Calcination temperature dependent structural modifications, tailored morphology and luminescence properties of MoO3nanostructures

Liu, Rapid synthesis of free- standing MoO 3 /Graphene ﬁlms by the microwave hydrothermal method as. cathode for bendable lithium batteries, J[r]

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Original Article

prepared by sonochemical method

a Department of Physics, Sai Vidya Institute of Technology, VTU, Bengaluru 560 064, India

b Research and Development Center, Bharathiar University, Coimbatore 641046, India

c 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

a r t i c l e i n f o

Article history:

Received 11 September 2017

Received in revised form

1 November 2017

Accepted 2 November 2017

Available online 10 November 2017

Keywords:

Superstructures of MoO 3

Sonochemical

Sonication time

CIE

CCT

a b s t r a c t

MoO3nanoparticles were prepared by a surfactant assisted sonochemical method Final products were calcined at 180C, 400C, and 600C resulting in thef-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 Kubelka-Munk function Photoluminescence spectra exhibited a strong emission peak at ~438 nm due to the hexa-coordinated [MoO6]5þdz 2edyztransitions 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/)

1 Introduction

extensively studied due to their remarkable physico-chemical

properties in the mesoscopic state and prospective industrial

ap-plications such as catalysis, display materials, sensors, advanced

battery materials, photo-chromic and electro-chromic devices etc

[1e5] MoO3 exhibits superior intercalation chemistry and

crys-tallizes in different phases like orthorhombic (a-MoO3), monoclinic

(b-MoO3) and hexagonal (h-MoO3)[6,7] As compared toband h

nature Thea-MoO3phase has a distinct 2D layered structure in

which every layer consists of two sub layers stacked along the (010)

direction[8]

h-MoO3has potential applications in theﬁeld of photocatalysis

because it possesses zig-zag chains of [MoO6] octahedra which are

interlinked side by side with cis position, producing one dimen-sional tunnel structure[9] This structure helps electronehole pair disconnection under irradiation Hence it is useful for optical

electrodes of lithium ion batteries[13,14], light emitting diodes, etc

methods provide nanoﬁbers [16], nanorods[17], nanobelts [18], and nanowires[19], which are useful for various applications Op-tical properties of the materials can be enhanced by controlling

different morphologies affect directly the sensing of the excitation

enhance light harvesting from various light reﬂections, scattered within the cavities so that the efﬁciency of the excited light can be

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

the decrease in crystal size of MoO3considerably improved their optical properties due to the enhanced exposed surface area

* Corresponding author C.N.R Rao Centre for Advanced Materials, Tumkur

Uni-versity, Tumkur 572 103, India.

E-mail address: bhushanvlc@gmail.com (H Nagabhushana).

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.2017.11.001

Journal of Science: Advanced Materials and Devices 3 (2018) 77e85

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[30,31] Previous studies have revealed that mass production of

usually produce relatively less yield and require long reaction time

[38] In order to overcome these challenges, in this manuscript, the

nano-materials 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

surfac-tant dependent morphological changes, and blue light emitting

photoluminescence, which were not fully explored in previous

works[26e29]

2 Experimental

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

to get a clear solution and agitated at 180C for 1 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 60C 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

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

Fluorolog- 3 (Jobin Yvon) was utilized to measure the

photo-luminescence (PL) property[39]

3 Results and discussions

Fig 1presents the powder x-ray diffraction (PXRD) patterns of

MoO3NPs synthesized at different temperatures (180C, 400C

and 600C) At 180C, the PXRD pattern can be well indexed to

The lattice parameters and the unit cell volume were found to be

a¼ 4.00 Å, b ¼ 13.967 Å, c ¼ 3.710 Å and V ¼ 207.27 Å3

respec-tively Further when the sample was heated to 400C, the phase

-mono-clinic structure with JCPDS card No 47-1320 and upon increase in

place from theb-monoclinic to h-hexagonal structure with JCPDS

[CTAB; C16H33(CH3)3NBr, 1 g] surfactant was used while preparing

with different sonication times (1 he4 h).Fig 1(b) shows the

times with a 1 g CTAB concentration

It was observed that neither the sonication time nor the

sur-factant affected the crystal structure ofa-orthorhombic MoO NPs

However, it was noticed that the crystallite size was greatly affected

by these two parameters Further the 4 h sonication time was used

as the standard duration and the procedure was repeated for different weights of CTAB from 1 g to 4 g (Fig 1(c)) The detailed variation of crystallite size in all these cases was tabulated in

Table 1 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 crystal-lite size

Morphology of the products was analyzed to get a better un-derstanding of the formation and growth mechanism of the prod-uct.Fig 2(a) shows SEM images of thea-MoO3NPs calcined at

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 calci-nation temperature, bond strength and type of bonds It can be noted that, as shown inFig 3(a), the morphologies of h-MoO3NPs

mecha-nism of MoO3due to the presence of HCl of 10% concentration is proposed as follows:

Mo7O246anions would join with protons to form H2MoO4ﬁrst

MoO3NPs, which could give out as the nuclei The equivalent re-action processes are shown below[41]:

Further the effects of sonication time and CTAB concentration on

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 re-sidual stresses that favor an asymmetrical chemical environment in the reaction system[26] The reaction temperature and time pro-mote the nucleation and growth ofﬂower like h-MoO3NPs[42]

controlled nucleation at a controlled reactant species, ii) growth of

through Ostwald ripening, and iv) inter-particle interaction with controlled reaction time and temperature, leading to the formation

of 3D hierarchicalﬂower-like microspheres[43,44] TEM images shown inFig 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 non-uniform dispersibility.Fig 6d shows the SAED pattern which pro-vides the information of nanorods of polycrystalline MoO3NPs 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

different temperatures and different sonication times were

(Fig 7) Both the spectra show a strong reﬂectance response be-tween 420 and 570 nm which indicate that the sample shows high H.S Yogananda et al / Journal of Science: Advanced Materials and Devices 3 (2018) 77e85

78

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absorption in the visible region Here, the optical transition was

from the vacant“d” orbitals of the cation (Mo6þ) and“p” orbitals of

the oxygen ions with lone pair of electrons (O2)[45] The MoO3

sample obtained with 4 h sonication exhibits a wider adsorption

sonication time of 1 h, 2 h, 3 h and 4 h The absorption peak at

530 nm was due to the intrinsic absorption at the semiconductor

to estimate the energy band gap[47,48]

FðRÞ ¼ð1 RÞ2

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)n¼ A(hyEg), where n¼ 2 for a direct allowed transition, and n ¼ 1/2 for an in-direct allowed transition, A is the constant, and hyis the photon energy[49] The linear part of the curve was extrapolated to (F(R)

hy)1/2¼ 0 to get the indirect band gap energy The estimated Eg

values for the samples prepared under different sonication times

shown inFig 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 600C calcined sample with an emission wavelength of 438 nm is shown inFig 9(a) The spectrum consists of single excitation peak positioned at 324 nm

Fig 9(b) shows the PL spectra of MoO at different temperatures

Fig 1 (a) PXRD patterns of MoO 3 nanostructures calcined at different temperatures (a-180 C,b-400 C and h-600 C, a sonication time of 4 h) (b) PXRD patterns of MoO 3

nanostructures for different sonication times (a calcination temperature of 600C, CTAB- 1 g) (c) PXRD patterns of MoO 3 nanostructures for different surfactant (CTAB) con-centrations (the sonication time of 4 h and the calcination temperature of 600C).

Table 1

Detailed variations of particle size and bandgap values of MoO 3 under different conditions.

Temperature

( c)

Band gap

E g (eV)

Particle size

D (nm)

Sonication time (h) with CTAB (1 g) and 600 C calcination

Band gap

E g (eV)

Particle size

D (nm)

Surfactant CTAB (g) with 4 h sonication and 600 C calcination

Particle size

D (nm)

H.S Yogananda et al / Journal of Science: Advanced Materials and Devices 3 (2018) 77e85 79

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Fig 2 SEM images ofa-MoO 3 (a) nanorods calcined at 600C and (bed) SEM images of thea-MoO 3 under different magniﬁcations.

Fig 3 SEM images of h-MoO 3 (a) nanorods calcined at 600C and (bed) SEM images of the h-MoO 3 under different magniﬁcations.

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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

which is a reﬂection of a radiative recombination of inter band electrons and holes in MoO3crystals[51e53] Enhancement in PL intensity was observed at a lower wavelength (438 nm) due to the

Fig 4 SEM images of MoO 3 nanostructures for different sonication times (a) 1 h, (b) 2 h, (c) 3 h, and (d) 4 h.

Fig 5 SEM images of MoO 3 nanostructures for different CTAB concentrations (a) 1 g, (b) 2 g, (c) 3 g, and (d) 4 g.

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Fig 6 TEM images (aec), SAED pattern (d), and EDAX pattern (e) ofa-MoO 3 nanostructures.

Fig 7 (a) DRS ofa-MoO 3 nanostructures calcined at different temperatures (b) DRS of MoO 3 nanostructures for different sonication times of 1 h, 2 h, 3 h and 4 h.

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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

tempera-ture The 600C 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),

Molybde-num vacancies (VMo), interstitial oxygen (Oi), interstitial

Molybde-num (Moi), antisite oxygen (O), F-centers (created by oxygen ion

vacancy acquired by 2 electrons) or Fþ-centers (created by oxygen

ion vacancy acquired by 1 electron) or surface states The defects in

MoO3were created due to bond breaking and surface stress created

by large surface to volume ratio Due to these defects Fþ and F centers were converted to F-aggregates like F2, F2þ, F2 þ The energy

levels of these defects centers be present in the forbidden energy

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

calculated and shown inFig 10(a), conﬁrming that MoO3can be used as blue light emitting diodes Moreover, it was well-known

Fig 8 (a) Band gap analysis ofa-MoO 3 nanostructures calcined at different temperatures (b) Band gap analysis ofa-MoO 3 nanostructures for different sonication times of 1 h, 2 h,

3 h and 4 h.

Fig 9 (a) Excitation spectra ofa-MoO 3 nanostructures at a 438 nm emission wavelength (b) Emission spectra ofa-MoO 3 nanostructures at a 324 nm excitation wavelength.

Fig 10 (a) CIE diagram and (b) CCT diagram ofa-MoO nanostructures.

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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)

chroma-ticity coordinates to (U0, V0), the CCT value was found to be 1968 K

in-tensity is in warm region (Fig 10(b))[58]

4 Conclusion

MoO3crystal structures respectively for different calcination

tem-peratures of 180C, 400C, and 600C SEM images show that the

samples exhibited hierarchical morphologies such as nano rods and

ﬂowers CTAB surfactant and sonication time were observed to play

vital roles in obtaining different morphologies The band gap of the

samples was determined to be in the range 3.4e3.6 eV, indicating

that the prepared samples were wide band gap semiconductors PL

and CIE results showed, MoO3NPs are potential materials for blue

light emitting phosphors in display devices The CCT conﬁrms that

the product is a potential material for warm white light emitting

diodes

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