Two-Dimensional Materials for Environmental Applications

MXenes are an intriguing class of two-dimensional early transition metal (M) carbides, nitrides, or blends with surface terminal groups like ‒OH, ‒F, –O, etc. The synthesis of the first MXene Ti₃C₂ was reported in 2011 from its parent Ti₃AlC₂ (MAX phase). Since then, MXenes have gained enormous attention for an extensive range of applications due to their peculiar properties, such as high specific surface area, excellent conductivity, tailorable interlayer spacing, tunable surface chemistry with easy functionalization, hydrophilic nature, and good mechanical strength. Considering the rich pool of lucrative properties of MXenes, they have been widely employed in environmental remediation applications such as photocatalysts, membranes, electrochemical separation techniques like capacitive deionization (CDI), and adsorbents for heavy metals, organic dyes, radioactive ions, etc. MXenes have also been studied for utilization in electrocatalytic sensors for pollutant detection, solar desalination, anti-biofouling and antibacterial agents. This book chapter mainly sheds light on the recent advances and accomplishments of MXenes in the aforementioned fields, focusing on improving their properties through various strategies. Moreover, the fundamental aspects, structural features, and different synthesis methodologies of MXenes are summarized. The challenges to overcome and future research directions to realize the full potential of MXene materials for environmental remediation applications are also emphasized.

The number of nanomaterials that are suitable for many applications has increased with the 2011 discovery of two-dimensional (2D) transition metal carbides as well as nitrides (MXenes). MXenes are a new class of 2D materials that are quickly gaining popularity for various uses in the fields of medicine, chemistry, and the environment. MXenes but also MXene-composites or hybrids have several desirable properties, including a large surface area, outstanding chemical stability, hydrophilicity, excellent thermal conductivity, and environmental compatibility. MXenes have therefore been utilized in the creation of lithium-ion batteries, semiconductors, and hydrogen storage. The remediation of contaminated groundwater, surface waters, industrial and municipal wastewaters, as well as the capture and conversion of hydrogen, are just a few of the environmental applications where MXenes have recently been used. These applications frequently outperform those for traditional materials. MXene-composites can deionize via Faradaic capacitive deionization (CDI) as well as adsorb a range of organic and inorganic contaminants when employed for electrochemical applications. The applications of MXenes as well as its composites/hybrids for water treatment and CO₂ capture and conversion, are conversed in this chapter as per the literature. We have also discussed the challenges with regard to the utilization of Mxene and its materials in wastewater remediation, along with drawn conclusions.

Water scarcity has been a grave concern because of increasing urbanization and industrialization activities, unrestrained exploration of natural sources, and depletion of water table. Water pollution causes severe health hazards and jeopardizes biodiversity and the aquatic ecosystem. Therefore, wastewater treatment technologies for providing clean water through sustainable and economic approaches are gaining increasing interest. Adsorption and photocatalytic degradation of organic pollutants by a wide range of activated carbons and nanostructured materials have been addressed for their adsorptive separation and mineralization. The invention of graphene in 2004 has propelled immense interest in different types of two-dimensional (2D) nanostructured materials for a diversified range of energy and environmental applications. The 2D nanostructured analogues of graphene, viz. MoS₂, WS₂, h-BN, g-C₃N₄, MXenes, and their composites of high accessible surface area, controlled surface functionalities, and tunable band-gap have shown excellent performance for wastewater treatment. The chapter covers a comprehensive overview of various types of pollutants in water and recent developments on their adsorptive removal and photocatalytic mineralization/conversion by nanostructured MoS₂, WS₂, h-BN, g-C₃N₄, MXenes, and their nanocomposites, heterostructures, and hybrids. The structural, surface, textural, and chemical properties of 2D nanomaterials are reviewed to highlight their roles in the adsorptive removal and photocatalytic degradation of organic pollutants. The chapter also covers futuristic opportunities, environmental sustainability, and technological challenges in preparing and applying inorganic analogues of graphene for wastewater treatment.

The alarmingly rising environmental pollution adversely affects the sustainable growth of modern civilization. Scientists have persistently been putting tremendous efforts over the decades to develop environment benevolent technologies to overcome this major challenge. Photocatalysis is one such technology which needs renewable solar energy and abundantly available water resources as driving forces for pollutants’ degradation. In addition, the selection of an appropriate semiconductor is highly essential to degrade toxic organic compounds, hazardous heavy metals and noxious gases into harmless products efficiently. Among various semiconductor photocatalysts, g‑C₃N₄ (GCN) is considered a robust photocatalyst because of several fascinating properties like metal-free chemical nature, visible-light-responsive activity with moderate band gap of 2.7 eV, tunable electronic structure, facile synthesis, low cost, high thermal and chemical stability. However, low surface area (∼10 m² g⁻¹), high rate of charge carriers recombination, incomplete solar spectrum absorbance and inadequate valence band position (1.4 eV vs NHE) are some of the limitations due to which expected photocatalytic performance of GCN is yet to be achieved. Therefore, modification strategies such as exfoliating bulk GCN into nanosheets, incorporating foreign elements into its crystal structure and heterostructure formation have been employed to overcome these limitations to achieve high photocatalytic efficiency. In this chapter discusses the basic principle of photocatalytic pollutant degradation over a semiconductor surface. Recent developments in modification strategies to enhance the photoactivity of GCN have been summarised systematically. Photocatalytic applications of GCN-based photocatalysts with respect to environmental remediation are presented in this chapter. The challenges and future perspectives in designing GCN-based photocatalysts for efficient performance towards environmental applications are addressed along with the conclusion.

Many bacterial species have developed the ability to tolerate multiple drugs, and hence there is a severe threat in treating infectious diseases. Since the conventional drugs are becoming largely ineffective, there is an urgent need to find novel antibacterial strategies. To this end, the development of nanomaterials, mainly two-dimensional (2D) nanomaterials, has emerged as a new class of antimicrobial agents that demonstrate strong antimicrobial properties and are less susceptible to bacterial resistance. Various mechanisms associated with the antibacterial activity of 2D nanomaterials have been exhibited, including physical or mechanical damage, the release of controlled drug/metallic ions, multi-mode synergistic antibacterial activity, oxidative stress, and photothermal/photodynamic effects. In addition to the detailed mechanisms involved in antibacterial activities, this chapter discusses various types of 2D nanomaterials used for antibacterial activities. For better understanding, the 2D nanomaterials are classified into carbon and non-carbon based for antibacterial activities. The present challenges and possible future directions for the development of 2D nanomaterials with advanced antimicrobial properties are also discussed.

Water is an essential part of living things as the life cycle, and water cycle are inextricably linked. The development of industrialization caused the water to become hazardous by emitting dyes, pollutants, organic wastes and other harmful chemicals into the water bodies. It’s a big challenge to treat wastewater for human health and to preserve the ecosystem. In recent years, graphene-based photocatalytic materials have gained much attention to eliminate liquid pollutants. This is due to the tremendous optical, electrical and physicochemical properties of this unique two-dimensional material. On integration with other semiconducting, metallic or polymeric materials, graphene remarkably boosts the photocatalytic activity of materials toward contaminants destruction. The high charge carrier mobility, high surface area and excellent mechanical strength of graphene-based nanocomposites make them suitable for photocatalytic applications. This book chapter will focus on the recent significant advances in developing graphene-based photocatalytic materials. The principle of photocatalysis, the basic properties of graphene and the mechanism of how the photocatalytic efficiency against the removal of the liquid pollutant can be enhanced when coupled with graphene has been discussed in this book chapter. Furthermore, current challenges and future recommendations for developing graphene-based photocatalysts are also discussed.

Adsorption methods have been employed for pollution control as well as cleanup all around the world. Composite materials have been the most suitable candidates for high-standard adsorption systems. So also, when merged with graphene or its derivatives, they become very effective candidates for adsorbing environmental contaminants found in water. The combined effect of graphene oxide, as well as its engineered material nanostructures (hybrids, composites, etc.), has also been shown to significantly contribute to the adsorption of heavy metals, toxic organic chemicals (colorants, diverse volatile organic compounds (VOCs), pesticides, chemical fertilizer, drugs), as well as other suspended particles pollutants of water, particularly industrial effluents. The broad surfaces of graphene oxide's derivatives and nanocomposites are bonded with a variety of reactionary oxygen-containing functionalities, giving them exceptional stability and adsorption efficiency in an aqueous environment. This also enables them to be recycled for numerous adsorption–desorption cycles. The present chapter discusses all of these graphene-based materials, their adsorption phenomena, and their application to water to cleanse and purify it.

Environmental concerns regarding water shortages due to industrialization and pollution have led to escalation in research towards efficiency in wastewater treatment and desalination. To date, nanotechnology is the most effective solution towards water shortages and is currently used for wastewater treatment and desalination. Among the emerging nanosheets, two-dimensional (2D) nanosheets have gained much attention since graphene was discovered in the fabrication of cost-effective and sustainable membranes for environmental remediation. Recently, 2D nanoengineered membrane technologies have revealed a new potential for removing hazardous compounds from our surroundings. The present chapter gives an overview of the concept of membrane technology, membrane fabrication techniques and the importance of 2D nanomaterials in the desalination and wastewater treatment membranes. Firstly, popular fabrication methods for membranes, such as electrospinning, drop-casting, spin-coating, solution casting and phase inversion, will be discussed. This will be followed by the application of 2D nanoengineered membranes incorporated with graphene, MXenes, molybdenum disulfide (MoS₂) and other nanosheets, in their 2D form, for excellent improvement in desalination and wastewater treatment. Notably, the chapter emphasizes the wide range of membrane applications as well as their potential and challenges for use in the development of nanotechnology-based environmental remediation.

The global environmental pollution crisis is an ever-increasing issue due to human-induced factors such as urbanization and industrialization. Toxic environmental pollutants such as pesticides, dyes, polycyclic aromatic hydrocarbons, heavy metals, etc., threaten human health and the environment. Therefore, the development of rapid, affordable, selective, and sensitive sensing platforms for determining toxic environmental pollutants is a significant necessity. Today, many methods with the features to overcome the disadvantages of traditional sensors are being developed. Among these, electrochemical sensors stand out with features, such as short analysis time, high sensitivity, versatility, miniaturization, and low cost. Due to their unique chemical and physical characteristics, two-dimensional (2D) nanomaterials, such as graphene and its derivatives, transition metal oxides, graphitic carbon nitride, and metal dichalcogenides, are widely used to improve electrochemical sensors performance thanks to their high surface area, porosity, and catalytic effects. In this chapter, the determination of the most important toxic environmental pollutants in various environmental samples with 2D nanomaterials-based optical and electrochemical sensors are overviewed, covering the years between 2016 and 2022.

The development of sustainable solutions for meeting escalating needs, such as clean energy and safe drinking water, is of the utmost importance to the modern world. Hydrogen as a fuel can be worthiest for this purpose, and further generating it from wastewater via green routes, i.e. photo/electrocatalytic splitting, can make it a sustainable solution, overcoming challenges of wastewater treatment simultaneously. In this chapter, we have discussed different materials that can be utilized as photoelectrocatalyst focusing on 2D materials for hydrogen generation from wastewater (textile, pharmaceutical, food industry, etc.). The potential catalytic properties of transition metal dichalcogenides (TMDs), transition metal oxides (TMOs), MXenes, graphene, nitrides, carbides, and their hybrids are discussed for the same. The standard diagnostic parameter for evaluating photoelectrocatalyst is photo response, incident photon to current efficiency, faradaic efficiency, and wastewater treatment in terms of percentage degradation, COD, TOC, etc., are presented and compared for 2D materials. Further, material performance in terms of the band gap, appropriately positioned valence and conduction bands, stability, economics, etc., are also compared for the wastewater systems. Last but not the least, the future outlook of the field is also presented with respect to challenges and research directions to tap this important unused energy source, i.e. wastewater.

Toxic environmental pollutants include chemical contaminants such as heavy metals and pesticides. Moreover, pesticides are used for modern extensive agricultural practices, affecting human health badly. Meanwhile, pharmaceutical and industrial wastes, personal care products, and endocrine disruptors can be cited as chemical pollutants. Environmental monitoring has been important in providing better safety measures in various sectors of life. Fabricating a new sensing platform with high selectivity and sensitivity is the most needed environmental detection tool. Electrochemical sensors draw attention to on-site environmental analysis. They are selective and sensitive toward electroactive compounds, fast, moveable, and cheap. Due to its important properties, many researchers use two-dimensional metal–organic frameworks (2D MOFs). The applications of 2D MOFs are broad ranges, including sensors, catalysis, gas adsorption, gas separation, supercapacitor, and so on. 2D MOFs are based on the coordination of unique metal ions and organic ligands, including two-dimensional layered structures. The typical 2D materials consist of transition metal carbonitrides (Mxenes), graphene, transition metal disulfide compounds (TMDC), silene and carbon nitride, and hexagonal boron nitride. The chapter focused on 2D MOFs materials, sensor design strategies, and environmental application. Moreover, this chapter summarizes and evaluates recent advances in the development and application of 2D MOFs-based sensors for determining toxic environmental pollutants.

The fascinating properties of two-dimensional (2D) nanomaterial, such as excellent mechanical strength, a high portion of active sites, ease of functionalization and tuning the physical and chemical characteristics, are attracting researchers to host their applications in various fields, including wastewater treatment. Among various 2D nanomaterials, 2D MoS₂ has stand out as a promising alternative inorganic analogue of most explored 2D graphene due to its unique characteristics such as high active surface area, low cost, excellent mechanical strength, small band gap and the possibility of surface functionalization. The excellent water remediation characteristics are attributed to the controlled morphology, specific nano-sized properties, abundant availability, and variable surface chemistry of MoS₂ nanomaterials. Additionally, the selectivity of MoS₂ towards water contaminants promotes its application in water purification. This chapter presents the recent progress, future prospects and challenges of 2D MoS₂-based nanomaterials in water remediation techniques such as adsorbent, photocatalyst, membrane and antibacterial agent. The mechanism behind the water treatment process using 2D MoS₂ is also explained. This chapter will provide a platform to the researchers, who are focused on exploring the application of MoS₂-based materials in water purification. The research demands for future water applications of 2D MoS₂ nanomaterials are also identified.

Two-dimensional (2D) metal oxides consisting of stacked layers possess exceptional structure properties such as highly exposed surface-active sites, large interlayer spacing, tunability for exfoliation into single or few layers with improved mass transport and high specific surface area. The combination of these properties makes 2D materials applications very fascinating in photocatalysis because of the enhanced photoactivity performance. In this chapter, significant advancements in the design, synthesis, structure properties and photoactivity performance of 2D metal oxides in CO₂ reduction reaction are discussed. Specifically, a comprehensive insight into understanding the inter-play between strategic design of the 2D metal oxides and their multidimensional heterostructures (0D/2D, 1D/2D, 2D/2D and 2D/3D) with controlled nanoscale interface structure properties correlated to photocatalytic activity performance in CO₂ reduction reaction are highlighted. The construction strategies of 2D metal oxides heterojunctions with other materials such as non-metals, metals dopants, and metal oxides providing high interfacial intimate contacts for improved charge carriers’ separations and transfer for improved high-performance CO₂ photocatalytic conversion are critically discussed.

Recently, nano-engineered two-dimensional (2D) materials have gained immense interest in various applications, including CO₂ capture. The precise atomic structure of 2D nanomaterials introduced various significant characteristics required for specific applications. Increasing levels of CO₂ in the environment is a concerning topic for surviving a sustainable life on Earth. Therefore, CO₂ capture and conversion into useful products have been recognized as the best approach to reduce the CO₂ level in the atmosphere. To capture CO₂, several materials have been studied and emphasised about their advantages and disadvantages. The recent progress in 2D materials, especially graphene-based materials, has shown their potential in CO₂ capture. Graphene-based materials, transition metal dichalcogenides (TMDCs), 2D transition metal oxides (TMOs), MXenes, boron nitrides, carbon nitrides, 2D metal–organic frameworks (MOFs) etc., are the various examples of 2D materials, which have been investigated for CO₂ capture. This chapter aims to provide a brief overview of the recent advantages in the nano-engineering of the various 2D materials for CO₂ capture. In particular, the recent development of emerging strategies such as doping, defects engineering, hetero-structural designing, and architectural functionalization of 2D nanomaterials for enhanced CO₂ capture are discussed thoroughly. The challenges and future outcomes have also been highlighted, which will open the directions for future research.