CHAPTER 2

Chapter 2 Experimental Methodology

2.1 Introduction 2.2 Synthesis of nanomaterials. 2.2.1 DC arc plasma reactor setup for material processing 2.2.2 Microwave processing of materials 2.2.2.1 Microwave Induced Plasma processing System setup 2.2.2.2 Multimode Microwave setup 2.3 Analytical techniques for characterization nanophase powders 2.3.1 X- ray diffraction study 2.3.2 X-Ray photoelectron spectroscopy 2.3.3 Surface area measurement (BET method) 2.3.4 Electron Microscopy (SEM and TEM) 2.3.4.1 Scanning electron microscopy (SEM) 2.3.4.2 Transmission Electron Microscopy (TEM) 2.3.5 DTA&TGA 2.3.6 Vibrating sample magnetometer (VSM) 2.3.7 Ultraviolet visible spectroscopy (UV/vis) 2.3.8 Raman spectroscopy 2.3.9 Photoluminescence spectroscopy

2.4 Conclusion 2.5 References.

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Chapter 2 Experimental Methodology

2.1 Introduction This chapter provides the detailed description of the experimental setups for (i) DC arc plasma (reactor) & (ii) microwave processing systems used for the synthesis of nanophase materials and (iii) other analytical-measuring methods such as of X-ray diffraction, X-ray photoelectron spectroscopy, surface area using BET method, scanning and transmission electron microscopy (SEM / TEM), UV-Visible absorption spectroscopy, thermal analysis (DTA / TGA), vibrating sample magnetometer (VSM), Raman and Photoluminescence spectroscopy.

2.2 Synthesis of nanomaterials 2.2.1 DC arc plasma reactor setup for material processing The experimental DC arc plasma setup used for the synthesis of various nano­ materials is as shown in figure 2.1 (a). The plasma reactor consisted of a multi port stainless steel chamber (12 inch in diameter) with a multiple gas inlet facility providing various mixtures of gases, which can be incorporated as plasma forming gas. For inserting the arcing electrodes two opposite ports were used with ceramic bushes for insulation purpose. The electrodes were fabricated in such a way that they could be easily moved linearly along the arcing direction. The aluminum flange was used for covering this reactor from the top. The DC power supply (Keje Electric Co.) used in the synthesis process has specifications of open circuit voltage of 70 Volt and short circuit current of 200 Ampere. This got fitted on a separate trolley. After carrying out series of experiments another modified setup was developed. The modified setup is shown in the figure 2.1 (b).

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Figure 2.1 Open arc plasma reactor (a) original set up and (b) modified setup.

The modified setup consisted of water cooled dome for collecting the nanopowders. The arc reactor was a stainless steel chamber (with a diameter = 28 cm and height = 53 cm).

It has number of ports of different sizes for providing various facilities as follows: 1. Separate ports for anode and cathode. The Cathode is mounted at an angle of 40° with respect to the vertical axis.

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2. Two ports with 25 mm diameter each with feed-through, which are kept at a distance of 22.5 cm from the arc zone for the in-situ measurements of thermal and electrical parameters 3. A view port of 25 mm diameter for observation of the arc plasma process 4. Port for different gas inputs and a port for evacuation 5. A port with special quartz window for coupling the emission spectrometer for emission characteristics studies 6. The present chamber is demountable into two sections. Bottom and top sections hold the anode and cathode assemblies respectively 7. A separate water cooling arrangements for both the sections. Further, the top chamber has a provision to cool the collector dome using liquid nitrogen. It helps in getting better powder yield during the synthesis. 8. The cathode assembly is cooled using argon gas shroud The mass flow meters having measurement capacity from 2.5 to 40 lit / min are connected for monitoring and maintaining the flow of different gases like air, nitrogen, oxygen and argon. A rotary vacuum pump (of 100 liters capacity) can be connected for evacuation of the chamber which can lower the reactor chamber pressure up to 5 x 10"3 torr. The anode holder was made up of copper cup (diameter = 6 cm & thickness = 2.5 cm) which was kept in contact with a stainless steel co-axial tube. Inlet and outlet tubes for water flow into it were fitted. The metal to be evaporated was placed in the copper cup and tightened with screws so that anode cannot get lifted up with the cathode when it does get fused occasionally. The anode is metal slab (or rod / block/ foil) of interest preferably having dimensions: diameter = 5 cm and thickness = 2 cm. For a cathode there are hollow co-axial tubes (2 nos.) with gas flow arrangement. The end of co-axial tube has a special provision for mounting the cathode element, which is generally a tungsten rod (diameter = 2 mm & length = 7 cm). A graphite (a good electron emitter) cathode can also be used with rod (diameter = 6 mm & length = 5 cm). Figure 2.2 (a) and (b) show the schematic diagram and the photograph for the experimental setup of the DC arc plasma system respectively.

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i'tium ajaigc (a)

IT - -^

Figure 2.2 (a) Schematic diagram and (b) photograph of the modified DC arc reactor.

Both the anode and cathode are movable over a distance of 7.5 cm. The cathode is oriented at an angle of 60° with respect to the anode. Thermocouples are connected inside the chamber which enables the measurement of the temperature inside the reactor. The detail

40 CHAPTER 2 description of the method of synthesis of nanophase oxide powders of aluminum, iron and zinc using different plasma parameters, is given in the Chapter No.3 2.2.2 Microwave processing of materials 2.2.2.1 Microwave Induced Plasma Processing [MIPS] System setup The rectangular waveguide (RWG), designated as WR-430 [11], in the S-Band [12] was fabricated indigenously using the design details and specifications as found elsewhere [13]. The phosphor-bronze (M.P. = 1010°C and having composition 90 % Cu + 10 % Sn) material was used for the fabrication of RWG. The RWG was fabricated using the standard casting techniques in a single piece. They were milled and polished to a fine accuracy. The RWG was fabricated in following three sections as shown figure 2.3 (a).

(i) TEJO3 applicator with three screw tuner facility (ii) Input guide (magnetron coupling) and (iii) An adjustable short circuit

(a) N F P j-^Hr^-a

QS-

IK

5? tfl D2-45 GHz 750 W

MICROWAVE GENERATOR {Magnetron)

Figure 2.3 (a) MIPS setup

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(b)

Figure 2.3 (b) photograph for complete MIPS processing setup

An isolator was added to protect the magnetron (microwave source, frequency 2.45 GHz, power = 750 W) from excessive reflected power. The entire set up has been identified as Microwave Induced Plasma System [MIPS] is as shown in figure 2.3 (a). An adjustable choke coupling is placed Ag/4 from the end with incorporation of the quartz sintering tube. The photograph for complete setup of MIPS processing is shown in figure 2.3 (b).

The Quartz resonating tube The double walled quartz tube was used for the processing purpose. The outer wall of tube is provided for flowing helium gas which was used as a coolant gas. A special aluminum head attachment was designed incorporating a novel quartz hanger. This is used for easy sample translation and rotation. This was suspended within the TE103 applicator. The complete assembly was designed to work up to a pressure of 1 militorr. The quartz tube was placed at a distance of lg /4 (wavelength of guide = Xg) from the end, this was to ensure the maximum E-field penetration. The sample diameter was chosen to be < (kg 12), so that the E- field does not vary too much across the sample length. The technical details evaluated from the literature [11] are given in table 2.1 for understanding of the wave guide.

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Table 2.1 Technical specifications of RWG S.No. Parameter Specification 1. Guide wavelength Xg -15.3 cm

2. Free space wavelength h, = 12.24 cm at f0 = 2.45 Hz 3. Width a =10.922 cm 4. Height b = 5.461 cm 5. Wall thickness = 5 mm 6. Comer radius <= 1mm

7. Three screw tuner placed at distance 3Xg/8

8. Resonator length (LR) 3 (kg 1 2) = 22.95 cm 9. Xc (TEio) Cutoff = 2a 10. Distance of the quartz sintering tube from the end = V4 11. Skin depth (for phosphor bronze) c = 3.29xl0"4cm 12. Attenuation conductor loss Oc = 0.1846 Np/m 13. Skin Resistance (for phosphor bronze) R = 0.032 £2

The magnetron was coupled to the RWG in such a way that there is complete grounding between the body of magnetron and the RWG for safety precautions so that there are no leakages of microwaves. The tuning is properly adjusted by characterizing the RWG with a standard Vector network analyzer coupled to the S-parameter test set. The temperature in the reactor tube was measured using the optical pyrometer (CHINO, Pyrostar model IR-U, China) by with and without detuning the plasma. In some cases, a single wall quartz rube was used for processing of different materials. The detail description for the synthesis of carbon n:. v\ubes and processing of materials by using Microwave Induced Plasma System [MIPS] is given in the Chapter No.4.

2.2.2.2 Multimode Microwave setup In some material processing experiments, convection domestic microwave oven [KENSTAR, Model No.: OM-29ECF) with grill was used. It operates at a frequency of 2.45 GHz with MW energy output of 1 kW. It is with adjustable power output up to 1 kW. It's a

43 CHAPTER 2 multimode cavity type reactor. In microwave heating, the heat is always generated within the material itself due to interaction of microwaves with dipoles.

Figure 2.4 An experimental setup for material processing using multimode microwave oven.

So, due to volumetric heating flow of heat and thermal gradients are quite different from those observed in conventional heating. Here microwave energy is introduced into the reaction chamber remotely with no direct contact between energy source and the reactants. Figure 2.4 shows the experimental setup for processing of materials using domestic microwave oven. The detail description for the microwave processing using multimode microwave oven is given in the chapter no. 4.

2.3 Analytical techniques for characterization nanophase powders 2.3.1 X- ray diffraction study The X-ray diffraction technique was used for phase identification and for determining the average particle size. The information regarding crystal structure and identification of the material is provided sufficiently using the versatile technique of X-ray diffraction. Bragg's diffraction condition is used to evaluate the inter-planar spacing. The condition is: 2d sinG = n X

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Where, I = wavelength (-1-2 A0) of X-ray (For Copper target: Cu Ka: k = 1.542 A0) 6 = Angle of diffraction d = Inter planar spacing in A0, n = Order of diffraction XRD technique is also used to measure the crystallite size in a powder sample based on certain limitations. The lines in the powder diffraction pattern are of finite width, but for small particle sizes enhanced broadening takes place. As the size of particle decreases there is corresponding increase in the broadening. This limit is eventually reached with particle diameters in the range of about 20-100 A0, in which case the lines become so broad that they can effectively disappear into the background radiation. The Debye - Scherrer formula [6(a)] is used to determine the particle size. d = k XI (B cos G) Where, d = particle diameter in A0 (or crystallite size in case of nanophase materials), k = Shape factor (~ 0.9) P = Full width at half maxima (FWHM) of the diffracted line corresponding to the diffracted angle 9. Above relations contains two sources of errors [7]: (i) instrumental broadening and (ii)

correction for the k aia2 splitting.

Experimental details and sample preparation for XRD X-ray diffractometer (Phillips (Model PW1840, Bruker D8 Advance Physics) was used for recording the XRD spectra using the conventional Bragg-Brentano geometry. It was used to analyze all the as-synthesized samples of nanomaterials for investigating their phases and particle sizes. The as synthesized powder was lightly ground in an agate mortar assembly. This was then placed on a quartz holder and sprayed evenly. The holder was then placed in the chamber for diffraction. The diffracted X-ray beam was collected using a silicon diode array detector. The powder method is used to determine the values of lattice parameters precisely.

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2.3.2 X-Ray photoelectron spectroscopy (XPS) This is an important quantitative spectroscopic technique that measures the empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy (KE) and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. It requires ultra-high vacuum (UHV) conditions. [14, 15] . In the present work, X-ray photoelectron spectroscopy was used mainly for investigation of stoichiometry and purity of as-synthesized nanophase materials. Among the different spectroscopic technique this is the most versatile generally applicable surface spectroscopic technique. Photoelectron spectroscopy is an important technique for investigations and characterization of the electronic structure of surfaces, solids, liquids, molecules and atoms. This gives energy distribution of occupied electron states. Here absorption of a quantum of energy hv and ejection of photoelectron is related to the binding energy of the electron in the target atom. In this process incident photon transfers its entire energy to the bound electron and element identification is provided by measurement of the energy of the electrons that escape from the sample without energy loss. This is neatly observed from the figure 2.5. Assuming that the Fermi level of the sample and spectrometer align, the binding energy of any core level as shown in figure 2.5 is the energy separation between the core level and the Fermi level of the sample and the photoemission of electrons is governed by the Einstein's relation:

hv = E BE + Ek th Where, EBE= Binding /ionization energy of the k species of electron in the material Ejc = Kinetic energy of the ejected electron.

On introducing correction terms for the spectrometer work function 0sp and sample charging

0sa, the above relation becomes

hv = EBE + Ek+0sp+0sa

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SOFT X-ray HEMISPHERICAL Mg-ka X-ray SOURCE ENERGY ANALYSER ENERGY = 1253.6 eV

INCIDENT PHOTON ELECTRON ELECTRON 3^9 OPTICS DETECTOR SAMPLE

ULTRA HCK3-HVACU1K M | CHAMBER, 10 "9 toir

Figure 2.5 Schematic of an Experimental setup used in XPS

For conductive samples in electrical contact with an electron spectrometer the Fermi levels of the spectrometer and the samples will be coincident. For semiconductors and insulators the Fermi level lies somewhere between the filled valence band and the empty conduction band, in which case one has to choose the reference level more carefully.

Experimental details for XPS X-ray photoelectron spectra were recorded using XPS spectrometer (ESCA-3 MKII, VG Scientific Co. UK) at a base pressure > 10~9 torr, X-ray excitation energy = 1253.6 eV (Mg ka photons) and pass energy = 50 eV, giving an overall resolution of about -v 1.1 eV. This work was carried out at Centre for material characterization (CMC), National Chemistry Laboratory (NCL), Pune. The angle between the detector and the X-ray flux direction was constant and equal to 90°, while the measurements were made at an electron take off angle of 70°. The calibration of the spectrometer and charging correction were carried out by determining the binding energy of C Is level (at 285 eV). All the spectra were corrected for the charging effects. A full scan of 1000 eV was carried out to check for the impurities if any, The short scans for Al 2p and O Is core levels are recorded for detail analysis of stoichiometry and electronic structure of surfaces.

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2.3.3 Surface area measurement Brunauer, Emmett and Teller (BET) method It is well known fact that the fine particles exhibit a larger surface area as compared to the bulk solid due to the increased surfaces exposed. Whenever a matter is divided into smaller particles new surfaces must be produced with a corresponding increase in surface area. In addition to particle size, shape of the particle contributes to the powder surface area. A sphere shape exhibits the minimum area to volume ratio while a chain of atoms bonded only along the chain axis, will give the maximum area to volume ratio. Although the particulates can assume all geometric shapes, most particle size measurements area based on the "equivalent spherical diameter". This is the diameter of a sphere, which would behave in the same manner as the test particle being measured in the same instruments. When a surface of a solid (i.e. adsorbent is exposed to a gas i.e. adsorbate), gas molecules are get adsorbed for a finite amount of time. Depending on the interacting forces there are chemisorptions (chemical bonding) and / or physisorption (Vander Waals forces). Physisorption occurs at low temperature and becomes significant at critical temperature of the adsorbate. The critical amount of volume V of adsorbed gas depends on (i) the pressure P at which adsorption takes place and (ii) the nature of adsorbate & adsorbent. This pressure is normalized by the saturated pressure Po at a given temperature. Adsorption theories deduce the monolayer

capacity Vm defined as the quantity of the adsorbate needed to cover the surface with a molecular monolayer. The total surface of the adsorbent is then derived by dividing Vm by the area occupied by one molecule. The surface area of the powder was measured using the gas adsorption technique, analyzed using the BET (Brunauer, Emmett and Teller) adsorption isotherm (type II) by single point method [8]. The analyzers (Micromeritics, Model Flow sorb II, USA and SMART SORB 91, Smart Instruments Co. INDIA) at NCL and Departments of Physics, University of Pune were used for the surface area measurements [9]. The setup for SMART SORB BET surface area analyzer is as shown in figure 2.6.

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Figure 2.6 Experimental Setup for SMART SORB BET surface area analyzer By enveloping each particle of a powder sample with an adsorbed gas and finding out the amount of gas adsorbed one can compute the surface area of a given sample powder accurately.

Experimental details and description of the method Initially, the weight of sample was taken. To remove moisture, the sample was kept in 'U' tube at 150°C for 15 min. Meanwhile the helium-nitrogen gas mixture (70 % He + 30 % N2) was passed through the BET set up at a flow rate of 100 seem for 15-20 min. This helps in flushing out the air present in the tubes of setup. The presence of air may affect the heater element by oxidizing it when BET setup is switched on. Initially, an offset adjustment has to be carried out by 'Resistor Bridge' balancing by using 'balance pot'. The 'U' tube containing the sample was attached to the setup and the gas mixture was then allowed to pass through it.

A LN2 flask was kept around the U tube such that the sample gets immersed in the LN2. During this process the counter shows the varying 'adsorption count'. After the counter stabilizes it is reset and LN2 flask is then removed and replaced with a beaker containing normal water. The nitrogen adsorbed on the sample due to increase in the temperature is again exhibited by the counter reading. This is the 'desorption count' value. A syringe

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containing a measured quantity of pure N2 gas was then introduced into the apparatus and the count value was noted as the 'calibration count'. All these noted counts and temperature values were used for computing the specific surface area value using a standard formula as given below. The same procedure was used for all nanophase powder samples.

Surface area = 4.38 Vm,

Where, Vm = volume of molecular monolayer.

2.3.4 Electron Microscopy (SEM and TEM) The electron microscopy is the well known technique capable of providing micro- structural information over a wide range of magnification and offers an ideal solution to the difficulties presented by the resolution limitations of the conventional optical (light) microscope. It is capable of providing information on the atomic scale when favorable parametric conditions are maintained. Depending on the instrument: TEM or SEM, the resolution can fall somewhere between < 1 nm and 20 nm respectively. In general, SEM images are much easier to interpret than TEM images.

2.3.4.1 Scanning electron microscopy (SEM) SEM is used for the morphological studies of the samples. This characterization utilizes the medium energy electrons in the typical range 5-50 kV (the low energy counter part of TEM) in a fine beam scanning the specimen [10]. Both, X-rays and secondary electrons are emitted by the sample; the former are used for chemical analysis (energy dispersive X-ray analysis: EDS) and the later are used to build up an image of the sample surface which is displayed on the screen (secondary electrons imaging). An EDS helping in understanding the constituents of the materials treated as well as synthesized in different plasmas. EDS gives precise concentration of the elements present in terms of atomic percentage. The interaction of this electron beam with the specimen provides lot of information related to the surface as shown in figure 2.7. Incident electron beam and interactions in SEM gives a large area image as compared to TEM (where scanning is done in small part areas).

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Incident SEM / EPMA Beam \ SEM X-rays Primary baekscattered elections (Atonic (TJliougli-tliiekiiesis! number and composition topograpnic mibmiation) uirbimatiori) Secondary electrons

Tliin transmitting specimen

jQielasfically Transmitted Elastically •scattered electrons electrons scattered electrons (Energy loss) (Tluougli-tlucloiess (Miaostnicnual information TEM composition from images and information ciystallogiapluc inrbnnation using diffraction patterns)

Figure 2.7 Interaction of electron beam with the specimen

Experimental details and sample preparation for SEM JEOL Analytical Scanning Electron microscope (Model JSM 6360A) was used for SEM studies at Department of Physics, University of Pune, Pune.. The sample was properly dispersed using the ultrasonicator for 15 min in a beaker containing little amount of double distilled water. It was allowed to dry and then used for SEM study. The nonconductive samples were coated with gold / platinum before subjected them for SEM and EDS studies. The powder was coated with gold / platinum using a JEOL JFC-1600 auto fine coater. The thickess of coated film was about 30 nm.

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2.3.4.2 Transmission Electron Microscopy (TEM) The basic principle and technique used for TEM imaging is as described below.

Electron beam wavelength Diffrocting plones A in crysiol

Specimen

Crystal plane Crystal plane (h,k,l,) (M2l2>

Transmitted beam

Objective [ens -- £

wald sphere bjective Aperture Back ioca\ plane)

Image plane

^Diffraction (h2k2tz pattern

ELECTRON SOURCE (GUN

CONDENSER LENS J SAMPLE

OBJECTIVE LENS

DIFFRACTION PLANE

INTERMEDIATE IMAGE

PROJECTOR LENS

VIEWING PLANE (Fluorescent screen or photographic film )

Figure 2.8 Basic system components of a TEM The basic system components of a TEM are as shown in the figure 2.8. In the transmission mode for examining the sample, it must be thinner than about 2000 A°. This is

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because the electrons interact with matter strongly and are further get completely absorbed by the thick specimen. The electrons emitted from tungsten filament (electron gun) are accelerated through a high voltage (typically 100 kV). The wavelength is related to the accelerating voltage V by X = h / (V (2meV) Where, m and e are mass and charge on electron.The electron wavelengths are much smaller than X-ray wavelengths used in diffraction experiments, e.g. ~ 0.04 A° at 90 kV accelerating voltage, consequently the Bragg angles for diffraction are small and the diffracted beams are concentrated into a narrow cone centered on the un-diffracted beam. These electrons are focused using electromagnetic lenses. The condenser lenses are used to control the size and angular spread of the electron beam that is incident on the sample. These lenses produce a fine parallel beam of electrons at the specimen position and a range of interaction takes place with incident electron as shown in the figure 2.8. Transmitted electrons then pass through the sequences of lenses- objective, intermediate and projector — and form a magnified image of the sample on a fluorescent viewing screen. The diffracted and un-scattered electrons form the basis for the conventional mode. This is the bright field imaging mode. In the dark field mode only the diffracted beams from the particle of interest are allowed to recombine to form the image, using this technique one can view crystal defects such as dislocations, stacking faults, anti-phase and twin boundaries. The resolving power of the microscope is dependant on wavelength and quality of the objective lens (a function of the spherical aberration in the lens) which produces the first image. From the figure 2.8 one finds that if the photographic recording plane is at a distance "L" then the radius "r" of the diffraction rings is related to the interplanar spacing "d" by the relation.

Camera constant: XL = rd hki. Where, the small angle approximation is used (Braggs Law) and has the usual significance. Thus the camera constant can be known and the structure of material can be determined from the ring pattern. The topography of the particles can be studied using TEM.

The experimental details and sample preparation for TEM JEOL1200 EX, Japan (at CMC, NCL, Pune) Analytical Transmission Electron microscope was used for morphological studies of powders. For this the small quantity of powder was well dispersed in isopropyl alcohol and was kept for ultrasonicaton for 15 min.

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to make the dispersion more uniform and also to separate the individual particles. A microlitre (uL) drop of this solution was then placed on a carbon coated copper grid of 400 meshes (Germany). The samples thus prepared were kept for drying. This grid was then placed inside the column for imaging. The accelerating voltage of the electron was fixed at 120 kV for both the modes. 2.3.5 Differential thermal analysis (DTA) & Thermo gravimetric analysis (TGA) In order to complete the thermal analysis several test and measuring instruments are designed to characterize materials based on their behavior under various heat conditions. Differential thermal analysis has long been applied in the study of minerals such as clay minerals, carbonates, sulphates and zeolites, sulfides and arsenides, graphite. Thermal analysis instruments predominantly use thermocouples, platinum resistance thermometers, and thermistors. When the sample undergoes a transformation, it will either absorb (endothermic) or release (exothermic) heat. In differential thermal analysis, the T°C difference between a reactive sample and a non-reactive reference is determined as a function of time, providing useful information about the temperatures, thermodynamics and kinetics of reactions. Thermo gravimetric analysis determines the weight gain or loss of a condensed phase due to gas release or absorption as a function of temperature. It also measures changes in weight of a sample with increasing temperature. Moisture content and presence of volatile species can be determined with this technique. Computer controlled graphics can calculate weight percent losses. Differential thermal analysis (DTA) set up measures the difference in temperature between a sample and a thermally inert reference as the temperature is raised. The plot of this differential provides information on exothermic and endothermic reactions taking place in the sample. Temperatures for phase transitions, melting points, crystallization can all be determined using the computer controlled graphics package. Optimum sample size is 50-100 mg and ideally the sample should be ground to 100 mesh. In combined DTA-TGA (simultaneous thermal analysis: STA) system both thermal and mass change effects are measured concurrently on the same sample (DTA, TG, DTG).

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In our experimental studies the DTA / TGA were used for the determining the phase transformations (Seiko Model TG/DTA32). The thermal analyses were performed on a Rheometric Scienific Instrument (MODEL STA 1500+, USA).

2.3.6 Vibrating Sample Magnetometer (VSM) The vibrating sample magnetometer has become a widely used instrument for determining magnetic properties of a large variety of materials: diamagnetic, paramagnetic, ferromagnetic and anti-ferromagnetic. The magnetic properties of solids are very important, and attempts to understand them have led to a deep insight into the fundamental structure of many solids, both metallic and nonmetallic. Therefore accurate measurements of magnetic properties of solids have a great significance. The Vibrating Sample Magnetometer is the instmment used to measure the magnetic moment which is the most fundamental quantity in magnetism of solid samples. When a sample material is placed in uniform magnetic field, a dipole moment proportional to the product of the sample susceptibility times the applied field is induced in the sample. If the sample is made to undergo sinusoidal motion as well, an electrical signal will be induced in suitably located stationary pick-coils. This signal, which is at the vibration frequency is proportional to the magnetic moment, vibration amplitude and vibration frequency. The instrument has a special feature to overcome the problem of dependence of induced signal on vibration frequency and amplitude. The signal is made to depend on magnetic moment of the sample only. The instrument displays the magnetic moment of the sample in e.m.u. units. The schematic diagram of the typical VSM setup is as shown in figure 2.9. The voltage V measured across the sensing coils in a VSM can be expressed as the product of four contributing sources: V = M.A.F.S Where, M = magnetic moment of the sample A = amplitude of vibration F = frequency of vibration S = sensitivity function of the sensing coils. EG & G PAR 4500 Vibrating Sample Magnetometer ( available in CMC NCL Pune) was used for recording the magnetisation (M) and coercivity (Ec) data at room temperature.

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Vibration mui lii

Figure 2.9 Schematic diagram of the typical VSM setup

2.3.7 Ultra violet visible spectroscopy

Ultraviolet-visible spectroscopy spectrophotometry (UV/VIS) involves the spectroscopy of photons and spectrophotometry. It uses light in the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this region of energy space molecules undergo electronic transitions. The instrument used in ultraviolet-visible spectroscopy is called a UV/vis spectrophotometer. It measures the intensity of light passing through a sample (I), and compares it to the intensity of light before it passes through the sample (I0). The (quantity) ratio (I / Io) is called the transmittance, and is usually expressed as a percentage (%T). UV-vis spectroscopy is usually applied to molecules and inorganic ions or complexes in solution. The concentration of an analyte in solution can be determined by measuring the absorbance at some wavelength and applying the Beer-Lambert Law The absorbance 'A', is based on the transmittance:

A = -log(%T)

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The method is most often used in a quantitative way to determine concentrations of an absorbing species in solution, using the Beer-Lambert law:

A « - log1D(J/J0) = e • c • L

where, A is the measured absorbance, To is the intensity of the incident light at a given wavelength, / is the transmitted intensity, L the pathlength through the sample, and c the concentration of the absorbing species. For each species and wavelength, e is a constant known as the molar absorptivity or extinction coefficient. This constant is a fundamental molecular property in a given solvent, at a particular temperature and pressure, and has units of 11M* cm or often AUIM* cm.

The basic parts (figure 2.10) of a spectrophotometer are a light source (often an incandescent bulb for the visible wavelengths, or a deuterium arc lamp in the ultraviolet), a holder for the sample, a diffraction grating or monochromator to separate the different wavelengths of light, and a detector. The detector is typically a photodiode or a CCD. Photodiodes are used with monochromator, which filter the light so that only light of a single wavelength reaches the detector. Diffraction gratings are used with Charge Coupled Devices, which collects light of different wavelengths on different pixels.

mphochrpmator \U7 detector lens sample lamp amplifier readout cuvette

Figure 2.10 Schematic diagram of a single-beam UV/vis spectrophotometer

A spectrophotometer can be either single beam or double beam. In a simple single- beam UV/Vis spectrophotometer, the white light from the source (bulb) passes through the cuvette containing the sample, is reflected onto a diffraction grating, and focused onto a CCD detector. In a single beam instrument (such as the Spectronic 20), all of the light passes through the sample cell. The value of !<, must be measured by removing the sample.

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L ^ reference detector mono- K'- lamp chromator ratio sample detector

Figure 2.11 Schematic of a dual-beam UV-Visible spectrophotometer

In a double-beam instrument (figure 2.11) the light is split into two beams before it reaches the sample. One beam is used as the reference; the other beam passes through the sample. Some double-beam instruments have two detectors (photodiodes), and the sample and reference beam are measured at the same time. In other instruments, the two beams pass through a beam chopper, which blocks one beam at a time. The detector alternates between measuring the sample beam and the reference beam.

Samples for UV/Vis spectrophotometric study are most often liquids, although the absorbance of gases and even of solids can also be measured. Samples are typically placed in a transparent cell, known as a cuvette. Cuvette is made of high quality quartz typically rectangular in shape, commonly with an internal width of 1 cm. (This width becomes the path length, L, in the Beer-Lambert law.) Thus UV/VIS spectroscopy can be used to determine the concentration of a solution. It is necessary to know how quickly the absorbance changes with concentration. This can be taken from references (tables of molar extinction coefficients), or more accurately, determined from a calibration curve.

Absorption spectra

An absorption spectrum is the absorption of light as a function of wavelength. The spectrum of an atom or molecule depends on its energy level structure, and absorption spectra are useful for identification study of compounds. Measuring the concentration of an absorbing species in a sample is accomplished by applying the Beer-Lambert Law. When atoms or molecules absorb light, the incoming energy excites a quantized structure to a

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higher energy level. Electrons are promoted to higher orbital by ultraviolet or visible light, vibrations are excited by infrared light, and rotations are excited by microwaves.

2.3.8 Raman spectroscopy This is a spectroscopic technique used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system. The fundamental Raman Effect occurs when light impinges upon a molecule and interacts with the electron cloud of the bonds of that molecule. The incident photon excites one of the electrons into a virtual state. [16] It relies on inelastic scattering, or Raman scattering of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system. Infrared spectroscopy yields similar, but complementary information. For the spontaneous Raman Effect, the molecule will be excited from the ground state to a virtual energy state, and relax into a vibrational excited state, which generates Stokes Raman scattering. If the molecule was already in an elevated vibrational energy state, the Raman scattering is then called anti-Stokes Raman scattering.

Method

Typically, a sample is illuminated with a laser beam. Light from the illuminated spot is collected with a lens and sent through a monochromator. Wavelengths close to the laser line (due to elastic Rayleigh scattering) are filtered out and those in a certain spectral window away from the laser line are dispersed onto a detector. Raman spectrometers typically use holographic diffraction gratings and multiple dispersion stages to achieve a high degree of laser rejection. In the past, PMTs were the detectors of choice for dispersive Raman setups, which resulted in long acquisition times. However, the recent uses of CCD detectors have made dispersive Raman spectral acquisition much more rapid. A molecular polarizability change, or amount of deformation of the electron cloud, with respect to the vibrational coordinate is required for the molecule to exhibit the Raman effect. The amount of the polarizability change will determine the intensity, whereas the Raman shift is equal to the

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vibrational level that is involved. Spontaneous Raman scattering is typically very weak, and as a result the main difficulty of Raman spectroscopy is separating the weak in-elastically scattered light from the intense Rayleigh scattered laser light.

This is non-destructive technique and measurements can be made over a wide range of temperatures or pressures. It can provide unique information about vibrational and electronic properties of the material. Even though it is not a direct method, it can also be used to determine the structure of the material and allows the identification of materials through the characteristic vibrations of certain structures. Because the Raman intensity of a vibration or phonon in a crystal depends on the relative directions of the crystal axis and the electric wave polarization of the incident and scattered light, it may also be used to determine the orientation of nanotubes in polymer matrices or within nanotube bundles. The existence of nanotubes can be proved using Raman spectra. Raman spectroscopy has been used to determine the diameter of nanotubes, the diameter distribution of nanotube bundles and the structural properties of nanotubes. The unique one-dimensional molecular nature of SWNTs makes the resonance. The identification of the (n, m) indices, or alternatively of the diameter and the chirality of an individual SWNT is possible using this method. By knowing the (n,m) vector, the dependence of all the features of the spectra on the diameter, chiral angle, laser excitation energy and other parameters can be worked out in detail [17]. Therefore, the spectrum of SWNT bundles can be interpreted and the effect of nanotube-nanotube interactions can be deduced The fact that the RBM vibrational frequencies are sensitive to the nanotube diameter and that the intensity of the spectra is proportional to the number of nanotubes. 2.3.9 Photoluminescence spectroscopy Photoluminescence is a process in which a chemical compound absorbs photons (electromagnetic radiation), thus jumping to a higher electronic energy state, and then radiates photons back out, returning to a lower energy state. Technically, it is "luminescence arising from photo excitation [18]. The period between absorption and emission is in the order of 10 nanoseconds. Technique: This is a contact less, sensitive and nondestructive method of probing the electronic structure of materials. Light is directed onto a sample, where it is absorbed and

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imparts excess energy into the material in a process called photo-excitation. One way this excess energy can be dissipated by the sample is through the emission of light, or luminescence. In the case of photo-excitation, this luminescence is called photoluminescence. The intensity and spectral content of this photohjminescence is a direct measure of various important material properties. Photo-excitation causes electrons within the material to move into permissible excited states. When these electrons return to their equilibrium states, the excess energy is released and may include the emission of light (a radiative process) or may not (a non-radiative process). The energy of the emitted light (photoluminescence) relates to the difference in energy levels between the two electron states involved in the transition between the excited state and the equilibrium state. The quantity of the emitted light is related to the relative contribution of the radiative process. One measures the light emission vs. wavelength under fixed optical excitation and the emission peaks are usually interpreted in terms of band gap, impurity contributions, heterostructure, interface morphology, excitons, stresses and defects in the material. The PL characterizations were carried out using a PERKTN ELMER LS 55 Luminescence spectrometer. It consisted of a double spectrometer. Xenon flash tube was used as an excitation source. The path of radiation is shown in the set up figure 2.12.

Figure 2.12 Schematic showing the Path traveled by radiation in PL set up

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The accuracy of measurement is lnm. The energy from the source is focused by the ellipsoidal mirror M (E) 5 and reflected by the torroidal mirror on to the entrance slit of excitation monochromator. The monochromator consisted of a grating (1440 lines/mm), spherical mirror, entrance and exit slit. The majority of excitation beam is transmitted to the sample area via focusing torroidal mirror M (T) 1 and a small portion is reflected by the beam splitter onto the reference PMT. Energy emitted by the sample is focused by the torroidal mirror M(T)1 on to the entrance slit of the emission monochromator ( speed range 200-900nm & zero order). Applications of Photoluminescence study are (i) Band Gap Determination and (ii) Impurity Levels and Defect Detection in materials.

2.4 Conclusion

Two most important techniques, for synthesis and processing of nanomaterials, namely involving thermal plasma and microwave radiations have been described in details in this chapter. Different characterization techniques, a few of which were rigorously used, have been described in details; and few others which were marginally used have been briefly mentioned. Although the information is quite general it was necessary to keep the continuity of the thesis.

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2.5 References [1] R. Siegel, S. Ramasamy, H. Hann, L. Zongguan, L. Ting, R. Gronsky, J. Mater. Res, 3 [6](1988) 1367 [2] P. Madhu kumar, N.D Sali, S.V. Bhoraskar, V.K. Rohatgi in P.K. Ghosh (Ed.), National symposium on Plasma Science, 8-10 Nov. 1995, held at IIT kanpur, India, Proc. Natl. Symp. Plasma Science, PHI, 1996. [3] P.A. Dokhale , N.D Sali, P. Madhu kumar S.V. Bhoraskar, V.K. Rohatgi ,V.N. Bhoraskar, S. Badrinayanan S.K. Date Mater. Sci. Eng. B49 (1997) 18-26. [4] P. Madhu kumar , C. Balasubramanian, N.D. Sali , S. V. Bhoraskar, V.K. Rohatgi, S. Badrinayanan Mat Sci. Eng B63 (1999) 215-227. [5] P.S. More N.D. Sali, Y.B. Kholam, S.K. Date Mater. Lett. 58 (2004) 1020-1025. [6] (a) Elements of x-ray diffraction, B.D. Cullity, 2nd Ed (1978) Addison Wesley, MA, USA (b) Powder diffraction file, JCPDS data sheets, Swarthmore, PA, (1967). [7] S.F. Bartram, in Adv. in X-Ray analysis, Vol. 4 (1960) 40, ED.: W.M. Mueller, R.C. Rau, in Adv. in X-ray analysis, Vol. 6 Plenum Press, NY, USA (1962). [8] Powder surface area and porosity, S. Lowell, J.E. Shields Chapman and Hall, London, UK (1991) 3rd Ed. [9] P. Madhukumar Ph.D. thesis, Pune University, Physics Dept. July 1995 [10] Scanning electron microscopy and X-ray microanalysis, 2n ed. Plenum press, NY and London, (1992) J.I. Goldstein, A.D. Roming Jr., D.E. Newbury, C.E. Lyman, P. Echlin, C. Fiori, D.C. Joy, E. Lifshin. [11] Constantine A. Balanis Advanced Engineering Electromagnetic John Wiley and sons Inc., USA, (1989)352-392. [12] L. Sivan, Microwave tube transmitters, Chapman and Hall, London UK, 1st ed. (1994) [13] Om P. Gandhi, Microwave engineering and Applications Pergamon Press, Inc., UK, (1989)67-83,241-277. [14] Handbooks of Monochromatic XPS Spectra, Volumes 1-5, B.V.Crist, published by XPS International LLC, 2004, Mountain View, CA, USA [15] Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, ed. J.T.Grant and D.Briggs, published by IM Publications, 2003, Chichester, UK

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[16] Gardiner, D.J. (1989) Practical Raman spectroscopy Springer-Verlag. ISBN 978- 0387502540. [17] Qing Zhao and H. Daniel Wagner 10.1098/rsta Sept (2004)1447, Phil. Trans. R. Soc Lond. A (2004) 362, 2407-2424 [18 ] International Union of Pure and Applied Chemistry "photochemistry" Compendium of Chemical Terminology Internet edition.

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