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Vol. 9. Issue 1.
Pages 67-75 (January - February 2020)
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Vol. 9. Issue 1.
Pages 67-75 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.030
Open Access
A novel route for controlling and improving the texture of porous structures through dual growth of alumina nanoparticles and carbon nanotubes using explosion process of solid fuel
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Osama Sabera,b,
Corresponding author
osmohamed@kfu.edu.sa

Corresponding author at: Department of Physics, College of Science, King Faisal University, P.O. Box 400, Al-Ahsa 31982, Saudi Arabia.
, Abdullah Aljaafaria, Adil Alshoaibia, Mohammed Al-Yaaric, Mostafa Osamac
a Department of Physics, College of Science, King Faisal University, P.O. Box 400, Al-Ahsa 31982, Saudi Arabia
b Petroleum Refining Department, Egyptian Petroleum Research Institute, P.O. Box 11727, Nasr City, Cairo, Egypt
c Chemical Engineering Department, College of Engineering, King Faisal University, P.O. Box 380, Al-Ahsa 31982, Saudi Arabia
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Tables (2)
Table 1. Composition of alumina-CNTs nanocomposites.
Table 2. Adsorption data derived from adsorption–desorption isotherms for the prepared alumina nanoparticles and alumina/CNTs nanocomposites using BET, De Boer and BJH methods.
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Abstract

Micro- and meso-porous structures of alumina have attracted attention for their potential use in different applications. In this research, a novel and facile route was introduced for dual growth of alumina nanoparticles and carbon nanotubes together to fabricate nanocomposites at low temperature 250°C through explosive processes of solid fuel. In this trend, series of alumina species with and without carbon nanotubes were prepared and characterized by X-ray diffraction, Raman spectra and transmission electron microscopy.

The surface properties of the alumina-CNT nanocomposites were characterized and compared with the prepared nanoparticles of alumina by adsorption–desorption system. The specific surface area of the prepared alumina-CNT nanocomposites was increased from 257.9 to 307.7m2/g and 314.8m2/g with growing CNTs inside the porous structure of alumina. These increments were observed because of the dual growth of nanoparticles and nanotubes by which new micropores inside their nanocomposites were created. When the source of CNTs was changed from ethanol to methanol, pure mesoporous structure with narrow pore size distribution was observed for the alumina-CNT nanocomposite. In addition, the surface area and the total pore volume increased to be 324.9m2/g and 0.673cm3/g; respectively.

The detonation technique of an explosive solid has been used for the first time to improve and control the porous structure of alumina through dual growth of CNTs and alumina at low temperature to meet the special requirements of the markets of catalysis and water purification.

Keywords:
Microporous structure
Mesoporous structure
Alumina texture control
CNT-alumina dual growth at low temperature
Detonation of explosive solids
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1Introduction

Alumina is one of the most widely used structural engineering materials in petroleum processes [1,2] and in membrane technologies [3]. The organized mesoporous alumina is considered a very interesting molecular sieve because it exhibits a narrow pore size distribution comparing with the conventional alumina [4]. Many researchers focused on synthesis of the organized mesoporous alumina through different methods [5,6]. The structure of mesoporous alumina was stabilized by doping with the rare earth metals [7]. The synthesis and potential uses of mesoporous alumina were described by Cejka [8]. The ordered mesoporous alumina with amorphous walls was prepared through a sol–gel method under special conditions by Niesz et al. [9]. Ray et al. studied the effect of surfactants on the structure of mesoporous alumina [10].

On the other side, carbon nanotubes (CNTs) drew the researchers’ attention because of their astonished properties, such as high thermal and chemical stability, high porosity, large specific surface area and low density [11].

Therefore, incorporating alumina with CNTs seems as an attractive approach to get a porous nanocomposite. Although, alumina-CNT composites synthesis has been achieved [12], most previous works has used the conventional mixing between CNTs and alumina species or growth one of them in the presence of the other. Currently, the conventional mixing technique is the only available way for producing alumina-CNT nanocomposite because growth and production of CNTs are usually performed above 600°C through chemical vapor deposition (CVD) or plasma techniques. These techniques led to non-homogeneous composites because of the agglomeration behavior of CNTs produced from the van der Waals attraction among its sidewalls [12].

To overcome such drawbacks, many researchers relied on changing the hydrophobic character of CNTs to be easily dispersed in liquid environment through attaching hydrophilic functional groups to the side walls of CNTs [13]. For this purpose, sulphuric acid solutions and concentrated nitric acid were used as oxidizing agents for CNTs. However, these strong conditions could cause clear damage and defects on the structure of CNTs.

In the present study, in order to obtain homogeneous dispersion of CNTs in the matrix of alumina and strong interfacial bonding between CNTs and the matrix, new approach for dual growth of both carbon nanotubes and alumina nanoparticles together at 250°C has been employed through detonation method of an explosive solid.

Some researchers have used unusual techniques for producing nanoscale materials [14]. In this trend, Kroke et al. [15] have prepared CNTs using detonative decomposition method. Wang et al. [16] investigated sulfur effect on CNTs growth using detonation-assisted chemical vapor deposition. Recently, Zhao et al. [17,18] fabricated CNTs by gaseous detonation method through ignition of mixed gas consisting of ferrocene vapor, methane, and oxygen. In the same year, Huber et al. [19] have used the high explosive detonation of Composition B-3 (40% TNT (Trinitrotoluene), 60% RDX (Cyclotrimethylenetrinitramine)) for producing novel nanocarbons. These researchers indicated that denotation and explosive denotation are considered useful techniques to reduce the high external energy for producing CNTs. Although, they have used gaseous and liquefied explosives, no one tried to use solid explosives. However, they did not succeed for decreasing the process temperature below 300°C. In addition, they did not solve the problem of aggregating behavior of CNTs.

In case of the solid fuel, several compositions of ammonium nitrate are well-known explosives [20]. Metallic fuels like aluminum hydroxide gel assisted in more energetic and aggressive combustion of the propellant [21]. It means that ammonium nitrate with aluminum hydroxide is considered a strong source for detonation process. In addition, the solid fuel takes small space and is easily confined inside the porous structure of aluminum hydroxide to obtain dispersed CNTs at lower temperature. Furthermore, auto-ignition and explosion of ammonium nitrate take place inside the confined spaces of the porous structure of alumina species. Above 230°C, the decomposition goes as the following exothermic reaction [20]:

8NH4NO35N2+4NO+2NO2+16H2O

It means that 8mol of ammonium nitrate produce 27mol of gases (high pressure and temperature). Therefore, the inorganic solid fuel (ammonium nitrate) is more effective than the gaseous and the liquefied fuel for producing homogeneous nanocomposites.

In the present study, a novel and facile technique is used for dual growth of both CNTs and alumina nanoparticles through decomposition of alcohol in the presence of solid propellant or explosive solid fuel. This method favors the growth of CNTs at lower temperature than the other techniques. At the same time, alumina nanoparticles start to grow through boehmite structure. In addition, the effect of the alcohol concentration on the growth of CNTs has been investigated and the growth of CNTs and alumina structure has been observed. Furthermore, different sources for CNTs growth have been examined as well.

2Materials and methods2.1Synthesis of aluminum species and explosive compound

Aluminum nitrate Al(NO3)3·9H2O was used as precursor for alumina species and ammonium bicarbonate (NH4HCO3) was the source for producing ammonium nitrate as an explosive solid. Cetyltrimethyl ammonium bromide (CTAB) surfactant acted as a template. Aluminum hydroxide and ammonium nitrate were synthesized by sol–gel process. Initially, one liter of aqueous solution was prepared by dissolving 0.06mol of aluminum nitrate and 0.0003mol of CTAB in deionized water. An aqueous solution of ammonium bicarbonate (10wt.%) was added drop by drop under continuous stirring until the gel was formed. After one hour of extra stirring, the mixture aged for 48h. The white gel was filtrated and dried under vacuum for 5h at 80°C. Part of the dried gel was calcined at 600°C to prepare alumina nano-particles for comparison.

2.2Dual growth of carbon nanotubes and alumina nanoparticles

Appropriate amount of the prepared dried gel was mixed with 350ml of alcohol (methanol or ethanol). Then, the mixture was placed in a pressurized vessel equipped with a temperature controller unit. The thermal process of the solid was achieved under super critical conditions (temperature=250°C and pressure=100bar). By the slow heating rate, the temperature of the mixture became higher than the critical temperature. Accordingly, the pressure of the autoclave reached a value higher than the critical pressure. At that moment, the pressure of the autoclave was released by slightly opening the autoclave valve. An inert gas (Argon) was used during the autoclave fluxing process to remove any remaining gases. Since the vapors of the solvent were replaced by the inert gas, no liquid condensation occurred. The products were calcined at 600°C in the presence of air.

To study the effect of gel and alcohol on the growth of CNTs, two different amount of the dried gel and two different kinds of the alcohol were used in this process as shown in Table 1.

Table 1.

Composition of alumina-CNTs nanocomposites.

Sample  S. alumina/S. CNTs ratioa  Dried gel  Alcohol 
AC-1  0.05  15Ethanol 
AC-2  0.09  25Ethanol 
AC-3  0.09  25Methanol 
a

S. alumina/S. CNTs Ratio is the ratio of alumina source to CNTs source.

2.3Characterization techniques

X-ray diffraction (XRD) analysis was carried out using Bruker-AXS, Karlsruhe, Germany with Cu-Kα radiation (λ=0.154nm). Raman spectra measurements were performed using LabRAM HR Evolution, Horiba-Jobin Yvon Technology, with laser 633 ULF and grating groove density of 300grooves/mm. Thermal gravimetric analysis (TGA) was carried out in TA thermogravimetric analyzer (series Q500). Differential scanning calorimetry (DSC) analysis was carried out using TA series Q 600, with a heating rate of 10°Cmin−1. Transmission Electron Microscopy (TEM) was carried out at room temperature JEM 2100F with an acceleration voltage of 200kV.

2.4Surface characterization

The surface properties and porous structures of alumina at the nano-scale were measured by a full adsorption–desorption isotherm of nitrogen gas at 77K (−196°C). These adsorption–desorption processes were carried out by a Quanta-chrome Nova sorption system. From adsorption isotherm, specific surface area, pore volume and average pore radius were calculated by using the Brunauer–Emmett–Teller (BET) equation. In addition, surface area and porosity were detected by applying the t-method. The analysis of desorption data was performed to obtain pore size distribution and surface areas according to Barrett, Joyner, and Halenda method [22].

3Results and discussion3.1Production of aluminum hydroxide saturated with ammonium nitrate

The XRD diagram of the dried gel is shown in Fig. 1a. It showed strong peaks at 2θ=17.98, 22.48, 28.94, 32.96, 36.14, 37.78, 39.86 and 40.16° agreeing with the crystalline phase of ammonium nitrate (as in JCPDS file No. 47-0867). No peaks were observed for aluminum species.

Fig. 1.

(a) X-ray diffraction patterns and (b) thermal analysis of the prepared gel.

(0.11MB).

It means that the dried gel, produced from the reaction of ammonium bicarbonate with aluminum nitrate, consists of amorphous form of aluminum hydroxide and crystalline structure of ammonium nitrate as shown in the following reaction:

Al(NO3)3+3NH4HCO33NH4NO3+Al(OH)3+3CO2

Thermal analyses of the dried gel were used to determine the saturated amount of ammonium nitrate inside aluminum hydroxide. The differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) of the dried gel were shown in Fig. 1b. The DTA curve showed five endothermic peaks agreeing with the five phase modifications of ammonium nitrate. The first three endothermic peaks observed at 39, 69.9 and 130.1°C verified the three phase state transitions of ammonium nitrate as reported in literature [23].

The fourth endothermic peak at 166.5°C represented the absorption of the ammonium nitrate melting heat. The fifth endothermic peak at 261.4°C is attributed to the absorption heat of the gradual decomposition of ammonium nitrate and dehydroxylation reaction of aluminum hydroxide. Above this temperature, there is a sharp exothermic peak for the complete decomposition of ammonium nitrate.

The corresponding TG curve showed weight loss of about 77.7 up to 299°C. Further heating above 299°C, the DTA curve showed one broad exothermic peak. This peak is associated with small weight loss (4.01%). This weight loss may be due to the crystallization of aluminum oxide. Finally, the residual weight of 18.2% indicated that aluminum hydroxide was saturated with three times of its weight with ammonium nitrate according to the above reaction stoichiometry.

3.2Characterization of alumina-CNT nanocomposites

XRD pattern of the nanocomposite AC-1 revealed weak peaks matching with the standard diffraction pattern of gamma alumina structure (JCPDS card no. 1-1303) as shown in Fig. 2(a). In addition, the broad weak peak at 2θ=26.1° suggested the presence of CNTs. By increasing the percentage of aluminum, XRD results revealed that the structure of the nanocomposite AC-2 is similar to that of AC-1 confirming presence of gamma alumina as shown in Fig. 2(b). In case of changing the CNTs source from ethanol to methanol, XRD results of the nanocomposite AC-3 showed the weak peaks of gamma alumina as seen in Fig. 2(c). It means that aluminum hydroxides were dehydrated and calcined to form aluminum oxide. At the same time, ethanol decomposed to build CNTs. However, the peak of CNTs is not clear in the diagrams of the prepared nanocomposites because of the homogeneous distribution of CNTs inside the matrix of alumina.

Fig. 2.

X-ray diffraction pattern of (a) AC-1 at 600°C, (b) AC-2 at 600°C (c) AC-3 at 600°C and Raman spectra of (d) AC-1 at 250°C, (e) AC-1at 600°C, (f) AC-2 at 600°C and (g) AC-3 at 600°C.

(0.29MB).

To illustrate the presence of CNTs and its combination with alumina, Raman spectra of the prepared nanocomposites are shown in Fig. 2(d–g). The nanocomposite AC-1 exhibited the characteristic bands of CNTs. A distinguished feature of the D and G bands, due to the contribution of the carbon nanotubes, was observed at 1100–1541cm−1. As shown in Fig. 2(d), the overlap between the D and G bands focusing at 1267cm−1 corresponds to a lower degree of graphitic ordering of CNTs. Also, Fig. 2(d) shows two Raman lines at 2785 and 3539cm−1 indicating the overtone of the “D” and “G” modes (2D and 2G). The Raman spectrum of aluminum hydroxides was observed at 688cm−1.

By thermal treatment at 600°C, Fig. 2(e) showed broad band indicating again overlap between the D and G bands of CNTs. According to the results of Inbaraj et al. [24], we could explain the meaning of this overlap. Inbaraj et al. [24] reported that the characteristic Raman peaks of alumina were observed at 1373 and 1403cm−1. It means that there is difficulty for getting four bands in this small region because the D and G bands of CNTs are adjacent to the alumina bands and almost merged with them. Also, this overlap confirms the good dispersion of CNTs inside the alumina structure. Using Gaussian functions, the D and G bands of CNTs were split and observed at 1346 and 1501cm−1; respectively. In addition, the maximum peak observed at 1473cm−1 was due to alumina nanoparticles as shown in Fig. 2(e).

The origin of the D band is explained as disorder-induced features due to the lattice distortion. The D band is frequently referred to as the defect band [25]. Its intensity relative to the G band is often used to determine the quality of nanotubes. The ID/IG ratio of pure CNTs is 0.8–0.9 [25]. In case of the nanocomposite AC-1, The ID/IG ratio was 1.7 indicating a lower degree of graphitic ordering of CNTs. After thermal treatment at 600°C, the ID/IG ratio became 0.93 confirming that CNTs became more ordered and possessed significantly lower defects.

By increasing the percentage of aluminum, the presence of CNTs in the nanocomposite AC-2 was confirmed by the Raman results. Fig. 2(f) shows broad band at 1476cm−1 indicating combination between CNTs and alumina structure. The D and G bands of CNTs were observed at 1352 and 1528cm−1 while the alumina band was at 1476cm−1. The ID/IG ratio of AC-2 was 0.86, which is related to the CC stretching mode of CNTs representing their crystalline order. When the CNTs source was changed from ethanol to methanol, the broad band of the nanocomposite AC-3 shifted to lower wavenumber 1451cm−1 and its ID/IG ratio increased to become 0.97 as shown in Fig. 2(g). The peak at 1451cm−1 seems to appear due to alumina structure while the D and G bands of the CNTs shifted to lower values 1330 and 1481cm−1. This finding indicates that different kinds of CNTs are produced by this technique depending on the carbon source.

TEM images of the nanocomposite AC-2 confirmed the individual dispersion of CNTs through the structure of gamma alumina as shown in Fig. 3(c) shows group of alumina nanoparticles assembling into agglomerates with constructing pores at the nano scale because the nanoparticles tend to share common faces in order to maximize packing density through self-assembly. Also, it displayed that branches of individual CNTs are extended from the porous structure of alumina. In addition, Fig. 3(d) indicated that CNTs combine with alumina nanoparticles to build porous structure of nanocomposites. Furthermore, Fig. 3(b) revealed that the length of CNTs is short.

Fig. 3.

TEM images of the nanocomposite AC-2 at 600°C.

(0.71MB).

In the composite AC-3, Fig. 4 showed that the individual and homogeneous dispersion of CNTs become clearer through the agglomeration of alumina nanoparticles. This finding implies that methanol is a favorable source of CNTs production by this technique. At the same time, the alumina nanoparticles were also observed. TEM images showed that the alumina particles formed a network structure through which CNTs were homogeneously distributed. It is important to mention that CNTs surrounded by alumina nanoparticles assure the interaction between CNTs and alumina particles as shown in Fig. 4(c, d). By crack bridging, strong CNT–Al2O3 interface was suggested by many researchers. Balani and Agarwal [26] confirmed the possibility of enhanced interfacial bonding between Al and C indicating that crystals with high surface energy try to adhere to new surfaces in order to minimize their overall energy. Also, the study of Keshri et al. [27] concluded that there is a strong interaction between CNTs and Al2O3 at the atomic interface. In our study, the crack bridging was produced during the explosion process and growth of both CNTs and aluminum oxides according to the following equation:

2Al2O3+6CAl4C3+3CO2

Fig. 4.

TEM images of the nanocomposite AC-3 at 600°C.

(0.77MB).

Fig. 4(c, d) shows that the nanoparticles of aluminum oxides are attached with the tubes of carbon confirming the interaction between them. Thus, the texture of alumina was strongly improved because of these interactions as shown later in the textural characteristics of alumina-CNT nanocomposites.

It means that the dual growth of CNTs and alumina nanoparticles together created a link between them. Also, Fig. 4(c, d) shows that the sample AC-3 has longer CNTs than that of the sample AC-2 suggesting that changing the source of CNTs may change the dimensions and the type of CNTs inside the texture of alumina.

3.3Textural characteristics of alumina-CNT nanocomposites

The broad applications of alumina are mainly due to its texture. Generally, for porous materials, improvement of their surface properties is considered an important factor for expanding their applications. Therefore, full adsorption–desorption isotherms of nitrogen at 77K (−196°C) for the prepared alumina-CNT nanocomposites were performed. Also, the alumina nanoparticles without CNTs were prepared and used for comparison. Nitrogen adsorption–desorption isotherms are shown in Figs. 5–7. Also, Table 2 contains the adsorption data calculated by BET, De Boer and BJH methods and included total pore volume, specific surface areas and average pore size.

AC-1 and
alumina nanoparticles).

'> Surface properties of AC-1 at 600°C: (a) nitrogen adsorption–desorption isotherms, (b) Vl-t plot and (c) Pore sizes distribution ( AC-1 and  alumina nanoparticles).
Fig. 5.

Surface properties of AC-1 at 600°C: (a) nitrogen adsorption–desorption isotherms, (b) Vl-t plot and (c) Pore sizes distribution (

AC-1 and
alumina nanoparticles).

(0.16MB).
Fig. 6.

Surface properties of AC-2 at 600°C: (a) nitrogen adsorption–desorption isotherms, (b) Vl-t plot and (c) pore sizes distribution.

(0.17MB).
Fig. 7.

Surface properties of AC-3 at 600°C: (a) nitrogen adsorption–desorption isotherms, (b) Vl-t plot and (c) pore sizes distribution.

(0.15MB).
Table 2.

Adsorption data derived from adsorption–desorption isotherms for the prepared alumina nanoparticles and alumina/CNTs nanocomposites using BET, De Boer and BJH methods.

Sample  SBET (m2/g)  Sext (m2/g)  Sint (m2/g)  SBJH (m2/g)  VpBJH (cm3/g)  VMP (cm3/g)  RpBJH (nm) 
Alumina nanoparticles  257.9  211.9  45  296.8  0.315  0.046  2.376 
AC-1  307.7  229.7  78  360  0.381  0.074  1.875 
AC-2  314.8  223.7  91.1  320  0.326  0.078  1.867 
AC-3  324.9  324.9  500  0.673  2.376 

SBET, specific surface area calculated by BET method; SExt, external surface area calculated by De Boer method; SInt and VMP, internal surface area (micropore surface area) and micro pore volume calculated by De Boer method; SBJH, VpBJH and RpBJH, specific surface area, total pore volume and pore radius calculated by BJH method, respectively.

According to Brunauer and Emmett’s classification [28], type IV isotherm was observed for the nanocomposite AC-1 as shown in Fig. 5(a). It exhibits hysteresis loop ended at relative pressure in the range 0.1 and 0.2. The measurements of progressive addition of nitrogen gas are represented in the lower branch of the loop while the measurements of progressive withdrawal are observed in the upper branch. The filling and emptying processes of the mesopores, which are happened by capillary condensation, produce this hysteresis loop. According to the IUPAC classification [28], the shape of the loop allows assigning the isotherm to the H2 type.

It means that the nanocomposite AC-1 consists of porous aggregates and the lattice of cross-linked pores formed its internal free space. At beginning of the desorption process, the polymolecular adsorbate layer situated at the external surface of the aggregate started to withdraw at the high values of the relative pressure P/Po. Thus, it showed linear region of the desorption branch. While, with decreasing the relative pressure, the region of the isotherm knee appears because of the desorption from the pores located in the aggregates. The calculated surface parameters from this isotherm showed that the nanocomposite AC-1 has high specific surface area (SBET=307.7m2/g) as seen in Table 2. Also, Table 2 indicated that both the total pore volume (Vp) and the average pore size (rp) of the nanocomposite AC-1 are high.

The high total pore volume and pore size of this nanocomposite could be attained by creating void surface area (pores) and by either fabricating small particles or clusters where the surface to volume ratio of each particle is high. The t-method was applied to distinguish between the micropores and the mesopores [28]. Table 2 shows that both the micropore surface area (Sint) and the external surface area (Sext) of the nanocomposite AC-1 are 78 and 229.7m2/g; respectively. The mesoporous texture of the nanocomposite AC-1 was confirmed by the Vl−t plots in Fig. 5(b) which represented upward deviation at t>0.5. The Vl−t plot consists of linear segment and upward deviation. Occurrence of capillary condensation in the mesopores causes the upward deviation. After all the mesopores and the micropores are filled with nitrogen adsorbate and adsorption continued with the multilayer mechanism, the t-plot shows a linear segment.

Analysis of the branch of the isotherms’ desorption in the hysteresis region of the nanocomposite AC-1 by using the BJH method showed the pore size distribution, surface area and pore volume, as presented in Table 2. According to this analysis, the majority of pores of the AC-1 nanocomposite has sizes confined within the narrow range between 2 and 4 nm (radius), as shown in Fig. 5c. A maximum pore size of 1.875nm (radius) was observed in the curve of pore size distribution of the nanocomposite AC-1. By calculating the average surface areas from both BET and BJH methods and using the theoretical density of aluminum oxide to be 3.98g/cm3, the particle size was assessed to be 4.5 nm by assuming that the particles are spherical.

By comparing the results of AC-1 and alumina nanoparticles without CNTs, it is clear that the surface area of alumina increased from 257.9 to 307.7m2/g because of growing CNTs during construction of the porous structure of alumina. This enhancement of surface area was achieved because of the dual growth of the nanoparticles and the nanotubes by which new micro pores in addition to the porous structure of alumina were created as shown in Table 2. Therefore, the internal (micropore) surface area increased from 45m2/g (in case of alumina without CNTs) to 78m2/g (for AC-1 nanocomposite). Also, in the same trend, the micropore volume increased from 0.046 to 0.074cm3/g. Therefore, it can be concluded that the growth of CNTs during the construction of alumina improved the texture of alumina through creating new micropores via aggregates of the nanoparticles of alumina around CNTs.

This speculation was confirmed by increasing the percentage of alumina source as seen in the sample AC-2. The adsorption data derived from adsorption–desorption isotherm of the alumina-CNT nanocomposite (AC-2) using BET, De Boer and BJH methods are summarized in Table 2. Fig. 6(a) shows nitrogen adsorption–desorption isotherm of the calcined sample of AC-2. It showed typical type IV isotherm and has a hysteresis loop in the area of capillary condensation for the relative pressure of nitrogen P/Po0.20. No significant difference in the shape of hysteresis loop was observed for AC-2, where it belongs to the H2 type of the IUPAC classification [28]. It indicated that AC-2 has similar porous structure to AC-1 with internal free space being formed by the lattice of cross-linked micro- and meso-pores. By increasing the ratio of source of alumina to source of CNTs up to 0.09, the BET surface area of the calcined AC-2 was higher than that of AC-1. The main factor of this increment in surface area is due to the rise in the internal surface area and the micropore volume from 78m2/g and 0.074cm3/g (for AC-1) to 91.1m2/g and 0.078cm3/g (in case of AC-2); respectively. Hence, as the percentage of alumina source increases the aggregates of the alumina nanoparticles around CNTs increases and thus leads to the creation of new micropores inside the texture of nanocomposite confirming our suggestion for the effect of the growth of CNTs during construction of the texture of alumina.

However, changing the source of CNTs from ethanol to methanol added new factor for controlling the alumina texture. The clear enhancement of both the BET surface area and the total pore volume of the calcined sample of AC-3 were observed although the microporous structure inside the alumina structure disappeared as shown in the nitrogen isotherm in Fig. 7 and the adsorption data in Table 2.

The nitrogen adsorption–desorption isotherm of the calcined sample AC-3 displays a type IV isotherm with H1 hysteresis loop, characterized for being a mesoporous material. It means that the shape of hysteresis loop changed from H2 to H1. Total pore volume was 0.673cm3/g and no microporous volume was detected. It means that no microporosity could exist in the calcined sample AC-3 which is an important difference between this material and the other prepared alumina nanoparticles and nanocomposites. These results suggest that changing the source of CNTs may change the dimensions and the type of CNTs inside the texture of alumina leading to the creation of new mesopores instead of micropores. This explanation was confirmed by the TEM images of AC-3 that revealed long CNTs inside the texture of alumina.

To explain this phenomenon, we apply the heat of enthalpy of each alcohol. The standard heat of enthalpy of gas phase methanol is −201.0kJ/mol while the ethanol has −235.0kJ/mol [29]. It is reasonable that the standard heat of enthalpy affects the growth rate of CNTs because an alcohol molecule having a lower absolute value of a standard heat of enthalpy is more easily decomposed. It means that methanol is more favorable than ethanol for producing longer tubes of CNTs. Consequently, the outer diameter of the methanol-based CNTs is different from that of the ethanol-based CNTs [30].

4Conclusion

The present study has a dual aim for improving and controlling the texture of alumina and introducing novel approach for producing CNTs-based nanocomposites. For this purpose, series of alumina-CNTs nanocomposites were successfully prepared and characterized by X-ray diffraction, Raman spectra and transmission electron microscopy. The texture of the prepared nanocomposites was studied by low temperature nitrogen adsorption–desorption system. In case of using ethanol as a source for CNTs, two porous nanocomposites were produced with clear enhancement of their surface areas. These enhancements were due to the creation of new micropores because of growth of short CNTs with formation of alumina nanoparticles.

By changing the source of CNTs from ethanol to methanol, pure mesoporous structure of alumina-CNTs nanocomposite was observed with narrow pore size distribution. The surface area and the total pore volume of the produced mesoporous structure increased to be 324.9m2/g and 0.673cm3/g; respectively. Disappearance of microporous structure and increasing mesoporous structure were attributed to creating new mesopores instead of micropores because of the dual growth of both long CNTs and alumina nanoparticles.

Finally, it could be concluded that the dual growth of both CNTs and alumina nanoparticles together improved and controlled the texture and the surface properties of their nanocomposites.

Conflict of interest

The authors declare no conflicts of interest.

Acknowledgments

The authors gratefully thank the Deanship of Scientific Research at King Faisal University (Saudi Arabia) for funding and providing the facilities required for this research as a part of the Research Grants Program (No. 17122002).

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