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CHAPTER - V

5. Effect of Calcination Temperature on Surface Morphology and Size of

Ionic liquid Capped ZnO Nanoparticles by Sol-gel Method

5.1. Introduction

A versatile smart, multifunctional and semiconductor ZnO material has attracted

considerable attention due to its widespread applications [1, 2]. The sol-gel method for the

preparation of zinc oxide nanoparticles has its unique advantages such as simple, fast, better

homogeneity, controlled stoichiometry and easy industrial production [3]. Most of the

organic solvents employed for the synthesis are highly toxic, flammable and even explosive.

Ionic liquids (ILs) which has no negative environmental and safety problems like that of

volatile organic solvents are explored and received much attention in inorganic synthetic

protocols [4].

Among the various types of ionic liquids, imidazolium based ILs is the most

commonly used [5]. Thermal annealing is widely employed to improve the crystal quality and

to study the structural defects of nanomaterials. Detailed investigations are required to

explain the grain size effect on the temperature dependent behavior, which is very much

essential from the technological perspective. Because the particle size and annealing

temperature have a close relation, with increases of annealing temperature, particle size

increases [6-8]. Annealing process may be responsible for the occurrence of dislocation and

adsorption/decomposition on the surface, thus the structure and stoichiometric ratio of the

material would change [9]. There are many reports on the effect of calcination temperature on

zinc oxide synthesis [10], but the same effect was not yet reported in IL capped synthesis.

Hence in the current study an effort was taken to study the change in size, morphology and

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structure of ZnO under various calcination temperatures in [BMIM] BF4 ionic liquid capped

pathway. A comparative study is taken into account between ionic liquid and conventional

solvent water in ZnO nanoparticles synthesis. The Zn(OH)2 were subsequently annealed at

different temperatures (100, 300 & 500°C) for studying the effect of thermal treatment on the

surface morphology and the size of the ZnO nanoparticles. Stabilization of ZnO nanoparticles

is attained by capping their surface with the organic ligand of ionic liquid. Structural, thermal

behaviour and morphological properties of the synthesized nanoparticles were characterised

by XRD, TGA and SEM with EDX analysis. The schematic representation of ionic liquid

capped ZnO nanoparticle synthesis under different calcinations is given below (Fig. 5.1).

Figure 5.1: Schematic representation of calcination effect on ZnO nanoparticles

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5.2. Results and Discussion

5.2.1. Phase structure of ZnO nanoparticles by XRD

The phase structure and crystallinity of ZnO nanoparticles synthesised using ionic

liquid and conventional solvent at different calcination temperatures are characterised from

XRD analysis are shown in Figure 5.2.1(A) & 5.2.1(B). XRD pattern of synthesized ZnO

nanoparticles with two different solvents depicts a set of diffraction peaks corresponding to

(100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) planes in close

correlation with lattice planes of hexagonal wurtzite structure of ZnO. In both cases, sharp

peaks with prominent intensity reflect the high crystallinity in ZnO nanoparticles and the

detectable peaks matches well with the JCPDS card (36-1451). All of the characteristic peaks

of ZnO lie in the range of 20°<2θ<80° it further confirms the form of metal oxide (ZnO). The

crystallite size of ZnO nanoparticles calculated using Scherrer’s equation are 28, 34 and 41

nm with ionic liquid and 43, 52 and 59 nm with water solvent under the calcination

temperature 100, 300 and 500°C respectively and tabulated in Table 5.2.1.

Generally elevation of increasing calcination temperature produces increase in

particle size due to aggregation and also unevenly sized particles. Crystallite size of ZnO

nanoparticles increases gradually by the elevation of calcination temperature in ionic liquid

compared to conventional solvent as medium. When ionic liquids (ILs) are used as template,

the nucleation rate is increased due to their low interface tension and it has control over the

shape of each particle. Hence calcination of ZnO prepared in ionic liquid (IL) template

produced controlled size nanoparticles of definite crystallinity with identical shape and size.

The XRD pattern of both solvents emphasizes the purity of ZnO nanoparticles by the absence

of unidentified peaks. Increase of crystallinity with increasing calcination temperature was

75

observed by the increase of intensity of the diffraction peaks thereby confirms the statement

of previous reports [6].

5.2.2. Morphology of ZnO nanoparticles by SEM

The particle size and morphology change between the low and high calcination range

of ZnO nanoparticles prepared in two different solvents are investigated by scanning electron

microscopic analysis. SEM images of ZnO nanoparticles prepared with conventional solvent

at different temperature were recorded under two different magnifications are displayed in

Figure 5.2.2(A). The structure of spherical shaped granular particles without much

agglomeration is received for the ZnO at 100°C. When temperature increases to 300°C and

500°C, the spherical shaped morphology changed to irregular and uneven granular particles

without any boundary due to the lack of aptitude to control agglomeration of particles by the

conventional solvent (water). SEM images of ZnO nanoparticles prepared in [BMIM] BF4 is

depicted in Figure 5.2.2(B) [a = 100°C, b = 300°C and c = 500°C]. At 100°C, the morphology

of ZnO nanoparticles exhibit tiny aggregative granule shaped nanoparticles, whereas the

temperature increased to 300°C and 500°C, the morphology changes to ball like uniform

shaped particles and capsules shape nanoparticles with very fine agglomeration respectively.

The average particle sizes of individual ZnO nanoparticles from the SEM images based on

the scale bar provided are around 50, 100, and 150 nm for 100, 300 and 500°C, respectively.

5.2.3. Thermal stability of ZnO nanoparticles by TGA

The relative weight loss of the zinc hydroxide precursor synthesized without and with

ionic liquid during the heat treatment was studied by thermogravimetric analysis. The TG

analysis was carried out from room temperature to 800°C and depicted in Figure 5.2.3. From

the TGA curve of zinc hydroxide in water solvent (a), the intact weight loss of about 16%

associated with the heating process occurred in three phases is understood. A significant

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weight loss of about 8% happened upto 366°C is envisaged for the removal of water which is

adsorbed on the surface of the material. The second stage weight loss of about 7% occurred

between 366°C to 400°C can be attributed to the complete crystallisation of zinc oxide from

the amorphous nature. Oxidation of residue compounds is occurred above 400°C with the

accompanying weight loss of 1%.

The TGA curve of the zinc hydroxide precursor with ionic liquid (b) presents three

stages upto 800°C is given in Figure 5.2.3. The first stage weight loss (5%) between room

temperature to 250°C associated with the elimination of water molecules and surface

hydroxyl groups on the synthesized particles. A sharp down fall weight loss of about 25%

observed in the second stage from 250°C to 350°C may be due to the crystallisation and

formation of zinc oxide nanoparticles. Further weight loss of about 10% may be happened

due to the decomposition of ionic liquid, [BMIM] BF4 between 350°C to 623°C. There is no

reasonable weight loss above 600°C.

5.2.4. Energy dispersive x-ray spectroscopy

The EDX spectrum of ZnO nanoparticles synthesized at different calcination

temperatures by employing the conventional and ionic liquid solvent were recorded along

with the SEM measurement are shown in Figure 5.2.4(A) & 5.2.4(B). The formation of ZnO

nanoparticles were confirmed by the presence of Zn and O peaks in both solvents at all

investigated temperatures. The significant difference in the EDX spectrum of ZnO

nanoparticles in ionic liquid capped medium is the peak corresponding to carbon along with

Zn and O, which could be from the carbon skeleton of imidazolium cation. The presence of

carbon atom peak elucidates the capping of ionic liquids on the synthesized ZnO at all

calcination temperatures even at higher temperature of 500°C.

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5.2.5. Formation of ZnO nanostructures

The reason behind the controlled size particles with well defined morphology at all

calcination temperature ranges are based on the spectral features of the ionic liquid as

template. Nucleation and growth of crystal was controlled by the ionic liquid. IL itself is

existing as cations and anions as follows [(BMIM)x (X)x-n]n+

cations and [(BMIM)x-n (X)x]n-

anions where X= BF4-. The anion surrounds the [Zn(OH)4]

2- by coulombic force of attraction

which polarises the precursor and facilities dehydration to form ZnO nanoparticles.

Meanwhile the negatively charged hydroxyl ion of [Zn(OH)4]2-

repulse the anionic part of

ionic liquid, in order to counter act the repulsion cationic part forms the second layer over the

anionic part of ionic liquid followed which ZnO nanoparticles formed by the dehydration of

the precursors. In the presence of ionic liquid the anisotropic growth of ZnO is totally

controlled by capping the newly formed ZnO particle with cation of IL which could not

provide sites for other ZnO particles to occupy interstitial sites, therefore further growth is

totally restricted it helped to get a controlled size discrete nanoparticle of well-defined

geometry. Generally size of nanoparticles increases with increase of temperature due to the

aggregation of individual particles. The same case occurs in our study also but due to the

capping of ionic liquid on the surface of ZnO nanoparticles, increase of size occurs in a

controlled rate due to the thermally robust property of ionic liquid.

ZnO nanoparticles are having size and shape dependent characteristics [11]. For example,

50-150 nm was used as a sunscreen additive to absorb UV light and enhances the sun

protection and with capsule shape was used in biomedical applications and sensing devices

[12, 13]. Therefore the preparation of ZnO nanoparticles of varied size in discrete state with

specified geometry such as granules, balls and capsule shapes would fulfil the need in

different areas of application.

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5.3. Conclusion

In this study the effect of temperature on the size and morphology of ZnO

nanoparticles synthesized by conventional and ionic liquid solvent were reported. The

morphology of zinc oxide nanoparticles synthesized with conventional solvent changes to

irregular and uneven surface from the spherical shaped granules with increasing calcination

temperature. Use of ionic liquid as solvent in this investigation provides environmentally

benign and greener route and also tune the size and shape of nanocrystals. The use of

additives as shape directing/templating agents can be eliminated by employing ionic liquids

due to its multifunction such as template, solvent, templating agent and reactant during the

particle formation process. Specifically interesting particles including granules, balls and

capsule shapes of ZnO can be synthesized by this route. The special quality of ionic liquid

induces to act as entropic driver for spontaneous, well defined and extended ordering of

nanoscale materials. An ionic liquid control the crystal growth and collects the nanoparticles

just by condensed manner. Ionic liquids replace the use of harmful solvent and stabilizing

agent in inorganic synthesis of nanoparticles. This study concludes that this method opens up

a new way for preparing nanoparticles of metals, metal oxides, metal alloys, semiconductive

metals and their alloys.

79

References

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Journal of Pharmaceutics, 434, 374 (2012).

[2]. Z. Jijun and C. Zhongfang, Journal of Computational and Theoretical Nanoscience, 8,

2397 (2011).

[3]. S. Maensiri, P. Lackul and V. Promarak, J Cryst Growth, 289, 106 (2006).

[4]. M. Ramalakshmi and M. Sundrarajan, Asian journal of chemistry, 25, 3081 (2012).

[5]. M. Ramalakshmi and M. Sundrarajan, Materials research bulletin, 48, 618 (2012).

[6]. M. Hamadanian and V. Jabbari 5th SASTech 2011, Khavaran Higher-education

Institute, Mashhad, Iran. May 12-14.

[7]. Kuan Y. Cheong, Hartini Hussin and Khairul Ismail, ECS Trans, 3, 115 (2006).

[8]. C. C. Vidyasagar, Y. Arthoba Naik_, T. G. Venkatesha and R. Viswanatha, Nano-Micro

Lett. 4, 73 (2012).

[9]. L.C. Nehru1 M. Umadevi1 and C. Sanjeeviraja, International Journal of Materials

Engineering, 2, 17 (2012).

[10]. Z. Qidong and X. Tengfeng, J Phys Chem C, 111,17145 (2007).

[11]. D. Wei1 and H.E. Unalan, Nanotechnology, 19, 424006 (2008).

[12]. L. Wang and M. Mohammed, J Mater Chem, 9, 2871 (1999).

[13]. M. Movahedi, E. Kowsari, and I. Yavari, Material Lettrs, 31, 3856 (2008).

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Legends

Table

Table 5.2.1: Size of ZnO nanoparticles at different calcination temperatures from XRD

Figures

Figure 5.1: Schematic representation of calcination effect on ZnO nanoparticles

Figure 5.2.1 (A): XRD pattern of ZnO nanoparticles synthesized via water solvent at different

calcinations temperature (100-500°C)

Figure 5.2.1 (B): XRD pattern of ZnO nanoparticles synthesized via ionic liquid at different

calcinations temperature (100-500°C)

Figure 5.2.2 (A): SEM images of ZnO samples prepared via water solvent at different

calcinations temperature (100-500°C)

Figure 5.2.2 (B): SEM images of ZnO samples prepared via ILs at different calcinations

temperature (100-500°C)

Figure 5.2.3: TG analysis of ZnO nanoparticles a) without IL and b) with IL

Figure 5.2.4 (A): EDX spectra of water mediated ZnO samples prepared at different

calcinations temperature (100-500°C)

Figure 5.2.4 (B): EDX spectra of ionic liquid mediated ZnO samples prepared at different

calcinations temperature (100-500°C)

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Table 5.2.1: Size of ZnO nanoparticles at different calcination temperatures from XRD

Solvent

Calcination temperature (°C)

FWHM

Peak (deg)

Mean particle size (nm)

Water

100

300

500

0.1900

0.1580

0.1400

43

52

59

[BMIM]BF4

100

300

500

0.3800

0.2789

0.2331

22

28

34

82

Figure 5.2.1 (A): XRD pattern of ZnO nanoparticles synthesized via water solvent at

different calcinations temperature (100-500°C)

100C

(10

0)

(00

2)

(10

1)

(10

2)

(11

0)

(10

3)

(20

0)

(11

2)

(00

4)

(20

2)

Inte

ns

ity

(a

.u)

300C

10 20 30 40 50 60 70 80

500C

2

83

Figure 5.2.1 (B): XRD pattern of ZnO nanoparticles synthesized via ionic liquid at

different calcinations temperature calcinations temperatures (100-500°C)

10 20 30 40 50 60 70 80

500°C

2

(20

2)

(10

3)

(00

4)(1

12

)

(20

0)(11

0)

(10

2)

(10

1)

(00

2)

(10

0)

100C

3000C

In

ten

sit

y (

a.u

)

84

Figure 5.2.2(A): SEM images of ZnO samples prepared via water solvent at different

calcinations temperatures (100-500°C)

85

Figure 5.2.2 (B): SEM images of ZnO samples prepared via ILs at different calcination

temperatures (100-500°C)

86

100 200 300 400 500 600 700

84

88

92

96

100

366.30C

233.07CW

eig

ht

(%)

Temperature (°C)

95.00C

a) Zn(OH)2 precursor without ionic liquid

100 200 300 400 500 600 700

60

70

80

90

100 b) Zn(OH)2 precursor with ionic liquid

622.59C

347.17C

95.00C

Weig

ht

(%)

Temperature (°C)

Figure 5.2.3: TG analysis of ZnO nanoparticles a) without IL and b) with IL

87

Figure 5.2.4 (A): EDX spectra of water mediated ZnO samples prepared at different

calcination temperatures (100-500°C

88

Figure 5.2.4 (B): EDX spectra of ionic liquid mediated ZnO samples prepared at

different calcination temperatures (100-500°C)