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Reaction temperature dependent shape-controlled studies of copperoxide nanocrystals To cite this article before publication: Janki Shah et al 2018 Mater. Res. Express in press https://doi.org/10.1088/2053-1591/aacacc

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Oxide nanocrystals

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Reaction temperature dependent shape-controlled Studies of Copper-

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Janki Shah1, Mukesh Ranjan2, Sanjeev K. Gupta3*, A. Satyaprasad2, Sunil Chaki4 and Yogesh Sonvane1* Advanced Materials Lab, Department of Applied Physics, S.V. National Institute of Technology, Surat 395007, India 2 3

FCIPT, Institute for Plasma Research, Sector-25, Gandhinagar 382044, India

Computational Materials and Nanoscience Group, Department of Physics, St. Xavier's College, Ahmedabad 380009, India

P. G. Department of Physics, Sardar Patel University, Vallabh Vidyanagar, Gujarat 388 120, India

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Abstract

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In the developing era, metal oxide nanomaterials are intensively pursued due to their prominence application in different applied and technological fields. The transition metal oxide, Copper oxide is a dominant candidate for magnetic storage devices, sensor, and solar energy transfer as a heat absorber, super capacitors and mainly as a good catalyst in chemical reactions. Here, CuO nanostructures with different shapes (nanoparticle, cubelike, rectangular, nanobar and nanorod) are synthesized by precipitation method from CuCl2 precursors. The CuO all structures are characterized by X-ray

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diffraction for the structural study. CuO different shapes morphological phenomena are carried out from SEM and TEM. The thermal properties are calculated by recording thermo-curves, viz. thermogravimetric (TG), differential thermogravimetric (DTG). Thermogravimetric analysis revealed CuO all structures show weight loss at 340K to 380K and 1000K to 1250K region because of water evaporation and combustion of organic compounds respectively. Activation energy, Arrhenius factor,

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activation enthalpy, activation entropy and Gibbs free energy for the decomposition of CuO were determined using the Coats-Redfern (CR) method for all shaped structures. Keywords: CuO, Shape-controlled, SEM-TEM, Thermogravimetric analysis, Coats-Redfern (CR) method

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Introduction In the universe, there are numbers of metal oxides available in nature, but some of them are most useful in accordance with their applications of day to day life in science and technology. Metal oxides

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play a significant role in interdisciplinary areas of physics, chemistry, and materials science [1–6]. The metal elements show a large multiplicity of oxide compounds [7]. The structural geometry along with the electronic structure of the compound can be changed to distinguish it as a metal, semiconductor or insulator. Oxide materials are used in many technological applications like sensors, fabrication of microelectronics circuits, piezoelectric devices, and coatings for the passivation of surfaces against corrosion, fuel cells and as catalysts. As an example, catalysis is mostly used in all industrial applications based on oxide materials as a supporter or active phase of the system. The product of billion

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dollar value produced every year in chemical and petrochemical industries by the use of oxides and metal-oxides catalysis [8]. Catalysis or sorbents which contain oxides are very much useful to control environmental pollution by removing CO, NOx, and SOx compounds produced throughout the

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combustion of fossil-derived fuels [9,10]. Besides, the most active field of the semiconductor industry involves the use of oxides [11]. Thus, most of the chips used in electronic components contain an oxide material like CuO because of high-Tc superconductors and giant magnetoresistance of material [12]. CuO has been used for the preparation of organic-inorganic nanostructured due to its novel behavior as high electrical and thermal conductivity as well as high temperature durability [13]. Hence, various morphologies of CuO nanostructures have been fabricated for the fundamental and practical importance

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[14].

Oxide nanoscaled materials show different physical and chemical properties depending upon their size and shape morphologies [5,15–20]. The size and shape controlling morphologies of nanomaterials is one of the important, challenging and effective ways to obtain properties [21–26]. CuO has a favorable bandgap which is very much useful for solar energy conversion and it could be used in

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solar cell window preparation. As another application in nanofluid, it acts as a coolant in refrigerators and mixing with carrier it can be used as a thermal heat transfer to enhance productivity of any engineering system like TiO2, Al2O3 etc. There are many reports on to synthesis in different shapes like cube, rod, prisms, wires using different methodologies [27,28]. At the nanoscale regime, the properties

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of materials are strongly affected by their shape and dimensions. It is expected that various shaped CuO 2

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synthesized and multidimensional nanostructured material is valuable and more attractive to study its intrinsic characteristics for use in existing applications. In recent years, the shape-controlled synthesis of nanostructured CuO has attracted significant interest [29–35]. Among CuO nanostructures, many works

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are reported on 1D nanomaterials [32,33,36–38]. Recent research shows that 2D and 3D structure of CuO nanomaterials is widely focused by different synthesis techniques [39,40]. Highly dimensional structures of CuO are prepared by many scientists, for examples Hsieh’s group synthesized well-ordered CuO nanofibers in large scale production on the basis of self catalytic growth mechanism [41]. Zeng’s group has successfully fabricated mesoscale organization of CuO nanoribbons [29,30]. Yang’s group has synthesized CuO nanoribbons arrays on a copper surface [32,42,43].

In the present work, CuO nanostructures with different shapes (nanoparticle, cubelike,

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rectangular, nanobar and nanorod) are synthesized through a very basic synthetic route, which involved the precipitation of copper salts with concentrated NaOH solutions at room temperature. Here, different shapes of CuO nanocrystals were prepared by a traditional precipitation method of controlling synthesis

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conditions. The resulted X-ray diffraction (XRD) shows the dehydration of Cu(OH)2 into crystallite CuO after calcined at 450°C/4h. The structural analyses of different shapes are observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). CuO nanocrystals had moderately equiaxed shape with average crystallite sizes ranging from 22-27 nm. The results indicate that shape-controlled synthesis of nanomaterials may present a great opportunity for the design of catalysts with desired properties. In this manuscript, the authors present the thermal properties study of CuO with different shapes. The thermal properties study was carried out by recording thermo-curves,

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viz. thermogravimetric (TG) and differential thermogravimetric (DTG). The kinetic nonmechanistic Coats-Redfern (CR) method was employed to calculate the thermal kinetic parameters of the CuO nanostructure samples.

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1. Synthesis of CuO nanocrystals with different shapes

Copper oxide nanocrystals were synthesized by precipitation method using different crystal growth heating temperature. The copper chloride precursor was dissolved in 100ml distilled water to make 0.1 M concentration. The next step is (0.5M) NaOH solution was drop wise added into aqueous solution

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under vigorous stirring by magnetic stirrer (2h) until pH reached to 14. Black precipitates were obtained 3

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in mother solution. The mother liquor is kept at different room temperatures. After that, final product was collected by filtration (Whatman No. 1 filter papers), and washed with deionized water to remove any possible ionic remnants, till pH reached by 7. Subsequently, the washed precipitates were heated at

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different reaction temperature and calcined all the samples at 450°C/4h. All synthesis conditions for different nanocrystal shapes are mentioned in below Table 1.

Table: 1 Comparative study for different size and shapes of the synthesized CuO nanocrystals with respect to same precursor (CuCl2) having different synthesis conditions. Shape & Avg. crystallite size of Nanocrystals

Synthesis conditions

1

Room temp 24h Reaction temp: 60°C/12h Calcined: 450°C/4h

2

Room temp 15h Reaction temp: 80°C/12h Calcined: 450°C/4h

Cubelike 23.09 nm

3

Room temp 10h Reaction temp: 110°C/12h Calcined: 450°C/4h

Rectangular 26.08 nm

Room temp 8h Reaction temp: 160°C/12h Calcined: 450°C/4h

Nanobar 26.92 nm

Room temp 5h Reaction temp: 180°C/12h Calcined: 450°C/4h

Nanorod 23.59 nm

Nanoparticles 22.06 nm

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Sample

The results of this study have shown that the morphology of the nanocrystals is strongly influenced by the reaction temperature. The particle size and shape is most affected parameters in synthesis process where molecular weight determines the propagation and termination. As the temperature increase in

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synthesis process as mentioned in our work, the termination rate also increases and the molecular weight decreases. While increasing temperature the time viscosity decrease of the system and Brownian motion of droplets enhanced, so rate of collision of particle accelerated and particle size increased.

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Synthesis Reaction Mechanism

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In aqueous solution, when copper salt CuCl2 dissolved in distilled water it dissociates into [Cu(H2O)6]2+ ions which is responsible for the sky blue color and Cl- anions. These ions are partially coordinated with copper ions in [Cu(H2O)6]2+ where six water molecules completely surrounded by Cu2+ ion. The

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availability of the copper ion in the system is more favoured as compared to water is weak [44]. At higher then room temperature, CuO is formed as below reactions [45]: Cu (aq)+2OH (aq)  Cu(OH) (aq) +

-

2

(1)

Cu(OH) (aq)+2OH (aq)  Cu(OH) (aq)  CuO(s)+2OH (aq)+H O -

2-

2

(2)

-

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2

Nanocrystals shaped within reverse micelles undergo further growth or aggregation yielding particles “larger” than their initial nanodroplets which can be resulted as multimodal distribution.

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When copper salt is solvating in water, four water molecules bounded the Cu 2+ to make square structure 2of Cu(OH) 4 and remaining two of them are placed on axis [46,47].

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The mechanism and formation of different shaped nanocrystals of CuO (e.g. nanocube, nanorod, nanowires, rectangle-triangle shaped etc) can be explained by the difference in crystal growth temperature rates and various directions of growth. The reaction temperature is most important parameter to affect crystal growth process. Addition to that the CuO nanocrystals formation is also followed by these reaction steps,

Cu Cl (aq)+2Na OH (aq)  Cu(OH) (aq)+2NaCl(aq) +2

-

+

-

(3)

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2

Cu(OH) (aq)+2OH (aq)  [Cu(OH) ] (aq) -

2

(4)

2-

4

[Cu(OH) ] (aq)   CuO(s)+2OH (aq)+H O 2-

(5)

-

heat

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2At lower temperature, when NaOH is added in aqueous solution the hydroxyl group of Cu(OH) 4 will

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be formed and make hydrogen bonds by interconnections. The direction growth would be leading to formation of all shaped of nanocrystals [48,49]. Though, close to room temperature (T ≤ 25°C) very less hydrogen bonds are destroyed. The remaining hydrogen bonds will lead to form a structure with mixed morphologies. When crystal growth temperature is increased (25°C < T < 180°C), the respected an

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equivalent increase in the nucleation and growth rates as well as the destruction of more hydrogen bonds. So for additional analysis on effect of reaction temperature 60°C to 180°C on particle size, shape, 5

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morphology and thermal properties will lead to a better understanding of the growth mechanism of the as-produced CuO nanostructures [50].

XRD

The XRD pattern of CuO nanopowders (with different reaction temperature) is plotted below as a Fig.1. X-ray diffraction is a basic process which gives details about crystal structures of the synthesized nanocrystals. The monochromatic CuKα radiations of wavelength λ = 0.15418nm is utilized in XRD as a source of energy 40 kV/35 mA and the graph is recorded in the range of 20º-80º 2θ. X-ray diffraction

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analysis (Fig.1) shows that all diffraction peaks are indexed to be monoclinic crystal structure of all structure of CuO with negligible traces of Cu(OH)2 residual by reason of reaction time. The broadening of the diffraction peaks suggests a nanometer scale crystallite domain size. XRD graph reveals that crystallite phases were obtained in synthesis condition after calcinations temperature (450°C/4h) with

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enhanced structural properties. So, at enough higher calcinations temperature 450°C/4h the intensity of the diffraction peak of CuO become stronger, which shows that the crystallization of copper oxide is complete and more inclusive due to agglomeration. While removing impurities from samples, the sharpe diffraction peaks were obtained with high intensity. XRD data of all samples (samples 1-5) with different reaction time compared with Joint Committee on Powder Diffraction Standards (JCPDS) card no. 01-80-1916. All diffraction peaks are denoted in below XRD graph and lattice parameters are

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a=4.69, b=3.42, c=5.13 and miller indices are identified.

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2. Results and discussion

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Fig.1 XRD graphs for different shapes of CuO nanostructure having different reaction temperatures.

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The average crystallite size is calculated from Debye-Scherrer formula (6) [51],

D

0.89  cos 

(6)

Where, ‘D’ denotes the average crystallite dimension which is perpendicular to the specimen surface, ‘0.89’ is the Scherer’s constant, ‘λ’ is the X-ray wavelength, ‘β’ indicates the full width at half

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maximum (FWHM) intensity and ‘θ’ is the Bragg’s angle. By applying Debye-Scherrer formula (6), the average crystallite size of nanocrystals is in the range of 22-27nm calculated. By changing reaction temperature of all samples of CuO, the position of their diffraction peaks are remaining same and no remarkable shifts are observed. In general, the diffraction peak pattern is same for all the CuO

2.2

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structures, but the reaction temperatures and shapes are different. Morphological studies of CuO nanocrystals The different morphologies of CuO nanocrystals were examined here by Scanning Electron

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Micrographs (SEM) and Transmission Electron Microscopy (TEM) which is shown below as Fig.2 (15). All the samples are prepared via precipitation method using CuCl2 copper salt with different reaction 7

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temperature. The microstructure- based texture properties are affected by changing experimental conditions like concentration, surfactant, reaction temperature time, heating rate etc. Reaction temperature is most affected parameter to nucleation growth. Increment in temperature causes

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coagulation of grain boundaries which tends to formation of clumps in structure [52]. As the reaction temperature increases the product tends to get agglomerate and started to increase the size from nanoparticles to nanorods and clustered were getting prepared from CuCl2 salt [53]. Here we have shown SEM images at 100-300nm range and TEM images at 20-500nm range with average crystallite size of 22-27nm.

The properties of nanomaterials are mainly depending on particle size, shape, morphology and specific surface area of synthesized sample. Some aspects strongly depend on the synthesis methods.

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Nano materials are separated from the bulk materials on the basis of surface to volume ratio. In nano scale sample preparation we observed that the size and shape quantization effects have lots of influence on the material properties [54]. Suleiman M, et al. [55] noted that CuO nanoparticles synthesized by different methods and different synthesis conditions such as solvents, methods, surfactants, reaction time

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and temperature which are useful terms to synthesis different size and shapes of CuO nanoparticles. Size and shape of clusters are affected by primary concentration of copper salt, reaction time and temperature. Ayask HK et al. made copper oxide nanoparticles by mechano-chemical process using copper hydroxide [56]. Wua R, et al. [57] investigated the effect of changing processing parameters on size and shape of CuO particles by changing temperature, molar ratio and nucleation and growth kinetics are also discussed. They prepared highly dispersed copper oxide nanoparticles having different

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morphological shapes like spherical-shaped, spindly-shaped and rod-shaped CuO particles. Study of different shapes synthesis process and affected basic parameters for CuO nanoshapes are reviewed by

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Singh et al. [58].

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Fig.2 SEM and TEM images of different shaped CuO nanostructures with different reaction

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temperatures.

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The effect of different reaction temperature on the formation and crystal growth of copper oxide nanopowders are investigated here after calcinated at 450°C/4h. Fig.2 (1) shows formation and crystal growth process of CuO nanoparticles at reaction temperature 60°C/12h with average crystallite size

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22.06nm. As reaction temperature increases to 80°C/12h, the clusters started to grow larger into cubelike structure (Fig.2 (2)) which having 23.09nm average crystallite size. A few rectangular shapes with 26.08nm emerge in the sample after 110°C/12h shown in Fig.2 (3). After reaching reaction temperature at 160°C/12h the crystal growth is started in particular direct and nanobar Fig.2 (4) with 26.92nm sized structure is formed. Even at 180°C/12h reaction temperature we observed nanorod like structure as shown in Fig. 2(5) with 23.59nm average crystallite size of structure.

A TEM images Fig 2 shows the presence of different size and shapes of distribution network of

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nanostructures with agglomeration in the range of 22-27nm. Figure 2(1) shows CuO nanoparticles with irregular shapes of 22.06nm crystallite size and agglomeration can be observed. This shape is mainly depend on synthesis temperature and copper salt precursors [59,60]. Wang et al. [61] synthesized CuO and Cu2O irregular shapes nanoparticles with particle size around 31-32nm which is also comparable

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with our analysis. Sun and Xia [27] mentioned the same reaction temperature concept with silver nanoparticles by reduction of silver nitrate salt. Their results conclude that morphology of synthesized samples is strongly depended on reaction conditions mainly reaction temperature. Another work is reported on different shapes of silver nanoparticles using chemical methods. In this work Wiley et al.[62] controlled reduction ratio by different reaction temperature. In their method, PVP surfactant is used as a stabilizer to prevent the aggregation of nanoparticles, act as a reducing agent and also as a

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substance to control the shape of nanoparticles.

In our experiment, as we change reaction temperature 80°C/12h the shape of nanostructure is going to change particles to cubelike shape after calcinations sample at 450°C/4h. Another way to synthesis cubelike structure is to increase concentration and it started to nucleate and grow into nanostructure [63,64]. Kim et al.[65] synthesized cubic-Cu2O by chemical synthesis with the help of

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PVA surfactant of and showed same cubelike structure. Ghosh et al. [66] suggested in calcinations process transformation of C2CuO4·nH2O to CuO take place but shape morphology remains same. But major contribution for shape changing is heating temperature when it started to shell formation. They observed same cubelike CuO shape about 8-15nm in TEM and the self assembled smaller particles are

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observed in SEM with cubelike agglomeration. Zeng et al. [67] worked on different method to change 10

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shapes of morphology. They changed the capping agents to form cubelike shape of silver nanoparticles with PVA surfactant.

Here after calcinations at 450°C/ 4h we observed rectangular structure at reaction conduction

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10h room temperature and heating temperature 110°C/12h. Lanje AS, et al. [68] formed rectangular structure of 5-6nm particle size with monoclinic structure of CuO by basic precipitation method. Wiley et al. [69] synthesized silver nanobars with varying proportions. They mentioned that showed nanobars are thinner then nanocube structures same symmetry shown in presented work. We have synthesized nanobar with changing parameter of reaction temperature as 160°C/12h.

Generally nanorods are fabricated by thermal and electrochemistry based methods [70–72]. We have used precipitation method to synthesis nanorod by 5h room temperature and 180°C/12h synthesis

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temperature. Aslan et al. [73] used wet chemical method for fabrication of silver nanorod on glass substrate. They prepared silver seed and then reduction is done with precursors to get nanorod of 2-4nm. Hence, Morphology of any nanostructures are mainly depend on reaction conditions and by controlling

Thermal analysis

The thermogravimetric (TG) and differential thermogravimetric (DTG) curves were recorded simultaneously of the different synthesized CuO nanostructure morphologies. The TG and DTG curves were recorded in the temperature range of 300 K and 1200 K. All the thermo-curves were recorded for the heating rate of 15 °C/min. The thermal curves were measured in nitrogen atmosphere. The recorded

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TG curves of the different nanostructure morphologies are shown in Figure 3(a). The analysis of the recorded TG curves clearly shows that the decomposition of all the synthesized CuO nanostructure morphologies happens by two steps. This is because there are two steps of weight loss observed in the analyzed complete temperature region. The two steps TG decomposition was further substantiated by the presence of two peaks in the DTG curves. The temperature ranges of decomposition steps,

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them different shapes can be observed.

percentage weight loss, DTG peak positions, etc. are tabulated in Table – 2.

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Fig. 3 The (a) TG and (b) DTG curves of different morphology CuO nanostructures.

Table: 2 Thermal parameters observed from thermo-curves of different morphology CuO nanostructures. Weight loss % Sample

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Temperature range (K) Step - I

Step - II

340 to 380

1000 to 1250

Nanoparticles

0.9

6.53

Cubelike

8.27

Rectangular

Total Weight loss %

DTG peak position at Temperature (K) Peak-2

13

361

1215

10.11

26

365

1141

0.7

10.63

12

350

1143

Nanobar

1.52

12.75

23

348

1112

Nanorod

5.3

14.02

19

357

1127

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The analysis of the data of Table 2 shows that in Step – I temperature range the cubelike CuO

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has maximum weight loss followed by nanorod. The other three morphology CuO nanostructures viz nanobar, rectangular and nanoparticles show less weight loss in Step – I temperature region. The Step – I weight loss may be arising due to dehydration of the nanocrystals [74]. Looking to the weight loss magnitude it seems the cubelike morphology of CuO entraps maximum water molecules followed by

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nanorod morphology. The nanobar, rectangular and nanoparticles morphology seem inept to enclose water molecules.

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Whereas in Step – II temperature range, the nanorod shows maximum weight loss followed by nanobar. While rectangular and cubelike morphologies show comparable weight loss magnitude. The least weight loss in this Step – II is seen for nanoparticles morphology. The nanoparticles morphology

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shows least weight loss in Step – I also, thus stating that the nanoparticles morphology is the most stable of all the morphologies of CuO. At high temperature range, in Step – II, since nanorod followed by nanobar shows maximum weight loss, it clearly states that these two morphologies of CuO are less stable compared to other studied morphologies.

Kinetic Parameters

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The kinetic parameters basically depend on the reaction rate. The TG data can be used to calculate the extent of reaction [75]. The kinetic analysis of the decomposition process requires the reaction model and Arrhenius equation. The reaction can be derived from TG data using below equation (7),

m0 - mt m0 - m f

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c

(7)

Here, mt represents the mass of the sample at particular temperature. The m0 and mf denote the initial and final masses of the reaction sample respectively. The invariance of the activation energy ‘Ea’, can be derived by means of expanded Friedman and modified Coats-Redfern methods [76]. The authors have used Coats-Redfern (C-R) for calculation of kinetic parameters of the decomposition stages of the

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analyzed samples [77]. The expression is illustrated by below equation (8),

log(log c-1 /T2 ) = log(AR/Eaβ) - Ea /2.303RT

(8)

Where, A is the Arrhenius factor, Ea is the activation energy, β the heating rate, T the absolute temperature, R the universal gas constant and c is function extent of the reaction. In contour to the above

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2.4

equation (8), the log(log c-1 /T 2 ) is plotted against 1/T for all the nanostructure morphologies. The CoatsRedfern [78] plots, Fig. 4(a,b), were drawn with the best fit model for the reaction. The activation energy (Ea) and pre-exponential factor (A) are calculated from the slope and intercept of the plots

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respectively.

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Figure 4 The Coats-Redfern (C-R) plots of the as synthesized varied CuO nanostructure morphologies

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for (a) Step - I and (b) Step - II temperature ranges.

The thermodynamic kinetic parameters of the synthesized different nanostructure morphologies like activation energy (Ea), enthalpy of activation (∆H*), entropy of activation (∆S*), and Gibbs free

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energy (∆G*) were calculated using below Equations (9-13);

Ea=2.303  slope  R/(1.6 1019  6.022 1023 )

(9) (10)

ΔH* =Ea - RT

(11)

ΔS* =2.303 R log(Ah/k BT)

(12)

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A=(kT/h)e(ΔS/R)

ΔG* =ΔH* - TΔS*

(13)

Where, Ea represents the activation energy, R is the gas constant, A is the Arrhenius constant, k B denotes the Boltzmann constant, h is Planck’s constant and T is temperature of DTG peak.

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The above kinetic parameters for different CuO nanostructure morphologies were determined for

the two temperature regions steps. The obtained values of different kinetic parameters for different CuO nanostructure morphologies are tabulated in Table- 3.

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Table: 3 Kinetic parameters of different morphology CuO nanostructures. 14

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DTG Peak

Ea

A

Δ H*

Δ S*

(K)

(kJ.mol-1)

(Sec-1)

(kJ.mol-1)

(J.K-1.mol-1)

(kJ.mol-1)

Step – I

Δ G*

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Sample

361

52.58

3.02E+01

42.48

-228.29

319.86

Cubelike

365

75.89

4.94E+02

66.41

-204.54

299.79

Rectangular

350

93.65

3.15E+03

84.15

-189.14

300.34

Nanobar

348

75.95

6.12E+02

66.71

-202.54

291.93

Nanorod

357

81.99

1.05E+03

72.62

-198.20

296.00

Nanoparticles

1215

49.78

2.64E+06

46.78

-123.58

91.40

Cubelike

1141

74.89

8.61E+09

71.86

-56.41

92.45

Rectangular

1143

65.78

Nanobar

1112

56.31

Nanorod

1127

65.74

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Step – II

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Nanoparticles

62.87

-73.29

88.53

4.67E+07

53.41

-99.39

88.00

6.87E+08

62.78

-77.25

90.35

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1.08E+09

The analysis of the data of Table- 3, states that in the temperature range of Step - I the activation energy is highest in the case of rectangular structured CuO and lowest in the case of nanoparticles structure morphology. Analogous to activation energy, the value of the enthalpy of activation is highest for rectangular nanostructure morphology and lowest in case of nanoparticles morphology in the Step- I

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temperature range. This clearly states that maximum heat absorption happens in case of rectangular nanostructure morphology and lowest heat absorption in case of nanoparticles morphology. The highest heat absorption in case of rectangular nanostructure samples should lead to maximum decomposition. Larger decomposition should be reflected in the form of highest weight loss. But the data of Table -2 shows the rectangular nanostructure morphology shows least weight loss in Step – I temperature range.

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Thus the fact is that absorption of heat does not lead to decomposition and weight loss. The heat absorption by the rectangular morphology samples gets utilized in the disorder. This observation is substantiated by the corresponding value of the change in entropy, which is highest in case of rectangular structure morphology. Further the data of Table -3 shows change in entropy is lowest in case

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of nanoparticles morphology stating minimum disorder. Thus it can be concluded that in lower 15

AUTHOR SUBMITTED MANUSCRIPT - MRX2-100765.R1

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temperature range of Step- I the rectangular structured CuO absorbs maximum heat but the heat is not utilized for decomposition and weight loss but gets converted into disordering of the sample. Also can be concluded that the least values of activation energy, the enthalpy of activation and entropy of

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activation is obtained in case of the CuO nanoparticles morphology stating them to be the most stable in the Step-I temperature range. This further substantiates the least weight loss observed in the nanoparticles morphology vide Table - 2 data.

The further analysis of Table - 3 shows that in Step -II temperature range the activation energy is highest in case of cubelike structure and lowest in case of nanoparticles. The corresponding enthalpy change value is highest in case of cubelike structure and lowest in case of nanoparticles. These observations clearly states that the maximum heat absorption happens in case of cubelike structure and

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minimum heat absorption happens in case of nanoparticles. The maximum heat absorption in cubelike morphology leads to decomposition and weight loss. This observation of decomposition of cubelike morphology is corroborated by the maximum weight loss being observed from the data of Table - 2. Thus the cubelike nanostructures are less stable at high temperature and nanoparticles are the most

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stable structure of CuO. These observations clearly substantiate the observation of earlier data of weight loss in TG curves. The TG data clearly mentions that the cubelike CuO nanostructures are most unstable and nanoparticles morphology is the most stable.

Conclusion

The nano-crystallite CuO with different size and shapes (nanoparticles, cubelike, rectangular, nanobar

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and nanorod) are synthesized by controlling a critical synthesis parameter- Reaction temperature. The precipitation method was used to synthesize CuO nanostructures by controlling the fictionalization of physical properties which is very basic, cost-effective and efficient method for shape-controlled synthesis process. In this process as the temperature increases, particle collision increases and size of

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nanostructures increases. As a result, all CuO structures have monoclinic phase with 22-27nm average crystallite size for different shapes. SEM and TEM images show good agreement with previously reported different shapes of CuO. The thermal analysis of the different morphological CuO nanostructures is recorded for the TG and DTG curves which clearly show that the decomposition happens by two steps weight loss. The Step – I weight loss is observed in the temperature range of 340K

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Page 16 of 22

to 380K, whereas Step – II weight loss is observed in the temperature range of 1000K to 1250K. The TG 16

Page 17 of 22

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analysis clearly states that the Step – I arise due to dehydration of the samples whereas Step – II arises due to removal of organic moieties from the samples. This conclusion is made on the basis of the magnitude of the temperature values in both the steps. The kinetic parameters were determined using the

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magnitude of the TG weight loss and employing the Coats-Redfern method. The analysis of the TG weight loss magnitude and the kinetic parameters stated that the nanoparticles morphologies of CuO are the most stable of all the studied morphologies. Acknowledgments

I would like to thank Mr. Shivam Kansara, (SVNIT, Surat), Dr. Jiten Tailor (SVNIT, Surat) and Mr.

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Arunsinh Zala (FCIPT, IPR) for their unparalleled support.

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