In situ growth of carbon nanotubes on fly ash substrates

3.1 Physical properties of fly ash

Fly ash is a by-product of power plant that use coal or heavy oil/crude oil as a fuel. The rising demand for power resources has resulted in considerable amounts of fly ash pollution, causing land resource depletion, soil and water pollution, and land acidity imbalance, all of which have adverse effects on the living environment of animals and plants [4,53]. Consequently, it has become a key concern for scholars to explore rational and comprehensive ways to utilize fly ash, while also broadening its application. Fly ash from heavy or crude oil has oxides that are similar to but in lower concentrations than fly ash from coal combustion. Oil fly ash contains more than 80% unburned carbon. Oil fly ash contains unburned hydrocarbons and differs from coal fly ash in particle morphology, density, and color. During the combustion process, coal/crude oil combustion particles become spherical and generate CO₂, resulting in loose and porous spherical particles.

The scanning electron microscope (SEM) images of coal fly ash and oil fly ash are shown in Figure 1. The unique structure and composition of fly ash have created opportunities for its use as a substrate for the preparation of nanomaterials. The carbon-rich composition provides an abundant source of carbon precursors necessary for CNT growth. The catalyst particles in the fly ash initiate CNT growth at high temperatures, the carbon precursors align around the catalyst particles and form small nanotubes known as nanotube seeds, which facilitate the initiating and length extending of CNTs.

Figure 1

SEM images of coal fly ash and oil fly ash. (a) Coal fly ash. (b) Oil fly ash.

Fly ash is a powdered solid waste that is expelled with flue gas after coal has been melted at high temperatures in a boiler. Variations in coal type and combustion conditions can result in different physical properties of fly ash. During the combustion process, coal combustion particles become spherical and generate CO₂, resulting in loose and porous spherical particles.

Fly ash is primarily composed of gray, black spherical particles and amorphous particles, featuring rough surfaces with a particle size range of 0.5–300.0 μm. Its density varies from 1.9 to 2.9 g/cm³, with a void ratio of 40–50%. Additionally, its specific surface area ranges from 300 to 500 m²/kg. The chemical composition of fly ash is dominated by Al₂O₃ and SiO₂, with a main physical phase that comprises porous vitreous and carbon grains. The crystalline phase is primarily composed of mullite, quartz, and hematite, while the non-crystalline phase accounts for more than 60% of fly ash, and features predominantly vitreous composition, making fly ash a classic volcanic ash material [54,55].

Rohatgi et al. [56] explore the relationship between fly ash’s chemical activity and its undirected vitreous content. The amorphous phase vitreous, due to its complex conformation, fails to crystallize or interact with molecules outside the microcrystals, resulting in high chemical internal energy and good chemical activity. The potential of fly ash to produce CNTs is highlighted as a result.

Furthermore, the geometric properties and pore structure of fly ash have led to its widespread use in various industries, including road building materials, wastewater treatment, and soil improvement, the utilization details are shown in Table 2. In construction projects, fly ash can be utilized as an auxiliary gel material with a similar composition to clay. It can also function as a raw material or additive to assist in cement production, as noted in Ref. [57]. Fly ash’s ability to improve the interfacial structure of concrete is discussed, leading to enhanced strength, compatibility, compactness, and reduced drying shrinkage.

In a study by Woszuk et al. [58], fly ash is explored as a low-cost, environmentally friendly filler in asphalt mixes. Grades F and C fly ash samples were added to replace the mineral filler in the experiment, resulting in asphalt mixes that met requirements for porosity, water resistance, and frost resistance. Finally, the use of fly ash as a matrix and raw material for CNT preparation is not expected to interfere with the overall modification effect of asphalt after modification.

In the realm of CNT synthesis, considerable progress has been made with regard to the utilization of fly ash as a viable raw material. Notably, Salah et al. [59] demonstrated the efficacy of carbon-rich oil fly ash in the preparation of high-quality CNTs via chemical vapor deposition, thereby confirming its potential as a suitable precursor and catalyst for nanotube growth. Similarly, in another study, Li et al. utilized fly ash as a catalytic carrier for CNT growth via chemical vapor deposition due to its well-suited composition of SiO₂, A₂O₃, and Fe₂O₃. Furthermore, successful synthesis of CNTs has also been achieved using Ni catalyst/fly ash substrates [60]. Despite these advancements, the control of growth temperature has been shown to significantly impact nanotube diameter, indicating a need for further refinement in the heating methodology and temperature control during the preparation process.

3.2 Methods for in situ-grown of CNTs

The significance of CNT synthesis with high purity and high yield cannot be overstated as it is a prerequisite for structural characterization, performance testing, and further research and application. To achieve this, there are several well-developed methods, such as CVD, arc discharge, and laser sintering. However, in situ-grown CNTs are commonly synthesized using chemical deposition and microwave heating methods.

3.2.1 CVD method

The CVD method has emerged as an industrially viable method to produce CNTs in large quantities due to its facile reaction control and high reactant purity. CVD utilizes horizontal, fluidized bed, or vertical reactors, with the former being the most common (Figure 2). To perform the test, small amount of fly ash was kept on a quartz boat and then placed into the quartz tube of the CVD system, the tube was first evacuated and heated to the desired reaction temperature, and then the precursor gas was injected at a certain rate to maintain the desired pressure of chamber until the growth time is up. During the process, the parameters such as reaction temperature, precursor gas compositions, gas pressure, and growth time need to be strictly controlled [37].

Figure 2

Chemical vapor deposition method [66].

The CVD synthesis of CNTs typically involves two steps: carbon-containing gas decomposition on the metal nanocatalyst, followed by the precipitation of CNTs on the catalyst surface [67]. The detailed process is as follows: (1) the carbon source gas was fed into the reactor at a certain flow rate; (2) the treated fly ash was introduced into the quartz tube reactor; (3) the temperature and pressure of the reactor were adjusted to reach the desired temperature and pressure values; (4) the CVD reaction was carried on for desired growth time, while the chamber pressure and temperature were maintained at the desired values; and (5) the sample was removed after the tube and sample were cooled to room temperature, and the products were collected using a powder collector.

Transition metals like Fe, Co, or Ni are commonly used as highly active catalysts to control the CNT growth as they decompose the carbon source by plasma radiation or heat and nucleate new CNT growth [68]. Moreover, catalysts are often doped with other metals to improve growth rates and lower reaction temperatures. Bimetallic catalysts, in particular, demonstrate better CNT growth rates owing to the synergistic effects of the two metal nanoparticles in the key catalytic steps of CNT preparation, as verified by Irigoyen et al [69]. By and large, hydrocarbons, such as methane, ethane, ethylene, acetylene, xylene, or a mixture of them, or gaseous carbon sources like isobutane and ethanol, are preferred as carbon sources in CNT vapor-phase deposition. The growth efficiency of CNTs, however, is primarily dependent on the reactivity and concentration of the gas-phase intermediates that arise via the decomposition of hydrocarbons into reactive substances and free radicals [70]. Thus, it is imperative to select a gas-phase intermediate that can be chemisorbed or physisorbed onto the catalyst surface to trigger CNT growth with the highest efficacy possible. The detailed information for in situ-grown of CNTs on fly ash substrate utilizing the CVD method are documented as shown in Table 3.

Table 3

In situ-grown of CNTs on fly ash substrate utilizing CVD method: detailed information

Ref.	Carbon source	Carrier gas	Temperature	Pressure	Growth time	substrate
[71]	C2H2	H2 and N2 (1:1)	650°C	Atmospheric	30 min	Fe(NO3)3·9H2O treated
[38]	C2H2	N2	750°C	Atmospheric	20 min	Carbon free fly ash
[37]	C2H2	Ar	650°C	Low pressure	20, 30, 40, 60 min	Carbon-free fly ash
[72]	Melamine	N2	800°C	Atmospheric	—	Original fly ash
[60]	C2H2	N2 and H2 (1:1)	400°C, 500°C, 600°C, 700°C, 800°C	A tmospheric	45 min	Ni(NO3)2·6H2O treated
[59]	C2H2	N2	650–700°C	Low pressure	50 min	Carbon-free fly ash

Yasui et al. [73] presented a thermal chemical vapor deposition method for the production of multiwalled CNTs (MWCNTs) on fly ash, which demonstrated a simple, cost-effective approach for high-volume synthesis of MWCNTs. The study established that MWCNTs can be efficiently synthesized by the thermal CVD technique, utilizing methane and ethanol gas, on fly ash surfaces that contain small iron concentrations. Dunens et al. [71] carried out an investigation employing fluidized bed chemical vapor deposition on industrial-grade MWCNTs, impregnating coal combustion fly ash with iron nitrate. The CNTs were characterized using various techniques such as Raman spectroscopy, thermogravimetric analysis, SEM, and transmission electron microscopy. The optimized catalyst resulted in a higher CNT yield of approximately 82.5% with reduced metal catalysis, using 5 wt% fly ash catalyst at a temperature of 650°C with ethylene as a carbon source. The Raman spectra analysis of un-impregnated fly ash, 2.5 wt% Fe fly ash, and 5 wt% fly ash catalysts, displayed in Figure 3, showed the ratio of the intensity of the G and D peaks, indicating the degree of graphitization of the carbon products. The G/D ratios were similar for the unimpregnated and 5 wt% catalysts, while the G/D ratio was lower for the 2.5 wt% Fe catalyst, implying the presence of more amorphous carbon phases in the reaction products. This study highlights the potential of fly ash as an effective growth matrix and catalyst for the synthesis of CNTs and for increasing the production efficiency of CNTs.

Figure 3

Raman spectra of three fly ash catalysts (a) non-impregnated fly ash, (b) 2.5 wt% Fe fly ash, and (c) 5 wt% fly ash catalysts [71].

CVD possesses several advantageous attributes, including its ability to produce both metal and non-metal thin films via atmospheric pressure or low vacuum reactions resulting in uniform coverage, high purity, and strong adhesion of thin layer coatings [74]. These properties allow for the precise manipulation of chemical morphology and crystal structure through deposition-related parameter adjustments to influence deposit properties. The use of CVD to produce CNTs has been shown to significantly improve purity [75]. The CVD method shows promise in the production of CNTs, as the structure of the nanotubes is heavily influenced by the carbon source employed and reaction temperature. In one study, He et al. [76] report that the utilization of CO and CH₄ as carbon sources yielded CNTs of varying diameters, while H2 was found to play a crucial role in the formation of larger CNTs. Higher H₂ concentrations lead to rapid aggregation of metal particles, which affects CNT size. Temperature also impacts catalyst activity and material decomposition, with high temperatures assisting in the growth of smaller-diameter CNTs [77].

In a recent study conducted by Salah et al. [78], it was discovered that carbon-rich fly ash exhibits ideal properties as a catalyst and precursor for the growth of CNTs via chemical vapor deposition. Various parameters were analyzed to evaluate their effects on the synthesis of CNTs from carbon-rich fly ash. After optimization, desirable results were obtained in terms of conversion, uniformity, and length of CNTs. In 2019, Li et al. [60] conducted experiments where CNTs were synthesized directly on fly ash particles, and CNT/fly ash composites were prepared. Active components such as SiO₂ and Fe₂O₃ in fly ash particles were utilized to synthesize CNTs on Ni catalyst/fly ash substrates via chemical vapor deposition. Properties and influencing factors of CNTs were also analyzed. The aforementioned studies provide an important means to convert environmentally hazardous landfill fly ash into useful carbon nanomaterials on a large scale.

However, the process of synthesizing CNTs by chemical vapor deposition using a reactor as a medium for transferring thermal energy tends to result in uneven and incomplete reaction due to the large temperature difference between the reactants. This has an impact on the mass production and quality of CNTs. To optimize CVD, achieving more uniform heat transfer is an important direction for future research.

3.2.2 Microwave heating method

The production of CNTs typically entails high temperature, high pressure, and high current density processes. Such processes invariably require inert gas protection to prevent the oxidation of carbon in harsh experimental conditions. Microwave heating facilitates high-frequency particle motions that are guided by electromagnetic waves, which generate “internal frictional heat” and increase material temperatures, representing a process of converting electromagnetic energy into heat energy [79]. Unlike conventional heating procedures, microwave heating eliminates the need for heat conduction and enables simultaneous heating of the interior and exterior of the material, ensuring fast and uniform heating (Figure 4), lower energy consumption, and effectively addressing the issue of uneven heating encountered in chemical vapor deposition techniques [80]. Given the exceptional characteristics of this heating process, it is ideal for initiating the nanocarbonization process at room temperature and under atmospheric air conditions.

Figure 4

Temperature propagation for different heating methods [80]. (a) Microwave heating. (b) Conventional heating.

The microwave heating approach employs conductive polymers, such as ferrocene, carbon fiber, graphite powder, and polypyrrole, as precursors that are rapidly heated by microwave radiation, aided by transition metals, such as copper wire, which undergo discharge reactions in the microwave field. CNT growth is initiated by the degradation of these precursors to iron nanoparticle catalysts in a very short time frame.

The method of microwave heating presents several advantages, such as high heating efficiency, simple instrumentation, and fast carbonization reaction, in the preparation of in situ-grown CNTs. The detailed information for in situ-grown of CNTs on fly ash substrate utilizing microwave heating method is are documented as shown in Table 4. The detailed process is as follows [81,82,83]: (1) the preprocessed fly ash was mixed with ferrocene powder at a certain ratio, then the mixed powder was put into a quartz crucible; (2) the metal wires were inserted into the powder and employed as the reaction triggers; (3) the quartz crucible was moved into a microwave oven and heated for a desired period of time; and (4) the quartz crucible was removed out of the microwave oven after the sample was cooled to room temperature, and the grown CNTs were collected from the crucible.

Table 4

In situ-grown of CNTs on fly ash substrate utilizing method: detailed information

Reference	Carbon source	Microwave absorber	Power (W)	Heating time (s)
[82]	Ferrocene	PPy coating	1,250	15–30
[47]	Ferrocene	PPy coating	1,250	30
[83]	Ferrocene	Graphite powder	1,800	5
[51]	Ferrocene	PPy coating, metal wires	800	20–40

Zhan et al. [81] conducted a successful study on the preparation of multiwalled CNTs on the surface of fly ash using the microwave heating method, demonstrating that an increase in the ratio of fly ash and ferrocene led to a gradual increase in the growth of CNTs. The highest density and most uniform distribution of CNTs were obtained when the ratio was 80/50. Conductive materials, such as graphene oxide and metallic copper wire, were found to have a promoting effect on CNT growth. In addition, researchers explored the preparation of CNTs using ferrocene pyrolysis in the microwave field and found that different carbon material samples had varying heating characteristics and reaction degrees. Microwave heating for the preparation of CNTs requires a suitable deposition carrier, where powdered carbon material acts as a good absorber and a deposition carrier for CNTs. Bajpai and Wagner [83] demonstrated that ferrocene, graphite, and carbon fibers are necessary for the rapid growth of CNTs using microwave heating. Dadras and Faraji [84] achieved ultra-fast growth of CNTs on anodic aluminum oxide templates by mixing precursor materials consisting of graphite and ferrocene and heating them in a microwave oven. The use of graphite resulted in uniform absorption of microwave radiation as well as rapid diffusion of heat. Liu et al. [82] grew CNTs in situ on the surface of fly ash using polypyrrole as a precursor and the microwave method, resulting in enhanced conductivity of fly ash wrapped by CNTs and improved mechanical properties and static toughness (Figure 5).

Figure 5

Schematic diagram of microwave-initiated in situ growth of CNT on the surface of fly ash particles [82].

In situ growth of CNTs using microwave heating has emerged as a promising method in nanotechnology owing to its efficient, uniform, and simultaneous heating properties. This technique offers advantages such as minimal temperature gradients, reduced incomplete reactions, and experimental simplicity, all of which lead to the fabrication of low-cost, high-quality CNTs when combined with fly ash. However, despite the potential of this technique, certain challenges need to be addressed before it can be widely adopted. These include the limited production of CNTs, which is not yet comparable to the overall process of the CVD method. Additionally, the current level of size and shape uniformity of the prepared CNTs remains inadequate and controlling the shape and size of these materials requires more precision. Notwithstanding, the existing industrial experience of the CVD method has paved the way for overcoming these limitations in the future.

3.3 Catalyst influence on the growth mechanism of in situ-grown CNTs

The growth of CNTs is influenced primarily by the selection of catalysts and transition metals, which possess distinct morphological characteristics that ultimately shape the formation of CNTs. Furthermore, the application of different preparation methods has the potential to induce variability in CNT structures.

Presently, two widely accepted methodologies for the growth of CNTs exist: “top growth” and “bottom growth,” both of which are illustrated in Figure 6. The “top growth” theory posits that the catalyst lies directly atop the carbon tube, enabling its continuous adsorption of small carbon-containing molecules and catalysis of molecule cleavage, resulting in a constant supply of carbon atoms. Following multiple diffusion processes, the carbon atoms are ultimately precipitated at the opposite end of the catalyst particle to form the carbon tube. In contrast, the “bottom growth” theory holds that the catalyst is situated at the lowermost end of the CNT, while the upper end remains sealed. Carbon atoms are continually introduced to the unsaturated bonds of the CNTs, subsequently entering the graphite layer network to form CNTs. Notably, one extremity of the CNT connects directly to the catalyst, whereas the opposing end is free of catalyst particles [85]. Both top and bottom growth models exhibit an outer diffusion rate of carbon atoms superior to that of the interior, ultimately resulting in the formation of a hollow tubular structure.

Figure 6

Schematic diagram of (a) top growth and (b) bottom growth models of CNTs [86].

In the realm of CNT preparation, the role of catalysts is of utmost importance, as their properties dictate those of the final product. The mode of preparation for catalysts directly shapes their structure, morphology, and properties. A plethora of catalyst preparation methods such as sol-gel, precipitation, microemulsion, ion exchange are presently available, and the variations in their properties resulting from these methods are well-documented. In the CVD method, the fly ash serves as not only the substrate to grow CNTs but also as the catalysts for the growth of CNTs, the component of fly ash includes SiO₂, Al₂O₃, and Fe₂O₃, iron is a well-known metal catalyst for CNT growth, and silicon and aluminum oxides were also good catalyst-supporting materials for CNT growth [37,59,87,88]. In the microwave heating method, the Fe₂O₃ comes from pyrolysis of ferrocene and also acts as a catalyst apart from the fly ash [81].

Several factors within the CNT preparation process directly influence the outcomes. The quality of the in situ-grown CNTs on fly ash substrate depends on the two critical parameters, that is, growth time and ratio of catalyst. During the in situ growth process, these parameters determine the diameter and length of the in situ-grown CNTs [78].

The length and diameter of the in situ-grown CNTs increase with increasing growth time [78]. Regarding the CVD technique, it is recommended that the growth duration yielding optimal outcomes lies within the range of 40–80 min [37,59]. Alternatively, in the context of the microwave heating method, it is suggested that a fly ash to ferrocene powder ratio of 1:1 be employed, accompanied by a heating duration of 20–40 s [50,51,78,81,82,83]. The effects of the ratio of catalyst is not yet clear at present. However, integrating the preparation of catalysts and CNT growth enhances the controllability of the preparation process, and this is where organometallic compound ferrocene offers exceptional capabilities [89]. As a metallocene complex containing iron atoms, hydrocarbon components, and the ability to effect homogeneous catalytic reactions, ferrocene offers a dual-purpose application in the preparation of CNTs as both a catalyst and a carbon source. In the context of the microwave heating method, it is suggested that a fly ash to ferrocene powder ratio of 1:1 be employed [50,81].

4.1 Microscopic morphology and mechanical properties

CNTs are a type of one-dimensional nanomaterial, consisting of carbon atoms arranged in a hexagonal pattern to form coaxial circular tubes with multiple layers. These tubes have a fixed inter-layer distance of approximately 0.34 nm and can have diameters ranging from 2 to 20 nm. CNTs possess exceptional mechanical properties, including an extremely high tensile strength of 50–200 GPa and an elastic strain of up to 12%. Moreover, their length-to-diameter ratio often exceeds 1,000:1, positioning them as high-strength fiber materials [90].

The CNTs self-grown through fly ash matrix offer several advantages, including high mechanical strength, large specific surface area, good adsorption properties, and excellent adsorption properties and electromagnetic wave absorption capacity [91,92,93,94,95,96,97]. These properties make them ideal catalyst carriers, and they can also improve the dispersion problem of CNTs in the modified matrix, leading to better control of the dispersion state and the formation of a stable conduction network. As shown in Figure 7, the in situ-grown of CNTs on the surface of the fly ash particles achieved the 3D self-assembly of the urchin-like CNTs particles, it solves the problem of agglomeration of ordinary 1D CNTs when they are used as modifiers. Thus, the dispersion performance of the in situ-grown CNTs outperforms commercial CNTs [81]. Given these exceptional physical properties and unique structural characteristics, there are abundant opportunities for the use of fly ash matrix CNTs in various applications.

Figure 7

The SEM images of the in situ-grown CNTs on fly ash substrate [60].

4.2 Wave absorption performance

The high surface area and small size of CNTs lead to an elevated atomic ratio, increasing their crystal defects and hanging bonds. This structure results in interfacial polarization and multiple scattering wave absorption mechanisms, thereby enhancing their dielectric loss properties. When nano-sized CNTs split at the electron energy level, they generate new wave absorption channels within the microwave energy range, leading to improved electromagnetic wave absorption properties [98]. Sun et al. fabricated an array of controlled catalyst-arranged CNTs through chemical vapor deposition. Morphological observations indicated an orderly perpendicular growth pattern, ultimately resulting in a jungle-like structure favorable for electromagnetic wave absorption. Under specific conditions, the effective absorption bandwidth achieved was 4.25 GHz [99].

Despite its excellent wave absorption performance, CNTs mainly rely on dielectric loss for electromagnetic wave absorption, have a single absorption mechanism, possess small impedance matching, and tend to agglomerate during wave-absorbing coatings’ preparation, negatively affecting their performance [100]. To combat this issue, researchers have proposed compounding CNTs with other materials, such as polymeric organic substances or magnetic media, to improve their wave absorption properties. Recently, carbon-based materials have been a focal topic in research, which can enrich the conductive network of CNTs, augment the interfacial polarization of the material, and enhance their wave absorption performance. The study by Yao et al. [101] established that graphene/CNT composites could enhance wave absorption performance via CNTs’ role in connecting the graphene layer-sheet structure, alongside the synergistic wave absorption effect they possess with graphene.

The intricate composition and porous nature of fly ash make it an incredibly promising candidate for the synthesis of materials that exhibit strong electromagnetic microwave absorption capabilities. Additionally, the distinctive clustering arrangement of microspheres found within fly ash microstructures contributes to enhanced reflections and heightened interfacial polarization [91,92,93]. Furthermore, previous research has indicated that the microwave absorption properties of CNTs (CNTs) can be improved through structural adjustments or the integration of conductive and magnetically lossy materials [94,95,96,97]. Exploiting the advantages of both fly ash and CNTs, the in situ growth of CNTs on fly ash substrates further amplifies their collective electromagnetic microwave absorption performance [102].

4.3 Frictional properties

The utilization of lubricant additives has been a proven method for enhancing the performance and environmental compatibility of lubricants while simultaneously mitigating energy loss and machine wear associated with friction. Nanomaterial is a noteworthy additive material that plays a vital role in the research and development of novel lubricants, whereby lubricating properties and mechanisms are correlated with their physical and chemical attributes, as well as their geometry [103]. Recent studies have demonstrated the considerable potential of carbon nanomaterials in the enhancement of frictional reduction and anti-wear properties, exerting a positive impact on lubricant properties as a result of their distinctive size, shape, and physiochemical properties [104,108,99,100,101,102,103,104,105,106,107,108].

Salah et al. [109] researched to investigate the tribological impact of a fly ash matrix CNT lubricant additive in comparison to other commercial carbon nanomaterials such as graphene, single-walled CNTs, and multiwalled CNTs. The experimental results based on friction coefficient tests established that fly ash matrix CNTs exhibited superior anti-friction properties in comparison to other carbon nanomaterials. The friction coefficient values for pure 500 SN base oil increased from 0.053 to 0.255, while for CNTs impregnated base oil, the friction coefficient values increased from 0.031 to approximately 0.227 as the load increased from 1 to 10 N. In comparison to pure oil, CNTs impregnated oil demonstrated a superior tribological performance at different loads. Additionally, when the concentration of fly ash matrix CNTs was 0.1 wt%, the coefficient of friction demonstrated a reduction of approximately 20%, whereas the rheological properties were minimally impacted. As a result, fly ash matrix CNTs display high potential as an optimal lubricant additive in reducing friction and improving fuel economy.

Bhaumik assessed the potential of multiwalled CNTs (MW-CNTs) as mineral oil additives to enhance load-carrying capacity and reduce wear resistance in their study [110]. Their experimental outcomes demonstrated a significant decrease in wear test and a notable increase in load-carrying capacity when compared to pure mineral oil. Carvalho et al. similarly investigated the effects of incorporating MW-CNTs on Al-Si CNT composites and observed a prominent reduction in wear loss [111]. These findings highlight the potential of carbon nanostructures as candidates for anti-wear and friction modification applications.

In addition, carbon nanomaterials have also been used as wear-resistant coatings (Table 5). Carbon nanomaterials have good organizational structures at both atomic and mesoscopic scales, which endow them with good conductivity. The outstanding mechanical and electrical properties of carbon nanomaterials over traditional coating materials make them very attractive for tribological and electromechanical applications in both nano/microscaled and macro-scaled systems.

Specifically, fly ash matrix-grown CNTs exhibit unique physical structure and exceptional mechanical properties, along with chemical stability and superior adsorption and dispersion qualities in a modified matrix, compared to unmodified CNTs. Additionally, they have excellent wave absorption properties, albeit with a relatively straightforward absorption mechanism, which can be improved by means of compounding with other materials. In the realm of lubricant additives, fly ash matrix CNTs have demonstrated favorable frictional properties, suggesting their potential efficacy in this field. Nevertheless, certain obstacles remain, the wear-resistant properties of the in situ-grown CNTs on fly ash substrates need to be further investigated, so as to evaluate their stability and durability in the lubricant oil.

5.1 As modifier

The in situ growth of CNTs possesses exceptional properties that have enabled its use in various fields, leading to several breakthroughs in research. The CNTs display a unique microstructure that exhibits favorable mechanical properties and characteristics of the matrix. When utilized as a material modifying agent, it effectively enhances the dispersion degree, thereby improving the modification effect. The detailed modification effects of in situ-grown CNTs in different materials are summarized in Table 6.

The aspect ratio and predominantly fibrous structure of CNTs allow for the formation of a complex network structure within the asphalt matrix with only a small amount of the material, effectively improving its toughness and strength [116]. Studies have demonstrated that the modification of asphalt with CNTs results in stronger intermolecular forces, contributing to an improved bonding performance of the modified asphalt. The enrichment of CNTs in the asphalt interface strengthens the mechanical anchoring between the polymer and the asphalt, while also increasing the roughness of the interface phase and producing a prominent cage-like structure of polymer-rich phase. This structure effectively improves the high-temperature performance and aging resistance of asphalt [117,118,119]. In their investigation of the effects of CNT/SBS composite-modified asphalt, [120,121] researchers found that the composite-modified asphalt displayed excellent temperature sensitivity. The addition of CNTs improved the high-temperature performance of the SBS-modified asphalt and enhanced its anti-aging stability. However, the low-temperature performance of the asphalt was adversely affected. The complex shear modulus of the composite-modified asphalt increased gradually with increasing CNT content, reducing the phase angle and elevating the rutting factor and fatigue factor. These results suggest that the addition of CNTs can enhance the rutting resistance of SBS-modified asphalt. However, it may also negatively impact the fatigue performance of asphalt pavement.

Numerous researchers have extensively investigated the efficacy of adding nano-materials into concrete, with a focus on enhancing concrete’s mechanical properties, durability, micro-mechanism analysis, and cracking resistance. According to a study by Lian et al. [122], the incorporation of CNTs into high-performance concrete has proven to be effective in mitigating the formation of cracks and improving the structure’s ultimate bearing capacity. The nano-additives have been observed to fill the internal micro-structure of concrete, increase the cohesion between internal aggregates, alter the material’s viscosity and fluidity, and ultimately improve the material’s compressive and splitting resistance, provided an appropriate dosage is utilized.

The in situ-grown CNTs from fly ash were also applied as reinforcing materials for different plastics, the tensile strength and Young’s modulus were effectively elevated by 20 and 38%, respectively, with 1–2 wt% in situ-grown CNTs [123,124]. Moreover, the in situ-grown CNTs on fly ash substrate inherit the benefits of fly ash and CNTs, the unique cluster structure further enhances its thermoelectric property and electromagnetic microwave absorption performance [81,102]. Thus, it might be utilized to design low-cost thermoelectric and electromagnetic microwave absorption materials.

In contrast to other frequently employed substrates, fly ash exhibits cost-effectiveness and easy accessibility owing to its status as a power plant by-product. For the simple microwave heating method, assume that the price of fly ash is US$ 30/t and that of ferrocene is US$ 20/kg, the preparation cost of in situ-grown CNTs on fly ash substrates is less than US$ 30/kg with the 1:1 mixing ratio of fly ash and ferrocene. While, the price of commercial CNTs is about US$ 140/kg [14], which is much higher than the preparation cost of in situ-grown CNTs on fly ash substrates. Moreover, fly ash has found extensive utility as a modifier in cement concrete, asphalt mixture, and various other major construction materials. Therefore, the integration of in situ-grown CNTs on the fly ash substrate holds great promise as a cost-effective nano modifier with extensive prospects for diverse applications.

5.2 As adsorbent

Both fly ash and CNTs were commonly used as adsorbents in environmental pollution management for the removal of heavy metals and dyes from water. Based on the inherent attributes of the fly ash matrix, characterized by large porosity and rough surfaces, and the desirable features of CNTs, such as small specific surface area, clear cylindrical hollow structure, and facile modification capabilities, in situ-grown CNTs have emerged as promising adsorbents with noteworthy research prospects. These advantageous characteristics have rendered CNTs suitable for various applications, such as wastewater treatment, removal of pollutants, and absorption of harmful gases. Several studies have been conducted to investigate the adsorption capability of CNTs toward a broad spectrum of pollutants, including heavy metals, inorganic matter, and organic matter [129,130]. In a recent study, Salah et al. [131] synthesized CNTs from fly ash and employed them as an adsorbent for the removal of Congo red dyes. Ultraviolet radiation was employed as a tool to enhance the adsorption of dyes. The synthesized CNTs were characterized by a range of analytical techniques, including SEM, transmission electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and infrared spectroscopy. Notably, the study showed that UV irradiation had a significant effect on the strength of the observed functional groups, resulting in improved adsorption capacity. Specifically, an increase in UV irradiation time from 0 to 30 min increased the adsorption capacity of Congo red from 9.1 to 26.5 mg/g, with the optimal pH being 4.5. The favorable adsorption results obtained demonstrate the potential of CNTs for the removal of dyes and other water pollutants, as depicted in Figure 8.

Figure 8

Maximum adsorption of various organic pollutants on pristine CNTs [132].

In the investigation conducted by Rajabi et al. [133], the adsorption properties of CNTs were examined in relation to various factors, such as dye concentration, pH value, and temperature. Further, the authors introduced a classification for anionic and cationic dyes. The results demonstrated that higher pH values provided better adsorption opportunities for cationic dyes, while lower pH values were more effective for anionic dyes. Additionally, the initial concentration of the dye played a significant role in the efficiency of the removal rate, as higher concentrations can result in saturation of the adsorption sites on the surface of the adsorbent, thereby reducing the removal efficiency [134]. Temperature is also a crucial factor that influences the adsorption capacity of CNTs. Higher temperatures allow increased interactions between the adsorbate molecules and CNTs, facilitating the adsorbate’s movement toward CNTs and enhancing adsorption [135]. As for the in situ-grown CNTs, the interaction between adsorbate and the urchin-like CNT particles could be adjusted by controlling the length and density of CNTs, so as to optimize its adsorption capacity.

5.3 As lubricant additive

The excellent tribological properties of CNTs make them highly sought after as lubricating additives in various lubricating oils. The in situ-grown CNTs on fly ash substrates were found to be superior to commercial multiwalled CNTs in terms of their frictional reduction and anti-wear properties when used as lubricant additives [136]. It was observed that a very low concentration of CNTs resulted in a significant decrease of the friction coefficient, to only 58% of the original value, indicating highly beneficial effects. Furthermore, the viscosity of pure 0.1 wt% CNT impregnated base oil was studied in the temperature range of 25–100°C, where it was found that the viscosity of CNT impregnated base oil did not undergo any significant changes. These observations suggest that cost-effective CNTs prepared from fly ash can be considered as highly efficient nanomaterials as lubricant additives. Similarly, in automotive engine lubricating oil, the addition of CNTs at 0.1 wt% causes a significant reduction in the friction coefficient, by about 25%, which is much better than commercial multiwalled CNTs [137]. Nora Nyholm’s study [138] revealed that the tribological behavior of carbon nanostructures is heavily influenced by their properties pertaining to structural integrity and chemical interaction. However, the theoretical understanding of the lubrication mechanism is still in its nascent stage. Additionally, the surface functionalization of CNTs can not only facilitate the dispersion of nanostructures but also enhance the inherent lubrication mechanism of nanostructures, providing a way to develop multifunctional additives. This opens up new avenues for future research into CNTs grown on fly ash matrices.

In utilizing fly ash as a precursor for CNT synthesis, notable advancements have been observed, indicative of the practicality of fly ash application in nanomaterials. The in situ growth of CNTs propelled by fly ash has demonstrated potential in reducing production expenses, mitigating environmental pollution, and promoting waste upcycling.