Journal of Science: Advanced Materials and Devices (2018) 77e85 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Calcination temperature dependent structural modifications, tailored morphology and luminescence properties of MoO3 nanostructures prepared by sonochemical method H.S Yogananda a, b, H Nagabhushana c, *, Ramachandra Naik d, S.C Prashantha e a Department of Physics, Sai Vidya Institute of Technology, VTU, Bengaluru 560 064, India Research and Development Center, Bharathiar University, Coimbatore 641046, India C.N.R Rao Centre for Advanced Materials, Tumkur University, Tumkur 572 103, India d Department of Physics, New Horizon College of Engineering, Bengaluru 560103, India e Research Center, Department of Science, East West Institute of Technology, VTU, Bengaluru 560091, India b c a r t i c l e i n f o a b s t r a c t Article history: Received 11 September 2017 Received in revised form November 2017 Accepted November 2017 Available online 10 November 2017 MoO3 nanoparticles were prepared by a surfactant assisted sonochemical method Final products were calcined at 180 C, 400 C, and 600 C resulting in the f-orthorhombic, b-monoclinic, and h-hexagonal structures of MoO3, respectively Variable morphologies were also seen from SEM images The energy band gap of the samples was estimated to be ~3.60 eV from diffuse reflectance spectra using KubelkaMunk function Photoluminescence spectra exhibited a strong emission peak at ~438 nm due to the hexa-coordinated [MoO6]5ỵd2e z dyz transitions The results show that the samples can be used as blue light emitting components of white light emitting diodes © 2017 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: Superstructures of MoO3 Sonochemical Sonication time CIE CCT Introduction Molybdenum oxide (MoO3) nanoparticles (NPs) have been extensively studied due to their remarkable physico-chemical properties in the mesoscopic state and prospective industrial applications such as catalysis, display materials, sensors, advanced battery materials, photo-chromic and electro-chromic devices etc [1e5] MoO3 exhibits superior intercalation chemistry and crystallizes in different phases like orthorhombic (a-MoO3), monoclinic (b-MoO3) and hexagonal (h-MoO3) [6,7] As compared to b and h phases, the a-MoO3 phase was thermodynamically more stable in nature The a-MoO3 phase has a distinct 2D layered structure in which every layer consists of two sub layers stacked along the (010) direction [8] h-MoO3 has potential applications in the field of photocatalysis because it possesses zig-zag chains of [MoO6] octahedra which are * Corresponding author C.N.R Rao Centre for Advanced Materials, Tumkur University, Tumkur 572 103, India E-mail address: bhushanvlc@gmail.com (H Nagabhushana) Peer review under responsibility of Vietnam National University, Hanoi interlinked side by side with cis position, producing one dimensional tunnel structure [9] This structure helps electronehole pair disconnection under irradiation Hence it is useful for optical devices [10], electrochromism [11,12], enhanced performance of electrodes of lithium ion batteries [13,14], light emitting diodes, etc [15] Different morphologies of MoO3 synthesized by various methods provide nanofibers [16], nanorods [17], nanobelts [18], and nanowires [19], which are useful for various applications Optical properties of the materials can be enhanced by controlling morphology and microstructure [20e23] MoO3 materials with different morphologies affect directly the sensing of the excitation light [24] Structures with plenteous cavities and multi holes enhance light harvesting from various light reflections, scattered within the cavities so that the efficiency of the excited light can be enhanced [25] Hydrothermally prepared MoO3 with multi hole structures was synthesized by Shen et al [26,27] and Deki et al [28] but their optical performance was not up to the mark Phuruangrat et al [29] synthesized MoO3 nanofibers of 50 nm diameter and 10e12 mm length by the hydrothermal method and showed that the decrease in crystal size of MoO3 considerably improved their optical properties due to the enhanced exposed surface area https://doi.org/10.1016/j.jsamd.2017.11.001 2468-2179/© 2017 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/) 78 H.S Yogananda et al / Journal of Science: Advanced Materials and Devices (2018) 77e85 [30,31] Previous studies have revealed that mass production of MoO3 nanomaterials using hydrothermal [32e34] and microwave [35e37] techniques is still challenging because these methods usually produce relatively less yield and require long reaction time [38] In order to overcome these challenges, in this manuscript, the novel sonication method was adopted to synthesize MoO3 nanomaterials In this method, high temperature can be reached in a very short duration to complete the reaction in a liquid mode [39] Since this technique is very simple to execute, it can be used for mass production with control over the morphology of the sample by tuning the pH of the precursor The presently synthesized sample exhibits importantly interesting properties like temperature dependent phase modifications, sonication time and surfactant dependent morphological changes, and blue light emitting photoluminescence, which were not fully explored in previous works [26e29] Experimental Raw materials taken for the synthesis of f-MoO3 nanoflowers were analytical grade stoichiometric amounts of 1.230 g of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24$4H2O) (AHM) 75 ml de-ionized (DI) water, 10% diluted hydrochloric acid (HCl) and ethanol in the ratio of 20:20:7 by volume were taken and mixed with AHM The final solution was sonicated for few minutes to get a clear solution and agitated at 180 C for h using ultrasonic bath Once the solution turned into precipitate of light-blue color, precipitate was centrifuged and washed with DI water and ethanol for several times Finally, the product was heated at 60 C for 16 h and calcined at different temperatures Further, the experiment was repeated by changing the sonication time and also prepared the samples by adding cetyltrimethyammonium bromide (CTAB) as surfactant Proposed samples were characterized by powder X-ray diffraction (PXRD) using Shimadzu X-ray diffractometer (Shimadzu 7000) with graphite monochromatized Cu-Ka radiation (l ¼ 0.15406 nm) The surface morphology was studied by Hitachi table top scanning electron microscope (SEM, Hitachi- TM 3000), Transmission electron microscope (TEM) of Hitachi H-8100, Kevex sigma TM Quasar, USA was used to study the crystallite size, composition and inter-planar spacing Spectrophotometer (Lambda e35, Perkin Elmer) was used to study the diffuse reflectance spectra (DRS) of the material Spectroflourometer equipped with Fluorolog- (Jobin Yvon) was utilized to measure the photoluminescence (PL) property [39] Results and discussions Fig presents the powder x-ray diffraction (PXRD) patterns of MoO3 NPs synthesized at different temperatures (180 C, 400 C and 600 C) At 180 C, the PXRD pattern can be well indexed to the a-orthorhombic structure with JCPDS card No 21-569 [40] The lattice parameters and the unit cell volume were found to be a ¼ 4.00 Å, b ¼ 13.967 Å, c ¼ 3.710 Å and V ¼ 207.27 Å3 respectively Further when the sample was heated to 400 C, the phase transformation took place from the a-orthorhombic to b-monoclinic structure with JCPDS card No 47-1320 and upon increase in the temperature to 600 C again the phase transformation took place from the b-monoclinic to h-hexagonal structure with JCPDS card No 29-0115 [40] Also cetyltrimethylammonium bromide [CTAB; C16H33(CH3)3NBr, g] surfactant was used while preparing with different sonication times (1 he4 h) Fig 1(b) shows the PXRD patterns of MoO3 NPs prepared under different sonication times with a g CTAB concentration It was observed that neither the sonication time nor the surfactant affected the crystal structure of a-orthorhombic MoO3 NPs However, it was noticed that the crystallite size was greatly affected by these two parameters Further the h sonication time was used as the standard duration and the procedure was repeated for different weights of CTAB from g to g (Fig 1(c)) The detailed variation of crystallite size in all these cases was tabulated in Table The time and temperature significantly influenced the chemical reaction, leading to the pathway for nucleation and growth of the resultant product In general, the rise in the reaction time and the reaction temperature allows the crystallite to nucleate, develop along precise growth sites, and assemble orderly, thus promoting highly crystalline samples with increased crystallite size Morphology of the products was analyzed to get a better understanding of the formation and growth mechanism of the product Fig 2(a) shows SEM images of the a-MoO3 NPs calcined at 600 C with h sonication time and g of CTAB concentration Fig 2(bed) shows SEM images of the same sample under different magnifications It was observed that the sample exhibits hexagonal shaped nanorods All atoms of these individual rods were at high energy, leading to the vibration and diffusion process The strength of vibration and diffusion of the solid was controlled by the calcination temperature, bond strength and type of bonds It can be noted that, as shown in Fig 3(a), the morphologies of h-MoO3 NPs of hexagonal smooth rods Fig 3(bed) shows SEM images of the same sample under different magnifications The growth mechanism of MoO3 due to the presence of HCl of 10% concentration is proposed as follows: Mo7O6À 24 anions would join with protons to form H2MoO4 first Upon sonication, H2MoO4 would dehydrate and form numerous MoO3 NPs, which could give out as the nuclei The equivalent reaction processes are shown below [41]: þ Mo7 O6À 24 þ H /H2 MoO4 (1) H2 MoO4 /MoO3 ỵ H2 O (2) Further the effects of sonication time and CTAB concentration on the morphology of the samples are shown in Figs and By increasing the sonication time, the surface of the particles gets destroyed; the re-crystallization process starts to take place by breaking individual rods which signifies surface dissolution This kind of phenomenon takes place mainly because of augmented kinetic and thermodynamic energies which initiate superior residual stresses that favor an asymmetrical chemical environment in the reaction system [26] The reaction temperature and time promote the nucleation and growth of flower like h-MoO3 NPs [42] hMoO3 NPs formation takes place in main stages including i) controlled nucleation at a controlled reactant species, ii) growth of hexagonal MoO3 phase, iii) development of 1D hexagonal rods through Ostwald ripening, and iv) inter-particle interaction with controlled reaction time and temperature, leading to the formation of 3D hierarchical flower-like microspheres [43,44] TEM images shown in Fig 6(aec) indicate the existence of both short and long rods of non-hexagonal geometry The density of the rods was very high and their agglomeration resulting in nonuniform dispersibility Fig 6d shows the SAED pattern which provides the information of nanorods of polycrystalline MoO3 NPs The spectra obtained from the energy dispersive X-ray spectra (EDX) analysis (Fig 6(e)) qualitatively confirmed the presence of Mo and the purity of the as-synthesized material The optical reflectance characterization of MoO3 NPs with different temperatures and different sonication times were conducted by measuring DRS in the range 300e700 nm wavelength (Fig 7) Both the spectra show a strong reflectance response between 420 and 570 nm which indicate that the sample shows high H.S Yogananda et al / Journal of Science: Advanced Materials and Devices (2018) 77e85 79 Fig (a) PXRD patterns of MoO3 nanostructures calcined at different temperatures (a-180 C, b-400 C and h-600 C, a sonication time of h) (b) PXRD patterns of MoO3 nanostructures for different sonication times (a calcination temperature of 600 C, CTAB- g) (c) PXRD patterns of MoO3 nanostructures for different surfactant (CTAB) concentrations (the sonication time of h and the calcination temperature of 600 C) Table Detailed variations of particle size and bandgap values of MoO3 under different conditions Temperature ( c) Band gap Eg (eV) Particle size D (nm) Sonication time (h) with CTAB (1 g) and 600 C calcination Band gap Eg (eV) Particle size D (nm) Surfactant CTAB (g) with h sonication and 600 C calcination Particle size D (nm) 180 400 600 3.6 3.6 3.6 40 36 32 3.4 3.4 3.4 3.4 38 36 34 32 33 36 37 38 absorption in the visible region Here, the optical transition was from the vacant “d” orbitals of the cation (Mo6ỵ) and p orbitals of the oxygen ions with lone pair of electrons (O2À) [45] The MoO3 sample obtained with h sonication exhibits a wider adsorption wavelength range The legend mentioned in Fig 7b indicates the sonication time of h, h, h and h The absorption peak at 530 nm was due to the intrinsic absorption at the semiconductor band gap [46] SchustereKubelkaeMunk (SKM) relation was used to estimate the energy band gap [47,48] FRị ẳ Rị2 2R (3) where R is the absolute reflectance of the sampled and F(R) is the so-called KubelkaeMunk function It was evident that the DRS for all samples increased with increasing wavelength The optical band gap (Eg) of phosphors was determined by (F(R) hy)n ¼ A(hyÀEg), where n ¼ for a direct allowed transition, and n ¼ 1/2 for an indirect allowed transition, A is the constant, and hy is the photon energy [49] The linear part of the curve was extrapolated to (F(R) hy)1/2 ¼ to get the indirect band gap energy The estimated Eg values for the samples prepared under different sonication times and temperatures were found to be in the range 3.4e3.6 eV as shown in Fig 8(a) and (b) It was predicted that the difference in Eg was due to the increase of carrier concentration, leading to the BursteineMoss effect [50] The excitation spectrum taken for the 600 C calcined sample with an emission wavelength of 438 nm is shown in Fig 9(a) The spectrum consists of single excitation peak positioned at 324 nm Fig 9(b) shows the PL spectra of MoO3 at different temperatures 80 H.S Yogananda et al / Journal of Science: Advanced Materials and Devices (2018) 77e85 Fig SEM images of a-MoO3 (a) nanorods calcined at 600 C and (bed) SEM images of the a-MoO3 under different magnifications Fig SEM images of h-MoO3 (a) nanorods calcined at 600 C and (bed) SEM images of the h-MoO3 under different magnifications H.S Yogananda et al / Journal of Science: Advanced Materials and Devices (2018) 77e85 81 Fig SEM images of MoO3 nanostructures for different sonication times (a) h, (b) h, (c) h, and (d) h Fig SEM images of MoO3 nanostructures for different CTAB concentrations (a) g, (b) g, (c) g, and (d) g with an excitation wavelength of 324 nm The PL spectra of all the samples show strong emission between 400 and 600 nm The emission spectra show a strong peak positioned at 438 nm may be due to the hexa-coordinated [MoO6]5ỵd2e z dyz transitions The commercial MoO3 has a weak emission peak at room temperature which is a reflection of a radiative recombination of inter band electrons and holes in MoO3 crystals [51e53] Enhancement in PL intensity was observed at a lower wavelength (438 nm) due to the 82 H.S Yogananda et al / Journal of Science: Advanced Materials and Devices (2018) 77e85 Fig TEM images (aec), SAED pattern (d), and EDAX pattern (e) of a-MoO3 nanostructures Fig (a) DRS of a-MoO3 nanostructures calcined at different temperatures (b) DRS of MoO3 nanostructures for different sonication times of h, h, h and h H.S Yogananda et al / Journal of Science: Advanced Materials and Devices (2018) 77e85 83 Fig (a) Band gap analysis of a-MoO3 nanostructures calcined at different temperatures (b) Band gap analysis of a-MoO3 nanostructures for different sonication times of h, h, h and h Fig (a) Excitation spectra of a-MoO3 nanostructures at a 438 nm emission wavelength (b) Emission spectra of a-MoO3 nanostructures at a 324 nm excitation wavelength decrease in grain size or an increase in the specific surface area [54,55] Further, it was observed from PL spectra that there was a gradual increase in emission intensity with increase in temperature The 600 C calcined sample has the highest emission intensity because calcination temperature and time have a direct effect on their PL intensities The emission peaks in the range 380e460 nm are due to surface defects such as Mo-vacancies or oxygen vacancies (Vo), Molybdenum vacancies (VMo), interstitial oxygen (Oi), interstitial Molybdenum (Moi), antisite oxygen (O), F-centers (created by oxygen ion vacancy acquired by electrons) or Fỵ-centers (created by oxygen ion vacancy acquired by electron) or surface states The defects in MoO3 were created due to bond breaking and surface stress created by large surface to volume ratio Due to these defects Fỵ and F centers were converted to F-aggregates like F2, F2ỵ, F2ỵ The energy levels of these defects centers be present in the forbidden energy gap of MoO3 The divergence in emission intensity between the samples prepared via different calcination, sonication time and variable CTAB concentrations may be attributed to higher density of defects present in the sample Therefore, the morphology plays a significant role in the PL emission [56,57], all the above photometric discussions show suitability of the sample for display applications The Commission International de I’E'clairage (CIE) chromaticity coordinates (x, y) [56] for the synthesized MoO3 samples were calculated and shown in Fig 10(a), confirming that MoO3 can be used as blue light emitting diodes Moreover, it was well-known Fig 10 (a) CIE diagram and (b) CCT diagram of a-MoO3 nanostructures 84 H.S Yogananda et al / Journal of Science: Advanced Materials and Devices (2018) 77e85 that the low color temperature was popular in solid-state lighting Thereby, the correlated color temperature (CCT) as one of the characteristics of phosphors are evaluated by using (x, y) chromaticity coordinates to (U0 , V0 ), the CCT value was found to be 1968 K for 600 C which was well below 5000 K indicates the light intensity is in warm region (Fig 10(b)) [58] Conclusion Sonochemically prepared MoO3 NPs showed three different phases including a-orthorhombic, b-monoclinic and h-hexagonal MoO3 crystal structures respectively for different calcination temperatures of 180 C, 400 C, and 600 C SEM images show that the samples exhibited hierarchical 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bond strength and type of bonds It can be noted... different temperatures (a-180 C, b-400 C and h-600 C, a sonication time of h) (b) PXRD patterns of MoO3 nanostructures for different sonication times (a calcination temperature of 600