Variation in melting point with size in encapsulated silver nanoparticles .

Navneeta Katyan

P09309

Centre for Excellence in Basic Sciences

Project Supervisor

Prof: Pushan Ayyub

Tata Institute of Fundamental Research

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DECLARATION

I, Navneeta Katyan studying in Centre for Excellence in Basic Sciences, University of

Mumbai, hereby declare that I have completed the project titled “Variation in melting

point with size in encapsulated silver nanoparticles” as part of my Master’s project in

the academic year 2013-14.

I further declare that the work and results submitted in this project is true and original to

the best of my knowledge.

Date :

Place : Mumbai

Navneeta Katyan

Centre for Excellence in Basic Sciences

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GUIDE CERTIFICATE

I, Prof. Pushan Ayyub hereby certify that Navneeta Katyan studying in Centre for

Excellence in Basic Sciences, has completed the project on “Variation in melting point

with size in encapsulated silver nanoparticles” under my guidance in the period Aug-

Nov 2013.

I further declare that the information presented in this project is true and original to the

best of my knowledge.

Date :

Place : Mumbai

PROF. Pushan Ayyub

Tata Institute of Fundamental Research

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Abstract

This study was motivated by an attempt to determine the size dependent variation in the

melting point of surface-encapsulated silver nanoparticles. Core-shell structures with Ag core

and an inert oxide (ZrO2 & SiO2) shell were synthesized via different chemical routes. Our aim

was to isolate silver nanoparticles from each other and to avoid thermal agglomeration and

grain growth during heating. The structural and compositional characterization of the

samples were done using XRD, FESEM, TEM and EDX. The melting transition was studied

using Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC).

Contents

1. Introduction

2. Synthesis techniques

2.1 Chemical synthesis of Ag (core) ZrO2 (shell) nanoparticles

2.2 Chemical synthesis of Ag (core) SiO2 (shell) nanoparticles

3. Characterization

3.1 X-ray diffraction

3.2 Electron microscopy

3.2.1 Scanning electron microscope

3.2.2 Transmission electron microscope

3.3 Energy dispersive X-ray spectroscopy

4. Thermal analysis

4.1 Differential thermal analysis

4.2 Differential scanning calorimetry

5. Results and discussion

5.1 Synthesis and study of Ag@ZrO2 nano-composites

5.2 Synthesis and study of Ag@SiO2 nano-composites

6. Conclusion

7. References

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1. INTRODUCTION

Nanostructured materials with a characteristic dimension of 1-100 nm represent one of the most

dynamic areas of modern science. The physical properties of small particles are a subject of intense

contemporary interest. As the size decreases to the nanometer (10-9m) scale, many of the

electronic, as well as magnetic, structural and thermodynamic properties are significantly altered

from those of the bulk. The two main reasons why materials at the nano-scale can have different

properties are increased relative surface area and quantum size effects. Nano materials have a much

greater surface area to volume ratio than their bulk forms, which can lead to greater chemical

reactivity and affect their strength. Also at the nano scale, quantum effects can become much more

important in determining the materials properties and characteristics, leading to discrete electronic

energy levels whose spacing depends inversely on the size. [1]

In this report, we study the size dependence of the melting point of shell-encapsulated silver

nanoparticles. Depression of the melting point of small particles below that of the bulk, when the

dimensions approach a few nanometers, has been known for a long time. The dependence of

melting on size is not restricted to any particular material; rather, it encompasses a wide variety of

materials from metals to semiconductors and to molecular organic crystals. The melting

temperature depression results from the high surface-to-volume ratio and the surface substantially

affect the interior “bulk” properties of these materials. The melting point depression was observed

in shell encapsulated gold nanoparticles. [2] SiO2 shell encapsulated gold particles were

synthesized via chemical route and their melting transition was determined using differential

thermal analysis (DTA) coupled to thermal gravimetric analysis (TGA) techniques. The result

showed clear melting endotherms in the DTA scan with no accompanying weight loss of the

material in the TGA examination. [2] The melting point reported for free silver particles is 962oC.

[3] In this project we have made an attempt to study the change in melting point of isolated silver

nanoparticles which are not in contact with each other and thus cannot grow in size while being

heated. Core-shell nanoparticles (ref Fig: 1.1) were synthesized so as to isolate silver nanoparticles

from each other and prevent them from growing in size. The silver core was encapsulated by a

shell made up of inert oxides such as SiO2, ZrO2 and ZnO. The shell acts as a nano-crucible for the

melting silver with little effect on the thermal analysis data. Recently, core-shell nanostructures

have by themselves become very attractive because of potential applications in microelectronics,

optoelectronics, catalysis, and optical devices.

Fig 1.1: Schematic of core shell structure.

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In this project, we have used two different chemical methods for the synthesis of silver nano core

shell particles. A number of samples were prepared by varying the reaction parameter in both the

processes so as to obtain silver nanoparticles in the size range from ≈ 2nm to 30nm. Samples were

synthesized, then their crystal structure was characterized using XRD (X-ray diffraction), study

of their surface morphology, shape, size and imaging were done using TEM (Transmission

electron microscopy), SEM(Scanning electron microscopy) and EDX (Energy dispersive X-ray

analysis) was performed to quantitative compositional analysis of the elements present.

2. Synthesis techniques.

2.1 Chemical synthesis of Ag (core) ZrO2 (shell) nanoparticles

Silver core and ZrO2 shell nanoparticles were synthesized in one-step synthesis [6]. The chemicals

used in the synthesis were reagent-grade silver nitrate (AgNO3), acetylacetone, zirconium (IV)

propoxide, acetone, DMF (Dimethyl Formaamide) and 2-propanol purchased from Alpha-Aesar

Co. and used without further purification. Two samples were prepared. In sample 1, a solution

containing equimolar (19.9 mM) amounts of zirconium (IV) propoxide and acetylacetone in 2-

propanol was prepared and for sample 2, a solution containing equimolar (39.8 mM) amounts of

zirconium (IV) propoxide and acetylacetone in 2-propanol was prepared A clear solution was

formed upon mild sonication. Another solution of 8.80 mM AgNO3 and 13.88 MH2O in DMF was

prepared for both the samples. A 40 mL sample of the first solution and 20 mL of the second

solution were mixed and stirred for about 10 minutes. Then the mixture was transferred to a heating

mantle and refluxed for 45 minutes for sample 1 and 90 minutes for sample 2. Reflux is a

distillation technique which involves the condensation of vapors and then returns this condensate

to the original system. The solution became green-black on heating. The color change was gradual.

Further refluxing of the solution resulted in the formation of a precipitate, which was dispersed by

sonication. Sonication is a process in which sound energy is applied at ultrasonic frequency (>20

kHz) to agitate particles in a sample. The colloidal material was precipitated by the addition of

toluene. The precipitate was washed repeatedly with toluene and dissolved in 2-propanol and then

dried. The cleaning procedure is important for removing the residues. Dry powders were not

dispersible (thus, the 2-propanol dispersions contained traces of toluene). Dry powders of both the

samples were collected.

The chemical reaction undergone in this process is

HCONMe2 + 2Ag+ + H2O 2Ag + Me2NCOOH + 2H+

Primary amine reduces silver ion to metal silver. The carbamic acid thus formed easily decomposes

to CO2 and Me2NH. The following two samples of Ag@ZrO2 core-shell structure were

synthesized: (1) size of silver core ≈ 12nm and b) size of core silver ≈30nm. This process can be

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adapted for the synthesis of large quantities of core-shell materials since they were stable for a

period of over one month.

2.2 Chemical synthesis of Ag (core) SiO2 (shell) nanoparticles

Another type of core-shell nano structure of Ag core and SiO2 shell was synthesized by chemical

method. The chemicals used in this synthesis were reagent-grade silver nitrate (AgNO3), ethylene

glycol (HOCH2CH2OH), polyvinylpyrrolidone (PVP, Mw 10,000), ammonium hydroxide

(NH4OH, 25% NH3 in H2O), acetone, and tetraethyl orthosilicate (TEOS), purchased from Alpha-

Aesar Co. and used without further purification. This is a two-step synthesis [7, 8, 9], first Ag

nanoparticles were synthesized and then added to TEOS solution for formation of core shell

structure. Five different samples were synthesized using this method by varying different reaction

parameter. The chemical reaction occurring in the process is explained below. PVP acts as a

protecting agent. Ethylene glycol molecule breaks into acetaldehyde and water molecule.

CH2OH-CH2OH CH3CHO + H2O

And acetaldehyde reduces Ag+ ion to metallic Ag.

2Ag+ + 2CH3CHO CH3CO-COCH3 + 2Ag + 2H+

When Ag+ ions are reduced into metallic Ag, the concentration of metallic Ag species in solution

will gradually increase and then reach super saturation, which triggers the nucleation of Ag nuclei.

For the formation of shell,

Fig 2.2: Diagram of the general procedure for the coating of colloids with silica.

Poly (vinylpyrrolidone) (PVP) was used as a coupling agent. It is an amphiphilic, nonionic

polymer that adsorbs onto metals like silver Ag and gold. Here PVP is adsorbed onto colloidal

solution of silver which is directly transferred into an ammonia/ethanol mixture where smooth and

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homogeneous silica coatings of variable thickness were grown by addition of tetraethoxysilane

(TES). Below synthesis flow chart is given for all the five samples.

Synthesis flow chart.

Hydrous PVP Anhydrous PVP

75 ml Ethylene Glycol + 10g PVP

Stirred at 500 rpm for 90 mins

Heated in an oil bath – 120 C̊, heating

rate 7.5 ̊C/min, for 30 mins

Cooled to room temp, centrifuged

for 3hrs at 9000rpm, ppt sample

collected. Washed twice with

ethanol and acetone and dried.

5 mg of silver

nanoparticle ppt

added to 50ml

of 5% NH4OH

soln + 2ml of

20% TEOS soln

in ethanol

Soln was stirred at 900 rpm for

2hrs and then ppt and dried.

5ml of 1mg/ml

silver solution

in ethanol

prepared 50ml

of 5% NH4OH

soln + 2ml of

20% TEOS soln

in ethanol

Sample 1

Ag@SiO2_1

75 ml Ethylene Glycol + 10g PVP

Stirred at 500 rpm for 90 mins

Heated in an oil bath – 120 C̊,

heating rate 7.5 ̊C/min, for 30

mins

Cooled to room temp, centrifuged

for 3hrs at 9000rpm, ppt sample

collected. Washed several times

with ethanol and acetone and dried.

5mg of silver nanoparticles

added to

50ml of 10%

NH4OH soln

+ 2ml of 20%

TEOS soln in

ethanol

Soln was stirred at 900 rpm for 2hrs

and then ppt and dried.

50ml of 2.5%

NH4OH soln

+ 2ml of 20%

TEOS soln in

ethanol

Sample 3

Ag@SiO2_3

Sample 4

Ag@SiO2_4

Sample 2

Ag@SiO2_2

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Sample 2

Ag@SiO2_2

Heated in an oil bath – 120 ̊C,

heating rate 10 ̊C/min, for 30

mins

Cooled to room temp, centrifuged

for 3hrs at 9000rpm, ppt sample

collected. Washed several times

with ethanol and acetone and dried.

5mg of silver nanoparticles added to 5g

PVP + 50 ml H2O solution and stirred at

500 rpm for 1hr. PVP modified Ag particles

were ppt by centrifugation and added to

Soln was stirred at 900 rpm for

2hrs and then ppt and dried.

50ml of 2.5% NH4OH soln +

2ml of 20% TEOS soln in

ethanol

75 ml Ethylene Glycol + 10g

PVP.

Stirred at 500 rpm for 90

mins.

Sample 5

Ag@SiO2_5

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In total five samples were synthesized from this method as mentioned in the flow charts. Then

the samples were characterized using various instruments and techniques mentioned in section 3.

3. Characterization

3.1 X-ray diffraction (XRD)

XRD is one of the most important tools available for characterization of crystalline materials. The

use of X-ray diffraction (XRD) for the structure analysis of solids is based on Bragg’s Law, which

relates the spacing between the adjacent (hkl) planes of the lattice and the glancing angle θ of the

X-ray beam having wavelength (λ).

𝒏𝝀 = 𝟐𝒅𝒉𝒌𝒍𝒔𝒊𝒏𝛉 (3.1)

The occurrence of peaks in the x-ray diffraction pattern from a periodic crystal is governed by

Bragg’s condition (3.1) for constructive interference.

The Xpert PRO MPD, PAN Analytic X-ray diffractometer was employed for all the measurements.

The intensity of the radiation reflected from the sample is measured as a function of angular

position by a solid state detector. The detector used was Xcelerator with Diffracted beam

monochromator. The operating voltage was 45kV and current was 40mA. The data is collected

using X-pert high score software. A schematic diagram of the instrument and its operation are

shown in figure 3.1. The X-ray tube is fitted with a Cu target as its characteristic alpha radiation

is suited for most inorganic crystals with moderate cell dimensions. Theoretically for an infinite

crystal, the Bragg’s reflection peaks whose positions are given by Eqn (3.1) are delta functions

(zero width and infinite intensity). However in reality, the reflection peaks have finite intensity

and non-zero width. For crystal sizes larger compared to lattice parameter, the main contribution

to peak broadening comes from the ‘instrument broadening’, which is caused by the finite spectral

width of the x-ray beam, its non-parallel nature, imperfect focusing, finite slit width etc. However,

for smaller crystallite size (<50nm) an additional broadening occurs which can be understood as

follows. If 𝜃𝑏 is the angle at which Bragg’s condition for constructive interference is satisfied by

a particular family of planes with interspacing 𝑑𝑏 , then the path difference between nth and (n+m)th

plane is 2𝑚𝑑𝑏𝑠𝑖𝑛θ𝑏 and the x-rays reflected from any two planes interfere constructively. For

another set of planes having path difference 𝑚𝜆 +𝜆

2 , they have destructive interference. For

intermediate values of path difference there is reduction in intensity by partial cancellation of x-

rays intensity, which is dependent on number of planes present in the crystallites. When the number

of planes is large, cancellation can occur over a very small deviation from θ𝑏 and in turn give rise

to sharp peak in x-ray spectrum. [10, 11] For small crystallites typically of nanometer size the

number of planes is restricted. Therefore the effective cancellation can take place only over a large

deviation from θ𝑏, resulting in considerable peak broadening. Thus the size of crystal can be

estimated from width of the Bragg reflection and is given by Scherrer formula [10]

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𝐶𝑥𝑟𝑑 =0.94𝜆

𝐵𝑐𝑜𝑠θ𝑏 (3.2)

where 𝐶𝑥𝑟𝑑 is the length of the crystal in the direction perpendicular to reflecting planes, B is the

FWHM (full width at half maximum) of the Bragg’s reflection in radians on the 2θ scale. It is

important to subtract the instrumental broadening from the observed line width to get a correct

estimate of broadening due to small size particles. The shape asymmetry of the particle can also

be estimated by measuring the size of particles along different crystallographic direction. The

component due to K𝛼2 was suppressed completely by using the software X-pert high score. The

angular speed can be selected from 0.02̊ to 30̊/min, with a step size ranging from 0.01̊ to 1.0̊.

Fig 3.1: Schematic of X-ray diffractometer X-pert PRO MPD, PAN Analytic.

3.2 Electron microscopy.

Electron Microscopes use a beam of highly energetic electrons to examine objects on a very fine

scale (nano scale). Electron Microscopes (EMs) function exactly as their optical counterparts

except that they use a focused beam of electrons instead of light to "image" the specimen and gain

information as to its structure and composition. The basic steps involved in all Ems are the

following: A stream of electrons is formed in high vacuum (by electron guns). This stream is

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accelerated towards the specimen (with a positive electrical potential) while is confined and

focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam. The

sample is irradiated by the beam and interactions occur inside the irradiated sample, affecting the

electron beam. These interactions and effects are detected and transformed into an image. In fig

3.2, the schematic diagram of light microscope and the two types of electron microscopes are

shown. We used both SEM (Scanning electron microscope) and TEM (Transmission Electron

Microscope) for sample characterization.

Fig 3.2: Schematic of light microscope, TEM and SEM.

Principle of electron-specimen interaction and image formation: When an electron beam interacts

with the atoms in a sample, individual incident electrons undergo two types of scattering - elastic

and inelastic. In the former, only the trajectory changes and the kinetic energy and velocity remain

constant. In the case of inelastic scattering, some incident electrons actually collide with and

displace electrons from their orbits (shells) around nuclei of atoms comprising the sample. This

interaction places the atom in an excited (unstable) state. Specimen interaction is what makes

Electron Microscopy possible. The interactions (inelastic) detected on the top surface of the sample

are utilized when examining thick or bulk specimens (Scanning Electron Microscopy, SEM) while

the electrons going through thin samples or foil specimens are detected below (Transmission

Electron Microscopy, TEM).The main differences between optical and electron microscopes are

the type of lenses required. In optical microscope lenses are made up of glass and have fixed focal

length whereas electron microscopy uses magnetic lenses. In next section details of two electron

microscope used during this project is mentioned.

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3.2.1 TEM (Transmission electron Microscope)

The schematic of the imaging system of Tem is shown in figure 3.3. TEM is a technique in which

an electron beam interacts and passes through a specimen. The electrons are emitted by a source

and are focused and magnified by a system of magnetic lenses. The electron beam is confined by

the two condenser lenses which also control the brightness of the beam, passes the condenser

aperture and “hits” the sample surface. The electrons that are elastically scattered consist the

transmitted beams, which pass through the objective lens. The objective lens forms the image

display and the following apertures, the objective and selected area aperture are used to choose of

the elastically scattered electrons that will form the image of the microscope. Finally, the beam

goes to the magnifying system that is consisted of three lenses, the first and second intermediate

lenses which control the magnification of the image and the projector lens. The formed image is

shown either on a fluorescent screen or on Computer screen.

Transmission electron microscopy (TEM) was employed to characterize the morphology and

microstructures of the Ag-synthesized nanoparticles. The model used for TEM: FEI Tecnai TEM.

The electron gun used in the instrument is LaB6 crystal with an operating voltage of 200kV.

Lanthanum Hexaboride (LaB6) Electron Gun: is a thermionic emission gun. It is the most common

high-brightness source. TEM samples were prepared by placing a drop of the nanoparticle

colloidal suspension on carbon-coated copper grids and allowing methanol to evaporate naturally

in air.

Fig3.3: Schematic diagram of the working principle of TEM.

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3.2.2 SEM (Scanning electron Microscope) In SEM, a source of electrons is focused in vacuum into a fine probe that is focused over

the surface of the specimen. The electron beam passes through scan coils and objective lens that

deflect horizontally and vertically so that the beam scans the surface of the sample (Fig: 3.4). As

the electrons penetrate the surface, a number of interactions occur that can result in the emission

of electrons or photons from or through the surface. A reasonable fraction of the electrons emitted

can be collected by appropriate detectors, and the output can be used to modulate the brightness

of a cathode ray tube (CRT) whose x- and y- inputs are driven in synchronism with the x-y voltages

restoring the electron beam. In this way an image is produced on the CRT; every point that the

beam strikes on the sample is mapped directly onto a corresponding point on the screen. [11] SEM

is suitable for surface topology and can also be used for chemical composition of the sample’s

surface since the brightness of the image formed by backscattered electrons is increasing with the

atomic number of the elements. This means that regions of the sample consisting of light elements

(low atomic numbers) appear dark on the screen and heavy elements appear bright.

The SEM employed in TIFR is the ZEISS Ultra FESEM and the electron source used is Field

Emission Gun. The field emission cathode is usually a wire of single-crystal tungsten fashioned

into a sharp point and spot welded to a tungsten hairpin. The significance of the small tip radius,

about 100 nm or less, is that an electric field can be concentrated to an extreme level. The

accelerating voltage of the instrument ranges from 0.1kV-30kV and beam current can go upto

100nA. The resolution of the instrument is 0.8nm at 30kV (STEM mode), 1nm at 15kV and 4nm

at 0.1kV. There are several detectors used for SEM imaging, the detectors and their functions are

summarized in table 3.1. Table 3.1: Different detectors employed in TEM and their functions.

Detectors Functions

In-lens (scintillator detector) Images surface structure

ASB (4 quad solid state detector) For compositional contrast

ESB (column mounted scintillator detector) For material contrast

SEI (Everhart-Thornley detector) For topography

Fig 3.4: Schematic of working principle of SEM.

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3.3 Energy-dispersive X-ray spectroscopy (EDX) EDX makes use of the X-ray spectrum emitted by a solid sample bombarded with a focused beam

of electrons to obtain a qualitative as well quantitative analysis of the sample. Qualitative analysis

involves the identification of the lines in the spectrum of elements in the sample. Quantitative

analysis (determination of the concentrations of the elements present) entails measuring line

intensities for each element in the sample and for the same elements in calibration standards of

known composition. A solid state detector is used because of its better energy resolution. Incident

X-ray photons cause ionization in the detector, producing an electrical charge, which is amplified

by a sensitive preamplifier located close to the detector. The ED spectrum is displayed in digitized

form with the x-axis representing X-ray energy (usually in channels 10 or 20 eV wide) and the y-

axis representing the number of counts per channel. An X-ray line (consisting of effectively mono-

energetic photons) is broadened by the response of the system, producing a Gaussian profile.

4 Thermal Analysis.

Thermal analysis is the technique which studies the variation in the physical properties of materials

as a function of temperature. Several methods are used for thermal analysis and are distinguished

from each other by the property they measure. In this project the thermal analysis of samples were

done using Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC).

Details of each techniques are given in next section

4.1 Differential Thermal analysis

This is a thermal analysis technique in which difference in temperature, ∆T, between a sample and

a reference material is measured when they are subjected to a controlled temperature program

(usually T increases linearly with time). Several mg of sample and inert reference are contained in

Al2O3 crucibles each with thermocouple, held in heating block. Both sample and reference material

are heated under carefully controlled conditions. If the sample undergoes a physical change or a

chemical reaction, its temperature will change while the temperature of the reference material

remains the same. That is because physical changes in a material such as phase changes and

chemical reactions usually involve changes in enthalpy, the heat content of the material. There is

a constant temperature difference ∆T between sample and reference since they have different heat

capacities. But when the sample undergoes an endo (exo) thermic change ∆T becomes different.

[11] The schematic of DTA is shown in fig 4.1 and fig 4.2 explains the various peak obtained from

a typical DTA spectrum,

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Fig 4.1: Schematic diagram of DTA instrument.

Fig 4.2: Typical spectrum obtained from DTA showing different peaks.

The spectrum in fig 4.2 is plot for analysis of a polymer which shows several features due to

physical and chemical changes, including, glass transition in which glassy, amorphous polymer

becomes flexible, ∆H = 0, but there is a change in Cp. Crystallization of amorphous polymer into

microcrystals is exothermic reaction with increase in ∆T whereas melting is an endothermic

reaction with decrease in ∆T.

4.2 Differential scanning Calorimeter (DSC)

A DSC is a thermal analysis technique which measures the difference in heat flow rate (mW =

mJ/sec) between a sample and inert reference as a function of time and temperature. A sample of

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known mass is heated and the changes in its heat capacity are tracked as changes in the heat flow.

This allows the detection of transitions like melts, glass transitions and phase changes. In a heat

flow DSC, the endothermic peaks are those events which require energy point up, because the

instrument must supply more power to the sample to keep the sample and reference furnaces at the

same temperature. The reverse logic applies to exothermic events where energy is released.

The model employed in TIFR is STA 449 F1 Jupiter® – Simultaneous TGA-DSC. It has a

temperature range of -150̊C -2400̊C. Heating and cooling rates: 0.001 K/min to 50 K/min

(dependent on furnace), has a weighing range upto 5000 mg with TGA resolution of 0.025 µg. The

DSC resolution is < 1 µW. This instrument was used to perform both DTA, DSC.

5. Results and discussion.

5.1 Synthesis & study of Ag@ZrO2 nano composites sample.

5.1.1 Structural and compositional characterization.

Ag core and ZrO2 shell nanoparticles (Ag@ZrO2) were synthesized by chemical synthesis (ref sec

2.1). The table 5.1 gives the summary of the Ag@ZrO2 samples synthesis.

Table 5.1: Summary of reaction parameters for synthesis of Ag@ZrO2 samples

Samples Solution 1 (conc) Solution 2 (conc) Reflux time

Ag@ZrO2_1 19.9mM 8.80mM 45 minutes

Ag@ZrO2_2 39.8mM 8.80mM 90minutes

XRD was done on both the samples Ag@ZrO2_1 and Ag@ZrO2_2. The XRD diffraction pattern

of both the samples is shown in fig 5.1. The peaks were analyzed using X-pert high score software

and the peak matching was done with the help of database already present in the software.

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Fig 5.1: X-ray diffraction pattern of Ag@ZrO2_1 and Ag@ZrO2_2 samples.

From the diffraction pattern it can be seen that there is a hump (a) in the spectra at 28̊, this can be

from the amorphous zirconia present in the sample. There are no peaks corresponding to unreacted

zirconium in the diffraction pattern. There are four peaks of silver identified in the diffraction

pattern from which it can be concluded that silver is present in the sample in crystalline form. The

silver peaks matched with database of cubic silver hence the silver was present in cubic (3C) phase.

The peaks of the second sample is sharp because it was scanned for a long time with a smaller step

size so as to minimize the signal to noise ratio and to record better statistics

Then the crystal size of both the samples were calculated using the Scherrer formula [ref sec

3.1].The crystal size was calculated along the [111] diffraction line as that peak had maximum

intensity.

Scherrer formula - CXRD=0.9𝜆

𝐵𝑐𝑜𝑠𝛩. B = FWHM of the peak; Θ = Bragg angle.

For Ag@ZrO2_1: CXRD =12nm and for Ag@ZrO2_2: CXRD =31nm. So, it was concluded that by

varying the concentration of solution 1 (ref table 5.1) nano composites with different particle size

of silver were formed.

Elemental analysis of samples Ag@ZrO2_1 and Ag@ZrO2_2 was done by EDX. (See fig 5.2)

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Fig 5.2: Elemental analysis spectrum of Ag@ZrO2_1, EDX data obtained for both the samples after quantifying

elements.

From the EDX data (ref table 5.2), the atomic % and weight % of Ag, Zr and O was obtained. It

can be seen from the atomic % ratio of Zr and O is 1:2, concluding that ZrO2 is present in the

sample along with silver. So if we combine the results obtained from XRD and EDX, then

crystalline silver in 3C phase and amorphous ZrO2 is present in both the samples.

To obtain the information about the topography and morphology of the samples Ag ZrO2_1 and

Ag@ZrO2_2 SEM imaging was done. Fig 5.3 shows the image of both the samples obtained from

SEM.

Fig 5.3: SEM image of sample Ag@ZrO2_1 and Ag@ZrO2_2.

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In fig 5.3 the bright spots are that of silver because silver has a higher atomic number Z=47 and

the dark patch is of zirconia matrix. Hence it was confirmed that silver was present in elemental

form embedded in amorphous ZrO2 matrix. Then TEM imaging was done to find out whether core

shell structure has been formed or not, how silver is distributed in the sample and size variation of

silver nano particles. Fig 5.4 shows the TEM image of Ag@ZrO2_1.

Fig 5.4: TEM images of silver nanoparticles embedded in zirconia matrix.

It can be seen that silver nanoparticles are spherical in shape. From the low magnified image (left)

of the sample, size variation of silver nanoparticles were estimated. The image on the right shows

the crystal planes of single silver particle again confirming that silver is present in crystalline form.

The histogram shown in fig 5.5 shows the size distribution of silver nanoparticles.

Fig 5.5: Histogram showing size distribution of silver nanoparticles in Ag@ZrO2_1.

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From the above histogram the mean size of particles is found to be 13 nm which is in good

agreement with the size of silver nanoparticles found using Scherrer formula which was 12nm.

The median of the distribution is 12 nm.

The TEM image of Ag@ZrO2_2 sample was also done to find out the distribution of silver and is

shown in fig 5.6.

Fig 5.6: Core-shell structure of silver nanoparticles in Ag@ZrO2_2.

From the TEM image in fig 5.6, it can be seen that the dark spots are of silver are spherical in

shape. A magnified image on the right shows silver nanoparticles surrounded by amorphous

zirconia shell. The thickness of the shell is ≈ 1nm. The histogram in fig 5.7 shows the size

distribution of silver nanoparticles in AgZrO2_2.

Fig 5.7: Histogram showing size distribution of silver nano particles in Ag@ZrO2_2

N=20

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From the above histogram the mean size of particles is found to be 28.28 nm which is in good

agreement with the size of silver nano particles found using Scherrer formula which was 30nm.

The median of the distribution is 28 nm.

Hence, we find from a combination of XRD, EDX, SEM, and TEM analysis that while sample

Ag@ZrO2_1 consists of ≈ 12nm crystalline (3C) silver nano particles embedded in amorphous

ZrO2 matrix, sample Ag@ZrO2_2 consists of ≈ 30nm crystalline (3C) silver nano particles

surrounded by amorphous ZrO2 shell of thickness ≈ 1nm.

5.1.2 Thermal analysis.

The melting temperatures of dried powders of the samples Ag@ZrO2_1 and Ag@ZrO2_2 were

determined using the DTA and TGA techniques simultaneously. A computer-controlled Jupiter

449 F1 DTA-TGA apparatus was used, and each sample was scanned from room temperature to

1100 °C, above the melting point of bulk silver which is 962̊C[3]. The chamber was heated at a

rate of 10°C per minute, and the temperature, mass of both the sample and the reference were

monitored at all times. The heating and cooling curve of bulk silver is shown in fig.5.8. Both

heating and cooling curve were analyzed. The onset temperature of heating curve gives the

melting point of bulk silver which is about 957.4̊C.

Fig 5.8: DTA/TGA data for bulk Ag with heating and cooling which shows melting transition.

950 ̊C

22

880 900 920 940 960 980 1000 1020 1040

-3

-2

-1

0

1

Tm=957.4

oC

H

(mW

/mg

)

cooling

heating

Tm=956.3

oC

Temp (oC)

Fig 5.9: DSC data for bulk Ag with heating and cooling which shows melting transition.

The melting point was obtained from DTA data in terms of the onset temperature for both heating and

cooling curves. The fact that melting temperature measured from heating and cooling curves differ by 7̊C

can possibly be ascribed to rather high heating rate 10C̊/min. The DSC was also performed on bulk silver

and data showed melting transition of silver in both cooling and heating curves. Here also the melting

temperature measured from heating and cooling curves differ by 1oC can possibly be ascribed because of

instrument error. Next the DTA measurements were done on both Ag@ZrO2_1 & Ag@ZrO2_2 samples.

The heating and cooling curve obtained for both the samples is shown in fig 5.10 and fig 5.11.

Fig 5.10: DTA/TGA data for Ag@ZrO2_1 (left) and Ag@ZrO2_2 (right) where heating curve shows three

transitions.

23

Fig 5.11: DTA/TGA data for Ag@ZrO2_1 (left) and Ag@ZrO2_2 (right) where cooling curve shows only

melting transitions

From the data obtained it can be seen that there is no appreciable change in the melting point of

silver nanoparticles as compared to their bulk counter parts. The change in the slope of two lines

gives melting point. From the intersection of tangent drawn from both the lines an estimate of

melting point can be estimated (heating curve). The melting point of sample Ag@ZrO2_1 is found

to be 957.4̊C and that of sample Ag@ZrO2_2 is 953.5̊C. The melting point transition is a reversible

transition as it is seen in both heating and cooling curve. The heating curve also shows two other

peaks. The first peak correspond to first order phase transition, which is of evaporation of water

molecule from the samples at 100̊C. It is an irreversible transition as it is not seen in cooling curve.

The second peak corresponds to the phase transition of zirconium from amorphous to crystalline

at around 545̊C. This is also an irreversible transition as it is not seen in cooling curve. So heating

and cooling curve of DTA also gives information about the transition whether it is reversible or

irreversible.

Now the two main possibilities for no appreciable change in melting points of nano composites as

compared to their bulk counterparts can be because;

Melting point is independent of size in this size range of silver nano particles (10 – 30nm).

Silver nanoparticles grew in size after amorphous ZrO2 shell was broken and Zr crystalized.

To check this, XRD and SEM was done on both the samples after DTA. The spectrum shown in

fig 5.12 shows the XRD diffraction pattern of both the samples after DTA. It can be seen that other

than silver peaks several other peaks have emerged.

24

Fig 5.12: XRD diffraction pattern of Ag@ZrO2_1 before and after DTA.

The peaks labelled in red are silver peaks and silver is in cubic (3C) phase, whereas peaks labelled

in blue are zirconia peaks in monoclinic phase. So after heating amorphous zirconia changes to

crystalline monoclinic phase. The size of zirconia found using Scherrer formula along [110] 4th

peak, was found to be ≈ 50nm. The XRD diffraction pattern for Ag@ZrO2_2 is shown in fig 5.13.

Fig 5.13: XRD diffraction pattern of Ag@ZrO2_2 before and after DTA.

25

From the diffraction pattern in fig 5.12 and 5.13, it can be seen that the FWHM of the silver peak

along [111] has decreased after DTA proving that the silver nanoparticles have grown in size (from

Scherrer formula CXRD=0.9𝜆

𝐵𝑐𝑜𝑠𝛩 , B= FWHM). The size of the silver particles in Ag@ZrO2_1 and

Ag@ZrO2_2 was ≈ 200nm, showing that after heating growth in silver particles were uniform.

There were several peaks of zirconia identified in monoclinic phase. This confirmed that zirconia

has become crystalline from amorphous on heating. SEM imaging was also done to see the

morphology of the sample. From the images (see fig 5.14) it was found that isolated single silver

particles had come in contact with each other and grown in size of ~ μm. It also confirmed that

zirconia shell was broken and grain growth occurred for zirconia of size ≈ 50nm.The change in

melting point is observed for size < 30 nm but since the silver had grown in size after zirconia

crystallized in monoclinic phase at around 545̊C, the nanoparticles of silver behaved as bulk silver

and no appreciable change was observed in its melting point.

Fig 5.14: SEM images of Ag@ZrO2 after DTA.

5.2 Synthesis & study of Ag@SiO2 nano-composites sample.

5.2.1 Structural and compositional characterization.

Ag core and SiO2 shell nanoparticles (Ag@SiO2) were synthesized by chemical synthesis (ref sec

2.2). The table 5.2 gives the summary of the Ag@SiO2 samples synthesis.

10μm

200nm

26

Table 5.2: Summary of reaction parameters for synthesis of Ag@SiO2 samples

Samples amount of silver NH4OH +

ethanol soln

TEOS +

ethanol

soln

ml ml

Ag@SiO2_1 5ml (1mg/ml) 50 (5%

NH4OH) 2

Ag@SiO2_2 5mg 50 (5%

NH4OH) 2

Ag@SiO2_3 5mg 50 (2.5%

NH4OH) 2

Ag@SiO2_4 5mg 50 (10%

NH4OH) 2

Ag@SiO2_5 5ml (1mg/ml) 50 (5%

NH4OH) 2

XRD was done on all the five samples Ag@SiO2_1 –Ag@SiO2_5. The XRD diffraction pattern of

Ag@SiO2_1 is shown in fig 5.15.

Fig 5.15: XRD diffraction pattern of Ag@SiO2_1 sample.

From the diffraction pattern it can be seen that no silver peaks were present. In all the Ag@SiO2

samples the XRD diffraction pattern showed no silver peaks. This can be due to the fact that the

amount of silver present in the sample is < 5% by weight. So, elemental analysis of all the five

27

samples Ag@SiO2_1 –Ag@SiO2_5 were done by EDX. (See fig 5.16)

Fig 5.16: Spectrum of Ag@SiO2_1 sample from EDX, Table showing the atomic% and weight% of

different elements.

From the EDX spectrum it was confirmed that Si, O and Ag was present in the samples. As it can

be seen from the above table the amount of silver present in the sample was ≈ 0.1% there was no

XRD peaks of silver obtained in the previous analysis. It can also be seen that the atomic % ratio

of Si and O is 1:2, concluding that amorphous SiO2 is present in the sample along with silver

Silver nanoparticles synthesized in step one of chemical route using hydrated PVP were imaged

using TEM, (fig 5.17). It can be seen that silver nanoparticles are well isolated from each other

and are uniformly distributed. The particles are spherical in shape. A high magnified image in the

right shows the crystal planes of silver particle.

28

Fig 5.17: TEM image of silver nanoparticles.

Fig 5.18: Histogram showing size distribution of silver nanoparticles synthesized from hydrous PVP.

The size distribution of isolated silver nano particles were estimated. From the above histogram it

can be seen that the mean size of silver is 13nm and median of the distribution is at 15nm.

To study the topology and morphology of the samples SEM imaging was done. The SEM image

of sample Ag@SiO2_1 and Ag@SiO2_2 is shown in fig 5.19.

29

Fig 5.19: SEM images of Ag@SiO2_1 and Ag@SiO2_2.

It can be seen that the amorphous SiO2 is spherical in shape and uniformly distributed in both the

samples.. The size of SiO2 sphere is around 113nm in Ag@SiO2_1 and ≈ 107nm in Ag@SiO2_2.

Then TEM imaging was done to find out whether the structure is core shell or not. Fig 5.20 shows

the TEM image of Ag@SiO2_1 sample.

Fig 5.20: TEM image of Ag@SiO2_1, crystalline planes of silver is seen.

From the above image it can be seen that silver is embedded in silica spheres but have not formed

core shell structure. The crystalline silver is embedded in amorphous silica matrix. The TEM

images of Ag@SiO2_2 is shown in fig 5.21. It can be seen that there are several small spherical

silver particles embedded in large silica sphere.

30

Fig 5.21: TEM image of Ag@SiO2_2, silver particles embedded in zirconia shell.

For the next three samples anhydrous PVP was used for the synthesis. Silver nanoparticles

synthesized from anhydrous PVP were imaged using TEM to study its shape and size. Fig 5.22

shows the TEM image of isolated silver nanoparticles.

Fig 5.22: TEM image of silver nanoparticles synthesized using anhydrous PVP.

From the above image it can be seen that silver particles are well isolated and spherical in shape.

Silver is present in crystalline form since crystal planes are seen in TEM. There are variations in

size and size distribution was estimated using a histogram.

31

Fig 5.23: Histogram showing size distribution of silver nanoparticles synthesized from anhydrous PVP.

From the above histogram it can be seen that the median of the distribution is ≈ 17nm. The mean

particle size is 14.5nm.

In the synthesis of sample Ag@SiO2_3 the concentration of NH4OH was reduced from 5% to 2.5%

in 50ml ethanol solution and in sample Ag@SiO2_4 the concentration was increased from 5% to

10% in 50ml ethanol solution. SEM imaging was done to study the topology and morphology of

the samples. The SEM image of sample Ag@SiO2_3 and Ag@SiO2_4 is shown in fig 5.24.

Fig 5.23: SEM images of Ag@SiO2_1 and Ag@SiO2_2.

32

From the SEM image it can be seen that SiO2 are spherical in sample and uniformly distributed.

The size of SiO2 spheres in Ag@SiO2_3 was ≈ 243nm and for Ag@SiO2_4 it was ≈ 130nm. So

the concentration of NH4OH in ethanol solution affected the nucleation rate. Therefore it was

concluded that NH4OH is acting like catalyst in this reaction, when more amount of NH4OH is

added nucleation rate increased and particles were grown in smaller size (sample Ag@SiO2_4)

and when NH4OH was less nucleation growth slowed down and particle size increased (sample

Ag@SiO2_5).

The TEM imaging of both the samples Ag@SiO2_4 and Ag@SiO2_5 was done. The image is

shown in fig 5.25.

Fig 5.25: TEM image of Ag@SiO2_3 and Ag@SiO2_4, silver particles encapsulated in zirconia shell.

33

From the above image it can be seen that crystalline silver nanoparticles are embedded in

amorphous SiO2 spheres. From the magnified image it can be seen that isolated silver particles are

surrounded by amorphous shell of thickness ≈ 1nm.

In the synthesis process of sample Ag@SiO2_5, two-step process (ref sec 2.2 flow chart). The

XRD diffraction pattern showed no peaks of silver. To study the morphology and topography of

the sample SEM imaging was done. The image is shown in fig 5.26.

Fig 5.26: SEM image of Ag@SiO2_5, uniformly distributed SiO2 spheres.

From the above image it can be seen that the sample is spherical in shape and uniformly distributed.

The spheres are ≈ 130 nm and some smaller spheres of ≈ 90 nm are also present.

In the near future we plan to perform DSC to find out the melting transition in all the five samples

of Ag@SiO2 prepared.

6. Conclusion

We report a study of the particle size dependence of the melting point of silver

nanoparticles encapsulated in an inert oxide shell (ZrO2 or SiO2).

34

By using a one-step chemical synthesis, and varying the concentration of zirconium

propoxide, we could synthesize: (a) 13nm Ag nanoparticles dispersed in an amorphous

ZrO2 matrix (nanocomposite) and (b) 30nm Ag nanoparticles encapsulated by an

amorphous 1nm ZrO2 shell (core shell).

The mean size of silver nanoparticles obtained by electron microscopy and x-ray

diffraction line broadening (Scherrer method) were found to be in close agreement.

Thermal analysis (DTA/TGA) of bulk silver (reference sample) showed that melting point

transition occurs at TM = 957 C.

Thermal analysis (DTA/TGA) of (a) Ag/ZrO2 nanocomposite showed TM = 957 C, and

(b) Ag/ZrO2 core shell structure showed TM = 953 C. Thus there was no appreciable

change in melting point for these two samples as compared to their bulk counter parts. This

can be due to:

1. Melting point being independent of size in this size range for Ag nanoparticles (10–

30nm).

2. Growth of Ag nanoparticles after crystallization of amorphous ZrO2 capping layer.

The XRD pattern of the samples (performed after DTA) showed sharp peaks of ZrO2 with

Ag, confirming that amorphous ZrO2 has become crystalline. SEM images of samples

showed that grain growth had occurred in both silver (size>100nm) and zirconia (size

>50nm).

Another set of five samples with core-shell structure having Ag core and SiO2 shell were

prepared by a two-step chemical synthesis. In the first step Ag nanoparticles were

synthesized with a mean size of 16nm. This was followed by formation of the core-shell

structure. Structural characterization of Ag@SiO2 samples were done using XRD, TEM,

SEM and EDX. There were samples with varying size of SiO2 spheres in the range 100-

250nm.

Measurement of the melting point of Ag@SiO2 samples using DSC could not be completed

due to instrumental malfunction.

7. References 1. Michae Quinten, Size and Quantum Size Effects in Very Small Nanoparticles, Wiley,

2011

2. Kimberly Dick, T. Dhanasekaran, Zhenyuan Zhang, and Dan Meisel, Size-

Dependent Melting of Silica-Encapsulated Gold Nanoparticles, JACS, 2012.

3. Teresa Bondora, The periodic Table of elements, futureworld publishing

international, 2011.

4. Poole and Owens, Introduction to Nano technology, Wiley, 2007.

5. Guozhong Cao, Nanostructures and nanomaterials – synthesis and properties,

Imperial College Press, 2006.

35

6. Renjis T. Tom, A. Sreekumaran Nair, Navinder Singh, M. Aslam,C. L. Nagendra,|

Reji Philip,K. Vijayamohanan and T. Pradeep, Freely Dispersible Au@TiO2,

Au@ZrO2, Ag@TiO2, and Ag@ZrO2 Core-Shell Nanoparticles: One-Step

Synthesis, Characterization, Spectroscopy, and Optical Limiting Properties,

Langmuir 19, 3439-3445, 2013.

7. Tao Gao, Bjørn Petter Jelle, Arild Gustavsen, Core–shell-typed Ag@SiO2

nanoparticles, J Nanopart Res 15, 1370, 2013.

8. Dongjo Kim, Sunho Jeong and JoohoMoon, Synthesis of silver nanoparticles using

the polyol process and the influence of precursor injection, Nanotechnology 17,

4019–4024, 2006

9. Christina Graf, Dirk L. J. Vossen, Arnout Imhof, and Alfons van Blaaderen, A

General Method To Coat Colloidal Particles with Silica, Langmuir, 19, 6693-6700,

2003

10. Warren B, X-ray Diffraction, Addison-Wesley, Reading, MA, 1969.

11. Janos Fendler, Nanoparticles and nanostructured films, preparation,

characterisation and application, Wiley-VCH, 1998.

Acknowledgement.

This research project would not have been possible without the support of many

people. I wish to express my gratitude to my supervisor, Prof. Pushan Ayyub

who was abundantly helpful and offered invaluable assistance, support and

guidance. Deepest gratitude are also to the members of the lab in which I

worked, especially Miss Smita Gohil without whose assistance this study would

not have been successful. I would also like to convey thanks to the Tata Institute

of Fundamental Research for providing the laboratory facilities. I also wish to

express my love and gratitude to my friends; for their understanding &

encouragement throughout the duration of my project.