NGUYEN THI MINH NGUYET “STUDY THE PURIFICATION AND SURFACTANT ASSISTED DISPERSION OF MULTI-WALLED CARBON NANOTUBES MWNTs IN AQUEOUS SOLUTION” Major: Technology of High Molecular and Co
INTRODUCTION
Since their discovery by Iijima [1], carbon nanotubes (CNTs) have attracted considerable attention due to their exceptional mechanical, thermal, and electrical properties These unique properties have facilitated interest in CNTs for a wide array of applications including biomaterials [2], multi-functional composites [3,4], and electronic components [5] However, their high aspect ratio and propensity to aggregate into bundles makes disentanglement and dispersion non-trivial processes limiting commercial applicability [6] Dispersing nanotubes in solvents typically involves chemical treatment to enable debundling while simultaneously driving favorable interactions between the nanotube surface and supporting solvent Covalent methodologies rely on directly binding organic moieties to nanotube sidewalls or defect sites [7] Unfortunately, such bonding disrupts the intrinsic sp 2 hybridized network that gives rise to the nanotXEHVảH[FHSWLRQDOSURSHUWLHV>@ ,Q contrast, non-covalent approaches focus on spurring non-disruptive interactions VXFK DV ʌ±ʌstacking, adsorption, or Coulomb interactions through insertion of a chemical bridging agent These approaches preserve the deloFDOL]HG ʌ-electron network of the nanotube sidewall ensuring minimal perturbation of the defect sensitive electrical and thermal properties [9] Surfactants and polymers are generally selected as the chemical bridging agents of choice [6]
In Vietnam, numerous studies have been carried out on nano materials, especially carbon nanotubes The Institute of Materials Science is one of the earliest institute produced CNTs successfully in 2002 After a few years, many different institutes such as The International Training Institute for Materials Science (ITIMS) and Institute of Engineering Physics - Hanoi University of Technology, R&D center of Saigon High Tech Park have also shown their interest in this new material by producing MWNTs in large scale but the quality of products was restricted Furthermore, the National Key Laboratory of Materials and Electronic Device- Materials Science Institute - Vietnam Science - Technical Institute has also given an
18 economical large ± scale in production of MWNTs In 2009, Prof Phan Hong Khoi and Phan Ngoc Minh [10] applied successfully MWNTs in tungsten tips for field emission devices as well as Ni-MWNTs, Cr-MWNTs composite plating film In the same year, Bui Van Ga et al published his investigation about the super- hydrophobicity of PS/MWNTs composite which could be applied in the biogas storage [11] Recent years, some groups in Ho Chi Minh University of Technology have studied CNTs synthesis and application processes and obtained acceptable results Cao Duy Vinh et al have succeeded in investigating the MWNTs' functionalization process by mixture of sulfuric acid and nitric acid [12] Furthermore, by blending modified MWNTs with PVA, he has shown the potential results in improving the electricity conductivity of this composite [13] Prof Nguyen Huu Nieu et al [14] have also used modified MWNTs as the supporter for fabricating magnetic iron (III) oxide
Until now, there is no report in studying the dispersion of MWNTs using surfactants Knowing the very important role of dispersing process, this thesis will be focusing on the following issues: x Purifying and evaluating the effectiveness of purification process x The role of purification in the dispersion of carbon nanotubes x Providing new method for fast dispersing carbon nanotubes in aqueous solution using different surfactants (Sodium Dodecyl Sulfate (SDS) and Triton X ± 100) x Determining the extinction coefficient ± a very important parameter for quantitative assessment of carbon nanotube dispersions x Comparing the dispersing power of two surfactants: SDS and Triton X ±
100 x Determining the optimum CNT-to-surfactant ratio for each surfactant x ,QIOXHQFHRIS+YDOXHRQWKHVWDELOLW\RI0:17VảVXVSHQVLRQ
OVERVIEW
CARBON NANOTUBES
The carbon phase diagram at high pressure (>1 GPa) is shown in Figure 2.1 below The phase diagram presents several main features:
Solid lines represent equilibrium phase boundaries A: synthesis of diamond from graphite by catalysis; B: P/T threshold of very fast solid-solid transformation of graphite to diamond; C: P/T threshold of very fast transformation of diamond to graphite; D: single crystal hexagonal graphite transforms to retrievable hexagonal- type diamond; E: upper ends of shock compression/quench cycles that convert hex- type graphite particles to hex-type diamond; F:upper ends of shock
20 compression/quench cycles that convert hex-type graphite to cubic-type diamond;
B, F, G: threshold of fast P/T cycles, however generated, that convert either type of graphite or hexagonal diamond into cubic-type diamond; H, I, J: path along which a single crystal hex-type graphite compressed in the c-direction at room temperature x The transition line, the boundary between the graphite and stable diamond regions, runs from 1.7GPa/ 0°K to the graphite/diamond/liquid triple point I at 12GPa/5000°K x The graphite/liquid/vapor triple point, the graphite/vapor phase boundary and the liquid/vapor phase boundary occur at pressures too low for scale of diagram (not presented here) x The melting line of graphite extending from the graphite/liquid/vapor triple point at 0.011 GPa/ 5000°K to the graphite/ diamond/ liquid triple point at 12 GPa/5000°K ± The dotted line (diamond GFB) represents the graphite-diamond kinetic transformation under shock compression and quenches cycles ± The diamond melting line runs at high P and T , above the triple point
In the above sections, we discussed the various ways that carbon atoms bond together to form solids These solids are the allotropes (or polymorphs) of carbon They have the same building block but with different atomic hybrid configurations: sp 3 (tetragonal), sp 2 (trigonal) or sp (digonal)
Figure 2.2 Model of carbon allotropies
These allotropic solids can be classified into three major categories (Figure 2.2):
The sp 2 structures include graphite, the graphitic materials, amorphous carbon, and other carbon materials
The sp 3 structures involve diamond and lonsdaleite (a form detected in meteorites)
The fullerenes and nanotubes are the third and fourth allotropes of carbon and consist of a family of spheroidal or cylindrical molecules with all the carbon atom sp 2 hybridized
These allotropes are sometimes found in combination such as some diamond-like carbon (DLC) materials produced by low-pressure synthesis, which are actually mixtures of microcrystalline diamond and graphite Recent investigations have revealed the existence of a series of diamond poly-types such as the 6-H hexagonal diamond and phase of carbon based on a three-dimensional network but with sp 2 bonds This phase could be harder than diamond, at least in theory
To understand the structure and properties of nanotubes, the bonding VWUXFWXUHDQGSURSHUWLHVRIFDUERQDWRPVDUHGLVFXVVHG¿UVW$FDUERQDWRPKDVVL[ HOHFWURQV ZLWK WZR RI WKHP ¿OOLQJ WKH V RUELWDO 7KH UHPDLQLQJ IRXU HOHFWURQV ¿OO the sp 3 or sp 2 as well as the sp hybrid orbital, responsible for bonding structures of diamond, graphite, nanotubes, or fullerenes, as shown in Figure 2.3
In diamond, the four valence electrons of each carbon occupy the sp 3 hybrid RUELWDODQGFUHDWHIRXUHTXLYDOHQWıFRYDOHQWERQGVWRFRQQHFWIRXURWher carbons in the four tetrahedral directions This three-dimensional interlocking structure makes diamond the hardest known material Because the electrons in diamond form covalent ı bonds and no delocalized ʌ bonds, diamond is electrically insulating The electrons within diamond are tightly held within the bonds among the carbon atoms These electrons absorb light in the ultraviolet region but not in the visible or infrared region, so pure diamond appears clear to human eyes Diamond also has a high index of refraction, which makes large diamond single crystals gems Diamond has unusually high thermal conductivity
In graphite, three outer-shell electrons of each carbon atom occupy the planar sp 2 hybrid orbital to form three in-plane ı bonds with an out-of-plane ʌ orbital (bond) This makes a planar hexagonal network Van der Waals force holds sheets of hexagonal networks parallel with each other with a spacing of 0.34 nm The ı bond is 0.14 nm long and 420 kcal/mol strong in sp 2 orbital and is 0.15 nm and 360 kcal/mol in sp 3 FRQ¿JXUDWLRQ7KHUHIRUHJUDSKLWHLVVWURQJHULQ-plane than diamond
In addition, an out-of-plane ʌ orbital or electron is distributed over a graphite plane and makes it more thermally and electrically conductive The interaction of the loose ʌ electron with light causes graphite to appear black The weak Van der Waals interaction among graphite sheets makes graphite soft and hence ideal as a lubricant because the sheets are easy to glide relative to each other
A CNT can be viewed as a hollow cylinder formed by rolling graphite sheets Bonding in nanotubes is essentially sp 2 However, the circular curvature will FDXVHTXDQWXPFRQ¿QHPHQWDQG ı± ʌ rehybridization in which three ıbonds are slightly out of plane; for compensation, the ʌ orbital is more delocalized outside the tube This makes nanotubes mechanically stronger, electrically and thermally more conductive, and chemically and biologically more active than graphite In addition, they allow topological defects such as pentagons and heptagons to be incorporated into the hexagonal network to form capped, bent, toroidal, and helical nanotubes whereas electrons will be localized in pentagons and heptagons because of UHGLVWULEXWLRQRIʌHOHFWURQV)RUFRQYHQWLRQZH call a nanotube defect free if it is of only hexagonal network and defective if it also contains topological defects such as pentagon and heptagon or other chemical and structural defects
Figure 2.3 Bonding structures of diamond, graphite, nanotubes, and fullerenes: when a graphite sheet is rolled over to form a nanotube, the sp 2 hybrid orbital is deformed for rehybridization of sp 2 toward sp 3 RUELWDORUı- ʌERQGPL[LQJ7KLVUHK\EULGL]DWLRQ VWUXFWXUDO IHDWXUH WRJHWKHU ZLWK ʌ HOHFWURQ FRQ¿QHPHQW JLYHV nanotubes unique, extraordinary electronic, mechanical, chemical, thermal, magnetic, and optical properties
25 Fullerenes (C60) are made of 20 hexagons and 12 pentagons The bonding is also sp 2 , although once again mixed with sp 3 character because of high curvature The special bonded structures in fullerene molecules have provided several surprises such as metal±insulator transition, unusual magnetic correlations, very rich electronic and optical band structures and properties, chemical functionalizations, and molecular packing Because of these properties, fullerenes have been widely exploited for electronic, magnetic, optical, chemical, biological, and medical applications
,LMLPD ZDV ¿UVW WR UHFRJQL]H WKDW QDQRWXEHV ZHUH FRQFHQWULFDOO\ UROOHG graphene sheets with a large number of potential helicities and chiralities rather than a graphene sheet rolled up like a scroll as originally proposed by Bacon Iijima initially observed only MWNTs with between 2 and 20 layers, but in a subsequent SXEOLFDWLRQLQKHFRQ¿UPHGWKHH[LVWHQFHRIVLQJOH-walled carbon nanotubes (SWNTs) and elucidated their structure The exact properties of CNTs are extremely sensitive to their degree of graphitization, diameter (or chirality), and whether they are in single wall or multi wall form (Figure 2.4a and b, respectively) Single-walled carbon nanotubes (SWNTs), which are seamless cylinders, each made of a single graphene sheet, were first reported in 1993 Multi-walled carbon nanotubes (MWNTs), consisting of two or more seamless graphene cylinders concentrically arranged, were discovered two years previously [17]
Figure 2.4 Computer-generated images of carbon nanotubes [17]
In order to describe fundamental characteristic of the nanotubes, two vectors, C h and T, are introduced in Figure 2.5 [15]
Figure 2.5 Chiral vector and unit cell of CNT
Ch is the vector that defines the circumference on the surface of the nanotube connecting two equivalent carbon atoms
The chiral angle is used to separate carbon nanotubes into three different classes by their electronic
Where: and are two basic vectors of graphite n and m are integers n and m are also called indexes and determine the chiral angle
27 The chiral angle is used to separate carbon nanotubes into three different classes by their electronic properties
Figure 2.6 Rolling the graphite sheet on different directions
In three classes of nanotubes, armchair carbon nanotubes are metallic (a degenerate semi-metallic with zero band gap), zig-zag and chiral nanotubes can be semimetals with a finite band gap if n-P LLEHLQJDQLQWHJHUDQGQPRU semiconductors in all other cases (Figure 2.6)
Carbon nanotubes have many magnificent properties that have attracted researchers of many disciplines into intensive studies [16, 17, 18] A compilation of the mechanical, thermal and electrical properties are discussed below
The most important electrical property is the conductance/resistivity of any material Another property is the maximum current density Carbon based materials have shown interesting electrical properties Graphite is a good conductor ZKHUHDVGLDPRQGLVDYHU\JRRGLQVXODWRU&DUERQQDQRWXEHVảFKLUDOLW\LVUHODWHGWR its electrical behavior For a given (n, m) nanotube, armchair carbon nanotubes are metallic (a degenerate semimetal with zero band gap), zig-zag and chiral nanotubes can be semimetals with a finite band gap if n-P LLEHLQJDQLQWHJHUDQGQP or semiconductors in all other cases
Several experiments have concluded that CNTs behave like quantum wires
In a study [19], multi walled nanotubes were found to exhibit ballistic conductance
In another investigation [20] the resistivity of ropes of metallic SWCNTs was found out to be 10-FPDW.,QWKHVDPHVWXG\WKHFXUUHQWGHQVLW\LVUHSRUWHGWREH greater than 107A/cm 2 Another study [21] reports that the current density can be increased as high as 1013 A/cm2 The extremely small size and superior electrical properties makes carbon nanotubes very suitable for small scale electronics applications
COMMON SYNTHESIS TECHNIQUES
There are many techniques used to produce MWNTs or SWNTs Methods such as electric arc discharge, laser ablation and chemical vapour deposition techniques are well established to produce a wide variety of CNTs These methods are described in following sections
2.2.1 Arc Discharge and Laser Ablation [24]
Figure 2.7 Schematic of an arc-discharge apparatus, along with electron microscopy pictures of the products with doped and pure anodes
The first method that was successfully used to synthesize CNTs in small quantities was the arc discharge method (Figure 2.7) Opposing anode and cathode terminals made of 6-mm and 9-mm graphite rods respectively are placed in an inert environment (He or Ar at ~500 Torr) A strong current, typically around 100 A (DC or AC), is passed between the terminals generating arc-induced plasma that evaporates the carbon atoms in the graphite The nanotubes grow from the surface of these terminals A catalyst can be introduced into the graphite terminal Although MWNTs can be formed without a catalyst, it has been found that SWNTs can only be formed with the use of a metal catalyst such as iron or cobalt
A process called Laser Ablation (Figure 2.8), first developed in 1995, uses a similar principle to produce nanotubes Carbon is evaporated at high temperatures
32 from a graphite target using a powerful and focused laser beam In the most basic laser ablation technique, a 1.25-cm diameter graphite target is placed in a 2.5-cm diameter, 50-cm long quartz tube in a furnace controlled at 1200 o C and filled with 99.99% pure argon to a pressure of 500 Torr A pulsed Nd-YAG laser beam at 250mJ (10 Hz) is focused using a circular lens and the beam is swept uniformly across the graphite target surface The nanotubes, mixed with undesired amorphous carbon, are collected on a cooled substrate at the end of the chamber
Both of these methods have limited potential for scale-up Solid graphite must be evaporated at >3000°C to source the carbon needed, the nanotubes produced are in an entangled form, and extensive purification is required to remove the amorphous carbon and fullerenes that are naturally produced in the process
Figure 2.8 Schematics of a laser ablation set-up [25]
Figure 2.9 Schematic of CVD deposition oven [26]
In the CVD process, growth involves heating a catalyst material to high temperatures (160±1200 0 C) in a tube furnace using a hydrocarbon gas through the tube reactor over a period of time The basic mechanism in this process is the dissociation of hydrocarbon molecules catalyzed by the transition metal and saturation of carbon atoms in the metal nanoparticle Precipitation of carbon from the metal particle leads to the formation of tubular carbon solids in a sp 2 structure.
The characteristics of the carbon nanotubes produced by CVD method depend on the working conditions such as the temperature and the pressure of operation, the volume and concentration of hydrocarbon gas such as methane, acetylene, methylene or carbon monoxide, the size and the pretreatment of metallic catalyst, and the time of reaction.
APPLICATIONS
Because of their special physico-chemical properties, CNTs are expected to play a major role in numerous applications
34 The exceptional mechanical strength of nanotubes makes them also attractive as tips for scanning probe microscopies Such tips are usually silicon cantilevers or metal wires that are etched to form a sharp point They can achieve high resolution because of small protusions, but they seldom survive a tip crash (accidental contact with the observation surface) Nanotubes are in this respect far more resistant The great advantage of nanotubes is however their slender shape and well-defined end Because of their large aspect-ratio, they are able to reach down deep trenches and to image sharp topographies with good resolution when attached to the silicon cantilevers of conventional atomic force microscopes
Carbon nanotubes are ideal candidates for novel molecular devices because of their electronic properties depend rather on their geometry than on doping by impurities, which results in high thermal stabilities There have been thus several realizations of carbon-based electronic devices, more specifically of transistors
Such a switching device that consists of one semiconducting single-wall nanotube connected to two metal electrodes (Figure 2.11) By applying a voltage to a gate electrode, the nanotube can be switched from a conducting to an insulating state, even at room temperature
Figure 2.11 Diagram of nanotube transitor [29]
Researchers demonstrated chemical sensors based on individual single- walled nanotubes [31] Upon exposure to gaseous molecules, the electrical resistance of a semiconducting SWNT was found to dramatically increase or decrease These nanotube sensors exhibit a fast response and a substantially higher sensitivity than that of existing solid-state sensors at room temperature An individual nanotube can be furthermore used to detect different types of molecules The selectivity is achieved by adjusting the electrical gate to set the SWNT in an initial conducting or insulating state
Figure 2.12 Electrical response of a semiconducting SWNT to NO2 gas molecules The evolution of the conductance with time depends clearly on the gas flow [30]
Using a carbon nanotube instead of traditional silicon, researchers have created the basic elements of a solar cell that hopefully will lead to much more efficient ways of converting light to electricity than now used in calculators and on rooftops
Figure 2.13 In a carbon nanotube-based photodiode, electrons (blue) and holes
(red) - the positively charged areas where electrons used to be before becoming excited - release their excess energy to efficiently create more electron-hole pairs when light is shined on the device [31]
One of the first commercial applications of multi±walled carbon nanotubes is in its use as electrically conducting materials in polymer composites The combination of high aspect ratio, stiffness, mechanical strength, low density, small size and high conductivity makes carbon nanotubes ideal substitutes to carbon fibers as reinforcements in high strength, low±weight and high performance polymer composites In addition, incorporation of carbon nanotubes in plastics can potentially result in remarkable increase in the modulus and strength of structural materials
UNDERSTANDING SURFACTANTS IN DISPERSING CARBON
For many industrial applications a uniform and stable dispersion of particulate matter plays an important role This requirement is especially critical when submicron or nanometer-sized particles are involved In such ranges the surface chemistry controls the dispersion state of the particles within a final product It is extremely important to learn how to manipulate the surface properties in order to achieve a product with the desired properties
A surfactant's property of accumulation at surfaces or interfaces has been widely utilized to promote stable dispersions of solids in different media [32-36] Those ³ amphiphilic ´ molecules, i.e., compounds having both polar and apolar groups, adsorb at the interface between immiscible bulk phases, such as oil and water, air and water or particles and solution, act to reduce the surface tension The GLVWLQFW VWUXFWXUDO IHDWXUH RI D VXUIDFWDQW RULJLQDWHV IURP LWV àGXDOLW\ả WKH hydrophilic region of the molecule or the polar head group; and the hydrophobic region or the tail group that usually consists of one or few hydrocarbon chains Surfactants are classified according to the charge of their head groups, thus cationic, anionic, nonionic or zwitterionic are known
Two important features which characterize surfactants, namely adsorption at interface and self-accumulation into supramolecular structures, are advantageously used in processing stable colloidal dispersions The adsorption of surfactants onto inorganic and organic surfaces usually depends on the chemical characteristics of particles, surfactant molecules and solvent Thus, the driving force for the adsorption of ionic surfactants on charged surfaces is the Coulombic attractions, which are formed, for example, between the surfactant's positively-charged head group and the negatively-charged solid surface The mechanism by which nonionic surfactants adsorb onto a hydrophobic surface is based on a strong hydrophobic attraction between the solid surface and the surfactant's hydrophobic tail Once the
38 adsorption of surfactant molecules on particle surfaces is established, self- organization of the surfactant into micelles (aggregative structures of surfactants) is expected to occur above a critical micelle concentration (CMC)
The following sections will describe more details about surfactants
2.4.1 General structural features and behavior of surfactants [37]
Surfactants have a characteristic molecular structure consisting of a structural group that has very little attraction for the solvent, known as a lyophobic group, together with a group that has strong attraction for the solvent, called the lyophilic group This is known as an amphipathic structure (Figure 2.14) When a molecule with an amphipathic structure is dissolved in a solvent, the lyophobic group may distort the structure of the solvent, increasing the free energy of the system When that occurs, the system responds in some fashion in order to minimize contact between the lyophobic group and the solvent In the case of a surfactant dissolved in aqueous medium, the lyophobic (hydrophobic) group distorts the structure of the water (by breaking hydrogen bonds between the water molecules and by structuring the water in the vicinity of the hydrophobic group) As a result of this distortion, some of the surfactant molecules are expelled to the interfaces of the system, with their hydrophobic groups oriented so as to minimize contact with the water molecules The surface of the water becomes covered with a single layer of surfactant molecules with their hydrophobic groups oriented predominantly toward the air Since air molecules are essentially nonpolar in nature, as are the hydrophobic groups, this decrease in the dissimilarity of the two phases contacting each other at the surface results in a decrease in the surface tension of the water On the other hand, the presence of the lyophilic (hydrophilic) group prevents the surfactant from being expelled completely from the solvent as a separate phase, since that would require dehydration of the hydrophilic group The amphipathic structure of the surfactant therefore causes not only concentration of the surfactant at the surface and reduction of the surface tension of the water, but also orientation
39 of the molecule at the surface with its hydrophilic group in the aqueous phase and its hydrophobic group oriented away from it
Figure 2.14 General structure of surfactant
The chemical structures of groupings suitable as the lyophobic and lyophilic portions of the surfactant molecule vary with the nature of the solvent and the conditions of use In a highly polar solvent such as water, the lyophobic group may EHDK\GURFDUERQRUÀXRURFDUERQRUVLOR[DQHFKDLQRISURSHUOHQJWKZKHUHDVLQD OHVVSRODUVROYHQWRQO\VRPHRIWKHVHPD\EHVXLWDEOHHJÀXRURFDUERQRUVLOR[DQH chains in polypropylene glycol) In a polar solvent such as water, ionic or highly polar groups may act as lyophilic groups, whereas in a nonpolar solvent such as heptane they may act as lyophobic groups As the temperature and use conditions
HJ SUHVHQFH RI HOHFWURO\WH RU RUJDQLF DGGLWLYHV YDU\ PRGL¿FDWLRQV LQ WKH structure of the lyophobic and lyophilic groups may become necessary to maintain surface activity at a suitable level Thus, for surface activity in a particular system the surfactant molecule must have a chemical structure that is amphipathic in that solvent under the conditions of use
40 The hydrophobic group is usually a long-chain hydrocarbon residue, and less often a halogenated or oxygenated hydrocarbon or siloxane chain; the hydrophilic group is an ionic or highly polar group
From the commercial point of view surfactants are often classified according to their use However, this is not very useful because many surfactants have several uses, and confusions may arise from that The most accepted and scientifically sound classification of surfactants is based on their dissociation in water a Nonionics
These types of surfactants have no charge on their polar head-group They do not ionize in aqueous solution, because their hydrophilic group is of a non- dissociable type, such as alcohol, phenol, ether, ester, or amide A large proportion of these nonionic surfactants are made hydrophilic by the presence of a polyethylene glycol chain, obtained by the polycondensation of ethylene oxide They are called polyethoxylated nonionics b Anionic surfactants
In solution, the head of anionic surfactants is negatively charged They are dissociated in water in an amphiphilic anion, and a cation ,which is in general an alcaline metal (Na+, K+) or a quaternary ammonium They are the most commonly used surfactants They include alkylbenzene sulfonates (detergents), (fatty acid) soaps, lauryl sulfate (foaming agent), di-alkyl sulfosuccinate (wetting agent), lignosulfonates GLVSHUVDQWV HWFô $QLRQLF VXUIDFWDQWV DFFRXQW IRU DERXW RI the world production
These surfactants bear a positive charge on the polar portion in solution A very large proportion of this class corresponds to nitrogen compounds such as fatty amine salts and quaternary ammoniums, with one or several long chain of the alkyl type, often coming from natural fatty acids These surfactants are in general more expensive than anionics, because of the high pressure hydrogenation reaction to be carried out during their synthesis As a consequence, they are only used in two cases in which there is no cheaper substitute, i.e (1) as bactericide, (2) as positively charged substance which is able to adsorb on negatively charged substrates to produce antistatic and hydrophobant effect, often of great commercial importance such as in corrosion inhibition d Zwitterionic
When a single surfactant molecule exhibit both anionic and cationic dissociations it is called amphoteric or zwitterionic This is the case of synthetic products like betaines or sulfobetaines and natural substances such as aminoacids and phospholipids
Some amphoteric surfactants are insensitive to pH, whereas others are cationic at low pH and anionic at high pH, with an amphoteric behavior at intermediate pH Amphoteric surfactants are generally quite expensive, and consequently, their use is limited to very special applications such as cosmetics where their high biological compatibility and low toxicity is of primary importance
The past two decades have seen the introduction of a new class of surface active substance, so-called polymeric surfactants or surface active polymers, which result from the association of one or several macromolecular structures exhibiting hydrophilic and lipophilic characters, either as separated
42 blocks or as grafts They are now very commonly used in formulating products as different as cosmetics, paints, foodstuffs, and petroleum production additives
Figure 2.15 Some common types of surfactants 2.4.3 Micelle formation by surfactants [38]
One of the fundamental properties of surfactants is their tendency to be adsorbed at the interface or surface of a given system The interface indicates a boundary between any two immiscible phases; the term surface denotes an interface where one phase is usually air
At lower surfactant concentration, most physicochemical properties do not vary significantly However, at a certain concentration of surfactant, an abrupt change in these properties takes place
43 The concentration of surfactant at which this change occurs is known as the Critical Micellar Concentration (CMC) Below the CMC, surfactant molecules are present in the form of individual molecules, whilst above the CMC, surfactant molecules are present in the form of aggregates These aggregates are known as micelles and the process of their formation is known as micellization (Figure 2.16) A low CMC value indicates micelle formation at a low surfactant concentration
CHARACTERIZATION METHODS
Characterization tools are very important in the study of materials to determine their physical, chemical properties and potential in applications This section presents a variety of techniques that have helped to reveal structure and properties of catalyst, carbon nanotubes products
The simplest scanning electron microscope has an electron gun and a condenser lens system to produce a focused electron beam A very fine electron beam is focused at the surface of the specimen in the microscope and scanned across in a pattern of parallel lines The most important factors for scanning microscopy are the emission of the secondary electrons and re-emission or reflection of high energy backscattered electrons from the primary beam Collecting synchronously emitted electrons allows to record images of nanometric objects The intensity of emission of both secondary and backscattered electron is sensitive to the angle at which the electron beam strikes surface The emitted electron current is collected and amplified
In my thesis, this method is the standard characterization method It provides information about the density, surface of catalysts and carbon nanotubes It is not a method to measure the quality or the structure of the grown CNTs, since it is limited in resolution The typical resolution of a SEM is about 2 to 5 nm
Transmission Electron Microscopy allows to reach high resolution in observation of nano materials This is an imaging technique whereby a beam of electrons is focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film, or to be detected by a charge- coupled device camera (CCD) This method is a powerful technique that allows to determine the number of walls in a MWNT or to image the isolated SWNTs residing inside a SWNT bundle TEM has played an important role in investigating the structures of CNTs
Raman spectroscopy is a technique based on the scattering of monochromatic light, usually from a laser Inelastic scattering appears when the frequency of photons of monochromatic light changes upon with a sample interaction The photons of the laser light are absorbed by the sample and subsequently re-emitted Frequency of the re-emitted photons is shifted up or down in comparison with the original monochromatic frequency, which is known as the Raman Effect This Raman shift provides information about vibration, rotational, and other molecular modes Raman spectroscopy can be used to study solid, liquid, and gaseous samples Raman spectroscopy is a widely used tool to characterize material composition, sample temperature, and strain from analysis of the material specific phonon mode energies It requires very little sample preparation and a rapid, non-destructive optical spectrum is easily achieved
In CNTs, this technique can be used to determine the graphic level of the material It is commonly accepted that it exists a G band that represents the graphic level of the tubes and a D band that is responsible for the introduction of disorder in the system (sp 3 , defects and so on)
Thermal Gravimetric Analysis (TGA) is a simple analytical technique that measures the weight loss (or weight gain) of a material as a function of temperature
As materials are heated, they can lose weight from a simple process such as drying, or from chemical reactions that liberate gasses Some materials can gain weight by reacting with the atmosphere in the testing environment Since weight loss and gain are disruptive processes to the sample material or batch, knowledge of the magnitude and temperature range of those reactions are necessary in order to design adequate thermal ramps and holds during those critical reaction periods
In this study, based on the residual mass, we can partly judge the purity of CNTs
Infrared spectroscopy (IR spectroscopy) is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum It covers a range of techniques, the most common being a form of absorption spectroscopy As with all spectroscopic techniques, it can be used to identify compounds and investigate sample composition
IR radiation does not have enough energy to induce electronic transitions as seen with UV Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states
For a molecule to absorb IR, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule The alternating electrical field of the radiation (remember that electromagnetic radiation consists of an oscillating electrical field and an oscillating magnetic field, perpendicular to each other) interacts with fluctuations in the dipole moment of the molecule If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration
From the IR spectroscopy of materials, we can identify the composition of the sample by comparing it with the reference spectroscopy of each substance (about the wavelength range)
2.5.6 Ultraviolet Visible (UV-Vis) spectroscopy
Ultraviolet/visible spectroscopy is useful as an analytical technique for two reasons First it can be used to identify some functional groups in molecules and
53 secondly, it can be used for assaying This second role determining the content and strength of a substance is extremely useful
)RU PRVW VSHFWUD WKH VROXWLRQ REH\V %HHUảV /DZ 7KLV states that the light absorbed is proportional to the number of absorbing molecules ± i.e to the concentration of absorbing molecules This is only true for dilute solutions A second law ± /DPEHUWảV ODZ ± tells us that the fraction of radiation absorbed is independent of the intensity of the radiation Combining these two laws gives the Beer±Lambert law: log 10 I 0 , İOF
I 0 = the intensity of the incident radiation
I = the intensity of the transmitted radiation İ WKHPRODUDEVRUSWLRQFRHIILFLHQW l = the path length of the absorbing solution (cm) c = the concentration of the absorbing species in mol dm -3
Two useful pieces of information are the mRODUDEVRUSWLRQFRHIILFLHQWİand Ȝmax which is the wavelength at which maximum absorption occurs These two SLHFHVRILQIRUPDWLRQDUHVRPHWLPHVHQRXJKWRLGHQWLI\DVXEVWDQFH+RZHYHULIİDQG Ȝmax are known for a compound the concentration of the solution can be calculated This is the most common application used in this thesis
METHODOLOGY AND EXPERIMENTAL
MATERIALS
All materials used in experiments are shown in Table 3.1
Table 3.1: Materials used in experiments
Density: 1.179 Kg/l Molar mass: 36.46 g/mol Boiling point: 48 0 C Melting point: -27.32 0 C
Multi-walled carbon nanotubes (MWNTs)
Highly foaming Molecular formula: NaC 12 H 25 SO 4
Physical state: clear liquid Boiling point: > 200 0 C Freezing/Melting point: 2 0 C Solubility in water: Soluble Density: 1.061
APPARATUSES
Elma (T460H) sonication bath was the main tool in purification process x Power:600W x Frequency: 37KHz x Temperature range: 30-80 0 C
Almost dispersing processes were carried out with a horn sonicator (Sonic Vibracell VC505) with a cylindrical tip (10 mm end cap diameter) The output SRZHUZDV¿[HGDW:thus delivering energy of 1100±1200 J/min
Figure 3.2 Ultrasonic processor Sonic Vibracell VC505 3.2.3 Magnetic and hotplate stirrer
Magnetic and hotplate stirrer, supplied by Stuart Barloworld± England, was used in the purification process with some characterizations shown below: x Power: 1300W x Range temperature: 45-450 0 C x Range speed: 100-1500 rpm
Figure 3.3 Magnetic and hotplate stirrer
Nabertherm furnace with the temperature range from 100-1280 0 C and time controller was used to anneal the MWNTs in ambiance The temperature precision is ±5 0 C
Figure 3.4 Nabertherm furnace 3.2.5 Centrifugal machine
,Q RUGHU WR UHPRYH WKH SUHFLSWDWHV LQ 0:17Vả VXVSHQVLRQ FHQWULIXJDO machine supplied by Hettich centrifuge-EBA 21-Germany was used The maximal speed is 5000rpm and time can be adjusted from 1 minute to 60 minutes
Figure 3.5 EBA 21 centrifugal machine 3.2.6 Universal oven
Memert (UNB 400) Universal oven-Germany was used to dry the samples after purifying and in some other needed cases The maximal temperature can be reach is
300 0 C and the highest heating rate is 20 0 C/mins
Figure 3.6 Memert (UNB 400) Universal oven-Germany 3.2.7 pH meter
To be able to measure pH, we need a tool which is sensitive to the hydrogen ions that define the pH value Mellter Toledo ± Germany supplied the pH meter with high accuracy (0.001) in order to determine pH value of suspension fast and efficiently In this thesis, we used this tool to investigate the influence of pH values on the dispersion of carbon nanotubes
Figure 3.7 Melter Toledo pH meter
EXPERIMENTAL
7KHFRPSOHWHSXUL¿FDWLRQSURWRFROLVDWZR-step process The first one is an air oxidation to burn amorphous carbon, the second one is an acid treatment to remove the unprotected metallic catalysts A schematic representation of this procedure is shown in Figure 3.8 The detailed processes are as follows
Figure 3.8 Schematic flowchart of MWNTs purification
In gas phase oxidative purification, MWNTs are purified by oxidizing carbonaceous impurities at 460 0 C for 24 hours under an oxidizing ambiance High temperature oxidation in air is found to be an extremely simple and successful strategy for purifying MWNTs, which are metal free and have fewer defects on tube walls This sample will be identified as M 460
SEM, TEM, TGA, XRF, Raman
SEM, TEM, TGA, XRF, Raman
60 Although the merits of air oxidation are obvious, it has a drawback that the metal nanoparticles cannot be directly removed, and further methods are needed The commonly used oxidants for liquid phase oxidation include HNO3 [70±72],
H2O2 or a mixture of H2O2 and HCl [73±75], a mixture of H2SO4, HNO3, KMnO4 and NaOH [76-79], and KMnO 4 [80,81] The shortcomings of this method are that it causes reaction products on the surface of CNTs, adds functional groups, and destroys CNT structures (including cutting and opening CNTs) In order to overcome this limitation, hydrochloric acid is offered as an agent to purify MWNTs It is inexpensive, capable of removing metal catalyst and no secondary impurities are introduced
150mg sample M 460 was refluxed in solution of 75g HCl 37% and 75g H 2 O, 1h ultrasonication and then stirring for 24h at 60 0 C Products were filtered and re- washed in DI water several times to remove completely residual hydrochloric acid and named M-P
The solid samples obtained after each step of purification were characterized by Scanning Electron Microscopy (SEM) (JEOL-JSM-7401F), Transmission Electron Microscopy (TEM) (JEOL-JEM-1400), Thermo Gravimetric Analysis (TGA) (STA 409 PC), Raman (Labram 300) and FTIR
3.3.2 Determine the extinction coefficient of MWNTs using UV-Vis spectroscopy
In addition to purification process, we used two surfactants: Sodium Dodecyl Sulfate (SDS) and Triton X-100 to make stable suspensions of MWNTs
A practical outcome of this study is to investigate the dispersion of MWNTs and formation of stable aqueous suspensions The concept ³VWDELOLW\´ can be defined in different ways, depending on the chosen reference In this study, we introduce the term ³VXVSHQGDELOLW\´, which we define as the concentration of
61 MWNTs that remains in suspension after centrifugal process for a fixed concentration of surfactants or initial MWNTs
Characterization of nanotube dispersions by using UV±Vis spectroscopy allows calculation of nanotube suspendability through application of the Beer± /DPEHUWảODZZKLFKLVGH¿QHGDV
Where: A is absorbance b is the path length of the absorbing solution (cm) c is the concentration of MWNTs (mg/ml) İLVWKHH[WLQFWLRQFRHIILcient (cm 2 mg -1 )
Beer±Lambert correlation plots are constructed through absorbance measurements of known concentration dispersions to obtain the extinction FRHI¿FLHQW İ ,Q WKLV ZRUN %HHU±Lambert correlation plots were constructed at wavelengths of 500 nm This particular wavelength was chosen to avoid peaks attributable to surfactants and the solvents
The dispersion samples of MWNTs used to estimate the extinction coefficient were prepared following to the procedure below (Figure 3.9):
(a) Dissolving Sodium Dodecyl Sulfate (SDS) in water to form an aqueous solution of SDS
(b) Mixing purified-MWNTs (M-P) in the aqueous solution of SDS with a probe-type of ultrasonic oscillator to achieve a uniform dispersion of CNTs
Wherein the SDS has a concentration about 1%wt.in the aqueous solution, the weight ratio of SDS:MWNTs is 10:1 [83] and the probe-type ultrasonic oscillator is operated intermittently (Solution 1)
62 After that, we centrifuged the solution for 20 minutes at 3000 rpm to remove precipitate (non-dispersed material) at the bottom of the test tube The stable suspension (Solution 2) is then filtered and washed several times in deionized water to remove surfactant This sample is annealed in the furnace at decomposition temperature of SDS (determined through TGA) and denoted as M-P f TGA and FTIR measurement would be used to inspect that surfactant was eliminated completely after multi-step treatment The aim of this step is to partly remove bundle MWNTs (big size MWNTs) to achieve new source of MWNTs with highly amount of uniform and individual MWNT that can be dispersed homogeneously for precisely determining extinction coefficient
Figure 3.9 Schematic of multi-step treatment to make the stable suspension for estimating the extinction coefficient
SDS (1% wt.) dissolved in water
Filtering and washing with DI annealing
Dried-M-P after removing SDS (denoted M-P f )
SDS (1% wt.) dissolved in water Mixing with horn sonicator
63 The procedures in (a) and (b) were then repeated but the purified-MWNTs was replaced with M-P f and the weight ratio of SDS:M-P f was 40:1 [83] to get a uniform suspension for determining the extinction coefficient We carried out the centrifuging again to ensure that the solution homogeneously dispersed and no precipitate received at the bottom of the test tube (using UV-Vis spectroscopy for testing) The obtained concentration of dispersed MWNTs (M-Pf) is too high to get a measurable absorbance in the spectrometer Hence, the dispersion would be diluted 10 times to obey the Beer ± /DPEHUWảODZ
From this equation, with known concentration of MWNTs (M-Pf WKH İ would be calculated Similarly, the extinction coefficient of MWNTs would also be calculated when using Triton X-100 dispersant with the same procedures like SDS
3.3.3 Comparing the dispersing power of the two surfactants: SDS and Triton-
In order to compare the dispersing power of the two surfactants, dispersions of MWNTs were prepared at concentrations spanning from 0.2 to 2.5 mg/ml, keeping the concentration of surfactant (1%) constant These 14 samples were mixed with probe-type ultrasonic oscillator for 2h in order to get completely surfactant-coated MWNTs
Dispersions were analyzed via UV±Vis spectroscopy and the maximum extractable concentration of MWNTs (at 1% surfactant concentration) was determined for each of the surfactants as follow equation:
64 Where: c1 is concentration of MWNTs recovered in solution c is concentration of MWNTs originally taken in surfactant
This parameter is the measure of dispersion of carbon nanotubes in solution
3.3.4 Determination of optimum CNT to surfactant ratio using UV±Vis spectroscopy
,Q RUGHU WR ¿QG WKH RSWLPXP &17-to-surfactant ratio for each surfactant, a different set of experiments were carried out In these experiments, concentration of surfactants was varied from 0.5 to 1.5% in steps of 0.1%, keeping the amount of MWNTs constant Constant MWNT concentrations chosen in these experiments were the maximum extractable concentrations of MWNTs (determined in the 3.3.3 experiments for 1% surfactant concentration) Again, these samples were analyzed using UV±Vis spectroscopy
3.3.4 Comparing the stability of MWNTs before and after purification
In order to compare the stability of MWNTs before and after purification, dispersions of raw MWNTs (M-raw) and purified-MWNTs (M-P) were prepared at the same concentration of 1.5 mg/ml These two samples would be centrifuged at
3000 rpm for 20 minutes to remove the residue The received suspensions were assessed via UV-9LVVSHFWURVFRS\EDVLQJRQWKHİYDOXH getting above
3.3.5 Preparation of dispersion of MWNTs in various pH values
The pH effects of dispersing MWNTs in aqueous surfactant solutions were undertaken using the two surfactants described above The adsorption experiments at various pH values in the range of 2 ± 12 were performed These pH values were adjusted by addition of concentrated hydrochloric acid for the acidic range and sodium hydroxide for the basic range, making use of a Melter Toledo pH meter
RESULTS AND DISCUSSIONS
Properties of MWNTs source
In this thesis, raw MWNTs (purity 95%) produced by chemical vapor deposition (CVD) method, were obtained from Timesnano (China) The main impurities coexisting with MWNTs were catalyst nano-particles (transition metal or metal oxide), amorphous carbon, graphite, multi-shell carbon nanocapsules XRF, SEM, TEM, TGA and Raman were used to assess the properties of MWNTs source
Analysis from XRF revealed that MWNTs consist of Nickel (Ni) particles Figure 4.1 shows the peaks at 7.468KeV (KA) and 8.266KeV (KB) corresponding to Ni elements
Figure 4.1 XRF spectra of M-raw
Carbon element cannot appear in the spectra due to the limitation of the software of XRF instrument
67 Figure 4.2 presents SEM and TEM images of raw materials The raw MWNTs contain numerous carbon nanotubes with the tubular structure, but also a significant amount of metal nanoparticles (black dots in the figure)
Figure 4.2 a) SEM image and b) TEM image of raw MWNTs
From TGA result (Figure 4.3), the crude MWNTs were heated to 900 0 C with heating rate of 5 0 C/min in pure oxygen to remove completely carbonaceous materials and determine percentage of catalyst
According to this result, we can see that:
68 x The residue (%wt.) of raw MWNTs after annealing at 900 0 C is 6.44%wt, which means that the amount of carbonaceous is about 93.5% and this residual part is Nickel catalyst ,Wảs relatively in accord with supplierảV information (purity of MWNTs is 95%wt.carbonaceous) x The initial oxidative temperature is 415 0 C while the ending oxidative temperature is 610 0 C that depicts a good evidence for existence of amorphous carbon in raw MWNTs This result is also suitable for studies of other researchers [83-86] They showed that the amorphous carbon and graphite carbon were destroyed in the temperature range of 320-500 0 C
Through Raman spectroscopy of raw MWNTs, the properties of carbon products can be determined In this section, Raman spectroscopy is used to determine the existence of defect sites in the structure of MWNTs
Raman experiment conditions for MWNTs samples are presented below: ắ Excitation: He-Ne laser (632.8nm) ắ Model: Labram 300 ắ Configuration: Single ắ Time of integration: 120s
In the high-frequency range of Raman spectra, we observe G-band (~1575-1581cm -
1), D-band (~1324-1327cm -1 DQG'ả-band x The D band is usually attributed to the presence of amorphous or disordered sp 2 carbon in the CNTs samples and other forms of carbon, such as rings along with defects on nanotube walls, vacancies, heptagon-pentagon pairs x The G-band originated from in-plane tangential stretching of the carbon- carbon bonds in graphene sheets x 7KH'ảEDQGZKLFKis a weak shoulder of the G-band at higher frequencies and also a double resonance feature induced by disorder and defects
69 The properties of MWNTs can be determined following D-peak and G-peak, so, in our experiment, it is not necessary to FKDUDFWHUL]H WKH 'ả IHDWXUH RI 5DPDQ spectra
Peaks are separated using /RUHQW]LDQ FXUYH ¿WWLQJ SURFHGXUH >7] of OriginPro 8
The Lorentzian lineshape is described by:
Here y is the intensity at energy x, y o is a constant shift and x o $ȦDUHWKH center, area and full width at half maximum (FWHM) of the peak, respectively
Figure 4.4 Raman spectra of M-raw
The appearance of D peak in Raman spectra of M-raw (Figure 4.4) prove that crude MWNTs product contains amount of amorphous carbon and defect sites LQ0:17VảVWUXFture
In this section, we evaluated generally properties of MWNTs source; these results would be used in next sections to assess efficiency of purification as well as dispersion process.
Purification process
The air oxidation is a necessary step in purification process This step bases on the difference thermal oxidation rates exist between MWNTs and other carbonaceous particles The oxidative treatment has two effects: ắ To remove carbonaceous impurities such as amorphous carbon and graphite particles ắ To expose the catalyst nano-particles enclosed in amorphous or graphite carbon for being easily dissolved by acid treatment
The following step is using HCl to dissolve catalyst nano-particles that exist in the raw materials
TEM and SEM images have traditionally been the most important techniques for characterizing carbon nanotubes In this section, TEM would be used to measure the diameter of MWNTs, qualitatively evaluate the structure as well as the purity of MWNTs samples Because of the lower resolution of SEM, it would be only used to provide a general view of the shape of MWNTs after each purification steps Figures 4.5 and 4.6 below show respectively SEM and TEM images of MWNTs samples before and after the each step of purification process
Figure 4.5 SEM images of a) M raw and b) M-P
SEM images of oxidized MWNTs samples at 460°C for 24 hours in air and M-P after treating with hydrochloric acid are shown in Figure 4.5 Almost samples still exhibit the original tubular shape of MWNTs Nevertheless, it is very difficult to distinguish MWNTs with amorphous carbon, graphite nano-particles and catalyst in sample by SEM Therefore, other methods would be used to evaluate the efficiency of acid treatment step
From TEM images (Figure 4.6), we can observe the unchanged structure of MWNTs after the air oxidation step Besides, amorphous carbon surrounding MWNTs are decomposed under high temperature and help to reduce diameter of MWNTs a Mraw b M-P
Figure 4.6 TEM images of a) Mraw, b) M460 and c) M-P
Table 4.1 7KH0:17VảRXWHUGLDPHWHURI0raw and M460
Samples Average outer diameter (nm)
Diameter of MWNTs was calculated based on TEM images by using the ImageJ software [89] These results are presented in Figure 4.6 and Table 4.1: the diameter of oxidized MWNTs is around 15.8nm, while the diameter of raw MWNTs is higher (16.9nm) This difference takes part in to the affirmation of the decomposition of amorphous carbon on MWNTs surface Besides, images also prove that the structure of MWNTs is not changed significantly after purification process The diameter of purified MWNTs is nearly the same with M 460 (15.7nm for M-P and 15.8nm for M460) (Table 4.1) because the acid treatment step just remove catalyst nanoparticles
In addition, the differences between TEM images of M-P and M 460 are identified rather clearly that the catalyst impurities are almost dissolved in HCl so the density of black points significantly decrease in Figure 4.6c This is a strong evidence for the efficiency of purification process
To calculate the percentage of catalyst nano-particles existed in MWNTs before and after each purification process, MWNTs samples were analyzed by thermal gravimetric analysis (TGA) with heating rate of 5 0 C/min up to 900 0 C, in pure oxygen to remove completely carbonaceous materials The results are summarized in Table 4.2 and Figure 4.7
74 From TGA result (Figure 4.7), we can see that after the air oxidation step, there is only the carbonaceous materials in raw MWNTs were burn, the catalyst nano- particles were exposed and still remained in the sample leading to the percentage of metal oxide nano-particles increase (6.44%wt for Mraw opposed to 26.30%wt for
M 460 ) In next steps, these metal oxide nano-particles were etched away by HCl It is seen from the results that the residual weight drops to a rather low level after the acid treatment, the percentages of residue reduced to 5.4%wt (Table 4.2) which means that the catalyst particles was efficiently removed
For being easy to understand, we assume that: m1, m2, m3 correspond to the content of MWNTs, carbonaceous impurities (amorphous carbon and graphite) and metal FDWDO\VW LQ WKH UDZ VDPSOH Pả Pả P3) is the remain of catalyst after acid treatment
(due to the removal of carbonaceous impurities)
(due to the removal of carbonaceous impurities and a part of catalyst nanoparticles)
% residue (for M-raw) < % residue (for M460) and % residue (for M460) > % residue (for M-3 ,WảV LQ DJUHHPHQW with the results in Table 4.2 and asserts the effectiveness of purification
Figure 4.7 TGA of MWNTs sample after each purification step
Table 4.2 The remained weight of the CNTs sample after each purification step
The chemical nature of nanotube exterior is not expected to change significantly during air annealing and acid treatment, and FTIR spectroscopy shows no difference spectra before and after purification process (Figure 4.8) Both of them contain the identified peaks including: x Broad peaks at 3400-2800 cm -1 corresponds to ±OH groups of water and/or carboxylic acid groups and C-H as defect sites x 1650-1500 cm -1 represents for -C=C- stretching groups in CNTs structure x 1720-1722 cm -1 and 1205-1218 cm -1 stand for the -C=O and -C-O- stretching indicating the introduction of carboxylic groups
Figure 4.8 FTIR spectroscopy of a) M-raw and b) M-P 4.2.4 Raman spectroscopy
Raman investigation shows the intensity ratio between a disorder peak (D- band) and graphitic peak (G ±band) of MWNTs samples after each purification step (Figure 4.9) The intensity ratio of the D and G band (I D /I G ratio) is a measure of the structural perfection of the nanotubes, i.e., a low I D /I G ratio is indicative for a high degree of perfection
Figure 4.9 Raman spectra of M460 and M-P
77 The decrease of ID/IG ratio from 2.52 for raw MWNTsto 2.24 for MWNTs samples after annealing at 460 0 C (Table 4.3) indicates that amount of amorphous carbon is removed from the raw MWNTs The ID/IG ratio of M-P (2.24) changes insignificantly comparing with M460 (2.18) It once again proves that there LVQảWDQ\ QRWLFHDEOHGLVWXUEDQFHLQ0:17VảVWUXFWXUH
Table 4.3 Comparison ID/IG ratio of Mraw, M460 and M-P
Samples ID IG ID/IG
The role of purification in the dispersion of MWNTs
In order to assess the impact of purification process on the stability of nanotube dispersions in water, we have explored each step
Gas-phase oxidation, via high temperature, furnace-assisted thermal annealing in the air can reduce the overall length distribution of carbon nanotubes After heating in the air, most nanotubes have sub-micron lengths as compared to several microns for untreated MWNT, but TEM images in Figure 4.6 do not show clearly these change because it depends heavily on the photographing technique
We also observe diameter reduction of MWNTs after the air oxidation step (Table 4.1) Nanotube diameter and nanotube length were found to be an important role in the dispersibility of MWNTs Recent studies [89, 90] have demonstrated that mere shortening and small diameter of carbon nanotubes without introduction of functional groups or solubilising agents substantially increases the stability of 0:17VảGLVSHUVLRQLQZDWHU
78 3XUL¿FDWLRQLVDOVRQHFHVVDU\WRHQVXUHUHPRYDORIH[WUDQHRXVFDUERQDFHRXV materials prior to absorption measurements Furthermore, the catalyst particulates encased in the nanotube ends are composed of high density metals such as Nickel in this case These high-Z elemental impurities have substantially different refractive indices in the visible light region when compared with graphitic materials [91] Thus, removal of these impurities is essential in accurately measuring light absorption and scattering from solubilized carbon nanotubes
Otherwise, the raw MWNTs can be separated in water under supporting of sonication process, however, this process cannot remove the catalytic nano-particles which have the big size And as a consequence, the suspended raw MWNTs in water are not stable and immediately aggregated after finishing sonication process Interestingly, the metal catalyst associates with the larger MWNT bundles rather than the individuals and smaller MWNT bundles If we take the acid treatment process, the transition metal nano-particles will be removed The disappearance of big-size catalytic nano-particles contributes to increase the dispersibility of MWNTs from microscale to nanoscale (Figure 4.10)
Figure 4.10 Schematic of state of suspended MWNTs at nanoscale in water after removing catalyst
In order to assess quantitatively the stability of carbon nanotube dispersions in aqueous media before and after purification, their optical density (absorbance) was reported The absorbance at a wavelength of 500 nm was converted into concentration using an extinction coefficient determined below Figure 4.11 shows
79 the absorbance of raw MWNTs and purified-MWNTs suspensions after centrifuging and then diluted 50 times
Wavelength (nm) a - raw MWNTs b - purified-MWNTs a b
Figure 4.11 Absorbance of 1.5mg/l suspension (diluting with a factor of 50) of raw-
MWNTs and purified-MWNTs after centrifuging at 3000rpm for 20 minutes (using
This result proves that the absorbance of purified-MWNTs dispersion is higher than raw MWNTs dispersion It means that after centrifuging, the real concentration of suspension of purified-MWNTs is higher than untreated MWNTs
Table 4.4 Remained concentration of suspension raw MWNTs and purified-
MWNTs samples after centrifuging at 3000rpm for 20 minutes
From the Table 4.4, the remained concentration of M-P suspension is 1.15 mg/ml, relatively higher than M-raw suspension (0.55 mg/ml) and this M-P solution can be stable at least 5 weeks (Figure 4.12) It once again confirms utmost important UROHRISXULILFDWLRQIRUFDUERQQDQRWXEXHVảGLVSHUVLRQ
80 After centrifuging at 3000 for 20 minutes
Figure 4.12 Images of suspensions of M-P at concentration of 1.15 mg/ml during various times (using SDS as dispersant)
Determining the extinction coefficient of MWNTs
In addition to purification process, we have used Sodium Dodecyl Sulfate (SDS) and Triton X-100 to make a stable suspension of MWNTs In general, adsorption of SDS and Triton X-100 on nanotube surfaces stabilizes their
81 dispersions in water due to formation of a micelle around the nanotube, whereby the hydrophobic tails of the surfactant wrap around the walls of the nanotube and the hydrophilic heads point outwards into the aqueous solution
The solution used to estimate extinction coefficient was prepared by mixing M-P f in the aqueous solution of SDS with the weight ratio of SDS: M-P f was 40:1, wherein M-Pf was the product after multi-step treatment to remove almost bundled purified-MWNTs The probe-type ultrasonic oscillator can help MWNTs well- dispersed in water in which a proper ionic surfactant is contained TGA and FTIR results (showed in Appendix) proved the effectiveness of washing SDS/M-P solution through a filter membrane to remove SDS They indicated that the surfactant can be completely removed by washing and thus the CNTs can be easily applied to various processes
This solution was once again tested by using UV-Vis spectroscopy Figure 4.13 compares the UV absorbance of solution of SDS/M-P f (40:1) after 45 mins sonicating, lasting 10 mins more and after centrifuging at 3000 rpm in 20 mins The obtained concentration of dispersed CNTs was too high to obtain a measurable absorbance in the spectrometer Hence, the dispersions were diluted 10 times to enable measurement From the image, the curves are almost overlapped It indicates that the mixture was completely dispersed, we can use this solution to calculate the extinction coefficient value
Figure 4.13 UV-Vis spectrum of SDS/M-Pf (40:1) solution a) after 45 mins sonicating b) after 55 mins sonicating and c) after centrifuging
The centrifuged suspension was diluted by a factor of 10, 20, 30, 40 and analyzed by using UV-Vis spectroscopy (Figure 4.14)
Wavelength (nm) a - 0 mg/ml b - 0.00625 mg/ml c - 0.0083 mg/ml d - 0.0125 mg/ml e - 0.025 mg/ml a b c d e
Figure 4.14 a) UV-Vis absorbance of M-Pf dispersed in the presence of SDS for different concentrations and b) Beer ± Lambert curve a b
83 Fig 4.14 shows the evolution of absorbance for different concentration of purified-MWNTs in water The higher concentration of MWNTs, the higher absorbance was received Plotting the absorbance at 500nm wavelength versus the concentration, we could see that the curve is almost linear, it means at 500nm, it followed the Beer-/DPEHUWảODZ
Table 4.5 The extinction coefficient value of M-Pf (using SDS as dispersant)
Similarly, extinction coefficient value of purified-MWNTs dispersed in Triton X-100 solution is also calculated following table 4.6 and Figure 4.15
Table 4.6 The extinction coefficient value of M-P f (using Triton X-100 as dispersant)
Wavelength (nm) a - 0 mg/ml b - 0.00625 mg/ml c - 0.0083 mg/ml d - 0.0125 mg/ml e - 0.025 mg/ml a b c d e
Figure 4.15 a) UV-Vis absorbance of M-Pf dispersed in the presence of Triton X-
100 for different concentrations and b) Beer ± Lambert curve
According to the calculation in Table 4.5 and Table 4.6, the average extinction coefficient at 500nm wavelength is equal to 45.52 cm 2 mg -1 for SDS and 43.17 cm 2 mg -1 for Triton X-100 These values are very close to the extinction coefficient obtained for suspensions of multi-walled FDUERQQDQRWXEHVİ500 = 46 ± 1.4 cm 2 mg -1 ) [92].
Comparing the dispersing power of two surfactants
Dispersions of MWNTs were prepared in a series of concentrations: 0.2; 0.3; 0.5; 1.0; 1.5; 2.0; 2.5 mg/ml, keeping the concentration of surfactant (1%) Concentration of MWNTs that really dissolved or dispersed into the solution can WKHQEHGHWHUPLQHGXVLQJWKHVSHFL¿FH[WLQFWLRQFRHƥcient of carbon nanotubes at QPİSDS = 45.52 cm 2 mg -1 DQGİTriton-X = 43.17 cm 2 mg -1 , in the Lambert±Beerả law With this knowledge, percentage recovery into the solution can be calculated as
85 Where c1 is concentration of MWNTs recovered in solution c is concentration of MWNTs originally taken in surfactant
This parameter is the measure of dispersion (or suspendability) of carbon nanotubes in solution
Figure 4.16 and 4.17 depicts the UV±Vis spectra of MWNTs with varying concentrations of nanotubes in surfactant solutions The next curve shows the Lambert±Beer dependence of absorption at 500 nm on the concentration of MWNTs
Figure 4.16 (a) UV-Vis spectra of carbon nanotubes in SDS solution (diluted with a factor of 50) and (b) Beer ± Lambert curve
Figure 4.17 (a) UV-Vis spectra of carbon nanotubes in Triton X-100 solution
(diluted with a factor of 50) and (b) Beer ± Lambert curve
In order to compare the dispersing power of surfactants in context, percentage extractability was calculated at different concentrations of nanotubes In all surfactants, percentage extractability vs concentration follows a Gaussian trend (Figure 4.18), which depicts a linear increase with increase in concentration of MWNTs until the maximum extractability limit is achieved This is presumably because at low CNT concentrations, the amount of surfactant is suƥcient to coat the carbon nanotube surface evenly Eventually, a concentration value is attained for which the surfactant amount is just suƥcient to disperse the carbon nanotubes; i.e., the maximum extractability limit is achieved at this point For subsequent increases in the concentration of nanotubes, the surfactant amount turns insuƥcient to fully disperse the agglomerates of CNTs, therefore decreasing the percentage extractability at high concentrations The maximum amount of MWNTs is extracted in the case of Triton X-100, where percentage extractability has gone as high as 92.80% It is estimated to be 82.94 % for SDS Maximum extractable MWNT concentration is found to be 1.6mg/ml and 2 mg/ml for SDS and Triton X-100, respectively, for the constant surfactant concentration (1%) Thus, the same amount of Triton X-100 can disperse large amounts of MWNTs as compared to SDS
% extractability Gauss Fit of % extractability
% extractability Gauss Fit of % extractability
Figure 4.18 Percentage extractability vs concentration trend of carbon nanotubes for (a) SDS and (b) Triton X-100
Dispersing power of surfactants can also be explained on the basis of their chemical structures (Figure 4.19) In order to disperse nanotubes in water, surfactant molecules orient themselves in such a fashion that hydrophobic tail groups face toward the nanotube surface while hydrophilic head groups face toward the aqueous phase, producing a lowering of the nanotube/water interfacial tension Thus, the dispersing power of the surfactant depends on KRZ ¿UPO\ LW DGVRUEV RQWR WKe nanotube surface and produces by these adsorption energy barriers of suƥcient height to aggregation Molecules having the benzene ring structure adsorb more strongly to the graphitic surface duH WR ʌ±ʌ VWDFNLQJ W\SH LQWHUDFtion [93,94] Generally, hydrophobic tail groups tend to lie Àat on the graphitic surface because graphitic unit cells match well with the methylene units of hydrocarbon chains [95] Thus, eƥciency of adsorption and consequently dispersing power of surfactants are greatly affected by the tail length of the surfactant Longer tails means high spatial volume and more steric hindrance, thus providing greater repulsive forces between individual carbon nanotubes [96] Besides this, surfactants with unsaturated bonds in their tail groups contribute more toward nanotube dispersion [96]
Figure 4.19 Chemical structures of (a) SDS and (b) Triton X-100
As one can see, SDS has greater hydrocarbon tail length, while Triton X-100 has smaller one The phenyl ring has an effective length of about three and one-half carbon atoms, while carbon atoms on branches contribute about one-half the effect of carbon atoms on straight alkyl chains Thus, Triton X-100 has an effective chain length of nine atoms only, which is shorter than SDS Thus, theoretically Triton X-
100 should exhibit lower dispersing power, contrary to experimental observations Such a paradigm departure from experimental observations is presumably because of the presence of benzene ring in the tail group of Triton X-100 7KLV ¿QGLQJSURPSWVXVWRFRQFOXGHWKDWZKHQHYHU³WDLOOHQJWK IDFWRU´DQG³EHQ]HQHULQJIDFWRU´ compete, the latter contributes more to dispersion of CNTs In total, Triton X-100 proves to be better than SDS.
Determination of optimum CNT to surfactant ratio using UV±Vis
In order to determine the optimum CNT-to-surfactant ratio for each surfactant, the concentration of surfactants was varied from 0.5 to 1.5% in steps of 0.1% while the MWNT concentration was kept constant Constant MWNT concentrations were selected to be 1.6 mg/ml and 2 mg/ml for SDS and Triton X-
100, respectively (chosen from the above experiments) Again, absorbance values were determined at 500 nm and percentage extractability of MWNTs was calculated for each sample There appears to be a Gauss fit between percentage extractability and the concentration of surfactant (Figure 4.20)
Percentage extractability attains maxima at 1% (Triton X-100) and 1.2% (SDS) surfactant concentrations With this knowledge, optimum CNT-to-surfactant ratio was calculated to be 1:7.5 and 1:5 for SDS and Triton X-100, respectively
% extractability Gauss Fit of % extractability
% extractability Gauss Fit of % extractability
Figure 4.20 Variation of percentage extractability with variation of concentration of surfactant for (a) SDS and (b) Triton X-100
Increase in percentage extractability with increase in surfactant concentration was on the expected lines However, after attaining maxima, percentage extractability decreased This problem can be explained in the light of the theory of micelle formation in surfactants Figure 4.21 depicts a schematic illustration of a plausiEOH PHFKDQLVP RI ÀRFFXODWLRQ RI &1Ts via surfactant molecules At high concentrations, the surfactant molecules form micelles in solution The size of these micelles keeps on increasing with increasing surfactant concentration due to interaction between groups of the same polarity Likewise, surfactant molecules form multilayers on nanotube surface when the concentration of surfactant is increased for a constant nanotube concentration As a consequence, surface
90 coverage by surfactant molecules becomes so high that portions of surfactant molecules extending into the liquid phase start interacting with others on neighboring CNTs [95] If the orientation of the outermost layer is such that the hydrophobic groups of surfactant molecules are forced to extend into the aqueous phase, then this interaction favors a reduction in their surface energies [96] This bridging of CNTs via extra surfactant PROHFXOHVFDXVHVÀRFFXODWLRQ)Oocculation of CNTs might be the reason behind the decrease in percentage extractability, which in turn decreases the dispersion of nanotubes at high surfactant concentration
Figure 4.21 0HFKDQLVPRIÀRFFXODWLRQRf CNTs via surfactant molecules [62] 4.7 (IIHFWVRIS+YDOXHVRQWKHVWDELOLW\RI0:17VảVXVSHQVLRQ
While dispersing nanotubes in aqueous medium is of paramount importance for numerous applications, many systems of interest operate under non-neutral pH conditions Thus, a detailed study on the pH effects of dispersing MWNTs in aqueous surfactant solutions was undertaken using the two surfactants described above
Nanotube dispersions were prepared in which the constant concentration of MWNTs and surfactants were the optimal values determined in above experiments
91 (4.5 and 4.6) The adsorption experiments were performed at various pH values in the range of 2 ± 12
From Figure 4.22, it is clear that the Triton X-100 surfactant based dispersions demonstrate negligible pH dependence with pH from 2 to 12 SDS shows some pH dependent solubility with a maximum being reached near neutral pH conditions (Figure 4.22) SDS stabilized dispersions are found to be lower under basic and acidic conditions
Figure 4.22 Suspendability of MWNTs in (a) SDS and (b) Triton X-100 solution at different pH
Generally, water - solid interfaces acquire a net charge of a particular sign to maintain electrical neutrality with the surface The structure of this electrical double layer depends on the pH of the medium and the type and amount of electrolyte present The zeta potential is a measure of electrostatic interactions of double layer Its value can be related to the stability of colloidal dispersion and has been used in the literature to explain the effect of pH on MWNT dispersions
The PZC or point of zero charge is defined as the pH at which the surface exhibits a net surface charge of zero
92 Zeta potential of the CNTs suspensions decreased with increasing pH (Figure 4.23), and the point of zero charge was estimated to be pH 6.5 [97] Then, the negative charges of the CNTs at high pH may come from deprotonation of oxygenated moieties on the nanotube surface
Figure 4.23 7KHLQÀXHQFHRIS+RQ]HWDSRWHQWLDOVRI&17V7KHVWDQGDUG deviations were calculated with three replications [97]
Three possible surfactant±CNT interaction mechanisms are: (a) electrostatic attraction or repulsion, (b) hydrogen bonding between the ±O± of polyethoxylate and ±OH and/or ±COOH groups of CNTs and (c) hyGURSKRELFDQGʌ±ʌLQWHUDFWLRQV[97] Following the results in Figure 4.22b, the constant adsorption at various pHs VXJJHVWVWKDWK\GURSKRELFDQGʌ±ʌLQWHUDFWLRQVZRXOGEHWKHPDMRUPHFKDQLVPVIRr the adsorption ofTriton X-100 by CNTs Adsorbed amount of Triton X-100 on the CNTs kept nearly unchanged when CNT surface charge changed from positive to negative with pH from 2 to 12 It means that electrostatic interaction had no VLJQL¿FDQW HIIHFW RQ the adsorption of surfactants to CNTs Furthermore, the polyethoxyl moiety of Triton X-100 and the hydroxyl/carboxylic groups on the CNTs surface may form hydrogen bonds [99] If hydrogen bond is a major mechanism of surfactants adsorption on CNTs, the adsorbed amount on CNTs
93 would decrease with increasing pH because of the transition from the -COOH group to the ±COO - group on the surface of the CNTs However, the constant adsorption within pH 2±12 (Fig.4.22b) could rule out the hydrogen bonding as a major mechanism regulating the adsorption of Triton X-100 onto CNTs
It has been extensively demonstrated that the main driving forces for adsorption of ionic surfactant molecules on charged surfaces are the Coulombic attractions between the surfactant heads and the charged surface groups from the solid, and the hydrophobic bonding between the surfactant tails [98] Figure 4.24 illustrates the pH dependence on the adsorption of anionic surfactant SDS Following to Figure 4.22a, the maximum suspendability achieved near neutral conditions (at pH around 6.5-7) at which CNT surface exhibit a net surface charge of zero At these conditions, Coulombic forces do not play a central role, but are overcome by the hydrophobic interactions between the surfactant tail and nanotube walls Under acidic conditions, the stable suspension decreases because Coulombic attractions start governing the adsorption process at low pH, i.e the negatively charged headgroups are more attracted by the positively charged surface Thus, it begins to appear the flocculation phenomena between the surfactant tails that make the poor solubility of MWNTs in strong acidic conditions (Figure 4.25) In basic mediums, the stability of MWNTs also decrease due to the repulsion generated between the negative heads of surfactant and the excess negative charge generated on the MWNT surface well above the PZC, which overcomes the attraction hydrophobic forces between the surfactant tail and the nanotube (Figure 4.24)
Figure 4.24 Schematic representation of surfactant assisted adsorption of nanotubes using the ionic surfactants SDS with deference to pH effects on the nanotube surface
Figure 4.25 Flocculation occurs via surfactant tails at low pH (acidic conditions)
CONCLUSIONS AND PERSPECTIVES
This study has demonstrated a facile but effective and non-destructive method for the purification and dispersion of multi-walled carbon nanotubes The stability of MWNTs in aqueous solution with two different surfactants (Sodium dodecyl sulfate (SDS) and Triton X-100) were investigated The results achieved are as follow: x We have developed a simple, effective and non-destructive method for purifying MWNTs MWNTs with a purity of 95% MWNTs were obtained using a multi-step method However not all tips of carbon nanotubes were opened in air oxidation, then the catalyst particles embedded in them could not be removed, a few percent (5%) of catalyst particles still remained in the sample x It was also observed the important role of purification for highly dispersing of MWNTs Purification process helps to reduce the length, diameter of the tubes, remove catalyst and debundle that enhancing the dispersibility of carbon nanotubes from micro-scale into nano-scale x The invention also provided a method for fast dispersing CNTs in aqueous solution and determining the extinction coefficient ± a very important parameter to assess quantitatively the stability of carbon nanotube in aqueous media In this method, the purified carbon nanotubes were added into an aqueous solution of two different surfactants (Sodium dodecyl sulfate (SDS) and Triton X-100), and then dispersed therein through horn sonicator By using UV-Vis tool and Lambert-%HHUả ODZ HTXDWLRQ, the extinction coefficient at 500nm wavelength of purified MWNTs in SDS and Triton X-
100 solutions were calculated as 45.52 cm 2 mg -1 and 43.17 cm 2 mg -1 , respectively x The dispersing power of two surfactants was compared Triton X-100 could disperse large amount of MWNTs as compared to SDS Maximum
96 extractable MWNT concentration was found to be at 1.6 mg/ml and 2mg/ml for SDS and Triton X-100, respectively x The key ¿QGLQJRIWKHSUHVHQWVWXG\LVWKHVLJQL¿FDQFHRIRSWLPXP&17- to- surfactant ratio The quality of nanotube dispersion deteriorates below or above this ratio Thus, surfactants should be in concentration just suƥcient to coat the nanotube surface, avoiding any excess, as an unnecessarily large amount of surfactant also decreases the nanotube dispersion We have standardized this ratio for the surfactants in context It was found to be 1:7.5, 1:5 for SDS and Triton X-100, respectively x The pH dependence of these surfactant assisted MWNT dispersions was also examined Not surprisingly, Triton X-100 surfactant was found to exhibit minimal pH dependence However, SDS was found to yield poor solubility under acidic and basic conditions Consequently, for highly acidic or basic aqueous media, it is preferable to utilize non-ionic Triton X-100 while anionic surfactant SDS enhances MWNT solubility at high level for near neutral conditions
As perspective of this work, we suggest: x Surfactants used to stabilize CNT dispersions often foam heavily Because WKHLQFRPLQJXOWUDVRQLFZDYHLVUHÀHFWHGDWWKHDLU±liquid interface, these air layers and foams prevent the wave from reaching the CNT surfaces We should investigate the additions of antifoam agents to the dispersing liquid to LPSURYHXOWUDVRQLFGLVSHUVLRQHI¿FLHQF\RI&17V x )DEULFDWLQJ KRPRJHQHRXV 0:17VSRO\PHU FRPSRVLWH 39$ HSR[\ô using noncovalently functionalized soluble MWNTs to improve mechanical, electrical and thermal properties of polymers
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APPENDIX Table A.1 The outer diameter of M raw , M 460 and M-P
Sample Outer diameter (nm) Average
Table A.2 % extractability calculated for various concentrations of M-P dispersed in 1% SDS solution
C (mg/ml) Absorbance C 1 (mg/ml) % extractability
Table A.3 % extractability calculated for various concentrations of M-P dispersed in 1% Triton X-100 solution
C (mg/ml) Absorbance C 1 (mg/ml) % extractability
Table A.4 % extractability calculated for concentration of 1.6 mg/ml M-P dispersed in various concentrations of SDS solution
Concentration of SDS (%) Absorbance C (mg/ml) C 1 (mg/ml) %extractability
Table A.5 % extractability calculated for concentration of 2 mg/ml M-P dispersed in various concentrations of Triton X-100 solution
Absorbance C (mg/ml) C 1 (mg/ml) %extractability
Figure A.1 TGA curve of SDS surfactant
Figure A.2 TGA curve of M-P and M-P f after removing SDS
Figure A.3 FTIR spectra of a Mraw , b M-Pf and c M-P with Triton X-100
Figure A.4 FTIR spectra of a Mraw, b M-Pf and c M-P with SDS