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ScienceDirect Materials Today: Proceedings 3 (2016) 54 – 63

National Conference on Emerging Trends of Advanced Functional Materials (NCAFM-2015)

Investigation and Comparison of Optical and Raman Bands of Mechanically Synthesised MoO3 Nano Powders A. Gopala Krishnaa, R.V.S.S.N Ravikumarb T. Vijaya Kumarc, S. Daniel Ephraima, B. Ranjithc, M. Pranoyc and Sundeep Dolaa* a Department of Nanotechnology, Jawaharlal Nehru Technological University, Kakinada, Andhra Pradesh, 533 003, India. b Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar, Andhra Pradesh, 522 510 c Department of Mechanical Engineering, K L University, Green Fields, Vaddeswaram, Guntur, 522 520, India.

Abstract

MoO3 Nano powders were synthesized using mechanochemical assisted synthesis technique at different time intervals at room temperature. The structural properties were analyzed by Powder X-ray Diffraction technique (PXRD), which reveals the average crystalline size, strain, dislocation densities and phase structure. The surface morphology and chemical composition of the sample is revealed by Scanning Electron Microscope (SEM) images and Energy Dispersive X-ray Spectroscopy (EDS) Spectra. The fundamental vibrational modes and Raman modes are demonstrated by FT-IR spectroscopy and Raman Spectroscopy and are compared. X-ray diffraction data is well matched with JCPDS data and confirms the RUWKRUKRPELFĮ- MoO3 phase structure of the Nano powders. The bandenergy is calculated from the UV-Spectra. The thermal properties are studies from TGDTA analysis. © 2016 2015Elsevier Ltd.All Allrights rightsreserved. reserved. © Elsevier Ltd. Selectionand andPeer-review Peer-review under responsibility of the Committee Members of National Conference on National Conference on Selection under responsibility of the Committee Members of National Conference on National Conference on Emerging Functional Materials (NCAFM-2015) EmergingTrends TrendsofofAdvanced Advanced Functional Materials (NCAFM-2015). Keywords: Nano powders, Mechanochemical synthesis, FT-IR, Raman Spectroscopy and UV-Vis spectra.

* Corresponding author. Tel.: +91 9701246017. E-mail address:[email protected]

2214-7853 © 2016 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of National Conference on National Conference on Emerging Trends of Advanced Functional Materials (NCAFM-2015) doi:10.1016/j.matpr.2016.01.121

A. Gopala Krishna et al. / Materials Today: Proceedings 3 (2016) 54 – 63

1.

Introduction

Transition metal oxides emerged as a new class of research materials due to their interesting applications as FDWDO\VLV¶V VHQVRUV DQG SKRWRFKURPDWLF GHYLFHV [1-4]. Among these materials Molybdenum tri Oxide (MoO 3), ntype semiconductor with a wide band gap (>2.7eV) is attractive due to its structural and optical properties [5-6]. Nanotechnology also provides a vast area of research in tribological applications [7]. Naturally Molybdenum oxides are classified in to thermodynamically stable orthRUKRPELF Į-MoO3 SKDVHDQGWKH PHWDVWDEOH PRQRFOLQLFȕ-MoO3 phase with ReO3 type structure. In general, molybdenum (IV) oxide exists in three different phase structures as WKHUPRG\QDPLFDOO\VWDEOHRUWKRUKRPELFĮ-MoO3) [8-9]PRQRFOLQLFȕ-MoO3) and hexagonal (h-MoO3) [10-11]. It has a vast area of applications such as photochromatic materials which changes colourless to blue by UV irradiation [12-13], smart windows [14], self-developing photography [12], conductive gas sensors [15], lubricants [16], and catalysts [17]. Various synthesis methods of MoO3 were also reported which includes hydrothermal synthesis [18-19], vapor-transportation [20], vapor-deposition [21], hydro/solvothermal treatment [22], chemical precipitation [23] and sol-gel method [24-25] in various different structures as rods [18], wires [26], belts, flowers [27-28], and platelets [22]. MoO3 is used as a solid lubricant as it offers excellent friction coefficient of nearly 0.2 at high temperature of about 700 oC [29]. The friction and wear behavior of MoO3 were analyzed at high temperatures with aluminum bronze and appreciable results were derived [30]. Nano composites of MoO3 based such as MoO3-CuO, MoO3-PbO, MoO3-CoO, MoO3-NiO and MoO3-ZnO are studied for their superior tribological properties at elevated temperatures [31-35]. Apart from frictional properties MoO 3 also have a numerous applications due to their unique thermal, optical and electric properties [36]. In the present work orthorhombic MoO 3 Nano powders were synthesized by simple ball milling technique in to five samples in different milling time proportions. The structural, optical and thermal properties were studied by Powder X-ray Diffraction (PXRD), Scanning Electron Microscope (SEM), Energy Dispersive X-ray Analysis (EDX), Fourier Transform Infrared (FT-IR) spectroscopy, Raman spectroscopy, UV-Vis absorption spectroscopy, Thermogravimetric analysis (TGA) and Differential Thermal Analysis (DTA). 2. Experimental Procedure 2.1 Materials Molybdenum tri oxide (MoO3) was purchased from Merck chemicals, India. The chemical reagents are of analytical grade and are used without any further purification. The chemicals used in the synthesis are of above 99% purity. 2.2 Synthesis Procedure MoO3 powders of mean size 5μm were considered in stoichiometric weight ratio and were mechanically activated using high energy planetary ball mill (Ratsch). The ball mill consists of 50 ml vial and balls of zirconium as medium in milling. Mechanical activation was performed using powder to ball ratio of 1:5. The milling was performed for 2, 4, 6, 8 and 10 hours respectively with an interval pause of 5 min after every 30 min and with a rotational speed of 350 rpm. The obtained mixtures were taken in a crucible and calcinated in air for 3 hours at 200oC. The samples prepared at different time intervals as mentioned were collected. 3. Characterization Powder X-ray diffraction (PXRD) patterns of MoO3 Nano powders were performed on a PANalytical XPert ProGLIIUDFWRPHWHUZLWK&X.ĮUDGLDWLRQȜ Å). The measurements were made at room temperature at a range of

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10o-80o RQ ș ZLWK D VWHS VL]H RI o. Scanning Electron Microscope (SEM) images were obtained from a Carl Zeiss SEM EVO with carbon coating and Energy Dispersive X-ray Spectroscopy (EDS) images were taken from ZEISS EVO 18. Fourier Transform Infrared (FT-IR) spectrum was recorded using the KBr pellets on Shimadzu IRAffinity-1 FT-IR spectrophotometer in the region 4000-400 cm-1. Raman measurements were obtained using mini Raman Microprobe Imaging system 3000 coupled to an optical microscope with spatial resolution of 1.5μm and spectral resolution 2cm-1. Optical absorption spectrum was recorded from JASCO V-670 Spectrophotometer in region of 200±1400 nm wavelength at room temperature. Thermogravimetry analyses (TGA) and Differential thermal analysis (DTA) were carried out at a ramp rate of 10 oC min-1, in nitrogen flow of 20 cm3min-1 using TGA/DTA-TA Instruments SDT Q600. 4. Results and Discussion 4.1 Powder X-Ray Diffraction analyses studies The crystal structural phases of the synthesized MoO3 Nano powders were performed by X-ray diffraction pattern. The XRD patterns of synthesised samples at different time intervals (2, 4, 6, 8 and 10 hours) are as shown in Fig. 1. The diffraction peaks exhibited by WKHVDPSOHVLQVLVWWKHSKDVHVWUXFWXUHDVRUWKRUKRPELFFU\VWDOV\VWHPĮMoO3) with corresponding lattice cell parameters a=3.962, b=13.858, c=3.697nm. The diffraction peaks of the Nano powders are well matched with the standard diffraction data of JCPDS file No. 05-0508 respectively. No change of phase structure is observed with respect to milling time.

Fig 1: X-ray Diffraction peaks of 2, 4, 6, 8, 10 hours ball milled MoO3 Nano powders

7KHH[LVWHQFHRIVKDUSGLIIUDFWLRQSHDNVDWșORFDWHGDW o, 23.37o 25.72o, 27.35o, 33.78o, 38.96o and 45.69 corresponding to (020), (110), (040), (021), (111), (060) and (202) reveals the formation of pure MoO 3 Nano powders. The estimation of crystalline size and lattice strain is performed by simple and powerful X-ray profile analysis [37]. o

a. Crystal size The average crystalline size of the prepared samples were calculated using the Debye-6FKHUUHU¶VIRUPXOD


[38] given in the Eq. (1). D=

଴Ǥଽఒ ఉ ୡ୭ୱ ఏ

nm

(1)

Where D is the crystalline size; ȜLVWKHZDYHOHQJWKRI;-UD\Ȝ QPIRU&X.Į ; ȕLVWKH)XOO:LGWK+DOI0D[LPXP):+0 RIWKH%UDJJVSHDNLQUDGLDQV șLVWKHGLIIUDFWLRQDQJOHRIWKHUHIOHFWLRQ The average crystalline size µD¶ is calculated from the diffraction peaks was found to be 34-16 nm. A small shift in the position of the diffraction peaks towards higher angles is noted. The peak broadening in the XRD pattern indicates the presence of small nanocrystals in the samples. There is no evidence of any impurity or bulk material remnant.

Fig 2: Average crystalline size of MoO3 Nano powders at different milling time.

The Fig 2 shows that the average crystalline size of the MoO3 reduces gradually with decreases in milling time. The least size was found at 10 hours ball milling which produced 16.3 nm sized crystals. The average crystal sizes at different time intervals can be seen in Table 1. b. Micro strain The shifting in the peak position and line broadening of the XRD pattern reveals the presence of micro strains in the sample. Lattice strain is a measurement of distribution of lattice constants arising from crystal imperfection such as lattice dislocation. The micro strains (‫ )ڙ‬induced broadening in powders due to crystal imperfection and distortion was calculated using the Eq. (2) [39]. From Fig. 3(a) the graph shows the increase of micro strain with respect to increase of milling time. The values of Micro strains MoO3 Nano powders synthesised at different milling intervals are tabulated in Table 1. ఉ௖௢௦ఏ

‫=ڙ‬ c. Dislocation Density

ସ

(2).

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The high speed rotations of ball and the vial produces a twist is in the crystal which results a screw like dislocations from ball milling. The grinding balls are accelerated through coriolis and centrifugal forces. A powerful impact forces are resulted between the balls and the sample material as the material is powerfully accelerated from one side to other in the vial. X-ray line broadening was used to estimate the dislocation densities in the samples. The dislocation density (į) was calculated by the Eq. (3). į '2)

(3)

Where D is the crystallite size.

Fig 3: (a) Micro strain and (b) Dislocation Densities of MoO3 Nano powders at different milling times.

The dislocation densities of the samples increase with the increase in milling time. This is shown from the Fig 3(b) and the values of dislocation densities of different samples synthesized at different time intervals are as shown in Table 1. Table 1. Comparison of average crystalline size, Micro strain, and Dislocation density of MoO 3 Nano powders synthesised at different milling time. Milling Time (hours)

Micro strain ‫ڙ‬î-3)

Dislocation density įî-15m-2)

2

Crystalline Size 6FKHUUHU¶V 34.6

0.02

4.32

4

30.2

0.27

3.84

6

26.6

0.31

2.97

8

20.4

0.35

3.36

10

16.3

0.37

1.73

4.2 Morphological Studies The morphological studies of MoO3 Nano powders are observed from SEM images. SEM analysis is an important characterisation technique for topographic study of the samples and gives the information regarding the growth mechanism, shape and size of the particles. Fig. 4 (a,b and c) show the images of MoO3 Nano powders taken


at different magnifications. The prepared samples contain rod like and spherical like structures, with irregular shape and dimensions. As the grain size from SEM images are the domains formed by the agglomeration of nanocrystalline [40], the grain sizes from the SEM images and the crystalline size calculated from the debyeVFKHUUHU¶V formula. The EDS spectra of MoO3 Nano powders are shown in the Fig. 4. (d and f). It was found that EDS data demonstrates that prepared samples are composed of Molybdenum (Mo) and Oxygen (O) species. No other element was found in EDS spectra results in purity of as synthesised sample.

Fig. 4. (a,b,c) SEM images and (d and e) EDS pattern of MoO3 Nano powders

4.3 FTIR and Raman Studies (a) FTIR Studies FT-IR spectra of MoO3 Nano powders are shown in the Fig. 5(a), which is acquired in the range of 4000400cm-1. The characteristic bands observed at 470, 619, 882 and 991 cm-1 are attributed to the fundamental vibrational modes of Mo=O [41]. The successive synthesis of MoO3 Nano powders is shown by the characteristic bands. The formation of as-synthesised Nano powders is also confirmed by the X-ray diffraction (XRD) and EDS spectra. The dominion band at 870 cm-1 is associated with the vibration of Mo-O-Mo bridging bonds [42]. The absorption bands detected at 1387 and 1643 cmí1 were associated with the vibration mode of the Mo±OH bond and the bending mode of adsorbed water, respectively [43]. As the samples were calcinated at high temperatures, the presence of stretching and bending bonds are relatively low. (b) Raman Studies Crystalline quality can be effectively evaluated using the Raman spectroscopy. Fig. 5(b) shows the Raman analysis bands corresponding to tKH FKDUDFWHULVWLF SKDVH RI Į-MoO3 Nano powders. The vibrational mode

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assignments of MoO3 are explained on the basis as described in the literature [44-47]. The number of physical properties of materials depends on the interactions between vibrational modes of molecules or lattices. According to WKHJURXSWKHRU\5DPDQDFWLYHPRGHVDUHVKRZQIRUĮ-MoO3) as 8Ag+8B1g+ 4Bg2+4Bg3 [48]. The vibrations in the MoO3 are due to the involvement of oxygen atoms, since the molybdenum atom is about six times heavier than oxygen atom. The vibrational modes occurring in the region of 1000-600 cm-1 correspond to MoO6 octahedral stretching modes and the modes between 400-200 cm-1 and lower modes below 200 cm-1 corresponds to MoO6 octahedral bending vibrations and lattice modes [45-47, 48]. From the Fig. 5(b) two main absorption patterns observed at 995, 819 cm-1 are available with MoO3 and corresponds to symmetric stretching of Mo=O and Mo-OMo. The Raman band assignments of MoO3 Nano powders are tabulated in Table 2.

Fig. 5. (a) FT-IR and (b) Raman spectra of MoO3 Nano powders.

Table 2: Vibrational Band Assignments of FT-IR and Raman spectra bands of MoO3 Nano powders FTIR

Band Assignment

Raman (cm-1)

Band Assignment

619

Mo-O

123, 151

Deformation Modes

882

Mo-O-Mo

282, 330, 376, 660

Mo-O (Bending)

991

Mo=O

812

Mo=O (Streching)

1384, 1643

Mo-OH

990

Mo-O-Mo (Streching)

4.4 Optical Studies The optical properties of synthesised MoO3 Nano powders were analysed by UV-Visible absorbance spectroscopy. Measuring the band gap is important in the semiconductor and nanomaterial industries. The band gap energy of insulators is large (> 4eV), but lower for semiconductors (< 3eV). 7KH WHUP ³EDQG JDS´ UHIHUV WR WKH energy difference between the top of the valence band to the bottom of the conduction band electrons are able to jump from one band to another.


Fig. 6: UV-Vis spectrum of MoO3 powders

In order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition, the band gap energy. Fig. 6 shows the UV-Visible absorbance spectrum of MoO3 Nano powders, where the optical edge is observed at 333 nm. The band gap energy is calculated as 3.73 eV by the Eq (4). %DQG*DS(QHUJ\( K &Ȝ (4) where h = Planks constant = 6.626 x 10-34 Joules sec, C = Speed of light = 3.0 x 108 meter/sec, Ȝ &XW RII wavelength = 333 x 10-9 meters. 4.5 Thermal Studies To understand the thermal stability and relative weight loss of the material, Thermogravimetric and Differential Thermal Analysis (DTA) plays an important role. The analyses are performed under nitrogen atmosphere of 20 cm3 min-1 at a ramp rate of 10o C min-1. The sample weights around 13.5 mg, the TGA and DTA curves of MoO 3 Nano powders are as shown in the Fig. 7. The MoO3 Nano powders show an excellent thermal stability up to 637 oC with a negligible weight loss of about 0.7 percent and without ant stage decomposition. Hence the result shows that MoO3 Nano powders show a higher flame resistance and thermal stability.

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Fig. 7. TG/DTA curves of MoO3 Nano powders

5. Conclusion Using a simple and inexpensive mechanical activation method, the synthesis of MoO 3 Nano powders were prepared. Powder XRD diffraction patterns of the prepared suggests the orthorhombic phase structured MoO3. The crystalline sizes are evaluated from the diffraction data through Debye-6FKHUUHU¶VIRUPXODZKLFKare in the order of nanometres. Morphology obtained from SEM images confirms the irregular shaped particle sizes and EDS spectra confirm the presence of constituent elements of the prepared samples. The characteristic vibrational bands of MoO 3 are revealed through FT-IR spectrum. The qualitative information of the samples was obtained from Raman spectroscopy. Band gap energy is calculated using UV-Visible absorbance spectrum. The TGA/DTA curves show high thermal stability up to 637 oC with a minor decomposition. Hence the MoO3 nanocomposites may be used as metal insulator semiconductors and also as a lubricant at high temperatures.

Acknowledgments The authors Dola Sundeep and S.Daniel Ephraim are thankful to Director Centralised Laboratory, Acharya Nagarjuna University, for providing FT-IR, UV-Vis and for his extreme guidance and support and are thankful to AICTE, New Delhi, India for providing financial assistance through AICTE - GATE meritorious fellowship (11889699051) to carry out the research. 7KH DXWKRU¶V DFNQRZOHGJH P.S.S.Rao JNTU Kakinada for Raman Spectroscopy analysis, OUCT Hyderabad for SEM, EDS and TG/DTA and IIT Madras is acknowledged for performing XRD analysis. References [1] A.F. Lamic-Humblot, P. Barthe, G. Guzman, L. Delannoy, C. Louis, Thin Solid Films 527 (2013) 96. [2] Y. Zhao, B. Yang, J. Xu, Z. Fu, M. Wu, F. Li, Thin Solid Films 520 (2012) 3515. [3] H.M. Martínez, J. Torres, M.E. Rodríguez-García, L.D.L. Carreño, Phys. B Con. Matter 407 (2012) 3199. [4] C.S. Hsu, C.C. Chan, H.T. Huang, C.H. Peng, W.C. Hsu, Thin Solid Films 516 (2010) 4839 [5] J.N.Yao, K.Hashimoro, A.Fujishima, Nature 533, (1992), 624-626 [6] F.Hamelmnn, K.Gesheva, T.Ivanova, A.Szekeres, M.Abroshev, U.Heinzmnn; J. Optoelectron. Adv. Matter, (2005), 393-399. [7] D.Sundeep, S.Daniel Ephraim; Use of nanotechnology in reduction of friction and wear; Vol. 1, (2014), IJIRAE, 2349-2163.

A. Gopala Krishna et al. / Materials Today: Proceedings 3 (2016) 54 – 63 [8] Song, J., Li, Y., Zhu, X., Zhao, S., Hu, Y., Hu, G.; Mater. Lett. 95, (2013), 190-192. [9] Nirupama. V, Uthanna S.; J. Mater. Sci. Mater. Electron. 21, (2010) 45-52. [10] P.Badica, J. Cryst. Growth Des., 7, (2007), 794-801. [11] N.A. Chernova, M.Roppolo, A.C.Dillon, M.S.Whittingham, J. Mater. Chem. 19, (2009), 2526-2552. [12] 7+HDQG-