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CHAPTER 3

CHARACTERIZATION OF SELENIUM NANOPARTICLES

AND COPPER NANOPARTICLES

3.1 INTRODUCTION

Nanocrystalline Se and Cu particles were prepared by using

suitable precursors and reducing agents by the chemical precipitation method

at room temperature and the prepared samples were annealed at different

temperatures. The method is simple costs low and can be easily used to

prepare large quantity of nano structured particles.

In the present study x-ray diffraction studies have been carried out

using PANalytical X-ray Diffractometer, surface morphology of the samples

has been studied using FEI QUANTA 200 Scanning Electron Microscope

(SEM) and the composition of the prepared samples has been studied by

energy dispersive x-ray analysis (JEOL Model JED -2300). The optical

properties have been studied using the absorbance spectrum recorded using

spectrophotometer (JASCO V-570).

3.2 CHARACTERIZATION OF SELENIUM NANOPARTICLES

3.2.1 Structural Studies

Figure 3.1 shows the X-ray diffraction patterns of as the prepared,

100⁰C and 200⁰C annealed selenium nanoparticles. The diffraction peaks

present at 2θ (degrees) of 23.57°, 29.73°, 41.28°, 43.68°, 45.43°, 51.72°,

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56.07°, 61.83°, 65.24° and 71.60° corresponds to (100), (101), (110), (102),

(111), (201), (112), (202), (210) and (113) planes of selenium. All the

diffraction peaks in the 2θ range corresponds to the hexagonal structure of

selenium with lattice constants a = 4.357Å and c = 4.945Å and are in good

agreement with the standard JCPDS data (JCPDS card No. 06-0362). The

sharpness of the diffraction peaks suggests that the prepared samples are well

crystallized. The (100), (101), (110), (102), (111), (201), (112), (210) and

(113) peaks of the 100°C and 200°C annealed selenium is slightly shifted

when compared to that of the as prepared selenium because annealed samples

are under a tensile strain and the amount of strain increases with increase in

annealing temperature. The crystallite size of selenium has been calculated

using Scherrer’s equation

where D is the grain size, K is a constant taken to be 0.94, λ is the wavelength

of the x-ray radiation, β is the full width at half maximum and θ is the angle

of diffraction. The crystallite size has been calculated and is found to be in the

range 30-50 nm for as prepared, 100⁰C and 200⁰C annealed selenium

nanoparticles.

qb

l

cos

KD =

65

10 20 30 40 50 60 70 80

(c)

(b)

(101)

(100)

Inte

nsity (

a.u

.)

2q (degrees)

(102)

(110)

(111)

(112)

(201)

(202)

(210)

(113)

(a)

Figure 3.1 X-ray Diffraction pattern of Se (a) as prepared, (b) 100⁰C

and (c) 200⁰C annealed samples

3.2.2 Raman Spectroscopy

Figure 3.2 shows the typical Raman spectrum of the as-prepared

selenium nanoparticles. Only one resonance mode at around 235 cm-1 was

observed and this is attributed to the stretching vibration of selenium which

exists for the hexagonal phase (A1 mode). The characteristic Raman

resonance absorption band for a-selenium and monoclinic selenium are

centred at 264 and 256 cm-1, respectively, which are not observed in this

spectrum. Thus, the A1 mode of Raman spectrum shows the presence of

selenium. Lucovsky et al (1967) and Rajalakshmi et al (1999) have reported

that the modes of selenium appears at A1 mode.

66

Figure 3.2 Raman spectra of as prepared Se nanowires

3.2.3 Surface Morphology Studies

Figure 3.3 shows the Scanning Electron Microscope (SEM) images

of the as prepared selenium, 100⁰C and 200⁰C annealed selenium

nanoparticles. The SEM image reveals that the selenium nanowires are of

uniform size with a mean diameter of 30 nm. It can be seen that nanowires are

of several micrometers in length with diameter ranging from 30 to 50 nm. It

can be clearly seen that the length of the selenium nanowires increase rapidly

with increase in annealing temperature, however, the diameters increase

slowly. The length of the selenium nanowires can be controlled by annealing

temperature.

Figure 3.4a shows the Transmission Electron Microscope (TEM)

image of as-prepared selenium nanowires. The image shows that the

nanowires are straight and uniform with an average diameter of about 30 nm.

Figure 3.4b shows the Selected Area Electron Diffraction (SAED) pattern of

as-prepared selenium nanowires. Selected area electron diffraction image

(Figure 3.4b) exhibit diffraction rings corresponding to the (100), (101),

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(110), (102), (111) and (201) directions of the hexagonal phase of selenium.

The d spacing values obtained for all the diffraction rings from the SAED

pattern match very well with that of hexagonal selenium. Figure 3.5a shows

the TEM image of 200⁰C annealed selenium nanowires. The TEM image

(Figure 3.5a) confirms the formation of selenium nanowires with several

micrometers in length and diameter of 50 nm. Figure 3.5b shows the SAED

pattern of 200⁰C annealed selenium nanowires. SAED image (Figure 3.5b)

exhibit diffraction rings corresponding to the (100), (101), (110), (102), (111),

(201) and (103) directions of the hexagonal phase of selenium.

Figure 3.3 SEM image of Se (a) as prepared, (b) 100⁰C annealed and

(c) 200⁰C annealed samples

(a)

1µm

1µm

(b)

(c)

1µm

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Figure 3.4 (a) Transmission Electron Microscope and (b) Selected area

electron diffraction images of as prepared Se nanowires

(a)

(201)

(111)

(102)

(110)

(101)

(100)

(b)

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Figure 3.5 (a) Transmission Electron Microscope and (b) selected area

electron diffraction images of Se nanowires annealed at

200⁰C

(103)

(201)

(111)

(102)

(110)

(101)

(100)

(b)

(a)

70

3.2.4 Compositional Analysis

Energy dispersive X-ray analysis was used to determine the

composition of the selenium particles. EDAX pattern of the synthesized

selenium particles is shown in Figure 3.6. Weak signals from the O atoms

have been recorded along with the strong peak corresponding to Se atom.

EDAX analysis indicates the presence of O and they are very low. Thus the

analysis indicates that the obtained selenium particles are quite pure.

Figure 3.6 EDAX spectra of the as prepared selenium nanowire

3.2.5 Optical Studies

Figure 3.7 shows the effect of annealing temperature on the UV-Vis

absorbance of Se nanowires. The absorbance increases with increase in

annealing temperature. The red-shift in peak position corresponds to increase

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in particle size during the annealing of Se nanowires. Kamalesh Mondal et al

(2010) has reported the UV-Vis absorbance of selenium.

Figure 3.7 Optical absorption spectra of Se (a) as prepared, (b) 100⁰C

annealed and (c) 200⁰C annealed samples

3.3 CHARACTERIZATION OF COPPER NANOPARTICLES

3.3.1 Structural Studies

Figure 3.8 shows the XRD pattern of the as prepared, 200°C, 400°C

and 600°C annealed copper particles. The diffraction peaks at 2θ (degrees)

values 43.49°, 50.64° and 73.32° corresponds to the (111), (200) and (220)

planes of face-centered cubic structure of copper with lattice constant

a = 3.602Å and is in good agreement with the standard JCPDS data (JCPDS

Card No. 85-1326). The diffraction peak of oxide phase has not been

detected, indicating that pure copper has been obtained. The sharpness of the

diffraction peaks suggest that the product is well crystallized.

(a)

(c)

(b)

72

Figure 3.8 X-ray diffraction pattern of Copper dendrite like structure

(a) as prepared, (b) 200⁰C (c) 400⁰C and (d) 600⁰C annealed

samples

The crystallite size has been calculated and is found to be in the

range 36-98 nm for as prepared, 200⁰C, 400⁰C and 600⁰C annealed Cu

nanoparticles.

3.3.2 Raman spectroscopy

Raman spectroscopy is a powerful tool to investigate the structural

properties of nanoparticles. Figure 3.9 shows a typical Raman spectrum of the

as prepared copper particles. Ji-Ming Hu et all (1997) has reported the raman

spectrum of copper(II). The Raman spectrum of a Cu sample reveals two

Raman active bands. The two allowed modes of Cu crystal appear at 342 cm-1

and 667 cm-1.

10 20 30 40 50 60 70 80

(d)

Inte

ns

ity

(a

.u.)

2q (degrees)

(11

1)

(20

0)

(22

0)

(c)

(b)

(a)

73

Figure 3.9 Raman spectra of as prepared Copper dendrite like structure

3.3.3 Surface Morphology Studies

Surface morphology of the synthesized Cu particles was studied

using Scanning Electron Microscopy (SEM). Figure 3.10 (a, b) shows the

SEM image of the as-synthesized copper particles. The synthesized particles

are found to have dendrite like structures. For the growth of copper dendrites,

copper nitrate (Cu (NO3)2) has been added to the capping agent PVP. The

copper precursor capped with PVP forms the back bone of the copper particle

and the sodium borohydride used reduces copper nitrate to copper particle at

room temperature. The copper dendrite like structure are prevented from

aggregation into larger ones by the presence of PVP macromolecules that

could chemically adsorb onto the surfaces of copper nanoparticles. The small

copper nanoparticles start to dissolve into the solution and grow onto large

nanoparticles of copper via a process known as Ostwald ripening. With the

assistance of PVP, these large copper nanoparticles were able to grow into

dendrite shaped structures. One possible function for PVP is to kinetically

control the growth rate of various faces by interacting with these faces

through adsorption and desorption. The slow dissolution of the small copper

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particles into the solution might also play a certain role in achieving an

anisotropic growth of the copper particles.

Figure 3.10 (a, b) SEM image of the as-prepared copper dendrite like

structure

The SEM image clearly indicates that the copper particles have

many multi-armed petal like structures arranged along the central axis. The

length of the petal is about 2µm and the central axis is about 10µm. The

structure shows that many short separated petals have grown from the central

axis and are arranged in an alternative manner to one above the other on

opposite sides to each other. The growth occurs in the central axis meanwhile

the lower petal also starts to develop and forms an inverted cone shape. The

10 μm

(a)

(b)

5 μm

75

structure resembles to that reported by (Chaoquan Hu et al 2007) where petals

with non porous and central trunk swelled with a sponge structure was

observed. Thus, it resembles the simple plant growth mechanism.

A possible growth process that occurred during the synthesis of

copper dendrite like structure is shown in Figure 3.11. In this it is clearly

revealed that small copper nanoparticles were formed by the addition of

sodium borohydride and when PVP is added to this the small particles get

enlarged to form rods. When the reaction rate increases with time these rods

form a dendrite like structure.

Figure 3.11 Schematic description of the growth process of hierarchical

copper dendrite like structures

During the growth process, sodium borohydride and PVP play a

crucial role in the formation of these morphologies. To understand the growth

mechanism of the copper dendrite like structure to copper tubular like

structure, time-dependent experiments were carried out by annealing the

particles at different temperatures 200°C, 400°C and 600°C for 1 hour. The as

prepared copper has been annealed at different temperatures 200°C, 400°C

and 600°C for 1 hour to get different structures of copper. Due to the increase

76

in annealing temperature the crystalline nature of particles gets changed such

that the structure enlarges and results in the formation of different structures.

Figure 3.12 (a, b) shows the SEM image of Cu annealed at 200⁰C. The SEM

image reveals that the copper dendrite like structure transforms to a randomly

oriented copper cactus like structure with less multi-armed needle like

structures arranged along a sponge structure with high porosity. The length of

the petal is about 4µm to 5µm. The structure shows that the central trunk

present in the dendrite has transformed into sponge like structure with

increase in size and has swelled. When the temperature is increased, it is clear

that a great number of pores appear along the trunk with enlargement of the

trunk and also there is reduction in the size of the petals leading to the

formation of needles without pores.

Figure 3.12 (c, d) shows the SEM image of Cu annealed at 400⁰C.

The SEM image reveals that the randomly oriented copper cactus like

structure has transformed to copper honey comb structure having many pores.

The length of the petal is about 3.8µm to 4.7µm. When there is increase in the

temperature, there is an increase in the diameter of the pores present on the

enlarged trunk and also there is disappearance of the needle like petals.

Figure 3.12 (e, f) shows the SEM image of Cu annealed at 600⁰C. It shows

that the copper honey comb like structure has transformed into a tubular

structure. The length of the tubular part is about 1 µm.

77

Figure 3.12 SEM images of copper (a, b) annealed at 200°C (c, d)

annealed at 400°C (g, h) annealed at 600°C

(a) (b)

(c) (d)

(e) (f)

78

Figure 3.13 Schematic description of the growth process of hierarchical

copper structures

Figure 3.13 shows that the growth process of the porous dendrites,

cactus, and honey comb and tubular like structures is similar to that of the

nonporous dendrites. Herein, we choose the nonporous dendrites as the model

because the reaction rate of it is relatively slower than that of porous dendrites

and different stages can be clearly obtained. With increase in the temperature,

copper dendrites are transformed into cactus, and honey comb and tubular like

structures with various diameters. The possible growth process demonstrating

the transformation of copper dendrite like structure to tubular like structure

with annealing temperature is as described schematically as shown in

Figure 3.13.

Figure 3.14a shows the Transmission Electron Microscope (TEM)

image of as-prepared copper dendrite like structure. The image confirms the

formation of dendrite structure. The image shows that the dendrite like

structure has many multi-armed petal like structures arranged along the

central axis. The length of the petal is about 2µm and the central axis is about

10µm. The structure shows that many short separated petals have grown from

the central axis and are arranged in an alternative manner as one above the

other on opposite sides to each other.

As prepared 400 ⁰C

600 ⁰C

⁰C

200 ⁰C

79

Figure 3.14 (a) Transmission Electron Microscope images and (b)

selected area electron diffraction image of as prepared

copper dendrite like structure

(a)

(b)

(220)

(200)

(111)

80

The result is consistent with the result obtained from SEM. Figure

4b shows the selected area electron diffraction (SAED) pattern of the as-

prepared copper dendrite like structure. Selected area electron diffraction

image (Figure 3.14b) exhibit diffraction rings corresponding to the (111),

(200) and (220) directions of the face-centered cubic structure of copper. The

d spacing values obtained for all the diffraction rings from the SAED pattern

match very well with that of face-centered cubic structured copper.

3.3.4 Compositional Analysis

Energy dispersive x-ray analysis was used to determine the

composition of the copper particles. EDAX pattern of the synthesized copper

particles is shown in Figure 3.15. Weak signals from the C and O atoms have

been recorded along with the strong peak corresponding to Cu atom. EDAX

analysis indicates the presence of C and O and they are very low. Thus the

analysis indicates that the obtained copper particle is quite pure.

Figure 3.15 EDAX spectra of copper

81

3.3.5 Optical Studies

Figure 3.16 Absorption spectra of Copper dendrite like structure (a) as

prepared, (b) 200⁰C, (c) 400⁰C and (d) 600⁰C annealed

samples.

The absorption spectra of copper dendrite like structure is shown in

Figure 3.16. Absorbance band at about 560 nm, was observed in the spectra.

The absorption edge of Cu dendrites shifts towards longer wavelength with

increase of annealing temperature. The shift of the absorption edge to the

longer wavelength range is due to the fact that the grain size increases with

increasing of annealing temperature. Thi My Dung Dang et all reported the

UV-VIS absorption spectra of copper. The red shift of absorption edge can be

related to the grain size increasing.

(a)

(b)

(c)

(d)