Bio-Inspired Synthesis of Carbon-Based Nanomaterials and Their Potential Environmental Applications: A State-of-the-Art Review

Abstract

Providing safe drinking water and clean water is becoming a more challenging task all around the world. Although some critical issues and limits remain unsolved, implementing ecologically sustainable nanomaterials (NMs) with unique features, e.g., highly efficient and selective, earth-abundance, renewability, low-cost manufacturing procedures, and stability, has become a priority. Carbon nanoparticles (NPs) offer tremendous promise in the sectors of energy and the environment. However, a series of far more ecologically friendly synthesis techniques based on natural, renewable, and less expensive waste resources must be explored. This will reduce greenhouse gas emissions and harmful material extraction and assist the development of green technologies. The progress achieved in the previous 10 years in the fabrication of novel carbon-based NMs utilizing waste materials as well as natural precursors is reviewed in this article. Research on carbon-based NPs and their production using naturally occurring precursors and waste materials focuses on this review research. Water treatment and purification using carbon NMs, notably for industrial and pharmaceutical wastes, has shown significant potential. Research in this area focuses on enhanced carbonaceous NMs, methods, and novel nano-sorbents for wastewater, drinking water, groundwater treatment, as well as ionic metal removal from aqueous environments. Discussed are the latest developments and challenges in environmentally friendly carbon and graphene quantum dot NMs.

1. Introduction

Undoubtedly, environmental contamination is one of the most serious issues confronting civilization today [1]. Thousands of tons of harmful chemicals are dumped into lakes, rivers, and seas every day. Heavy metals, textile dyes, herbicides, surfactants, and insecticides are examples of harmful contaminants found in the aquatic environment [2]. The removal of these harmful and poisonous pollutants from water bodies has received considerable attention in order to prevent their hazardous impact on the planet [3]. Day by day, new approaches are continually being investigated to remove toxins present in the air, water, and soil. In addition, due to the fast population increase, medications are used extensively in health care, and a considerable amount of these substances are pumped into the environment in decomposed or untapped forms [4,5]. As a result of the continuous enhancement in people’s housing conditions and the quick introduction of industrialization, humankind in the twenty-first century has confronted environmental challenges resulting from natural resource pollution. As a result, mineralization, eradication, conversion, and reduction of pollution molecules from the aquatic environment are sorely needed [6]. Furthermore, the world’s energy needs are mostly addressed by fossil fuels such as petroleum, natural gas, and coal, which are rapidly diminishing. One of the most significant challenges is the persistently increasing concentration of CO₂, the main greenhouse gas available in the atmosphere, as a result of rapid industrialization and excessive fossil fuel burning. Anthropogenic CO₂ emissions have considerably impacted on global climate change as a greenhouse gas. In this context, researchers consider the best techniques for overcoming the mentioned problems [7,8,9]. Water purification techniques have been extensively developed in chemical, coagulation, flotation, sedimentation, biological, physical, gravity separation, reverse osmosis, and filtration. However, due to the movement of pollutants from one phase to another or partial removal, the performance of these technologies is not particularly good for effectively cleaning polluted water [10,11,12,13]. Nanotechnology has picked much attention in recent years due to the remarkable physicochemical characteristics of nanomaterials (NMs) [14,15,16]. NMs have a higher surface-to-volume ratio than their bulkier competitors, resulting in increased reactivity and greater efficacy. In addition, compared to traditional techniques, NMs have the potential to leverage unique surface chemistry, enabling them to be synthesized and characterized or patched with functional groups which can target specific molecules of concerned pollutants for effective remediation of the environment [17,18]. Furthermore, systematic modification of nanoparticles’ (NPs) physical properties (such as size, shape, permeability, and chemical composition) may convey additional favorable qualities that ultimately enhance the material’s effectiveness for pollutant treatment. The wide surface modification chemistry of the NMs and its adjustable physicochemical parameters offer significant advantages over standard techniques associated with environmental contamination [19,20]. As a result, technologies that integrate multiple diverse materials (hybrids/nano-composites) to harvest desirable characteristics from each component are more economical, efficient, and durable than approaches focused on a particular nanotechnology field [21]. When compared to NPs alone, anchoring NPs to a framework is always an alternative approach to enhance the material’s sustainability [22]. NMs that have been functionalized with certain chemicals that target contaminant molecules of concern can enhance the specificity and effectiveness of NMs [23,24]. The NMs utilized to clean up pollution must not be another pollutant once they have been utilized. As a result, biodegradable materials are especially appealing in this environmental application field. Biodegradable NMs will enhance consumer trust and acceptance of technology by removing the need to dispose of material waste after treatment and providing a greener and safer approach for pollution remediation in the environment [25,26]. Additionally, emerging technologies that rely on the absorption of pollutants on a target-specific basis are particularly appealing because they address poor efficiency imposed by off-targeting. As a consequence, several studies have focused on integrating nanotechnology principles with the chemical and physical surface modification of NMs to obtain engineered NMs that can conquer several of the issues encountered during contamination remediation [27,28,29]. Some key challenges that must be considered when developing new NMs for environmental remediation include target-specific identification, facile and economical fabrication, non-toxic nature, biocompatible, reusability, and the ability for regeneration after use [30,31,32,33,34,35,36].

Whereas numerous “nano” forms of carbon are becoming increasingly popular over the years and are being vigorously explored more than ever around the world, which is certainly justified due to their incredible characteristics, one should not underestimate the astounding range of technological solutions that bio-based carbons can deliver [37,38,39,40]. Exclusively, the gigantic majority of porous carbons are considered those are cost-effective and display potential for environmental prominence. Porous carbons, for comparison purposes, are often used in industrial applications and have always been the subject of extensive interest in categories, such as gas storage [41], heat management [42], compression [43], detection and separation [44], solar energy storage [45], supercapacitors [46], electrochemical energy storage and conversion [47], batteries and fuel cells [48,49], catalysis [50,51], pollutants elimination in gas as well as liquid [52,53], photocatalysis [54,55,56], and protection against electromagnetic waves [57]. Interestingly, 62.1% of published data comprise the practical application and designing of carbon-based NMs for water mitigation practice (Figure 1a). Considering the significance of porous materials, the demand for natural, recyclable, low-cost, yet efficient precursors promises to be a worthwhile endeavor. Because of the wide range of potential compositions and porosity scales involved, bio-based raw materials can be considered for all of the uses as mentioned above [58]. Carbon includes a broad range of allotropic forms, ranging from 0-D to 3-D nano-structures, whereas some of the forms, such as graphene and its derivatives, are rising in popularity, especially when novel features are found and exploited in the introduction of unique functional NMs for environmental applications [59]. A survey based on carbon NPs exemplifies that from 2012 to 2021, searching the keywords “carbon nanoparticles for environmental remediation” displays approximately 10,500 publications. This depicts the keen apprehension of the scientific community for remediation of the environment via carbon-based NPs (Figure 1b). The present review comprehensively represents and encourages the scientific community to go further than traditional approaches and reconsider the primordial carbon forms in new green technologies for eco-friendly environmental applications. The significant aspect of the research represented in this review portrays the green fabrication of carbon-based NMs and highlights a huge spectrum of carbon-based NMs for a wide range of environmental uses.

2. Carbon Nanoparticles as a Promising Future Nanomaterial

Certainly, various innovative carbon-based NMs have been utilized to construct cost-effective techniques and slightly elevated water purification systems for real-time and continuous environmental monitoring; such NPs have also been widely used for effluent remediation, mitigation, and purification [60]. Carbon NMs are easily recyclable. They also efficiently preserve the water quality for quite a long time, are convenient, and demonstrate practicability of water supply while repetitively detecting biological and chemical pollution from synthetic, municipal, and industrial water waste [61]. This sub-part represents the potential of carbon NMs for future environmental treatment. Carbon-based NPs, such as carbon nanotubes (CNTs), carbon quantum dots (CQDs), graphene, and other derivatives, are used for the remediation of water and elimination of pharmaceutical waste as well [62,63]. Carbon NPs possess a number of unique forms and characteristics that are intimately connected to the carbon atom’s distinctive atomic structure. Carbon NPs are classified as zero-dimensional NPs (such as fullerenes and carbon quantum dots), 1-D NPs (such as carbon nanotubes and carbon nanofibers), 2-D NPs (graphene), and 3-D NPs (like carbon sponges) based on their distinctive dimensionality, morphology and size [64,65]. The most prominent 1-D carbon nanostructures are carbon nanotubes (CNTs) and carbon nanofibers. These carbon nanostructures are formed from rounded graphene sheets. Carbon nanotubes (CNTs) also possess a tube-like shape. Once carbon nanotubes (CNTs) are formed, the other derivatives, such as single-walled, double-walled and multi-walled carbon nanotubes are formed from them [66]. Recent studies also reported that CNTs could also efficiently eliminate metal ions such as chromium (III) as well as pharmaceutical wastes from water [67]. As per the demand for modern approaches, CNTs possess tunable characteristics, having exceptional mechanical strength and chemical stability, making them excellent contenders for a broad range of uses. Due to the astoundingly high surface-to-area ratio, improved chemical stability, cost-effective and limited effect on the environment that carbon-based NPs have, they are a great alternative for the degradation of environmental issues [68]. CNTs and allotropes with a cylindrical nanostructure are excellent nano-sorbents for the decomposition of diverse pollutants and could be used to remove pharma pollutants or their byproducts to common sorbents, such as activated carbon, clay, zeolite, and diatomite [69]. Graphene is primarily derived from single-layer graphite and has a lot of potential for environmental services. A 3-D carbon NM was formed by converting a 2-D NM into a 3-D microporous structure [70]. The 3-D carbon nanostructures are constructed using a range of fabrication procedures precisely described in the next section. The structure and interfacial interactions with encompassing bulk materials define the physicochemical characteristics of carbon nanostructures. Drugs with high water solubility get adsorbed in target tissues/cells in humans, animals, and plants as they are extremely persistent in the environment and stable under ordinary circumstances. Each year, thousands of tons of drug substances and pharmaceutical constituents are introduced into the environment, with excess medicines responsible for a major amount of this [71,72]. Additionally, they are re-converted to active components via metabolic processes, and pharmaceutical pollutants are particularly common in cosmopolitan sewage. Graphene-based NMs also treat wastewater because they have vital physicochemical characteristics, such as large surface area, facile functionalization, and hydrophobicity [72,73]. Graphene nanocomposites (NCs) also act as nano-sorbents for removing hazardous and noxious mixtures of numerous contaminants and catalytic decomposition of those pollutants using catalytic, visible light photocatalytic oxidation and reduction, and electrocatalytic processes. On the other hand, commercial production and usage are still major obstacles [74]. Nanoporous graphene sheets may filter and desalinate water by changing their pore size and pressure. However, their mechanical stability after increasing the pore number is the primary restriction [75]. Using the self-assembly fabrication technique, Yan et al. [76] fabricated N-doped sandwich shape graphene NCs for the deionization of electrodes. The formed heterocomposite delivered a specific large surface area, less electronic resistivity, and better adsorption ability with efficient, recyclable characteristics. They also represented that graphene sheets are applicable as selective barriers for water penetrability. Graphene sheets are superoleophilic and superhydrophobic in nature and thus can be applied as an absorbent for numerous other NMs, signifying their extreme selective nature, appropriate recyclability, and substantial absorption abilities of even more the 90%. The aqueous elimination of hazardous dyes by graphene derived nano-adsorbents has several benefits over CNT-based NMs because single-layered graphene or single-layered CNT shown two basal planes reachable for pollutant adsorption [77]. Furthermore, graphene and its derivatives can be easily synthesized via chemical/mechanical exfoliation of graphite without employing complicated equipment. Notably, the obtained graphene derived NPs are free from residues of catalytic reactions, thus hindering the necessity for further purification processing. Mostly in the case of graphene oxide, the nano-sorbents provide a large number of O₂-bearing functionalities, hence prohibiting further acid treatments from promoting reactivity and hydrophilicity of the NM [78]. Favorably, their interaction, such as electrostatic π-π and hydrophobic contacts with a variety of organic and inorganic pollutants, represents the potential of graphene oxides for acting as a good nano-sorbent for polluted water and wastewater [79]. The science world is positive about carbon nanostructured materials attaining considerably better sustainable energy conversion and waste management.

Carbon and graphene quantum dots represent an interesting group of NMs with extensive applications among nano-carbon materials [80]. Xu et al. [81] discovered quantum-sized carbon dots in 2004 (a few months before graphene counterparts), and Sun et al. [82] gave them the popular moniker “CQDs” in 2006. However, their use as heterogeneous photocatalysts only began lately. CQDs are carbon-based 10-nanometer NPs with a flat or quasi-spherical form, sp³ or sp² hybridized carbon atoms (i.e., diamond shape-like and graphene sheets like structure), composition-dependent fluorescence, and a variety of surface functional groups [83]. Graphene quantum dots (GQDs) primarily comprise a single graphene layer with a size not greater than 2 nm. Carbon-based nanodots, CQDs, graphene quantum dots GQDs, and carbon nitride dots are all these NMs that belong to the same carbon family [84]. These carbon-based nanostructures demonstrate intriguing involvement in non-biomedical regions, such as optoelectronic components and visible light photocatalysis, because of their eclectic properties, which include strongly size-dependent automated, electrochemical, and optical, as well as quantum size effects, easy surface modification, customizable composition, the ability to channel long-wavelength light, and ease of fabrication [85].

Furthermore, CQDs and GQDs are also promising candidates for biological and medical equipment, including therapeutics, bio-sensing, gene editing, and bio-imaging, owing to their advantages, which include nontoxicity, high photo-stability, remarkable electronic stability in vivo, biocompatibility, electro-chemiluminescence, non-linear optical feedback, intense and infinitely adaptable photoluminescence (PL) with the use of surface functional groups, and dot size, among other things [86,87]. Based on density functional theory (DFT), GQDs may be altered from deep UV to NIR; Chen et al. [88] demonstrated that by modulating the morphological properties, surface functional groups, particle size, edge configuration, defects in surface, and doping of heteroatom, results in the (near-infrared). Graphene (i.e., 0-D CQDs, 2-D graphene sheets), nano-diamonds and CNTs are the most well-known carbon-based NMs in NCs. GQDs might also self-assemble into honeycomb nanostructures or hollow microspheres [89]. In addition to having the potential to act as an alternative semiconductor, which has severe demerits of toxicity and high cost, carbon NPs could impact the quality of several nano/photocatalysts through energy band alignment, morphometric change, light adsorption augmentation, synergic coexistence, and charge transferal promotion, among other mechanisms [90]. Because of their high photon absorption, fully available surface, outstanding photo-stability, high miscibility, tunable band gap structure, and potential to fabricate heterostructures or hybrids with others, nanostructures have been widely used as efficient nano-photocatalysts for photocatalytic energy generation as well as in the visible light photocatalytic deterioration of pollutants. Figure 2 concisely represents the qualities of carbon-based materials and portrays their potential as a future NMs for a broad range of implementations in the environmental remediation sector.

3. Fabrication of Carbon Nanoparticles

The introduction of next-generation nano-carbon composites, such as sophisticated components with enhanced performance and multifunctionality, has piqued attention [91]. Carbon-based precursors, such as graphene and CNTs, are critical in assessing the long-term viability of the technology. To date, carbon precursors, such as acetylene, methane, benzene, toluene, and xylene, have been employed to synthesize CNTs [92,93]. However, because of their dependence on natural resources, carbon-based precursors will suffer the problem of shortage in the future. Thus, a more economical, convenient, and eco-friendly way is needed to produce carbon-based NMs [94,95]. Inexhaustible and cost-effective, natural antecedents as a reservoir of hydrocarbons might be a green choice for large-scale manufacturing of NMs. As a carbon substrate for the manufacturing of CNTs, natural precursors represent an attractive option because of their low cost and lack of scarcity in the near future. Instead of depleting a natural resource, every manufacturing plant must use renewable feedstock to comply with the green chemistry idea [96]. In addition, the process must be achieved by incorporating as many of the feedstock’s component atoms as feasible into the final product. Using natural carbon NM precursors has lately gained a lot of attention as a safe and ecologically friendly solution to a broad variety of applications. Massive amounts of carbon-based materials, such as CNTs and graphene, can already be manufactured using natural hydrocarbon antecedents and recyclable materials, proving that the themes of synthesis antecedents may be obtained in large numbers [97]. The combustion of plant-derived antecedents and residues, including textiles, seeds, bagasse, and hydrocarbons, has provided a variety of carbon products. According to studies on the ultrastructure of plant parts and carbon formed by them, plant-derived precursors display some basic hydrocarbon components, such as skeletal structures. Natural precursors have lower market pricing than fossil, gaseous, and liquid hydrocarbons [98]. The 1-D and 2-D carbon structures represent the basic raw natural hydrocarbon components. These natural precursors provide an ecologically friendly way to manufacture CNT_S and graphene on a wide scale for diverse environmental applications.

3.1. Conventional Fabrication Techniques

Graphene and CNTs are generally produced by synthesizing fossil hydrocarbons, which are costly and difficult to acquire. A catalyst working on gaseous fossil species originating from the thermal decomposition of hydrocarbons is generally accountable for the formation of graphene and CNTs [99,100]. It has been possible to manufacture several kinds of CNTs, carbon nanofibers (CNFs), vapor produced carbon fibre, and other categories of carbon nanoscale materials using a variety of different processes, including, when it comes to synthesizing nano carbon-based substances, some of the most popular methods, i.e., laser ablation, arc discharge, and CVD (chemical vapor deposition) [101]. Heterogeneous CVD processes essentially comprise passing a gaseous or liquid flow having a specific proportion of a hydrocarbon, such as xylene (C₈H₁₀), benzene (C₆H₆), toluene (C₇H₈), acetylene (C₂H₂), methanol (CH₃OH), ethanol (C₂H₅OH), hexane (C₆H₁₄), methane (CH₄), and carbon monoxide (CO) over the nano surface of metal particles, such as Cu and Fe, Co, Ni), in the furnace. For the fabrication of graphene and CNTs, solid carbon sources, such as polyimide, polymethyl methacrylate (PMMA), and coal, have been employed. The majorities of liquid and gaseous fossil hydrocarbons are poisonous or explosive and harmful to the environment and human health [102,103]. Figure 3 depicts the various gaseous and liquid hydrocarbon fossils often utilized to synthesize carbon-based NMs at higher temperatures using various metals acting as catalysts. Various techniques used to fabricate carbon-based NMs are chemical oxidation, atmospheric plasma-based fabrication, hydrothermal carbonization, microwave aided fabrication, ultrasonic techniques, solvothermal methodology, thermolysis, laser ablation approach, and electrochemical approach [104,105]. Different synthesis procedures allow for fine-tuning carbon-based nanostructures size, component elements, and characteristics. A strong oxidant oxidizes a carbon precursor to carbon dots in chemical oxidation. Carbon dot fabrication on a wide scale is a rather simple technology [106]. Unfortunately, a fundamental drawback of this approach is the lack of uniformity in the size distribution of the resulting particles. Laser ablation is a simple and quick method to fabricate carbon NPs in which a laser is used to irradiate a carbon target [107,108]. However, this approach has large energy consumption and a poor quantum yield. It is also a costly technology that does not secure NPs size control. Hu et al. [109] investigated the influence of laser pulse width on carbon dot size and shape. They demonstrated that the pulse width had a significant impact on the nucleation and proliferation of fabricated particles. The effect of carbon NPs size and surface states on optical properties and photoluminescence has been studied. The size of the fabricated NPs may be determined by varying the proportion of HNO₃ used for oxidative refluxing, and selective reduction with NaBH₄ changes the surface states. Electrochemical oxidation synthesizes carbon NPs with high purity and yield and adjustable shape and size. However, due to the technique’s extensive processing processes, it is only of limited utility. Graphite rod, carbon paste, and carbon fibre are the electrodes utilized [110]. Liu et al. [111] described an electrochemical synthesis that employs graphite as the working electrode, platinum foil as the counter electrode, and Ag/AgCl as the reference electrode. They created carbon dots with an average diameter of 0.2–4.0 nm that they utilized to detect Fe³⁺ ions in drinking water. Ultrasonic energy has been frequently employed for carbon dot fabrication because of the process’s simplicity, cost-effective, and absence of secondary contamination. Wu et al. [112] used an ultrasonic approach to fabricate amine-decorated carbon dots from graphite rods. The fabricated carbon dots were useful for detecting cobalt (II) ions in actual samples and sensing nucleic acids in living cells. Carbon dots were fabricated from urea, citric acid, and polyethylene glycol via ultrasonic processing as a lubricant [113,114,115]. The carbon core and tiny size, which provide a rolling effect, were associated with the lubricating action. The microwave approach, which requires a minimal reaction time, provides a simple and cost-effective synthesis. This simple process can produce size-controlled particles with a high yield, but it requires a lot of energy. In addition, the approach is limited by unpredictable reaction parameters [116,117]. Microwaves were used to carbonize a combination of citric acid and Citrus japonica to synthesize biocompatible luminous carbon dots for biological uses [118]. For the synthesis of simple, doped, and supported carbon-based NPs, the solvothermal approach, particularly the hydrothermal method, is the most widely used methodology [118,119,120,121]. It comprises a chemical reaction in a solvent such as water in a sealed pressure vessel. Environmental friendliness, very moderate working conditions, a one-step synthesis approach, and excellent dispersion in solution are all advantages of this technology. As a result, a broad range of precursors has been employed to synthesize carbon-based NMs using the hydrothermal approach, including aloe, papaya, cornstalk, water hyacinth, apple juice, and milk. It is an effective strategy for cost-effectiveness and convenience of use [122,123,124,125,126]. The thermal breakdown is an affordable solvent-free fabrication method, although the resulting particles lack a uniform distribution of NPs [127]. Despite significant advances in the sector of carbon-based NMs fabrication, the employment of demanding reaction conditions, hazardous precursors, and post-synthetic processes for surface passivation generally complicates the process and prevents their widespread adoption. The majorities of the papers have clear drawbacks, such as high-cost equipment, the use of corrosive chemicals, and non-homogeneity of the resultant NPs, and hence are not cost-effective. Various research groups have experimented with easier and lower-cost synthesis strategies and the use of environmentally acceptable precursors [106,128,129]. Many review articles have been published imparting the information regarding the use of hydrocarbons for the fabrication of carbon NMs, but only a few of them have discussed the utilization of waste products as a precursor [128,129]. Thus, there is an utter demand to the scientific community for green, economic, as well as renewable synthesis fabrication of these NPs for environmental applications.

3.2. “Green” Synthesis of Carbon-Based Nanomaterials

Naturally occurring plant hydrocarbon precursor and organic waste are renewable, low-cost, and widely available sources of hydrocarbon for the fabrication of various carbon nanostructures [130]. Thus, we can synthesize the carbon-based NMs on a large scale by utilizing these natural waste products [131]. As a source of carbon for high-quality CNT and graphene, renewable antecedents and waste products may provide a number of benefits [132]. These precursors generate a sustainable amount, are safe for the environment, and are cost-effective. In addition, a variety of usable energy and energy activities could benefit from the usage of high-quality carbon-based nanostructures naturally derived from antecedents and waste products [133]. The various types of natural precursors, edible food, and scrap material employed to fabricate numerous new innovative carbon nanostructures are depicted in Figure 4. For the green production of carbon-based NPs, diverse kinds of plant are employed as renewable carbon feedstocks [134]. The leaves, flowers, and roots are the most widely utilized plant. Shells, bark, stem, and peels are examples of plant biomass [135,136]. Pre-processing is necessary to reduce particle size and remove excess water from these feed sources [137]. Drying the feedstock under full sun saves energy in comparison to more traditional methods of dehydration. For hydrothermal synthesis, fruit juice is advantageous since it provides a “ready-to-use” solution, avoiding the need to revive dried plant components. However, this advantage comes from sacrificing the diversity of carbon seen in dry feedstocks. For example, Lotfy and colleagues generated citric acid from Aspergillus niger with a 90% yield using beet and maize steep liquors and salts as feedstocks [138]. There is a lot of citric acid in citrus fruits, which account for up to 8% of the dried fruit mass. Researchers have used L-glutathione and celery leaves to make carbon NPs via hydrothermal synthesis as part of hybrid green syntheses that combine renewable raw materials with renewable refined products.

Bio-oils also represent a potential candidate for the fabrication of CNTs due to their cost-effectiveness and easy availability, thus making them an excellent green antecedent for CNT production. A major component of eucalyptus oil has the chemical formula C₁₀H₁₈O, which comprises carbon, hydrogen, and a small proportion of oxygen. For instance, under atmospheric pressure, Ghosh et al. [98] employed eucalyptus oil as a catalyst and Fe/Co coated on zeolite as the catalyst support to establish a straightforward, and cost-effective technique for the production of single-walled carbon nanotubes (SWCNTs) with low energy consumption. This fabrication approach does not require any prior treatment of the catalyst precursor to manufacture an active catalyst precursor. SEM (Figure 5a) and HRTEM images (Figure 5b,c) of the as-grown SWCNTs are represented in Figure 5, demonstrating the development of thick CNTs on the whole surface of the zeolite particles. The SWCNTs formed are in bundles, as per HRTEM studies. Kumar et al. [139] employed sunflower oil to synthesize aligned bamboo-shaped nitrogen-doped carbon nanotubes (N-CNTs). SEM and TEM results (Figure 5d,e) revealed that fabricated nanotubes lie parallel to the quartz tube’s walls and have a consistent length of roughly 25 m. The N-CNTs in the form of bamboo were constructed from several segments with declining segment spacing and approximately equivalent numbers of layers, resulting in a bamboo-shaped structure. A cap of Fe catalyst particles is placed on top of the N-CNTs, and seven graphitic layers are used to recreate the hollow segments of the bamboo structures. In addition to the obvious natural hydrocarbon precursors, graphene and carbon nanotubes have indeed been derived from many diverse other eatable foodstuffs, such as honey and sugar, and fats and dairy products, such as butter, milk, and cheese, for graphene synthesis.

Seo et al. [140] reported a unique plasma approach for fabricating nano-graphene sheets employing five distinct eatable foodstuffs, including butter, honey, milk, cheese, and sugar, in various physical states. This plasma-enabled, low-temperature (450 °C) technique uses renewable natural resources and does not require any catalysts or hazardous precursors. The main advantage of this strategy is that it works for a variety of precursors, including honey, sugar, and milk. Figure 6 depicts the characteristics of several precursors in various stages. The authors showed how honey, butter, table sugar, and condensed milk might be converted into nano-graphene sheets without other catalysts or external heating when treated with hydrogen and argon plasma. Fabricated nano-graphene sheets are employed for bio-sensing applications. A convenient method for the fabrication of highly crystalline carbon dots has been proposed through a plasma-induced methodology. Wang et al. [141] reported a plasma-induced technique for synthesizing carbon dots utilizing inexpensive and natural chicken eggs as the precursor, and this approach may be utilized to make carbon dots from a variety of carbon sources. Thus, the prospects of these carbon dots as fluorescent inks for multicolor patterning utilizing inkjet and silk-screen printing have been expanded.

A novel NCs system based on CNTs, graphene, and its derivatives is evolving for use in multifarious applications in biology, chemistry, medicine, and physics. Recently, graphene and carbon nanotube production methods, such as spray pyrolysis and CVD, have made significant strides thanks to advances in carbon nanostructure technology. However, carbon precursors derived from fossil fuels, such as acetylene, benzene, methane, and other hydrocarbons, which are frequently used in the manufacturing of carbon-based nanostructures, have a limited stock, damage the environment, and will be unreachable in the foreseeable future due to a lack of available resources. It may be appropriate to use natural precursors, eatable products, and residual carbon-containing products as a benchmark in this situation. It is anticipated that the use of these resources as carbon source precursors would cut the cost and use of scarce fossil fuels and aid us in treating our environment with more sensitivity. The production of eucalyptus oil, turpentine oil, palm oil, biodiesel, sunflower seed oil, neem oil, castor oil, and other natural precursors may be done effectively, in an ecologically acceptable way, and in an environmentally friendly setting. In addition, there are a plethora of massive carbon-containing waste materials strewn across the environment. The chances are that they have been discarded, and so they are contaminating the environment with little to no chance. These materials can be used to make a variety of carbon nanostructures with ease (Figure 7). From surface properties to large-scale manufacturing, natural waste products from the creation of graphene and carbon nanotubes will continue to make significant contributions to the forefront of nanotechnology and related bulk commodities for many generations to follow.

4. Environmental Applications

Unique physicochemical features of NPs have made them an attractive candidate for use in bioremediation and improving performance in the renewable energy industry. NPs are present in nature, and some of them have been discovered to help the environment [142]. For more than a decade, environmental remediation utilizing NPs, also known as nano-remediation, has been effectively employed to clean or decontaminate the air, water, and soil [143]. Nano-remediation is among the most efficient alternatives since it provides in-situ treatment, eliminating the need to pump groundwater out for diagnosis and excavating to reach the desired region. The NPs are introduced into the appropriate site and carried along with the groundwater flow, disinfecting the water by immobilizing pollutants. Redox reactions represent the general process involved in decontamination. Surface water is treated with NPs for disinfection, purification, and desalination [144]. Heavy metals, viruses, and organic pollutants are the most likely contaminants. It is effective, obviating the requirement for chemicals that might result in secondary reaction products. NPs are primarily used to remediate sewage water and the resulting sludge. NPs replace traditional chemicals because they are less expensive, more efficient, and need less treatment volume. Nano-filtration is a mainly new membrane filtration approach for water purification that has remarkable use in the food and dairy sectors [145]. The utilization of NPs for renewable energy is the subject of much investigation. Solar cells with higher light and UV absorption and low reflection coatings have increased their efficiency significantly. Self-cleaning solar cells have been developed thanks to the hydrophobic features of certain NPs. Certain NPs with high thermal conductivity and heat absorption capacity cover boilers and solar concentrators to increase their thermal efficiency [146].

4.1. Water Treatment Using Carbon-Based Nanoparticles

Photocatalysis refers to the use of an appropriate catalyst to speed up photoreactions. Hydroxide free radicals are used in the majority of photocatalyzed processes. The water electrolysis process, made feasible by titanium dioxide, can be identified as the progenitor of the photocatalysis reaction. An important challenge was how to remove various pollutants from wastewater by using photocatalytic and adsorption techniques in combination to remove a broad spectrum of contaminants. Photosynthetic photocatalysis assists in the degradation of organic pollutants, which may then be removed through adsorption. Carbon nanotubes may be employed as photocatalyst carriers because of their porous structure, unique electrical structure, amazing chemical stability, strong absorption ability, and nanoscale hollow tube characteristic. Over the last few years, the remediation of industrial wastewaters from the urban and industrial sectors has garnered considerable attention. With a rapidly rising population, maintaining a consistent, clean water supply has become progressively difficult with a rapidly rising population, and severe drought has become common [147]. The detection of heavy metals in the environment, polycyclic aromatic hydrocarbons, total suspended solids, as well as organic and inorganic wastes would negatively impact the health of humans. Human care and development are inextricably related to basic sanitation and water availability. As a result, it has become necessary to treat wastewater streams to recirculate the cleaned water [148]. Several treatment methods employed include biological, chemical, and physical processes. Carbon-based NMs have also made their way into the wastewater treatment process, where they may be used as adsorbents to remove contaminants. As a result of their large surface areas and availability of surface adsorption sites, bi-functional CNTs have indeed been extensively investigated. It is being explored if functionalized single-walled nanotubes (also known as SWCNTs) and multi-walled nanotubes (also known as MWCNTs) can adsorb heavy metal contaminants, such as arsenic and chromium [149]. Ahamad et al. [150] prepared NH₂-MWCNTs@Fe₃O₄ to eliminate the toxic dye methylene blue (MB) from water. MB dye concentration, pH, temperature, and contact duration were examined and optimized as adsorption factors. The Langmuir isotherm equation determined a maximum adsorption capacity of 178.5 mg g⁻¹, greater than other adsorbents employed for MB adsorption. The recycling analysis revealed that NH₂-MWCNTs@Fe₃O₄ might be utilized for several adsorption cycles without losing significant adsorption capability. All of the results demonstrated that NH₂-MWCNTs@Fe₃O₄ might be an effective, readily separable, and reusable adsorbent for the adsorptive removal of MB dye from polluted water. Zhang et al. [151] described a facile and economic hydrothermal technique to fabricate TiO₂ layers coating on CNTs NC. The TiO₂ NPs were found exceedingly consistent, and the NCs’ layer thickness was adjusted. Meanwhile, MWCNTs operated as an electron transfer channel, thus enhancing photocatalytic efficacy substantially. To boost the photocatalytic effectiveness, they discovered that the shallow TiO₂ coating layer provided a quick way for electron transfer and was more efficient at separating carriers. The photo-oxidation and photo-reduction sensitivity of TiO₂ NPs were studied concerning the depth of the TiO₂ coating layer using photocatalytic dye degradation. Even under the best conditions, one-dimensional TiO₂@MWCNTs has a higher photocatalytic performance than the conventional anatase TiO₂ and P25. Saharan et al. [152] fabricated Sn doped MnO₂/CNTs via the co-precipitation technique. Fabricated NM was utilized to eliminate dyes via adsorption and for the electrochemical sensing of thiourea (Figure 8a). The results revealed that the fabricated NC showed 95% of adsorption within 30 min and recovered successfully. Sn doped MnO₂/CNTs exhibited exceptional activity in terms of sensitivity also. Graphene has taken on a new role in wastewater remediation due to its unique physicochemical and surface features. Because they are 2-D NMs with a single-atom graphite layer, graphene oxides and functionalized graphene have been highly sought after for their ability to cleanse available polluted stream flows, sequester heavy metal ions, rare-earth metals, and organic molecules. Graphene and its related materials are also being researched as possible photocatalysts because of their capacity to break down pollutants. For example, graphene possesses a better UV–visible light transmittance than other carbon compounds, superior electrical and thermal conductivities, and greater corrosion resistance than other carbon compounds. To reduce Cr(VI) under visible, UV, and NIR light irradiation, Wu et al. [153] produced a variety of hybrid photocatalysts with various decreased graphene weight ratios and tested their photocatalytic efficiency. The CuS/RGO demonstrates high optical absorption and catalytic performance under full sunlight irradiation because of its synergistic influence. One weight percent of RGO total immersion in the hybrid photocatalysts may reduce Cr(VI) by 90% under total UV-Vis-NIR light irradiation. CuS and RGO’s outstanding activity may be ascribed to robust absorption across the entire solar spectrum and effective charge segregation and transfer due to their similar energy levels and excellent interfacial contact. Photo-generated electrons have been found to influence the catalytic process. While CuS absorbs photon energy (Figure 8b), it is driven by photons to the conduction band (CB), resulting in electron-hole pairs (EHPs). Electrons and holes migrated to the photocatalysts surfaces to complete the redox reactions. The photocatalytic reduced Cr(VI) process may reduce Cr(VI) to Cr(III) because the Cr(VI)/Cr(III) potential (0.51 V vs. NHE) is more positive than the CuS CB (-0.38 V against. NHE) and work function of RGO (-0.08 V vs. NHE). Pure CuS has minimal photocatalytic activity because the photo-induced charge carriers are recombined in the bulk and surface of the material. As a result, RGO served as an effective electron acceptor to collect photo-induced electrons, considerably increasing separation efficiency and reducing charge carrier recombination, thereby boosting photocatalytic performance. New UV-Vis-NIR light photocatalyst technologies may be developed based on this study, which might be used to clean the environment and convert energy.

The use of graphene oxide to improve photocatalytic efficiency has gotten a lot of interest. Many contaminants have been effectively eliminated using graphene and its derivatives, including chromium, formaldehyde, bisphenol A, and tetracycline. Reduced graphene oxide/titanate nanotube hybrid NCs with different loading of reduced graphene were fabricated via a simple hydrothermal technique by Nguyen et al. [154]. Hybrid structures containing rGO have significant application potential because of the synergetic effect produced by the enhanced adsorption of the cationic dye methylene blue and the isolation of photogenerated electron–hole pairs. The photodegradation of MB by reduced graphene oxide/titanate nanotube (rGO/TNT) composites was significantly improved compared to other catalysts such as TiO₂ when exposed to UV and visible light. When evaluated with other artificial catalysts, the rGO/TNT composite containing 3 wt% rGO had the greatest decolorization and mineralization activity compared to the other catalysts; achieved about 100% decolorization and 77.4% mineralization after one hour of UV irradiation. However, 95% decolorization and 78.8% mineralization were achieved after three hours of visible light irradiation. P25 improved the removal of anionic dye MB compared to rGO/TNT composites. However, the 3% rGO/TNT composite exhibited the significant photocatalytic performance among the produced NMs studied. Based on these results, it is hypothesized that hybrid rGO/TNT composites may be employed as adsorbents and photocatalysts to efficiently and completely remove hazardous dyes from water prior to recycling. Other nano-carbon molecules being investigated in this sector include carbon quantum dots, fullerenes, and similar chemicals. Zhang et al. [155] used a hydrothermal process to fabricate Fe₂O₃/CQDs NCs that were effective semiconductor photocatalysts for the photo-degradation of toxic fumes such as methanol or benzene under visible-light conditions. The pathway for Fe₂O₃/CQDs NC with cubic morphological features of magnetic architecture and their photochemical use is depicted in Figure 9a–c. Because of the unique properties of CQDs, magnetic nano-hybrids can combine magnetic properties with fluorescence on one substrate. Because of the high electron storage capacity of CQD (shown in Figure 9d), electrons stimulated by photons emitted from Fe₂O₃ NPs may move freely across the CQD conducting system. An increase in the number of active oxygen radicals capable of removing dangerous gases may be achieved by the interaction of the adsorbent reductants/oxidants with the electron-hole pairs.

4.2. Carbon Nanomaterials for Hydrogen Evolution Reaction

As a dopant in carbon, hetero-atoms (e.g., B, P, N, S, or others) can change carbon’s physical and chemical characteristics to acquire more reactive sites. More crucially, this technique can synthesize carbon-based NMs that have a higher capability to adsorb the atomic/molecular species involved in catalytic processes while maintaining their conductive characteristics [156]. These heteroatom-doped structures might open the opportunities for the sustainable development of cost-effective metal-free catalysts with high activity and long spans. The difference in electronegativity and size of hetero-atoms and carbon has been reported to polarize nearby carbon atoms to assist the oxygen reduction reaction, which might be used in the hydrogen evolution [157]. The first design and synthesis of a 3D N-graphene-C₃N₄ hybrid film with a microstructure equivalent to van der Waals heterostructures were reported by Duan et al. [158]. A significant increase in catalytic activity was noticed in the novel material, making it a strong candidate for the next generations of HER catalysts (Figure 10a). It has also been discovered that superior structural characteristics of electrocatalysts, such as the richness of bare active sites, symbiotic pairing among porous carbon nitride (PCN) nano-layers and graphene sheets, hierarchical porous structure, and N-doping alteration graphene functionality, are associated with increased performance. Because of the packing of C₃N₄ within graphene sheets, the resulting catalyst displayed outstanding endurance over a long period when subjected to acidic conditions during cycling tests. Furthermore, this material was created using a straightforward approach that used low-cost precursors. As a result, it may be utilized to create a range of graphene-based 3D catalysts for an array of applicants, spanning hetero-catalysis, solar cells, and fuel cells, among others. Zhou et al. [157] reported that N-doped carbon-coated Co nanorod supported on a Ti, i.e., Co@NC/Ti, was fabricated by a two-step approach. Electrochemical experiments demonstrated that the electrocatalysts had a high HER activity, with an onset potential of −56 mV (vs. RHE), and a substantial current density (10 mA cm⁻² at −106 mV), a modest Tafel slope of 78.2 mV dec⁻¹, and good electrochemical durability. The findings might lead to a new technique for designing and constructing efficient HER catalysts using earth-abundant and low-cost components. Liu et al. [159] described that the fabrication of carbon nano-dots extracted from rapeseed flower bee pollens was utilized to combine with ultrathin gC₃N₄ nano-sheets in a hydrothermal technique to form 2-D/0-D type photocatalysts. CQDs efficiently enlarged visible light absorption zones by photosensitization and suppressed photo-induced carrier recombination. According to electron paramagnetic resonance technology, fluorescence spectroscopy, and photo-electrochemical studies, CQDs act as electron transfer mediation to enhance charge separation and lengthen the lifespan of photo-induced carriers. The best H₂ evolution of the UCN/CQDs-0.2% composite is 88.1 mol/h; the UCN/CQDs composites had a significant photocatalytic efficacy for H₂ release when exposed to visible light. Figure 10b represents a plausible mechanism for improved photocatalytic performance in UCN/CQDs composites. To begin with, CQDs acted as an electron acceptor, allowing photo-induced electrons generated from UCN under visible light to easily shuttle to the conducting band of CQDs, thereby delaying recombination and extending the lifespan of electron-hole pairs. Second, like organic dyes, π- conjugations CQDs were utilized as a photosensitizer to sensitize UCN and donate electrons to its conduction band, resulting in a longer visible light response area for photocatalytic H₂ release. Finally, because of the up-conversion photoluminescence capabilities of CQDs, they may absorb longer wavelength visible light and subsequently emit shorter-wavelength light, causing UCN to create additional charge carriers. However, because the yield of photocatalytic hydrogen generation matches the PL results, the photogenerated charge-separation efficacy is the most important element in the photocatalytic performance of UCN/CQDs composites. Therefore, the limitation of quick charge carrier recombination in CN photocatalysts has been effectively controlled by creating 2-D/0-D heterostructures, resulting in improved photocatalytic effectiveness. Zhang et al. [160] reported an in-situ hydrothermal technique to accomplish a facile and green one-pot fabrication of titanate nanotube/graphene NCs. This ecologically friendly approach involves the direct development of well-defined TNT (titanate nanotube) uniformly dispersed on GR (graphene) substrates. Their photocatalytic H₂ evolution capacity was greatly enhanced by adding GR to TNTs. An array of energy and environmental issues might be addressed by these TNT/GR NCs, which have unique 2-D morphological features and a one-pot green hydrothermal method. To better understand the photocatalytic mechanism of the hydrogen evolution reaction from CH₃OH aqueous solution, Figure 10c depicts a possible reaction mechanism. After being irradiated by a Xe-lamp, electrons in the VB of TNT were excited to the CB of TNT, which left only holes in the VB, and the high energy electrons in the CB of TNT moved effectively through the interfacial contact between TNT and the GR sheet in order to reduce H or directly reduce water/proton to form H₂. Meanwhile, methanol, the sacrificial agent, scavenged the photo-induced holes in TNT’s VB, limiting the recombination of charge carriers. It is important to note that when the loading percentage of GR surpasses 1%, black GR sheets restrict Xe-lamp illumination through the depths of the reaction solution and impede electron production from the interior of TNT/GR catalysts. A significant number of GR sheets also prohibited the expendable agents from coming into touch with the TNTs, which resulted in a decreased rate of H₂ formation. Furthermore, the H₂ evolution rate enhancement attained by combining TNT and GR is substantially greater than that investigated by Zhang et al., who fabricated TNP/GR NCs employing a one-pot hydrothermal approach with sacrificial agents Na₂S and Na₂SO₃. This is likely because, due to its 2-D p-conjugation structure, good electron conductivity, and the large contact area between TiO₂ and GR, GR acts as an electron-transfer channel and acceptor, extending the lifespan of charge carriers.

4.3. Carbon Nanomaterials for Gas Sensing

Technologies for identifying volatile organic compounds (VOCs) accurately and efficiently in fields, such as environmental gas monitoring, non-invasive medical diagnostics, and point-of-care activities, seem to be a promising direction in the development of future technologies. For one reason, VOCs are produced by diverse human activities, such as fuel burning, car exhaust emissions, and interior decor, resulting in the development of fine particulate matter and ozone (O₃), causing major environmental issues including pollution and chemical smog [161]. On the other hand, the majorities of VOCs have a nasty odour and are poisonous, neurotoxic, mutagenic, and carcinogenic. Moreover, even at low concentrations, long-term exposure to VOC emissions can cause vision problems, respiratory disease, and even death [162]. Therefore, tracking VOC gas data in the atmosphere plays an essential role in ensuring the safety of persons engaged in certain activities.

Furthermore, low-dimensional carbon nanostructures like CNT, graphene, and MXene have an enormous specific surface area, making them excellent sensors for volatile organic compounds (VOCs). Therefore, nano-carbon materials, such as graphene, MXene, and CNTs, are becoming more prominent in VOC gas sensor technologies [163,164,165]. Wang et al. [166] effectively fabricated mesoporous ultrathin SnO₂ nanosheets in situ modulated by graphene oxide (GO), employing graphene oxide as both a sensitizer and a template. The fabricated NMs has a structure of mesoporous 2-D nano-sheets made up of tiny nanocrystals, as per the morphological characterizations. Changing the heat treatment temperature helped alter nanocrystal grain sizes, specific surface area, and GO content in 2-D GO/SnO₂ NS. The link between heat treatment temperatures and sensing characteristics was also thoroughly investigated. The best sensing efficiency was mesoporous GO/SnO₂ NS-475, which had a response value (Ra/Rg) of 2275 toward 100 ppm HCHO at 60 °C.

Moreover, the HCHO preference of 2-D GO/SnO₂ NS was very high. To the unique sensitive properties, it is possible to ascribe structural regulation and composite sensitivity in the same system, including an adequate GO content, an ultrathin nanostructure with a unique extremely thin nanostructure, nanosized SnO₂ particles, and multiple mesopores. For the investigation of the detecting capacities of GO/SnO₂ NS-475, the optimal operating temperature was determined by testing the detection capability at temperatures ranging from 35 °C to 90 °C. Figure 11a shows the sensor’s reaction concerning the operating temperature. The response value was observed to rise from 40 to 60 °C for the detection of 100 ppm HCHO and then decrease from 60 to 90 °C, with a maximum value of 2275 at 60 °C. Figure 11b shows real-time variations depending on the resistance/response of the GO/SnO₂ NS-475 sensor to 100 ppm HCHO gas at 60 °C. The reaction and recovery times for GO/SnO₂ NS-475 were 81.3 and 33.7 s, correspondingly. Ultrathin nanosheet architectures decreased the transport channel for 2-D GO/SnO₂ NS, improving the gas sensor’s response/recovery. It can be observed in Figure 11c that the selectivity of HCHO for the identical amounts of alcohol, acetone, toluene, NH₃, and H₂O was 32, 105, 373 (1750), and 989 for each of these compounds. As per the comparison, these sensor nodes can tolerate exposure to liquids, some VOCs, and humidity, during regular use. HCHO solution was used to make gas mixtures containing interfering compounds such as ammonia, acetone, ethanol, toluene, and water. The objective was to detect HCHO in the atmosphere using the GO/SnO₂ NS-475 sensor selectively. Figure 11d demonstrates that GO/SnO₂ NS-475 has strong anti-interference effectiveness in a complex environment. The response to interfering vapors was substantially lower than that of pure HCHO. We also explored how well the GO/SnO₂ NS-475 held up through repeated tests in 10 ppm HCHO vapor and fresh air. The sensor’s reproducibility and durability were tested under these conditions. It is shown in Figure 11e that the sensitivity remained constant in intensity over all five cycles and that the curve was capable of completely returning to its pristine level, demonstrating repeatability and high stability. Figure 11f shows that after a month of evaluation, the sensor response value declined from 2270 to 1975, as shown by the arrow. Notably, the output reduction occurred primarily in the project’s first week. In order to determine the thermostability of the sensor, Raman spectra were acquired before and after operation at 60 °C for a week.

HCHO gas may be detected using SnO₂@rGO NCs made by Rong et al. [167]. As a result of the inclusion of the rGO, these SnO₂@rGO NCs had an increased sensing response of 40 to 100 ppm for HCHO with an improved reaction time of less than one second (even less than 10 s). Additionally, the development of rGO/SnO₂ heterostructures was credited with enhancing sensing performance. As a consequence of the electron and hole migration caused by the different work functions of SnO₂ and rGO, a heterojunction of rGO@SnO₂ is formed, as illustrated in Figure 12. The presence of the rGO@SnO₂ heterojunction regulated the electronic transport characteristics of SnO₂. The thickness of the electron depletion layer on the surface of the SnO₂ material was increased due to a reduction in the number of electrons in the material. Consequently, the resistance variability and sensing sensitivity were enhanced when HCHO was applied to the sensor. An explanation of the composite’s gas sensing process was also provided based on a working assumption based on the heterojunction.

MXene, like graphene structures, belongs to the 2-D materials category, which includes the newly found members of the 2-D layered materials consisting of transition metal carbides/nitrides [168]. Ti₃C₂Tx, transition metal carbide, was the first MXene material identified by etching away the Al atom from the Ti₃AlC₂ MAX phase, and it was also one of the most studied MXene materials. Such a methodology endows the Ti₃C₂Tx MXene with several functional groups, such as fluorine, oxygen, and hydroxyl groups, which improves Ti₃C₂TxMXene’s adsorption characteristics toward target VOCs gases and allows it to be readily tuned as a gas sensor for VOCs detection [169,170,171]. The ability of the Ti₃C₂Tx-based VOCs sensor to respond to VOC analyses at room temperature has been reported. For instance, in situ XRD measurements were used by Koh et al. [172] to evaluate the interlayer spacing change of Ti₃C₂Tx MXene upon gas introduction. To investigate the relationship between film swelling behaviour and gas sensor response, N₂ and target gases/vapours employed in gas sensing tests were successively introduced to Ti₃C₂Tx MXene films inside an XRD chamber. During the measurements, Ti₃C₂Tx MXene sheets expanded preferentially when ethanol vapour was added but not when CO₂ was injected. According to the findings, the concentration of intercalated sodium ions is crucial in regulating the degree of swelling when ethanol vapour is introduced.

Furthermore, the degree of swelling corresponded to the strength of the gas reaction, with Ti₃C₂Tx MXene films treated with 0.3 mM NaOH exhibiting the greatest degree of swelling and the greatest sensitivity to ethanol. As a result, it is critical to think about pre-intercalation to improve gas sensing performance in the future. This research contributes to our understanding of MXenes’ gas sensing mechanism and sheds light on how gas or vapour molecules interact with MXenes, which is crucial for sorbents, separation membranes, and other applications. Stanciu et al. [173] recently fabricated the Ti₃C₂Tx/WSe₂ hybrid using a simple surface-treating and exfoliation-based method, as illustrated in Figure 13a. Furthermore, the Ti₃C₂Tx/WSe₂ hybrid was coated on the polyimide (PI) substrate with Au interdigital electrodes using an inkjet printing method and acted as the sensing materials. Finally, by implementing a wireless monitoring system, as represented in Figure 13b, a wirelessly operating VOCs gas sensor was developed. The results showed that the Ti₃C₂Tx/WSe₂ hybrid sensor has a low LOD, reduced noise level, high accuracy, and rapid response/recovery times, paving the way for the formation of high-efficiency VOCs sensors for the next era of IoT. Another study [174] demonstrated sub-ppm VOC gas detection by simply increasing the sulfonation degree of poly(ether ether ketone)(PEEK) to 85% and mixing two types of carbon nano-fillers, CNT and fullerenes (as depicted in Figure 13c. Furthermore, the sulfonation degree of PEEK was discovered for the first time to be favorable to methanol gas selectivity, which was a promising strategy in the development of high selectivity CH₃OH sensing devices and systems.

4.4. Other Environmental Applications

As a result of the introduction of nanotechnology, a whole new universe of opportunities for developing a wide range of NMs with unique capabilities for the treatment of pollutants has widened. Nanostructures are used extensively in environmental remediation activities, such as the treatment of water, air, commercial and residential wastewater, soil, and sediments. There have been diverse efficient, ecologically benign, and economically viable nano-sized materials developed via nanotechnology, each with its unique features and the ability to detoxify commercial sewage and aquatic bodies [175]. Nano-sized materials are classified as nano adsorbents, nano-catalysts, and nano-membranes based on their properties. It has been demonstrated that nano-adsorbent materials may effectively remove contaminants from wastewater and the air. Biochar, carbon nanotubes, carbon nanofibers, graphene, metals and their oxides, silica, and clay have all been applied as adsorbent materials, as have CNTs, carbon nanotube fibres, and graphene [176,177,178]. Researchers have been paying close attention to NMs, such as metal oxides and semiconductors, in developing novel water treatment solutions. To break down of contaminants in wastewater, many types of nano-catalysts are utilized, and some catalysts also have antibacterial characteristics. As a result, they can also be used to remove biological pollutants [179]. In several therapeutic procedures, nano-membranes are employed. Pressure-driven wastewater treatment is an excellent method for removing pollutants from water. As a result of their small pore size, affordability, high efficiency, and environmental-friendliness, nano-membranes are commonly used to treat wastewater. When it comes to creating nano-membranes, metal NPs, non-metallic particles, and carbon-based NMs are all extensively employed [180]. It is also possible to enhance the surface characteristics of NMs by grafting functional groups onto their surfaces. This may be accomplished using a plasma technique, chemical modification, or a microwave treatment [181]. The plasma process is the most energy-efficient and ecologically friendly of the options since it requires less energy and is less harmful to the environment. Carbon-based NMs, which have been treated with metals or metal oxides, may also be capable of depleting toxic heavy metal ions [182]. According to experts, carbon-based materials will play an essential role in developing advanced energy storage systems in the near future. In addition to their extremely large surface area, mechanical capabilities, pore size distribution, and electrical characteristics, these materials are used in diverse research projects and are considered some of the most promising materials currently available. Materials, including carbon nanotubes, fullerenes, graphene, and carbon dots, are used in energy storage. Solar cells, commonly known as photovoltaic cells, were conceived by Regan and Graetzel in 1991 as a low-cost means of harvesting energy from the sun. They are now used worldwide. Photons of a given wavelength can be easily converted to energy by solar cells [183]. Thin films and crystalline silicon solar cells are the two available crystal silicon solar cells. Semiconductors such as crystalline silicon, III–V groups, cadmium telluride, and copper indium selenide/sulphide are the most popular materials utilized to produce first- and second-generation solar cells. Those economically practical devices with an easy synthesis and manufacturing procedure are included in the thin-film category [184]. Several semiconductor electrodes with a wide band gap are made possible by applying dye molecules. Among the materials used in dye-sensitized solar cells (DSSCs), platinum is the most frequent. DSSC counter electrodes are made of platinum. However, scientists are exploring alternatives due to their excessive cost and scarcity. DSSCs made of carbon-based materials have distinct characteristics, such as high conductivity, high electrochemical performance, and cheap cost, making them interesting alternatives to expensive platinum in DSSCs. All carbon-based materials, including graphene-based materials, have been applied as a medium to expedite electron transmission and maximize the dependability of photovoltaics [185,186,187,188]. The facilitating function (electron transport) of graphene-based materials is also utilized in other fields, such as batteries and fuel cells, to enhance their performance. Electrochemical energy storage devices, also known as supercapacitors, are promising next-generation energy storage devices for diverse uses, including hybrid vehicles, plug-in hybrids, portable electronic devices, and backup power cells [189,190,191,192,193]. This is due to multifarious features, including their high power density, excellent high stability, and very short charging time. However, even though carbon-based supercapacitors have several merits over metal-based supercapacitors, such as high energy density and cyclability, their low energy density prevents them from being used in batteries. When it comes to supercapacitors, 2-D carbon-based materials outperform 0-D and 1-D nanostructures in terms of performance. Graphene is one of the most well-known 2-D materials structures, and it is also one of the most studied. A higher level of interest in graphene than in other carbon compounds has been generated due to its high electrical conductivity, huge surface area, and big surface-to-volume ratio for applications in supercapacitors [194]. However, when utilized as electrode materials, graphene sheets have significant limitations due to their penchant for restacking, making them unsuitable for many applications. Due to the exposed surface of CNTs, which has the potential to be functionalized, and their high mechanical strength and electrical features, they are excellent candidates for energy storage [195]. Lithium-ion batteries are another kind of energy storage material, which stores energy in the form of chemical energy and maybe recharged several times. Li-ion batteries beat capacitors in terms of energy and power density, and they have the potential to mitigate greenhouse gas emissions by a significant amount. Because of their exceptional energy efficiency, they are applied in a wide range of electrical applications. Lithium-ion batteries have lately received great interest for their potential use in both research and industry [196]. Carbon materials are often considered to be the most suitable for Li-ion batteries. They are capable of being lithiated to some degree. In addition to determining the amount of lithium that is reversibly integrated into the carbon lattice, the faradic losses experienced during the initial charge–discharge cycle and the voltage profile experienced during charging and discharging are all determined by the structure of the carbon material. Carbon-based Li-ion batteries are predicted to surpass other metal-based technologies in energy density per unit area [197,198]. This could be because they have much more energy per unit area than other metal-based technologies.

In addition to having features such as low expense, compact size, flexible porosity, easy process ability, chemical modification simplicity, carbon-based NMs such as activated carbons, 1-D carbon nanotubes, and 2-D graphene nanosheets are promising candidates for technological applications in a variety of sustainable energy storage devices. In general, increasing the specific surface area of the carbon structure and the size distribution of the pores allows for increased electrochemical capacitance efficiency in terms of both power delivery rate and energy storage capacity, as seen in the figure below.

Table 1. Various carbon NMs employed for diverse environmental applications.

Analyte	Nanoparticles Extracted	Method	Application	References
Camphor Oil	Multiwall Carbon nanotube	CVD precursor	Photocatalytic	[199]
Chicken Eggs	Carbon Dots	Plasma Induced	Silk-screen printing, Inkjet	[141]
Neem Oil	Multiwall Carbon nanotube	Pyrolysis	Graphitization, Catalytic	[200]
Hibiscus tiliaceus	MWCNTs	Raman spectroscopy	Gas sensing or water remediation	[201]
Terminalia chebula	Graphene nanosheets	UV–Vis analysis	Nanoelectronics, Composites, fuel cells, supercapacitor, sensors and catalysis	[202]
Eucalyptus	rGO	Adsorption	Bioreduction	[99]
Rice Hull	Carbon Dots	Hydrothermal	Alcohol vapor detection	[203]
Chickpea	Fluorescent CNTs	Pyrolysis	Bioimaging	[204]
Larch	Carbon nanodots	Hydrothermal	Photocatalysis	[205]
Au@CQD	CQDs	-	Photocatalyst, H2 Production	[206]
CNNS@CQDs	Carbon Nanosheets and Quantum dots	-	Photocatalyst, H2 Production	[207]
Wheat husk	Carbon dots	Hydrothermal	Cellular Imaging, F- ion detection	[208]
Pineapple rind	CQD	Hydrothermal	determination of Hg2+, electronic security devices	[209]
Durian skin	Carbon dots	Pyrolysis	Supercapacitor Electrode	[210]
Apple rind	Carbon dots	charring	photodegradation	[211]
Tea leaf waste	CQD	Oxidative pyrolysis	Gefitinib sensor	[109]
CdS/BCN	Carbon NPs	-	CO2 Reduction, Photocatalyst	[212]
SiNW@CQDs	Carbon Quantum Nanodots	-	Optoelectronic Devices	[213]
Bi-NRs@NCNTs	Caarbon Nanotubes	-	CO2 Reduction, electrocatalyst	[214]
RuTiO2	MWCNTs	-	CPE (Chlorinated Polyethylene) Liquid Level Sensors	[215]
SWCNTs@Gold NPs	Carbon Nanotubes	-	Gas sensing	[216]
MO2C@CNT	Carbon Nanotubes	-	Hydrogen Generation	[217]
C@NiO	Carbon Dots	Calcination	Supercapacitor	[218]
Mn3O4@CNP	Carbon NPs	-	Supercapacitor	[219]
B@CNPs	Carbon NPs	Hydrothermal	Bioimaging	[220]

depicts a variety of published publications illustrating diverse carbon NMs created from green synthesis (plant extracts) that have been used for diverse environmental uses, as shown in the articles.

5. Conclusions and Future Prospectus

There is an urgent need for organic compounds and heavy hydrocarbons to address the growing demand for carbon-based nanostructures in emerging areas, including sensors, biomedicine, supercapacitors, and gas storage. In order to safeguard the environment and human health, wastewater treatment, especially industrial wastewater, is critical and required. Environmental pollution prevention has made considerable strides in recent years, as seen by this summary. Some sophisticated technologies that can be utilized for wastewater treatment include greener nanotechnology and sustainable NPs that can be used to remediate dangerous pollutants. Carbon-based NPs may have an advantage over other types of NPs in terms of infrastructure and resource use, as well as concerns regarding the dangerous qualities of NPs. Using CNTs, CQDs, GQDs, and graphene-based NPs to clean up the environment and save resources may be better than traditional treatment NMs. Despite the significant advancements in synthesis and the catalytic capabilities shown by green NPs, such as CNTs, CQDs, GQDs, and graphene-based NPs, future research should pay greater attention to the following issues:

• Innovative and revolutionary synthetic approaches are required to minimize the cost of NM synthesis and enhancing their catalytic performance.
• Bio-waste-derived NPs have been used to remediate wastewater.
• Magnetic NPs for water purification have been improved and reused.
• Deception of animal residues, such as bone, bristles, and eggshells, for NM fabrication.
• Natural minerals, such as bentonite, clays, zeolites, and montmorillonite, are being used as non-noxious, low-cost, and plentiful substrates for the biosynthesis of NPs and their environmental applications.
• For all of the aforementioned ideas, toxicity studies should be completed. Full analyses of NMs hazards and possible effects on human health and ecosystems, including potential cellular toxicities, should be investigated and evaluated.