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The present work reports on the solution combustion synthesis of CaMoO4 nanoparticles using molybdenum 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 CaMoO4 nanoparticles were subjected to the calcination at different temperatures in an ambient atmosphere.
Journal of Science: Advanced Materials and Devices (2019) 150e157 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Effect of calcination temperature on characteristic properties of CaMoO4 nanoparticles M Kusuma, G.T Chandrappa* Department of Chemistry, Central College Campus, Bangalore University, Bengaluru, 560001, India a r t i c l e i n f o a b s t r a c t Article history: Received 25 September 2018 Received in revised form February 2019 Accepted February 2019 Available online 12 February 2019 The present work reports on the solution combustion synthesis of CaMoO4 nanoparticles using molybdenum 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 CaMoO4 nanoparticles 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 crystallite size of scheelite tetragonal CaMoO4 nanoparticles 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 CaMoO4 nanoparticles 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/) Keywords: Nanoparticles Calcium molybdate Scheelite structure Solution combustion Photocatalytic activity Transmission electron microscopy Introduction In present years, molybdate family materials such as nanostructured 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 functionalized 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, * Corresponding author E-mail address: gtchandrappa@yahoo.co.in (G.T Chandrappa) Peer review under responsibility of Vietnam National University, Hanoi where molybdenum is blended with steel in the solid solution form whereas calcium oxide is left over as debris [4] CaMoO4 has attracted a flourishing 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-optic filters, catalyst, microwave applications, solid-state lasers, phosphor microparticles, energy storage, blue phosphor in fluorescent 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 deteriorating in luminescence Furthermore, as compared to other oxide materials, CaMoO4 also owns good chemical and physical properties [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 https://doi.org/10.1016/j.jsamd.2019.02.003 2468-2179/© 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/) M Kusuma, G.T Chandrappa / Journal of Science: Advanced Materials and Devices (2019) 150e157 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 MoO3 were homogenously mixed in an agate mortar Then, the powder was transferred to an alumina crucible and a thermal treatment of 800 C for 10 h was applied, using a heating rate of C 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 processing the reaction Here, two different surfactants were used during the synthesis, i.e ethyl 4-dimethylaminobenzoate (EDA) and 1,2,4,5-benzenetetracarboxylic dianhydride (BTD) In this typical synthetic route,  10À3 mol of molybdic acid (H2MoO4) and  10À3 mol of calcium nitrate were dissolved in 90 mL of water and the mixture was stirred for 15 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 140 C for 1, 2, and mins Then the obtained solution was washed with dist H2O ten times to neutralize the solution (pH ¼ 7) and finally, the precipitates were dried at 100 C 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 synthesis 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 process 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 nanomaterials In addition, the process is instantaneous, economical, energy 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 combustion 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 photocatalytic degradation at different pH conditions As per our knowledge, we are the first to use molybdenum metal powder for the synthesis of CaMoO4 nanoparticles as a molybdenum source 151 dissolving all the components In a typical reaction, 0.2 g molybdenum powder (dissolved in 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 undergoes thermal dehydration followed by froth formation with the liberation of gaseous products and results in voluminous CaMoO4 nanoparticles with fine particles of carbon content Subsequently, the product was calcined at the same temperature for about 20 to obtain pure carbon free CaMoO4 nanoparticles The obtained product was then calcined at 400 , 500, and 600 C for h in an ambient atmosphere 2.3 Photocatalytic test The photocatalytic activity of CaMoO4 nanoparticle was evaluated 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 absorption band maximum (664 nm for MB) using a UV-3101 PC UV-VISNIR scanning spectrophotometer (Shimadzu) The percentage of dye degradation rate was calculated by the following equation: Photodegradation rate ¼ Co À C  100 Co (1) where Co is the initial concentration at and C is the concentration at the regular time interval (t) 2.4 Characterization The powder X-ray diffraction (PXRD) measurements were performed on a PANalytical X'pert PRO MPD instrument with graphitefiltered 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 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 peroxopolymolybdic acid, calcium nitrate tetrahydrate (Mo: Ca ¼ 1:1) and citric acid as fuel (Oxidizer: Fuel ¼ 1:5) was prepared by Fig PXRD patterns of CaMoO4 nanoparticles calcined at different temperatures 152 M Kusuma, G.T Chandrappa / Journal of Science: Advanced Materials and Devices (2019) 150e157 Table Effect of calcination temperature on crystallite size, surface area and particle size of CaMoO4 nanoparticles Calcination Temperature (oC) 400 500 600 CM1 CM2 CM3 Crystallite size (nm) Surface area (m2g-1) Particle size (nm) 16 20 48 45.43 24.56 04.77 15e30 20e50 25e60 used for morphological studies and the transmission electron microscopy (TEM/HRTEM JEOL JEM 2100) was used to analyze the particle size of CaMoO4 nanoparticles Horiba Jobin Yvon - Fluorolog F3-111 was used to analyze the photoluminescence spectrum Results and discussion Fig 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 formula (D ¼ Kl/bCosq), all of the observed diffraction peaks can be perfectly indexed to those of the tetragonal phase of CaMoO4 with space group I41/a (no 88) as well as cell constants of (a) 5.22 Å, (c) 11.43 Å and Z ¼ (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 shows the dependence of crystallite size on calcination temperature The present CaMoO4 nanoparticles, synthesized through solution combustion route, resulted in higher BET surface area at the calcination temperature of 400 and 500 C (Table 1) as compared to Fig Nitrogen adsorptionedesorption isotherm and corresponding pore-size distribution curve of CaMoO4 nanoparticles calcined at different temperatures M Kusuma, G.T Chandrappa / Journal of Science: Advanced Materials and Devices (2019) 150e157 the CaMoO4 developed through a combustion route at 300 and 500 C by Manickam Minakshi et al., where BET surface area was found to be 20.6 m2g-1 at 300 C and 12.2 m2gÀ1 at 500 C, respectively [30] The nitrogen adsorption/desorption isotherms and corresponding pore size distribution curve of as-synthesized CaMoO4 nanoparticles at variable calcination temperatures are presented in Fig 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 400 C 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 in Table 153 The surface texture can be examined using scanning electron microscopy (SEM) The SEM images in Fig a-f explain the porous nature and morphology of CaMoO4 samples at different calcination temperatures from 400 to 600 C The SEM images of samples at increasing calcination temperatures show a significant change in morphology and its porous nature From SEM analysis, it is evident that relatively large low-density agglomerates are produced At increasing calcining temperature, continuous agglomeration of fine 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 Furthermore, an increase in temperature will produce isolated and connected particles with a decrease in the porous nature of the sample [31] The TEM images in Fig a, d and g exhibit the size and size distribution of well-dispersed irregular shaped CaMoO4 nanoparticles at different calcination temperatures from 400 to 600 C Fig SEM images of CaMoO4 nanoparticles calcined at different temperatures (a and b) CM1, (c and d) CM2 and (e and f) CM3 154 M Kusuma, G.T Chandrappa / Journal of Science: Advanced Materials and Devices (2019) 150e157 Fig 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 M Kusuma, G.T Chandrappa / Journal of Science: Advanced Materials and Devices (2019) 150e157 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 in Table A large divergence crop up between the sizes determined from TEM and PXRD in the present work Differences in particle size as estimated 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 samples are shown in Fig c, 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 variation 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 b, e and h) reveals that the lattice spacing of CM1 ¼ 0.2 nm corresponds to (200) plane, CM2 ¼ 0.3 nm coincides to (004) plane and CM3 ¼ 0.4 nm resembles to (101) crystalline plane, respectively Recently, Huerta-Flores et al [22] synthesized CaMoO4 by the traditional solid-state method and investigated its photocatalytic activity using tetracycline under UV-light irradiation Here, photocatalytic degradation of tetracycline resulted in a maximum of 84% of degradation by nearly h and 91% of degradation was improved by the addition of H2O2 Though, the present CaMoO4 synthesized using solution combustion technique exhibits photocatalytic degradation of MB under UV-light irradiation at the rate of ~96% for CM1, 66% for CM2 and 15% for CM3 at pH by 60 mins The profile of the photocatalytic degradation of methylene blue dye solution containing CaMoO4 nanoparticles calcined at different temperatures are used as a catalyst under varied pH conditions is illustrated in Fig The role of pH on the decolorization efficiency was studied in the pH range 3, and 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 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 (l ¼ 664 nm) was found to be arranged under the sequence of CM1 > CM2 > CM3 with pH > pH > pH 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 (Fig 6) in comparison with CM2 and CM3 with pH condition and 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 intermediates was relatively less favorable and hence less impulsive to enhance the degradation reaction rate Also, CM1 calcined at 400 C 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 155 Fig Photocatalytic degradation of MB dye solution at different pH media Furthermore, this optical property is sensible to the presence of energy levels within the band gap [20] Fig illustrates 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 mechanisms 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 CaMoO4 is attributed to intrinsic structural 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 Photocatalytic degradation of MB dye solution at pH 156 M Kusuma, G.T Chandrappa / Journal of Science: Advanced Materials and Devices (2019) 150e157 best CaMoO4 nanoparticles 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 University for extending financial support to carry out the present work References Fig PL spectra of CaMoO4 nanoparticles calcined at different temperatures Conclusion In this work, we have prepared scheelite CaMoO4 nanoparticles using facile solution combustion method The effect of calcination temperature on crystallite size, surface area, particle size, photocatalytic activity at three different pH conditions and luminescence properties of the nanocrystalline CaMoO4 were 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 CaMoO4 samples was affected by the increase in particle size and surface area The sample calcined at 400 C for h is the [1] L Krishna Bharat, G Seeta Rama Raju, Jae Su Yu, Red and green colors emitting spherical-shaped calcium molybdate nanophosphors 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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 University for extending financial... Influence of calcination temperature on the structure and porosity of nanocrystalline SnO2 synthesized by a conventional precipitation method, Int J Electrochem Sci (2014) 81e95 [32] Kusuma Manjunath,... but slow down the crystal growth development, supporting continues preparation of porous nanomaterials In addition, the process is instantaneous, economical, energy saving and no convoluted set-up