1 Introduction

The presence of water is indispensable for life, serving as a vital component for the sustenance of living organisms [1, 2]. Across time, water has always been acknowledged as a fundamental element in the sustenance of life. It undergoes several cycles, including evaporation, condensation, and transpiration. Presently, a pressing national concern revolves around ensuring access to clean water for drinking purposes. Statistics indicate that 2.5 million people require water for sanitation, while approximately one billion people lack access to clean drinking water [3]. Recognized as a natural gift, water holds significant importance for both agricultural and industrial activities. The scarcity of water adversely affects one in every six individuals on Earth [4]. Elevated levels of water pollution not only impact human health but also disrupt marine life [5]. An estimated 14,000 people lose their lives daily due to wastewater, highlighting that contaminated water not only leads to severe health issues but also has fatal consequences [6]. The contamination of freshwater sources is predominantly attributed to industrialization and urbanization [7]. With its versatile applications in washing, swimming, drinking, and cooking, water stands out as a pivotal element in our daily lives. Safeguarding all water sources is imperative as we find ourselves surrounded by two-thirds of water. Despite being the most extensively studied substance on Earth, water's intricacies are surprisingly misunderstood by the public and scientists working with it daily. In the current era of scientific advancements, nanotechnology is vital in advancing technologies and making significant strides, particularly in photocatalysis [8,9,10]. Metal and non-metal nanoparticles have various applications, such as energy conservation, water purification, and air purification [11]. Their versatility extends to pharmacological, biosensor, and photocatalytic activities through modifications in composition and structure. The high surface-to-volume ratio of these nanoparticles presents additional prospects for rapid and efficient chemical reactions. Nanotechnology is a highly researched field in materials science that utilizes nanoparticles as catalysts for breaking down dyes. This is due to their unique physical and chemical properties, which differ from those of bulk materials [12]. Nanomaterials can significantly alter any product’s design, manufacturing, and characteristics. When a material undergoes scaling down from macro to nano size, it profoundly influences its mechanical, optical, magnetic, electric, and antibacterial properties. The changes that occur in a substance are mainly due to the ratio of its surface area to volume and its reactivity on the surface. These factors are crucial in determining the substance's physical and chemical properties. A nanomaterial is generally defined as a substance with at least one dimension falling within the range of one to one hundred nanometers. The fields of electrical and computer technologies have greatly benefited from the capacity of these materials to create products that are robust, efficient, and user-friendly [13]. The positive impacts of nanoparticles are augmented in solar energy applications using diverse techniques like photocatalysis (A Comprehensive Review on Adsorption, Photocatalytic and Chemical Degradation of Dyes and Nitro-Compounds over Different Kinds of Porous and Composite Materials). As human society continually evolves, the demand for energy is consistently rising. Finite fossil fuel supplies are inadequate to meet the increasing global energy needs as humanity progresses and advances [14]. The widespread utilization of fossil fuels has given rise to numerous substantial environmental problems, negatively affecting human society's health and sustainable development. To address this, it is crucial to seek clean, renewable energy sources as alternatives to fossil fuels. In recent years, solar energy has emerged as a prominent candidate among sustainable and clean energy sources [15]. One of the ways to tackle energy and environmental issues is by transforming solar energy into chemical fuel, which presents a viable, cost-effective, and environmentally friendly solution [16]. Photocatalytic technology stands out among advanced conversion technologies, promising to generate clean, sustainable, and renewable chemical fuels. Despite facing significant challenges in the widespread use of highly active and durable photocatalysts, scientists are diligently engaged in exploring and producing exceptionally efficient photocatalytic materials [13]. There is optimism that these obstacles will be overcome shortly, leading to the development of highly efficient and feasible materials for industrial-scale implementation. This review article centers on examining hazardous dyes and CO2 gases contributing to water pollution. While various materials have been explored as photocatalysts for mitigating the mentioned pollutants, the specific emphasis in this review is on photocatalysts related to MXenes to reduce dyes and gases. MXenes, the first type of photo-catalysts developed from transition metal carbides and nitrides, known for their two dimensional (2D) nature, have experienced a proliferation in number since their initial discovery in 2011 [17]. MXenes are nanomaterials belonging to the 2D metal carbide/nitride/carbonitride category. These materials have a general formula of Mn+1XnTx, where M stands for a transition metal, X denotes carbon and nitrogen, and Tx represents functional groups on the surface. MXenes possess adjustable work functions, high specific surface areas, metallic electrical conductivity, excellent carrier mobility, and easy solution processability, rendering them well-suited for photovoltaic applications [18]. The discovery of MXenes, a group of thin two-dimensional materials made of transition metal carbides and nitrides, has sparked significant interest for scientists and researchers in exploring their synthesis, characteristics, and potential uses. MXenes have demonstrated exceptional performance across diverse domains, including energy storage, electrocatalysis, optoelectronics, healthcare, and environmental remediation [19].

Presently, over 40 distinct formulations of MXenes are available [20]. It is worth noting that the current count of these materials is not accurate, and it is anticipated that they will become the most prominent family of 2D materials ever found in the future. This is because they possess some unique features, such as high electrical conductivity up to 20,000 S cm−1, hydrophilicity, adjustability of size, large surface area, flexibility, layered structure, and complex surface chemistry [21]. MXenes proves highly valuable in diverse applications such as energy storage, optoelectronics, biomedicine, communications, and environmental applications. The size quantization effect, surface effect, and quantum confinement effect are generally prominent in 0D structured g-C3N4. Consequently, carriers created by photons can easily diffuse from the particle's interior to its surface. It provides an electron transfer channel with a direct current path for 1D structured g-C3N4 (primarily nanotubes and nanofiber). A large specific surface area with more active sites and well-developed porous channels for quick electron transfer are features of 3D structured g-C3N4 that are advantageous to photocatalytic performance because of their structural interconnectivity. Regarding 2D g-C3N4, its exceptional in-plane conductivity is attributed to the covalent bond located in the plane [22, 23]. Overall, the novelty of the present review article is comprehensive study with updated literature of MXenes compounds for environmental protection. Here MXenes stability was the main insight of this review article which will be useful for the new researchers that developed MXenes for various application. This review article will be a useful contribution for developing 2D materials.

2 Hazardous materials

An essential challenge to both health and a sustainable future arises from compromised water quality due to hazardous contamination. A considerable segment of the global population is reported to face a shortage of access to potable water, marking it as one of the most significant issues of this century. Despite the Earth's abundant water resources, comprising up to 71% of its surface, only 1% meets international standards for human consumption. Reports attribute water contamination primarily to industrial discharges, municipal discharges, and agricultural activities [24,25,26]. A multitude of detrimental substances, encompassing dyes, pesticides, heavy metals such as cadmium, lead, and mercury, as well as various organic compounds (polycyclic aromatic hydrocarbons, polychlorinated biphenyl compounds, etc.) and inorganic pollutants, contribute to water contamination. A few gases like NO2 and CO2 are also implicated in this issue. Short-term exposure to dyes and gases can result in skin irritation characterized by itching, redness, and burning symptoms. Inhaling fumes, dust, and gases from dyes can lead to respiratory and allergic problems [27]. Prolonged exposure to these gases and dyes can impact human reproductive hormones, and they are recognized as carcinogens, representing a significant cause of cancer. Moreover, long-term exposure can induce neurological problems, including headaches, memory loss, and dizziness [28]. Hazardous materials such as cadmium, lead, and mercury are commonly encountered and responsible for water pollution. These heavy metals infiltrate water through mining and industrial processes. Once introduced into the water, these metals are absorbed by the human body and accumulate, leading to various health issues, including kidney damage, neurological disorders, and growth-related problems, particularly in children [29]. Exposure to organic pollutants, such as herbicides, pesticides, and industrial chemicals, poses significant health risks, giving rise to conditions like hormonal disorders and various types of cancer. Organic compounds like polychlorinated biphenyls, which are liquid, function as carcinogens and contribute to reproductive and developmental issues. For instance, they can lead to menstrual irregularities in females and diminished sperm quality in males [30]. Photocatalysis is an effective process to degrade solid hazardous compounds, such as polycyclic aromatic hydrocarbons, produced during the incomplete combustion of fossil fuels. Phenolic compounds, known for causing allergic reactions with symptoms ranging from mild skin itching to severe breathing difficulties, are also addressed [31]. Certain phenolic compounds contribute to endocrine disruption and neurodevelopment disorders. In the same way, general antibiotics work in concert with host defences to weaken and reduce the virulence of microbial pathogens, making them more susceptible to phagocytosis and eventual eradication by the host's intrinsic antimicrobial systems. Nevertheless, antibiotics also affect host defences through a variety of additional mechanisms, some of which are advantageous and some of which are detrimental. Exacerbation of potentially harmful inflammatory responses is one of the harmful activities of agents that target the cell wall. This property encourages the release of pro-inflammatory microbial cytotoxins and components of the cell wall [32]. However, the specific emphasis in this review article is on the degradation of dyes and the reduction of CO2 through the photocatalytic route along with its stability for long term use. This approach aims to transform these substances into less hazardous compounds, thereby fostering a friendly and safe environment for living organisms [33].

3 Responsible dyes for water pollution

Incomplete adhesion during the colouring process leads to dyes becoming part of wastewater. Azo dyes, commonly used in colouring, are inherently highly carcinogenic [34]. Industries such as food, paper, and textiles extensively employ dyes for various purposes. Despite their enhanced stability and solubility, dyes pose threats to aquatic species and exhibit high persistence. Both the food and textile industries utilize a diverse range of colors to enhance the texture and aesthetics of their products [35]. High solubility and low price, synthetic colors are a popular choice in the food sector. In the food sector, synthetic colors, known for their high solubility and cost-effectiveness, are trendy. With over 10,000 colors used across various applications in the industrial sector, synthetic colors are reported to be produced in substantial quantities, reaching an annual total of 30,000 tons [36]. Numerous synthetic dyes possess carcinogenic properties, posing a threat to human health, while their ability to reduce light permeability into water bodies negatively impacts algae and other aquatic life [37]. In human biology, dyes can adversely affect enzyme levels such as gamma-glutamyl transferase and alkaline phosphatase, impacting the liver and the central nervous system. Allergic reactions, including itching, irritation, and redness, are common responses to certain dyes, with some containing airborne particles that, when inhaled, can lead to respiratory problems such as asthma. Contact dermatitis, characterized by inflamed, itchy, and red skin, is another skin issue associated with dyes. Prolonged exposure to high concentrations of synthetic dyes increases the risk of cancer, and dye molecules are known to cause endocrine disruption and systemic toxicity.

In both industries and agriculture, approximately one-third of freshwater is utilized, and this water often becomes contaminated with dyes, pesticides, and fertilizers. Among these pollutants, dyes are particularly concerning due to their hazardous effects, as even in minute quantities, they can significantly impact the appearance and quality of water [38, 39]. Dyes are categorized as either natural or synthetic. Plants and animals primarily produce natural dyes, whereas synthetic dyes are predominantly manufactured through human activities in industrial settings. These dyes are often called synthetic organic molecules, featuring a chromophore responsible for electron acceptance and auxochromes facilitating electron donation [40]. The chromophore plays a crucial role in imparting color to the dye [41].

Additionally, dyes are classified based on their solubility, falling into soluble and non-soluble dyes. Soluble dyes encompass crystal violet, green S, indigo carmine, Sudan red G, solvent violet 13, and Allura red AC, while non-soluble dyes include azo, sulfur, disperse, vat, and solvent dyes [42]. Other categories of soluble dyes include reactive dyes, acid dyes, and metal complex dyes. Degradation of insoluble dyes poses a challenge due to their high stability [43]. The aromatic groups in dyes contribute to their resistance to removal, and even in minute quantities, they can lead to water pollution [44]. It is estimated that 5–10% of the total dyes generated annually (approximately 7 × 105) end up in wastewater.

Another classification of dyes involves categorizing them as anionic, cationic, and nonionic. Cationic dyes typically contain amide groups, while anionic dyes feature SO−3 groups. Dyes exhibiting dissociation properties are categorized as nonionic [45]. Azo dyes contain an additional azoic group, leading to adverse effects such as vomiting, genetic mutations, and allergic reactions. These dyes exhibit enhanced stability against light, heat, and aerobic digestion. Dyes, in general, have diverse negative impacts on aquatic life. Textile dyes diminish the aesthetic value of water bodies, prompting an increased demand for chemical and biological oxygen. They hinder photosynthesis, impede plant growth, enter the food chain, contribute to recalcitrance and bioaccumulation, and may elevate toxicity, mutagenicity, and carcinogenicity [46]. Refer to Table 1 for the classification of dyes based on functional groups.

Table 1 Classification of dyes based on different functional groups
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4 Responsible gases for water pollution

The intensive aquaculture industry's quick growth has led to environmental issues, such as the discharge of excess nutrients into nearby water sources and the release of greenhouse gas (GHG) emissions. Fish in intensive aquaculture use less than 50% of the added nutrients, with the rest being discharged into natural water sources as effluent. In recent times, there has been a surge in interest in comprehending and solving GHG emissions from aquaculture. In the literature its stated that in 2014, the top 21 aquaculture-producing nations produced 6.04 Tg of CH4 emissions. Aquaculture-related N2O emissions increased from 146.1 Gg in 2009 to 601.9 Gg in 2030, making up 5.72% of all anthropogenic N2O emissions [51]. Rice fields are vital in transferring harmful gases from the soil to the atmosphere. Although some greenhouse gases occur naturally, most result from human activities, contributing to the greenhouse effect leading to global warming. These gases trap heat in the atmosphere by absorbing infrared radiation. While the natural greenhouse effect is necessary for life to thrive on Earth, the excessive increase of GHGs exacerbates it, leading to higher temperatures. Another hazardous gas, H2S, possesses an offensive odor, is colorless, readily soluble in water, and functions as a weak reducing acid. It is a significant contributor to sulfide pollution in seawater and is present in aerobic and anaerobic water conditions. H2S becomes a part of the environment through pollution from rivers and municipal sewage [52]. H2S exposure can lead to respiratory ailments such as pneumonia or bronchitis, skin burns, and eye inflammation. Inhaling even small amounts of H2S can induce nausea and headaches, while higher concentrations may result in fatalities. In polluted harbor waters, the H2S level can reach 50 ppm.

However, among the gases above, CO2 poses the most significant threat, causing severe environmental pollution that complicates life for organisms on the Earth's crust. The concentration of atmospheric CO2 has increased by more than 40% since the beginning of the industrial revolution. It has risen from around 280 parts per million (ppm) in the 1800s to the current level of 400 ppm [53]. According to the Scripps Institution of Oceanography at the University of California, San Diego, the Earth's atmospheric CO2 levels reached 400 ppm during the Pliocene Epoch, which took place around 5 million to 3 million years ago [54]. CO2 significantly contributes to global warming, trapping heat and elevating Earth's temperature, resulting in rising sea levels, droughts, and severe storms. Exposure to elevated CO2 levels can lead to symptoms such as shortness of breath, headaches, and dizziness [55]. Indications of mild CO2 contact may comprise headache and drowsiness. CO2 at higher concentration causes fast breathing, increased cardiac output, confusion, elevated blood pressure, and increased arrhythmias. Breathing oxygen exhausted air triggered by extreme CO2 concentrations can lead to death by suffocation. The enormous energy depletion has not only triggered worldwide energy disaster, but also caused in disproportionate CO2 productions. Among the different methods to decrease the consequence of CO2 productions photocatalysis compromises an attractive approach to alleviate the problem of energy disaster and worldwide climate change via harnessing solar energy to unswervingly reduce CO2 into valuable chemical feed stocks or carbon-based fuels [56]. Photocatalysis is a valuable approach to converting CO2 into less harmful and beneficial products like methane, carbon monoxide, methanol, and formaldehyde. Various photocatalysts, including Titanium dioxide, Zinc oxide, Cerium oxide, Bismuth vanadate, Iron oxide, Nickel oxide, Cobalt oxide, and Tungsten trioxide, have been utilized for this purpose to mitigate CO2 levels [57]. In this review article, our primary focus will be on the degradation of dyes and the reduction of CO2 using MXene materials.

5 Different methods for wastewater treatment

Various conventional methods are employed to remove organic pollutants from wastewater, including coagulation, adsorption [58], physicochemical treatment [59], chemical precipitation [60], chemical degradation [61], ion exchange [62], biological treatment [63], and electrochemical treatment [64]. Water treatment technologies such as adsorption and coagulation are already accessible, and while they alter the phase of contaminants, they persist in the environment in a less favorable form [65]. Traditional water treatment techniques like sedimentation, filtration, and membrane technology are costly and can produce secondary contaminants [66]. Adsorption, a clean and physical process, is utilized to eliminate dyes from water to safeguard marine life. However, it still encounters challenges in terms of industrial implementation [67]. The adsorption process is limited as it relies on factors such as the reaction temperature, availability of binding sites, porosity, agitation speed, and dye size [48]. Another method involves the chemical degradation of pollutants by reducing agents like sodium borohydride, zinc amalgam, lithium, aluminum, and hydride, among others [68]. While this process facilitates rapid organic pollutant degradation, it has drawbacks, including environmental pollution due to reducing agents and the high cost associated with this method [69]. Among the methods mentioned above, photocatalysis is an environmentally friendly, cost-effective, and straightforward technique that occurs in the presence of sunlight and an appropriate photocatalyst [70]. Photocatalysis is applied to degrade various dyes and gases, such as CO2 [66]. Researchers have shifted their focus to photocatalysis as an intriguing oxidation process for eliminating various organic pollutants and hazardous gases. Through photocatalysis, these harmful compounds undergo mineralization, forming carbon dioxide, water, and mineral acids at ambient temperature. Photocatalysis stands out as a sophisticated oxidation process, primarily due to its capacity to generate hydroxyl (●OH) and superoxide (●O2−) free radicals along with holes (h+) [71]. This semiconductor-based method is both effective and cost-efficient [72]. Photocatalysis exhibits superior efficiency, enhancing the degradation of the mentioned pollutants through semiconductor materials [73]. Additionally, it utilizes selectively specific catalysts to degrade pollutants. Notably, photocatalysis operates under mild reaction conditions, such as ambient pressure and temperature, making it a preferred technique among researchers [74]. Various methods employed in wastewater treatment and their respective advantages and disadvantages are outlined in Table 2. However, this review will specifically concentrate on photocatalysis for the degradation of diverse dyes and the reduction of CO2.

Table 2 Different methods for wastewater treatment
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6 General mechanism of photocatalysis

Photocatalysis plays a pivotal role in the degradation of various pollutants, for which semiconductors are preferred due to their suitable band gap [75]. The band gap of semiconductors can be further enhanced through doping [76]. In the process of photocatalysis, the reaction rate increases in the presence of light, which is primarily responsible for initiating a chemical reaction. When a photocatalyst absorbs light, electrons transition from the valence band to the conduction band, creating an electron–hole pair When a metal oxide is exposed to UV or visible light, the OH- anions oxidize to generate ●OH free radicals, and O2 is reduced to form O2−, facilitating the conversion of toxic substances into less harmful forms such as CO2 or H2O [77]. Figure 1 illustrates the typical mechanism of photocatalysis. The photochemical reaction mechanism unfolds as follows: When incident light rays with energy surpassing the band gap of the prepared catalyst strike its surface, electrons are excited from the valence band to the conduction band [78,79,80]. This excitation leads to the formation of holes on the catalytic surface, generated by removing electrons from the valence band. These holes then initiate the oxidation of water adsorbed on the catalyst's surface, resulting in the formation of hydroxyl radicals (OH•) [81], as illustrated in Eq. (1). The complete mineralization of organic pollutants produces carbon dioxide, water, and inorganic acids as byproducts [82]. Simultaneously, an excited electron in the conduction band plays a role by reducing dissolved oxygen on the catalyst surface, as depicted in Eq. (2). This dual process contributes to the degradation of pollutants [83].

Fig. 1
figure 1

Typical mechanism of photocatalysis

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Oxidation Reaction:

$$OH^{ - } + h^{ + } \to OH^{ \bullet }$$
(1)

Reduction Reaction:

$$O_{2} + e^{ - } ?O_{2}^{(o - )}$$
(2)

Photogenerated electrons and holes recombine that results in release of heat and return to grounded state as shown in Eq. (3).

Recombination:

$$h^{ + } + e^{ - } ?Heat$$
(3)

During this process, electrons function as reducing agents, while holes are potent oxidizing agents. They can reduce and oxidize contaminants, transforming them into degraded products [84].

7 Different materials for photocatalysis

Various types of photocatalysts have been employed, including metal oxides (such as zinc, cerium, vanadium, titanium, chromium, cadmium, iron, strontium, bismuth, palladium, platinum, and molybdenum). These metal oxides can absorb light and generate electrons and holes, further responsible for oxidizing organic pollutants [85, 86]. Since TiO2 is abundant on Earth, has a strong redox capability, is inexpensive, nontoxic, and chemically stable, it is a highly studied semiconductor photocatalyst and a benchmark in the field of ultraviolet (UV) photocatalysis. The utilization of titanium dioxide (TiO2) in the energy and environmental domains has consequently gained prominence. These applications include photocatalytic evolution of hydrogen (H2) and oxygen (O2), reduction of carbon dioxide (CO2), decomposition of hazardous substances, nitrogen fixation, organic synthesis, and so forth [23].

Another category of useful photo-materials is metal sulfides, known for their quantum confinement effect, which is a crucial factor making them suitable as photocatalysts [87]. Various other photocatalysts such as graphene oxide (GO), reduced graphene oxide (rGO), mixed oxides, different composite materials, clay-based materials, carbon materials, layer double hydroxides (LDHs), bio-based materials, metal organic framework (MOFs), MXenes, and COFs have also been utilized [88]. Additionally, crystalline carbon nitride has a stronger interlayer force and a shorter π-π interlayer distance, which encourage the production and movement of charge carriers and make it more favourable for activating oxidants. Moreover, the light absorption capacity of crystalline carbon nitride is significantly stronger than that of PCN because of the narrow band gap caused by the activation of n → π* excitation [89, 90]. The properties of boron nitride (BN), such as its high specific surface area and high chemical stability, make it a promising material for water remediation [91]. The stabilized property, increased activity, and response to visible light (Vis) of ultra-thin carbon nitride (UCN) have attracted a lot of interest as a metal-free catalyst [92].

While all the materials mentioned above serve as useful photocatalysts, they come with certain drawbacks, including low efficiency, environmental toxicity in some cases, limited surface area, and large particle size. Some photocatalysts may not efficiently absorb light, restricting their utility to a full spectrum of light, encompassing both UV and visible regions. This limitation hinders their effectiveness in photocatalytic applications. Consequently, in recent decades, researchers have preferred MXenes photocatalysts to degrade various pollutants. MXenes exhibit favourable properties and enhanced efficiency as photocatalytic materials [93,94,95]. MXenes, characterized by outstanding conductivity ranging from 6000 to 8000 S/cm, are notable for a range of commendable properties that extend beyond their conductivity. Their two-dimensional structure provides a large surface area, facilitating enhanced interactions with various substances [96]. This feature makes MXenes promising for a wide array of applications, including but not limited to energy storage, catalysis, and sensor technologies [97]. Furthermore, the impressive mechanical strength and conductivity of MXenes position them as robust materials suitable for structural applications and reinforcement in composite materials [98]. MXenes demonstrate commendable chemical stability under diverse environmental conditions, ensuring prolonged application durability [99]. The surface chemistry of MXenes is adjustable, allowing for precise modifications and functionalization that tailor their properties to specific applications such as adsorption, sensing, and catalysis [100]. Moreover, certain MXenes exhibit high optical transparency in the visible and near-infrared spectrum, creating opportunities for their use in optically transparent applications like conductive films or coatings. These inherent characteristics collectively highlight the extensive potential of MXenes across various technological domains, from electronics to biomedical devices, making them materials of significant interest and potential for innovation in modern applications [101,102,103]. The primary goal of this review paper is to describe the efficiency of MXenes in photocatalysis for different dyes and CO2 gas with their complete mechanism, synthesis methods, aiming to contribute to a cleaner as well as more environmentally friendly living space. The building blocks of MXene photocatalysts, along with their band gaps, are presented in Table 3. The subsequent sections will specifically focus on MXenes materials for photocatalysis.

Table 3 The building blocks of MXenes photocatalyst
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8 MXene-based materials

MXenes and MXenes-based composites constitute a captivating class of materials that have exhibited promising applications in various fields, including energy storage, antimicrobial/antiviral agents, supercapacitors, batteries, desalination, water treatment, electrochemical sensors, diagnosis/imaging, and cancer theranostics. These materials have also been employed as photocatalytic due to their robust photothermal activity, multimodal imaging potential, biocompatibility, large surface area, low toxicity, electrical conductivity, and improved hydrophilicity. Their easily adjustable elemental makeup, uniform layered structure, and excellent electrical conductivity have made them subjects of extensive investigation in photocatalytic applications [107]. Current research on MXenes emphasizes the importance of surface charge engineering to manipulate their surface properties, aiming to enhance their potential for photocatalytic applications. MXenes must exhibit effective targeting competence, selectivity, intelligent targeting capabilities, and suitable stability and biocompatibility in diverse environments [108]. Among the most developing families of MXenes are carbides, nitrides, and carbonitrides, garnering increased attention due to their exceptional characteristics [109]. The general formula for MXenes is Mn+1XnTx, where X represents carbon/nitrogen, M stands for a transition metal, and T stands for OH, O, F. An exemplary MXene, Ti3C2Tx, possesses favorable surface properties and high conductivity [110,111,112]. The first reported MXene, TiAlC2, was synthesized by Naguib et al. in 2011 through a hydrofluoric acid (HF) etching process. Subsequent studies by Naguib et al. on the Ti3C2Tx derivative revealed its oxidation in the presence of air for 30 s, forming thin sheets of graphitic carbon embedded in TiO2 [113,114,115]. Zou and coworkers synthesized urchin-like structure MXenes with oxo groups using FeCl3 solution, demonstrating superior adsorption capacity for chromium [116]. Photocatalysts based on MXenes represent a promising frontier in environmental remediation and sustainable energy, showcasing notable effectiveness in degrading dyes and converting CO2 into environmentally friendly products. These advanced materials, distinguished by their outstanding properties, including a large surface area, strong electrical conductivity, and impressive photophysical attributes, have emerged as ideal candidates for various photocatalytic applications [117]. When considering the degradation of dyes, MXene-based composites have increased photodegradation capability compared to their unmodified counterparts. This enhanced performance is attributed to active sites within a porous structure, thereby augmenting their photodegradation activity [118]. The photocatalytic dye degradation facilitated by MXene involves an intricate interplay between surface dye adsorption and the generation of reactive oxygen species when exposed to light irradiation [119]. Additionally, in CO2 conversion, MXene demonstrates a strong affinity for CO2 adsorption and exceptional capability to capture and separate charge carriers. These attributes position MXene as an effective co-catalyst in the photocatalytic reduction of CO2 [120]. However, the precise mechanism underlying MXene's role in CO2 reduction remains a subject of ongoing investigation. To gain a better understanding of its functionality, researchers have employed theoretical frameworks and computational modelling [120]. Although MXene-based photocatalysts show significant promise for dye degradation and CO2 conversion, gaining a deeper understanding of their complex mechanisms and exploring optimization strategies is crucial. Further research efforts are necessary to uncover the full extent of their capabilities and enhance their performance for practical applications in addressing environmental challenges. The performance of materials is fundamentally linked to their dimensions, which directly impact reaction sites [121, 122]. MXenes are materials known for their exceptional versatility across various fields thanks to their diverse dimensions, encompassing thickness, surface area, and morphology [123, 124]. The thickness of MXene layers plays a pivotal role in determining their applications. Thinner layers exhibit amplified surface-to-volume ratios, rendering them suitable for energy storage devices such as batteries and supercapacitors [123, 124]. Conversely, thicker MXene layers provide heightened mechanical strength, making them ideal for structural applications [123, 124]. Variations in surface area among MXenes influence their adsorption and catalytic capabilities. MXenes with higher surface area possess increased active sites, making them favorable for applications in catalysis, sensing, and filtration [123, 124]. The unique morphologies of MXenes, spanning from nanosheets to nanoribbons, significantly impact their characteristics. Nanosheets are favored for applications in transparent conductive films, whereas nanoribbons are employed in reinforcing polymers or functioning as sensors, thanks to their distinctive shapes [123, 124]. Thanks to their adaptability and structural diversity, researchers can customize MXenes for a wide range of applications, including energy storage, catalysis, sensing, biomedical uses, and beyond [123, 124]. Ongoing exploration into MXenes across different dimensions unveils innovative possibilities, influencing the landscape of multifaceted applications across diverse scientific disciplines [123,124,125]. Because of its metallicity and work function, MXene can serve as a co-catalyst [126]. The transfer of electrons from metal to semiconductor establishes an ohmic contact [127,128,129]. Due to the superior metallic conductivity and adjustable optical characteristics of the precursor Ti3C2Tx MXene, TiO2-based materials derived from Ti3C2Tx (Tx = OH, O, or F as surface terminal groups) have recently gained interest in photocatalysis; these materials have efficient solar light harvesting and charge separation [90]. Figure 2 illustrates various applications of MXene.

Fig. 2
figure 2

Various application of MXenes

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8.1 Synthesis of MXenes

The synthesis of MXenes involves a range of intricate methodologies, each designed to produce these versatile materials with specific properties and functionalities. One prevalent technique revolves around the selective etching of the "A" layer from the MAX phase, a process acknowledged for its fundamental role in MXene production [130]. A significant approach in MXene synthesis focuses on exfoliating and separating layers from bulk MXene into fewer layers, accomplished through diverse techniques. These methods involve meticulous approaches that peel and detach layers, enabling the production of MXene nanosheets with controlled thickness and improved properties [131]. Another noteworthy technique involves molten-salt-assisted approaches, a sophisticated method that has exhibited tremendous potential in producing high-quality MXene materials with exceptional yields. This approach is valued for its capability to engineer MXene structures with desirable characteristics, highlighting superior properties suitable for specific applications [132]. Moreover, direct synthesis methods have surfaced as an innovative avenue in MXene production. These techniques enable a faster and more precisely controllable process for synthesizing MXenes by reacting metal, carbon, and metal halide salts. This method allows for the developing of MXene-based composite materials with customized properties, opening avenues for applications in various fields like electromagnetic interference shielding, energy storage, and electrochemical energy conversion [133]. The progress in these synthesis methods has unleashed the capability to create MXene-based composite materials with diverse functionalities. These materials are applied across various domains, from shielding against electromagnetic interference to augmenting energy storage capacities and facilitating efficient electrochemical energy conversion processes [134].

The conventional method for synthesizing MXenes involves the selective etching of the MAX phase, where aqueous HF is utilized to induce the formation of a three-dimensional (3D) structure resembling an accordion. Throughout the etching process, surface-terminating groups are also generated, reflected in the chemical formula of MXenes, which includes (T) to denote terminating groups. George et al. carried out the synthesis reaction for Ti2AlN etching in a mixture of hydrochloric acid (HCl) and KF, producing Ti2N [48]. Ghidiu et al. introduced a safer synthesis method for Ti3AlC2 that eliminates the use of HF [135]. The etching process of Ti3AlC2 is carried out in the absence of HF, utilizing NH4HF2 as an organic polar solvent, as described by Natu et al. [136]. Connecting two-dimensional MXenes leads to the synthesis of micro and hollow sphere MXenes, reducing repulsion forces between the 2D nanosheets [108]. To create porous 3D MXenes, a reaction involving HCl, PVP, and sodium thiosulfate is initiated to form sulfur particles, which are then mixed with 2D MXenes [137,138,139,140]. The stability of MXenes is enhanced through hydrogel formation, which is achieved by combining GO and the 2D dimension [110]. Electrospinning has emerged as a new synthesis method for MXenes formation [140, 141]. Although MXenes nanowire and nanotube structures are yet to be reported, the formation of conical scroll MXenes has been observed [142]. MXenes nanobelts can be synthesized through the electrochemical method, where the potential plays a crucial role in this preparatory process [143]. A varied array of synthesis techniques provides a versatile toolkit for constructing MXene-based composites, allowing researchers to investigate and leverage their numerous applications in advanced technologies. The synthesis routes for MXene are illustrated in the Fig. 3 [144].

Fig. 3
figure 3

Synthesis of MXene via different route. Reprinted from Ref. [144] with permission

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8.2 Applications of MXene and composites of MXene

MXenes have found applications across various fields, such as water splitting, organic transformation, degradation of dyes, and reduction of CO2. MXenes' stability and metallic qualities have made them popular substrates for hybrid electrocatalysts used in water splitting. Nonetheless, optimizing MXenes for exceptional hydrogen/oxygen evolution reaction (HER/OER) performance has proven to be challenging [145]. Because of their exceptional hydrophilicity, metal-like conductivity, high surface area/volume ratio, and rich surface chemistry, two-dimensional (2D) transition metal carbides, carbonitrides, and nitrides (MXenes) are regarded as a promising class of 2D nanomaterials in the photocatalytic and electrocatalytic water splitting application space [146]. Our primary emphasis here is on their roles in the degradation of dyes and the reduction of CO2, which will be elaborated upon in the following sections.

8.2.1 MXenes as photocatalyst for dyes degradation

MXenes serve as highly efficient catalysts or co-catalysts for the photodegradation of organic compounds. We utilized a hydrothermal treatment approach to embellish Ti3C2Tx MXenes with silver and palladium nanoparticles [147]. MXenes, which are a type of two-dimensional transition-metal carbides, nitrides, or carbonitrides, have been receiving much attention due to their unique characteristics such as high conductivity, structural and chemical stability, and the abundance of hydrophilic functional groups (-OH, -O, and -F) present on their surface. These properties make them effective in the photocatalytic degradation of methylene blue (MB) and rhodamine B (RhB) and have made them a popular choice among carbon-based nanomaterials [148]. These attributes position MXenes as a versatile platform for constructing composites in photocatalytic systems. Notably, titanium carbide Ti3C2Tx within MXenes contains a significant proportion of Ti, capable of undergoing surface oxidation to generate TiO2/Ti3C2Tx. The improved photocatalytic performance of these MXenes nanocomposites can be attributed to the formation of anatase TiO2 particles on the MXenes substrate surface [149]. The presence of noble metals further enhances performance, leveraging the surface plasmonic resonance effect and promoting charge separation [150]. Literature suggests that introducing noble metals enhances the photocatalytic activity in AgNPs/TiO2/Ti3C2Tx compared to PdNPs/TiO2/Ti3C2Tx photocatalysts. Moreover, the slightly accelerated kinetics observed in AgNPs/TiO2/Ti3C2Tx may be associated with the size and distribution of nanoparticles. MXenes quantum dots (QDs) find extensive application in the degradation of organic pollutants, serving as a co-catalyst to enhance light absorption capability. The combined form of Ti3C2/g-C3N4 has a thickness of 2–3 times greater than individual layers, measuring 4.5/4.7 nm. Therefore, 2D MXenes are effectively employed as photocatalysts. Shao and colleagues constructed these through hydrothermal processes, while the Guo group used hydrothermal methods to synthesize 2D materials [151]. Hybrid Bi2WO6/Nb2CTX has proven efficient in photocatalysis for the degradation of TC-HCl, MB, and RhB [152]. MXenes have been employed to degrade various dyes, as illustrated in Fig. 4 [153] along with the corresponding by-products.

Fig. 4
figure 4

MXene for the conversion of various organic pollutant and gases into useful and harmless compounds. Reprinted from Ref. [153] with permission

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8.2.1.1 Mechanism for dyes degradation over MXenes

MXenes function as photocatalysts by absorbing photons, generating oxygen-reactive species like hydroxyl radicals, which are subsequently utilized in degrading dyes into less harmful products. Additionally, MXenes serve as electrodes in electrochemical processes, facilitating enhanced electron transfer for degradation. The Fenton-like reaction is also augmented by MXenes, which utilize hydrogen peroxide to generate hydroxyl radicals [154]. As a result, dyes undergo degradation, transforming into environmentally non-toxic forms. When exposed to visible light in photocatalytic conditions, electron–hole pairs are generated, giving rise to O2 and OH radicals [155]. These highly reactive radicals play a crucial role in breaking down organic pollutants in water solutions, converting them into benign byproducts such as CO2 and H2O [156]. In the overall mechanism, when BLFO/Ti3C2 is photoexcited, it produces electron–hole paired charge carriers, as depicted in Eq. 4. Large surfaces provide numerous active sites, potentially increasing the likelihood of extensive interactions between dye molecules and the nanohybrid. As the process initiates, superoxide anion radicals •O2– are generated, and •OH– radicals emerge through the reaction of OH– with holes. This intricate series of reactions contributes to the effective degradation of organic pollutants in the water solution [157,158,159].

When electrons engage with the catalyst, H2O molecules transform OH. The radicals formed during this process include •O2– and •OH–, displaying high reactivity towards organic pollutants like CR, resulting in their degradation into harmless byproducts such as CO2 and H2O [159]. The degradation mechanism is outlined below in Eq. 4–9. The efficiency of different types of MXenes in dyes degradation is illustrated in Table 4.

$$Hv+ {M}_{n+1}{X}_{n}T\to {e}^{-}+ {h}^{+}$$
(4)
$${M}_{n+1}{X}_{n}+{e}^{-}\to {e}^{-}\left({Trap sites M}_{n+1}{X}_{n}\right)$$
(5)
$$e^{ - } (Trapsites\,\,{\rm M}_{n + 1} X_{n} )\, + \,{\rm O}_{2} \to \bullet \,{\rm O}^{2 - }$$
(6)
$$H_{2} O + h^{ + } \to OH^{ - }$$
(7)
$$OH^{ - } + Dye Molecules \to CO_{2} + H_{2} O \left( {Degradation Byproduct} \right)$$
(8)
$$Dye Molecule + \, \bullet O^{2 - } \to CO_{2} + H_{2} O \left( {Degradation Byproduct} \right)$$
(9)
Table 4 Different MXenes efficiency in dyes degradation
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8.2.1.2 MXenes for CO2 reduction

MXenes have emerged as highly efficient materials for the photocatalytic reduction of carbon dioxide, owing to their unique surface properties and tunable surface functionalities. The adsorption mechanism of carbon dioxide on MXenes has received considerable attention, with the Ghosh research group providing insightful contributions illuminating this interaction's intricacies [168]. While other materials, including metal–organic frameworks (MOFs), graphene-based materials, zeolites, metal nanoparticles, and transition metal dichalcogenides (TMDs), have been extensively explored for carbon dioxide reduction [169, 170]. MXenes present promising opportunities in this regard due to their versatile chemistry and potential for property tuning [171]. The surface terminations of MXenes, such as hydroxyl, oxygen, or fluorine, contribute to their hydrophilicity and enhance their adsorption capabilities [172]. In the realm of sustainable carbon dioxide reduction strategies, MXenes have paved the way for new possibilities [173]. Recognized for their abundant active sites and electron acceptor properties, MXenes have become promising materials for various gas adsorption applications [148, 174,175,176,177]. Integrating MXenes with other compounds has attracted significant interest in enhancing their functionalities. An exemplary study by Anasori et al. [174], demonstrated the combination of MXenes with Co–Co nanosheets using an in-situ Metal–Organic Framework (MOF)-derived method. This innovative approach involved the calcination of urea along with Ti3C2 and urea, resulting in the formation of a heterojunction known as Ti3C2/g-C3N4. While the intricacies of this method require further elucidation, the unique synthesis technique holds the potential for expanding the gas adsorption capabilities of MXenes [174]. The amalgamation of MXenes with Co–Co nanosheets and the creation of the Ti3C2/g-C3N4 heterojunction through a sophisticated synthesis process signifies a noteworthy advancement in material science, calling for deeper exploration and a comprehensive understanding of its underlying mechanisms in gas adsorption applications [174, 178].

MXenes have garnered attention from researchers exploring their potential as photocatalysts for CO2 reduction. Li et al. conducted a study introducing CsPbBr3/Ti3C2Tx MXene aerogels as an efficient photocatalyst for CO2 reduction. The resulting CsPbBr3/Ti3C2Tx MXene aerogels demonstrated significant photocatalytic activity in CO2 reduction, achieving a complete electron consumption rate of 112.6 µmol g−1 h−1. This performance surpassed that of the pristine powders of CsPbBr3 NC by 6.6 times. The enhanced photocatalyst performance can be attributed to robust light absorption, effective charge separation, and CO2 adsorption on the surface of CsPbBr3/Ti3C2Tx MXene aerogels [179]. Otgonbayar et al. presented a nanocomposite involving MXene (Ti3C2), Cu2O, and Fe3O4 with magnetic properties designed for CO2 reduction. Notably, the developed nanocomposite exhibited satisfactory stability over one month, emphasizing its long-term usability [180]. In another study by Liu et al., a novel MXene (V4C3-MXene) was introduced for CO2 reduction, illustrated in Figure [181]. The research emphasized the influence of resonance between stored electrons and holes on the crystal of V4C3-MXene, which heightened with ambient temperature up to 369 °C, expediting the photothermal catalytic CO2 reduction reaction. During the CO2 reduction reaction at 250 °C under simulated sunlight irradiation (3 h), V4C3-MXene achieved a CO yield of 95.68 μmol⋅g−1⋅h−1, with a selectivity of 96% for CO. This study not only experimentally confirms the photothermal catalytic CO2 conversion by V4C3-MXene with a localized surface plasmon resonance (LSPR) effect but also proposes a promising research direction for designing and fabricating MXene full-spectrum photothermal photocatalysts as depicted in Fig. 5 [181]. When sunlight illuminates V4C3-MXene, the incident rays induce vibrations of transverse magnetic (TM) waves in the V4C3-MXene electrons, accelerating the excitation of the surface-dispersed holes in the crystal into high-energy hot holes (depicted as green particles) and inducing a localized surface plasmon resonance (LSPR) effect. This LSPR effect reacts with H2 molecules, dissociating them into *2H+ (with an energy of E = 0.41 eV) due to the TM resonance action effect of the localized holes generated in V4C3-MXene, the bound electrons in the crystal become excited, lagging the holes. After migrating to the surface and following the LSPR effect lag diagram, these electrons form high-energy hot electrons. The LSPR effect lags the surface in this process, and the absorbed CO2 gradually forms *CO2– intermediates (CO2 + e– → *CO2–). Subsequently, *CO2– engages with •OH, and the hot electrons dissociated from H2O, produced in the reaction, contribute to forming the intermediate HCO3– (*CO2– + •OH + e– → HCO3–). HCO3– gains electrons and H+, further decomposing into •OH and *CO (HCO3– + e– + H+  → 2•OH + *CO). The *CO dissociates from the surface of the photocatalyst, producing CO molecules (*CO → CO).

Fig. 5
figure 5

Graphical representation of CO2 reduction over V4C3-MXene. Reprinted from Ref. [181] with permission

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Numerous researchers have reported various MXenes, and while it is impossible to cover all of them here, the literature consistently confirms that MXene stands out as an excellent candidate for CO2 reduction compared to other materials, owing to its unique properties. Table 5 provides a detailed overview of recently reported MXenes for CO2 reduction, along with their respective efficiencies.

Table 5 The efficiency of different types of MXenes in CO2 reduction
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8.2.1.3 Fundamental mechanism for CO2 reduction over MXenes

The fundamental principle of photocatalytic CO2 reduction (CO2CRR) is depicted in Fig. 4. The CO2CRR primarily involves three sequential steps. In the initial step, charge generation occurs, wherein electrons in the conduction band (CB) of semiconductors (photocatalysts such as MXenes) absorb photons with energies surpassing the band gap energy. This excitation leads to the creation of electron–hole pairs. The band gap, valence band (VB), and conduction band (CB) can be determined using UV–Vis absorption spectrum, ultraviolet photoelectron spectroscopy, and the Mott–Schottky method. The expressions for VB and CB are calculated as EVB = χ—Ee + 0.5 Eg and ECB = EVB—Eg, where χ, Ee, Eg, ECB, and EVB represent electronegativity, free electron energy on the hydrogen scale (4.5 eV), bandgap, position of CB, and position of VB, respectively. During the second step, photoinduced charge transport and separation occur on the photocatalyst's surface. However, it is essential to note that rapid recombination of charge carriers occurs during this process on the surfaces of the photocatalyst or in the bulk within a few picoseconds to nanoseconds. This rapid recombination is attributed to the higher Coulomb force, which is not conducive to the desired photocatalytic reaction. Moving on to the third step, charge redox reactions occur wherein electrons and holes play a crucial role in initiating reduction and oxidation reactions on the surface of the photocatalyst. These reactions include CO2 reduction, N2 reduction, and the degradation of organic pollutants. The specific steps involved in CO2 reduction include the requirement of a high potential of -1.9 V in the presence of one electron due to the stable nature of CO2 molecules, making the reaction more challenging to proceed. The presence of multi-electron CO2 reduction reactions involving protons can enhance the efficiency of CO2 reduction. Depending on the number of electrons (2e−, 4e−, 6e−, and 8e−, respectively) and protons involved in the reactions (Eq. 10–14), the products of CO2 reduction include carbon monoxide (CO, −0.53 eV), formic acid (HCOOH, −0.61 eV), formaldehyde (HCHO, −0.48 eV), methanol (CH3OH, −0.38 eV), CH4 (−0.24 eV), and other hydrocarbons [83,84,85], as depicted in Fig. 6 [181]. This indicates that the intricate reduction pathway involves breaking C–O bonds, forming C–H bonds, and radical dimerization [86, 87]. The photocatalytic CO2 reduction proceeds in conjunction with the reduction of water to produce hydrogen (H2, −0.41 V) and the oxidation of water to yield oxygen (O2, 0.82 V) processes. The effective photocatalytic CO2 reduction reactions require the accumulation of proton-assisted multi-electron processes at the reaction site of the photocatalyst [182].

Fig. 6
figure 6

Graphical representation of photocatalytic CO2 reduction in the presence of MXene. Reprinted from Ref. [182] with permission

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$${CO}_{2 }+ {2H}^{+}+ {2e}^{-} \to HCOOH {E}_{redox}^{0}= -0.61V$$
(10)
$${CO}_{2 }+ {2H}^{+}+ {2e}^{-} \to CO+ {H}_{2}O {E}_{redox}^{0}= -0.53V$$
(11)
$${CO}_{2 }+ {4H}^{+}+ {4e}^{-} \to HCHO+ {H}_{2}O {E}_{redox}^{0}= -0.48V$$
(12)
$${CO}_{2 }+ {6H}^{+}+ {6e}^{-} \to {CH}_{3}OH+ {H}_{2}O {E}_{redox}^{0}= -0.38V$$
(13)
$${CO}_{2 }+ {8H}^{+}+ {8e}^{-} \to {CH}_{4}+ {2H}_{2}O {E}_{redox}^{0}= -0.24V$$
(14)

MXenes, as photocatalysts, absorb photons to generate electron–hole pairs, subsequently forming oxygen-reactive species such as hydroxyl groups. These species facilitate the conversion of carbon dioxide into less toxic forms like carbon monoxide, methane, and methanol. This transformative process reduces CO2 emissions, making the environment less polluted. The efficiency of various MXenes in CO2 reduction is detailed in Table 5. MXene has become crucial in numerous studies to mitigate CO2 emissions through reduction processes. Researchers have explored diverse methodologies harnessing MXene's unique properties to drive efficient CO2 conversion. In a groundbreaking study, researchers focused on creating a catalyst by leveraging MXene-regulated Ag-ZnO interfaces. This innovative approach yielded a catalyst with nearly 100% CO Faraday efficiency and remarkably high partial current density [183]. These advancements have the potential to substantially enhance the efficiency of CO2 conversion processes, opening the door to more sustainable and eco-friendly practices. Another intriguing exploration involves MXene's role as a modification material for a boron-doped diamond (BDD) working electrode. This application effectively reduces overpotential during CO2 electroreduction and facilitates the generation of formic acid [184]. This outcome underscores MXene's ability to finely adjust electrode surfaces, promoting more efficient CO2 conversion pathways yielding valuable products. The integration of MXene as a co-catalyst in photocatalytic CO2 reduction processes has garnered attention for its exceptional electrical conductivity and expansive active area. These properties significantly boost the overall efficiency of CO2 conversion, showcasing MXene's potential in optimizing photocatalytic reactions [120]. Furthermore, investigators have delved into the synergistic effects arising from the integration of MXene with other materials, such as Cu2O/Fe3O4 and polymeric carbon nitride (CN), resulting in ternary nanocomposites. These complex combinations have demonstrated heightened photocatalytic CO2 reduction activity, increasing yields of valuable products like CH3OH [185, 186]. This exploration highlights MXene's adaptability as a component in multifaceted nanocomposites, leveraging its properties to enhance the efficiency of CO2 conversion and facilitate the generation of valuable chemical compounds. Collectively, these diverse studies underscore the significant potential of MXene in revolutionizing CO2 reduction processes. The innovative utilization of MXene in various configurations and applications exemplifies its crucial role in advancing sustainable technologies, mitigating carbon emissions, and fostering a more environmentally conscious future.

8.3 MXenes stability

MXenes are promising candidate for numerous applications due to its unique physical and chemical properties. Typically, MXenes have better for photocatalysis, but still undergoes its unwanted features in this situation, including weak stability, self-stacking, instability in aqueous conditions, fragile, and suboptimal optical absorption. MXenes are highly active chemically, and easily oxidized in open environment, leading to the deterioration in photocatalysis. Furthermore, the fragile interlayer interactions between the template and MXene flakes make it perplexing to developed an integrated structure [197]. 2D morphology, hydrophilic properties and rich functional groups at MXene surface, researchers established MXene composite, introducing with unified porous configuration, comprehensive and better photocatalytic properties, decent wettability and photothermal capability [198]. Pristine MXenes are not so valuable for photocatalysis due to their instability as stated before. Consequently, use of supportive and covering template is essential, which frequently in the form of different 3D polymeric network, wood, carbonaceous materials, and composite etc. The supporting templates will give backing, stability and reduce recombination of generated photocharges in MXenes-based photocatalysts during the photocatalytic technology.

8.4 Factors effecting photoactivity of MXene-based photocatalysts

The below factors highly effect the photoactivity of MXene-based photocatalysts: Band gap engineering is the process of doping MXenes with different elements to alter their bandgap, which increases their photocatalytic activity and allows them to absorb a greater range of wavelengths [199]. Doping can also cause MXenes to change morphologically, adding surface-rich moieties that increase their photocatalytic activity. MXenes and other semiconductors can form heterojunctions that aid in the transfer of photogenerated electrons and holes, thereby decreasing recombination and boosting photocatalytic activity. Heterojunctions can also improve light absorption by increasing photocatalytic activity by combining materials that can absorb a greater range of wavelengths [200]. MXenes can sustain their photocatalytic activity over time by virtue of doping and heterojunction formation, which can also increase their stability. Doping and heterojunction formation together have the potential to have synergistic effects that increase photocatalytic activity even more than the total of their individual parts [201]. In addition to enhancing MXenes qualities, doping MXene has allowed it to reach new frontiers in fields like sensors and catalysis, among many others. "Doping" is the process of adding foreign elements to 2D materials to modify their chemical and physical characteristics. Their applications were greatly expanded by the controlled doping of graphene and its derivatives with heteroatoms [202]. There are two methods to accomplish doping: either by substituting or adding heteroatoms to the lattice or by surface functionalization using molecules that donate or remove electrons. The development of new MXene-based systems was impacted by the effectiveness of doping techniques in previously 2D systems, providing a fresh perspective for theoretical and experimental [203]. Doping for the M, X, or T components of MXenes has thus been studied, either during the MAX phase or after exfoliation, and the results have been categorized as (a) M-doped, (b) X-doped, or (c) T-substituted. All three positions (M, X, T) can be occupied by heteroatoms like N, P, S, or O. Halides can take the place of T elements, and transition metals like Mo, Cr, Ru, etc. can be added to replace one or both M and X positions in the doped MXenes [204].

Two synthetic pathways are available for achieving doping through compositional engineering of 2D MXenes: (i) in-situ (bottom-up) and (ii) ex-situ (top-down) strategies. The dopant is added for the 3D MAX phase synthesis in-situ for example, by sintering, and the doped MXenes are then exposed through selective etching and exfoliation [205]. The main consequences of this method are substitutions at M and X, which vary according to the heteroatom dopant and transition metal that are employed. Ex-situ strategies have generally involved post-synthesis modifications e.g., by hydro/solvo-thermal, heat treatment, or plasma, which, depending on the heteroatom and conditions pursued, allow for doping or substitution at either/both X and T [206].

8.5 Advantages and disadvantages of MXenes

MXenes distinguish themselves through their multifaceted characteristics, encompassing advantageous features and areas that warrant additional refinement and scrutiny. These materials exhibit numerous positive attributes, driving their exploration across various applications. Notably, MXenes demonstrate exceptional biocompatibility, positioning them as highly suitable candidates for deployment in nerve cell regeneration and nerve reconstruction [98]. Their distinctive layered structure, impressive electrical conductivity, and mechanical robustness add to their appeal in energy conversion and storage [207]. MXenes showcases substantial surface area, stability, outstanding electrical conductivity, and facile surface functionalization. These qualities position MXenes as promising contenders for applications in cancer nanotheranostics [208]. Moreover, their exceptional morphological features, including remarkable surface areas and effective heat conductance, make them valuable in gas storage and air purification [209].

MXenes exhibit increased flexibility, mechanical strength, and conductivity [210]. They also demonstrate heightened stability, maintaining their structural integrity even under elevated temperatures [211]. The surface area of MXenes is substantial, and they offer flexibility of functionalization through the attachment of different groups. Precursor molecules for MXenes, such as carbides and layered ceramics, are readily accessible [21]. While MXenes share numerous advantages with COFs, they are not without limitations. The economic cost of MXenes is notably higher due to the precursor materials and synthesis techniques involved and as a photocatalyst it have recombination problem [212]. Industrial-scale production poses challenges as it necessitates a multi-step process and is sensitive to moisture and air. Additionally, the available compositions of MXenes are somewhat limited [213]. Despite these challenges, MXenes face obstacles that hinder their widespread adoption. Addressing concerns related to enhanced stability and diversifying synthesis methods is crucial to broadening the applicability of MXenes. Furthermore, addressing long-term in vivo biosafety issues is imperative for safe utilization in biological systems [214]. These challenges highlight key areas where additional research and development are essential to unlock the full potential of MXenes. While MXenes exhibit promising features, their utilization requires careful consideration, especially in the context of future applications. This involves overcoming current limitations and devising effective strategies for optimal and sustainable use across diverse fields.

Several methods are used to overcome the recombination problem of Mxenes such as surface passivation is applied, which entails coating the MXene surface with an appropriate substance. You can passivate using either organic or inorganic materials [215]. The separation of photogenerated electron–hole pairs can be aided by the formation of heterojunctions between MXenes and other semiconductors with appropriate oh radics. Thus, the optoelectronic performance of the MXene is enhanced [216]. Composite formation and doping with metal polymer are also used to overcome the recombination problem [75].

In summary, MXenes undeniably offer immense promise in various fields due to their remarkable properties and versatile nature. However, to fully realize their potential, ongoing research efforts to address limitations, enhance stability, diversify synthesis methods, and ensure long-term biosafety are crucial. With concerted efforts focused on optimization and advancement, MXenes are poised to play a transformative role in numerous technological domains.

9 Prospects and challenges

MXenes have emerged as promising candidates for photocatalytic applications, particularly in the mitigation of hazardous dyes and gases. Despite their remarkable potential, there exist notable challenges that must be addressed to fully exploit their capabilities. A significant hurdle lies in the low quantum efficiency exhibited by MXenes, which currently hinders their practical applications. To unlock their full potential, intensive research efforts are required to enhance their quantum efficiency and explore innovative strategies that can augment their overall photocatalytic performance. Strategies may include the development of novel synthesis techniques or the integration of these materials with other advanced materials to create synergistic and highly efficient photocatalytic systems.

Another critical challenge is the stability and durability of MXenes, particularly when exposed to harsh environmental conditions. Improvements in their stability are paramount to ensuring sustained and reliable performance in real-world applications. Researchers need to focus on developing methodologies that enhance the robustness of these materials, making them resilient in various environmental settings.

To achieve the full efficacy of the photocatalytic technology, the important difficulties hindering large-scale photocatalytic applications for the environmental cleaning and solar hydrogen generation necessary to be resolute. The key noteworthy obstacle to realizing this objective is the production of remarkably active photocatalytic materials on a large-scale via low-energy, economical and environmentally friendly production methods. The following problem is selecting the perfect arrangement of substrate, immobilization method and the respective material. In addition, complications rise in choosing the essential design and geometry of photocatalytic devices depending on the mode of operation and conditions.

Despite these challenges, the future prospects for MXenes are promising. The ongoing advancements in synthesis techniques and the strategic integration of these materials with other functional counterparts hold the potential to yield highly efficient photocatalytic systems. Furthermore, envisioning MXenes as an integral component in conjunction with renewable energy sources, such as solar energy, could lead to the development of sustainable and environmentally friendly solutions for tackling issues related to hazardous dyes and gases.

Looking ahead, research endeavours should concentrate on optimizing the inherent properties of MXenes to elevate their photocatalytic performance and overcome existing challenges. These materials stand as key players in the battle against environmental degradation and health hazards associated with hazardous dyes and gases. By unravelling their untapped potential and addressing the challenges they present, researchers can lay the foundation for ground-breaking and sustainable solutions, ushering in a cleaner and healthier environment.

10 Conclusion

This comprehensive review has explored the potential of MXenes as photocatalytic materials, highlighting their ability to address environmental and health concerns caused by hazardous dyes and gases. The review emphasizes these materials' remarkable properties and versatile applications, which play a pivotal role in addressing contemporary challenges. Despite their promise, the review also identifies the challenges that prevent the effective utilization of these materials. These challenges include enhancing stability and efficiency and addressing biosafety concerns. Therefore, concerted research and development efforts are necessary to overcome these challenges. MXenes hold immense promise in the face of growing demand for cleaner, sustainable, and renewable energy sources. These materials can significantly mitigate environmental degradation and foster a more sustainable future. However, it is crucial to continue research and development efforts to unlock their full potential and overcome the challenges they face. Addressing the outlined challenges will be pivotal in harnessing these materials' capabilities for widespread application and impact. This review is a comprehensive resource for researchers, scientists, and engineers in photocatalysis. Its insights into the properties, applications, challenges, and prospects of MXenes serve as a valuable foundation for further exploration and advancement in this dynamic and crucial domain.