Introduction to nanomaterials: synthesis and applications

CHAPTER 3

Introduction to nanomaterials: synthesis and applications R. Jose Varghese1,2,3, El hadji Mamour Sakho2,3, Sundararajan Parani2,3, Sabu Thomas1, , Oluwatobi S. Oluwafemi2,3 and Jihuai Wu4 1

International and Inter University Centre for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India 2 Department of Chemical Sciences (Formerly Applied Chemistry), University of Johannesburg, Doornfontein Campus, Johannesburg, South Africa 3 Centre for Nanomaterials Science Research, University of Johannesburg, Johannesburg, South Africa 4 Professor of Materials and Chemistry, Vice-President of Huaqiao University, Director of Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Director of Institute of Materials Physical Chemistry, Huaqiao University, Xiamen, Fujian, P.R. China

Contents 3.1 Introduction to nanotechnology 3.1.1 History of nanotechnology 3.1.2 Size effects of nanomaterials 3.1.3 Carbon nanomaterials 3.2 Quantum dots 3.3 Metal nanoparticles 3.4 Synthesis of nanomaterials 3.4.1 Top-down approaches 3.4.2 Bottom-up approaches 3.5 Conclusion References Further reading

75 75 79 80 84 84 86 86 88 90 91 95

3.1 Introduction to nanotechnology 3.1.1 History of nanotechnology Nanotechnology involves the synthesis and application of materials in dimensions of the order of a billionth of a meter (1 3 1029). This categorizes them under ultrafine particles. Fig. 3.1 reveals the size comparison of the nanoparticles against different living and nonliving species. The properties of nanoparticles vary from their bulk counterpart and their chemistry [1]. The electronic structure, reactivity, and thermal and mechanical properties Nanomaterials for Solar Cell Applications DOI: https://doi.org/10.1016/B978-0-12-813337-8.00003-5

© 2019 Elsevier Inc. All rights reserved.

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Figure 3.1 Size comparison of different structures (living and nonliving).

Figure 3.2 Mollusk shell.

tend to change when the particles reach the nanoscale. Through nanotechnology, we can build materials and devices with control down to the level of individual atoms and molecules. In the past two decades, there were reports of colloids and nanoparticles designed by nature [2,3]. Abalone shells (Fig. 3.2) are an example of nature’s Nanoassembly [4]. The shells of these mollusks are made by nanopatterning of calcium carbonate, which is same as limestone, but harder. The molecules in these shells are clumped and stacked up in a row pattern that makes them much harder. Another wonder of nanotechnology in nature is the spider, which synthesizes silk (Fig. 3.3) from protein polymer to form a fiber with strength similar to high-tensile steel [5]. Nanotechnology has been applied by humans for over a thousand years unknowingly from painting to making steel. Medieval stained-glass

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Figure 3.3 Spider web.

Figure 3.4 Medieval stained-glass of an ancient church.

(Fig. 3.4), which originated in Europe, is one of the examples of the oldest nanotechniques known in history. Different staining of the glass is due to the entrapment of different nanoparticle in the glass matrix, which was unintentional. For instance, the ruby red color was due to the entrapment of gold nanoparticles (AuNPs) in the glass matrix, whereas the formation of silver nanoparticles (AgNPs) within the glass matrix was responsible for the deep yellow color [6,7].

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Deruta ceramics is an iridescent ceramic material developed in Italy during the early medieval age. The metallic glaze of this material was due to the presence of copper and silver particles in nanometer range (Fig. 3.5) [8]. The Chinese used AuNPs to create a red color in the ceramic porcelains (Fig. 3.6). The Lycurgus Cup (Fig. 3.7) is a dichroic cup made by Romans in the 4th century. The color of the cup changes with respect to the incident light. When it is looked at in reflected light or daylight, it appears green. However, when light is shown into the cup and transmitted through the glass, it changes to red. This color variation is due to the presence of gold and AgNPs [9]

Figure 3.5 Deruta maiolica plate.

Figure 3.6 Chinese ceramic porcelain.

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Figure 3.7 Lycurgus Cup.

In 1861, James Clark Maxwell produced the first color photograph. Later, in 1883, American inventor George Eastman designed a film consisting of a long paper strip coated with an emulsion containing silver halides that are in nanoparticle range. So, the technology based on nanosized materials is not new. Michael Faraday, in 1857, published an article in “Experimental relations of gold (and other metals) to light” in which he attempted to explain how metal particles affect the color of church windows [10]. Richard Zsigmondy, the 1925 Nobel Prize Laureate in chemistry, proposed the term “nanometer.” Later, Richard Feynman, an American theoretical physicist, presented a lecture on “There’s plenty of room at the bottom,” at the meeting of American Physical Society on December 29, 1959, which led to the birth of theoretical nanotechnology [11,12]. He proposed a theory on manipulation of individual atoms to make new small structures having very different properties.

3.1.2 Size effects of nanomaterials The size of the nanoparticles has a great influence on their properties (Fig. 3.8). When a particle is in its bulk state compared to its size in its microscale, there is not much difference in its properties. However, when the particle reaches a size less than 100 nm, the properties changes significantly compared to its bulk state. In this scale (1 100 nm), quantum size

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Figure 3.8 Schematic representation of properties of nanoparticle.

effects decide the properties of particles, such as chemical, thermal, mechanical, optical, electrical, and magnetic [12 14]. The size-dependent properties of AuNPs have been explained well in the last few decades [7,9,10,14,15]. Fig. 3.9 shows the size-dependent color of AuNPs. At nanoscale, gold particle exhibits purple color different from the bulk, which was yellow colored. This color change is attributed to the change in their band type from continuous to discrete due to confinement effect (Fig. 3.10). These quantum effects in the nanoscale are the basic reasons behind the “tunability” of properties. By simply tuning the particle size, we can change the material property of our interest (such as fluorescence).

3.1.3 Carbon nanomaterials The element carbon plays an important role in nature due to its ability to bond in so many ways. If we investigate the nature of most important biological compounds, such as DNA and proteins, they are largely based on carbon linked to nitrogen, hydrogen, and oxygen. Carbon with

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Figure 3.9 Size color dependence of gold particles.

Figure 3.10 Schematic representation of confinement of electron and change in band gap with size.

electronic configuration of 1s2 2s2 2p2 can form different crystalline and amorphous materials, because it can exist in sp2 sp3 and sp1 hybridization. For a long time, carbon was thought to exist only in two allotropic forms, such as graphite and diamond. The difference in properties of the above materials is explained through its structure. When we look into graphite, the large amount of delocalized bonding, promotes the electrical conductivity of graphite, whereas the strong covalent bond of sp3 carbons in diamond promotes the hardness of the diamond. But, in 1985, the research focus on carbon changed after the discovery of fullerene, another form of carbon (Fig. 3.11) [16]. This led to the birth of synthetic carbon

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Figure 3.11 Structure of fullerene.

Figure 3.12 Structure of graphene sheet [19].

nanomaterials. This discovery was followed by carbon nanotubes (CNTs) in 1991 and graphene at 2004 [17,18] (Figs. 3.12 and 3.13). The fullerenes were discovered accidentally by Kroto and Smalley in 1985 when they observed a strange result in the mass spectra of vaporized carbon. C60 is found to be most stable fullerene. C60 consists of 60 equivalent carbon atoms, which is sp2 hybridized. The approximate diameter of reported C60 is 0.7 nm [16]. CNT is described as rolled up graphene sheets of one atom thick and made up of hexagonal rings similar to benzene rings of carbon atoms. The properties such as metallic and semiconducting properties of this material depend on the chirality of carbon atom. The actual discovery of CNTs is credited to Sumio Lijima, who worked

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Figure 3.13 Structure of carbon nanotube [20].

at NEC Corporation in 1991 and synthesized CNT with the help of the arc-discharge method. The reported CNT diameter was about 4 30 nm and length of 1 µm [21]. These structures were similar to a Russian doll-like coaxial packing that was later named multiwall CNTs (MWCNT). Later, in 1993, Lijima et al. and Bethune et al. reported the CNTs that were composed of single-layered graphene with a diameter of 1.37 nm [16,21] and named single-walled CNTs (SWNT). CNTs have been applied as a filler in composite material to improve structural properties, electronic applications, solar cells/batteries, and also in biologicals as sensors. Graphene is composed of a one-atom-thick sheet of sp2-bonded carbon atoms arranged in hexagonal pattern. This structure is considered as the basic building block of other carbon nanomaterials. For example, it can be rolled into CNTs or stacked into a graphite. It can be transformed into a fullerene by the addition of pentagons. From the electronic point of view, graphene is considered to be zero gap semiconductor, more like metals. This unique property led to many electronic properties, like ballistic transport (transport of charge carriers in a medium), pseudospin chirality, and conductivity in the absence of charge carriers that can be utilized for future applications [23]. Graphene has the fastest electron mobility, greater than silver; high mobility of temperature-independent charge carriers (200 times higher than Si); and effective Fermi velocity similar to the speed of light. Graphene has good mechanical and thermal properties. Due to its unique properties, graphene can replace graphite, CNTs, and metals. Graphene found its application as transparent electrodes, solar cells, and photoelectrodes [24]. The electrical conductivity can be tuned by doping with different impurities. The higher mobility can be utilized for high-frequency applications. The transparent property can be utilized for touch screens and solar cells in which it can replace the expensive indium titanium oxide.

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Basically, the physical and chemical properties of fullerens, CNTs, and graphene are related to each other. Compared to CNTs, which has various types (helicites, single-walled, multiwall), graphene is reported to be a more uniform material. The covalent modification to graphene is possible from both sides, compared to CNTs and fullerene, which exhibit exoand endofaces. The applications of these nanostructures are vast. Practically, the applications for the bulky ball are quiet few. More research is done in the utilization of CNTs. Graphene can be used extensively to replace steel, because it can be recyclable and manufactured in a sustainable way.

3.2 Quantum dots The realization of dependency of band gap against its particle size in 1980 was the birth of quantum dots (QDs). From then, scientists started to study the excited electronic states of smaller nanocrystals (2 10 nm). The results of these studies were compared with their bulk counterpart, and it was learned that these small nanocrystals displayed unique electronic properties compared to those of bulk semiconductors. This variation in property is also due to its high surface-to-volume ratio and quantum confinement [25]. The quantum confinement occurs when the size of the particle is smaller than the excitonic Bohr radii (distance between electron hole pair). For example, Bohr exciton radius of the bulk PbSe is 46 nm with a band gap 0.28 eV. But as if the size of PbSe was reduced to 4.8 nm, the band gap changes to 0.82 eV, which gives a strong confined blue shift of .500 m eV compared to the bulk PbSe. The most observed property is the fluorescence of these materials where they can exhibit different wavelengths depending on the size of the particle. When the size of the particle is smaller than Bohr radius, the energy level will become quantized (Pauli’s exclusion principle). This discretion of energy level can be compared to the molecules rather than bulk materials.

3.3 Metal nanoparticles Similar to QDs, metal nanoparticles also exhibit size-dependent optical properties that are called surface plasmon resonance (SPR). The most studied SPR is of silver and AuNPs. When the metal nanoparticle is photoactivated, the plasmon will couple with excitation light and result in

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an increase in the electromagnetic field in the particle [19,20,26 31]. The interaction between the incident photon and electric field will lead to the scattering and absorption of light. The research on surface plasma resonance is very active in scientific community and industry [32]. AuNPs: AuNPs are reported to be the most stable metal nanoparticles. Size-dependent electronic and optical properties are very well studied. As we discussed earlier, the size-dependent color change of AuNPs explains the surface plasma band (SPB). The SPB is reported to be absent for AuNPs with a diameter less than 2 nm. AuNPs are mostly used in medical diagnostics, catalysis, optics, solar cells, and as inks, sensors, and in surface coating. They are also used as the electron-dense labeling agents in the areas of histochemistry and cytochemistry. AgNPs: The unique property of AgNPs compared to other metal nanoparticles is their antimicrobial property. AgNPs are reported to be very effective against bacteria, viruses, and other eukaryotic microorganisms [33,34]. Even though it has been applied in many commercial products, the toxicity of AgNPs to the useful microorganisms and human body is yet to be evaluated. The mechanism of antibacterial property of Ag nanoparticle is under debate. Some reported that there is an interaction between AgNPs and the bacterial membrane that leads to the damage in cell walls [34]. Another explanation is the creation of reactive oxygen species when the AgNPs inhibit the respiratory enzyme of bacteria [35,36]. The absorption of AgNPs is reported to be in the range from 380 to 450 nm by localized SPR [37,38]. The antibacterial property was also reported to be dependent on the SPRs [39]. Copper nanoparticles (CuNPs): Cu is a naturally abundant material with low-cost synthesis procedure [40 44]. The application of CuNPs in various fields is restricted by their instability due to oxidation. Research has been conducted in order to solve this issue by making more complex structures with CuNPs, like core/shell CuNPs. To prepare highly active, selective, and stable nanocatalysts, CuNPs can be anchored to different supports, such as iron oxides, SiO2, carbon-based materials, and polymers. Cu is represented among 3D transition metals and has some unique physical and mechanical properties. Cu has four oxidation states (Cu0, Cu11, Cu21, Cu31), which lead to one and two electron pathways. Due to these unique properties, CuNPs have found applications in nanocatalysis [45 50].

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3.4 Synthesis of nanomaterials 3.4.1 Top-down approaches 3.4.1.1 Mechanical milling Through mechanical milling, size of bulk material can be reduced to nanoscale. This method can also be utilized to blend different phases. For large-scale production, this method is more suitable. The basic principle behind this process is the transfer of energy to the sample from the balls during the process [51]. Commonly used ball materials are steel and tungsten because of the fact that dense materials are required for the milling purpose [52]. The temperature generated during the process depends on the kinetic energy of the ball and the sample powder characteristics. The final particle shape and structure also depends on the strain-rated during the collision of the balls [53].

3.4.1.2 Mechanochemical processing During the milling, there can be deformation, fracture, and welding of powder if the conditions are not right. There is a development of plastic deformation of the powder due to the development of shear bands that finally decompose into subgrains. When the milling process is continued, the subgrain size decreases to nanoscale [54]. The unique property of mechanochemical processing is the processing speed in yielding to nanoscale. The nanometer powder enhances the reaction kinetics, which induces chemical reactions that basically requires high temperature [55]. 3.4.1.3 Electroexplosion In 1962 Karioris and Fish discovered a method that can be used to generate aerosols of the following metals: Au, Ag, Al, Cu, Fe, W, Mo, Ni, Th, U, Pt, Mg, Pb, Sn, and Ta. The particle size formed was around 30 50 nm. This is basically the electrical explosion of metal wires that fabricated nanoparticles with high activity. High-density electric pulse (104 106 A/mm2) is passed through a metal wire, which results in the increase in heat (20,000°C 30,000°C) and explodes and forms products that are converted to nanoparticles by the passage of gas [55 57]. Through this method, it is possible to fabricate powders with unique properties that are difficult to fabricate through other methods.

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3.4.1.4 Sputtering It was first observed by Grove in 1852. He noted the sputtering of the cathode surface of the discharge tube by energetic ions and the material was deposited inside of the discharge tube. Today this technique is utilized by the bombardment of a sample surface with energetic gaseous ions, which results in the ejection of surface atoms or small clusters. The sputtering is performed in the following different ways: DC-diode, RF-diode, and mag-neutron sputtering. Argon plasma is mostly used for sputtering. Unlike other vapor phase techniques, there is no melting of the samples [58]. 3.4.1.5 Laser ablation Laser ablation (LA) is a complex process. The laser penetrates to the sample surface, depending on the wavelength of the laser and the refractive index of the target material. The high electric field generated due to laser light is enough to remove electrons from the bulk sample. The generated free electron collides with the atoms of the bulk sample, in which transfer of energy occurs. This leads to the heating of the surface, which is followed by vaporization [59]. When the laser flux is high enough, the material will transfer to the plasma state, including atoms, molecules, ions, clusters. The pressure difference between the seed plasma and the atmosphere lead to a rapid expansion and cooling of the plasma. LA takes place in either a vacuum or gaseous environment. LA combined with a tube furnace is called a pulsed LA technique. This technique allows better control over growth temperature, flowing gas type rate, and pressure. 3.4.1.6 Lithography Micro and nanolithography technology have been utilized for decades to fabricate integrated circuits. This technique can create patterns with size in the nanometer range. Lithography is usually combined with deposition and etching to yield high-resolution topography. Lithography can be divided into masked lithography and maskless lithography. In masked lithography, masks or molds will be used to fabricate patterns. The types of masked lithography include photolithography [60 64], soft lithography [65,66], and nanoimprint lithography [67 72]. Maskless lithography yields arbitrary patterns without the use of masks. For example, electron beam lithography [73 75], focused ion beam lithography [76,77], and scanning probe lithography [78,79].

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3.4.1.7 Aerosol-based techniques Aerosol is a mixture of solid/liquid particles in a gaseous environment. The advantage of aerosol methods over others is due to its high-purity product yield with less toxicity to the environment. Aerosol methods are classified as follows: (1) furnace method; (2) flame method; (3) electrospray; (4) chemical vapor deposition (CVD); and (5) physical vapor deposition method. Electrospray method is reported to be the most favorable method to fabricate nanoparticle production, but the process is slow [80]. 3.4.1.8 Electrospinning Electrospinning technique was developed during the last decade (from 2012) for the fabrication of continuous fibers in submicron to nanometer scale range. This method is utilized to fabricate nanofibers of polymers, metals, ceramics, and composites. Nanoparticles are mixed with polymers and electrospun to produce scaffolds. Electrospinning is also used for the assembly of nanoparticles through the alignment with fibers and thus reduce the Gibbs Free energy [81]. The other advantage of electrospinning is that it does not require any functionalization process, it needs only a solvent that can disperse nanoparticles and dissolve the polymer. Electrospinning basically depends on the high electrostatic forces. Factors that influence the electrospinning process are polymer concentration, solution viscosity and flowrate, electric field intensity, the work distance, and air humidity.

3.4.2 Bottom-up approaches 3.4.2.1 Chemical vapor deposition In this method, a sample material is deposited in a vapor medium with the help of chemical reaction. The material is deposited in the form of thin film, powder or a single crystal. The advantages of the CVD is its excellent throwing power, thin films with uniform thickness with low percentage of porosity, and selective deposition on desired pattern. CVD is applied in thin films for dielectric, conductors, passivation layer, oxidation layer, conductive oxides, tribological and corrosion resistant coatings, heat resistant coatings, etc. Other applications include fabrication of solar cells and high-temperature fiber composites [82]. 3.4.2.2 Plasma arcing Plasma is basically an ionized gas. Plasma is created by conduction of electricity through gas with the help of a potential difference that is created

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Figure 3.14 Plasma torch.

between two electrodes. A plasma torch (Fig. 3.14) is used to generate a contracted plasma arc, using an inert gas. For example, an arc-discharge method is typically used to fabricate MWNTs and SWNTs. For this, it generally involves the use of a graphite electrode as the anode and cathode. The electrodes get vaporized by the passage of DC current (B100 A). When the arc discharge is completed for a period of time, carbon rods are created at the cathode side. This method is usually used to fabricate MWCNTs, but with the help of metal catalyst such as Fe, Ni, Mo, SWCNT can be fabricated [83]. 3.4.2.3 Wet chemical methods This method is used to synthesize uniform nanoparticles with desired size. This method achieved a great success because of the control over size, shape, and crystallinity. Wet chemical synthesis is mostly used to synthesize inorganic nanomaterials due to low cost and easy application. This method also has many disadvantages when it comes to the industrial scale due to long mixing time and uncontrollable nucleation and growth [84].

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3.4.2.4 Solvothermal/Hydrothermal synthesis The reaction occurring in solvents contained in sealed vessels by heating to their critical point under autogenous pressure is called hydrothermal/ solvothermal process. It is basically a crystallization process that consists of crystal nucleation and its growth. Particle morphology can be controlled by tuning the temperature, pH, and reactant concentrations [85]. 3.4.2.5 Reverse micelle method Surfactants are made up of hydrophilic head and a hydrophobic chain. These amphiphilic molecules can self-assemble into variable structures under certain conditions. For reverse micelles the structure is characterized through polar cores formed with hydrophilic heads. These solutions can be called microemulsions, which are thermodynamically stable and optically transparent. Water oil microemulsions consist of droplets in 5 100 nm size range. The reverse micelles can be tuned to nanometer scale by changing the parameters, such as molar ratio of water, to surfactant. With the adsorption of surfactants with the inorganic materials, it is possible to use these micelles for the synthesis of nanostructures with controlled morphology [86]. 3.4.2.6 Sol gel method This method is mainly used to synthesize metal oxide NPs and mixed oxide composites with desired nanostructures. Typical sol gel method includes the following steps: hydrolysis, condensation, and drying process. Initially, the metal precursor undergoes hydrolysis and yields metal hydroxide, followed by condensation to form gels. The final gel is dried and converted to xerogel/aerogel [87].

3.5 Conclusion The field of nanotechnology has found applications in all the fields, and this chapter has reviewed the field at an introductory level. It has been seen that nanomaterials are different from their bulk moieties and cannot be studied as same as bulk or small molecules due to their unique properties in nanoscale. The properties of nanomaterials depend on composition, chemistry, particle dimension, and interactions with other materials. Their intrinsically small dimension and higher surface area is critically studied. There are various methods to fabricate nanoparticles, depending on type of material and composition. Even though there are lot of advantages,

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unique characteristic of nanoparticle is a big concern in determining their toxicological and ecotoxicological properties and will be discussed in further chapters.

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