Structural and Optical Properties of V2O5-MoO3- ZnO

Transactions of the Indian Ceramic Society

ISSN: 0371-750X (Print) 2165-5456 (Online) Journal homepage: http://www.tandfonline.com/loi/tcer20

Structural and Optical Properties of V2O5-MoO3ZnO Glass-Nanocomposite System Anindya Sundar Das, Madhab Roy, Debasish Roy, Satchidananda Rath & Sanjib Bhattacharya To cite this article: Anindya Sundar Das, Madhab Roy, Debasish Roy, Satchidananda Rath & Sanjib Bhattacharya (2016): Structural and Optical Properties of V2O5-MoO3ZnO Glass-Nanocomposite System, Transactions of the Indian Ceramic Society, DOI: 10.1080/0371750X.2016.1175321 To link to this article: http://dx.doi.org/10.1080/0371750X.2016.1175321

Published online: 21 Jul 2016.

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Date: 21 July 2016, At: 23:36

Platinum Jubilee Volume

Trans. Ind. Ceram. Soc., vol. 75, no. 2, pp. 1-6 (2016). © 2016 The Indian Ceramic Society ISSN 0371-750X (Print), ISSN 2165-5456 (Online) http://dx.doi.org/10.1080/0371750X.2016.1175321

Structural and Optical Properties of V2O5-MoO3-ZnO Glass-Nanocomposite System Anindya Sundar Das,1, 2 Madhab Roy,1 Debasish Roy,2 Satchidananda Rath3 and Sanjib Bhattacharya4;* 1

Department of Electrical Engineering, Jadavpur University, Jadavpur, Kolkata – 700 032, India Department of Mechanical Engineering, Jadavpur University, Jadavpur, Kolkata – 700 032, India 3 School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar – 751 007, India 4 Department of Engineering Sciences and Humanities, Siliguri Institute of Technology, Darjeeeling – 734 009 (WB), India 2

Downloaded by [Mr Anindya Sundar Das] at 23:36 21 July 2016

[MS received October 09, 2015; Revised copy received February 15, 2016; Accepted April 02, 2016]

ABSTRACT The present work points to highlight the physical, structural and the optical properties of some semiconducting V2O5-MoO3-ZnO glass-nanocomposites using density, molar volume, X-ray diffraction (XRD), field effect scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and UV-VIS absorption spectra. We have observed that addition of V2O5 increases (or decreases) the density (or molar volume) of the glassy system due to structural changes. Distribution of Zn3V2MoO11, Zn2.5VMoO8 and Zn2V2O7 nanoparticles has been confirmed from FESEM and XRD studies. It has been observed from FTIR spectra that the network structure depends upon isolated strongly deformed M oO 4 polyhedra and VO 4 metavanadate chains. Vibrations of MoO6 octahedra, Zn2V 2O6, Zn2V2O7, Zn3V2MoO11 and VO2 are observed from the Raman spectroscopic studies. The fundamental UV-VIS absorption spectra have been analyzed, which indicates indirect transitions. [Keywords: Semiconducting V2O5 doped zinc-molybdate glass-nanocomposites-, XRD, Raman spectroscopy, UV-VIS absorption, FESEM, FTIR]

Introduction A rising interest on transition metal oxide (TMO) doped glasses and glass-nanocomposites in possible use of various devices applications1–3 has lead to their extensive studies in recent years. A lot of work on their electrical transport and structure1–6 has been carried out so far. It is observed from the literatures 1–7 that their transport properties significantly depend upon the structures. Formation of vanadium-oxygen polyhedra surrounded by one or more ions may lead to changes in the electrical conduction via hopping of electrons. The short-range atomic order in semiconducting molybdenum tellurite glassy system has been revealed from neutron diffraction8 analysis. Report 9 on the electrical properties of molybdenum phosphate glassy system exhibits the fact that the modifier oxide greatly affects the magnitude of the conductivity and activation energy of those glasses containing the same glass forming oxide, MoO3. Dimitrov10 has successfully shown that zinc vanadate glassy system contains unmodified VO5 groups, modified VO 5 groups and V 2O 7 pyrovanadate units as major structural units. Hubert et al.11 have observed that zinc *Corresponding author; email: [email protected]

VOL. 75 (2) APRIL – JUNE 2016

borovanadate glassy system contains VO4 tetrahedra as the main coordination polyhedra for V atoms. It has been observed that VO5 bi-pyramids are transformed into VO4 tetrahedra when modifier oxide (ZnO) is added to vanadate glassy system.12 Comparing IR spectroscopic data with known crystalline structures of V 2O5-MoO3-ZnO glassy system,13 suitable structural models have been proposed. From this comparison, it is suggested that layered or chain structured VO5 groups, isolated V2O7 pyrovanadate units and MoO6 groups are the major structural units of this amorphous network. Introduction of molybdenum ions in glassy system produces a change in optical and electrical properties, which are related to the existence of molybdenum ions in different valence states Mo3+, Mo4+, Mo5+ and Mo6+.14 Molybdenum and vanadium containing glassy systems are potential candidates for practical applications as amorphous semiconductors and solid electrolytes. 15 The structure of TMO doped glassnanocomposites has not been studied extensively so far. Structural study and optical response of glassnanocomposite materials are of our interest to shed some light on their microscopic properties. In the present paper, the composition dependence of the physical behaviour, structural and optical properties

1


Results and Discussion Figure 1 shows the variation of densities of the asprepared glass-nanocomposites with V2O5 content. The relationship between density and composition of an oxide glassy system can be expressed in terms of molar volume (VM) for the glass system, which can be obtained using the formula VM =xiMi/

..(1)

where xi is the molar fraction, Mi is the molecular weight of the ith component and  is the density of the system.

2

MOLAR VOLUME (cm 3)

Experimental Procedures Glass-nanocomposite samples xV2O5-(1–x)(0.05MoO30.95ZnO) where x = 0.3, 0.5, 0.7, 0.9, 0.92, 0.93 and 0.95 were prepared from the reagent grade chemicals V2O5, MoO3 and ZnO. The appropriate amounts of V2O5, MoO3 and ZnO powders were thoroughly mixed and preheated in an alumina crucible in the temperature range from 800o to 900oC depending upon the composition. Here, solid state reaction starts at few degree higher than room temperature and during this process sublimation of MoO3 does not take place because of heat of reaction. The melts were equilibrated for 30 min and then quenched between two aluminium plates. Partially transparent glassnanocomposites of thickness ~0.1 mm were obtained for x = 0.30-0.95. X-ray diffraction (XRD) patterns of the samples were recorded using a Seifert (model 3000P) X-ray diffractometer. In this instrument, Ni filtered CuK radiation operating at 35 kV and 25 mA in a step scan mode was used. The step size was taken to be of 0.02o in 2 and a hold time of 2 s per step. The microstructure and particle distribution in glassy nanocomposites were explored by transmission electron microscopic (TEM, JEOL; model: JEM-2100 HR) studies. Density () of the samples was measured by Archimedes’ principle using acetone as an immersion liquid. The FTIR spectra of the bulk samples in transmission mode were recorded in a Shimadzu FTIR 8400S spectrometer at 25 oC and at relative humidity of 50-60%. For FTIR measurements, pellets of thickness 1 mm and diameter 13 mm were obtained by pressing a mixture of 1 part of glass and 60 parts of KBr at a pressure of 200 kg.cm–2. The optical absorption spectra of the powder samples in the reflectance mode were recorded in the wavelength range 200-500 nm at room temperature using JASCO UV-VIS absorption spectrometer; model: V-630. Room temperature Raman spectroscopy was performed in the 100-1700 cm–1 wavelength range. A laser line of 514 nm wavelength and 20 mW power from argon ion source focussed to a 5 m circular spot, was used as an excitation line for the Raman spectra measurements.

DENSITY (g.cm –3)

of V 2O5-MoO 3-ZnO glass-nanocomposites has been investigated. Interestingly, we have also studied vibration properties and estimated optical band gap of vanadate glass-nanocomposites formed with other non-traditional glass formers such as MoO3.

X

Fig. 1 – Density and molar volume of xV 2O5-(1–x)(0.05MoO30.95ZnO) glass nanocomposites

The variations of molar volume for the prepared glassnanocmposites with V 2O5 content are also included in Fig. 1. It can be seen from Fig. 1 that density decreases while molar volume increases with V 2O5 content within this range of solubility limit. It may be due to the fact that addition of V2O5 modifies the ZnO-MoO3 glassy network, leading to less connectivity and compactness of the glass structure. Addition of V2O5 naturally increases the density of the glassy system due to increase of the molecular weight. But, here the density decreases with increasing V 2O 5 content. The decrease in density with gradual increase of V 2O5 content may be due to the gradual increase of non-bridging oxygen, attributed to the change occurred in volume concentration.14, 16 In Fig. 1, the density and the molar volume show opposite behaviour. The increase of molar volume with the addition of V 2O 5 attributed to the increase in number of non-bridging oxygen14, 16 as well as different nanophases15, 17 of V2O5, dispersed in the glass matrix. The change in molar volume depends on the rates of change of both density and molecular weight. The molecular weight increases with increase of V2O5, and the density decreases. So it must be accompanied by an increase in molar volume. The XRD patterns of all the glass-nanocomposites are shown in Fig. 2a. The sample x = 0.7 does not show more peaks due to mismatch of different nanocrystalline planes. Different crystalline peaks arise because individual nanostructures merge together. As a result, the activation energy becomes much larger so that small polaron may be responsible for grain-boundary diffusion as well as surface and volume diffusion for x = 0.7. Different peaks for Zn 3V 2 MoO 11 , 18 Zn 2.5 VMoO 8 , 19 and Zn 2V 2 O 7 20 nanocrystallites and MoO3 rod-like structural phases19, 20 are observed. The crystalline volume fractions in asprepared samples have been determined from the relation,16 XhklXRD = Ihkl/(Ia+khkl Ihkl). Here, Ihkl represents the area under the crystalline (hkl) diffraction peak and Ia was chosen as the area under the first diffraction maximum of the XRD under consideration. The calibration constant khkl was chosen as khkl=Ihkl/Ia. It is computed from XRD data TRANSACTIONS OF THE INDIAN CERAMIC SOCIETY

(152) (152)

(240)

(421)

(240)

(132)

(042) (042)

(124) (133)

(033)

(111)

x = 0.9

(241)

Figures 4a, 4b and 4c show the FESEM images for x = 0.3, 0.7 and 0.92 respectively. FESEM images exhibit surface morphology as well as distribution of different nanoparticles. Actually, Zn3V 2MoO11, Zn2.5VMoO 8 and Zn2V 2O7 nanocrystallites and MoO 3 rod-like structural phases are confirmed from XRD (Fig. 2a) and JCPDS data sheets.17–20 Their distribution has also been confirmed from TEM (Fig. 2b) and FESEM (Fig. 4) pictures. On the basis of the above discussion, it should be claimed that Figs. 4a-4c represent the distribution of Zn3V 2MoO11, Zn2.5VMoO8 and Zn2V2O7 nanoparticles having average size of 30 nm and tiny MoO3 rod-like structures18–20 for x = 0.3. Figure 4b indicates amorphous like nature as discussed earlier. But the sizes of Zn 3 V 2 MoO 11 , Zn2.5VMoO 8 and Zn2V 2O 7 nanoparticles are found to decrease in Fig. 4b for x = 0.92, which is already confirmed from Fig. 3. Here, addition of V2O5 may play an important role for the structural changes of the as-prepared glassnanocomposites.

(134)

x = 0.7

x = 0.5

(116)

(152)

(421)

(a) (133) (313) (132) (311)

20

(240)

(042)

(133)

(132) (132)

(121)

(033)

(121) (211) (121) (033) (124) (133)

(111)

(022)

x = 0.93

(002)

(121)

INTENSITY (a.u.)

(022)

x = 0.3

40 60 2 (degree)

(b) 80

(a)

(b)

Fig. 2 – (a) X-ray diffractograms of xV 2O5-(1–x)(0.05M oO 30.95Z nO) glass-nanocomposites with different values of x, (b) TEM image for x = 0.9

The Debye-Scherer formula,21 t=0.89/(cos), where t denotes the average grain size of the particles,  stands for the X-ray wavelength (1.54 Å),  for the Bragg diffraction angle and  for the peak width in radians at half-height, is employed to estimate average size of different nanocrystallites dispersed in the glass matrices. The variation of average crystallite sizes of above mentioned nanophases with V2O5 content is presented in Fig. 3. It is observed that average crystallite sizes are almost same up to x = 0.90 and after that it is found to decrease abruptly. This result may be related to their structures, which is discussed later. The instrumental and strain broadening effect have been considered while estimating average crystallite size. AVERAGE CRYSTALLITE SIZE (nm)


(121)

(022)

that out of all nanophases formed in the glassy matrices, 30% phases are Zn 2.5 VMoO 8, 15% phases are Zn3V 2MoO 11 and 40% phases are Zn2V 2O7 nanocrystallites. Rest phases are dispersed MoO 3 rod-like structures. The TEM image for x = 0.9 is presented in Fig. 2b, which shows the distribution of nanocrystallites of average size 28 nm. It is also clear from Fig. 2b that these nanocrystallites are distributed in the glassy amorphous matrices. Though the amorphous phase is not clear from XRD patterns (Fig. 2a), TEM image represents the amorphous/glassy nature for x = 0.9.

X Fig. 3 – Estimated sizes of different crystallites with V2O5 content


(c)

Fig. 4 – FESEM images of xV2O5-(1–x)(0.05MoO3-0.95ZnO) glassnanocomposites for (a) x = 0.3, (b) x = 0.7 and (c) x = 0.92

Figure 5 shows the room temperature FTIR spectra in the transmittance mode recorded in the region 4000-400 cm –1 for xV 2 O 5 -(1–x)(0.05MoO 3 -0.95ZnO) glassnanocomposites. Here, different bands are assigned to different vanadate and molybdate structural units. All the spectra show a water band at 1620 cm–1. The band near 1000 cm–1 is due to the stretching vibration of isolated V=O vanadyl groups in VO5 trigonal bipyramid.13 This band becomes weaker with the increase of V2O5 content in the compositions except x = 0.93 as shown in the spectra. It is clear from Fig. 1 that the density decreases with the gradual increase of V 2O5 content. This result may be attributed to the gradual increase of non-bridging oxygen, which is the main cause of bond weakening with the increase of V2O5 content. The exception for x = 0.93 needs more study in near future. Another band in the 750-970 cm–1 range is assigned to the vibrations of isolated strongly deformed MoO4 polyhedra.22 Here, some of the disordered (V/Mo)O4 tetrahedra are linked to ZnO6 octahedra and/or

3

x = 0.95

INTENSITY (a.u.)

x = 0.93 x = 0.92 x = 0.9 x = 0.7

x = 0.5

x = 0.3

4000

3000 2000 1000 WAVE NUMBER (cm–1)

trigonal prisms by sharing corners to form a three dimensional framework. Thus with the increase of V2O5 content in the composition, VO 5 groups are gradually replaced by VO4 metavanadate chains.13 This result also indicates gradual increase of non-bridging oxygen or decrease in density (Fig. 1). Consequently, a solid phase of ZnMoO4 is formed.20 The Raman scattering spectra for x = 0.5, 0.9 and 0.93 are shown in Fig. 6. The strong band at 930 cm–1 for x = 0.5 may be assigned to the vibrations of Mo-O terminal bonds in MoOy polyhedra.22 But in the Raman spectra for x = 0.9 and 0.93, it is noted that 930 cm–1 peak is shifted to more intense peak near 995 cm–1 due to stretching vibration of molybdenum atom and unshared oxygen atom.23 Recent study23 reveals that peak at 995 cm–1 can be assigned to the vibration of MoO6 octahedra.23 It may be concluded that phase transformation of molybdenum oxide from polyhedron to octahedron takes place as the V2O5 content increases (MoO3 decreases) in the compositions. This phase transformation may occur due to the formation of some complex Mo-O-V structures. The probable reason for formation of this complex structure may be due to

() = (1/d)ln(I0/It)

..(2)

where I0 and It are the intensities of the incident and transmitted beams, respectively and d corresponds to thickness of each sample. The factor ln(I0/It) is the absorbance, A. From this absorption study, the optical band gap energy, Eg, can be obtained using the following general relation proposed by Mott and Davis28 h = [B(h–Eg)]r

..(3)

where the index r takes different values depending on the mechanism of interband transitions, B is a constant called band tailing parameter, h is the incident photon energy. Here, r = 2 in Eqn (3) represents a straight line, which is associated with indirect allowed transitions. Tauc’s plot29

T = 0.93

x = 0.9

x = 0.5

ABSORBANCE (a.u.)

x = 0.93

INTENSITY (a.u.)


Fig. 5 – FTIR spectra for xV2O5-(1–x)(0.05MoO3-0.95ZnO) glassnanocomposites

increase in non-bridging oxygen or decrease in density (Fig. 1) as V2O5 content increases. Bands in the range 150-800 cm –1 may be due to different vanadate compounds like Zn2V2O6, Zn2V2O7, Zn3V2MoO11 and VO2 observed in the spectra.24, 25 It is also noted that vibration modes of VO2, appeared for x = 0.5 at 167 and 274 cm–1 are absent for x = 0.9 and 0.93. It is also noteworthy that peaks at 712 and 428 cm–1 for x = 0.5 have been shifted to 695 and 400 cm–1 for x = 0.9 and 0.93. This peak shifting may correspond to another phase transformation from Zn2V2O6 to Zn2V2O7 vibration modes.24, 25 The observed absorption of Mo5+ ions for the formation of Zn3V2MoO11 unit could also be explained using Raman study. The radius of V5+ ion (0.59 Å) is close to the radius of Mo5+ ion (0.65 Å). Owing to similarity of ionic radius, a part of the Mo5+ ions in the MoO 3 is substituted by V 5+ ions to form Zn3V2MoO11 unit, where ZnO acts as a stabilizer.26 Absorption (UV-VIS) as a function of incident wavelength for different compositions has been studied and is shown in Fig. 7. The absorption coefficient () at different wavelengths may be estimated using the relation:27

T = 0.92

T = 0.90 T = 0.70

T = 0.50

RAMAN SHIFT (cm–1)

Fig. 6 – Raman spectra for xV 2O5-(1–x)(0.05MoO3-0.95ZnO) glass-nanocomposites with x = 0.5, 0.9 and 0.93 [o: Zn3V2MoO11,  Mo-O-V, *: for x = 0.5, Zn 2V 2O6 and for x = 0.93, Zn 2V2O7,  Mo-O-V and  for x = 0.5, Mo-O and for x = 0.93, MoO6]

4

T = 0.30

250

300 350 400 450 WAVELENGTH (nm)

500

Fig. 7 – Absorption spectra with wavelength for all xV2O5-(1–x) (0.05MoO3-0.95ZnO) glass-nanocomposites

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2.5

(h)½ (cm–½eV½)

x = 0.93

x = 0.5 x = 0.9 x = 0.7

1.5

1.0 x = 0.3

Conclusions Physical and structural studies such as density, molar volume, XRD and FESEM, and optical and vibrational studies such as UV-VIS, FTIR and Raman spectroscopy have been carried out for V 2 O 5-MoO 3 -ZnO glassnanocomposites. The change in molar volume depends on the rates of change of both density and molecular weight. However, the molecular weight increases with increase of V2O5 and the cross-linking density decreases, which must be accompanied by an increase in molar volume. Formation and estimation of Zn 3 V 2MoO 11 , Zn2.5VMoO8 and Zn2V2O7 nanoparticles are determined from XRD and FESEM studies. The FTIR studies suggest a transformation of VO 5 trigonal bipyramid into VO 4 metavanadate chains due to the insertion of isolated strongly deformed MoO 4 polyhedra and ZnO. Raman spectroscopic studies reveal some other vibrations of MoO6 octahedra, Zn2V2O6, Zn2V2O7, Zn3V2MoO11 and VO2 of glass-nanocomposites under investigation. Variation of optical band gap may be associated with the structural changes due to the addition of V2O5 content into the MoO3ZnO system. It is observed that the present system under investigation behaves as an indirect band gap semiconductor.

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Acknowledgements: The financial assistance for the work by the Council of Scientific and Industrial Research (CSIR), India via sanction no. 03(1286)/13/EMR-II is thankfully acknowledged. Centre for Research in Nanoscience and Nanotechnology, Kolkata, India is also acknowledged for providing FESEM facility. Authors also acknowledge Dr Joydeep Chowdhury and Mr Sannak Dutta Roy, Department of Physics, Sammilani Mahavidyalaya, Kolkata, India for providing support for UV-VIS absorption spectra.

2.0

Eg (eV)


is employed to illustrate the variation of (h)1/2 with h for different glass-nanocomposites as shown in Fig. 8a. To estimate the values of Eg, the linear region of the curves is extrapolated to meet the h axis at (h)1/2=0. Figure 8b depicts the variation of E g with compositions. It is observed from Fig. 8b that E g becomes smaller with increasing V2O5 for x = 0.7 and after that it increases up to x = 0.9 and then decreases with increasing V2O5 content. The estimated Eg values for all the samples lie in the same range as reported for semiconductors.30, 31 This behaviour may be associated with the structural changes that take place due to the addition of V2O5 content into the MoO3ZnO system. Formation of Zn3V2MoO11, Zn2.5VMoO8 and Zn2V2O7 nanoparticles and cross linking of V---O---M may be the possible reasons for this structural change. So it can be concluded that whatever be the mechanism of transition of the system under study, the present system behaves as an indirect gap semiconductor.28 The samples under consideration are amorphous semiconductors, which contain some localized states. No perfect band gap exists. Polaron hoping may take place from one localized state into another when the sample is thermally excited. The average distance between two consecutive localized states is supposed to be band gap energy for this particular amorphous semiconductor. Here, band gap energy for x = 0.7 is found to be minimum. The reason for this may be due to structural changes due to the formation of

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X Fig. 8 – (a) Tauc’s plots for xV2O5-(1–x)(0.05MoO3-0.95ZnO) glassnanocomposites considering r = 2 in Eqn (3), (b) variation of Eg with x


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