Carbon nanotube–metal oxide nanocomposites: Fabrication, properties and applications

Accepted Manuscript Carbon nanotube–metal oxide nanocomposites: Fabrication, properties and applications Shadpour Mallakpour, Elham Khadem PII: DOI: Reference:

S1385-8947(16)30661-1 http://dx.doi.org/10.1016/j.cej.2016.05.038 CEJ 15198

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

10 March 2016 19 April 2016 8 May 2016

Please cite this article as: S. Mallakpour, E. Khadem, Carbon nanotube–metal oxide nanocomposites: Fabrication, properties and applications, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.05.038

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Revised Carbon nanotube–metal oxide nanocomposites: Fabrication, properties and applications

Shadpour Mallakpour ∗, 1, 2, 3 Elham Khadem 1 1

Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan

University of Technology, Isfahan, 84156-83111, Islamic Republic of Iran 2

Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan,

84156-83111, Islamic Republic of Iran 3

Center of Excellence in Sensors and Green Chemistry, Department of Chemistry, Isfahan

University of Technology, Isfahan, 84156-83111, Islamic Republic of Iran

Abstract Carbon nanotubes (CNTs), a carbonaceous material, depend on morphology, size and diameter have highly unique properties in mechanical strength, thermal stability electrical conductivity, catalytic and adsorption. Over the past decades, combination of carbon nanotube with metal oxide is an effective way to build hybrid carbon architectures with fascinating new properties. The present review summarizes the recent advances on the principle and techniques of preparation, functionalization of the carbon nanotubes (CNT). On the other hand, it discusses the effects of combination of CNT with metal oxides such as aluminum dioxide, titanium dioxide, ∗

Corresponding author. Tel.; +98-31-3391-3267; FAX. +98-31-3391-2350.

E-mail address. Mallakpour)

[email protected],

[email protected],

1

[email protected].

(Shadpour

zinc oxide, iron oxide and etc and how they can be applied toward novel devices with remarkable properties for an extensive range of applications, over the past six years. This review provides the fundamental insights into the strategies for development of nano-hybrid with multifunctional properties for a broad range of applications such as sensor, supercapacitors, absorbent, photocatalytic, photovoltaic and etc. Keywords. Carbon nanotube, Metal oxide, Combination, Photochemical stability, Absorbent Contents 1. Introduction 2. Synthesis of Carbon Nanotubes 2.1. Functionalization of Carbon Nanotubes 3. Synthesis of Carbon Nanotubes/Metal Oxides 3.1. Ex-Situ Synthesis 3.2. In-Situ Synthesis 4. Investigation of Properties and Applications 4.1.

Properties

and

Applications

and

Applications

of

Carbon

Nanotubes/Aluminum

Dioxide

Nanotubes/Titanium

Dioxide

Nanocomposites 4.2.

Properties

of

Carbon

Nanocomposites 4.3. Properties and Applications of Carbon Nanotubes/Zinc Oxide Nanocomposites 4.4. Properties and Applications of Carbon Nanotubes/Iron Oxide Nanocomposites 4.5. Properties and Applications of Others Carbon Nanotubes/Metal Oxides Nanocomposites 5. Comparison Between of Nanotubes/Metal Oxides Nanocomposites 6. Conclusions Acknowledgements

List of Abbreviations Carbon Nanotubes

CNTs

Nanocomposite

NC 2

Metal Oxides

MOs

Aluminum Oxide

Al2O3

Titanium Dioxide

TiO2

Zinc Oxide

ZnO

Iron Oxide

Fe2O3

Tin Dioxide

SnO2

Manganese Dioxide

MnO2

Zirconium Dioxide

ZrO2

Single-Walled Carbon Nanotube

SWCNT

Multi-Walled Carbon Nanotube

MWCNT

Chemical Vapor Deposition

CVD

Transmission Electron Microscopy

TEM

Scanning Electron Microscopy

SEM

Field Emission Scanning Electron Microscopy

FESEM

Atomic Force Microscopy

AFM

Poly(Vinyl Alcohol)

PVA

Trichloroethylene

TCE

Methylene Blue

MB

Lactic Dehydrogenase

LDH

Lithium-Ion Battery

LIB

Dye-Sensitized Solar Cells

DSSC

Relative Standard Deviation

RSD

3

1. Introduction Metal oxides (MOs) have been an active field of researches due to their high modulus and strength at much higher temperature than common polymeric engineering materials, which make them appropriate for several applications. However, individual MOs cannot accomplish all requirements to develop new technologies and to solve the world’s most immediate problems. For example, applications of MOs are restricted due to their inherent brittleness and low fracture toughness, so they have been a continuous driving force to encourage researchers to employ novel strategies with the aim of reinforcing and toughening MO.[1-3] In many cases, the combination of two or more materials presences a composite with features that are superior to those of the individual components. Over the last two decades, carbonaceous nano-fillers such as graphite, diamond and fullerene, and carbon nanotubes (CNTs) have established part of widespread research and challenging due to their superior behaviors and interesting applications over other materials. Among them, CNT due to better structural and fascinating properties was attracted in science fiction novels and opened up a broad range of possible researches and functional applications.[48] In this scenario, one field of research that has been developing very quickly is the design, development and characterization of inorganic hybrid carbon-based nanostructures, generally consisting of MO and CNTs. Attaching CNTs with MOs cause to new functionalities in terms of electronic, optical and mechanical properties.[9-11] Various factors such as CNT shape, size, interaction, dispersion, alignment play an important role for fabrication of nanocomposite (NC).[12, 13] In a composite factors controlling such as the orientation of the CNTs, homogeneity of the composite, nanotube matrix adhesion, nanotube aspect ratio, and the volume fraction of nanotubes can have remarkable influences on the performance of the composites.[14] In recent years, preparation of CNT/MO composites have been extensively studied. Addition of CNTs to a ceramic matrix not only can change physicochemical properties, but also can provide multidisciplinary applications to adsorb hazardous pollution, photocatalytic, medical and etc. Depending on which materials is greater in composite, CNT/MO composite materials can be categorized into two groups; MO-decorated CNT and CNT-doped MO that herein both two groups will be discussed.[5] Fig. 1 shows CNTTiO2 interaction as an example (Fig. 1). In first group, oxidize CNT (CNT-COOH) is functionalized by attaching MO NPs i.e. nano-particulate MO (5 nm) deposited on CNT, either 4

by covalent or noncovalent interaction. In second group, CNTs are embedded within the MO matrix i.e. CNTs deposited on larger MO (100 nm).[2, 15, 16]

Fig. 1. CNT-TiO2 morphologies for promotion of reactive surface area and inter-phase contact. (a) Shows schematically a random mixture of TiO2 NPs and CNTs, (b) TiO2 coated CNTs, and (c) CNTs wrapped around large TiO2 NPs.[15]

An extensive number of literatures over the past decade sought to develop CNT/MO composites with potential applications.[16, 17] In this contribution, we survey the literature and highlight recent progress in the development of MO attached on the CNT material. This review aims to provide a broad and updated vision of effect of combination of CNTs with designated MOs such as aluminum oxide (Al2O3) titanium dioxide (TiO2), zinc oxide (ZnO), iron oxide (Fe2O3), tin dioxide (SnO2) and etc. in the field analytical chemistry by covering the period 2010–2015. 2. Synthesis of Carbon Nanotubes From a historical point of view, the discovery of CNT in 1991 [18] provided the revolutionary changes in the scientific community from both technological and a fundamental point of view. Carbon nanotubes according to the number of rolled up graphene layers forming the tube are consisted of either one or multiple rolled-up graphene sheets.[1, 19, 20] Single-walled carbon nanotubes (SWCNTs) are considered as single graphene sheet rolled into a cylinder with typical diameter of the order of 1.2-1.4 nm in magnitude. Multi-walled carbon nanotubes (MWCNTs) are studied as stacking of concentric cylinders of several graphene layers with an interlayer spacing of approximately 0.42 nm (4.2 Å) and a diameter typically can reach up to 140 nm in magnitude.[6, 19, 21] The rolling-up of the hexagonal lattice depending on the synthesis procedure of CNT, so in during chemical synthesis, the controlling their dimensions (diameter 5

and length), alignment (zig-zag, armchair or chiral) and the number of walls are very important. [9, 21] CNTs with sp2 bonding properties are either metallic or semiconducting, and their intrinsic electronic properties are highly dependent on morphology, size, diameter, chirality and the one-dimensional quantum confinement.[1, 6] Accordingly, the electrical properties of SWCNTs can vary from semiconducting in configuration of zigzag and chiral to metallic in armchair SWCNT. The metallic SWCNTs have long mean free path and can be used for interconnect materials, whereas the semiconducting SWCNT can be explored for building nanoelectronic devices. In semiconducting SWCNT, the band gap value is approximately scaled with the reverse of the diameter of tubes. Compared with SWCNT, MWCNTs are always metallic and reveal a zero bandgap. The MWCNT have a structural complexity and current carrying capacity similar to the metallic SWCNT. Also, each layer in MWCNT has different parameters and there are coupling between neighboring shells. Nevertheless, MWCNTs show advantages over SWCNT, such as low product cost per unit, enhanced thermal and chemical stability and easier fabricate due to easier control of the growth process. [22-24]. Based on the advantages use of MWCNT, a new research implemented to control the number of wall in the CNT and created a MWCNT with great electronic properties. Nosbi and et al. [25] investigated the effect of milling time on the morphology of the grown CNT (size, number of wall) and revealed the correlation among catalyst size, CNT and morphology. It was noticed that the milling time had a strong influence on MWCNT wall thickness, whereby increasing the milling time from 0 to 15 h reduced the number of walls from 29 to 12. Meanwhile, the diameter of MWCNT was decreased from 32.3 to 13.1 nm with increasing milling time.[25, 26] A comparison of the physical properties of MWCNT and SWCNT are summarized in Table 1.[2, 17] CNTs are generally produced using very useful and widespread methodologies include arc discharge, laser ablation, and chemical vapor deposition (CVD). Carbon source and heat source to achieve the desired operating temperature are two basic requirements in these methods.[6, 27] The synthesis of CNTs by the DC arc discharge of graphite was first described by Ebbesen and Ajayan in 1992.[5, 28, 29] In this method, CNTs are yielded from carbon vapor produced by an electric arc discharge between two high-purity graphite electrodes as anode and cathode (with or without catalysts), in the presence of inert gases like, He, N2 and CF4 for avoiding the oxidation of CNT.[24] The second method, laser ablation, was reported by Smalley 6

in 1995.[30] In this method, a piece of graphite target is vaporized using high energy laser beam under an inert gas (typically argon) at very high temperatures about 1200 ºC and at a constant pressure of 500 Torr. In 1998, CVD was depicted by Endo.[31] The principle of this method is that, the gas phase carbon source is transferred into the reaction chamber together with carrier gas under the high temperature (600-1000 ºC), and the gas will be decomposed to yield the carbon atoms on a coated catalyst substrate to generate fullerenes, CNTs and other sp2-like nanostructures. Compared to other strategies, CVD is as the most popular pathway for the production of CNTs on a reasonably large-scale owing to simplicity, high yield, low cost, the high deposition rate, good degree of control and economy.[24, 32] Table 1. Physical properties of MWCNTs and SWCNTs.[4, 17] Properties

SWCNTs

MWCNTs

Specific gravity (g/cm3)

0.8

<1.8

Elastic modulus (TPa)

∼1.4

∼0.3-1

Strength (GPa)

50-500

10-60

Electrical conductivity (S/cm)

102–106

103–105

Electron mobility (cm2/ (V s))

∼105

104–105

6000

2000

Coefficient of thermal expansion (×10 K )

1.1>

-1.37

Thermal stability in air (ºC)

600-800

600-800

Resistivity (µΩ cm)

50-500

50-500

Thermal conductivity (W/(m K)) -3

-1

2.1. Functionalization of Carbon Nanotubes Nowadays, CNTs are progressively used as nano-additive to advance the thermal and mechanical properties of organic and inorganic composites. However, it is remarkable that the pristine CNTs without any pre-treatment cannot efficiently interact with matrix. CNTs have tendency to stick together into the parallel ropes or the bundles by van der Waals forces along the length axis. This undesirable property may be related to high surface area and high aspect ratio of CNTs.[9, 21] To avert such issues, functionalization techniques are used to improve homogeneous dispersion 7

of CNTs into the matrix and enhance interaction between the CNTs and the matrix. Functionalization of CNTs is first step to prepare the NC and shows a key role in facilitating the binding, embedding or loading particles on the wall surface of CNTs. Consequently, it increases CNTs’ internal energy and introduces the active sites on the surface of CNTs.[33] Over the years, many surface functionalization approaches have been developed and optimized. They can be divided in to two main categories of physical and chemical modification.[24] The physical method employs the mechanical means such as ultrasonic, milling, crushing and friction to alter the surface physical and chemical structure and stimulate surface of CNTs. In contrast with physical method, chemical approach presents by covalent and non-covalent functionalization. The non-covalent modification, the delocalized π electrons of CNT generally combine with the conjugated compounds via the Van der Waals forces, π–π stacking interactions, hydrogen bonds and electrostatic forces. However, this method is much weaker than covalent bonding, but it can preserve the original structure and properties of CNTs, and does not destruct the sp2 hybridization of carbon atoms. Covalent modification usually accomplishes in tip defects as well as lateral wall of CNTs in the presence of high concentration acid and it follows by investing functional groups such as –COOH, –OH and –C=O– groups in the side wall of CNTs.[9, 34, 35] In this method, the hybridization of the carbon atom from sp2 converts to sp3 and leads to the disruption of the electronic structure of CNTs.[21] By contrast with SWCNTs, MWCNTs are more responsive to the covalent modification. Impact of the surface modification of SWCNTs may break some C=C double bonds and leave “holes” in the structure of nanotube, while in MWCNTs is only modified the outer wall.[36] According to data, the aggregation tendency of CNT reduces with increasing number of walls and reducing nano-curvature. On the other word, there is not the available interlayer spacing between the coaxial layers of MWCNT for adsorption.[37] Acid oxidation is the most commonly used method for functionalization of the CNTs surface. The structural information of the MWCNTs before and after acid-treated was retrieved from transmission electron microscopy (TEM) images in Fig. 2. It demonstrated that the acid treatment of the pristine MWCNTs introduce the nanoscale defects on the surface of MWCNTs with depth about 10-20 nm. The acid-treated MWCNTs have rough surface with mild asperities, without no slight change in the length.[38, 39] Nevertheless, in some reports, CNT by acid treatment of 3: 1 (volume ratio) concentrated H2SO4: HNO3 mixture could chop into the shorter 8

fragments with COOH groups.[38, 40] The acid-treatment process, the treatment time plays an important role in the average size of the defect nanodepths, so with increasing of the treatment time the average size of defect depth is increased in nanoscale. Functionalization of MWCNT provides the larger electrical repulsive force between the acid-treated MWCNTs, consequently it facilitates the CNTs dispersion and prevents from tangling and agglomeration of them. These functional groups make CNTs more hydrophilic and appropriate for the adsorption of hydroxyl groups of metal oxides and/or relatively low molecular weight and polar contaminants.[37, 41]

Fig. 2. (a) TEM image of the pristine MWCNTs. (b) TEM image of the acid-treated MWCNTs. The arrows indicate the position of the nanodefects. (c) High-magnification TEM image of the acid-treated MWCNT surface.[38] According to literatures, covalent attachment of polymers on the surface of CNTs can be presented by three strategies of “grafting to”, “grafting from” and “supramolecular strategy”. The first method is based on reaction of the specific functional groups on the CNTs surface with the reactive sites of prefabricated polymers chain. In contrast, the “grafting from” is a method based

9

on the anchoring of initiators onto the CNTs surface, subsequently a polymer brush generates followed by surface-initiated polymerization. Finally, the supramolecular strategy which is relied on the π–π interactions between the conjugated polymers and the CNTs surface.[42, 43] As already implied, carboxyl group is a common functional group created by aggressive treatment with a strong oxidizing acids (HNO3/H2SO4). They can react with other organic compounds via cycloadditions, electrophilic and nucleophilic additions, ozonolysis, halogenation, or radical reactions and produce polar functional groups such as amino, carboxyl, hydroxyl, phosphate or thiol on the wall of CNT.[17, 40, 44] Extensive articles have been recently published in studying the functionalization and dispersion properties of the pure CNTs. [45-48] For example, Mallakpourʼs research group followed this approach to apply treatment CNT in bio-safe polymers for improving thermal and mechanical properties. They utilized the different organic compounds such as 5aminoisophthalic acid [49], ascorbic acid [50], different natural amino acids [51], 5aminoisophthalic acid [52], p-aminophenol [53], glucose [54], and fructose [45] for treatment of MWCNTs. According to the results of these literatures, they proved that functionalization of CNT not only promote their stability dispersion in various solvents, but also enhance their interaction and compatibility for biomedical functions and organic polymers. As observed in Fig. 3, the functionalization reaction is provided a high surface roughness implying the disruption the sp2 carbon network of graphitic CNTs. Hariharasubramanian and et al. [55] investigated the change in the magnetic behavior of functionalized MWCNTs by 6-aminobenzothiazole and showed they can be a promising candidate in electrical motors and magnetic recording media. Inam and et al. [56] used gum arabic and sodium dodecyl sulphate for the dispersion of MWCNT and good interface between the matrixes and CNTs.

10

Fig. 3. TEM images of MWCNT-ascorbic acid [50]

3. Synthesis of Carbon Nanotubes/Metal Oxides Many methods have been developed to combine CNT with metal oxide. In this review, two main approaches toward the synthesis of CNT-based hybrid materials has been presented: i) in-situ approach, where the growth of CNTs are achieved within the same process and ii) ex-situ, or post-growth in which the decoration is performed in a separate step following the synthesis of CNTs. However, most papers reported the preparation of MOs supported on bulk CNTs and only a few papers reported the preparation of MOs supported on surface-grown CNTs. The synergistic effects of CNTs and MO fabricate CNTs/MO heterostructures possess the properties of the both which cannot attain each acting materials alone.[57, 58] The mechanical properties of the CNT/MO nanocomposite appertain to numerous factors. (i) the quantity and uniformity of CNTs; low amounts of CNTs is not sufficient for improving the mechanical properties of the composite, while with increasing the CNT content, the hardness and toughness increase to maximum values and then reduce with further increase in the CNT content. The higher amount of CNTs in MOs seems to favor the inhibition of the matrix grain growth, and hence hinder the densification. (ii) The quality of CNT; the MWCNTs compared to SWCNTs have more defective structures which will decrease the toughness of CNTs in the metal matrix. In other word, in MWCNT only outer layers can transfer load from 11

the matrix to the nanotubes, since their inner layers is free and cannot link to the MO. So, MWCNT affect very little to the load transfer. (iii) The consolidation conditions of the NCs are very important to enhance the mechanical properties.[20, 59]. 3.1. Ex-Situ Concerning ex-situ or building block approach, two hybrid components are first produced separately with their desire dimensions and morphology. Then either the MO NPs or the CNTs (or the both) are modified with the suitable functional group or linker molecule, and finally are combined by the linking agents through covalent bonding, van der Waals forces, π-π stacking, hydrophobic interaction, hydrogen bonds and electrostatic forces [60]. As a consequence, deposition is often restricted to the first layer, therefore the kind of functionalization and the strength of interaction can control the distribution and concentration of the MO NPs on the wall surface of CNT. The main advantage of this method is that control both the size and morphology of the nanoparticle is easy because there are already various techniques developed for the preparation of inorganic nanocrystals with controlled size and morphology.[12, 61, 62] 3.2. In-Situ Under in-situ synthesis, one of the hybrid constituents is produced in the presence of other; sometimes both components are synthesized simultaneously. The main benefit of this method is that CNTs act as a support and the MO can be settled as a continuous amorphous or singlecrystalline film with controlled thickness, or as separate units in the shape of NPs, nanorods, or nanobeads. As a result, one component controls the production of the other in terms of the size, crystal structure and especially morphology, and so can create interesting hybrids with new properties [60]. More binding sites like defects and functional groups can be introduced by acid treated, ultrasonication, microwave irradiation or γ-ray irradiation in the structure of CNTs. However, in these methods the performance of the composites might deteriorate because of the structural damage to the CNTs during the harsh treatment. In order to preserve the structure and pristine properties of the CNTs, the deposition process can be pursued in two phase, (i) the solution phase using electrochemical reduction of metal salts, electro deposition, sol-gel processing, and hydrothermal treatment with supercritical solvents, and (ii) the gas phase via chemical deposition (CVD) under oxygen-free environment or physical deposition (laser ablation, electron beam deposition, thermal evaporation, or sputtering) .[62-64] 12

4. Investigation of Properties and Applications 4.1. Properties and Applications of Carbon Nanotubes/Aluminum Oxide Nanocomposites The majority of studies accomplished on the CNT/MO composites, researchers have focused on the effects of CNTs with prominent mechanical, electrical and chemical properties. These properties are motivated the use of them as reinforcing agent in composite materials like polymer , metal and metal oxide [33]. So far, several groups have studied the preparation methods and electrical and mechanical properties of CNT covered and twisted with alumina.[65, 66] Among all MOs, alumina is one which has significant potential applications in many engineering fields because of its excellent properties like, chemical and thermal stability, high strength, wear resistance, etc. However, its applications are limited because of its low fracture toughness and most brittle. Presence of CNT can be an effective agent in reducing these defects in alumina matrix. For example, Yamamoto and et al. [38] incorporated MWCNT modified with an acid treatment into an alumina matrix to achieve the CNT/ceramic composites with the high toughness and the high temperature stability. The combination of only 0.9 vol% acid-treated MWCNTs to alumina enhanced the bending strength (689.6 ± 29.1 MPa) and fracture toughness (5.90 ± 0.27 MPa.m1/2) of the alumina ceramic about 27% and 25%, respectively, implying that stress transfer capability increase from the alumina to the MWCNTs [38]. Synthesis of CNT/Al2O3 NCs through CVD by addition of 7.39 wt.% CNT to alumina gives rise to 8.4% increase in hardness (9.98 GPa) and 21.1% increase in toughness (4.7 MPam1/2) over that of the pure Al2O3.[67] The study of several papers revealed that crack bridging, CNT pullout and crack deflection are the critical mechanisms for toughening in CNT/MO composites. Yamamoto et al. [68] experimentally studied the crack bridging behavior of a MWCNT/Al2O3 composite using a single nanotube pullout technique. They proved that a strong load transfer between the MWCNTs and the alumina matrix has been existed, and no pullout phenomena of MWCNTs have been detected. According to TEM images obtained the fracture surface, a diameter change in the MWCNT structure was evidently detected for a certain percentage of the MWCNTs, that this morphology was quite similar to a ‛sword-insheath’-type failure (Fig. 4). MWCNT before

13

pulling out from the alumina matrix was broke in the outer shells and then the inner core was pulled away, leaving fragments of the outer shells in the matrix.

Fig. 4. TEM images of the fracture surface of the composite acquired (a) low and (b) high magnification images [68]. After that, Nozaka and et al. [69] estimated the interfacial strength between the CNTs and the ceramic matrix using new testing method, named “inclined slit-based pullout method”. The average interfacial strength between the MWCNTs and the alumina matrix for 3 MWCNTs was equal to 19.2 ± 6.6 MPa, which is supported the validity of strategies presented to prepare tougher composites. Scanning electron microscopy (SEM) and TEM images for a single MWCNTs captured before and after their pullout tests, were displayed in Fig. 5. Embedded length of the MWCNTs into the alumina matrix was evaluated using the focused ion beam technique. According to this technique, single MWCNT had a protrusion length of ~3.8 µm and after its pullout had a length of ~6.7 µm.

14

Fig. 5. A series of SEM and TEM images of a single MWCNT captured (a) before and (b, c, d) after the pullout test. (d) TEM image showing a tip of the MWCNT that was damaged by the FIB processing.[69] The functionalization type of MWCNT is one of the key factors towards stabilization, and report impoundment in fracture toughness of composite.[43] For example in a research work, Kasperski and et al. [70] examined the influence of functionalization type on the microstructure, mechanical behaviors and likely reinforcement mechanisms for MWCNT-Al2O3 composites. The large scale of crack-bridging by CNTs was observed only when non-covalent functionalization was applied. It may be related to the high fracture toughness for composites with the longer CNTs that preserved the mechanical properties of the MWCNTs. While in MWCNTs (8 walls, 1.5 µm long or 20 walls, 10 µm long) with covalent functionalization, due to the chemical treatment and low fracture toughness, are not observed any crack bridging in the corresponding Al2O3-matrix composites. Moreover, it is evidenced that the shorter CNTs are dispersed better than the longer ones. In other work, they investigated the mechanical properties and electrical conductivity of double-walled carbon nanotube (DWCNT)/Al2O3 composite using three different methods and densified by spark plasma sintering. Composites prepared by in situ synthesis had highly homogeneous and presented higher electrical conductivity but intergranular carbon films induced a poor cohesion of the materials. In other hand, samples prepared by mixing using moderate sonication offered a fracture strength higher (+30%) and a fracture toughness of 5.6 MPa m1/2 similar to alumina. This is related to crack-bridging by nonfunctionalized CNTs, in spite of the absence of homogeneity of the CNT distribution.[71] In a different work, the group of Lu used noncovalent functionalization approach to make negatively charged CNT and prepared monodispersive alumina/CNT composite powder.[72] The prepared composite fibers due to high temperature stability, the high interface area and the potential of high fracture toughness are attractive candidates for catalyst carriers and damping materials. CNTs, due to the large surface area provided by the hollow cores and outside walls of nanotubes and temperature dependent resistivity, have been one of the most important active materials in the thermal, electrochemical and gas sensors. Incorporation of low percentages of CNTs into MO matrix has provided further point vantage for improving their sensing characteristics, such as increasing sensitivity and selectivity, lowering the detection limit, 15

extending detection capacities to an ever-increasing number of gases or pollution at room temperature [152]. Sharma and Islam [73] prepared composite sensing film by dispersing MWCNTs in alumina solution by a sol-gel process and studied the impact of alumina phases (γ, δ, θ, α) on the sensing performances. The surface morphology of composite was investigated using Brunaur, Emmet and Teller, field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). According to results, the porosity as well as surface area in case of γ-phase is highly enhanced vis-à-vis the other phases and these are led to improve performance of sensor (Fig. 6). The decrease in surface roughness after annealing from 450 ºC to 800 ºC is possibly because of the decrease in number of the free space (porosity) as well as surface area of the composite film, providing the usual space for gas molecules to absorb on the MWCNTs surface.

Fig. 6. AFM images of composite samples (a) non-annealed MWCNTs-alumina composite, and MWCNTs-alumina composite annealed at (b) 450 ºC, (c) 800 ºC and (d) 1000 ºC.[73] Sehrawat et al. [74] fabricated Al2O3/CNT composites based temperature sensor. This composite showed the temperature coefficient of resistance or sensitivity of -0.56%/ºC at a loading level of 4 wt% that was ~1.5 times higher than the conventional metals and semiconductors. In other work, CNTs were dispersed into the poly(vinyl alcohol) (PVA) solutions, and then mixed with AlCl3-based gel, followed by high temperature sintering at a temperature up to 1150 ºC in argon to form Al2O3/CNT (99/1 by weight) composite. The reduction in surface area from 135 m2/g to 57 m2/g is ascribed to the phase transition from γ to α during high temperature

16

sintering. Therefore γ-Al2O3/CNT composites can be used as high temperature catalyzing carrier.[2] Barzegar-Bafrooei et al. [75] investigated phase development and stability of boehmite-CNTs composite at different temperatures. According to results, by increasing the amount of CNTs in boehmite sol stabilized γ-Al2O3 phase, the specific surface area and the mean particle size of resultant nanopowders increased and decreased, respectively. According to observations of Gallardo-Lo´pez and his workers [76], composites with 1 vol% SWCNT exhibited Vickers hardness similar to monolithic Al2O3, but a 25 % diminution was found for higher SWCNT contents. Values of hardness obtained for monolithic Al2O3 and composites were compared to other literatures. This decrease is attributed to the fact that SWCNT located at the grain boundaries are a softer phase than the Al2O3 matrix. The literatures various reasons for decrease of hardness with higher SWCNT content were presented, such as soft phases at the Al2 O3 grain boundaries, poor adherence CNT-ceramic grain, graphitic (lubricant) nature of CNTs and poor dispersion of CNTs in the Al2O3 matrix (agglomerates), which could negate the enhancement of the mechanical properties attained by the grain refinement.[77, 78] Change of stiffness in effect of changes of the CNT content in the matrix can be ascribed to adverse effects associated with poor densification. Since CNTs have a strong influence over the grain size, density, fracture mode in the composite, so these could be responsible for the lower flexural strength in NCs.[79] Yamamoto [80] and his co-workers prepared hybrid Al2O3 composites reinforced with four types of MWCNTs having different mechanical characteristics and almost the same diameter and length by using a simple mechanical mixing method followed by pressureless sintering at a temperature of 1400 ºC. They reported that the grain size of the Al2O3 matrix is increased in response to increase in the MWCNT percentage, and is led to reduction in the mechanical properties. Also, enhancement in the mechanical properties such as the bending strength and the fracture toughness may be attributed to the grain refining impact by increasing the MWCNTs and the energy-dissipation by the MWCNT debonding and pullout. In a recent communication, Sarkar et al. [66] described that high temperature sintering might be influenced in removing of amorphous carbon from MWCNTs and exhibit superior connectivity between the tubes resulting in increased DC electrical conductivity of the NCs. The observation of TEM image (Fig. 7) showed that at temperature ˃1500 ºC for 2 h in Argon a thin layer of external graphene layers of MWCNTs (35 wt%) encapsulation was formed on the Al2O3 grains that 17

supplied a prominent interface between MWCNT and Al2O3 and helped in grain refining of sintered NCs (This sample was labeled with G17).

Fig. 7. TEM images of G17 showing (a) attachment of CNTs with Al2 O3 grain through graphene layer encapsulation; scale bar. 50 nm and (b) HRTEM image of the layered interface; scale bar. 5 nm. [66] In another study, a novel approach was proposed to synthesis of Al2O3/CNT NCs by heterogeneous nucleation. Hardness and fracture toughness of the fabricated MWCNT/Al2O3 composites were measured by Niihara equations and introduced an increase 53% and 30% over that of pure Al2O3 sintered under the same sintering condition, respectively.[63] In the prepared MWCNT/Al2O3 NCs by pressureless sintering in static argon, maximum increase (22%) in thermal conductivity over unreinforced Al2O3 (~39 W/m K) was offered by 0.15 vol% MWCNT/Al2O3 specimen, while 0.3 vol% MWCNT/Al2O3 specimen presented the highest fracture toughness (KIC~5 MPa m0.5) and flexural strength (σFS~260 MPa) than those of the pure Al2O3.[81] Bocanegra-Bernal [82] investigated the effect of a mixture of SWCNTs and MWCNTs on the fracture behavior, hardness and fracture toughness of the Al2O3 matrix. Structures comprising both SWCNT and MWCNT not only retained the high specific surface area and the small effective pore size associated with SWCNTs, but also preserved the substantial macro-porosity associated with MWCNTs. According to the present results, Al2 O3 ceramics reinforced with MWCNTs exhibited a better performance in contrast to Al2O3 with additions of SWCNTs and Al2O3 reinforced with mixture of MWCNTs and SWCNTs. However, mechanical behaviors of

18

overall NCs were lower than the pure Al2O3, which could be related to the poor and inhomogeneous dispersion of CNTs in the alumina matrix (Fig. 8 ).[82]

Fig. 8. Variation of the Vickers hardness and fracture toughness with the relative density for the different nanocomposites.[82] The studies done on the mechanical properties and potential toughening mechanisms of Al2O3/CNT composite implied that MWCNTs were embedded not only at the grain boundaries, but also within the alumina grains and provided a synergistic combination of toughening mechanisms in this composite. The synergy among conventional fiber pullout caused toughening and stretching/uncoiling toughening by embedded MWCNTs at the trans-granular positions. This is a key factor for significant improvement in the fracture toughness and performance of the hybrids (Fig. 9).[83]

19

Fig. 9. (a) Crack bridging by shorter nanotube pullouts presumably at the transgranular positions (b and c) Stretching/disentangling of CNT in 6.4 vol.% of MWCNT/alumina composite presumably at the intergranular position (large white arrow) while the black arrows show presumably shorter nanotube pullouts at the transgranular positions. Schematic illustration of pullout induced toughening at the transgranular positions and stretching/disentangling toughening at the grain boundaries by MWCNTs.[83] Other CNT/Al2O3 composites continuing 0.15 to 2.4 vol.% of CNT were prepared by Sarkar et al. [84] via simple wet mixing of as-received commercial precursors followed by pressureless sintering. The NCs containing 0.3 vol.% MWCNT induced ~23% and ~34% improvement in hardness and fracture toughness, respectively, than monolithic Al2O3. While NCs with higher percentage of CNT offered lower improvement in the mechanical properties. It is may be due to the presence of clustered and non-uniformly dispersed CNT in effect of Van dar Waal’s attraction force. As shown in Fig. 10, in batches with the lowest amount of MWCNT, bamboostructured MWCNTs were individually dispersed in the Al2O3 matrix (Fig. 10a), while with increasing CNT content, clustering of MWCNTs in the matrix increased gradually and remained agglomeration in the NC. 20

Fig. 10. Morphology of (a) MWCNT/Al2O3 batche (0.15 vol.%) powder showing individual MWCNTs; (b) MWCNT/Al2O3 batche (2.40 vol.%) powder showing agglomerated MWCNTs.[84] Carbon nanotubes as adsorption have attracted significant attentions due to possess individual feature contributed to their excellent removal capacities.[85, 86] The alumina composite reinforced by CNTs was synthesized by growing CNT over Fe and Ni-doped active alumina through CVD and washed with acid to produce nanofloral clusters. According to analysis, maximum adsorption capacity of nanofloral clusters for Cr(VI) and Cd(II) in the optimum pH was 264.5 and 229.9 mg g-1, respectively. The presence of various factors such as activated alumina, CNT, amorphous carbon and various surface functional groups such as carboxyl, carbonyl and hydroxyl present in the clusters can be caused high capacity in the as-synthesis composite.[87] Asmaly et al. [88] studied the effect CNT/Al2O3 for highly effective adsorption of 4-chlorophenol and phenol from aqueous solutions. The results of SEM and BET indicated that the alumina NPs were deposited on the nanotubes wall, and the surface area enhances from 155.5 m2/g of pure CNTs to 227.5 m2/g of CNT/Al2O3, therefore improved its absorption efficiency for the removal of phenol and 4-chlorophenol from aqueous solution. Alumina decorated onto the surface of MWCNT was considered as an effective and promising adsorbent for simultaneous removal of Cd(II) ion and trichloroethylene (TCE) from contaminated groundwater. The adsorption mechanism of Al2O3/MWCNTs toward Cd(II) ion and TCE is mainly involved in the electrostatic interactions, the hydrogen bond interactions and the protonation or hydroxylation of Al2O3 (Fig. 11). The maximum adsorption capacity of Al2O3/MWCNT was estimated 19.84 mg/g and 27.21 mg/g for TCE and Cd(II) ion, respectively, showing high adsorption ability compared to Al2O3, MWCNTs and the functionalized MWCNTs.[85] 21

Fig. 11. The schematic presentation of Cd(II) ion (a) and TCE (b) interaction with Al2O3/MWCNTs.[85]

The Other number of studies over the past decade were seek to develop CNT/Al2O3 composites with different properties that one part of them were presented in Table 2. Table 2. Representative summery of CNT/Al2O3 synthesis rout along with their performance and property Hybrids

Synthesis rout

High light

Ref.

CNT/Al2O3

ultrasonic spray The low percolation threshold in DC conductivity [89] 22

pyrolysis spark

and was attained by the addition of only 0.18 wt% of plasma the dispersed MWCNT in the Al2O3 matrix

sintering process CNT/Al2O3

Sol-gel

The sensor with lowest MWCNT concentrations [90] showed higher sensitivity to trace level ammonia (NH3) gas, due to possess enormous porosity and the

available

surface

area

in

CNT-alumina

composite. MWCNT/Al2

in

O3

plasma

situ-spark Improvement in the tensile-bending load bearing [64] ability was attributed to the strong ceramic-induced inter-wall shear resistance engineered in MWCNT structure.

CNT/Al2O3

CVD

By contrast with pure Al2O3, the CNT/Al2O3 [59] mixture showed 8.4% and 21.1% increase in hardness and toughness, respectively.

CNT/Al2O3

In-situ

Alumina-coated MWCNTs revealed an effective [91] sorbent for the lead ions removal from aqueous solution with pH range of 3-7.

CNT/Al2O3

Pressure-less

Improvement in fracture toughness of CNT/Al2O3 [92]

sintering

composites is attributed to the pinning effect of the CNTs at the grain boundaries, however with increasing CNT content above 1 vol.% the fracture toughness is decreased due to the presence of porosity and clusters of CNTs in the composites.

CNT/Al2O3

Sol-gel

The granular CNTs/Al2O3 hybrid not only showed [93] the high mechanical strength, but also used as satisfactory adsorbent for removing some organic micro-pollutants

such

as

carbamazepine

and

diclofenac sodium. CNT/Al2O3

CVD

Addition of 1.5 wt% CNTs showed a 24% increase [94] 23

in the relative fracture toughness, which attributed to uniform dispersion of CNTs, CNT bridging, crack deflection and the strong CNT/Al2O3 interface. MWCNT/Al2 O3

In situ

The

maximum

adsorption

capacity

of

the [95]

MWCNT/Al2O3 composites for the removal of reactive Red 198 and Blue 19 dyes were evaluated 91.54% and 93.51%, respectively, which have an extensive usage in the textile industry

4.2. Properties and Application of Carbon Nanotubes/Titanium Dioxide Nanocomposites Titanium dioxide is the other superior candidate for composites based MO. Nano-TiO2, due to special properties such as chemical and photochemical stability, non-toxicity, and capability for the photo oxidative destruction has received extension attention in gas sensors, photo and thermal catalysts, and photoelectron catalysts.[96-98] Physiochemical properties of CNTs reinforced TiO2 matrix composites have been intensively studied. In recent years, TiO2 NPs incorporated with CNT have been employed to construct modified electrodes in the electrochemical analysis of some biologically compounds. TiO2 by providing more active sites at the electrode surface can be used in the wide potential windows (instead of Au-NPs), and developed the stability of the electrode, as a result enhances the repeatability of the electrode reaction.[15, 99] Mao et al. [67] proposed a glassy carbon electrode based on MWCNTs/TiO2 NCs dispersed in 3-dehydroabietylamine-2-hydroxypropyl trimethylammonium chloride as cationic surfactant, for the determination of mercury in river water and industrial wastewater samples. In TiO2 nano-semiconductor, there is some limitations such as fast recombination between photogenerated electrons and holes, relatively low electrical conductivity and ineffective utilization of visible light that reduce practical electrochemical performance. Hybridizing with CNTs is one of the most facile and effective approaches to minimize its limitations. By dispersion of photocatalytic TiO2 NP on the MWCNT surface as good support could produce many active sites for the photocatalytic degradation. In this system, the excited electrons in the conduction band of TiO2 are transferred to the CNTs during photo-irradiation of the composite, 24

and prevented the recombination of electron–hole pairs in the TiO2. Therefore it can improve antibiofouling properties of the mix matrix prepared membrane.[100-102] In essence, TiO2 is an n-type semiconductor, but in the attendance of CNTs, photogenerated electrons may easily migrate in the direction of the CNT surface with lower Fermi level. Therefore CNT as an electron sink leave a surplus of holes in the valence band of the TiO2, which can move to the surface and react; therefore the TiO2 wonderfully behaving as a p-type semiconductor.[15] Other advantages of TiO2 combined with CNT are formation the high quality active sites to facilitate the adsorption of contaminations on the catalysts and improvement the visible light photocatalytic properties.[103] Yan and et al. [102] synthesized TiO2 NPs coated CNT through the precipitation of TiCl4 precursor on the acid oxidized MWCNTs and used of them in polyethersulfone. Preparation stages of TiO2 decorated MWCNT were presented in Fig. 12. The experiments showed that the MWCNT/TiO2 was a good modifier for the formation of antibiofouling microporous polyethersulfone nanofiltration membrane, which this behavior explained by improvement in hydrophilicity and photocatalytic oxidizing activity and diminishing of surface roughness of the membranes.[102] According to TEM image of MWCNT/TiO2, the numerous nano-TiO2 with sizes in the range of 10–20 nm were uniformly attached on the surface of CNTs (Fig. 13).

Fig. 12. The schematic of preparation of TiO2 coated MWCNTs.[102]

25

Fig. 13. TEM image of TiO2 coated MWCNT.[102] Liu et al. [103] studied the photocatalytic activities of CNT/TiO2 composites fabricated by hydrothermal method using titanic acid nanotubes as the TiO2 precursor for the degradation of methylene blue (MB). According to results of analysis, when the hydrothermal reaction time is equal or greater than 6 h, titanate nanotubes in the composites entirely transform into anatase TiO2 NPs (Fig. 14). In this experiment, CNT facilitates visible light absorption, so plays an important role in injecting electron into TiO2 conduction band and triggers the formation of very reactive radicals superoxide radical ion O2•‾ and hydroxyl radical OH• [104], which are the essential species for the degradation of MB.

Fig. 14. TEM and HRTEM images of CNT/TiO2-12 h. Mechanism of the improved visible-light photocatalytic activity of the CNT/TiO2 composite.[103]

TiO2 NPs decorated on CNT have been established to yield positive effects on the photochatalytic activity, in applications from wastewater treatment to H2 production, by inducing synergies between constructors. Silvaand et al. [105] studied the use of CNT/TiO2 composite for 26

the photocatalytic production of H2 from biomass-containing aqueous solutions, namely from methanol and saccharides. The processes of functionalization of the MWCNT and the deposition of TiO2 play an important role in the nucleation and binding of TiO2 and influence the photocatalytic activity of the synthesized hybrids. The photocatalytic study done by Djokić and et al. [106], the maximum UV-A induced activity was found for TiO2/oxidative-MWCNT, while the TiO2/amino functionalized-MWCNT catalysts showed somewhat lower degradation rates. This the development of photocatalytic activities was ascribed to the more effective electron transfer properties of the oxygen- than amino-containing functional groups, which improves the efficient charge transportation and helps in reducing electron-hole recombination. For the first time, Tarigh and et al. [107] exerted a simple, quick and efficient method for the fabrication of a recoverable and effective MWCNT/TiO2 photocatalyst by electrostatic attraction. The SEM and TEM images, TiO2 NPs were uniformly spread on the CNT surface (Fig.

15). MWCNTs in TiO2–MMWCNTs are mainly responsible for the absorbent the

Malachite Green and transfer it to the surface of TiO2. In addition, MWCNT acts as a photosensitizer and by generating of electron under the irradiation of UV light forms superoxide radical ion and/or hydroxyl radical which is agent for degradation of Malachite Green.[107, 108]

Fig. 15. (a) SEM image and (b) TEM image of magnetic MWCNT-TiO2.[107]

27

Zhou and et al. [108] investigated the photocatalytic activity and adsorption performance pure TiO2, physical mixture of TiO2+SWCNT, TiO2/SWCNT and Degussa P25 (as reference) before and after UV irradiation (Fig. 16). On the basis of results, bonding TiO2/SWCNT composites showed the highest photocatalytic activity in destruction of organic pollutants, related to role of SWCNT in obtaining the large surface area in the composite.

Fig. 16. Photodegradation of rhodamine B aqueous solution for P25, TiO2, TiO2+SWCNT (mixture) and the bonding TiO2/SWCNT composite.[108]

Similarly in other studies, aqueous pollutants including methylene blue [109-112], benzene derivatives [113], and carbamazepine [114] were efficiently photodegraded by CNT/TiO2 composites. They reported that the bond of carbon–oxygen–titanium can expand the light absorption towards longer wavelengths and therefore potentially leading to the enhancement of the photocatalytic activity.[115] A photoelectrochemical (PEC) lactic dehydrogenase (LDH) biosensor based on a MWCNT/TiO2 NC was developed for detecting lactate in milk (Fig. 17).[101] Analytical performance of sensor was considerably improved and a dynamic range 0.5 to 120 µM, a sensitivity of 0.0242 µA µM-1 and limit of detection of the biosensor of 0.1 µM were estimated.

28

Fig. 17. The fabrication process and photoelectrochemical reaction mechanism of MWCNT TiO2 NP|LDH| nicotinamide adenine dinucleotide (NAD+)| Indium tin oxide. [101]

As reported in literature, TiO2 NPs due to their high ionic conductivity and unique chemical properties could improve the electrochemical performance of anode materials. Huang et al. [116] used of TiO2 NPs to decorate CNT by mechanically blending and reported that the TiO2/CNT NCs possess an improved cycling stability and higher reversible capacity than the pure CNTs. Gao et al. [117] studied the effect of the TiO2 particles on the electrochemical performance of CNT and showed that the as-synthesized TiO2/CNT composite films containing 19 wt.% of tetrabutyl titanate can be promised as binder-free flexible electrodes for lithium-ion battery (LIB) applications comparing to the pristine CNT. Such observations can be attributed to the relatively larger surface area and pore volume comparing with pristine CNT films, providing more active site for Li+ intercalation and give rise to a better ion storage capacity. In another recent report by Yan and co-workers, electrochemical performances such as exceptional cycling stability, good high rate durability, and reduced resistance of TiO2/CNT hybrids were improved than that of the previously reported TiO2 based supercapacitors.[100] The prepared TiO2/CNTs with beneficial synergistic nanosized effects demonstrated a reversible capacity of 200 mA.h g-1 at a current density as high as 0.1 Ag-1 as an anode in LIB. Recently, Zhang et al. [118] indicated that the removal rate of MB for TiO2/Fe3O4/MWNTs NC containing 30 wt% TiO2 content estimated 92.4% after 100 min UV irradiation, which was about 2 times of pure TiO2. Yan et al. [119] synthesized titania supported on the CNTs through a facile but effective solvothermal reaction. Then, they examined the effectiveness and selectivity of the CNTs/TiO2 composites as absorbent in the enrichment and analysis of phosphopeptides from various complex samples by MALDI-TOF MS. As shown in the SEM and TEM images (Fig. 18), numerous tiny TiO2 NPs are homogeneously coated on the individual CNT to decrease the interface surface energy. The diameter of the TiO2 NPs is almost 5-10 nm.

29

Fig. 18. (a) SEM and (b) TEM images of TiO2 NPs coated carbon nanotubes. The inset in (b) is the enlarged image showing the complete encapsulation of CNTs by TiO2 NPs. [119] There are a various routs that can be utilized to fabrication CNT/TiO2 hybrids comprising, solgel synthesis [96], mechanical mixing, electro-spinning, electrophoretic deposition and chemical vapor deposition that along with their effective properties were highlighted in Table 3. Table 3. Representative summery of CNT/TiO2 synthesis route along with their significant performance and property. Hybrids

Methods

Highlights

Ref.

CNT/TiO2

biomimetic

Direct contact between CNT and TiO2 [120]

mineralization

introduced a band gap narrowing and large surface area and thereby enhanced visible light photocatalysis.

CNT/TiO2

Electrospinning

CNT/TiO2 exhibited high capability to adsorb [121] and photodecompose of Rhodamine B without additional activating processes, than that of commercial porous carbon.

CNT/TiO2/gra phite plate

Sol-gel

CNT/TiO2 immobilized on thin graphite plate [98] as a nano-photocatalyst, enhanced adsorption ability of methyl orange in aqueous solution because of the high specific area, effective pore-sizes and available inter-spaces.

30

CNT/TiO2

Sol–gel

Nanocomposite with 12.8% CNT exhibited [122] maximum increase in photocatalytic activity for decomposition of methylene blue while the decrease in activity was found to composite with higher CNT loading (20%).

MWCNT/TiO2

sol–gel

In MWCNT/N, Pd co-doped TiO2 composite, [96] nitrogen

was

led

to

increase

visible

photoactivity, and palladium was led to decrease charge recombination. MWCNT/TiO2

atomic

Carbon doping yielded a band-gap state near [123]

layer deposition

to the TiO2 valence band and speeded light absorption to the visible region.

CNT/TiO2

layer-by-layer

CNT/TiO2 nanowire film was more suitable [124]

nanowire film

deposition

for usage as photocatalytic active filtration owing to its less pore blockage.

CNT/TiO2

catalytic

Gas

sensing

material

based

on [97]

pyrolysis

TiO2/MWCNTs composite shown the lowest threshold electric field required to draw current, and also displayed the highest sensitivity of 16% to ethanol.

CNT/TiO2

In situ

Nanohybrids

significantly

improved

the [125]

performance of the anode electrode of microbial fuel cells (MFC). The results revealed

that

the

current

density

and

coulombic efficiency of CNT@TiO2-MFC increased to 4.8 Acm-2 and 24.5% while in TiO2-MFC were 2.7 Acm-2 and 11.3%, respectively.

31

CNT/TiO2

Sol–gel

The

CNT-TiO2

microextraction mechanical efficiency,

coated fiber

strength, and

solid-phase [126]

indicated high

excellent

good

extraction

thermal

and

chemical stability compared to TiO2 and CNT alone.

4.3. Properties and Applications of Carbon Nanotubes/Zinc Oxide Nanocomposites Zinc oxide is a semiconductor oxide with unique optical and electrical properties including wide band gap (3.37 eV), large excitation binding energy (60 eV), superior electron communication features, low cost and environmental friendliness. The deposition of ZnO on the CNT sidewall can create NC with the excellent performance in various applications such as photocatalysts [127], electron emitters [128], field effect transistors [129], optical switches [130], photodetectors [131] and sensors.[132] In ZnO NPs, the recombination rate of the photo-induced electron-hole pairs is generally faster than the surface redox reactions, resulting in the low photocatalytic capability. One of the efforts to suppress the electron–hole recombination is use of MWCNTs as an electron sink. In semiconductor MWCNT/MO NCs, MWCNTs remarkably enhance the migration of photo-induced electrons in the photocatalytic process, resulting will synergistically enhance the photocatalytic performance through the efficient separation of the photo-generated electron/hole pairs (e−/h+) on the surface of ZnO.[133-135] Zinc oxide nanowire/MWCNT NCs synthesized by one-step hydrothermal, proved that the photocatalytic efficiency of as synthesized NC is 3 times higher than that of pure ZnO nanowires.[134] In a report, Wayu and his group [136] controlled the morphology and the size of ZnO in the preparation of electrocatalytically active ZnO/COOH-MWCNT using hydrothermal treatment temperature, prior to attachment to modified CNT for improving sensitivity and selectivity in the detection of H2O2. High activity for H2O2 reduction was attained when nanocomposite precursors with a roughly semi-spherical morphology formed by increasing the hydrothermal treatment temperature from 50 to 90 ºC. It is indicative of high selectivity in H2O2 detection from a host of other interfering analytes.

32

Chang et al. [137] studied the UV photoresponsivity and photovoltaic effects of ZnO/CNT hybrid structure. Their results showed a very fast UV photo-responsivity (up to 18 times) and photovoltaic efficiency (up to 4 times) compared with the pure ZnO film. Presence of MWCNTs can enhance the catalytic activity of ZnO NPs for the acetaldehyde removal [138] and cyanide degradation in aqueous media [139] under UV irradiation. Zinc oxide NPs are coated on MWCNTs via thermal hydrolysis method and provided a great enhancement in photocatalytic activity of as-prepared MWCNT/ZnO NC. Khongchareon and et al. [140] investigated the effect of addition of MWCNTs in different percentages (0, 2, 3, 5, 7, 9 wt%) into the gel polymer electrolyte of ZnO dye-sensitized solar cells. According to results, highest efficiency of 0.75% was achieved with 5 wt% of the MWCNT. This enhancement can be attributed to the suppression of charge transfer resistance at the ZnO photoelectrode/electrolyte interface. The proper introduction of MWCNT effectively decreases the charge transfer resistance of the electrolyte material and leads to the increase of the shunt resistance which can be confirmed by electrochemical impedance spectra.[141] Suriani and et al. [142] emphasized role of CNTs/ZnO sample in enhancement field electron emissions (FEE) performance. The turn-on and threshold field of the CNTs/ZnO sample were 4.6 and 6.7 V/mm, which compared to pristine ZnO showed the better FEE performance. The presence of the highly conductive CNTs created more electrons in the sample, causing to lower turn-on and threshold fields and higher emission currents in the CNTs/ZnO NC sample. Yang and et al. [143] fabricated the ZnO nanoparticle-coated SWNT network sensors using rf magnetron sputtering for investigating dimethyl methylphosphonate (DMMP) gas sensing properties. The resistance of SWCNT/ZnO was in range of 103 to 105, implying that the current path was along the SWCNT network rather than through the ZnO NPs. Consequently, the surface/interface characteristics of ZnO NPs may alter from p-type to n-type depending on the deposition time. Hmar And et al. [144] studied role of ZnO/MWCNT in dielectric permittivity and ac conductivity of flexible, transparent, high dielectric and photoconductive of the PVA thin film. ZnO/MWCNT-PVA exhibited superior dielectric behavior which may be attributed to the large interfacial area, grain boundaries, dislocations, vacancies and dangling bonds in ZnO nanosheets, inducing the positive and negative space charges and space-charge polarization. Assembling of ZnO NPs on the surface of CNTs can lead to an effective optimization of the electromagnetic parameters.[145] Owing to the reaction of an amorphous carbon layer on the tube wall of CNTs 33

with the oxygen atoms of ZnO lattice is generated of oxygen vacancies in lattice during sintering. This process produces a higher concentration of charge carriers in ZnO for more relaxation polarization and dielectric loss in electromagnetic field. In literatures from the XRD result and the zoomed-out/magnified TEM images are used for characterization of the structure of ZnO/CNT composite. For example in the XRD pattern, the diffraction peak at 2θ = 26º is attributed to the graphitic reflection of the MWNTs and the other peaks represented ZnO NPs crystallized in a hexagonal lattice structure. Finally from sharpest peaks could calculate size of the NPs by Debye-Scherrer equation. These values were in good agreement with the results of the TEM analysis.[146] The crystalline structure and surface morphology ZnO/CNT composites prepared by sol-gel, not only exhibited the hexagonal wurtzite structure with (100), (002), (101), (102) and (110) peak orientation plane for composites with CNT-doped 0.1%w/v but also showed that the ZnO grains size was increased with annealed temperatures.[128, 147, 148] More recently, Safa reported that UV-detection parameters such as time of response, photo responsivity, sensitivity and signal to noise ratio of ZnO/CNT composites synthesized by chemical precipitation process are much bigger than that of pure ZnO nanourchin and commercial nanopowder.[133] In the CNT impregnated sample, photogenerated electrons by migrating to conduction band of CNT are led to effectively separate the photogenerated pairs and inhibit them from recombination. Also, the highly conductive CNTs found a conductive transportation pathway in the ZnO framework for direct transportation of charge carriers. The results showed that CF4 plasma treatment of ZnO/CNTs is an effective way to increase the electrical conductivity and form field emitter devices.[149] Amongst all the samples, specimen exposed to 30 min of CF4 plasma, had lowest threshold field of 2.1 V/µm. The SEM images of ZnO/CNTs showed the morphology of the 30 min specimens is thin with some sharp wrinkles rises up on the surface. Recently, Moyo and et al. [150] prepared the sensor by modification of glassy carbon (GC) electrode with MWCNT/ZnO composite. They displayed outstanding electrocatalytic properties in the direction of oxidation of triclosan by giving higher currents and lower oxidation peak potential compared to the bare GC electrode. Aravinda et al. [151] used magnetron sputtering as a facile, green and highly efficient method for coating ZnO on functionalized MWCNT. These composites were used for super capacitor electrodes and

34

demonstrated good cycling stability in the repetitive charge/discharge test. A representative selection of other CNT/ZnO hybrids and their applications are summarized in Table 4.

Table 4. Representative summery of CNT/ZnO synthesis rout along with their significant performance and property

Hybrids

Methods

Analyte

Highlight

Ref.

ZnO-

chemical route urea

Sensitivity and a long shelf-life [152]

MWCNT/indium

deposition

of

biosensor

hybrid

was

estimated about 43.02 µA mM−1

tin oxide

cm−2 and more than 4 months (>16 weeks) respectively. Graphene-

one-pot

CNT/ZnO

method

Glucose

The as-synthesized composite [153] with appreciable repeatability, reproducibility and remarkable stability

exhibits

enhanced

response to glucose. ZnO/CNT

Ball mill

By increasing CNT content (0.1- [154]

−

1 wt.%) in ZnO, DC electrical conductivity was increased from 6.55 × 10−5 to 4.01 × 10−3 S/cm and optical band gap values was decreased compared to un-doped ZnO. ZnO/CNT

thermally

The investigations showed field [155]

−

physical

enhancement factor, β, for pure

vapour

CNTs is 2324 and for the

deposition

ZnO/CNT

synthesized

by

sputtering and oxidation are 2374 and 2574, respectively. So

35

coated CNTs by oxidation have more efficient emissivity and are the best emitter. ZnO/CNT

atomic

layer −

Nanocomposite was applied for [156]

deposition

UV photodetectors in two types p- and n-type

SWCNT/ZnO

Chemical bath −

The performance of the dye- [157]

deposition

sensitized solar cells (DSSC) depend

on

the

loading

of

SWCNTs, as the higher amount of SWCNTs (0.5 wt%) have no effective in the improvement performance

of

ZnO-based

DSSC. SWCNT/ZnO

sol-gel

NO2

SWCNT/ZnO gas sensor due to [158] high sensitivity and favorable response properties was applied in detecting NO2 gas at levels of just 1 ppm and at temperatures close to room temperature

CNT/ZnO

radio-

ZnO/CNT nanocomposites due [159]

−

frequency

to possess photoluminescence

magnetron

and

sputtering

properties can be potentially

increase

combined

field

emission

with

piezotronic

existing

devices

for

applications such as variable depth E-beam lithograph ZnO/MWCNT

ultrasonication

H2O2

Electrode

modified

composites, electrochemical 36

as

with [132] an sensing,

exhibited stability and selectivity and used for on-site, real time monitoring of H2O2, in a welldefined

swimming

pool

environment ZnO/CNT

Ultrasonic

The results showed present 5

−

[160]

wt% CNT provide straight routes to facilitating electron transfer in DSSC, so that power conversion efficiency of ZnO/CNT is 6.25%, which is 35.57% higher than the pure ZnO (4.61%).

4.3. Properties and Applications of Carbon Nanotubes/Iron Oxide Nanocomposites Iron oxide (i.e. Fe2O3 and Fe3O4) is one of the promising magnetic materials which makes other composite with CNTs. It has stood as one of the most attractive magnetic MO and has received widespread attentions due to its unique physical and chemical properties and various advantages such as high reversible capacity, rich abundance, low cost, and eco-friendliness.[161, 162] According to literatures [163, 164], the implantation of iron oxides on the surface of CNTs are accomplished by physical and/or chemical methods. CNTs after encapsulation with Fe3O4 exhibit super-paramagnetism. In pretreated MWCNTs, the magnetic saturation value is the slight more than primary MWCNTs, which may be resulted from the existence of Fe [164]. Rajarao et al. [16] illustrated simple, rapid, solventless, economic, environmental and scalable method to preparation MWCNTs/iron oxide NPs from thermal decomposition of metal formats without use of any solvent or reducing agent. Diffraction peaks obtained the XRD pattern (2θ. 33.1, 35.6, 49.5 and 54.5°) confirm the formation of iron oxide NPs on MWCNT in the presence of oxygen. The results obtained from the effect of heat treatment temperature on the structure and the magnetic properties of the MWCNTs/α-Fe2O3 composites revealed that the composites treated at 450 ºC and 600 ºC have good magnetic behavior and exhibit the ferromagnetic property. whereas

37

the composites treated at 750 ºC due to the Fe3O4 crystals growth and agglomeration of MWCNTs reveal the paramagnetic properties.[165] Magnetic particles is other candidate used to adsorb pollutant from aqueous or gaseous effluents. After adsorption, they can be separated from the medium by a simple magnetic process. Incorporating of the magnetic particles with CNT yields the promising adsorption materials due to their highly porous and hollow structure, large specific surface area and strong interaction between aromatic molecules and the highly polarizable graphite sheets of CNTs.[166] Ji and et al. [167] reported the effect of MWCNT/Fe3O4 and as-prepared MWCNTs/Fe3O4 NCs modified with 3-aminopropyltriethoxysilane (MWCNTs/Fe3O4–NH2) as adsorbent to investigate the adsorption kinetics and adsorption isotherms of tetrabromobisphenol A (TBBPA) and Pb(II) from wastewater. The maximum adsorption capacities for TBBPA and Pb were attained by MWCNTs/Fe3O4–NH2 in optimum PH. After adsorption, both adsorbents were separated by an external magnetic field within several seconds. So observed in TEM image (Fig. 19), Fe3O4 NPs are uniformly decorated onto the surface of MWCNTs. The presence of MWCNT among Fe3O4 NPs prevents more intimate contact and magnetic interaction and further improves the aggregation between pure Fe3O4 NPs.

Fig. 19. (a–c) are the TEM images of MWCNTs/Fe3O4 nanocomposites under different magnifications.[167] The adsorption of 1-naphthylamine on MWCNT/iron oxides/β-cyclodextrin composite (denoted by MWCNTs/iron oxides/CD) was studied by Hu and et al. [166]. On the basis of results, MWCNTs/iron oxides/CD by the hydrophobic interaction and the π–π interaction play a key role in the removal of the organic pollutants (Fig. 20). According to values of standard enthalpy (∆Hº) and standard entropy changes (∆Sº) the adsorption of 1-naphthylamine on as-prepared

38

composite is endothermic and spontaneous and maximum adsorption capacity is equal with 200 mg/g.

Fig. 20. The proposed mechanisms of 1-naphthylamine adsorption on MWCNTs/iron oxides/CD.[166] Adsorption behavior of multiwall CNT/iron oxide magnetic composites showed that pH and ionic strength influenced the pre-concentration and solidification of Ni(II) and Sr(II) (Fig. 21).[168] As results, adsorptions of Ni(II) and Sr(II) are increased with increasing pH and decreasing ionic strength. By increasing PH, the negative surface charge is caused by the deprotonation of functional groups on the surface of the magnetic composite and facilitated the attraction between the anionic surface sites and cationic metal ions.

Fig. 21. Effect of ionic strength on Sr(II) adsorption onto the magnetic composites as a function of pH, at Cinitial [Sr(II)] = 0.018 mg/L, m/V = 0.6 g/L and T = 25 ± 2 ºC.[168] 39

Asmaly and et al. [169] investigated effect of Fe2 O3 impregnated CNTs in the removal of phenol from water. Dispersion of Fe2O3 NPs coated on the nanotube surface was displayed in Fig. 22. It can be seen that Fe2O3 NPs were homogenously dispersed on the surface of CNTs with an average particle size ~6 nm. So, their surface area enhanced and hence increased the number of sites for adsorption of phenol by the π-π electron-donor-acceptor interaction and/or hydrophobic effect.

Fig. 22. (a) Back scattering FE-SEM images for CNT–Fe2O3 (b) TEM image of CNTs (c) TEM image of CNT–Fe2O3.[169] CNT-iron oxides magnetic composites have been successfully applied as adsorbent for removal of different targets from water such as Europium [170] in the presence of poly(acrylic acid) as a surrogate and Cr(III).[171] In order to improve the electric conductivity Fe3 O4 and reinforce the electrode structure Pang and et al. [161] designed MWCNT/Fe3O4 composite by electrodeposition of Fe3O4 in the presence of dispersed MWCNT. The nano-porous MWCNT/Fe3O4 composites demonstrates the reversible capacity of 601.0 mAh/g after 60 cycles that this value was 67.3% of the first discharge capacity. The improved reversible capacity, capacity retention, and high-rate performance than the other samples are concluded as a consequence of the porous structure of Fe3O4, better conductivity of porous Cu substrate and MWCNTs, and the morphology change of Fe3O4 NPs upon the addition of MWCNTs. The SWCNT/α-Fe2O3 hybrid films were fabricated by a simple heat treatment of the as-synthesized SWCNT macro-films. The SWCNT/α-Fe2O3 hybrid film shows the highest Li+ chemical diffusion coefficient, and thus possesses an optimal electrochemical performance in terms of the highest specific capacity over 1000 mAh/g and an excellent cyclic stability up to 100 cycles.[172] The improved electrochemical performance can

40

be attributed to the following factors: (1) SWCNT macro-films with higher conductivity and flexibility can facilitate the charge transfer processes and accommodate the volumetric change of the α-Fe2O3 NPs; (2) The porous walls of the nanotubes increase the electrode-electrolyte contact area, shorten the distance of diffusion and provide more reaction sites for lithium ions. (3) The CNTs increase the conductivity and hinder the aggregation of the Fe2O3 NPs during the insertion/extraction processes of lithium ions. In order to improv performance of carbon paste electrode for effective electrochemical hydrogen production was applied the iron oxide in bulk, nano sized particles, and CNT/Fe2O3.[173] By determineing of the hydrogen evolution reaction performance at the optimum conditions was characterized that the electrocatalytic activity of modified electrodes was in the order of Fe2O3-CNT>nano-Fe2O3>bulk Fe2O3. In other work prepared a biosensor based on catalase using a CNT/Fe3O4 interface for detection of H2O2 in milk samples and determination of quality of milk.[174] As a result, the linear range of the prepared amperometric sensor was between 1.2 and 21.6 µM with a quick response time of less than 1 sec. This may be related to the synergy of CNT and Fe3O4 NP in electron transfer between catalase enzyme and the electrode. One of the medical activation of magnetic CNT is determination of aconitine alkaloids (obtained from Aconitium, a vegetable used in traditional Chinese medicine) in human serum samples by extraction with MWCNT-Fe3 O4 and subsequent HPLC measurement.[175] Under optimal conditions, the recoveries of spiked serum samples were detected between 98.0% and 103.0% and RSDs were estimated from 0.9% to 6.2%. A one-step synthetic by the in situ formation was applied by Morales-Cid et al. [176], who prepared magnetic-CNTs from FeCl3. 6H2O and a suspension of MWCNTs in ethylene glycol. The obtained hybrids were used for sampling and extraction efficiency in the determination of both fluoroquinolones (FQs) and two quinolones (Qs) at trace levels by ultra-performance liquid chromatography (UPLC). The recoveries sample were in the range 78.8-102.4% with the relative standard deviations (RSD)s less than 2.4%. So, a sorbent phase with magnetic susceptibility facilitates separation of the solid material from the solution by means of an outwardly applied magnetic field. There are the other several publications associated with the preparation and properties of CNT/Fe3O4 NC materials, listed in Table 5

41

Table 5. Representative summery of CNTs/iron oxide synthesis rout along with their performance and property Magnetic

analytic

High light

Ref.

sorbent Polydimethylsilo xane

6

phthalic By coupling the magnetic solid-phase extraction [177]

– acid esters

MWCNT/Fe3O4

with GC-MS instrument, the recovery values of the samples estimated from 91.5 to 97.8% with the RSDs <10.64% in bottled water samples

MWCNT/Fe3O4

5

phthalate Magnetic solid phase extraction-chromatography- [178]

monoesters

mass

spectrometry

method

applied

to

the

recoveries of the specimens evaluated from 92.6% to 98.8% with the RSDs less than 10.7% in human urine samples. MWCNT/Fe3O4

13 phthalate The recovery of these compounds and RSD value [179] esters

were in the range of 86.6–100.2% and of less than 10%, respectively in drinking water.

Magnetic-

Cationic dyes High adsorption capacity and fast adsorption rate [180]

modified

obtained from dye-containing wastewater

MWCNT CNT/iron oxide

Arsenic (As)

The maximum adsorption capacities were ranged [181] 47.41 and 24.05 mg.g-1 for As(V) and As(III) using an external magnetic field

Carboxylated-

Cu(II), Co

MWCNT/Fe3O4

The good dispersion stability, acid and alkali [147, resistivity, magnetic properties in deionized water, 182] and Cu(II), Co removal were obtained by hybrid

MWCNT/Iron oxide

Eu(III)

The large sorption capacity and the magnetic [183] separation of Eu(III) on the hybrids was strongly dependent on pH values and weakly dependent on 42

ionic strength MWCNT/Fe3O4

polycyclic

The recoveries and RSDs of analyzed PAHs were [184]

aromatic

determinate 81.3%-96.7% and less than 12.7%,

hydrocarbons respectively in grilled meat samples (PAHs) MWCNT/Fe3O4-

bisphenol A Hybrid was applied to the determination of BPA [185]

magnetic

(BPA)

by HPLC with the recoveries of 87.3-95.4% in tap

imprinted

and natural water samples

polymers MWCNT/Fe3O4

BPA,

Results obtained tap water, river water and snow [186]

bisphenol F, water show that the recovery of these compounds bisphenol

F was in the range of 88.5–115.1% with RSD less

diglycidyl

than 10%.

ether MWCNT/Fe3O4

sulfonamide

In extraction of sulfonamides based on magnetic [187] separation from foodstuff sample (eggs), the limits of detection was obtained in the range of 1.4-2.8 ng g-1.

MWCNT/Fe3O4

4 agents

nerve The recovery 59.7-90.6% and limits of detection [188] and 0.05-1.0 ng mL-1 were measured for four highly

their markers

toxic nerve agents and their five non-toxic markers in water by GC after magnetic-solid phase extraction.

3-

Heavy metal Thiol-functionalized CNTs/Fe3O4 NCs showed the [189]

mercaptopropyl

ions

superparamagnetic performance and high specific

triethoxysilane-

surface area for heavy metal ions removal with

CNT/Fe3O4

Langmuir isotherm model.

β-cyclodextrin

Zn(II)

CD/MWCNT/iron oxide can support long term use [190]

43

(CD)-

as a cost-effective material in the decontamination

MWCNT/iron

of Zn(II) from aqueous solutions with minimum

oxide

replacement costs

Starch-

Anionic and High contact surface between magnetic MWCNT [191]

MWCNT-iron

cationic dyes

oxide

and

dye

prevents

from

the

formation

of

agglomeration and facilitates the diffusion of dye molecules to the surface of MWCNTs.

MWCNT/nanoiron oxide

Cr(III)

Dependent on PH value, the absorption capacity of [171] magnetic CNT for chromium ions removal enhanced by increasing the agitation speed

4.5. Properties and Applications of Other Carbon Nanotubes/Metal Oxides Composites In addition to discussed cases, there are some reports on CNT hybrids with inorganic MOs such as SnO2, ZrO2, and MnO2 for exceptional performance in applications such as gas sensors, photovoltaics, enhanced ability to trap electrons, heterogeneous catalysis, and photocatalysis [192-194]. According to literatures, SnO2, as one of the most important functional metal oxides and potential candidates for production anode material of LIBs, has a high theoretical capacity. This MO by anchoring on the surface CNT can store the large amounts of lithium during cycling and enhance battery performance in term of capacities for high performance free standing anodes.[195-198] For instance, a hybrid of unzipped CNTs and SnO2 nanorods was synthesized for improving reversible capacity (to 856 mA h/g at 0.1 A/g), cycling stability and rate capability at various current densities, which was much higher than that of bare SnO2 and unzipped CNTs.[199] Nitrogen-doped multiwall carbon nanotubes (N-MWCNTs) prepared from npropylamine were excellent electric conductors which by decorating the SnO2 NPs on the surface of N-MWCNTs could be used as the cathode materials for electrochemical reduction of CO2 to formate.[200] Insights into the electrochemical fields express that the hybrid composite indicates exceptional cycling performance with high reversible capacity, enhanced redox capacitance and 44

interfacial capacitance in contrast with SnO2 nanocrystals, implied the stable interfaces and the robust structure of the hybrid composite.[197] Mehdinia and et al [201] asserted that the MWCNTs/SnO2 NC can be used as a desirable anode materials for the mcrobial fuel cells (MFCs). The as-prepared hybrids had the high conductivity and the large unique surface area owning to the free interspaces between the constituents, which provided a straight expressway for efficient Li+ transport. These features propose the unique electrochemical properties which can be used as the high-performance anodes for electrochemical energy-storage for MFCs and LIBs.[201-203] In a research work by Li and et al. [203] SnO2/MWCNT NCs were synthesized by decomposition and oxidation of tin(II) 2-ethylhexanoate within the MWCNT networks in air at a low annealing temperature (350 ºC) for 1 h. In prepared CNs, MWCNT was preserved and served as ideal support for depositing of SnO2 NPs and hindering their aggregating. The results analysis obtained in Fig. 23 proved that SnO2 NPs with average size of ~5 nm were well dispersed either on the surface or within the MWCNTs network. The lattice fringes (0.33 nm) in HRTEM measurements is indexed to the (110) reflection of the pure tetragonal phase of the crystalline SnO2.

Fig. 23. (a) XRD pattern, (b) SEM and (c-d) HRTEM images of the SnO2-MWCNT nanocomposites obtained by annealing the C16H30O4Sn-MWCNTs precursor in air at 350 ºC for 1 h.[202] 45

Karami Horastani et al. [204] showed that by adding appropriate amount of SWCNT to the SnO2 matrix enhances the electrical conductivity and the methane sensitivity of SnO2 gas sensor. They suggested that the two different chemical and electrical mechanisms could interpret the sensitivity behavior of the samples as the main reasons for this process. Among pseudocapacitive transition-metal oxides, manganese dioxide (MnO2) has recently received a great deal of attention as electrode material in electronics devices like lithium ion battery, super capacitors, sensors and others. These are due to high theoretical specific capacitance, low cost, environmental friendliness, and natural abundance of MnO2. However, the poor electrical conductivity and significant volume expansion/contraction during repeated cycling processes limit MnO2 from practical use in the electrochemical devices.[205-210] One of the strategies to overcome this drawback, is to deposit a thin layer MnO2 NPs onto the external and the internal surface of highly conductive materials like CNTs or graphene. Loading MnO2 onto CNTs architectures can be administered through various techniques such as chemical coprecipitation, thermal decomposition, redox reaction, sol-gel, and electrodeposition.[208, 210] Zirconium dioxide (ZrO2) is the other MOs from kind of n-type semiconductor with a wide band gap (~5.8 eV). The most of CNT/ZrO2 bulk composites display either a decrease [211] or slight increase [212, 213] of the fracture toughness compared to that of the corresponding ZrO2. Kasperski and et al. [216] prepared DWCNT/yttria-stabilized zirconia (YSZ) composite powders by spark plasma sintering (SPS) and reported decrease the fracture strength compared to YSZ (7 7 vs 10.3 MPa

m1/2

), nevertheless the value reported is among the

highest reported up to now (for 1.2 wt% CNT). The weak interfacial bonding between the CNTs and zirconia as well as lack of homogeneity in composite have been pointed out as responsible for the lower fracture toughness of the composites in comparison with zirconia. In other work by Shen et al. [214] observed that a reduced Vickers hardness and increased fracture toughness were obtained with the addition of CNTs. Increase in fracture toughness of ZrO2/MWCNT composite can be related to the degree of interfacial bonding between MWCNTs and zirconia grains. This phenomenon is due to new toughening mechanism as a result of the incorporation of CNT at grain borders and to the capability of CNT to prevent grain border sliding and creep at high temperature. Also, the stretching/uncoiling and pulling out of the CNTs create elastic energy and

46

friction to dissipate the crack energy. So, the improvement in the fracture toughness is controlled by the combined effect of crack bridging and CNT pull-out. Wang et al. [215] reported ZrO2/MWCNT with the mesoporous structure not only can achieve a high surface area (312 m2/g) and a bimodal mesoporous structure (3.18 nm and 12.4 nm) but can also develop the electrical conductivity with the combination of conductive CNTs. The activation energy of the zirconia-CNT composites shows that there is a significant reduction in the activation energy above the percolation threshold of the composite with higher content of CNTs. As a result, the percolating network of CNTs generates an easier diffusion pathway for the metal ions at the grain boundary.[216] These few examples demonstrated the great potential applications of these composites and illustrated that the next technological boundaries will be expanded by understanding and optimizing substance mixtures and their synergistic functions, rather than through a superior understanding and application of a specific material. 5. Comparison between of CNT/MOs We observed that CNTs/MO composite materials can be fabricated by a range of different methods which most widely applied methods are mechanical mixing of, sol-gel, electrospinning methods, atomic layered deposition and chemical vapour deposition (CVD). However uniform coating of MO on CNTs may be accomplished by CVD and electro-spinning methods, these techniques are somehow not easy, require specialized equipment. The sol–gel process is undoubtedly the simplest and the cheapest one, although they usually lead to a heterogeneous coating of CNTs by MO. The application of sol-gel technique in composite preparation has some advances, e.g., its versatility and the possibility to achieve high purity materials, high stability due to chemical binding between coating and support, and possibility to control the coating thickness. [15, 73] In prepared CNT/MO, the final properties and performance of composite extremely depend on the type and quality of CNT, the chemical nature, the structure, the size, and the crystallinity of the inorganic phase and used method for preparation of composite. So in this case, the final composites are not just the sum of the original components, but rather entirely new materials with new properties. A synergistic effect often accomplishes from the close proximity of the two phases through size domain effects and the nature of the interfaces. Considering this

47

items, we cannot have a clear comparison on the properties each of composites. In following, a summary of special applications of each CNT/MOs is expressed. As inferred from text, alumina’s applications are limited because of its low fracture toughness and most brittle, in spite of that has high hardness, good oxidation resistance and chemical stability. So, mechanical response and failure mode of straight CNTs after embedment into the alumina ceramic were studied by various researcher and almost more of them showed incorporation of small amounts of CNTs into MO improve mechanical and physical properties and lead to more complete densification, and restrict alumina grain size. [2, 33] CNT unlike many adsorbents have various features such hollow and layered nanostructure with high aspect ratio, large accessible external surface area, light mass density, easily modified surfaces and well developed mesopores which by supporting of metal oxides like Al2O3, TiO2, specific ZnO and Fe2O3 not only can hinder from the formation of aggregates of CNT but also can apply for the removal pollutions. The metal oxide such as TiO2 and ZnO have been applied in many field including photocatalysis, sensors, solar cells, and lithium-ion batteries (LIBs). To achieve high conversion efficiency, MO photoelectrode should possess interconnection and continuity between nanoparticles to prevent charge-carrier recombination, achieve efficient electron flow, and ensure efficient current collection at the back contact of the photoelectrode. [160] CNT as a good support for MO provide straight paths to facilitating electron transfer and repressing recombination rate by decreasing grain boundaries and the traveling length of photoelectrons before reaching the back contact.[102, 217] One the most important application CNT/MO is in sensors. The mutual integration of the CNT and MO nanoparticles (ZnO, Fe2O3, and Al2O3) proved as effective the thermal, electrochemical and gas sensors due to their synergistic effect which in this field iron oxides are attractive candidates as nanointerfaces for sensing applications. 6. Conclusions CNT/MO hybrids have the possible to become next-generation functional materials for wideranging applications with noticeable influences on society, such as photocatalysis, supercapacitors, photovoltaic, LIB and sensor. As the push towards a ‘greener’ or ‘environmentally friendly’ future continues, the commercial use of hybrids with high efficiency and low cost will escalate. This review article has introduced the most properties and recent 48

applications (period 2010-2015) of the series CNT/MO nanocomposites such as CNT/Al2O3, CNT/TiO2, CNT/ZnO and CNT/Fe2O3 (or Fe3O4) that offer great potential in multidiscipline applications. In order to, at first step has summarized the methodology of preparation, functionalization and activation of the nanotubes along with their advantages and summarized main methods for preparation of CNT/MO nanocomposites. Some case-studies have been carried out referring to a realistic use of CNT/MO nanocomposites as adsorbents that have excellent adsorption ability for many kinds of pollutants like inorganic and organic compounds. In many case investigated the mechanism of improving the photo-catalytic activity based on adsorption competence of the nanotube and the transfer of promoted electrons from MO to CNT. Generally in a hybrid, the type and quality of CNT and NPs have a clear impact on the final performance of the CNT/MO hybrids. The hybrid behavior broadly ascribes to two factors (i) the type and properties of components, related to synthesis method, defects, functionalization etc and (ii) quality of coating in term of dispersion, size, concentration of metal oxides.

Acknowledgments Authors acknowledge the support of the Research Affairs Division of Isfahan University of Technology (IUT), National Elite Foundation (NEF), Iran Nanotechnology Initiative Council (INIC), and Center of Excellence in Sensors and Green Chemistry Research (IUT).

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Captions of Fig. s Fig. 1. CNT-TiO2 morphologies for promotion of reactive surface area and inter-phase contact. (a) Shows schematically a random mixture of TiO2 NPs and CNTs, (b) TiO2 coated CNTs, and (c) CNTs wrapped around large TiO2 NPs [15] Fig. 2. (a) TEM image of the pristine MWCNTs. (b) TEM image of the acid-treated MWCNTs. The arrows indicate the position of the nanodefects. (c) High-magnification TEM image of the acid-treated MWCNT surface.[38] Fig. 3. TEM images of MWCNT-ascorbic acid. [50] Fig. 4. TEM images of the fracture surface of the composite acquired (a) low and (b) high magnification images.[68] Fig. 5. A series of SEM and TEM images of a single MWCNT captured (a) before and (b, c, d) after the pullout test. (d) TEM image showing a tip of the MWCNT that was damaged by the FIB processing.[69] Fig. 6. AFM images of composite samples (a) non-annealed MWCNTs-alumina composite, and MWCNTs-alumina composite annealed at (b) 450 ºC, (c) 800 ºC and (d) 1000 ºC.[73] Fig. 7. TEM images of G17 showing (a) attachment of CNTs with Al2O3 grain through graphene layer encapsulation; scale bar. 50 nm and (b) HRTEM image of the layered interface; scale bar. 5 nm. [66] Fig. 8. Variation of the Vickers hardness and fracture toughness with the relative density for the different nanocomposites.[82] Fig. 9. (a) Crack bridging by shorter nanotube pullouts presumably at the transgranular positions (b and c) Stretching/disentangling of carbon nanotube in 6.4 vol.% of MWCNT/alumina composite presumably at the intergranular position (large white arrow) while the black arrows show presumably shorter nanotube pullouts at the transgranular positions. Schematic illustration of pullout induced toughening at the transgranular positions and stretching/disentangling toughening at the grain boundaries by MWCNTs.[83] Fig. 10. Morphology of (a) MWCNT/Al2O3 batche (0.15 vol.%) powder showing individual MWCNTs; (b) MWCNT/Al2O3 batche (2.40 vol.%) powder showing agglomerated MWCNTs.[84] 66

Fig. 11. The schematic presentation of Cd(II) ion (a) and TCE (b) interaction with Al2O3/MWCNTs.[85] Fig. 12. The schematic of preparation of TiO2 coated MWCNTs.[102] Fig. 13. TEM image of TiO2 coated MWCNT.[102] Fig. 14. TEM and HRTEM images of CNT/TiO2-12 h. Mechanism of the improved visible-light photocatalytic activity of the CNT/TiO2 composite.[103] Fig. 15. (a) SEM image and (b) TEM image of magnetic MWCNT-TiO2.[107] Fig. 16. Photodegradation of RhB aqueous solution for P25, TiO2, TiO2+SWCNT (mixture) and the bonding TiO2/SWCNT composite.[108] Fig. 17. The fabrication process and photoelectrochemical reaction mechanism of MWCNT TiO2 NP|LDH| nicotinamide adenine dinucleotide (NAD+)| Indium tin oxide. [101] Fig. 18. (a) SEM and (b) TEM images of TiO2 NPs coated carbon nanotubes. The inset in (b) is the enlarged image showing the complete encapsulation of CNTs by TiO2 NPs. [119] Fig. 19. (a–c) are the TEM images of MWCNTs/Fe3O4 nanocomposites under different magnifications.[167] Fig. 20. The proposed mechanisms of 1-naphthylamine adsorption on MWCNTs/iron oxides/CD.[166] Fig. 21. Effect of ionic strength on Sr(II) adsorption onto the magnetic composites as a function of pH, at Cinitial [Sr(II)] = 0.018 mg/L, m/V = 0.6 g/L and T = 25 ± 2 ºC.[168] Fig. 22. (a) Back scattering FE-SEM images for CNT–Fe2O3 (b) TEM image of CNTs (c) TEM image of CNT–Fe2O3.[169] Fig. 23. (a) XRD pattern, (b) SEM and (c-d) HRTEM images of the SnO2-MWCNT nanocomposites obtained by annealing the C16H30O4Sn-MWCNTs precursor in air at 350 ºC for 1 h.

Table 1. Physiochemical properties of MWCNTs and SWCNTs. Table 2. representative summery of CNTs/Al2O3 synthesis routs along with their performances and properties

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Table 3. representative summery of CNTs/TiO2 synthesis routs along with their significant performances and properties Table 4. representative summery of CNTs/ZnO synthesis routs along with their significant performances and properties Table 5. Representative summery of CNTs/iron oxide synthesis routs along with their performances and properties

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Superior properties and application of selected metal oxides deposited on carbon nanotubes Shadpour Mallakpour *, Elham Khadem

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Highlight
The preparation and properties of MOs combined with CNTs are outlined.
The distinctive chemistry and functionalization of CNTs have been summarized.
The utility of CNT/MO composites for a broad range of applications have been described.
The efficiency of several classes of CNT/MOs in sensor, absorbent, photovoltaic and etc are presented.

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