Effect of calcination temperature on characteristic properties of CaMoO4 nanoparticles

The effect of calcination temperature on crystallite size, surface area, particle size, photocatalytic activity at three different pH conditions and luminescence properties of the nano[r]

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Original Article

Effect of calcination temperature on characteristic properties of

Department of Chemistry, Central College Campus, Bangalore University, Bengaluru, 560001, India

a r t i c l e i n f o

Article history:

Received 25 September 2018

Received in revised form

4 February 2019

Accepted 4 February 2019

Available online 12 February 2019

Keywords:

Nanoparticles

Calcium molybdate

Scheelite structure

Solution combustion

Photocatalytic activity

Transmission electron microscopy

a b s t r a c t

The present work reports on the solution combustion synthesis of CaMoO4nanoparticles using mo-lybdenum metal powder as the‘Mo’ source and citric acid as a fuel To understand the effect of thermal treatment on the crystallinity, particle size, surface area and photocatalytic degradation at different pH conditions, combustion-derived CaMoO4nanoparticles were subjected to the calcination at different temperatures in an ambient atmosphere The powder X-ray diffraction patterns of calcined samples were used to substantiate the effect of calcination on phase formation and crystallite size The average crys-tallite size of scheelite tetragonal CaMoO4nanoparticles was found to increase with an increase in calcination temperature Transmission electron microscopy images illustrate the average particle size varying in the range of 10e70 nm The surface area of CaMoO4nanoparticles decreases with an increase

in the calcination temperature Photocatalytic degradation of MB dye under UV-light illumination was found to be affected by thermal treatment

© 2019 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

In present years, molybdate family materials such as

nano-structured alkali e earth metal molybdates have attracted the

interest of many researchers due to their practical, industrial and

broad potential applications They represent an important group

of ternary metal oxides, which contains large bivalent cations,

where metal molybdates of general formula AMoO4(A¼Ca, Sr, Ba)

is one of them and acts as the oxide-based host materials which

gives excellent luminescent properties due to its feature of energy

transfer from host lattice to the dopant ions [1] Among metal

molybdates family, calcium molybdate (CaMoO4) existed in the

scheelite structure, which known as a mineral powellite, [2] is

most important with high potential energy and has been

func-tionalized in cryogenic scintillation detectors and double beta

decays[3]

Powellite (CaMoO4) chronically arises as a secondary mineral in

the oxidation zone of molybdenite deposits Calcium molybdate is a

vital industrial product used as an add-on material to steel and for

the smelting of ferromolybdenum since it is more economical Then

calcium molybdate is reduced by iron due to the smelting of steel,

where molybdenum is blended with steel in the solid solution form whereas calcium oxide is left over as debris[4]

CaMoO4has attracted aflourishing attention because of its high chemical stability, intense and broad charge transfer band emerging from tetrahedron MoO4 unit [5] CaMoO4 possesses promising and potential applications in various domains like phosphor microparticles and acousto-opticfilters, catalyst, micro-wave applications, solid-state lasers, phosphor microparticles, en-ergy storage, blue phosphor influorescent lighting devices and MASER materials, in medical applications as scintillator, humidity sensors, Li-ion batteries, fluorescent lamps, photoluminescence, nanopigment etc[5e7] Also, this material is greatly translucent and admits a broad range of light to pass through without deteri-orating in luminescence Furthermore, as compared to other oxide materials, CaMoO4also owns good chemical and physical proper-ties[1]

So far, numerous synthetic and property studies have been made on CaMoO4 nanoparticles using different fabrication techniques, for example,ﬂux method[8], sonochemical route[9], coprecipitation method [10], chemical solution decomposition method [11], hydrothermal method [12], microwave radiation method [13], solegel [14], solution-phase rapid-injection-based route[15,16], pulsed laser ablation[17,18], molten salt method[19], polyol method[20], auto-combustion route[21]etec

* Corresponding author.

E-mail address: gtchandrappa@yahoo.co.in (G.T Chandrappa).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2019.02.003

Journal of Science: Advanced Materials and Devices 4 (2019) 150e157

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Recently, Ali M Huerta-Flores et al reported the synthesis of

alkaline-earth molybdates through a solid-state method Firstly,

equimolar quantities of metal and MoO3were homogenously mixed

in an agate mortar Then, the powder was transferred to an alumina

crucible and a thermal treatment of 800C for 10 h was applied,

using a heating rate of 3C per minute[22] F.K.F Oliveira and his

group synthesized calcium molybdate by co-precipitation method

followed by the microwave-assisted hydrothermal system for

pro-cessing the reaction Here, two different surfactants were used

dur-ing the synthesis, i.e ethyl 4-dimethylaminobenzoate (EDA)

and 1,2,4,5-benzenetetracarboxylic dianhydride (BTD) In this typical

synthetic route, 5 103 mol of molybdic acid (H2MoO4) and

5 103mol of calcium nitrate were dissolved in 90 mL of water and

the mixture was stirred for 15 min followed by the addition of

ammonium hydroxide to the solution until the pH reached by 14 to

intensify the rate of hydrolysis between Mo and Ca ions Later, the

fabrication was done by transferring the solution into a Teﬂon

autoclave and annealing at 140C for 1, 2, 4 and 8 mins Then the

obtained solution was washed with dist H2O ten times to neutralize

the solution (pH¼ 7) and ﬁnally, the precipitates were dried at 100C

for 24 h[23] In this manner, a surfactant-free hydrothermal route

[3], polyol method[20], reverse-microemulsion method[6], solegel

route[14], co-deposition in water/oil phase method [24]etc also

comprise effectual synthesis of homogenous nanoparticles and

powders Nevertheless, long fabrication time, low production rate,

multifaceted process are the typical troubles of these methods

Indeed, there is an immense demand for economically viable

syn-thesis techniques Hence, we eradicated these intricacies of the

above-cited methods by setting up a lively aspect known as solution

combustion route The solution combustion process is a promising

technique and a facile method which takes only a few minutes for the

preparation of various metal oxide nanoparticles[25,26] This

pro-cess is simple, fast and utilizes the self-sustained exothermic reaction

between the oxidizer and fuel, to attain a high temperature and

liberate tremendous gaseous product within a time of seconds These

features endorse crystallization but slow down the crystal growth

development, supporting continues preparation of porous

nano-materials In addition, the process is instantaneous, economical,

en-ergy saving and no convoluted set-up is required[25e27]

Thus, in the present work, we report the synthesis and study of

CaMoO4(powellite) nanoparticles using a facile solution

combus-tion synthetic route at three different temperature, i.e 400 ,

500, and 600 C in order to understand the effect of thermal

treatment on crystallinity, particle size, surface area and

photo-catalytic degradation at different pH conditions As per our

knowledge, we are theﬁrst to use molybdenum metal powder for

the synthesis of CaMoO4nanoparticles as a molybdenum source

2 Experimental

2.1 Chemicals

Molybdenum metal powder, calcium nitrate tetrahydrate,

hydrogen peroxide (30%), citric acid and methylene blue dye were

procured from Merck Ltd and used without further puriﬁcation

2.2 Sample preparation

The synthesis of CaMoO4 nanoparticles has been achieved

through solution combustion approach An aqueous solution of

peroxopolymolybdic acid as molybdenum source was prepared by

dissolving molybdenum powder in 30% solution of H2O2[28]

An aqueous solution containing stoichiometric ratios of

perox-opolymolybdic acid, calcium nitrate tetrahydrate (Mo: Ca¼ 1:1)

and citric acid as fuel (Oxidizer: Fuel ¼ 1:5) was prepared by

dissolving all the components In a typical reaction, 0.2 g molyb-denum powder (dissolved in 5 mL H2O2), 0.4922 g calcium nitrate tetrahydrate and 2.1903 g citric acid were used The obtained homogeneous solution was pre-heated on a hot plate until the formation of the viscous gel and then placed in a mufﬂe furnace maintained at 400 ± 10 C The reaction solution boils and un-dergoes thermal dehydration followed by froth formation with the liberation of gaseous products and results in voluminous CaMoO4 nanoparticles withﬁne particles of carbon content Subsequently, the product was calcined at the same temperature for about 20 min

to obtain pure carbon free CaMoO4 nanoparticles The obtained product was then calcined at 400 , 500, and 600C for 2 h in an ambient atmosphere

2.3 Photocatalytic test The photocatalytic activity of CaMoO4nanoparticle was evalu-ated using methylene blue dye in water by varying the pH under the illumination of UV-light A 120 W high-pressure mercury lamp with a wavelength of 253 nm was used as UV-irradiation source

An aqueous suspension (100 mL) containing 10 ppm methylene blue and 0.15 g of as-synthesized sample was placed in a 250 mL pyrex dish For the reaction in different pH media, the initial pH of the suspension was adjusted by addition of either NaOH or HCl solutions

To establish the adsorption/desorption equilibrium, the suspensions were magnetically stirred before illumination in the dark for 30 mins

at room temperature At a speciﬁc time interval, the sample solution was withdrawn and isolated from the catalyst by centrifugation The supernatants were analyzed by recording variations in the absorp-tion band maximum (664 nm for MB) using a UV-3101 PC UV-VIS-NIR scanning spectrophotometer (Shimadzu) The percentage of dye degradation rate was calculated by the following equation:

Photodegradation rate¼Co C

where Cois the initial concentration at 0 min and C is the con-centration at the regular time interval (t)

2.4 Characterization The powder X-ray diffraction (PXRD) measurements were per-formed on a PANalytical X'pert PRO MPD instrument with graphite-ﬁltered CuKa radiation source (a ¼ 1.541 Å) Nitrogen adsorptionedesorption measurements were carried out at 77 K using a gas sorption analyzer (Quantachrome Corporation NOVA 1000) Scanning electron microscopy (SEM, JEOL-JSM-6490LV) was

Fig 1 PXRD patterns of CaMoO nanoparticles calcined at different temperatures.

M Kusuma, G.T Chandrappa / Journal of Science: Advanced Materials and Devices 4 (2019) 150e157 151

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used for morphological studies and the transmission electron

microscopy (TEM/HRTEM JEOL JEM 2100) was used to analyze the

particle size of CaMoO4nanoparticles Horiba Jobin Yvon -

Fluo-rolog F3-111 was used to analyze the photoluminescence spectrum

3 Results and discussion

Fig 1 shows the powder X-ray diffraction (PXRD) pattern of

CaMoO4 nanoparticles calcined at different temperatures Our

results specify that there are no phase changes irrespective of

calcination while there is an increase in the average crystallite size

The increase in crystallite size at higher calcination temperatures

evidences the development of larger sized particles during the

calcination and is attributed to the coalescence of small grains through grain boundary diffusion[29] Thus, using Scherrer's for-mula (D¼ Kl/bCosq), all of the observed diffraction peaks can be perfectly indexed to those of the tetragonal phase of CaMoO4with space group I41/a (no 88) as well as cell constants of (a) 5.22 Å, (c) 11.43 Å and Z¼ 4 (JCPDS No 29e351) No peaks of other impurity phases are detected in the patterns, suggesting that CaMoO4 nanoparticles with high phase purity can be easily attained by this solution combustion synthesis.Table 1shows the dependence of crystallite size on calcination temperature

The present CaMoO4nanoparticles, synthesized through solu-tion combussolu-tion route, resulted in higher BET surface area at the calcination temperature of 400 and 500C (Table 1) as compared to

Table 1

Effect of calcination temperature on crystallite size, surface area and particle size of CaMoO 4 nanoparticles.

Calcination Temperature ( o C) Crystallite size (nm) Surface area (m 2 g -1 ) Particle size (nm)

Fig 2 Nitrogen adsorptionedesorption isotherm and corresponding pore-size distribution curve of CaMoO

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the CaMoO4 developed through a combustion route at 300 and

500C by Manickam Minakshi et al., where BET surface area was

found to be 20.6 m2g-1 at 300 C and 12.2 m2g1 at 500 C,

respectively[30]

The nitrogen adsorption/desorption isotherms and

correspond-ing pore size distribution curve of as-synthesized CaMoO4

nano-particles at variable calcination temperatures are presented inFig 2

It is found that irrespective of calcinations, the isotherm of all the

samples exhibited the characteristics of type IV isotherm with

discrete hysteresis loops in the range of P/Po> 0.1, which suggests

the presence of a mesoporous structure of the adsorbent as per

Brunauer-Deming-Deming-Teller classiﬁcation The hysteresis loop

is prominent in the sample CM1 calcined at 400C This can be

ascribed to the well-proportioned mesoporous nature with the

highest BET surface area when distinguished to CM2 and CM3 The

dependence of speciﬁc surface area of the samples on calcination

was calculated using the BrunauereEmmetteTeller (BET, nitrogen,

77 K) method and is showed inTable 1

The surface texture can be examined using scanning electron microscopy (SEM) The SEM images in Fig 3 a-f explain the porous nature and morphology of CaMoO4 samples at different calcination temperatures from 400 to 600C The SEM images of samples at increasing calcination temperatures show a signi ﬁ-cant change in morphology and its porous nature From SEM analysis, it is evident that relatively large low-density agglom-erates are produced At increasing calcining temperature, continuous agglomeration ofﬁne particles will ensue and when the agglomeration is concluded, rapid particle growth would take place Therefore at high calcination temperature, the pores

in the materials are trapped ensuing in the shrinkage Further-more, an increase in temperature will produce isolated and connected particles with a decrease in the porous nature of the sample[31]

The TEM images inFig 4a, d and g exhibit the size and size distribution of well-dispersed irregular shaped CaMoO4 nano-particles at different calcination temperatures from 400 to 600C

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Fig 4 TEM images- (a) CM1, (d) CM2, (g) CM3; HRTEM images- (b) CM1, (e) CM2, (h) CM3; and SAED pattern e (c) CM1 (f), CM2 and (i) CM3.

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The TEM images of samples at diverse calcination temperatures

show a considerable transformation in particle size The values of

particle size determined from the TEM images are given inTable 1

A large divergence crop up between the sizes determined from TEM

and PXRD in the present work Differences in particle size as

esti-mated from TEM and PXRD are attributed to the fact that PXRD

measures crystallite size while TEM measures particle size, where a

particle may reside with the combination of several crystallites

[29] Selected area electron diffraction (SAED) patterns of all

sam-ples are shown inFig 4c, f and i The discrete spotty rings in the

SAED pattern validate the good and polycrystalline nature of all

samples The variations observed between the SAED patterns of

samples calcined at different temperatures are owed to the

varia-tion of the particle size Smaller sized particles (~15 nm) turn out a

continuous ring pattern due to the presence of a large number of

single crystal particles in the speciﬁed selected area But, when the

particle size increases at higher calcination temperature, the spots

will be far from each other and this is evidently perceptible in the

SAED patterns Thus, a superior agreement is found between the

d spacing values calculated from the PXRD patterns using Bragg's

formula and from the SAED patterns HRTEM image (Fig 4b, e and

h) reveals that the lattice spacing of CM1¼ 0.2 nm corresponds

to (200) plane, CM2 ¼ 0.3 nm coincides to (004) plane and

CM3¼ 0.4 nm resembles to (101) crystalline plane, respectively

Recently, Huerta-Flores et al.[22]synthesized CaMoO4by the

traditional solid-state method and investigated its photocatalytic

activity using tetracycline under UV-light irradiation Here,

pho-tocatalytic degradation of tetracycline resulted in a maximum of

84% of degradation by nearly 3 h and 91% of degradation was

improved by the addition of H2O2 Though, the present CaMoO4

synthesized using solution combustion technique exhibits

photo-catalytic degradation of MB under UV-light irradiation at the rate of

~96% for CM1, 66% for CM2 and 15% for CM3 at pH 9 by 60 mins

The proﬁle of the photocatalytic degradation of methylene blue

dye solution containing CaMoO4nanoparticles calcined at different

temperatures are used as a catalyst under varied pH conditions is

illustrated inFig 5 The role of pH on the decolorization efﬁciency

was studied in the pH range 3, 7 and 9 under UV-light illumination

The pH of the solution is adjusted before irradiation UVeVis

spectra of decayed methylene blue dye with the variation of time

from 0 to 60 mins exhibit the maximum absorption wavelength at

664 nm

Under all three pH conditions i.e., acidic, neutral and basic, the

absorbance intensities of methylene blue were gradually decreased

in the presence of all CaMoO4 nanocatalysts with respect to the

time Thus, the photocatalytic activity upon UV-Visible irradiation

(l¼ 664 nm) was found to be arranged under the sequence of

CM1> CM2 > CM3 with pH 9 > pH 7 > pH 3 That is to say, there

was a signiﬁcant change in MB dye concentration in basic condition

and CM1 exhibits nearly 96% degradation by 60 mins at pH 9 (Fig 6)

in comparison with CM2 and CM3 with pH condition 3 and 7 This

implies that basic condition is favorable towards the formation of

the reactive intermediates, i.e hydroxyl radicals, which further help

in enhancing the reaction rate and favorable condition Besides, in

neutral and acidic conditions, the formation of reactive

in-termediates was relatively less favorable and hence less impulsive

to enhance the degradation reaction rate Also, CM1 calcined at

400C exhibits high surface area as compared to CM2 and CM3

(Table 1) Since the nanoparticles with higher surface area create a

superior contact area with the target material for adsorbing a

higher amount of dye molecules[32,33], the effect of surface area

on the photocatalytic activity can be observed here

In all-purpose, PL emission is considered as a potent tool to

acquire information on the electronic structure and degree of a

structural organization at a medium range of the materials

Furthermore, this optical property is sensible to the presence of energy levels within the band gap[20].Fig 7illustrates the room temperature-recorded photoluminescence spectra of CaMoO4 nanoparticles calcined at different temperatures using the same excitation wavelength of 385 nm All three CaMoO4 samples exhibited intense emission band in between 500 and 600 nm The emission spectra were measured at room temperature for the reason that, this optical property behavior can be inﬂuenced by the temperature

Here, all samples show a strong emission with the maximum centered at around 526 nm and 527 nm which exhibits green emission This emission band is typical of the multiphonon and multilevel process, that is, a system in which relaxation occurs by several paths involving the participation of numerous states within the band gap of the materials However, the origin and the mecha-nisms of PL emissions for metal molybdates, have not been entirely well recognized yet Several elucidations have conversed in the literature regarding the origin and mechanisms liable for the PL emissions of molybdates According to Compos et al and Marques

et al., green PL emission in CaMoO4is attributed to intrinsic struc-tural disorder in the CaO8eMoO4 clusters of tetrahedron groups [20,34]

Ryu et al investigated the dependence of PL properties on crystallinity and morphology and Liu et al concluded that, the charge transfer transitions into [MoO4]2e complex possibly considered to be the main cause liable for the green PL emissions [35,36]

Fig 5 Photocatalytic degradation of MB dye solution at different pH media.

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4 Conclusion

In this work, we have prepared scheelite CaMoO4nanoparticles

using facile solution combustion method The effect of calcination

temperature on crystallite size, surface area, particle size,

photo-catalytic activity at three different pH conditions and luminescence

properties of the nanocrystalline CaMoO4were investigated The

obtained results signify that the degree of crystallinity and particle

size of nanoparticles increase with increasing the calcination

temperature whereas the surface area decreases The degradation

activity of CaMoO4samples was affected by the increase in particle

size and surface area The sample calcined at 400C for 2 h is the

best CaMoO4nanoparticles for the photocatalytic process with high surface area

Conflict of interest The authors confirm that this article content has no conflict of interest

Acknowledgements One of the authors Kusuma M is thankful to Bangalore Univer-sity for extendingﬁnancial support to carry out the present work References

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