The present study describes the growth of carbon nanotubes (CNTs) from liquefied petroleum gas (LPG) on an Fe2O3/Al2O3 precatalyst via a chemical vapour deposition (CVD) process without hydrogen. The obtained multiwalled CNTs exhibit a less-defective structure with an identical external diameter of tubes of around 50 nm. The growth mechanism of CNTs suggests that the Fe2O3/Al2O3 precatalyst is reduced to Fe/Al2O3 during the synthesis process using the products of LPG decomposition, and the tip-growth mechanism is suggested. The resulting CNTs are surface-modified with potassium permanganate in the acid medium and used as an adsorbent for copper from aqueous solutions. The Langmuir and Freundlich isotherm models are employed to evaluate the adsorption data, and the maximum adsorption capacity of Cu(II) is 163.7 mg·g−1.

1. Introduction

In 1991, carbon nanotubes (CNTs) were detected by Iijima (Japan) in the carbon soot of graphite electrodes during arc discharge. The tubes were formed by the rolling of graphene sheets (single layer of carbon atoms (C-sp2) arranged in a hexagonal lattice) [1]. With a cylindrical structure and nanometer-sized diameter, CNTs have been extensively known for many outstanding physical properties such as extraordinary mechanical durability, very high electrical and thermal conductivity, or optical properties [1], and because of this, the market for CNTs is heating up by many scientists in the world. In 1994, Guo et al. synthesized CNTs from carbonaceous gas using laser ablation [2]. Despite high-quality nanotubes, both of the arc discharge and the laser ablation were not popularly used due to very high temperature (around 3000°C) resulting in complicated equipment and low efficiency. The third method used by Endo et al. in 1993 for synthesizing CNTs [3] is chemical vapour deposition (CVD). Until present, CVD has been a widespread method due to low growth temperature (less than 1000°C), simple furnace, high efficiency, and high purity of the product [4].

During the chemical vapour deposition, the carbon feedstock in the form of hydrocarbon was decomposed at a specified temperature into carbon gas deposited on transition metal catalysts such as Fe, Co, or Ni. Numerous studies report that the synthesis of CNTs using pure hydrocarbon sources such as CH4 [5], C2H2 [6], ethanol [7], or CO [8] is usually expensive because of the complex purification process. This results in high price of the obtained CNTs. Meanwhile, the price of natural carbon feedstock such as natural gas or liquefied petroleum gas (LPG) is much lower than that of pure hydrocarbons. Due to this advantage, LPG is popularly employed for the growth of CNTs by many scientists [912]. The growth of CNTs takes place when the amount of carbon in the solid solution with metal exceeds the saturation state [13, 14]. Therefore, in all previous studies, the metal oxide precatalyst that diffuses on the substrate (such as silica, alumina, or zeolite) has to be reduced into metal by hydrogen before the decomposition of hydrocarbon [1519]. This flow of hydrogen is maintained to prevent the obtained CNTs from oxidation until the synthesis terminates. The utilization of a large amount of hydrogen at high temperature for a long time not only raises the price of CNTs but also increases the danger during the operation of the furnace. As a result, the fabrication of CNTs must be carefully controlled. There is a little amount of research that recommends the fabrication of CNTs on the catalyst of Co and Ni without hydrogen [9, 20]. For that reason, the synthesis of CNTs without hydrogen is of considerable interest.

Heavy metal contamination in water is now an exigent problem for humans. Long-term exposure to toxic heavy metals can have carcinogenic effects on the central and peripheral nervous system and the circulatory system. The issue of “Lead and Copper Rule” in 1991 by the United State Environmental Protection Agency limits the concentration of lead and copper allowed in public drinking water at the consumer’s tap [21] and stresses on the popularity and seriousness of these two heavy metals. Copper poisoning can cause stomach and intestinal distress, liver or kidney damage, and complications of Wilson’s disease in genetically predisposed people. Therefore, the removal of copper from water is an essential study. Regarding the removal of Cu(II) with CNTs, many studies conducted so far have shown comparatively low values of maximum Cu(II) adsorption capacity [2228]. The carbon nanotubes are known as an excellent adsorbent for heavy metals [2932]. However, in the aqueous solution, the sorption ability is reduced because of the weak dispersion of CNTs caused by their noble nature. The adsorption behaviour of CNTs might be remarkably enhanced by surface modification. This modification using HNO3, H2SO4, KMnO4 H2O2, or NaClO increases the effective surface area of CNTs [3335].

In the present study, CNTs are successfully synthesized from LPG via the CVD method using an Fe2O3/Al2O3 precatalyst without an initial flow of hydrogen. Nitrogen is used to create a noble medium to prevent the oxidation of CNTs. The mechanism of CNT growth is addressed. The obtained CNTs are modified using potassium permanganate and used as an adsorbent for copper from aqueous solutions.

2. Experimental

2.1. Materials
2.1.1. Preparation of Precatalyst

The precatalyst Fe2O3/Al2O3 with 25.9% of Fe2O3 (w/w) was prepared from Fe(NO3)3·9H2O (Daejung, Korea) and Al2O3 (Daejung, Korea). Briefly, 50 mL of 0.35 M·Fe(NO3)3 solution was prepared and then added dropwise onto a 4 g-Al2O3 thin layer. The mixture was dried at room temperature for 10 hours and then at 100°C for 24 hours before calcination and at 500°C for 3 hours to completely pyrolyse Fe(NO3)3 to Fe2O3. The obtained red-brown precatalyst was ground and directly employed for the synthesis of CNTs. The SEM image of Fe2O3/Al2O3 precatalyst (Figure 1) exhibits uniform sphere particles with a size from 50 to 70 nm.

2.1.2. Growth of Carbon Nanotubes

CNTs were synthesized from LPG (Dung Quoc oil refinery, Quang Ngai, Vietnam) via CVD using the Fe2O3/Al2O3 precatalyst. The temperature of the furnace is automatically controlled. A small ceramic boat containing 0.4 g of the Fe2O3/Al2O3 precatalyst was placed in the centre of the furnace. 60 mL·min−1 of nitrogen gas was blown through the furnace during the process. The furnace was heated to the growth temperature at 800°C. At this temperature, 100 mL·min−1 of LPG was blown into the furnace for 2 hours for CNTs to be grown up. The furnace was then cooled down to room temperature under the nitrogen flow. The product containing CNTs and Fe/Al2O3 catalyst was collected from the boat.

2.1.3. Surface Modification of Carbon Nanotubes

A quantity of 0.25 g of the resulting CNTs was ultrasonically suspended in 25 mL of a solution containing 0.1 M·KMnO4 (Merck) and 0.5 M·H2SO4 (Merck) at 40°C for 2 hours. The surface-modified CNTs were separated, washed with deionized water until neutral, and dried at 80°C until constant weight. The suitable conditions of modification were determined on the basis of Cu(II) adsorption capacity. The concentration of Cu(II) was determined using atomic absorption spectrometry on Analyst 800 device (PerkinElmer, America).

2.2. Methods
2.2.1. Characterization of Materials

The crystal phase of the obtained CNTs was determined using X-ray diffraction (XRD) (RINT2000/PC, Rigaku, Japan) with λCuKα = 1.54 Å. The elemental composition of CNTs was obtained from the energy-dispersive X-ray spectrum (EDS) (Hitachi S4800, Japan). The morphology of CNTs was observed using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) (Hitachi S4800, Japan). The STEM-EDS and HAADF-STEM (high-angle annular dark field) studies were used to determine the elemental composition of catalyst particles located inside the tubes. The SAED (selected area electron diffraction) and FFT (fast Fourier transform) measurements on the HR-TEM (high-resolution transmission electron microscopy) device (Chemistem, Germany) were used to determine the lattice parameters of the material. The BET surface area of CNTs was analysed using N2 adsorption and desorption on the BELSORP-mini device (Japan).

2.2.2. Adsorption Studies

All the working solutions of Cu(II) were further diluted from the stock solution containing 1000 mg·L−1·Cu(II) (Merck).

The maximum Cu(II) adsorption capacity (qm) of the surface-modified CNTs was determined on the basis of isothermal data. The samples including surface-modified CNTs and 50 mL of Cu(II) solution with the amount of Cu(II) ranging from 10 to 60 mg·L−1 were stirred at 30°C for 80 min. Here, the amount of surface-modified CNTs was fixed at 0.2 g·L−1. The equilibrium amount of Cu(II) in the solution was determined after the adsorption. The adsorption capacity of Cu(II) was calculated according to the following equation:where qe is the Cu(II) adsorption capacity; C0 and Ce are the Cu(II) concentrations before and after adsorption, respectively; m is the mass of the surface-modified CNTs; and V is the volume of the Cu(II) solution.

3. Results and Discussion

3.1. Characterization of the Material

As can be seen, the XRD pattern of the synthesized product shown in Figure 2(a) exhibits characteristic peaks at 26.22° and 42.92° corresponding to the (002) and (100) diffraction planes of hexagonal graphite (JCPDS card files, No 41–1487), respectively [36]. Cao et al. [37] also confirm that these peaks are used to demonstrate the carbon crystal phase. This result proves that CNTs are the main product of the synthesis.

The EDS spectrum of CNTs (Figure 2(b)) reveals that the main composition of the product is carbon with a high-intensity peak at around 2.5 kV accounting for 91.24% of carbon. The low peaks of O, Al, and Fe observed with small weight percents are induced by Fe2O3/Al2O3 precatalyst.

The morphology of the synthesized CNTs is observed in the SEM images (Figure 3) and the STEM images (Figure 4). It is obvious that practically all tubes are long, less defective, and uniform. Some small particles appear on the tube, referring the remaining of the catalyst. The tube structure of CNTs is evidenced from the STEM images of the sample (Figure 4). The internal and external tube diameter is 15.2 ± 1.2 nm and 50 ± 2.3 nm (n = 50), respectively. The material might be multiwalled CNTs because the tube wall is relatively thick (16-17 nm).

The Russian Doll model [38] is recommended to describe the arrangement of graphene layers. High-magnification STEM images of CNTs (Figure 5) show that the wall of CNTs is thick and include many graphene layers arranged in concentric cylinders.

The synthesized CNTs exhibit a BET specific surface area of about 134 m2·g−1. The N2 adsorption and desorption isotherms (Figure 6) are of type II according to the IUPAC classification, which is assigned to the multilayer adsorption of a macroporous material.

Before surface modification, the catalyst of Fe/Al2O3 in the product was removed with HCl·0.1 M. The modification causes several changes in bare CNTs. The EDS spectrum of bare CNTs shows that the material comprises only carbon, while the surface-modified CNTs exhibit some oxygen and manganese in the material (Figure 7). The absence of Fe and Al in the bare sample might be because the Fe/Al2O3 catalyst is removed by washing with HCl. The amount of oxygen demonstrates the presence of the functional groups containing oxygen appearing on the surface of CNTs. The small amount of manganese might be the product of KMnO4 reduction.

The presence of functional groups on the CNTs was studied using FT-IR spectroscopy (Figure 8). The absorption bands assigned to –OH groups of carboxylic acid, alcohol, and water appear at around 3442 and 2920 cm−1. Similarly, the band showing the presence of C=O groups in –COOH appears at around 1627 cm−1. Wang et al. [22], Li et al. [25], and Moosa et al. [30] found similar characteristic bands for the surface-modified CNTs. The weak peak at the wavenumber of around 1550 cm−1 might be the evidence of C=C groups in the graphite structure.

Figures 9 and 10 show the SEM and TEM images of CNTs before and after surface modification. Generally, the tube structure remains after oxidization with KMnO4. However, if we look closely into the microstructure of bare CNTs, we can see the tubes break resulting in shorter structures after modification. This is also observed in the Raman spectra (Figure 11). The D band (D-disorder) located at 1319 cm−1 shows the amorphous carbon and structural defects; the graphite structures are proved by the G band (G-graphite) at 1567 cm−1. The G′ band at 2642 cm−1 is an overtone of the D band. The density of defects in the CNT structure could be estimated using the ratio of integrated intensities of the D and G bands (ID/IG). The larger the value of the ID/IG and ID/IG′ ratios, the higher would be the defect density [39]. The value of ID/IG for the surface-modified CNTs is 1.97, larger than that for bare CNTs (1.76), indicating some defects in the CNT structure.

The specific surface area of surface-modified CNTs measured using the BET method is 178 m2·g−1, larger than that of bare CNTs (134 m2·g−1). The rupture of CNTs indicates the formation of defects, i.e., the increase of the amount of pentagon and heptagon defects, resulting in the increase of the surface area [40].

3.2. Growth Mechanism of CNTs

The most commonly accepted growth mechanism of CNTs via the catalytic CVD method was postulated by Baker in the early 1970s [41], Yan et al. [42], Kumar and Ando [43], and Tessonnier and Su [44]. According to this mechanism, the hydrocarbon gas decomposes on the front-exposed surfaces of the metal particles to release hydrogen and the carbon gas dissolves in the metal particles. Since the solubility of carbon in the metal particle is temperature dependent, the precipitation of excess carbon will occur at the colder zone behind the particle, thus allowing the solid filament to grow with the same diameter as the width of the catalyst particles. Before the growth of CNTs, the precatalyst, which is metal oxide particles (such as Fe2O3 diffused on Al2O3 carrier), must be reduced to metal particles (Fe diffused on Al2O3) by hydrogen. This is one of the important periods in the synthesis of CNTs.

The present study skips this period and directly uses Fe2O3/Al2O3 as a precatalyst to synthesize CNTs. The mechanism of CNT growth is demonstrated from some extra analyses. Some dark particles observed inside the tubes on the STEM image of the CNT sample (Figure 4) might be nanoparticles of α-Fe.

Figure 12 shows that the signals of carbon, iron, aluminium, and oxygen atoms are expressed with different colours including red, blue, light-green, and Berlin blue, respectively. The appearance of dense signals indicates a high concentration of atoms. Figure 12(b) shows that in the observed field, the concentration of carbon is high. Iron concentrates on the catalyst particles inside the tube (Figure 12(a)). A low concentration of oxygen and aluminium on the catalyst particles infers two points: (i) Fe2O3 might be reduced to Fe and (ii) Fe atoms are separated from the Al2O3 substrate during the growth of CNTs. This indicates that the growth mechanism might be tip-growth (Figure 13) [45].

The overlap of the STEM-EDS mapping of iron and oxygen (Figure 14) shows that there is no significant increase of oxygen signals at the iron positions. This partly proves that the particles inside the tubes observed on the STEM image of CNTs are iron nanoparticles, not iron oxides.

The SAED and FFT measurements performed on HR-TEM device are used to determine the distance between atomic planes (Figure 15). The measured values perfectly match the theoretical values of lattice parameters of α-Fe as shown in Table 1 reflecting the reduction of Fe2O3 to α-Fe during the synthesis of CNTs. Theoretically, Fe2O3 is completely reduced to Fe by C at the temperature higher than 565°C or by H2 at the temperature equal to 425°C. Hence, the reduction of the precatalyst could completely take place at the growth temperature (800°C) by C and H2 formed from the decomposition of LPG.

In conclusion, the growth of CNTs still takes place on the metal nanoparticle catalyst. Main reactions suggested during the synthesis of CNTs are as follows:

3.3. Cu(II) Adsorption onto Surface-Modified CNTs
3.3.1. Surface Modification of CNTs

(1) Effect of KMnO4 Concentration. The Cu(II) adsorption capacity of the CNTs increases gradually from 62.1 to 103.7 mg·g−1 with the increase of KMnO4 concentration from 0.05 to 0.3 M and then remains practically stable around 103 mg·g−1 (Figure 16). This is because KMnO4 increases the amount of oxygen-containing groups (–OH, –C=O and–COOH) on the surface of CNTs and thus promotes Cu(II) adsorption. Unlike surface-modified CNTs, bare CNTs exhibit much lower Cu(II) adsorption capacity (7.6 mg·g−1). This indicates the crucial role of CNTs modification when CNTs are used in metal adsorption.

The morphology of the surface-modified CNTs samples indicates that KMnO4 with higher concentration destroys the tubes causing the tubes to break down to shorter ones (Figure 17). The tubes break considerably when the concentration of KMnO4 reaches 0.2 M and beyond.

The XRD patterns of bare CNT and surface-modified CNT samples (M0.1 and M0.25) (Figure 18) show that when using 0.25 M KMnO4, the crystal phase of carbon disappears and the material becomes amorphous. For the bare CNT and KMnO4-M0.1-modified sample, the characteristic diffraction peak of the graphite phase appears at 2θ of 26.22° corresponding to the (002) plane (JCPDS card files, no. 41–1487).

In conclusion, with the concentration of KMnO4 from 0.2 to 0.5 M, the CNT structure collapses completely and the obtained materials change into amorphous carbon. Therefore, the most suitable concentration of KMnO4 should be at 0.1 M, which is consistent with what reported by Slobodian et al. [46] when they oxidize the CNTs network embedded in elastic polyurethane. However, Zhang et al. [47] use the 0.313 M·KMnO4 solution to modify CNTs.

(2) Effect of Modification Temperature and Time. The range of modification temperature was from 40 to 80°C. Although the Cu(II) adsorption capacity of surface-modified CNTs is high (around 102 mg·g−1) in the temperature range from 50 to 80°C, most tubes break and the number of very short tubes increases with temperature (Figure 19). At 40°C, the material exhibits relatively high Cu(II) adsorption capacity (80.8 mg·g−1) and the tube length remains unaltered. The modification of CNTs was carried out for a period from 0.5 to 5 hours under ultrasound. The material modified with the 0.1 M·KMnO4 concentration at 40°C for 2 to 5 hours exhibits unvaried Cu(II) adsorption capacity at around 81 mg·g−1. Therefore, the 2-hour modification under ultrasound is suitable for the obtained CNTs.

3.3.2. Adsorption Isotherm Study

The Langmuir and Freundlich isotherm models [32, 48] were used to evaluate the adsorption with nonlinear equations as follows:where Ce is the equilibrium concentration of Cu(II) in the solution after adsorption; qe is the Cu(II) adsorption capacity of modified CNTs calculated from equation (1); qm is the maximum Cu(II) adsorption capacity; KL is the Langmuir constant which is related to the strength of adsorption; and KF and n are the Freundlich constants [48].

Figure 20 illustrates the nonlinear correlation between qe and Ce corresponding to Langmuir and Freundlich isotherm models. The determination coefficient for the Langmuir model (R2 = 0.962) is similar to that for the Freundlich model (R2 = 0.980). This indicates that the adsorption is in the monolayer form with the multienergy surface. High determination coefficients imply that 97-98% of the variability of the dependent variable accounts for the equilibrium capacity.

The maximum Cu(II) adsorption capacity (qm) calculated from the Langmuir nonlinear equation is 163.7 mg·g−1. This indicates that Cu(II) adsorption onto modified CNTs takes place favorably and irreversibly, and the modified CNTs are a good adsorbent. The maximum Cu(II) adsorption capacity of our material is much higher than that of other modified CNTs (Table 2).

4. Conclusions

The CNTs were successfully synthesized from LPG using the CVD method without an initial H2 flow. The characterization of the obtained CNTs confirms that the purity of the product is about 91.2% (w/w) of hexagonal graphite. The tubes have a multiwalled structure, and they are long, less-defective with an internal and external tube diameter of around 15 and 50 nm. The BET surface area of the obtained CNTs is 134 m2·g−1. The tip-growth mechanism of CNT formation is suggested. The precatalyst in the form of iron oxide is reduced to iron using hydrogen and carbon formed from the decomposition of hydrocarbon. The synthesized CNTs were surface-modified with KMnO4, and the obtained material exhibits high Cu(II) maximum adsorption capacity at 163.7 mg·g−1.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflicts of interest.