Navigating The World Of Membrane Filters: Types, Uses, And Benefits (Ⅱ)

Preface: In the previous part of this article, Navigating the World of Membrane Filters: Types, Uses, and Benefits (1), we discussed an overview of filtration membranes and introduced the two more common types of filtration membranes, polymeric filtration membranes, including Polyethersulfone (PES) and Polyvinylidene Fluoride (PVDF), and Ceramic Filtration Membranes, and described their preparation and applications. We also introduced their preparation and applications. Stanford Advanced Materials (SAM) will continue to introduce you to other types of membranes.

5 Nanostructured Membranes

5.1 Titanium Dioxide (TiO2) Nanotube Membranes

5.1.1 What Are Titanium Dioxide (TiO2) Nanotube Membranes

Since the discovery of carbon nanotubes in 1991, tubular structure nanomaterials have attracted much attention due to their unique physicochemical properties and promising applications in microelectronics, applied catalysis, photovoltaic conversion, etc. TiO2, with its advantages of good ultraviolet absorption, high dielectric constant, and stable chemical properties, is widely used in the fields of photocatalysis, solar cell coatings, anticorrosion, air purification, and wastewater treatment and other fields. Titanium dioxide nanotubes typically have a diameter of a few to tens of nanometres, while the length can range from a few hundred nanometres to several micrometres. This nanoscale size allows titanium dioxide nanotubes to have a high specific surface area, high capacity and special photovoltaic properties, making titanium dioxide nanotube membranes suitable for wide-ranging applications in photocatalysis, photovoltaic device preparation, sensor preparation, and the resulting related reaction areas such as water and air purification and treatment.

Fig. 6 Microstructure of Titanium Dioxide Nanotubes

5.1.2 Synthesis Methods for Titanium Dioxide (TiO2) Nanotube Membranes

Common preparation methods for TiO2 nanotube thin films include the solution method, vapour phase deposition method, and the electrochemical method, among which the solution method is the most commonly used due to the advantages of a simple and inexpensive process, as well as the ability to control the size morphology better.

The solution method is based on TiO2 precursors in solution, and under specific conditions (e.g., temperature, pH, solvent, etc.), TiO2 nanotube films are formed by controlling the processes of precipitation, dissolution, and crystal growth. The advantages of the solution method for the preparation of TiO2 nanotube films include simplicity of preparation, lower cost, and suitability for large-area preparation.

Vapour phase deposition is a method that utilises TiO2 precursors in the gas phase to form thin films by depositing them onto the surface of a substrate in a high-temperature environment. This method includes both chemical vapour deposition (CVD) and physical vapour deposition (PVD) types. In the CVD method, a TiO2 film is formed by feeding a gaseous precursor compound into a reaction chamber and decomposing and depositing it onto the substrate surface at high temperatures. In the PVD method, a physical process (e.g., sputtering, evaporation) is utilised to convert the solid TiO2 source material into a gaseous state, which is then deposited onto the substrate surface. Advantages of the vapour deposition method for preparing TiO2 nanotube films include fewer impurities produced during the preparation process and higher film quality.

Electrochemical methods utilise electrochemical reactions to deposit TiO2 nanotubes on the electrode surface. A common electrochemical method is anodisation, in which an oxide layer is formed on the surface of a substrate by applying a voltage in a specific electrolyte, and this oxide layer is then used as a template to grow TiO2 nanotubes under specific conditions. The advantages of electrochemical preparation of TiO2 nanotube films include the simplicity of the preparation process, ease of handling, and the fact that it can be performed at room temperature.

5.1.3 How Are Titanium Dioxide (TiO2) Nanotube Membranes Used

1. Water Treatment: Titanium dioxide nanotube membranes can be used in water treatment for micropollutant removal and water quality improvement. Its high specific surface area and photocatalytic properties enable it to effectively adsorb and degrade pollutants such as organic matter, heavy metal ions, and microorganisms in water, facilitating water purification and disinfection. For example, combining titanium dioxide nanotube membrane with photocatalytic technology can be stimulated to produce active oxygen species through the irradiation of ultraviolet light, to remove organic pollutants and bacteria in water.

2. Air Purification: Titanium dioxide nanotube membranes can also be used for air purification, removing airborne organics, VOCs (volatile organic compounds), formaldehyde, and other harmful gases. Similar to applications in water treatment, the photocatalytic properties of titanium dioxide nanotube membranes can be utilised to irradiate ultraviolet light onto the membrane to promote the degradation and removal of harmful gases.

3. Particulate Matter Filtration: Although titanium dioxide nanotube membranes are primarily photocatalytic, their nanoscale tubular structure also makes them capable of filtering particulate matter to a certain extent. Although this filtration performance may not be as efficient as other filtration materials, it still has a certain filtration effect in specific application scenarios and can be used as an additional filtration layer.

5.2 Graphene Oxide (GO) Membranes

5.2.1 Introduction of Graphene Oxide (GO) Membranes

Graphene oxide (GO) is an oxide of graphene, which is more active than graphene due to the increase of oxygen-containing functional groups on the graphene after oxidation and can improve its properties through various reactions with oxygen-containing functional groups. Graphene oxide flakes are the product of chemical oxidation and exfoliation of graphite powder. Graphene oxide is a single atomic layer, which can be readily expanded to tens of micrometres in lateral size. As such, its structure spans scales typical of general chemistry and materials science. Graphene oxide can be regarded as a non-traditional type of soft material with properties of polymers, colloids, thin films, and amphiphilic molecules.

Graphene oxide has a large amount of oxygen content (e.g., hydroxyl groups, carboxyl groups, etc.), which forms defects and functional groups between the graphene layers, leading to the formation of microporous structures in the interlayer gaps. These microporous structures give graphene oxide filter membranes a high degree of surface area and permeability. These microporous structures can be used for both physical filtrations, i.e., selectively blocking or allowing molecules in liquids or gases to pass through according to the size of the micropores, and removing suspended solids, solutes, microorganisms, and so on. The functional groups on the surface of the graphene oxide filtration membrane can also chemisorb with solute molecules so that solute molecules are adsorbed or attached on the surface of the filtration membrane, thus removing organic matter, heavy metal ions, and other pollutants in the liquid or gas. At the same time, the functional groups on the surface of the graphene oxide filtration membrane can be positively or negatively charged, and these charge effects can affect the adsorption and distribution of the solute molecules on the surface of the filtration membrane, thereby enabling selective filtration of specific solutes.

In addition, some graphene oxide filter membranes exhibit photocatalytic activity; when exposed to light, the graphene oxide on the surface can generate reactive oxygen species, such as hydroxyl radicals and superoxide ions, which can oxidise and degrade organic matter, thereby facilitating the degradation and removal of organic pollutants in water.

Fig. 7 Structure of Graphene Oxide (GO)

5.2.2 Different Preparation Methods of Graphene Oxide (GO) Membranes

Graphene oxide is obtained through the oxidation reaction of graphene; generally, there are two methods: the Hummers method and the Brodie method.

1. Hummers Method: Graphene is mixed with concentrated sulphuric acid and stirred to ensure full contact, then nitric acid is added and the reaction is stirred at below 5°C, after which cooled hydrogen peroxide is added to the reaction, and a large amount of water is added to dilute the reaction solution at the end of the reaction, and graphene oxide is obtained by filtration, washing, drying, and other steps.

Fig. 8 Preparation of Graphene Oxide by Hummers Method

2. Brodie Method: Graphite powder and concentrated nitric acid are mixed, while stirring, cold sulphuric acid is added, nitric acid oxidation of the graphite reaction produces NO2; after the end of the reaction, a large amount of water is added to dilute the reaction solution, followed by filtration, washing, drying, and other steps to obtain graphene oxide.

Graphene oxide is often made into thin films by the coating method, chemical vapour deposition method, and hydrothermal method.

1. Coating Method: The steps are relatively simple; graphene oxide powder is added to the appropriate amount of solvent and stirred evenly to make it dispersed, the solution is evenly coated on the substrate to make it dry, and then repeat the above steps until the thickness is appropriate.

2. Chemical Vapour Deposition (CVD): Graphene oxide powder is placed in a high-temperature furnace and heated to over 700°C. One or more gases containing carbon sources (such as methane, ethylene, etc.) flow into the reaction chamber, and the carbon source gases decompose at high temperatures to form graphene, which reacts with the oxides on the surface of graphene oxide to generate graphene oxide films.

3. Hydrothermal Method: Compared with the chemical vapour deposition method, the reaction temperature required is lower; the graphene oxide powder is added to the appropriate amount of solvent, heated to the appropriate temperature, and then the reducing agent (such as hydrogen, ammonia, etc.) is added to the reaction system, and the reducing agent in the hydrothermal conditions reduces the graphene oxide to obtain the film.

5.2.3 Various Graphene Oxide (GO) Membranes Application Scenarios

1. Water Treatment and Air Purification: Graphene oxide membrane can not only carry out conventional filtration, but its molecular selectivity makes it feasible to achieve desalination, oil-water separation, etc. Meanwhile, its microporous structure and oxidised components can also remove organic matter as well as heavy metal ions, effectively eliminating particles, solutes, and pollutants.

2. Molecular Separation: The microporous structure of the graphene oxide filtration membrane can regulate the permeability and selective separation of molecules, thus holding potential application value in gas separation, solvent separation, molecular screening, and so on. For example, a graphene oxide filtration membrane can be employed for CO2 capture, gas separation, and organic purification.

3. Biomedicine: Graphene oxide filtration membrane demonstrates good biocompatibility and biosorption, making it useful in the fields of biosensing, bioseparation, and bioanalysis. For instance, graphene oxide filtration membranes can be applied for cell culture, protein separation, and DNA capture.

4. Energy: Graphene oxide filtration membranes find usage in devices such as batteries, supercapacitors, and fuel cells in the energy sector as ion transport membranes and electrolyte membranes to improve the performance and stability of the devices.

5.3 Carbon Nanotube (CNT) Membrane

5.3.1 Properties of Carbon Nanotube (CNT) Membrane

Carbon nanotube (CNT) is a hollow tube formed by curling graphite flakes. The carbon atoms in carbon nanotubes are hybridised and bonded in an sp2 fashion, with a six-membered ring as the basic structural unit, which gives carbon nanotubes a high Young's modulus and makes them a material with high fracture strength that is not easily damaged in bending situations. Carbon nanotube films are two-dimensional carbon nanotube network structures formed by individual carbon nanotubes physically or chemically filled with arrays of freely arranged carbon nanotubes, with properties related to the carbon nanotube conformation, orientation, degree of defects, and length-to-diameter ratio. Carbon nanotube membranes have a highly nanoscale pore structure and large specific surface area, resulting in a substantial surface area, advantageous for the adsorption and separation of solutes. Its pore structure has nanoscale dimensions, which effectively obstructs solutes, such as particles, organic molecules, etc. Despite the nanoscale pore structure, carbon nanotube filtration membranes exhibit high permeability, facilitating the rapid passage of solutes and reducing filtration resistance. Carbon nanotubes also possess good chemical stability, high mechanical strength, and flexibility, enabling them to adapt to various environments while maintaining their structural properties. Carbon nanotube filtration membranes can be prepared using various approaches by adjusting the structure, density, number of layers, and other parameters to regulate the performance to meet the needs of different application scenarios.

Fig. 9 Schematic Structure of Different Forms of Carbon Monomers

5.3.2 Synthesis Approaches for Carbon Nanotube Filtration Membranes

1. Chemical Vapour Deposition (CVD): Carbon source gases typically used include hydrocarbons such as ethylene and methane, while metal catalysts such as iron, nickel, cobalt, etc., are usually chosen for the catalyst. The substrate to be deposited (e.g., silicon wafer, glass wafer, etc.) is placed in a reaction chamber to ensure that the substrate surface is clean and flat. The reaction chamber is heated to an appropriate temperature and then extracted to a certain vacuum level to ensure the purity and stability of the gases during the reaction process. The carbon source gas and catalyst gas are introduced into the reaction chamber through a gas supply system to control the gas flow rate and flow volume. The carbon source gas dissociates on the catalyst surface to generate carbon atoms, which are subsequently deposited on the substrate surface to form carbon nanotubes. The growth time of the carbon nanotubes is controlled, typically ranging from minutes to hours, to control the length and density of the nanotubes. Prolonged growth results in longer and denser carbon nanotubes. At the end of growth, the supply of carbon source and catalyst gas is stopped and the reaction chamber is cooled to room temperature. At the end of the reaction, the residual gas in the reaction chamber is removed by supplying an inert gas such as nitrogen or argon.

2. Coating Method: The carbon nanotube suspension is coated on the substrate surface by spin-coating, spraying, brushing, or rolling. During the coating process, parameters such as the coating speed and the rotation speed of the coating head can be controlled to manage the thickness and uniformity of the film. After coating, the coating is placed in a ventilated area or on a heated bench to induce solvent evaporation. After complete solvent evaporation, drying is performed to form a uniform carbon nanotube film. Optionally, the carbon nanotube film is heat-treated to enhance the crystallinity and mechanical properties of the film. The heat treatment conditions can be adjusted as needed, and are usually performed under an inert gas atmosphere.

3. Filtration: Commonly used filter membrane materials include polycarbonate (PC), polyester (PET), and polyamide (Nylon) membranes, while the pore size is typically selected based on the desired film thickness and permeability. The carbon nanotube suspension is filtered onto the filter membrane by vacuum or pressure. Filtration operations can be performed using equipment such as vacuum filtration funnels or membrane filters.

4. Stripping Method: Common stripping methods include mechanical stripping, where the carbon nanotube film is directly stripped from the substrate using stripping tools (e.g., tapes, scrapers, etc.); chemical stripping, where the grown carbon nanotube film is placed in an appropriate solvent or solution so that the bond between the film and the substrate is damaged to enable the stripping; and thermal stripping, where the substrate or the film is heated to make it thermally expanded or contracted to disrupt the bond between the substrate and the film to allow for stripping.

5.3.3 Utilisation of Carbon Nanotube (CNT) Membranes

A specific application of carbon nanotubes, in addition to functional applications similar to other types of filtration membranes, includes their use as reverse osmosis membranes. Reverse osmosis membrane is a membrane separation technology capable of separating impurities, ions, microorganisms, etc., from water; it finds wide usage in the fields of drinking water, industrial wastewater treatment, and seawater desalination. However, reverse osmosis membranes face challenges of low flux and low processing efficiency. To address these issues, scholars introduced carbon nanotubes into reverse osmosis membranes. Carbon nanotubes possess excellent properties, including high specific surface area, high strength, and high conductivity, which can form a proton conductor channel in the reverse osmosis membrane and enhance the flux. Concurrently, carbon nanotubes can also adsorb ions, microorganisms, and other impurities in water, which can effectively improve the water purification efficiency and longevity of the reverse osmosis membrane. As of now, reverse osmosis membranes based on carbon nanotubes have entered commercial use, demonstrating significant results in the fields of drinking water, seawater desalination, and others. Future research and preparation technology for carbon nanotube materials will be further developed, with ongoing improvements in the flux and processing efficiency of reverse osmosis membranes.

Table 2 Comparison of TiO2 Nanotube, GO, and CNT Properties

6 Metal Organic Framework (MOF)-based Membranes

6.1 What Are MOF Membranes

Metal Organic Framework (MOF) is a class of crystalline porous materials with periodic network structure formed by interconnecting inorganic metal centres and bridging organic ligands through self-assembly. MOF is an organic-inorganic hybrid material, also known as coordination polymer, which has both the rigidity of inorganic materials and the flexibility of organic materials. The metal-organic skeleton is a coordination polymer formed by the self-assembly of polydentate organic ligands containing oxygen, nitrogen, etc., and transition metal ions, which differs from both inorganic porous materials and general organic complexes. The backbone-type structures in different dimensions are mainly determined by the coordination interactions between organic ligands and metal ions as well as hydrogen bonding. The residual reactants and solvent small molecules during the synthesis process occupy the pores of the skeleton structure, with the removal of small molecules by activation treatment leaving a persistent pore structure. Moreover, the size and structure of the pores can be changed by the structure of organic ligands and the type of metal ions in the synthesis raw materials to control the specific surface area and porosity to suit different applications. Currently, metal-organic skeleton materials used with nitrogen-containing heterocyclic organic neutral ligands or mainly with carboxyl-containing organic anionic ligands can be synthesised in large quantities, showing great potential for development and application in modern materials research.

6.2 How to Produce MOF Membranes

1. In-situ synthesis method: according to the specific surface properties of the carrier itself, the carrier is directly put into the synthesis system, and under certain conditions, the carrier surface and the film-forming night directly contact and thus react, to prepare a continuous membrane. The in-situ synthesis method is straightforward and easy to operate, facilitating large-scale production; however, it can be challenging to prepare continuous MOF membrane due to the differing chemical properties between MOF materials and carriers, resulting in a reduced rate of crystal nucleation that may lead to poor bonding between the membrane and the carrier.

2. Crystal seed secondary growth method: this method first employs the hydrothermal approach to enable crystal seed growth on the substrate, following which membrane layer growth occurs through crystal nucleation, leading to the condensation reaction between groups on the porous substrate surface, ultimately forming covalent bonds with zeolite grains. However, this method has limitations as the filter membrane may not withstand high temperatures.

Fig. 10 Schematic Synthesis of a MOF Film: PSS@ZIF-8 Film

6.3 How Are MOF Membranes Used

In addition to functional applications similar to other types of filtration membranes, MOF membranes can be employed to treat heavy metal ions. MOF membranes possess a highly ordered porous structure formed by metal ions and organic ligands through ligand chemical bonding. This porous structure exhibits a tunable pore diameter and size, providing numerous adsorption sites and channels, favourable for the adsorption and embedding of heavy metal ions. Consequently, MOF films can be used in the field of water treatment for removing heavy metal ion pollutants, such as lead, cadmium, and mercury, from groundwater, industrial wastewater, and municipal wastewater. The highly controllable pore sizes and surface functionalisation of MOF films enable efficient adsorption and selective separation of specific heavy metal ions, contributing to adsorption treatment and recovery for environmental remediation and wastewater treatment processes.

7 Composite Filter Membrane

Composite filter membranes differ from traditional single-material filter membranes as they combine two or more materials to optimise their respective strengths and mitigate deficiencies, thereby achieving more efficient and reliable filtration. These materials can include polymers, ceramics, metals, and nanomaterials. Each material possesses unique physical, chemical, and mechanical properties and can be flexibly combined according to different filtration requirements.

In lithium-ion batteries, a PVDF-MOF Composite Membrane with a continuous MOF layer serves as a high-performance diaphragm. The uniform pore structure and sub-nano channels with connected open metal sites in the continuous MOF layer generate uniformly distributed Li+ flux, inhibiting the formation of dendritic protrusions and improving electrochemical performance.

Fig. 11 PVDF-MOF Composite Separator with Continuous MOF Layer [5]

In the field of seawater desalination, membrane distillation (MD) has emerged as an alternative seawater desalination methodology that can substantially reduce capital costs and energy consumption. In the MD process, nearly 100% of the non-volatile impurities are removed, with no limitation on the feed water concentration, while the pressure-driven reverse osmosis (RO) process has reduced potential for processing high salinity solutions with low water recovery. Volatile components are separated from the feed mixture utilising a microporous hydrophobic membrane, operating below the boiling point of the feed liquid. For MD applications, polymeric materials with low surface energy, high thermal stability, chemical stability, and inertness are often preferred. Polytetrafluoroethylene (PTFE) and Polyvinylidene Fluoride (PVDF) are considered the primary commercially available membrane materials for Vacuum Membrane Distillation (VMD) due to their high thermal stability and hydrophobicity. PVDF and PTFE are optimal polymers for VMD applications owing to their excellent chemical resistance and durability. These properties allow PVDF to withstand the aggressive chemical environments often encountered in VMD systems, ensuring long-term operational reliability. PTFE plays a key role with its non-stick properties and excellent resistance to high temperatures. In VMD, PTFE enhances membrane performance and effectively prevents fouling, ensuring unobstructed and efficient vapour transport throughout the membrane during distillation. In VMD applications, the synergistic use of PVDF and PTFE improves the durability, chemical resistance, and operational efficiency of the overall membrane system.

Fig. 12 Flow Chart on Preparation of Microporous PVDF-PTFE Composite Membrane [6]

8 Conclusion

Filter membranes made of different materials are utilised in varying fields due to their differing characteristics and can be selected according to specific needs in addition to the basic filtration process. Stanford Advanced Materials (SAM) can not only provide a wide range of filtration membrane products but also deliver professional selection advice, which you can consult immediately. 

Related Reading:

Case Study: Membrane Filters

References:

[1] Khayet M, Feng C, Khulbe K, et al. Preparation and characterisation of polyvinylidene fluoride hollow fibre membranes for ultrafiltration[J]. Polymer, 2002, 43(14).

[2] Li M, Cheng S, Zhang J, et al. Poly(vinylidene fluoride)-based composite membranes with continuous metal–organic framework layer for high-performance separators of lithium-ion batteries[J]. Chemical Engineering Journal, 2024, 487.

[3] Hu W, Zhang F, Tan X, et al. Antibacterial PVDF Coral-Like Hierarchical Structure Composite Film Fabrication for Self-Cleaning and Radiative Cooling Effect.[J]. ACS Applied Materials & Interfaces, 2024.

[4] Wei Y, Li K, Li P, et al. Enhanced ceramic membranes filtration by PS pre-oxidation with CuO assisted FeSO4 catalytic for NOM removal in drinking water treatment[J]. Separation and Purification Technology, 2024, 345.

[5] Ceramic membranes and their application in food and beverage processing[J]. Filtration and Separation, 2000, 37(3).

[6] Mala M M, S. S, S. F, et al. Sea and brackish water desalination through a novel PVDF-PTFE composite hydrophobic membrane by vacuum membrane distillation[J]. Discover Chemical Engineering, 2024, 4(1).