of thermo-plasmonic nanoparticles (TNPs) into thereduced graphene oxide (rGO) matrix is performed preferably through vacuum filtration.In certain embodiment, the TNP-rG°Composite obtained from the integration step issubsequently processed to fabricate the final microporous hydrophobic reduced graphene oxide(rGO) membrane incorporating thermo-plasmonic nanoparticles (TNPs).In certain embodiments, the annealing of the KOH activated rGO is performed at a temperaturein the range of 150-300 °C in argon atmosphere.In a certain embodiment, the inert gas is argonIn certain embodiment, the resulting membrane exhibits a pore size of about 0.1-1 μm, totalthickness of the rGO membrane along with nylon substrate is about 100 μm to 200 μm, whereinthe developed rGO membrane has the thickness ranging from 20-100 μm and a water-contactangle greater than about 113 °.In an embodiment of the present disclosure, the microporous hydrophobic membrane isemployed in an air-gap membrane distillation (AGMD) unit comprising a feed channel, an airgap, and a condensation surface.The TNP-decorated microporous rGO membrane is grown over carbon cloth / stainless steelmesh using a specially designed hydrothermal autoclave of the required dimensions. The TNPdecorated rGO membrane is supported by a stainless-steel mesh, which serves as a spacerwithin the AGMD module.In an air-gap membrane distillation (AGMD) configuration, the feed solution is in directcontact with one side of the membrane, while the opposite side is separated from thecondensing surface by an air gap typically having a thickness in the range of 3-10 mm. The airgap functions as an insulating layer that minimizes heat loss to the surroundings and enablesthe establishment of a significant temperature difference across the membrane.The thermo-plasmonic nanoparticles (TNPs) are positioned on the surface of a hydrophobicreduced graphene oxide (rGO) membrane that faces the feed solution. When exposed tosunlight, the TNPs absorb solar radiation and convert it into localized heat at the membrane-feed interface. A transparent glass window is positioned above the feed channel to allow solarradiation to directly reach the membrane surface, thereby enhancing localized heating. Thephotothermal activity of the TNPs generates a temperature gradient between the feed andpermeate sides of the membrane, resulting in a partial vapor pressure difference that drivesvapor transport through the microporous membrane and across the air gap. The vapor passingthrough the air gap condenses on a cooler condensation plate located on the permeate side,where the purified water is collected.The TNP-integrated microporous rGO membrane is grown over a carbon cloth or stainlesssteel mesh substrate using a specially designed hydrothermal autoclave of the desireddimensions. The stainless-steel mesh acts as a mechanical support and spacer within theAGMD module, ensuring membrane stability, uniform solar heat distribution, and efficientvapor flux during operation.AGMD offers advantages such as enhanced heat transfer, reduced fouling, improved masstransfer, and flexibility in module design. It is particularly suitable for desalination applicationsand can handle high salinity or feedwaters with variable quality.The reduced graphene oxide (rGO)-based membranes distillation technology provided by thepresent disclosure offers several advantages over traditional polymeric membranes,particularly in terms of cost-effectiveness and performance. The rGO-based membranes aremore economical and offer enhanced chemical resistance and thermal stability, making themsuitable for a wider range of applications. Additionally, these carbon-based membranes can beeasily scaled up for industrial use. The TNP incorporated rGO membrane helps in efficientabsorption of solar energy and localised heating of the membrane feed interface by directlyutilizing the solar radiation without requiring a separate spacer. The direct exposure to solarenergy also provides an inbuilt natural mechanism for combating biofouling and microbialgrowth, both on the membrane surface and within the feed solution. Solar irradiance helpsmaintain the membrane's hydrophobic state with a contact angle greater than 110°, ensuringthat the membrane maintains its hydrophobic state. Furthermore, this approach eliminates theneed for energy-intensive heating of the feed solution, thus the membrane surface temperaturecan be maintained between 50°C and 100°C. Thus, rGO-based membranes offer a moresustainable, energy-efficient solution for membrane distillation.EXAMPLES:The present disclosure is further exemplified by following examples. However, these examplesshould not be construed as limiting the scope of the embodiments herein.In the present disclosure, the inventors have demonstrated the ffabrication and characterizationof TNP@rGO Membrane Developed of reduced graphene oxide membraneThe graphene oxide (GO) precursor was synthesized by a Hummers' method, wherein syntheticgraphite powder of 100 mesh size was oxidized using potassium permanganate (KMnO₄) in a1:9 ratio of the amount of graphite to the amount of KMnO₄, with the reaction maintained for14 hours under controlled conditions. The GO thus obtained was subsequently reduced to rGO,which provided the desired hydrophobicity, conductivity, and structural integrity required forlater membrane fabrication and thermo-plasmonic nanoparticle integration.As part of design of experiments (DOE), a Taguchi L9 array was implemented to systematicallyvary the reaction parameters namely, oxidant ratio, reaction temperature, and reaction time.Nine samples were thereby synthesized to identify the optimal oxidation state. The oxidationstates of each sample were visually distinguished by color variation as shown in figure 5. Theselection of the most suitable GO sample for membrane fabrication was based upon the UV Visible spectroscopy as shown in Figure 3. The peaks observed in the UV-Visible spectrumcorresponded to π-π* transition (230-235 nm) and a shoulder peak of n-π* transition (300-310 nm), as illustrated in figure 4. Among the nine samples, only those exhibiting a sharp π-π* peak and well-defined n-π* shoulder indicative of moderate oxidation and uniform sheetstructure were selected for further use. Sharper π-π* peaks indicated thinner, fewer-layeredGO nanosheets, which were essential for the self-alignment of GO sheets during membraneformation.The carbon-based membrane was fabricated by vacuum filtration. The highly oxidized GO wasdeposited onto a nylon substrate of pore size 0.22 and 0.45 μm. After deposition, the GOmembrane was exposed to solar radiation for 3 hours to induce partial reduction. The reductionof GO to rGO was confirmed by X-ray diffraction (XRD), comparing the patterns of graphite,GO, and rGO, as illustrated in Figure 7.The SEM image of the selected GO sample revealed good stacking of the GO sheets andformed a layer-by-layer structure suitable for membrane formation, as shown in Figure 8. Uponexposure to solar radiation, the membrane surface color changed from yellowish-brown toblack, indicating successful photothermal reduction to rGO. The reduction was performedunder a solar simulator. The membrane was dried at 60 °C and tested for hydrophobicity usingcontact angle measurement. The contact angle initially measured about 37° and increased toabout 113° after 3 hours of solar irradiation as illustrated in figure 17. A contact angle ofapproximately 110° indicates that the membrane surface is highly hydrophobic, meaning thatwater droplets remain nearly spherical and do not penetrate the surface pores. This strong waterrepellence ensures that the pores remain unwetted, a critical property for sustaining vapor phase transport in membrane distillation processes.Integrated of the TNP's to the rGO membraneApproximately 2 mg of gold nanoparticles gold nanoparticles (AuNPs) of average diameter 60nm were deposited onto the surface of the rGO membrane. The 60 nm gold particle showcasedthermoplasmonic behaviour. The membrane was kept under the solar to monitor the plasmonicheating effect, with the localized surface temperature monitored using an infrared camera.Followed by the nanoparticle integration, the fabrication of the final TNP@rGO membranewas carried out. The integrated membrane was freeze-dried to remove any residual solventsand to preserve its microporous structure, followed by annealing under an inert atmosphere toenhance interfacial bonding between the rGO sheets and the gold nanoparticles. The resultingTNP@rG°Composite membrane was then vacuum-filtered and stabilized to obtain a self supporting, hydrophobic, microporous membrane suitable for solar-assisted membranedistillation. The surface morphology and nanoparticle distribution on the fabricated membranewere confirmed by Scanning Electron Microscopy (SEM) and Energy-Dispersive X-rayAnalysis (EDAX), as illustrated in Figures 11-14. SEM and EDAX analyses confirmed thepresence of thermo-plasmonic nanoparticles deposited over the rGO membrane surface. TheSEM micrographs showed uniform spherical nanoparticle distribution, while the EDAXspectra confirmed elemental gold. The quantitative compositional data obtained from EDAXare summarized in Table 1, confirming successful incorporation of the thermo-plasmonicnanoparticles. Table 1 quantifies the elements detected i.e. carbon, oxygen and gold confirmingthat gold nanoparticles have been deposited on the reduced graphene oxide membrane. Table1 showing the weight and atomic mass percentage got from the EDAX analysis.Table 1 presents the elemental composition of the thermo-plasmonic nanoparticle-integratedon to the reduced graphene oxide (TNP@rGO) membrane as EDAX spectroscopy. The dataconfirms the successful incorporation of gold nanoparticles onto the rGO membrane andprovides quantitative verification of the elemental distribution on the membrane surface. Theanalysis shows that carbon (C) and oxygen (O) constitute the major surface elements, withweight percentages of 59.47 wt % and 25.79 wt %, respectively. The gold (Au) signal, observedthrough its characteristic M-line emission, accounts for 14.74 wt % of the total composition.This confirms the presence of uniformly dispersed gold nanoparticles on the membrane surface.Comparative Analysis:T°Confirm thermoplasmonic behavior, of the developed membrane a comparative evaluationwas conducted between the thermo-plasmonic nanoparticle-integrated reduced graphene oxide(TNP@rGO) membrane and a non-integrated rGO membrane. Both the membranes weresubjected to solar irradiation for about 2 hours, and the surface temperature rise was monitoredusing an infrared camera. The membrane with gold nanoparticles attained a maximum surfacetemperature of about 69.8 °C as illustrated in figure 18, whereas the membrane withoutnanoparticles reached only about 44.9 °C, as illustrated in figure 19. This temperaturedifference of nearly 25 °C confirmed the strong thermo-plasmonic effect imparted by theintegrated gold nanoparticles, which efficiently absorbed solar radiation and converted it intolocalized heat at the membrane surface.The performance evaluation of the developed microporous hydrophobic membrane was donein an Air-Gap Membrane Distillation (AGMD) Unit.Example 2 - Performance Evaluation in Air-Gap Membrane Distillation (AGMD) UnitThe TNP@rGO membrane was evaluated in an air-gap membrane distillation (AGMD) setupto assess desalination performance and energy efficiency. A feed solution containing saline orcontaminated water was circulated through the feed channel at a temperature of about 60-80°C, while a cooling fluid, such as distilled water, flowed counter-currently on the permeate sideat a temperature of about 15-20 °C. The air gap, having a thickness of about 3-10 mm, actedas a thermal barrier minimizing conductive heat loss and promoting condensation efficiency.During operation, the TNP@rGO membrane demonstrated a stable permeate flux of 2.81kg / m2 hr and over a continuous testing of around 1 hours. The membrane maintained itshydrophobicity contact angle at 113o and microporous structure, showing negligible fouling orpore wetting throughout the test.Comparative Example - rGO Membrane without Thermo-Plasmonic NanoparticlesFor comparison, a control rGO membrane without thermo-plasmonic nanoparticles wasevaluated under identical AGMD conditions. The control membrane showed a lower steadystate surface temperature of 44.9 °C under solar illumination, resulting in a significantlyreduced vapor flux.Results and InterpretationThe results clearly demonstrate that the integration of thermo-plasmonic nanoparticles into therGO matrix enables localized solar-driven heating, enhanced vapor flux, and improved saltrejection during membrane distillation. The TNP@rGO membrane not only achieved superiordesalination performance but also exhibited enhanced thermal stability, hydrophobicity, andresistance to fouling, validating the synergistic effect between the plasmonic nanoparticles andthe hydrophobic rGO membrane.Thus, the inventive TNP@rGO membrane addresses the key challenges in membranedistillation namely, temperature polarization, energy inefficiency, and membrane wettingthereby representing a significant advancement over conventional carbon-based membranes.Characteristization and TestingFigure 20 presents the TEM micrographs of KOH-activated reduced graphene oxide (rGO),clearly displaying the formation of nanopores with an average pore diameter of approximately5 nm. These pores arise due to the chemical activation mechanism of KOH at elevatedtemperatures (typically >200-300 °C), wherein KOH reacts with the carbon lattice of rGO,etching the basal planes and introducing structural defects. This activation process generates ahighly porous network within the graphene sheets.Figure 21 provides the corresponding pore size distribution analysis, obtained using ImageJsoftware. The majority of pores fall within the 4-7 nm range, with the peak frequency centeredaround ~5 nm. The calculated mean pore size (5.29 nm) further confirms that the activationprocess predominantly produces mesopores (2-10 nm). Such a mesoporous architecture ischaracteristic of effective KOH activation and is advantageous for applications requiringenhanced mass transport and high surface accessibility.Figures 22 and 23 depict the exploded and assembled configurations, respectively, of themembrane test cell employed for evaluating nanoporous membranes, including rGO-basedmembranes. The labelled components illustrate the arrangement for membrane placement,sealing integrity, fluid inlet / outlet pathways, and provisions for temperature control. Together,these figures outline the functional design of the test assembly used for systematic membraneperformance assessments.ConclusionThe experimental analyses confirmed that the GO prepared using the optimized DOEparameters provided uniform, exhibited uniform oxidation and thin layer stacking, enabling theformation of a mechanically stable reduced graphene oxide (rGO) membrane. The developedrGO-based microporous membrane (pore diameter 0.1-1 μm, porosity >80%, thickness 100-200 μm, plays a crucial role in the performance of the TNP@rGO membrane. The pore sizeensures selective vapor transport while preventing pore wetting, the high porosity enhancesvapor flux and interfacial evaporation efficiency, and the optimized thickness minimizesconductive heat loss while maintaining mechanical integrity. It offers a scalable, carbon-basedalternative t°Conventional polymeric membranes, while providing superior chemicalresistance and thermal stability.The hydrophobic nature of the rGO framework (contact angle >110°) ensured that themembrane remained non-wetted and structurally stable throughout operation. Overall, theTNP@rGO membrane achieved superior thermal efficiency, vapor flux, and salt rejectioncompared to the unmodified rGO membrane. The disclosure thus provides an energy-efficient,durable, and solar-responsive membrane platform for water purification and desalinationapplications.

Claims

1. A microporous hydrophobic membrane comprising: - a hydrophobic microporous reduced graphene oxide (rGO) membrane; - a thermo-plasmonic nanoparticles; wherein the thermo-plasmonic nanoparticles (TNPs) are integrated onto the membrane surface of the hydrophobic microporous rGO membrane.

2. The microporous hydrophobic membrane as claimed in claim 1, wherein the thermoplasmonic nanoparticles comprise metal nanoparticles selected from aluminium (Al), gold (Au), silver (Ag), copper (Cu), platinum (Pt), carbon nanotubes (CNTs), or graphene oxide (GO).

3. The membrane as claimed in claim 1, wherein the integration of the thermoplasmic nanoparticles into the reduced graphene oxide (rGO) matrix is carried out by either by an in-situ or an ex-situ approach.

4. The microporous hydrophobic membrane as claimed in claim 1, exhibits a pore size of about 0.1-1 μm, porosity a minimum of 80%, and a water-contact angle a minimum of 110º.

5. A method for preparing the microporous hydrophobic membrane as claimed in claim 1 comprises: a) developing a thermo-plasmonic nanoparticles (TNPs); b) developing a porous rGO membrane; c) integrating the thermo-plasmonic nanoparticles onto the surface of the rGO membrane to obtain a microporous hydrophobic rGO membrane incorporated with the thermoplasmonic nanoparticles; and d) treating the membrane obtained in step (c), by freeze drying and annealing under an inert atmosphere.

6. The method as claimed in claim 5, wherein the membrane obtained in step (c) is annealed at temperature ranging from 150-300 °C.

7. The microporous hydrophobic membrane as claimed in the claim 1 as and when employed in an air-gap membrane distillation (AGMD) unit.

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