Nanoalchemy: Unveiling the Power of Carbon Nanostructures and Carbon–Metal Nanocomposites in Synthesis and Photocatalytic Activity

Abstract

Due to a rise in industrial pollutants in modern life, the climate and energy crisis have grown more widespread. One of the best ways to deal with dye degradation, hydrogen production, and carbon dioxide reduction issues is the photocatalytic technique. Among various methods, catalytic technology has demonstrated tremendous promise in recent years as a cheap, sustainable, and environmentally benign technology. The expeditious establishment of carbon-based metal nanoparticles as catalysts in the disciplines of materials and chemical engineering for catalytic applications triggered by visible light is largely attributed to their advancement. There have been many wonderful catalysts created, but there are still many obstacles to overcome, which include the cost of catalysts being reduced and their effectiveness being increased. Carbon-based materials exhibit a unique combination of characteristics that make them ideal catalysts for various reaction types. These characteristics include an exceptional electrical conductivity, well-defined structures at the nanoscale, inherent water repellency, and the ability to tailor surface properties for specific applications. This versatility allows them to be effective in diverse catalytic processes, encompassing organic transformations and photocatalysis. The emergence of carbon-based nanostructured materials, including fullerenes, carbon dots, carbon nanotubes, graphitic carbon nitride, and graphene, presents a promising alternative to conventional catalysts. This review focuses on the diverse functionalities of these materials within the realm of catalysis materials for degradation, hydrogen production, and carbon dioxide reduction. Additionally, it explores the potential for their commercialization, delving into the underlying mechanisms and key factors that influence their performance. It is anticipated that this review will spur more research to develop high-performance carbon-based materials for environmental applications.

1. Introduction

Rapid urbanization and population increases have a negative impact on the aquatic environment. The textile industry is not the only anthropogenic activity that seriously pollutes the environment; food industries, paper and pulp industries, and other manufacturing sectors, such as pigment industries, have also been hit [1,2,3]. During the processing phases, a variety of chemicals are used, resulting in effluents and other associated pollutants including heavy metal ions [4,5]. The discharge of metal ions and dyes into the environment causes environmental pollution that is harmful to both the flora and the wildlife. Organic contamination, which typically consists of hazardous and non-biodegradable substances like antibiotics or dye, is one of the main issues. Multiple scientific investigations have demonstrated a correlation between exposure to dye molecules and heavy metal ions, even at minute concentrations, and the development of various health complications in humans and other organisms. These complications can have severe, even fatal, consequences for the exposed populations [6]. There are numerous approaches to treating wastewater, including filtration, adsorption, coagulation, and photocatalytic destruction [7,8]. The latter holds a lot of potential as it not only eliminates contamination but also induces disintegration. Because there is no secondary contamination, photocatalysts outperform other commonly used methods that entail the transition of pollutants from one phase to another. Additionally, the procedure is quick, frequently uses a natural light source, and can be completed under ambient settings, and some of its other highlighted features are illustrated in Figure 1. Finding and creating new photocatalytic materials is a significant continuing research area because of the benefits listed above [9].

To completely break down organic pollutants (dyes) through photocatalysis, the material needs three key properties: an efficient absorption of light, especially at relevant wavelengths; a minimized recombination of electron–hole pairs; and the promotion of rapid electron transfer during redox reactions [10,11]. The most prevalent element on Earth is carbon when it comes to the production of sustainable products [12]. Nature utilizes a specific element, referring to oxygen, in combination with hydrogen to form water (H₂O), a readily available and efficient energy storage medium. Analogously, carbon-based materials are emerging as a critical component in next-generation technologies for harnessing renewable energy sources. These applications encompass biofuels, carbon dioxide conversion (reduction), catalysis, energy storage devices, and photocatalysis, among others [13,14]. The cost-effective, environmentally benign, and sustainable properties of carbon make it a compelling candidate for photocatalytic applications in industrial and large-scale processes. Its remarkable physicochemical and electrochemical characteristics have established carbon as a versatile material in various fields, including solar energy devices and optoelectronics. Notably, the development of carbon-based nanostructured materials has surged in recent times due to their significant potential. As illustrated in Figure 2, these nanostructures exhibit exceptional properties that render them highly suitable for photocatalytic applications like CO₂ reduction, hydrogen generation, organic transformations as catalysts, the photodegradation of organic dyes, water splitting, and carbon dioxide emission [15,16]. The potential applications of these materials encompass photocatalytic hydrogen generation under visible-light irradiation, serving as high-performance electrode materials for supercapacitors, facilitating water-splitting processes, and promoting the reduction of carbon dioxide. Strong chemical and thermal stability, environmental friendliness, low cost, ease of processing, and low framework density are only a few of the appealing qualities of materials based on carbon. It now signifies an important research issue in current research reports.

Photocatalysis has emerged as a promising approach to address environmental pollution and energy crises through the degradation of toxic contaminants, hydrogen production, and carbon dioxide reduction. Anthropogenic CO₂ emissions necessitate innovative mitigation strategies. While carbon capture and storage offer potential solutions, their energy intensity and cost limitations underscore the need for alternative approaches. CO₂ photoreduction presents a promising avenue, enabling CO₂ conversion into valuable products through renewable energy-driven processes. Rising CO₂ emissions necessitate urgent mitigation strategies. While renewable energy sources offer alternatives to fossil fuels, their current limitations ensure the continued dominance of hydrocarbons. Consequently, CO₂ recycling technologies have gained prominence. Photocatalysis, harnessing solar energy, emerges as a sustainable approach to CO₂ conversion compared to energy-intensive methods like carbon capture and storage.

The development of novel catalysts with unique functionalities has facilitated the exploration of heterogeneous catalysis. This approach offers advantages such as efficiency under visible-light irradiation and compatibility with organic reaction systems [17]. Gold (Au) and Silver (Ag) nanoparticles (NPs) on graphene sheets demonstrated an increased photocatalytic performance during the years 2015 to 2016. Metallic nanoparticles (MNPs) present themselves as attractive candidates for enhancing photocatalytic activity due to their dual functionality as both light absorbers and catalytic centers. This unique ability to synergistically combine photon capture and catalytic deed significantly broadens the latent applications of MNPs in catalysis [18,19]. This review encompasses the preventative (environmental protection, public health safeguards) and restorative (wastewater treatment, remediation strategies, pollutant transformation) applications of carbonaceous nanomaterials in environmental remediation. We commence by presenting a concise overview of carbonaceous nanoparticles and their associated characteristics. Subsequently, the focus shifts to the most prominent applications of carbon-based nanostructured materials in catalysis, specifically the degradation of organic model pollutants, hydrogen production, and carbon dioxide reduction.

This review article delves into the significance of carbon and its diverse allotropes (structural forms) as conductive buttress materials within the field of photocatalysis. It provides a critical assessment of the current state of this research area, highlighting the persistent challenges and offering insights into its future prospects. The substantial challenges associated with their development have limited the exploration of carbon-based metal nanocomposites as photocatalysts for processes like organic dye pollutant degradation, hydrogen production, and carbon dioxide reduction. To conclude, this review aims to elucidate the significance, future potential, and broader implications of this burgeoning research field.

2. Nanostructured Materials Based on Carbon: Principles and Characteristics

The mid-20th century witnessed the emergence of carbon-based nanostructured materials as catalyst supports in a wide range of chemical transformations employed within the industrial and manufacturing sectors [20]. In recent years, the development of carbon materials with tailored properties, particularly a high surface area, exceptional stability/recyclability, and enhanced charge carrier transport, has garnered significant attention within the environmental remediation field [21,22]. The benefits of carbon-based materials and their accomplishment in heterogeneous catalytic reactions can be enumerated as follows. These benefits include (i) chemical steadiness in the medium (acid and base), (ii) strong thermal constancy, (iii) less corrosion capability, (iv) extraction from the reaction mixture, (v) hydrophobic behavior, and (vi) lower cost [23,24,25].

Carbon is crucial to the evolution of the Earth. It can make reliable promises in a variety of ways. The two primary naturally occurring carbon allotropes are diamond and graphitic materials. Diamond exhibits exceptional hardness due to its strong sp³ hybridized C-C bonds in a three-dimensional network. Conversely, graphite possesses a layered structure with weak van der Waals forces between adjacent layers, rendering it a highly effective lubricant. Over the past three decades, scientists have discovered a diverse range of carbon-based nanostructures, including fullerenes, carbon nanotubes (CNTs), graphene, mesoporous carbon, and others (as depicted in Figure 3). Owing to their exceptional physicochemical characteristics, these carbon-based nanostructured materials exhibit significant potential for visible-light photocatalytic water treatment applications, as evidenced by Figure 4.

2.1. Fullerene (C₆₀)

During the 1960s, the architect Buckminster Fuller created geodesic domes that resembled cages (Bucky balls as shown in Figure 5). This allotrope was given the name fullerene in honor of the architect. Fullerene (C₆₀) is the third allotrope of carbon [26,27]. The discovery of C₆₀ in 1985 significantly increased the number of allotropes of carbon that were previously only known from graphitic materials and diamond. A fullerene is an molecule made entirely of carbon atoms, which can be shaped into an ellipsoid, hollow sphere, or tube. Carbon atoms are joined in hexagonal and pentagonal rings to form C₆₀. Because of its specific morphology, compact size, electrochemical stability, and well-organized structure, C₆₀ can be used in diverse applications of energy conversion systems [28]. Fullerene (C₆₀) exhibits a remarkable combination of physical and chemical properties, including tunable solubility through functionalization of its good thermal conductivity, high electrical conductivity, low mass density (1.65 g cm⁻³), surface groups, and exceptional mechanical properties. Furthermore, C₆₀ allows for the modification of its chemical functionalities, enabling the tailoring of its pore structure for specific applications.

Fullerenes, exemplified by C₆₀, are characterized by a closed, cage-like structure reminiscent of a soccer ball. This configuration is achieved by the assembly of 20 hexagonal and 12 pentagonal carbon rings, covalently bonded along each edge. Notably, the bonds connecting hexagons exhibit shorter bond lengths compared to those connecting a pentagon and a hexagon, resembling a double bond configuration [30,31,32]. At extremely high pressures and temperatures, C₆₀ is remarkably stable. Fullerenes are only mildly soluble in aromatic solvents like toluene and insoluble in water. C₆₀ has a reactivity toward nucleophiles that is comparable to that of an olefin missing an electron. Due to the above-mentioned advantages, C₆₀ is used in various applications such as organic photovoltaics (OPV), as a catalyst, as a portable power device, for biohazard protection and water purification, and as a polymer additive [33].

2.2. Carbon Nanotubes (CNTs)

Carbon nanotubes (CNTs) are carbon allotropes that are found in the fullerene structural family and have extremely small diameters (nanometer and micrometer-sized) with 1D structure. They are proposed as cylindrical objects with end caps made of hemispheres of fullerene structure and a center hollow core formed by cylinders of rolled-up graphene sheets [34,35,36], and their various shapes are featured in Figure 6. Their performances entirely depend on the type of helix they have, and as a result, they behave either as a metal or a semiconductor. The synthesis of carbon nanotubes (CNTs) can be categorized into three primary methodologies: (i) the production of single-walled carbon nanotubes (SWCNTs), (ii) the fabrication of double-walled carbon nanotubes (DWCNTs), and (iii) the growth of multi-walled carbon nanotubes (MWCNTs). SWCNTs are compared to a sheet of graphite that is one atom thick, wrapped into a cylinder, and then covered with a fullerene hemisphere. DWCNTs are coaxial nanostructures with one SWCNT nestled inside the other, made of exactly two SWCNTs [37,38,39]. MWCNTs are comparable to nanotubes within nanotubes.

The history of SWCNTs began two years after that of MWCNTs, and they may be divided into three groups based on their crystallographic configurations, including chiral, armchair, and zigzag (as depicted in Figure 6) [41,42]. These arrangements primarily depend on how precisely the graphene sheet is wrapped up. The chirality of a carbon nanotube (CNT) is determined by the chiral vector Ch = na1 + ma2 within the hexagonal lattice, where (n, m) are integers representing the number of steps along the unit vectors a1 and a2. This (n, m) notation defines three distinct classifications of carbon atom orientations around the CNT circumference: ‘zigzag’ (n = m and m ≠ 0), ‘armchair’ (n = m = 0), and ‘chiral’ for all other cases. Notably, chirality significantly impacts the transport properties of CNTs, particularly their electrical conductivity. A specific (n, m) CNT is classified as metallic if (2n + m) is a multiple of 3, and as semiconducting otherwise [43]. The electrical properties of multi-walled carbon nanotubes (MWCNTs) exhibit greater complexity compared to single-walled carbon nanotubes (SWCNTs). This arises from the presence of multiple graphene sheets within MWCNTs, where each layer can possess a distinct chirality. The interplay of these various chiral arrangements within MWCNTs presents a significant challenge in accurately predicting their electrical behavior.

CNTs are an effective material for water filtration because of their fascinating thermal and electrical properties. Carbon nanotubes (CNTs) exhibit exceptional electrical conductivity and can display either semiconducting or metallic behavior dictated solely by their specific chirality. This unique property is attributed to the transfer of electrons from the conduction band of TiO₂ to the CNT structure. The CNT acts as an efficient electron sink, extending the time period of photogenerated charge carriers within the TiO₂/CNT composite system.

CNTs further contribute to the enhanced photocatalytic efficiency of TiO₂ by providing a significantly improved surface area (ranging from 200 to 400 m² g⁻¹). The favorable energetic alignment between TiO₂ nanoparticles and CNTs, arising from the lower Fermi level of CNTs, facilitates efficient electron transport from the conduction band of TiO₂ to the CNT surface. This rapid electron transfer process minimizes the recombination of photogenerated electron–hole pairs within TiO₂, thereby extending their lifetime and promoting their participation in photocatalytic reactions.

Beyond their exceptional mechanical properties, carbon nanotubes (CNTs) exhibit remarkable physical characteristics, particularly superior electrical and thermal conductivity compared to other carbon allotropes. This unique combination of properties makes them highly versatile materials for various applications, including thermal conductors, conductive adhesive, structural and thermal materials, field emission, energy storage, biological applications, air and water filters, ceramics, catalytic supports, fibers, and others [41].

2.3. Carbon Dots (CDs)

The year 2004 marked the inadvertent identification of zero-dimensional (0D) quasi-spherical carbonaceous nanostructures, subsequently designated as carbon dots (CDs), with an average diameter of ~10 nm during the purification process of single-walled carbon nanotubes [43,44]. Carbon dots (CDs) can be broadly classified into three primary categories: graphene quantum dots (GQDs), carbon nanodots (CNDs), and carbonized polymeric dots (CPDs) (as illustrated in Figure 7). These zero-dimensional (0D) carbonaceous nanomaterials typically exist as nanosized particles dispersed in aqueous or other solvent media. The majority of CDs possess a core comprised of hybridized sp²/sp³ carbon atoms and are decorated with various surface functional groups. For instance, GQDs exhibit anisotropic surface/edge functionalities or interlayer defects and possess lateral dimensions exceeding their height. They are typically composed of mono- or multi-layered graphene sheets at the nanoscale. The photoluminescence (PL) properties of CDs are dictated by a complex interplay of factors depending on their specific type. In GQDs, the size of the π-conjugated domains and the structure of surface/edge functionalities exert the primary influence on their optical behavior. Conversely, CDs and CPDs generally exhibit spherical cores decorated with surface groups. For CDs, the presence of multi-layered graphitic structures within the core leads to PL emission dominated by intrinsic state luminescence and the size-dependent quantum confinement effect. CPDs, on the other hand, are hybrid nanostructures composed of aggregated or cross-linked carbonaceous cores encapsulated within polymer chain shells. The molecular state of the polymer and the specific cross-linking structure are the key determinants of the optical properties exhibited by CPDs [45]. The majority of CDs have a carbon core inside and a lot of functional groups including carbonyl, hydroxyl, carboxyl, and epoxy on the surface. With the help of these functional groups, CDs have excellent water solubility and photoluminescence (PL) quantum characteristics.

The diverse structural characteristics of CDs are significantly influenced by the selection of synthetic pathways. These pathways can be broadly categorized into two main approaches: ‘bottom-up’ and ‘top-down’ approaches. Breaking up huge carbonaceous materials into smaller carbon structures is a top-down strategy. On the contrary, bottom-up methods use chemicals like amine compounds, citric acid, ascorbic acid, glucose, etc., as carbon precursors to create CDs [47]. The CDs types share the same basic nontoxic, simple to manufacture, water-soluble, tunable fluorescent, photostable features, high photobleaching resistance, stable colloidal solution, and ecological friendliness, and these are all attributed to the 0D carbon-based nanomaterials [44,48]. CDs have emerged as a versatile material platform, attracting significant interest for their potential applications in various fields, including photovoltaics (solar cells), optoelectronic sensors, photocatalysis, multi-chromatic printing, and bioimaging [49]. CDs have been engaged due to their improvement in the semiconductor photocatalytic capabilities [50]. The extended photo-responding range and highly effective charge separation made possible by CDs have greatly increased activity. Upon exposure to sunlight, CDs classified as organic semiconductors possess the ability to generate photoinduced electron–hole pairs. Additionally, CDs exhibit amphoteric behavior in suspensions or solutions, functioning as both electron donors and acceptors. This unique property allows them to act as an essential energy transfer intermediary. CDs typically absorb light in the 240–320 nm range, while this range can be extended to a tiny portion of the visible-light spectrum. Bright blue luminescence was visible in the CDs suspension when exposed to 365 nm UV light, demonstrating that CDs have down-conversion fluorescent characteristics [43,51]. The amount/types of dopants in the carbon core, the dimensions of the π-conjugated domains, and the absorption characteristics of CDs are all controlled by the synthesis techniques and different precursors.

2.4. Graphene

The discovery of graphene in 2004, a single atomic layer of sp² hybridized carbon atoms achieved through mechanical exfoliation, has ignited significant scientific and engineering interest due to its remarkable properties [52,53]. Beyond its record-breaking status as the strongest and thinnest material discovered to date, graphene exhibits a remarkable array of exceptional chemical and physical properties. These properties are fundamentally linked to its unique two-dimensional honeycomb lattice structure comprised of sp² hybridized carbon atoms [54,55]. As a result, graphene is acknowledged as the fundamental component of all-dimensional carbon materials. It has exceptional and distinctive qualities, including a high mobility of charge carriers (>200,000 cm² V⁻¹ S⁻¹ at electron densities of 2 × 10¹¹ cm⁻²), thanks to its lamellar structure. Graphene’s exceptional mechanical properties, including its outstanding Young’s modulus (around 1.0 TPa) and high spring constants (ranging from 1 to 5 N m⁻¹), have attracted significant scientific attention. Additionally, its potentially high specific surface area (approximately 2630 m² g⁻¹) makes it a promising candidate for various applications. Furthermore, graphene boasts a remarkable thermal conductivity (around 5000 W m⁻¹ K⁻¹) and excellent optical transmittance (approximately 97.7%), further expanding its potential applications [56]. The unique combination of structural, electrical, optical, and physiochemical properties exhibited by graphene has positioned it as a highly attractive material for a broad range of technological applications. These include nanoelectronics, electrical, biochemical and chemical sensing, H₂ production and storage, drug delivery, photovoltaics, polymer composites, intercalation materials, and catalytic applications [57].

Within the diverse applications of graphene, photocatalysis holds promise for water treatment. This approach involves the development of metal nanoparticle-embellished graphene nanostructures, which function as efficient photocatalysts for the degradation of pollutants in water. Heterogeneous photocatalysis is acknowledged as an effective and environmentally benign technique that is gaining attraction in the scientific community. This is mainly because graphene’s sheet-like shape allows for electron delocalization due to the sp² property of its carbon bonds [58,59]. A graphene sheet has remarkable electrical conductivity, which might be further amplified by incorporating several metal nanoparticles into the sheet-like structure of graphene to impact the photocatalytic activities of photocatalysts based on graphene. Based on current research, it is evident that metal-based graphene photocatalysts are essential for achieving extraordinarily high levels of photocatalysis efficiency [60]. A significant challenge hindering the practical implementation of metal-based photocatalysts lies in their inherent limitations, including a low quantum yield, compromised stability, and insufficient light absorption under visible-light irradiation. Fortunately, the strategic integration of metal nanoparticles on graphene presents a viable approach to overcome these limitations and enhance the overall functionality of these photocatalysts.

2.5. Graphitic Carbon Nitride (g-C₃N₄)

The system known as g-C₃N₄ is a π-conjugated n-type semiconductor that is hybridized in Sp² with a unique polymeric structure associated with tri-s-triazine and s-triazine-based patterns [61,62]. It can take on several polymorphic forms based on the ratio between the two combining atoms, as well as the configuration of the carbon and nitrogen atoms. It mostly crystallizes into five distinct polymorphic phases: the cubic phase, graphitic phase, α-phase, β-phase, and quasi-phase [63]. These phases have bandgaps ranging from 5.5 to 2.7 eV, minimal compressibility, and exceptional hardness that is comparable to that of diamond. Diverse band structures are produced when carbon and nitrogen atoms with different valence states are present. g-C₃N₄ stands out for having numerous great qualities, including a moderate bandgap (≈2.7 eV), which allows the harvest of visible light at a wavelength lower than 460 nm; the van der Waals effect; high polymeric flexibility; and thermal compatibility (≈600 °C) [64,65]. Furthermore, g-C₃N₄ can be readily obtained by employing readily available carbon/nitrogen-containing precursors (like urea, cyanamide, thiourea, and melamine) via thermal, alkaline, and acid exfoliation techniques.

Graphitic carbon nitride (g-C₃N₄), a metal-free polymeric semiconductor containing tri-s-triazine units, has gathered significant interest for its remarkable potential in photochemistry and photocatalysis. Its appealing bandgap structure aligns well with visible-light absorption, making it a promising candidate for various applications, including water splitting, photocatalytic degradation of organic pollutants, CO₂ reduction, and organic synthesis. Additionally, g-C₃N₄ boasts several attractive features, such as low cost, simple synthesis procedures, good chemical stability, and environmentally friendly characteristics. However, intrinsic limitations of pristine g-C₃N₄ hinder its photocatalytic efficiency. These limitations include a low specific surface area, which restricts the number of available reaction sites; rapid recombination of photogenerated electron–hole pairs, which diminishes their ability to drive photocatalytic reactions; and insufficient light absorption, particularly in the visible region, which limits the utilization of solar energy [66]. Three mechanisms are typically involved in each photocatalytic reaction: electron–hole charge production and separation, catalytic surface reactions, and photon absorption [62]. To address the intrinsic limitations of pristine g-C₃N₄ and enhance its photocatalytic activity, various modification strategies have been explored. These strategies include elemental and molecular doping, the creation of mesoporous g-C₃N₄, exfoliation to 2D nanosheets, combination with conductive materials, the construction of nanocomposite structures with other semiconductors, and dye sensitization [67].

3. Synthesis Methods of Various Carbons

3.1. Synthesis of Carbon Nanotube

The process factors of the carbon nanotube (CNT) synthesis technique, which regulate the CNTs’ development mechanism and characteristics, include the reaction time, gas flow rates, temperature of growth, pressure, size, and concentration of the catalyst, carrier gases, substrates, and type of precursors used for carbon. The process factors listed above need to be adjusted in order to produce a better yield of CNTs that is purified and well-structured. In contrast to other methods, the previous researchers tirelessly used arc discharge (AD) [68,69], laser ablation (LA) [70,71], and chemical vapor decomposition (CVD) [72,73] techniques to create several forms of carbon nanotubes (CNTs) with various crystallographic topologies. They all have certain benefits and drawbacks that lead to different development outcomes, which make it easier for us to choose a certain technique for producing CNTs with the necessary qualities. A thorough review is nearly impossible to accomplish, given the volume of research papers and the variety of experimental setups used in the CVD, AD, and LA procedures. The subsequent sections will delve into a detailed examination of the process factors employed during the synthesis of CNTs. This analysis will encompass a comprehensive overview of the CVD, AD, and LA techniques.

3.1.1. Arc Discharge (AD)

Scientists have been keen on using a technique called arc discharge to make C₆₀ ever since they figured out a way to create it in large amounts using this method. This is because of the technology’s high efficiency, ability to manufacture nanotubes at a specific place, and tremendous potential for commercial procedure. When compared to other processes, it is the most traditional, straightforward, efficient, and adaptable method that is widely utilized in the production of carbon nanotubes. Numerous research publications on CNTs synthesis using an AD technique with incremental improvement have been published since the 1990s. Of them, only a small number of studies involving the large-scale synthesis of CNTs have been published. Our research showed that making well-organized carbon nanotubes (single-walled, multi-walled, and double-walled) using the arc discharge method is challenging. This difficulty arises because we cannot fully control how the nanotubes grow during this process, and our knowledge about how they form in general is limited. To overcome this, we need to better understand and manage the effects of different settings used in the arc discharge experiment. This will allow us to get the most out of this technology and improve how we control the structure of the nanotubes. Arora and Sharma have already conducted some work on understanding the growth process of nanotubes in the arc discharge method [68]. This also elucidated critical parameters necessary for the initiation and progression of the arc discharge (AD) process.

(i)

The applied current is the technique’s most important and vital limitation, and it requires rigorous examination to obtain the best AD.

(ii)

It is still unknown how temperature affects the CNTs’ development process during preparation.

(iii)

Electrode size is a crucial factor, and further evaluations are necessary to understand the precise role that electrode geometry and shape play in the creation of CNTs.

(iv)

Graphite serves as the predominant carbon source for the arc-discharge process. However, investigations into the utilization of more economical precursors, including coal and carbon black, are ongoing.

Their analysis indicates a predominant use of direct current (DC) power supplies for arc generation, characterized by substantial current fluctuations. This stands in contrast to alternating current (AC) and pulsed DC methods. The study identified the need for the further optimization of experimental parameters governing CNTs synthesis. Additionally, theoretical investigations and the development of mathematical simulations are crucial to elucidate the CNTs growth mechanism and replicate it within the arc discharge (AD) process. The literature review revealed the utilization of various inert gases during CNTs production to mitigate carbon oxidation at elevated temperatures [41]. In an effort to overcome the constraints imposed by vacuum environments and specific ambient gases, researchers have devised a novel methodology for CNTs synthesis. This method prioritizes simplicity, rapidity, and cost-effectiveness, enabling the maximization of external energy input during the production process.

3.1.2. Laser Ablation (LA)

SWCNTs are mostly produced using this more recent and advanced pulsed laser vaporization method, usually known as the LA process. This methodology entails the high-temperature heating of a graphite target within an inert gaseous environment. Subsequently, the target undergoes vaporization induced by various laser sources. Notably, the Smalley group at Rice University pioneered the utilization of laser vaporization, or the double-pulse laser oven technique, for the synthesis of SWCNTs bundles exhibiting a narrow diameter distribution, as established in 1995. Similar to the AD synthesis method, this process facilitates the vaporization of the carbon target material, followed by its subsequent deposition onto a substrate. In a comparative investigation of SWCNTs synthesis via pulsed arc discharge and pulsed laser techniques, Roch et al. discovered that the SWCNTs produced by both methods had similar characteristics, including high yield and similar diameter, as well as similar nucleation and growth mechanisms [74]. Extensive research has explored the application of LA for CNTs synthesis. This body of work investigates the influence of various process parameters on the final product’s yield and quality. The parameters explored include a broad spectrum of laser types, pulse durations (nanosecond, microsecond, and continuous wave), and wavelengths [75].

Laser ablation investigations have explored a progression of laser systems, transitioning from pulsed dual-laser configurations to continuous-wave lasers, with the objective of augmenting the production rate [71]. The LA technique exhibits several advantages that position it as a potentially superior method for SWCNTs synthesis. These advantages include enhanced product purity, demonstrably increased production yield, and a streamlined post-processing methodology. Additionally, the LA process presents itself as a more environmentally benign approach. LA is a cool way to make tiny tubes of carbon called SWCNTs. It seems to be better than other methods because it makes purer tubes, more of them, and is easier to clean up afterward. It might also be better for the environment. The scientists are still figuring out exactly how LA blasting affects how these tubes grow. There are some mysteries they need to solve, like how big and small clumps of carbon leftover from blasting affect the tubes, how the tiny clusters of metal catalysts used to make the tubes form and grow, how the carbon and metal interact, and what makes the tubes stop growing at a certain point. Figuring this out will help them to improve LA blasting to make even better tubes [76]. It is evident that the majority of CNTs generated by the LA synthesis process are of the SWCNTs type and are primarily synthesized in an argon environment.

3.1.3. Chemical Vapor Deposition (CVD)

Researchers have focused increasingly on the CVD approach because of its many benefits, which include low-temperature synthesis, energy efficiency, scalability, high-quality products, ease of use, and more control over the process. CVD facilitates the progress of CNTs by introducing carbon-containing precursors in the gas phase over a catalyst supported on a substrate. Subsequently, a high-temperature energy source induces the decomposition of the gas molecules into reactive carbon species in the form of atomic carbon. A comparison of CNT synthesis approaches was recently carried out by a number of researchers, who then published their conclusions following a critical evaluation. This evaluation focused on defining the techniques’ potential for use as large-scale, capabilities, efficacy, and economically viable production [73,77]. They discovered that CVD appears to be a very effective method for producing high-quality CNTs at a high yield when compared to other CNTs synthesis approaches.

Following a thorough examination, they presented some crucial details regarding the catalytic CVD process, mentioning the following:

(i)

The type of catalyst used has a significant impact on the diameter of synthetic CNTs.

(ii)

Understanding the association between the catalyst and the features of synthesized CNTs is essential for optimizing the use of CNTs in various applications.

(iii)

Further thorough research is required to determine the rationale behind the addition of different contaminants during the synthesis.

(iv)

Researchers must anticipate new and improved methods because the quality of synthesized CNTs is currently subpar, and there is a lack of total control over CNTs’ chirality.

(v)

During the synthesis process, the development of CNTs is significantly influenced by the substrate. It is yet unknown how exactly the kind of substrate utilized and the sort of CNTs produced relate to one another.

Therefore, more thorough research is required to determine the substrate’s hidden involvement in the synthesis of CNTs. The study also revealed a significant cost disparity between CNTs production and the synthesis of other carbon nanomaterials. This highlights the necessity for research into novel, cost-effective carbon sources to minimize the economic burden associated with CNTs synthesis via CVD. The CVD technique employs various precursors of carbon and process variables that influence the final product. Notably, temperature and precursor pressure (often referred to as carbon flow) exert critical control over the morphology, structure, and growth rate of the resulting CNTs. The experiment observed the formation of well-aligned, defect-free CNTs at lower precursor temperatures and pressures. Conversely, higher temperatures and pressures resulted in the production of CNTs with misalignments and imperfections [78]. Additionally, they discovered that as the precursor pressure dropped and temperature rose, CNTs’ diameter and wall count decreased.

3.2. Synthesis of Carbon Dots

In contrast to metal-based nanomaterials, carbon dots (CDs) may be inexpensively and easily synthesized on a large scale using a variety of carbon-based precursors. The carbonization and post-functionalization of carbon-rich organic compounds serve as the foundation for several synthetic techniques.

3.2.1. Top-Down

Top-down methods are mostly used to break down large carbonaceous materials into nanoparticles under a variety of reaction circumstances, including a very acidic environment, high pressure and/or temperature, or oxidation. Chemical oxidation [79,80], laser ablation [81], ultrasonic exfoliation [82], and electrochemical oxidation [83] are examples of common top-down techniques that appear in Figure 8. For example, under severe oxidation conditions, carbon-based materials with varying dimensions of carbon black, graphene, and carbon nanotubes (CNTs) can be exfoliated to create (quasi) 0D-CDs with diameters smaller than 10 nm.

Using graphite rods as the cathode and anode, Li et al. proposed a straightforward electrochemical method for producing high-quality CDs in an alkaline ethanol/water solution combination. The resultant CDs had a high PL quantum yield of 12% and good UCPL characteristics. They ranged in size from 1.2 to 3.8 nm. These CDs have applications in bioscience and energy technology as highly effective catalysts and fluorescent indicators [84]. Ming et al. introduced an easy-to-use electrochemical method for creating CDs with excellent purity. This device employed graphite rods as the anode and cathode at a DC power source of 15–60 V, with clean water chosen as the electrolyte. The solution is in dark yellow and mostly composed of graphite oxide and CDs of various sizes. A light brown hue was seen in the aqueous solution of CDs in the described treatment. The average diameter of the CDs was approximately 4.5 nm, and they were evenly distributed throughout the aqueous solution [85]. These CDs are potential building blocks for sophisticated photocatalyst synthesis because of their respectable photo-response to visible light.

Under ultrasonic radiation, a sequence of events including dehydration, polymerization, and carbonization will cause a brief spike in carbon nucleation, after which the nuclei will develop into nanoparticles. For instance, Park and colleagues utilized leftover food as a precursor to create green carbon nanodots (G-dots) using an ultrasonic technique at ambient temperature [82]. These G-dots’ high surface oxygen-containing group contents contributed to their exceptional solubility in aqueous solution. The straightforward and eco-friendly process enables the large-scale manufacturing of green CDs, which typically have a 2–4 nm size. The G-dots were suitable for biomedical applications because they demonstrated strong photostability, photoluminescence characteristics, good size uniformity, and non-toxicity.

A common method for creating CDs with a variety of nanostructures and morphologies is laser ablation. A particular amount of nano-carbon materials was added to a solvent (acetone, ethanol, or water) and ultrasonically sonicated constantly while the glass cell was exposed to 532 nm laser light. Additionally, magnetic stirring was employed to lessen the immaculate carbon elements’ gravitational setting. Following laser irradiation, the supernatant CDs were collected and placed on the copper grid.

3.2.2. Bottom Up

Primarily, CDs are prepared from carbon-rich precursors by bottom-up procedures such as solvothermal [86], pyrolysis [87], hydrothermal [88,89], and microwave techniques [90] as shown in Figure 9. These techniques enable the production of CDs from tiny molecules or polymer precursors in a moderate environment.

For instance, Jiang et al. produced a sizable amount of CDs in a matter of minutes using a microwave technique. This experiment involved microwave pyrolysis, dialysis, and freeze-drying of the precursors of phosphoric acid and ethanolamine. The majority of the generated CDs exhibited very long afterglow lifetimes ranging from 1.4 to 10 s, even though when exposed to UV (365 nm) light, some mild blue FL may be released from the pale and yellow CDs powder. It was discovered that the amorphous polymer-like structure of CDs, surface heteroatom (N and P) dopants, and intra-particle linkages were the primary causes of the observed ultralong phosphorescence of CDs [91,92]. These CDs have the potential to be used as a security ink for sophisticated anti-counterfeiting methods. Furthermore, Song developed a simple and inexpensive microwave technique that uses urea and citric acid as precursors to create CDs in 4–5 min. When the reactants are treated under microwaves, the translucent solution changes to a clustered dark-brown precipitate. Following synthesis, the resultant CDs exhibited exceptional stability and a maximum quantum yield reaching 17%. Their average size distribution ranged from 1 to 5 nanometers (nm). The low or nil toxicity of these CDs was confirmed by toxicological testing. With their unique photoluminescence features, cheap cost, low toxicity, and great biocompatibility, CQDs have demonstrated their promise for high-end manufacturing and may be utilized as a biocompatible fluorescent ink for a variety of applications [50].

The preparation of CDs has made considerable use of hydrothermal techniques. A hydrothermal technique was used to manufacture N-CDs with single-state fluorescence, producing a large number of surface poly vinyl alcohol (PVA) chains. The graphitized cores may be isolated from π–π interactions, and the aggregation of CDs can be inhibited by the chains. In the solution state, these N-CDs displayed an apparent fluorescence redshift. Zhu et al. also used a hydrothermal technique to condense and carbonize a combination of ethylene diamine and citric acid to produce a high quantum yield of polymer-like CDs [93]. When exposed to UV light, these CDs, which were utilized as printing inks, showed intense fluorescence at a concentration of 1 × 10³ mg mL⁻¹.

A more environmentally friendly method for creating CDs is pyrolysis. Crysmann et al. created CDs under different pyrolysis settings using ethanolamine and citric acid. The photoluminescence characteristics of the generated CDs were strongly influenced by the precursor ratio and pyrolysis temperature. The as-synthesized CDs could display a strong photoluminescence spectrum at a molar ratio of 1:3, which was mostly caused by fluorophores that included amides. The unique PL feature of CDs would arise from the production of carbonate nuclei at higher temperatures. CDs with a strong PL spectrum will be secured under ideal pyrolysis conditions.

In general, big carbon compounds are mostly broken down into nanoparticles via top-down processes. The process operated simply and was appropriate for a variety of raw materials. The produced CDs have certain functional groups attached to the surface and a structure that is somewhat comparable to that of virgin materials. The top-down methods make it simple to see the lattice structure of CDs. However, the production of CDs is rather low, and the particle size is not consistent. Furthermore, the majority of the CDs produced by the bottom-up method originate from small molecules or polymer precursors by carbonization and dehydration. The partial carbonization of the resulting CDs will result in the formation of a polymer structure on their surface. In contrast to the top-down strategy, controlling the size and form is rather easy using the bottom-up method. On the surface, heteroatoms can be doped. The large-scale manufacturing, cheap cost, and environmental friendliness of the bottom-up techniques make them adaptable and enable the development of functional CDs with desirable features at an upmarket level [86,92]. As the extent of carbonization progresses, the polymeric architecture and luminescence properties will exhibit a marked decrease. Conversely, the degree of graphitization and electrical conductivity will experience a significant enhancement.

3.3. Synthesis of Graphitic Carbon Nitrate (g-C₃N₄)

The inherent layered structure of as-synthesized bulk g-C₃N₄ materials translates to comparatively low specific surface areas despite the theoretical potential for an exceptionally high specific surface area in perfectly dispersed g-C₃N₄. To unlock the full potential of g-C₃N₄, strategies to break apart these stacked layers are crucial. This review provides a concise overview of the various exfoliation techniques employed for g-C₃N₄.

Yang et al. reported a sonication-assisted liquid exfoliation strategy for the production of thin-layer nanosheets of g-C₃N₄ from bulk. Their investigation revealed that isopropyl alcohol, characterized by its low boiling point, serves as an effective solvent for the exfoliation of bulk g-C₃N₄ under continuous sonication [94,95]. The nanosheets of g-C₃N₄ were produced with a very thin layer (about 2 nm), which is necessary to achieve the material’s large surface area of 384 m² g⁻¹. Employing electrochemical impedance spectroscopy (EIS) as an analytical tool, researchers have observed that the thin-layer g-C₃N₄ nanosheets’ electron transfer resistance, which is calculated from semicircular Nyquist plots, was 75% lesser than that of the bulk nanosheets of g-C₃N₄. This suggests that the nanosheets’ capacity for charge transport and separation has improved. The photoluminescence (PL) spectra of g-C₃N₄ provided additional evidence for this, showing that following exfoliation, there was a noticeably decreased PL intensity, which is indicative of a lower rate of recombination of electrons and holes by photo-induction [94,96]. She et al. employed a sonication technique to exfoliate bulk g-C₃N₄, yielding thin layers with a thickness ranging from approximately 3 to 6 atoms (ca. 0.9–2.1 nm). The specific surface area of the resulting material was measured to be around 3 m² g⁻¹. Thus, an additional surface area of around 32 m² g⁻¹ was attained [97]. Similarly, EIS research showed that exfoliation reduced g-C₃N₄ electron-transfer resistance by 60%. Furthermore, during visible-light irradiation, a greater photocurrent was achieved for thin layer g-C₃N₄, in accordance with transient photocurrent measurements. The combined photoelectrochemical analysis revealed enhanced photoinduced charge carrier transport and separation within the thin-layer g-C₃N₄ nanosheets [98].

Beyond organic solvent-based exfoliation techniques, bulk g-C₃N₄ can also be delaminated into thin layers using acidic or basic aqueous solutions. Xu et al. combined concentrated H₂SO₄ with g-C₃N₄ produced from dicyandiamide in deionized water, then sonicated the mixture to exfoliate [99]. This exfoliation process utilizes H₂SO₄ intercalation within the bulk g-C₃N₄ interlayers, facilitating the production of single-layer g-C₃N₄ nanosheets with a thickness of approximately 0.4 nm. These nanosheets exhibit a significantly increased specific surface area (ca. 206 m² g⁻¹) compared to bulk g-C₃N₄ (ca. 4 m² g⁻¹) [100]. Photocurrent and EIS measurements provided evidence for the enhanced transport and separation of photogenerated charge carriers within these single-layer nanosheets of g-C₃N₄. In a separate study, Sano et al. employed hydrothermal treatment with a NaOH solution for the exfoliation of melamine-derived g-C₃N₄ [101]. The resultant g-C₃N₄ displayed a substantial reduction in particle diameter, the formation of a mesoporous structure, and a corresponding increase in specific surface area from approximately 8 m² g⁻¹ to 65 m² g⁻¹.

Designing Nanostructure of g-C₃N₄

Owing to its inherent polymeric nature and structural flexibility, g-C₃N₄ can be engineered into diverse morphologies by utilizing appropriate templates. Notably, researchers have successfully synthesized a variety of g-C₃N₄ nanostructures, including porous morphologies, hollow spheres, and one-dimensional (1D) nanostructures. A detailed overview of these structures is provided below.

(i)

Porous g-C₃N₄

Porous photocatalysts are of significant interest due to their inherent advantages. The porous structure offers a high surface area and a network of channels that facilitate mass diffusion, charge migration, and the separation of photogenerated species. To achieve this desirable morphology in g-C₃N₄, hard and soft templating techniques are frequently employed. These techniques allow for precise control over the porosity structure of g-C₃N₄ by utilizing various templates.

Chen et al. utilized cyanamide as a precursor for the synthesis of ordered mesoporous g-C₃N₄. This material exhibited a high surface area of 239 m² g⁻¹ and a pore volume of 0.34 cm³ g⁻¹. Notably, the pores within the synthesized g-C₃N₄ possessed an average diameter of approximately 5.3 nm, which is demonstrably smaller than the 10.4 nm pores of the employed SBA-15 template [102]. This observation is unsurprising as, in SBA-15 templated materials, the pore size is inversely proportional to the template’s wall thickness rather than the pore diameter itself. To enhance the interaction between cyanamide and the silica template, Zhang et al. implemented a pre-treatment step involving diluted HCl [103]. Next, vacuum and sonication were used to improve the cyanamide molecule penetration. This pre-treatment with diluted HCl resulted in the formation of ordered mesoporous g-C₃N₄ with a significantly increased surface area of 517 m² g⁻¹ and a larger pore volume of 0.49 cm³ g⁻¹. In a separate study, Fukasawa et al. employed cyanamide as a precursor to synthesize inverse opal g-C₃N₄ with ordered mesostructures. Their approach utilized brimming assemblies of regular-sized silica nanospheres as the template. This technique offers precise control over the pore size of the resulting g-C₃N₄. By systematically varying the diameter of the silica nanospheres from 20 nm to 80 nm, the researchers were able to tune the average pore size of the g-C₃N₄ product within a range of 13 nm to 70 nm [104]. Interestingly, an inverse relationship was observed between the pore volume and surface area of the synthesized g-C₃N₄. The sample containing the smallest pores (20 nm) exhibited the highest pore volume (1.70 cm³ g⁻¹), while the sample with the largest pores (70 nm) displayed the greatest surface area (230 m² g⁻¹). This highlights the inherent trade-off between these two properties in porous materials. It is noteworthy that the ordered porous structures of g-C₃N₄ hold promise as hard templates for the fabrication of size-tunable and well-defined Ta₃N₅ nanoparticles.

Beyond templated methods, soft templating approaches like Pluronic P123 have yielded porous g-C₃N₄ with a high surface area and extended light absorption. Recently, a bubble-templating technique using thiourea/urea and dicyandiamide as precursors emerged as a simple and nontoxic route for synthesizing nano-porous g-C₃N₄ [105]. The thermal decomposition of urea or thiourea during the treatment process generates gas bubbles, which act as templates to create a porous g-C₃N₄ structure. This porous material exhibits a significantly increased surface area compared to g-C₃N₄ synthesized solely from dicyandiamide.

(ii)

1D Nanostructures

One-dimensional (1D) nanostructured photocatalysts, including nanobelts, nanorods, nanotubes, and nanowires, offer the potential for tailored electronic, optical, and chemical properties. This ability to precisely control these properties through material design makes 1D nanostructured materials highly attractive for optimizing photocatalytic activity.

Li et al. employed an anodic aluminum oxide (AAO) template as a directing agent for the thermal condensation of cyanamide. This approach yielded well-defined g-C₃N₄ nanorods with a uniform diameter of approximately 260 nm [106]. The confinement effect imposed by the AAO template promotes enhanced crystallinity and preferential orientation within the g-C₃N₄ nanorods, leading to improved charge-carrier mobility. Additionally, the synthesized nanorods exhibit a more favorable valence band (VB) position, translating to a higher oxidation power for photocatalysis. Furthermore, the researchers successfully fabricated mesoporous g-C₃N₄ nanorods by employing SBA-15 nanorods as a template through a nanocasting approach [107]. These synthesized g-C₃N₄ nanorods exhibited a well-defined morphology with a diameter of approximately 100 nm and a high surface area ranging from 110 to 200 m² g⁻¹. Additionally, the presence of well-defined mesopores throughout the structure makes them ideal candidates for applications involving the loading of various metal nanoparticles or for use in catalysis and photocatalysis.

Bai et al. reported a template-free approach for the synthesis of g-C₃N₄ nanorods. Their method utilized g-C₃N₄ nanoplates as precursors, which were themselves prepared through the thermal treatment of dicyandiamide at 550 °C for a duration of 4 h [108]. A straightforward reflux process was used in a mixed solvent system of methanol and H₂O to convert the initial g-C₃N₄ nanoplates into nanorods. This process likely involves a rolling, exfoliation, and regrowth mechanism. The resulting g-C₃N₄ nanorods possess a higher proportion of active lattice facets and exhibit fewer surface imperfections. Both of these characteristics are highly desirable for enhanced photocatalytic activity [109,110].

A large surface area is often accomplished for g-C₃N₄ using porous structures, which can give an abundance of reactive sites. In the meantime, the target reactants and g-C₃N₄ interactions can be enhanced by the porous structures acting as efficient channels [111,112]. Furthermore, g-C₃N₄ 1D nanostructures could have the required specific surface area and charge carrier mobility.

4. Catalytic Applications Using Carbon-Based Metal Nanocomposites

As a catalyst, metal nanoparticles frequently experience problems with agglomeration, sintering, and dissolving. Carbon-based metal nanocomposites have emerged as demonstrably effective catalysts. These materials offer several advantages, including the ability to mitigate catalyst degradation, enhance catalytic activity, and improve overall robustness [113,114,115]. The possible approaches for improving photocatalysis performance are also suggested in Figure 10.

Numerous engineering techniques have been widely developed in recent decades in an effort to enhance the kinetic processes mentioned below and satisfy thermodynamic criteria. These engineering approaches may be separated into two categories: kinetic and thermodynamic, which are summed up in Figure 11. In the domain of photocatalysis, two primary thermodynamic strategies are employed to optimize material performance: the development of photocatalysts responsive to a wide range of light wavelengths and the modification of semiconductors with inherently large bandgaps through compositional engineering. Conversely, to enhance the often-challenging processes of charge separation, transport, and utilization within photocatalysts, researchers focus on material design strategies that promote efficient charge transfer. These strategies include the creation of high-efficiency charge-transfer nanostructures, well-defined heterojunction interfaces, materials with highly reactive surfaces, and the incorporation of cocatalysts. Favorably, the multi-functional graphene made all of these solutions possible. From a thermodynamic perspective, graphene holds promise for the development of wide-spectrum responsive photocatalysts or as a dopant for modifying wide-bandgap semiconductors. However, its potential extends beyond thermodynamics. Graphene can also be strategically utilized from a kinetic standpoint to enhance photocatalytic activity. This can be achieved through the creation of highly reactive surfaces and cocatalysts, well-defined high-quality heterojunction interfaces, and high-efficiency charge-transfer nanostructures.

Because of their specific characteristics, including those listed below, an assortment of carbon-based materials, including fullerenes, CNTs, CDs, graphene, and graphitic carbon nitride, may be employed as catalyst supports:

(i)

Holds up well in both harsh alkaline and acidic environments.

(ii)

Greater surface area and better dispersion than other common catalyst supports (such as alumina and silica).

(iii)

Possibility to control surface chemistry and porosity, as well as being favored for metal material production and carbon/carbon-based material recovery by combustion.

The captivating properties and exceptional controllability of carbon-based materials, particularly carbon-based metal nanocomposites, have garnered significant interest in the field of visible-light-driven catalysis. This interest stems from the ability to tailor their morphology and composition, thereby precisely tuning their catalytic performance.

4.1. Degradation of Dye

Dye-contaminated wastewater poses a significant environmental challenge. While various treatment methods exist, carbon-based metal nanocomposites in photocatalysis have emerged as a promising approach for its remediation. Carbon-based metal catalysts have emerged as a promising field in chemical and materials engineering, particularly for visible-light-driven applications. While significant advancements have been made, cost reduction and performance enhancement remain key challenges. Carbon-based nanomaterials offer a potential alternative to conventional catalysts [117]. Graphene, carbon nanotubes, carbon quantum dots, and graphitic carbon nitride have emerged as prominent carbon-based metal oxides for photocatalyst development (Table 1). Their stability, earth abundance, and cost-effective synthesis render them attractive for this application.

4.1.1. Carbon Nanotubes (CNTs)

One-dimensional (1D) carbon nanostructures known as CNTs are notable for their large surface areas, interesting optical and electrical capabilities, unique physicochemical qualities, and enormous aspect ratios. The unique properties exhibited by CNTs position them as promising candidates for the development of novel photocatalytic materials. Due to the straightforward fabrication technique and relatively inexpensive cost, numerous literature investigations have been conducted on the method of creating metal nanocomposites based on CNTs. CNTs primarily serve to construct composite photocatalysts, which are actually incorporated with various semiconductors. Due to their extraordinary properties like their structure and excellent mechanical strength, CNTs have gained significant attention and also serve as aspiring candidates for novel composite materials [118,119,120,121]. The electronic properties of CNTs can be precisely tailored, ranging from semiconducting to semi-metallic or metallic, depending on their specific helicity and diameter. This control over electronic structure is coupled with their inherently high surface area, well-defined layered architecture, and abundant surface π-electrons, making them ideal adsorbents. Additionally, the unique curvature of CNTs can promote the formation of highly active sites on their surface, further enhancing their adsorption capabilities. Therefore, a metal nanocomposite based on CNTs could be employed to successfully degrade organic colors.

The photocatalytic efficacy of various carbon nanotube (CNT)-based nanocomposites has been explored across a range of pollutants, with notable results. For instance, a WO₃/CNT nanocomposite synthesized via the sol-gel and hydrothermal method achieved the complete degradation of tetracycline within just 60 min [122]. Similarly, a CNT/CuS–MD composite prepared through in situ coprecipitation exhibited a 92.4% degradation efficiency for Rhodamine B (RhB) dye over 120 min [123]. A CNT/LaVO₄ nanocomposite, synthesized using the hydrothermal method, showed 81% tetracycline degradation in 180 min [124]. A CNTs/ZnO/MoS₂ composite, also synthesized hydrothermally, demonstrated 76% aniline degradation within 120 min [125]. Additionally, the MgO@CNT nanocomposite, produced via US-assisted hydrothermal synthesis, achieved 100% degradation of sulfadiazine in 80 min [126]. These results underscore the potential of CNT-based nanocomposites in efficient photocatalytic applications across diverse contaminants.

4.1.2. Graphene

The two-dimensional (2D) carbon-based substance known as graphene has a great deal of promise for use. Graphene’s remarkable properties stem from its unique atomic structure. It comprises a single layer of carbon atoms arranged in a hexagonal lattice, covalently bonded through sp² hybridization. This one-atom-thick sheet grants graphene an exceptional mechanical strength, exceptional thermal conductivity, superior electrical conductivity, a high surface area, and impressive physicochemical stability [127,128,129]. Modern graphene’s sheet-like structure, which is based on hybridization with metal nanoparticles, has gained attention for its advanced catalytic and photon-based characteristics. This is principally caused by the carbon’s sp² hybridization structure, which allows for electron delocalization. Furthermore, graphene exhibits a near-zero bandgap due to minimal overlap of the valence band (VB) and conduction band (CB) overlap, making it a viable option for use in photocatalysis.

The photocatalytic performance of various graphene-based nanocomposites has been evaluated against a range of pollutants, demonstrating significant degradation efficiencies. A GO/TiO₂ nanocomposite, synthesized via the hydrothermal method, achieved a remarkable 99.84% degradation of amoxicillin within 60 min [130]. Metal nanoclusters of silver and gold integrated with graphene, prepared through chemical synthesis, completely degraded 4-nitrophenol in 175 min [131]. A Ag–Au–rGO nanocomposite, also produced via the hydrothermal method, demonstrated an impressive 100% degradation of 4-nitrophenol in just 360 s [132]. Another graphene-based composite, graphene/TiO₂/g-C₃N₄, achieved 83.5% tetracycline degradation within 80 min using the hydrothermal method [133]. Furthermore, the CeO₂/CdS/rGO nanocomposite, synthesized hydrothermally, exhibited 90% degradation of ciprofloxacin within 120 min [134]. These findings highlight the efficacy of graphene-based nanocomposites in photocatalytic applications, offering potential solutions for the degradation of various environmental pollutants.

4.1.3. Graphitic Carbon Nitride (g-C₃N₄)

Recently, graphitic carbon nitride has emerged as a cutting-edge subject of study in the field of catalyst development. Graphitic carbon nitride (g-C₃N₄) is a polymeric semiconductor with a layered structure resembling that of graphite. It is composed entirely of carbon and nitrogen atoms. The unique combination of its sheet-like morphology, intriguing bandgap energy, excellent thermal stability, and synthetic accessibility makes g-C₃N₄ a promising candidate for the development of high-performance photocatalysts [135,136]. Zhang et al. and Wang et al. independently proposed the utilization of sacrificial templates for the creation of porosity within g-C₃N₄. This approach aims to enhance the surface area and porosity of the material, thereby promoting mass transfer efficiency and ultimately leading to the improved photocatalytic performance of the tailored g-C₃N₄ photocatalysts [137,138]. The photocatalytic activity of g-C₃N₄ is limited by its intrinsic bandgap of approximately 2.7 eV. This restricts light absorption to wavelengths shorter than 460 nm, which corresponds to the region of the solar spectrum. As a result, numerous bandgap techniques have been extensively explored, in addition to atom- and molecular-level doping, to attain enhanced photocatalytic efficacy and enhance the g-C₃N₄ photon harvesting potential.

The photocatalytic efficiency of g-C₃N₄-based nanocomposites has been explored with promising results. For instance, a Au + g-C₃N₄ nanocomposite synthesized via thermal polycondensation achieved 99% degradation of Rhodamine B (RhB) dye within 120 min [139]. Similarly, a graphene/TiO₂/g-C₃N₄ composite prepared through the hydrothermal method demonstrated 83.5% degradation of tetracycline within 80 min [133]. These results underscore the potential of g-C₃N₄-based materials in effectively breaking down organic pollutants in various environmental applications.

Table 1. Carbon-based metal nanocomposite photocatalyst.

S. No.	Photocatalyst	Synthesis Method for Photocatalyst	Target Pollutant	Performance of the Photocatalyst Used	Reference
1.	WO3/CNT	Sol-gel/hydrothermal	Tetracycline	100% for 60 min	[122]
2.	CNT/CuS–MD	In situ coprecipitation	RhB dye	92.4% for 120 min	[123]
3.	CNT/LaVO4	Hydrothermal method	Tetracycline	81% for 180 min	[124]
4.	CNTs/Zno/MoS2	Hydrothermal method	Aniline	76% for 120 min	[125]
5.	MgO@CNT	US-assisted hydrothermal	Sulfadiazine	100% for 80 min	[126]
6.	Au/TiO2@CNTs	Micro/nanobubble	Gaseous styrene	69.2% for 6.58 h	[140]
7.	Au@CNT@TiO2	Chemical synthesis	Simulated sunlight	97% for 60 min	[141]
8.	Au + GC3N4	Thermal polycondensation	RhB dye	99% degradation (120 min)	[139]
9.	GO/TiO2	Hydrothermal method	Amoxicillin	99.84% within 60 min	[130]
10.	Metal nanocluster (Ag and Au)/graphene	Chemical synthesis	4-Nitrophenol	100% in 175 min	[131]
11.	Ag–Au–rGO nanocomposite	Hydrothermal method	4-Nitrophenol	100% in 360 s	[132]
12.	Graphene/TiO2/g–C3N4	Hydrothermal method	Tetracycline	83.5% within 80 min	[133]
13.	CeO2/CdS/rGO	Hydrothermal method	Ciprofloxacin	90% within 120 min	[134]
14.	ZnO-modified SiO2 nanospheres	Wet impregnation	Congo red	83%	[142]
15.	TiO2(Rod)/MoS2	Solvothermal method	Ciprofloxacin	93% within 60 min	[143]
16.	Carbon dots	Green synthesis	Congo red	90%	[144]
17.	Carbon dots/CNTs	Chemical vapor deposition	Methyl blue	99%	[145]
18.	CdS/ZnO/1 wt% GO	Hydrothermal method	Methyl orange	99% in 60 min	[146]
19.	GO-Carbon Nanotubes-Ni	Chemical vapor deposition	RhB dye	100% in 120 min	[147]
20.	CdS/1 wt% RGO-carbon nanotubes	Hydrothermal method	Methyl blue	62% in 30 min	[148]
21.	rGO/ZnS/CuS	In situ microwave method	Ofloxacin	86% in 140 min	[149]
22.	BiVO4/rGO aerogel	Hydrothermal method	Formaldehyde	60% in 120 min	[150]

Table 1. Carbon-based metal nanocomposite photocatalyst.

S. No.	Photocatalyst	Synthesis Method for Photocatalyst	Target Pollutant	Performance of the Photocatalyst Used	Reference
1.	WO3/CNT	Sol-gel/hydrothermal	Tetracycline	100% for 60 min	[122]
2.	CNT/CuS–MD	In situ coprecipitation	RhB dye	92.4% for 120 min	[123]
3.	CNT/LaVO4	Hydrothermal method	Tetracycline	81% for 180 min	[124]
4.	CNTs/Zno/MoS2	Hydrothermal method	Aniline	76% for 120 min	[125]
5.	MgO@CNT	US-assisted hydrothermal	Sulfadiazine	100% for 80 min	[126]
6.	Au/TiO2@CNTs	Micro/nanobubble	Gaseous styrene	69.2% for 6.58 h	[140]
7.	Au@CNT@TiO2	Chemical synthesis	Simulated sunlight	97% for 60 min	[141]
8.	Au + GC3N4	Thermal polycondensation	RhB dye	99% degradation (120 min)	[139]
9.	GO/TiO2	Hydrothermal method	Amoxicillin	99.84% within 60 min	[130]
10.	Metal nanocluster (Ag and Au)/graphene	Chemical synthesis	4-Nitrophenol	100% in 175 min	[131]
11.	Ag–Au–rGO nanocomposite	Hydrothermal method	4-Nitrophenol	100% in 360 s	[132]
12.	Graphene/TiO2/g–C3N4	Hydrothermal method	Tetracycline	83.5% within 80 min	[133]
13.	CeO2/CdS/rGO	Hydrothermal method	Ciprofloxacin	90% within 120 min	[134]
14.	ZnO-modified SiO2 nanospheres	Wet impregnation	Congo red	83%	[142]
15.	TiO2(Rod)/MoS2	Solvothermal method	Ciprofloxacin	93% within 60 min	[143]
16.	Carbon dots	Green synthesis	Congo red	90%	[144]
17.	Carbon dots/CNTs	Chemical vapor deposition	Methyl blue	99%	[145]
18.	CdS/ZnO/1 wt% GO	Hydrothermal method	Methyl orange	99% in 60 min	[146]
19.	GO-Carbon Nanotubes-Ni	Chemical vapor deposition	RhB dye	100% in 120 min	[147]
20.	CdS/1 wt% RGO-carbon nanotubes	Hydrothermal method	Methyl blue	62% in 30 min	[148]
21.	rGO/ZnS/CuS	In situ microwave method	Ofloxacin	86% in 140 min	[149]
22.	BiVO4/rGO aerogel	Hydrothermal method	Formaldehyde	60% in 120 min	[150]

4.2. Hydrogen Production

The escalating energy crisis and environmental degradation necessitate sustainable, cost-effective, and environmentally benign energy solutions like water splitting. Developing affordable, stable, and earth-abundant nanostructured photocatalysts is paramount. To achieve this, a comprehensive understanding of photocatalyst engineering, reactor design, and performance optimization is essential. The present environmental problems and energy crisis can be mitigated by the efficient development and use of solar energy using photocatalysis [151,152,153]. For hydrogen evolution reactions, a variety of semiconductor materials have been employed, including TiO₂, WO₃, g-C₃N₄, CdS, and ZnS, which are tabulated in Table 2. For practical uses, these materials must possess exceptional photocatalytic activity and excellent stability, which they do not now possess.

4.2.1. CNTs

Numerous metals and their oxides serve as efficient catalysts for hydrogen generation on carbon nanotube (CNT) surfaces, as well as for synthetic fuel and polymer precursor production. CNTs exhibit limited solubility due to their hydrophobic nature and weak van der Waals interactions. Nevertheless, their versatile functionalization enables enhanced solvation in diverse solvents. CNTs, characterized by an exceptional conductivity, electroluminescence, surface area, and tensile strength, exhibit promising photocatalytic potential [153]. Combining CNTs with other catalytic materials enhances photocatalytic performance. CNTs/CdS composites, synthesized through solvothermal decomposition, demonstrate superior hydrogen production compared to pure CdS [154]. Iron oxide nanoparticles, while possessing advantageous properties, suffer from aggregation issues. MWCNTs/Fe₂O₃ nanocomposites, prepared via in situ flame synthesis, exhibit efficient photocatalytic hydrogen generation. TiO₂, limited by UV absorption and charge recombination, benefits from modification with carbonaceous materials. TiO₂/FCNTs composites, synthesized via arc discharge, significantly enhance hydrogen production under solar irradiation [155,156]. Kang et al. synthesized a CuO/NiO photocatalyst supported on CNTs via the hydrothermal method. Eosin Y and triethanolamine enhanced visible-light-driven hydrogen production, reaching a rate of approximately 1.0 mmol h⁻¹ g⁻¹ [157].

The photocatalytic hydrogen production efficiencies of various CNT-based nanocomposites have been investigated using different light sources. A ZnO/CNTs composite synthesized via atomic layer deposition achieved 1 µmol h⁻¹ g⁻¹ under UV–visible light [158], while a CNTs/CdS composite produced through the hydrothermal method exhibited a significantly higher rate of 52 µmol h⁻¹ g⁻¹ under Xe lamp irradiation [159]. A CuO/NiO/CNTs nanocomposite, created using the impregnation–calcination method, also reached 1 µmol h⁻¹ g⁻¹ under visible light [157]. Notably, a TiO₂ + CNTs composite synthesized via wet impregnation demonstrated an impressive rate of 2134 µmol h⁻¹ g⁻¹ under solar light [160]. A TiO₂(Rod)/MoS₂ composite, prepared by the solvothermal method, achieved the highest hydrogen production rate of 7415 µmol g⁻¹ under a 300 W Xe lamp, highlighting its superior photocatalytic performance [143].

4.2.2. Carbon Quantum Dots (CQDs)

Carbon quantum dots (CQDs) are efficient photosensitizers for solar-driven hydrogen generation. They exhibit photocatalytic activity across the visible spectrum and demonstrate long-term stability. CQDs offer advantages in terms of cost, toxicity, and scalability compared to other photocatalysts [161]. Their high surface area and electron storage capacity enable diverse photocatalytic applications. CQD–semiconductor composites effectively degrade organic dyes under visible and infrared light, with conjugated CQD structures crucial for optimal performance. CQD-copper composites outperform pure copper nitrate for hydrogen production. TiO₂-CQD nanocomposites exhibit enhanced photocatalytic activity due to increased electron energy [162]. Yu et al. synthesized a novel CQDs/ZnFe₂O₄ composite photocatalyst using a facile hydrothermal method. The composite, containing 15% CQDs by volume, demonstrated a remarkable eightfold increase in the transient photocurrent compared to pure ZnFe₂O₄, suggesting significantly enhanced charge carrier separation and transfer efficiency.

A Z-scheme carbon nanodot/WO₃ hybrid photocatalyst exhibited a remarkable hydrogen production rate of 1330 mmol h⁻¹ g⁻¹ under xenon lamp illumination. CQDs/P25 composites demonstrated enhanced photocatalytic hydrogen evolution compared to pure P25 without the need for noble metal cocatalysts. A hydrothermally prepared 5% CDs/NiCo₂O₄ composite significantly improved hydrogen and oxygen evolution rates compared to pristine NiCo₂O₄, attributed to efficient charge separation and transfer facilitated by uniformly loaded CDs [163].

The photocatalytic hydrogen production capabilities of carbon quantum dots (CQDs) and their composites have been demonstrated under xenon lamp irradiation. CQDs synthesized via the hydrothermal method achieved a rate of 423.7 µmol h⁻¹ g⁻¹ [164]. Enhancing the system, a CQDs/WO₃ composite, also prepared hydrothermally, significantly boosted the production rate to 1330 mmol h⁻¹ g⁻¹ [165]. Additionally, a CuO/CF/TiO₂ nanocomposite, synthesized through wet impregnation and calcination, achieved an impressive 2000 µmol h⁻¹ g⁻¹, showcasing the potential of these materials in efficient hydrogen generation [166].

4.2.3. Graphene

Incorporating nanomaterials within graphene layers enhances graphene’s properties, making it a prime candidate for photocatalytic nanocomposite fabrication. Graphene-based hybrids excel in oxygen and hydrogen evolution reactions. Metal-doped graphene exhibits a superior photocatalytic activity due to efficient charge separation. TiO₂’s stability, oxidative capacity, affordability, and low toxicity render it a prominent photocatalyst, albeit limited to UV light absorption. Integrating TiO₂ with graphene addresses challenges in adsorption, charge separation, and light absorption [145,167]. A hydrothermal-synthesized G/TiO₂ composite with 5% graphene demonstrated a hydrogen production rate of 86 mmol h⁻¹ g⁻¹ under UV–visible light irradiation without a cocatalyst [168]. Graphene/Cu₂O composites synthesized via CVD exhibit superior semiconducting properties and efficient hydrogen evolution. Cu₂O/doped graphene produces hydrogen at 19.5 mmol h⁻¹ under visible light without additives.

Pure CdS suffers from limitations in hydrogen production. Graphene/CdS clusters, synthesized solvothermally, exhibit an enhanced surface area and hydrogen evolution rate. Bismuth tungstate (Bi₂WO₆), with its suitable bandgap, is effective for organic pollutant degradation and shows promise for hydrogen generation. Graphene integration improves Bi₂WO₆ photocatalytic performance [169]. A graphene/TiO₂ composite demonstrated efficient hydrogen production under UV–visible light. Graphene/Cu₂O composites excel in hydrogen evolution. Cu₂O/doped graphene produces hydrogen under visible light without additives. Pt-TiO₂, GO-TiO₂, and ternary GO-Pt-TiO₂ hybrids have been synthesized for hydrogen generation. These are tabulated in Table 2.

The photocatalytic hydrogen production efficiencies of various graphene-based nanocomposites have been assessed using different light sources. A TiO₂ + Graphene composite exhibited a rate of 86 µmol h⁻¹ g⁻¹ under UV–visible light [170], while a CuO + Graphene composite, synthesized via pyrolysis, achieved 19.5 µmol h⁻¹ g⁻¹ under visible light [171]. A CdS + Graphene nanocomposite, prepared through the hydrothermal method, significantly outperformed with a rate of 175 µmol h⁻¹ g⁻¹ under visible light [172]. Additionally, a G/MoS₂/TiO₂ composite, also hydrothermally synthesized, achieved 165.3 µmol h⁻¹ g⁻¹ under a xenon arc lamp [168], and a graphene/CdS composite recorded a rate of 70 µmol h⁻¹ g⁻¹ under xenon lamp irradiation [159], highlighting the versatility and efficiency of graphene-based materials in photocatalytic applications.

4.2.4. Graphitic Carbon Nitride (g-C₃N₄)

Graphitic carbon nitride (g-C₃N₄) offers low toxicity, biocompatibility, environmental benignity, and scalable production. Its advantageous properties for photocatalytic hydrogen evolution (HER) include efficient light absorption, an ample surface area, favorable surface characteristics, robust elemental stability, and economic viability. The material’s structural stability enables diverse approaches for constructing g-C₃N₄ heterojunctions [173,174]. Despite combinatorial efforts, g-C₃N₄ exhibits a suboptimal photocatalytic performance due to rapid electron–hole recombination, limited visible-light absorption (460 nm cutoff), and surface hydrogen inhibition. The hydrogen production rates range from 0.1 to 4.0 mmol h⁻¹ g⁻¹. Nonetheless, g-C₃N₄ remains crucial for hydrogen generation, CO₂ reduction, pollutant degradation, and oxygen activation studies.

A TiO₂/g-C₃N₄ nanocomposite demonstrated a substantial increase in hydrogen production compared to pure g-C₃N₄. Ag-loaded g-C₃N₄ has emerged as a versatile photocatalyst, with Ag quantum dots/g-C₃N₄ composites exhibiting a significantly improved hydrogen evolution efficiency compared to pristine g-C₃N₄. Ag/g-C₃N₄ nanostructures have also shown promise in NOx removal. Tungsten oxide (WO₃) is a promising material due to its visible-light absorption, chemical stability, and electron mobility. It finds applications in photocatalysis, gas sensing, and electrochromic displays. WO₃ and g-C₃N₄ exhibit similar band structures. The hydrothermal synthesis of WO₃/g-C₃N₄ composites yields high hydrogen production rates. Further, enhancement is observed with WO₃/g-C₃N₄/Ni(OH)x and optimized WO₃/g-C₃N₄ nanomaterials, demonstrating the potential of WO₃-based composites for efficient hydrogen generation [175,176].

The hydrogen production efficiencies of various g-C₃N₄-based nanocomposites have been explored under 350 W xenon lamp irradiation. Pure g-C₃N₄, produced by chemical exfoliation, achieved a remarkable rate of 54.13 mmol h⁻¹ g⁻¹ [177]. The Ni₂P/0.5-Fe³⁺ doped g-C₃N₄ composite, synthesized via an in situ growth process, reached 397 µmol h⁻¹ g⁻¹ [178], while the g-C₃N₄/FexP composite, prepared by thermal treatment, yielded 83.2 µmol h⁻¹ g⁻¹ [179]. Additionally, the g-C₃N₄/CdS composite, fabricated through vapor deposition, showed a rate of 392.8 µmol h⁻¹ g⁻¹ [180]. A hybrid Ag/CN-TiO₂/g-C₃N₄ composite, created using a bamboo leaf-assisted thermal treatment, achieved 96 µmol h⁻¹ g⁻¹ under UV–LED light [181]. Finally, the g-C₃N₄/SiC composite, synthesized via in situ heating, produced hydrogen at a rate of 182 µmol h⁻¹ g⁻¹, illustrating the broad potential of g-C₃N₄-based materials in photocatalytic hydrogen generation [182].

Table 2. Carbon-based metal nanocomposites for H₂ production rate.

S. No.	Photocatalyst	Synthesis Method of Photocatalyst	Light Source	Performance of Photocatalyst	Reference
1.	ZnO/CNTs	Atomic layer deposition	UV–Visible	1 µmol h−1 g−1	[158]
2.	CNTs/CdS	Hydrothermal	Xe lamp	52 µmol h−1 g−1	[159]
3.	CuO/NiO/CNTs	Impregnation–calcination method	Visible	1 µmol h−1 g−1	[157]
4.	TiO2 + CNTs	Wet impregnation method	Solar	2134 µmol h−1 g−1	[160]
5.	TiO2(Rod)/MoS2	Solvothermal method	300 W Xe lamp	7415 μmol g−1	[143]
6.	MoS2–PSWCNT	Electrostatic restacking approach	400 W Xe lamp	7475 µmol h−1 g−1	[183]
7.	AgBr/bCNTs/TiO2	Deposition and reflux method	35 W HID Xe lamp	477 ppm	[184]
8.	ZnFe2O4 + CNT	Hydrothermal	Mercury Lamp	18 µmol h−1 g−1	[185]
9.	CNT/ g-C3N4	Calcination method	L9.ED lamps	654.8 µmol h−1 g−1	[186]
10.	FCNTs/TiO2	Wet impregnation method	Solar	13107 μmmol h−1 g−1	[187]
11.	g-C3N4 + graphene	Impregnation method	350 W Xe lamp	0.451 mmol h−1 g−1	[100]
12.	G/MoS2/TiO2	Hydrothermal	Xenon arc lamp	165.3 µmol h−1 g−1	[168]
13.	Graphene/CdS	Hydrothermal	Xenon lamp	70 µmol h−1 g−1	[159]
14.	g-C3N4	Chemical exfoliation	350 W Xe lamp	54.13 mmol h−1 g−1	[177]
15.	Ni2P/0.5-Fe3+ doped g-C3N4	In-situ growth process	350 W Xe lamp	397 µmol h−1 g−1	[178]
16.	g-C3N4/FexP	Thermal Treatment Process	350 W Xe lamp	83.2 µmol h−1 g−1	[179]
17.	g-C3N4/CdS	Vapor deposition method	350 W Xe lamp	392.8 µmol h−1 g−1	[180]
18.	Ag/CN-TiO2/g-C3N4	Bamboo leaf-assisted thermal treatment	UV–LEDs (3 W, 420 nm) 80.0 mW cm−2 and 1 cm2	96 µmol h−1 g−1	[181]
19.	g-C3N4/SiC	In situ heating	350 W Xe lamp	182 µmol h−1 g−1	[182]
20.	TiO2 + Graphene	-	UV–Visible	86 µmol h−1 g−1	[170]
21.	CuO + Graphene	Pyrolysis	Visible	19.5 µmol h−1 g−1	[171]
22.	CdS + Graphene	Hydrothermal	Visible	175 µmol h−1 g−1	[172]
23.	Pt-TiO2 + rGO	Photocatalytic reduction	Solar	1075.6 µmol h−1 g−1	[188]
24.	LaNiO3 + rGO	Photocatalytic reduction	UV–Visible	3.22 mmol h−1 g−1	[189]
25.	CQDs	Hydrothermal	Xenon lamp	423.7 µmol h−1 g−1	[164]
26.	CQDs/WO3	Hydrothermal	Xenon lamp	1330 mmol h−1 g−1	[165]
27.	CuO/CF/TiO2	Wet impregnation and calcination process	Xenon lamp	2000 µmol h−1 g−1	[166]

Table 2. Carbon-based metal nanocomposites for H₂ production rate.

S. No.	Photocatalyst	Synthesis Method of Photocatalyst	Light Source	Performance of Photocatalyst	Reference
1.	ZnO/CNTs	Atomic layer deposition	UV–Visible	1 µmol h−1 g−1	[158]
2.	CNTs/CdS	Hydrothermal	Xe lamp	52 µmol h−1 g−1	[159]
3.	CuO/NiO/CNTs	Impregnation–calcination method	Visible	1 µmol h−1 g−1	[157]
4.	TiO2 + CNTs	Wet impregnation method	Solar	2134 µmol h−1 g−1	[160]
5.	TiO2(Rod)/MoS2	Solvothermal method	300 W Xe lamp	7415 μmol g−1	[143]
6.	MoS2–PSWCNT	Electrostatic restacking approach	400 W Xe lamp	7475 µmol h−1 g−1	[183]
7.	AgBr/bCNTs/TiO2	Deposition and reflux method	35 W HID Xe lamp	477 ppm	[184]
8.	ZnFe2O4 + CNT	Hydrothermal	Mercury Lamp	18 µmol h−1 g−1	[185]
9.	CNT/ g-C3N4	Calcination method	L9.ED lamps	654.8 µmol h−1 g−1	[186]
10.	FCNTs/TiO2	Wet impregnation method	Solar	13107 μmmol h−1 g−1	[187]
11.	g-C3N4 + graphene	Impregnation method	350 W Xe lamp	0.451 mmol h−1 g−1	[100]
12.	G/MoS2/TiO2	Hydrothermal	Xenon arc lamp	165.3 µmol h−1 g−1	[168]
13.	Graphene/CdS	Hydrothermal	Xenon lamp	70 µmol h−1 g−1	[159]
14.	g-C3N4	Chemical exfoliation	350 W Xe lamp	54.13 mmol h−1 g−1	[177]
15.	Ni2P/0.5-Fe3+ doped g-C3N4	In-situ growth process	350 W Xe lamp	397 µmol h−1 g−1	[178]
16.	g-C3N4/FexP	Thermal Treatment Process	350 W Xe lamp	83.2 µmol h−1 g−1	[179]
17.	g-C3N4/CdS	Vapor deposition method	350 W Xe lamp	392.8 µmol h−1 g−1	[180]
18.	Ag/CN-TiO2/g-C3N4	Bamboo leaf-assisted thermal treatment	UV–LEDs (3 W, 420 nm) 80.0 mW cm−2 and 1 cm2	96 µmol h−1 g−1	[181]
19.	g-C3N4/SiC	In situ heating	350 W Xe lamp	182 µmol h−1 g−1	[182]
20.	TiO2 + Graphene	-	UV–Visible	86 µmol h−1 g−1	[170]
21.	CuO + Graphene	Pyrolysis	Visible	19.5 µmol h−1 g−1	[171]
22.	CdS + Graphene	Hydrothermal	Visible	175 µmol h−1 g−1	[172]
23.	Pt-TiO2 + rGO	Photocatalytic reduction	Solar	1075.6 µmol h−1 g−1	[188]
24.	LaNiO3 + rGO	Photocatalytic reduction	UV–Visible	3.22 mmol h−1 g−1	[189]
25.	CQDs	Hydrothermal	Xenon lamp	423.7 µmol h−1 g−1	[164]
26.	CQDs/WO3	Hydrothermal	Xenon lamp	1330 mmol h−1 g−1	[165]
27.	CuO/CF/TiO2	Wet impregnation and calcination process	Xenon lamp	2000 µmol h−1 g−1	[166]

4.3. CO₂ Reduction

Accelerated industrialization and population growth have intensified CO₂ emissions, exacerbating global warming. While renewable energy sources offer potential alternatives, the prevailing reliance on fossil fuels necessitates innovative CO₂ mitigation strategies. Photocatalysis, utilizing solar energy to convert CO₂ into value-added products, presents a sustainable and promising approach compared to energy-intensive carbon capture and storage methods [190]. Developing photocatalysts with efficient light absorption, charge separation, and suitable redox potential remains a significant challenge [191]. While various visible-light-active materials have been explored, practical applications are often hindered by factors like instability, photo corrosion, and toxicity. Consequently, carbon-based metal oxides have emerged as a promising class of photocatalysts (Table 3).

4.3.1. CNT

The impact of CNT incorporation and copper doping on the photocatalytic performance of TiO₂-based nanocomposites synthesized via supercritical fluid methods was examined. The results indicated a pronounced enhancement in CO₂ photoreduction compared to conventional TiO₂. While the CNT/TiO₂ ratio exhibited a critical impact on catalytic activity, the effect of copper doping was less pronounced. The optimized composite demonstrated significantly elevated CO and CH₄ production rates relative to commercial products, suggesting the potential of this approach for improved CO₂ conversion [192,193]. CNT/TiO₂ composites have demonstrated enhanced photocatalytic CO₂ reduction capabilities. CNTs act as efficient electron acceptors and transporters, prolonging the lifetime of photogenerated charge carriers and boosting overall photocatalytic activity. Studies have shown that CNT/TiO₂/Ni catalysts outperform both TiO₂/Ni and TiO₂ alone in CO₂ photoreduction under visible-light conditions [194,195].

The catalytic performance of various CNT-based nanocomposites for the production of diverse chemical fuels has been studied using different synthesis methods. A AgBr/CNT composite, prepared via the deposition–precipitation method, was effective in producing methane, methanol, CO, and ethanol [196]. A CNT/TiO₂ composite, synthesized using the supercritical method, achieved production rates of 8.1 μmol g⁻¹ h⁻¹ for CO and 1.1 μmol g⁻¹ h⁻¹ for CH₄ [197]. The CoP/CNT composite, created through an in situ method, demonstrated an impressive CO production rate of 19,755 μmol g⁻¹ h⁻¹ [198]. Additionally, a ZnO/CuO/CNT composite, produced via the hydrothermal method, facilitated the generation of ethanol, oxalic acid, and formaldehyde, showcasing the versatility and effectiveness of these CNT-based materials in catalytic applications [199].

4.3.2. Carbon Dots

Carbon dots (CDs) exhibit versatile photocatalytic capabilities, acting as standalone catalysts or components within complex systems. Their optoelectronic properties enable enhanced light absorption, charge separation, and system stability. Unlike other metal-free carbon materials, CDs can catalyze CO₂ conversion under specific structural conditions. Graphitic carbon nitride (g-C₃N₄), while chemically stable and tunable, exhibits limited CO₂ adsorption and suffers from rapid electron–hole recombination [200]. Integrating carbon dots (CDs) into g-C₃N₄ composites enhances charge carrier separation. Song et al. pioneered the construction of a g-C₃N₄/CD heterojunction, demonstrating a significant increase in methane and carbon monoxide production attributed to efficient electron transfer from g-C₃N₄ to CDs.

CD-Cu₂O composites exhibit enhanced stability and light absorption compared to pure Cu₂O. CDs act as hole acceptors, facilitating water oxidation. Incorporating a carbon layer further improves light absorption, protects the semiconductor, and enhances electron transfer, leading to increased methanol production and selectivity in CO₂ photoreduction. Mengli Li et al. demonstrated the integration of carbon dots (CDs) with TiO₂ to extend light absorption beyond the UV spectrum [105,201]. N,S-doped CDs, synthesized using thiourea, were employed to further enhance photocatalytic activity by introducing N, O, and S functionalities [202]. These CDs acted as electron reservoirs, capturing photoexcited electrons from TiO₂ and facilitating CO₂ reduction to CO and CH₄ under visible and near-infrared light irradiation.

The hydrothermal synthesis of carbon dots (CDs)-based nanocomposites has demonstrated notable catalytic activity for the production of various chemical fuels. The N,S-CDs/TiO₂ composite efficiently generated CO and CH₄ [202], while the CL@CDs/Cu₂O nanocomposite produced both CH₄ and CH₃OH [203]. Additionally, the CDs/Ag composite was specifically effective in generating CH₃OH [204]. These results highlight the potential of CDs-based materials in the catalytic production of valuable chemical fuels.

4.3.3. Graphene

Graphene’s exceptional electronic and thermal properties, coupled with its tunable bandgap in the oxidized form (graphene oxide/reduced graphene oxide), have positioned it as a promising photocatalyst for CO₂ conversion. Initial studies demonstrated graphene oxide’s ability to convert CO₂ to methanol, albeit with low yields. To enhance efficiency, researchers have explored the integration of visible-light-absorbing metal complexes onto graphene oxide, resulting in improved CO₂-to-organic/hydrocarbon conversion [205]. TiO₂-graphene oxide (GO)/reduced graphene oxide (rGO) composites have shown enhanced visible-light absorption and charge separation compared to pure TiO₂. However, achieving effective GO/rGO-TiO₂ nanotube contact has been challenging due to nanotube morphology. Recent studies employing multi-leg TiO₂ nanotubes demonstrated successful GO/rGO wrapping, leading to an increased photocurrent and reduced charge-transfer resistance [206].

Several studies have explored the potential of TiO₂-graphene composites for CO₂ photoreduction. TiO₂-graphene nanosheets have demonstrated selective ethane formation, while boron-doped graphene-TiO₂ nanoparticles have shown enhanced CO₂ reduction activity [191]. Graphene oxide-functionalized TiO₂ and graphene-coated Pt-TiO₂ nanotubes have yielded methane as a primary product. Additionally, faceted TiO₂ nanocrystals supported on graphene exhibited an improved CO₂ conversion efficiency, with CO as the primary product. Graphene-TiO₂ hybrids have also been reported to produce methanol and formic acid under UV irradiation [207].

In Table 3, graphene oxide, synthesized via chemical exfoliation, has shown effective catalytic activity for the production of methanol (CH₃OH) [208]. This demonstrates the potential of graphene oxide as a valuable material in catalytic processes for chemical fuel generation.

4.3.4. Graphitic Carbon Nitride (g-C₃N₄)

Graphitic carbon nitride offers significant advantages for photocatalytic CO₂ reduction compared to its 0D and 1D counterparts. Its unique layered structure, large surface area, and tunable bandgap facilitate efficient light absorption and charge carrier separation. The reduced dimensionality enhances charge transport and minimizes recombination losses. Additionally, the abundance of surface defects and exposed active sites contribute to an improved photocatalytic performance [209]. The integration of Ag and B-P dopants into SnO₂/g-C₃N₄ composites enhanced photocatalytic CO₂ reduction, with Ag exhibiting superior electron-accepting properties compared to ZnO and TiO₂. This combination extended light absorption to the visible spectrum, significantly boosting CH₄ production [66]. Mn-based oxides, known for their catalytic activity and low toxicity, have also been coupled with g-C₃N₄ to enhance CO and CH₄ yields through a novel supra-molecular synthesis approach [210].

g-C₃N₄-based nanocomposites offer versatile applications. In₂O₃/g-C₃N₄ hybrids exhibit enhanced photocatalytic hydrogen generation and CO₂ reduction due to efficient charge transfer [190,211]. The optimized composite demonstrated significantly improved methane production compared to pristine materials, attributed to suppressed charge recombination as evidenced by reduced photoluminescence.

The catalytic performance of g-C₃N₄-based nanocomposites has been explored for CO and CH₄ production. The g-C₃N₄/OMC–CNT composite, synthesized using a US-assisted method, achieved production rates of 25.1 μmol g⁻¹ h⁻¹ for CO and 14.7 μmol g⁻¹ h⁻¹ for CH₄ [212]. Similarly, the g-C₃N₄/carbon nanosheets composite, prepared via the hydrothermal method, was also effective in generating CO and CH₄, highlighting the efficiency of these materials in catalytic applications [175].

Table 3. Carbon-based metal nanocomposite for CO₂ reduction.

S. No.	Photocatalyst	Synthesis Method of Photocatalyst	Yield Product	Reference
1.	AgBr/CNT	Deposition–precipitation method	Methane, methanol, CO, and ethanol	[196]
2.	CNT/TiO2	Supercritical method	8.1 μmol g−1 h−1 CO and 1.1 μmol g−1 h−1 CH4	[197]
3.	CoP/CNT	In situ method	19,755 μmol g−1 h−1 CO	[198]
4.	ZnO/CuO/CNT	Hydrothermal method	Ethanol, oxalic acid, and formaldehyde	[199]
5.	g-C3N4/OMC–CNT	US-assisted method	25.1 μmol g−1 h−1 CO and 14.7 μmol g−1 h−1 CH4	[212]
6.	CNT–TiO2	Sonothermal method/hydrothermal	2360 μmol g−1 h−1 methanol, 3246.1 μmol g−1 h−1 hydrogen, and 68.5 μmol g−1 h−1 formic acid	[198]
7.	Ag@AgBr/CNT	Deposition–precipitation/photoreduction	Methane, CO, methanol, and ethanol	[213]
8.	g-C3N4/carbon nanosheets	Hydrothermal	CO, CH4	[175]
9.	Graphene Oxide	Chemical exfoliation	CH3OH	[208]
10.	N,S-CDs/TiO2	Hydrothermal	CO, CH4	[202]
11.	CL@CDs/Cu2O	Hydrothermal	CH4, CH3OH	[203]
12.	CDs/Ag	Hydrothermal	CH3OH	[204]

Table 3. Carbon-based metal nanocomposite for CO₂ reduction.

S. No.	Photocatalyst	Synthesis Method of Photocatalyst	Yield Product	Reference
1.	AgBr/CNT	Deposition–precipitation method	Methane, methanol, CO, and ethanol	[196]
2.	CNT/TiO2	Supercritical method	8.1 μmol g−1 h−1 CO and 1.1 μmol g−1 h−1 CH4	[197]
3.	CoP/CNT	In situ method	19,755 μmol g−1 h−1 CO	[198]
4.	ZnO/CuO/CNT	Hydrothermal method	Ethanol, oxalic acid, and formaldehyde	[199]
5.	g-C3N4/OMC–CNT	US-assisted method	25.1 μmol g−1 h−1 CO and 14.7 μmol g−1 h−1 CH4	[212]
6.	CNT–TiO2	Sonothermal method/hydrothermal	2360 μmol g−1 h−1 methanol, 3246.1 μmol g−1 h−1 hydrogen, and 68.5 μmol g−1 h−1 formic acid	[198]
7.	Ag@AgBr/CNT	Deposition–precipitation/photoreduction	Methane, CO, methanol, and ethanol	[213]
8.	g-C3N4/carbon nanosheets	Hydrothermal	CO, CH4	[175]
9.	Graphene Oxide	Chemical exfoliation	CH3OH	[208]
10.	N,S-CDs/TiO2	Hydrothermal	CO, CH4	[202]
11.	CL@CDs/Cu2O	Hydrothermal	CH4, CH3OH	[203]
12.	CDs/Ag	Hydrothermal	CH3OH	[204]

5. Mechanism behind the Catalysis Used in the Degradation of Dyes, Hydrogen Production, and CO₂ Reduction

5.1. Degradation of Dye

In the context of dye effluent treatment, photocatalysis plays a crucial role. Light irradiation triggers the excitation of electrons from the valence band (VB) of the photocatalyst to the conduction band (CB). This process results in the generation of electron–hole pairs. The dye is totally broken down into nonhazardous byproducts (CO₂, H₂O, etc.) by a powerful oxidizing agent, the hydroxyl radical, which is produced as shown in Figure 11 [44,214,215,216]. There are three fundamental phases in the basic photocatalytic process, which include the following:

(i)

The photocatalyst absorbs light energy and creates a photoexcited electron–hole pair as the system’s charge carriers.

(ii)

The separation and passage of charge carriers.

(iii)

The charge carriers and surface species’ chemical interactions.

Light absorption exceeding the bandgap energy (Eg) of a photocatalyst excites electrons from the VB to the CB, creating electron–hole pairs. O₂ is produced when photo-oxidizing H₂O molecules with VB’s holes, while H₂ is created when photo-reducing H⁺ with a CO₂ molecule. In photocatalysis, achieving efficient conversion relies on the appropriate alignment of the photocatalyst’s band edge potentials with the redox potentials of the target reaction. Ideally, the valence band (VB) edge potential (E_VB) should be positioned sufficiently negative relative to the oxidation potential (E_ox) of the universal solvent (typically water). Conversely, the conduction band (CB) edge potential (E_CB) should be more positive than the reduction potential (E_red) required for the desired product formation, all at the relevant operating pH. The adsorbed dye is decreased when positive holes formed by electron excitation take an electron from them. Furthermore, additional free radicals that are generated by the oxidation of H⁺ ions that are released when water molecules split go on to damage the dye. The single-electron reduction of CO₂ to CO^• radicals requires a highly negative reduction potential of approximately −1.9 eV. This poses a significant thermodynamic barrier due to the substantial energy input necessary to overcome this potential [217,218]. The rapid rate of electron–hole recombination has prevented this approach from developing photocatalysts with the necessary high activity and stability despite years of research and numerous studies in the field. In a photocatalytic process, approximately 90% of the electron–hole pairs generated upon illumination undergo recombination within a 10-nanosecond timeframe. Therefore, to overcome the above-mentioned disadvantages, photocatalytic reduction is enhanced by the inclusion of carbon compounds.

Despite the well-established efficiency of adsorption for extracting heavy metal ions from wastewater effluents, its applicability can be hindered in real-world environmental scenarios with complex matrices. These limitations are particularly evident in soil remediation efforts, river and lake pollution control, and the treatment of mine tailings. Metal ions can replace heavy metal ions to reduce the pollution caused by them, and metal ions can also be solidified and immobilized. Photocatalytic reduction, where a high-valent metal ion is exposed to visible light and undergoes reduction to a lower-valent state, serves as a highly effective method for precipitation and immobilization. Semiconductor heterojunction arrangements, classified into three categories based on their bandgap energies, play a crucial role in this process. In the Type 1, or straddling gap, heterojunction configuration, the CB minimum of Semiconductor 2 lies at a more negative potential compared to the CB minimum of Semiconductor 1. Conversely, the VB maximum of Semiconductor 2 is positioned at a more positive potential than the VB maximum of Semiconductor 1. Based on the principles of charge carrier transport, this band alignment facilitates the accumulation of photogenerated electrons within the material possessing the narrower bandgap. Because of this, a smaller Eg could lead to charge carrier recombination, which would lower the photocatalytic performance. In the staggered gap (Type 2) heterojunction configuration, the CB minimum of Semiconductor 2 resides at a lower energy level compared to the CB minimum of Semiconductor 1. Conversely, the VB maximum of Semiconductor 1 is positioned at a higher energy level than the VB maximum of Semiconductor 2. This band alignment promotes the directional transfer of photogenerated charge carriers. Electrons from the higher-energy CB of Semiconductor 2 migrate towards the lower-energy CB of Semiconductor 1. Conversely, holes in the VB of Semiconductor 1 can move to the more positive VB of Semiconductor 2. This spatial separation of electrons and holes in Type 2 heterojunctions leads to more efficient suppression of electron–hole recombination compared to Type 1 configurations. In the broken gap (Type 3) heterojunction configuration, both the CB minimum and VB maximum of Semiconductor 1 are located at lower energy levels compared to their counterparts in Semiconductor 2. This band alignment creates an insurmountable energetic barrier at the interface, effectively prohibiting the transfer of photogenerated electrons and holes between the two semiconductors. Consequently, Type 2 (staggered gap) heterojunctions are the most commonly employed configuration among the three types due to their efficient spatial separation of charge carriers, as previously discussed [219,220]. This design fosters the efficient separation of photogenerated electron–hole pairs, thereby suppressing their recombination during the charge-transfer process. This ultimately leads to an enhancement in the photocatalytic reaction efficiency.

5.2. Hydrogen Production

Photocatalysis is initiated by the absorption of photons by a semiconductor photocatalyst. Solar irradiation induces the excitation of electrons from the valence band to the conduction band, generating electron–hole pairs. Efficient charge separation and the migration of these carriers within the semiconductor matrix facilitate their interaction with target reagents. Effective photocatalysis mandates a semiconductor bandgap exceeding 1.23 eV, efficient charge carrier separation, and abundant surface active sites. For water splitting, the CB minimum must be below water’s reduction potential and the VB maximum above its oxidation potential. Photocatalysis comprises light absorption generating electron–hole pairs and their subsequent separation and migration, culminating in redox reactions producing hydrogen and oxygen (Figure 12). This endothermic process requires a Gibbs free energy input of 273.2 kJ mol⁻¹, equivalent to 1.23 eV per electron transferred [221,222]. For hydrogen evolution, the semiconductor CB must be more negative than the H⁺/H₂ redox potential. Conversely, oxygen evolution necessitates a VB more positive than the O₂/H₂O electrode potential (1.23 eV).

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2\left(\mathrm{g}\right)+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2\left(\mathrm{g}\right)$$

where ΔG = +273 kJ mol⁻¹.

Electron–hole recombination significantly hinders photocatalytic efficiency. Carbon-based metal oxides can enhance charge separation by rapidly transporting photoexcited electrons from the semiconductor conduction band. This exhibits superior electron affinity compared to most semiconductors, facilitating electron trapping or transfer to active sites. Prominent carbon-based metal oxides include graphene oxide, reduced graphene oxide, carbon nanotubes, carbon dots, and graphitic carbon nitride. Introducing a Schottky barrier at the metal–carbon-based metal oxide interface further promotes charge separation. The Fermi level disparity between metal catalysts and carbon-based metal oxide semiconductors drives charge transfer until equilibrium, inducing band bending and Schottky barrier formation [153]. This barrier effectively suppresses electron recombination, enhancing electron availability for hydrogen evolution.

5.3. Carbon Dioxide Reduction

The initial reduction of CO₂ to CO₂⁻ is thermodynamically unfavorable due to its highly negative reduction potential. Consequently, generating this intermediate is challenging without a photocatalyst possessing an even more negative potential. Conversely, the subsequent proton-coupled electron reduction steps for CO₂ conversion are more feasible due to less negative reduction potentials. Photocatalytic CO₂ reduction can yield a range of products, from formic acid and carbon monoxide to methanol and methane, through multi-electron reduction processes (

Table 4. The reactions undergone in the CO₂ reduction.

S. No.	Reaction	E (V) vs. NHE (pH 7)
1	CO2 + e− → CO2•−	−1.9
2	CO2 + 2e− + 2H+ → HCOOH	−0.61
3	CO2 + 2e− + 2H+ → CO + H2O	−0.53
4	CO2 + 4e− + 4H+ → HCHO + H2O	−0.48
5	CO2 + 6e− + 6H+ → CH3OH + H2O	−0.38
6	CO2 + 8e− + 8H+ → CH4 + 2H2O	−0.24

) [223,224,225].

The mechanism for the CO₂ reduction is shown in Figure 13. The effectiveness of the nanomaterial is based on CO₂ photocatalysts, which require several key steps. Initially, CO₂ molecules adsorb onto the active sites. Subsequently, light-induced electron–hole pair generation occurs, with electrons migrating to the catalyst’s active sites for CO₂ reduction [226,227]. The carbon-based products are desorbed from the nanocomposite surface. Simultaneously, sacrificial agents consume holes to prevent electron–hole recombination.

6. Factors Affecting the Carbon-Based Metal Oxides Nanocomposites as Catalysts

6.1. Dye Degradation

Several factors govern the rate and efficacy of photocatalytic dye degradation within a photocatalytic system. These factors include photocatalyst loading, solution pH, initial dye concentration, light intensity and irradiation time, reaction temperature, and dopant concentration. The optimal values of these parameters exhibit a high system dependency. Their variation is primarily influenced by the following factors: substrate characteristics, photocatalyst properties, and crystal structures.

6.1.1. Effect of pH

Solution pH plays a critical role in photocatalytic dye degradation. Variations in pH can modify the surface charge of the metal or metal oxide photocatalyst. This, in turn, affects the position of the CB and VB edges, consequently altering the reduction and oxidation potentials that govern the photocatalytic reaction [228,229]. The rate of a photocatalytic dye degradation reaction is heavily influenced by the adsorption behavior of the dye molecule onto the photocatalyst surface. Alterations in the dye’s surface adsorption characteristics can significantly impact the reaction rate. It is generally known that the pH value affects the rate at which various organic compounds degrade in photocatalytic reactions. The surface of a metal or metal oxide photocatalyst can undergo protonation or deprotonation depending on the solution’s pH. In order to better understand how the contaminant degrades, multiple pH values (between 3 and 11.5) were investigated. Under acidic conditions, photogenerated holes (h+) are believed to be the primary oxidizing species responsible for dye degradation. These holes can directly react with adsorbed dye molecules or indirectly react with surface-bound hydroxide (OH⁻) ions to generate hydroxyl radicals (OH•), which are highly reactive oxidants that contribute significantly to the degradation process. However, at higher pH values, electrostatic repulsion arises between the negatively charged photocatalyst surface and hydroxide anions. This Coulombic repulsion hinders the adsorption of OH⁻ ions onto the catalyst surface, thereby impeding the formation of OH^• radicals and consequently diminishing the photocatalytic degradation efficiency [230,231]. Due to its many functions, interpreting how pH affects the effectiveness of the dye photodegradation process is particularly challenging.

6.1.2. Effect of Dopant Concentration

Beyond the influence of photocatalyst heaping, the dopant concentration plays a distinct and critical role in photocatalytic dye degradation. This effect extends to the degradation efficiency of both anionic and cationic dyes, exhibiting a vibrant dependence on the dopant concentration [232]. The effective volume of dopants for the active performance of the photocatalytic dye degradation is determined, as per previous research, by taking different quantities of dopants and investigating them individually each time while maintaining a constant dye concentration. Deviations from the optimal dopant concentration can lead to a decline in the photocatalytic activity of the material. The catalytic activity improves as the number of dopants is increased for the following reasons: Both (i) vacancies and (ii) photoinduced electron/hole recombination centers are necessary for the atom to occupy additional oxygen. As a result, it is noted that raising the volume of dopants results in efficient photocatalytic dye degradation; nevertheless, the same is improved by ~4% when determining the ideal volume of dopants [233,234]. In order to increase the photocatalytic activity, it is implied that the same procedure should be followed to adjust and find the ideal level of dopants individually.

6.1.3. Impact of Photocatalyst Dose

Different photocatalysts have different lattice mismatches, which affects their photocatalytic activity. When more photocatalysts are present, the dye typically degrades more quickly in heterogeneous catalysis. The introduction of dopants can enhance photocatalytic activity by creating more active sites on the photocatalyst surface, facilitating the generation of OH^• and O₂^•−. These radicals play a crucial role in degradation, leading to the discoloration of the dye solution. However, exceeding the optimal dopant concentration can lead to a detrimental effect. Further increasing the photocatalyst amount beyond the optimal level can exacerbate the decline in photocatalytic activity. It raises the turbidity of the solution, which finally blocks light irradiation and lowers the amount of the relevant compound that is photoactivated [235,236]. Below the optimal dopant concentration threshold, the photocatalytic degradation performance suffers significantly. This decline can be attributed to the insufficient generation of reactive species. At the peak absorption wavelength of the photocatalyst, a greater number of photons are absorbed, leading to the enhanced production of reactive species. Another mechanism responsible for decreased dye degradation with increasing photocatalyst loading is the phenomenon of nanoparticle clusters. This aggregation reduces the effective surface area available for dye adsorption and subsequent photocatalytic reactions [237,238]. Optimizing the photocatalyst concentration is crucial. Using an excessive amount can lead to agglomeration and hinder performance, ultimately increasing the cost of the photocatalytic process. Unfavorable light scattering and minimal light penetration into the solution are seen when more photocatalyst is supplied. This phenomenon can be attributed to two opposing forces: electrostatic repulsion and confinement effects. The negatively charged surface of the photocatalyst can lead to electrostatic repulsion between the dye molecules, which are often also negatively charged. However, the relatively small pores within the catalyst can restrict the movement of the dye molecules, hindering their escape. In order to achieve a prospective photocatalytic activity, it is therefore important to determine the ideal concentration of photocatalytic loading in each activity.

6.1.4. Effects of Dye Initial Concentration

The photocatalytic degradation efficiency of a dye is directly correlated with the amount of dye adsorbed onto the photocatalyst surface. In simpler terms, only the dye molecules adsorbed onto the catalyst can actively participate in the photocatalytic degradation process, not the dye molecules remaining in the bulk solution. The initial concentration of the dye plays a critical role in photocatalytic degradation due to its direct influence on dye adsorption. Only the dye molecules adsorbed onto the photocatalyst surface can be degraded, highlighting the importance of optimizing the initial dye concentration for efficient photocatalytic treatment [228,239]. A constant concentration of photocatalyst and an increase in dye concentration can generally be used to lower the degradation percentage. The degradation percentage drops as the dye concentration rises because more dye molecules attach to the photocatalyst’s surface. High initial dye concentrations can impede degradation. They act as shields, absorbing light and reducing the production of OH^• radicals. Additionally, more catalysts may be needed, potentially causing light blockage or agglomeration, further limiting reactive species and hindering dye breakdown. A higher dye concentration demands more dye molecules to be excited and degrade [144,240,241,242,243,244]. At low dye concentrations, however, a contentious effect of reaction inhibition is produced by catching photons on their catalyst, which results in the creation of a large number of OH^• radicals. To efficiently carry out photocatalytic activity, the impact of the initial dye concentration must be investigated, and the ideal level must be chosen.

6.1.5. Importance of Surface Area on Photocatalyst Degradation

When the surface area of the nanomaterials is huge, their photocatalytic activity is improved. A decreased nanoparticle size translates to a higher surface area per unit volume. This enhanced surface area facilitates improved dispersion within a solution and strengthens the interaction between the nanoparticles and contaminant molecules. As a result, they become more effective in photocatalysis reactions.

Since all chemical reactions take place at the nanomaterial’s surface, surface shape must be taken into account when using nanomaterials as a photocatalyst. There have been attempts to enhance this surface area, usually through the use of minuscule particles suspended in liquids or shaped into porous films. Nanostructured materials with crystallite or grain sizes below 20 nm exhibit unique properties that deviate significantly from their bulk counterparts, making them a subject of intense scientific interest. This has also created opportunities for their use as a photocatalyst in a variety of contexts. The growth in the photocatalyst’s surface area is greatly influenced by the calcination temperature [245,246]. The particle size of the NPs rose when the calcination temperature was reduced, which in turn raised the surface area and eventually improved the photocatalytic activity. High surface area materials possess a greater abundance of active sites compared to their low surface area counterparts. This translates to an increased number of reaction sites for photocatalytic processes, ultimately leading to enhanced photocatalytic activity.

6.1.6. Impact of Light Intensity and Irradiation Time

The photocatalytic dye degradation rate exhibits a dependence on both irradiation time and light intensity. Studies have shown that at moderate light intensities (25 mW cm⁻²), the reaction follows a half-order dependence on the light intensity, whereas under low light intensities (0–20 mW cm⁻²), the rate exhibits a first-order linear dependence on increasing light intensity [247,248,249]. The photocatalytic dye degradation rate exhibits light intensity dependence. Under low light conditions, the rate follows a first-order kinetic relationship due to the dominance of electron–hole generation. As light intensity increases, electron–hole recombination becomes more competitive, leading to a transition to a zero-order dependence on light intensity.

Pseudo-first-order kinetic models predict a decrease in reaction rate with extended exposure time. This slowdown can be attributed to two primary factors: competition between the target dye molecules and accumulated intermediate products for the limited pool of hydroxyl radicals (OH^•) and the gradual deactivation of the photocatalyst’s active sites due to the deposition of strongly bound byproducts from the degradation process [250,251]. While photocatalytic activity can be enhanced with longer irradiation, this trend is not always linear and may be limited by factors like competition for reactive species and photocatalyst deactivation.

6.1.7. Temperature Effect

Temperature affects photocatalytic activity like a volcano. Higher temperatures generally improve activity, but too high a temperature leads to more recombination and less dye degradation. Conversely, very low temperatures also hinder the process. The photocatalytic process achieves maximum dye adsorption efficiency within a temperature window of 20–80 °C [252,253,254]. Photodegradation was found to be most efficient at high temperatures, whereas dye photodegradation was shown to be least efficient at low temperatures. This is because the dye molecules interact more and more with the photocatalyst surface. As a result, the adsorption rises, which favors the dye photodegradation.

6.2. In Hydrogen Production and CO₂ Reduction

Optimizing carbon-based photocatalyst performance for hydrogen evolution and CO₂ reduction necessitates a comprehensive understanding of the influential parameters. The carbon material dimensions, defect types, layer count, functional groups, conductivity, and interfacial interactions with semiconductors significantly impact photocatalytic activity. Strategic manipulation of these attributes is crucial for maximizing hydrogen production.

6.2.1. Defects

Nanomaterials exhibit defects at various scales, including point, line, plane, and volume defects. Point defects, such as vacancies, interstitials, and impurities, further categorize these structural imperfections. Defect-free photocatalysts are practically unattainable. These defects influence the electrical and surface properties, thereby impacting the charge distribution and photocatalytic activity. Defect-induced active sites serve as catalytic centers. Oxygen functionalities and other non-metallic groups within carbon-based metal oxides potentially enhance hydrogen generation capabilities [255,256]. Advanced carbon-based metal oxides often exhibit intrinsic defects, and graphene’s emerging properties suggest a similar defect-dependent photocatalytic behavior, though research is limited. Developing precise defect engineering methodologies remains crucial. While annealing and etching have been employed to induce defects, correlating specific defects to photocatalytic performance is hindered by the lack of precise control over defect generation [257,258].

6.2.2. Doping

Heteroatom doping enhances catalytic activity by introducing structural defects and modifying the physicochemical properties. Doping alters the electrical conductivity, electronic structure, and charge distribution within carbon-based metal oxides. These changes impact the surface area, active sites, and charge-transfer kinetics, influencing overall photocatalytic performance [259,260]. Despite advancements, precise atomic-level characterization and manipulation of catalytic centers remain challenging. Non-metallic heteroatom doping is the primary focus for enhancing photocatalytic hydrogen generation [261]. This process introduces defects and vacancies within the metal oxide nanomaterial structure. Identifying the specific heteroatoms, vacancies, or defects responsible for catalytic activity remains a significant challenge [262].

6.2.3. Dimensions

Photocatalyst efficiency is strongly correlated with material size. Zero-dimensional (0D) carbon nanomaterials, such as carbon quantum dots (CQDs), exhibit both photocatalytic and cocatalytic properties. While CQDs demonstrate strong light absorption and scattering, size reduction can exacerbate charge recombination. To mitigate this, CQD-based nanocomposites offer a promising strategy. One-dimensional (1D) CNTs surpass their 0D counterparts due to their higher surface area and enhanced light interaction, providing abundant active sites for photocatalytic processes [50,263]. CNTs exhibit efficient charge transfer, enhancing hydrogen evolution rates. However, their practical application necessitates optimized production and utilization methods. Two-dimensional (2D) graphene offers a superior photochemical stability, surface area, and mechanical and optical properties, positioning it as a promising candidate for advanced photocatalytic systems. While challenges in enhancing hydrogen generation efficiency and mass transfer persist, graphene surpasses its carbon dots and CNT counterparts due to its unique properties. Its ability to modulate band structure and efficiently absorb and scatter light contributes to improved charge transfer and hydrogen evolution [45,264,265]. The thickness of graphene significantly impacts photoinduced electron mobility, highlighting its critical role in optimizing photocatalytic performance.

7. Conclusions and Future Outlook

This review explores the diverse applications of carbon nanostructures (fullerenes, CNTs, CDs, diamond, graphene, g-C₃N₄) as catalyst supports. Each of these materials also offers unique properties and benefits. Additionally, we emphasize their outstanding application in the field of visible-light irradiated catalysis research, as well as their most recent application in the development of novel, diversified carbon-based catalysts. Advanced carbon-based catalysts demonstrate a high activity in electro-, photo-, and chemical-catalysis, driving their use in diverse fields: desulfurization, biosensing, chemical synthesis, energy storage/conversion (supercapacitors, batteries, fuel cells, solar cells), and contaminant degradation.

Despite advancements in carbon-based catalyst design and use, challenges remain:

(i)

The scientific community continues to face a significant hurdle as it anticipates the enormous performance of carbon catalysts.

(ii)

For further improvement of the catalytic performance, the catalysis mechanisms need to be clarified.

(iii)

It is ideal to use reasonable fabrication, manufacturing, and commercialization strategies.

(iv)

It is necessary to build novel, broadly applicable carbon-based catalytic techniques.

Recent research on metal–carbonaceous nanocomposites has primarily focused on optimizing fabrication methods. Due to their complementary properties, carbonaceous nanostructures and metal nanoparticles are now used in a wide range of novel processes, from water purification to the production of renewable energy. Before any true industrial applications, a number of activities must be considered. The mass production of high-quality, consistent carbon-based metal nanostructures remains a challenge. Carbonaceous supports offer promise for reducing charge recombination in these nanostructures, making them ideal for visible-light-driven environmental remediation. This review highlights the diverse properties of carbon-based metal nanostructures for photodegradation, hydrogen production, and CO₂ reduction applications.

New applications are expected to emerge for the following reasons:

(i)

Scalable and cost-effective methods for producing large quantities of carbon-based sheet-like structures are crucial for wider adoption.

(ii)

Understanding synergistic effects in carbon-based metal nanocomposites is key to optimizing their performance in catalysis, biosensing, and other applications.

(iii)

The precise fabrication of carbon-based metal nanocomposites not only prevents the restacking of carbonaceous sheets but also creates ideal platforms for doping/decorating with metal nanoparticles, enhancing their catalytic activity.

(iv)

The synergy between components in carbon-based metal nanostructures significantly accelerates visible-light catalytic materials.

(v)

Future advancements in carbon-based metal nanostructures hold promise for addressing energy and wastewater treatment challenges.

This review highlights recent advances in carbon-based metal nanocatalysts for photodegradation, hydrogen production, and CO₂ reduction applications, paving the way for innovative environmental remediation.