4.2. Gas Separation and Storage
Composite materials based on MOFs are observed to be a promising class of materials because they have the potential to combine the benefits of MOFs and other elements. Additionally, the disadvantages of each component are individually minimized in adsorption and separation systems
[103][34]. MOFs are the dominant component of MOFs/GO composites, which produce admixtures rather than composites with covalent bonds. By using the GO substrate as a ruling component and support, nanoscale MOFs will be chemically bonded over the top of the platform
[104][35].
MOF materials offer a lot of benefits for CO
2 capture. Al (HCOO)
3 (ALF), a form of aluminum, is also among the most basic MOFs. To address its cost and scalability, Evans et al.
[105][36] produced ALF characteristics from readily available and affordable starting materials, such as formic acid and aluminum hydroxide, as mechanical materials to examine the adsorption
[105][36]. To increase the capabilities of MOFs for the adsorption of CO
2, Zhao et al.
[106][37] produced a composite material of L-arginine-modified MOFs with GO (MOFs/GO-Arg). The bridging of L-arginine-modified GO with the linker MOF (Cu-BTC) was chosen due to its stability, simplicity of synthesis, and affordability. By adjusting the pore shape and chemical environment at the interface between MOFs and GO, the performance of CO
2 adsorption can improve.
4.3. Water Purification
To effectively remove different pollutants from polluted water and wastewater streams, MOF materials, a hybrid class of substances with a metallic center and organic linkers, are incredibly effective due to their porous nature
[108][38]. Pharmaceuticals, particularly antibiotics, are removed from contaminated water using MOF-based adsorbents and catalysts
[109][39]. Sewage treatment employs a variety of methods to remove environmental pollutants. Separation, absorption, photocatalytic degradation, and membrane filtration are the typical techniques
[110][40]. The development of GO/MOFs, which have higher hydrophilicity, fouling resistance, and selectivity, has sparked interest in the water industry
[111][41].
Textile wastewater is one of the most challenging for wastewater treatment due to the highly contaminating solid pollutants, such as dyes. In another study, Jafarian et al.
[112][42] synthesized a novel NF membrane to remove DIRECT RED 16 (DR16) dyes and humic acid from synthetic wastewater. To form this membrane, they deposited a thin layer of GO and a Zn-based metal-organic framework (ZIF-7) on a chitosan (CTS)-coated polyethersulfone (PES) substrate. The surface membrane was made rougher and more hydrophilic by adding GO-ZIF-7. According to the data, the 5GO-ZIF membrane (~94%) had the highest dye clearance rate. Additionally, the highly hydrophilic surface of the GO-ZIF layer and the biocidal activities of GO and zinc introduced by the GO-ZIF-7 nanocomposite improved the antifouling and anti-biofouling capabilities of the membrane. It was observed that overlaying the CTS membrane with the 5GO-ZIF nanocomposite layer lowered the pure water flux (11.4%), which may be a drawback for actual industrial applications. The increase in mass transfer resistance caused by the addition of layers comprising the GO-ZIF nanocomposite can be used to explain the decrease in pure water flux
[112][42].
4.4. Sensors
MOFs are combined with some conducting materials, such as carbon and conducting polymers, to produce materials for electrochemical sensors with good analytical performance. Graphene is an example of a well-known 2D substance that has gained interest due to its excellent electrical conductivity and surface area. By contrast, MOFs and graphene produced new physical and chemical properties
[114][43]. Due to their substantial specific surface area, high electrical conductivity, and superior chemical stability, graphene and its analogs are widely employed to develop electrochemical sensors. The MOF conductivity effectively increased by combining graphene and its analogs with MOFs
[115][44].
Environmental monitoring would greatly benefit from constructing a sensitive voltametric platform to examine the excessively toxic organic pollutant p-chloronitrobenzene (p-CNB). A composite made of nickel-based MOF (Ni-MOF) and GO was successfully synthesized by Gao et al.
[116][45] using simple and affordable chemical precipitation. The Ni-MOF/GO composite was modified to produce an electrochemical sensor for p-CNB. With the help of electrochemical impedance spectroscopy, distinguishing pulse voltammetry and cyclic voltammetry were used to examine the electrochemical characteristics of as-prepared sensors. Thus, the detection limit of the composite sensor was 8.0 nM (S/N = 3), and it demonstrated superior electrocatalytic performance towards the oxidation of p-CNB in the concentration range of 0.10–300.0 μM. Subsequent research revealed the production sensing platform to have good long-lasting stability, strong selectivity, and reproducibility
[116][45].
4.5. Catalysis
In many diverse industries, particularly the chemical sector, catalysts (homogeneous and heterogeneous) play a significant role. The industrial use of homogeneous catalysts is restricted because they frequently have a higher catalytic activity and are challenging to recover from the reaction solution. Contrarily, heterogeneous catalysts (supported catalysts) are easily removed from the reaction solution and repeatedly used. These catalysts have received significant attention when used in different reaction systems
[118,119][46][47]. Catalytic processes are necessary for over 90% of chemical industrial processes and over 20% of all chemical products. Catalyst production is a crucial process that requires extensive research to produce the high-performance catalysts currently in use
[120][48]. The MOFs-GO have become a new class of catalytic composite materials in the nanoarchitecture field because of their high surface area, unique electrical properties, magnificent conductivity, and hydrophilic nature. Nano-structuring porous catalysts would be a crucial way to expose accessible active sites to achieve high catalytic activity
[121][49]. MOFs-GO is a hybrid material that combines the distinct benefits of MOFs and GO and is ideal for immobilizing nanoparticles (NPs)
[122][50].
4.5.1. Electrocatalysts
The study of environmentally friendly energy has received significant attention from researchers due to an evident trend of rising energy consumption. The hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) are consequential electrochemical reactions used in fuel cells and batteries for energy storage. Graphene-based materials are frequently used in composite MOFs to create efficient electrocatalysts because of their low cost and high carrier conductivity
[125,126,127][51][52][53].
Gopi and colleagues
[128][54] produced the catalyst V-Ni
0.06 Fe
0.06 MOF/GO using an in situ technique. They performed excellent bifunctional electrocatalysis for the hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) and showed high durability in both acidic and alkaline mediums. The choice of V, Ni, and Fe redox metal nodes in combination with a highly porous MOF/GO composite for the water splitting reaction is motivated by the abundance of free carriers at the Fermi level of the V atom and the more active edge sites of the Fe atom. Moreover, the MOF-to-graphene MOF conducting mechanism was improved by 2D-graphene-combined MOF composites.
4.5.2. Photocatalysts
According to researchers, MOFs are highly efficient in the photocatalytic degradation of pollutants and organic dyes when used as catalysts in photocatalytic processes. The retrieval and separation of photocatalyst MOFs from the reaction mixture can be facilitated and improved through modifications. Thus, the performance of photocatalysis can be enhanced by GO
[130,131][55][56].
To improve the catalytic performance of MOFs, Jin et al.
[132][57] intensively hybridized them with GO; however, it is still unclear how the pore structure of MOFs affects the activity of GO/MOF hybrids. To investigate the connection between the pore structure of MIL-125 and the activity of the hybrid, they created a variety of GO/MIL-125 hybrids using the sonication process. The obtained GO/MIL-125(H) has an electron mediator, a significant surface area, displays an assistant photothermal effect, and has hierarchical pores that aid in the adsorption and photodegradation of toluene.
The photocatalytic activities of UiO-66 and GO (UiO-66_GO) nanocomposites for the degradation of carbamazepine (CBZ) were examined in a different study by Heu et al.
[133][58] using a one-step hydrothermal method. Under a range of circumstances, including GO loading, catalyst dosage, initial pollutant concentration, and solution pH, the nanocomposite for the degradation of CBZ was investigated. Increased surface area and porosity, a smaller bandgap, and better light absorption are all factors that were found to be responsible for GO’s enhanced photocatalytic activity. Additionally, the composite demonstrated notable stability and recyclability over five successive cycles of photocatalytic degradation. Modifying semiconductors with GO as an electron acceptor is a successful method for enhancing photocatalytic activity. Additionally, it serves as a great source of inspiration for the growth of additional GO-based composite photocatalysts and the use of UiO-66 in water or wastewater treatment methods. The UiO-66_GO nanocomposites demonstrated a high overall removal efficiency (>90%) in 2 h and a photocatalytic rate constant of up to 0.0136 min
−1. The results of the experiments supported the idea of a photocatalytic mechanism for enhanced CBZ photodegradation by showing that O
2• and OH
• are the responsible radicals for photocatalytic degradation. Further, using this composite material as a solar-base catalyst is possible due to its ability to absorb light in the visible light spectrum (400–700 nm)
[133][58].
4.6. Batteries
Fuel cells and large-capacity, inexpensive, and sustainable rechargeable batteries have drawn a lot of attention due to the new energy industry’s rapid development
[134][59]. Batteries are known to store relatively large amounts of energy compared to supercapacitors but have relatively low power delivery or uptake, a short life cycle, and thermal management issues
[135][60]. Due to their unique properties of the adaptable structure, high porosity, and abundance of active sites, MOFs have received extensive research as a typical inorganic–organic nanomaterial in the growth of battery electrodes
[136][61].
Li et al.
[140][62] successfully produced Ni(OH)
2-GO electrode materials using a two-step synthesis process. The numerous functional groups on GO nanosheets aid in the nucleation and growth of Ni-MOFs. The subsequent hydrolysis of Ni-MOF makes it possible to successfully prepare Ni(OH)
2-GO for its promising use in supercapacitors. Within Ni(OH)
2-GO, the GO content decreases to 7.41 wt%. The electron transfer between Ni(OH)
2 and GO readily occurs during the electrochemical reaction, drastically increasing the electrochemical activity of Ni(OH)
2.
4.7. Other Applications (Biomedical)
Numerous MOFs and MOF-derived nanomaterials have been developed and used in biomedicine for antibacterial mechanisms
[142][63]. In different studies, MOF composites have also been examined for bio-applications such as drug delivery, bio-imaging, and cancer treatment
[143][64]. There is not a thorough review report available that explains how to functionalize GO with MOFs for biomedical applications or how to use natural chemotherapeutic agents for cancer therapeutics
[144][65]. The amount of GO’s active surface available for interacting with bioactive molecules was constrained by its intense agglomeration. Combining it with other NPs, such as MOFs, is an appealing strategy for overcoming this restriction and enhancing GO’s efficiency in the biomedical sector
[145][66].
To compare their effectiveness as a vehicle for the anticancer drug carriers 5-Fu, Pooresmaeil and colleagues
[146][67] created three different types of chitosan-based microspheres: CS, chitosan-coated zinc-based MOF (CS/Zn-MOF), and a chitosan-coated hybrid of ZnMOF with GO (CS/Zn-MOF@GO). The ternary of hybrid CS/Zn-MOF@GO microspheres was found to have the highest amount of 5-fluorouracil (5-Fu) loading, at about 45%. The rough-surfaced, 5-Fu-loaded CS microspheres (5-Fu@CS/Zn-MOF@GO microspheres) displayed a pH-sensitive and sustained release pattern for the 5-Fu that was loaded. Therefore, the total amount of drug released over time at pH 5.0 was roughly twice that at pH 7.4. Ultimately, CS/Zn-MOF@GO microspheres demonstrated acceptable enzymatic biodegradability and good biocompatibility with the epithelial human breast cancer cell line MDA-MB 231. The ability of 5-Fu-loaded CS/Zn-MOF@GO microspheres to treat tumor cells was demonstrated by the cell viability of 41.2% following 48 treatments with 5-Fu@CS/Zn-MOF@GO microspheres
[146][67].
5. The Advantages and Challenges of Metal-Organic Frameworks/Graphene Oxide (MOFs/GO)
There are many applications for MOFs and materials based on graphene, especially in electrochemistry. The presence of GO during the production of MOFs has several benefits, which include the capacity to tune the particle size, quicker electron mobility, and chemical and thermal stability. The best performance under specific environmental conditions (pH, temperature, humidity, etc.) is necessary for MOF and graphene-based composites to operate in harsh environments, thus constraining their ability to develop. In this regard, MOF/graphene-based materials still have room for improvement
[148][68]. Additionally, the combination of GO and MOFs results in the creation of tiny holes, which leads to an increase in the dispersive strength of the MOFs, suppressed MOF aggregation, strong specific adsorption, and a high rate of CO
2 storage
[149][69]. GO can resolve the weak coordination bonds between metal nodes and organic ligands, guide MOF development, and lessen poor conductivity
[150][70]. During the production of MOF derivatives, the merge of graphene-based materials can stop high-temperature etching, break down the MOF structure, and perform other processes that would otherwise lower the specific surface area and active sites of MOFs
[151][71]. The composite MOFs/GO have hierarchical pore structures that provide an ideal space for oxygen atoms in GO, enhancing its stability
[152][72]. GO combined with MOFs decreases toxicity and exhibits exceptional electrochemical, mechanical, thermal, and electrical properties. Due to the diversity of MOF compounds and complex MOF–GO interactions, MOFs’ growth and structure orientations are still challenging to predict
[153][73].