Spectroscopic Investigation on Zinc Oxy Fluro Borate Glasses Doped with V₂O₅

Spectroscopic Investigation on Zinc Oxy Fluro Borate Glasses Doped with V₂O₅

1. Naresp*, B. Rupa Venkateswara Rao1,

   1Dept. of Physics,

   V.R. Siddhartha Engineering College, Vijayawada-52007, India

   Abstract – In this work, glass systems of the composition (10-x) ZnO30ZnF260B2O3: xV2O5 where (x = 0.2, 0.4, 0.6, 0.8,

   1.0 mol %) have been prepared by the conventional melt

   quenching technique. Optical absorption, IR and ESR of these glass samples has been investigated. Optical absorption spectra exhibits two broad absorption bands at about 640 and 1020 nm due to 2B22B1 and 2B22E transitions of VO2+ ions. With increase in the concentration of V2O5, the intensity of these peaks is observed to increase with a red shift. The IR spectral studies indicated that the glass samples contain various structural units with the linkages of the type BOB, VOV; the increasing content of V2O5 in the glass samples seemed to have weakened such linkages. The ESR results

   1. Vijay 2, P. Ramesh Babu2

      2 Dept. of Physics,

      Usha Rama College of Engineering & Technology,

      Telaprolu-521109, India

      alkaline earth and transition metal oxides but it does not form glass on their own [24-27].

      In a borate glass matrix, vanadium ions are easily dissolved due to some of the infrared vibrational bands of the structural groups of the vanadium ions and the BO3 and BO4 structural units are lying in the same region. The vanadium ions in the borate glass matrix can adopt two valence states, V4+ and V5+. The electrical conductivity of the material is expected to

      indicate that the ratio

      g g

      ||

      is observed to decrease

      influence by the presence of the vanadium ions due to polaron hoping between valence states V4+

      gradually with concentration of V2O5. Finally, the analysis of

      the results of OA, ESR spectra of the studied glass has indicated that a considerable proportion of vanadium ions do exist in V4+ state in addition to V5+ state with an increase in the concentration of V2O5.

      Key Words: Glasses, OA, IR, ESR.

      1. INTRODUCTION

         The borate glasses containing ZnO have found many technological applications in gas sensors, catalysts, solar cells, display panels etc., due to high in thermal conductivity, low in specific heat, soaring in insulating strength, low in thermal expansion coefficient and good transparency [1-4]. By adding ZnF2 to ZnOB2O3 glasses, it is expected to lower the viscosity and liquidus temperature to a considerable extent and also widens the glass-forming region of the system [5-7].

         A vanadium doped glasses are potential candidates in technological applications such as in electrical threshold, optical and memory switching devices, cathode materials for solid, devices and optical fiber [8-16] due their superior properties such as relatively high electrical conductivity, good semi conducting properties, chemical durability, low crystallization tendency and melting point [17-23]. V2O5 is a conditional glass former i.e., it readily forms glass with other glass formers or with modifiers such as alkali,

         V5+. There are available several polaron is hopping models to describe the conduction mechanism of vanadate glasses [28-30].

         The aim of the present work is to synthesize ZnO-ZnF2- B2O3 glasses doped with different concentration of V2O5 and investigate the structural changes due to the varied oxidation states of vanadium ion using optical absorption, IR and ESR spectral studies.

      2. EXPERIMENTAL METHODS

1. Preparation of glasses

   Zinc oxy fluro borate glasses doped with different concentration of V2O5 are prepared by using analytic grade chemicals of ZnO, ZnF2, H3BO3 and V2O5 as starting materials with 99.9% purity. The ratio of composition for the present study is mentioned in Table 1. These compounds in appropriate amounts (all in mol%) were mixed thoroughly in an agate motor and melted in an electric furnace in silica crucible around 950 0C for nearly 1 hour till a bubble free liquid is formed. The melt is then quenched at room temperature in air to form glass. The glasses so formed are annealed at 250 0C in another furnace and cooled to room temperature at the rate of about 10C per minute to relieve the structural stress.

2. MEASUREMENTS

   The X-ray diffraction patterns of the samples in the powder state are recorded on Xpert PRO, panalytical X-ray diffractometer. The Differential Scanning Colorimetric (DSC) traces for the present samples are recorded on universal V24.2 Build 107 differential scanning calorimeter with programmed heating rate of 10 0C /min in the temperature range 30-900 0C. The optical absorption spectra of well polished V2O5 doped glass samples are recorded on Model V-670 UV visNIR spectrophotometer in the wavelength region of 300-2100 nm. The IR transmission spectra of the present samples are recorded on a JASCO-FT/IR5300 spectrophotometer in the spectral region 400-2000 cm-1 using KBr pellets containing pulverized sample, which are pressed in a vacuum die at -680 MPa. The ESR spectra of the samples in the powder form are recorded on JEOL JES-TES100 X-band EPR spectrometer at room temperature.

   Table 1: Composition of glasses for the present study

   1. RESULTS

      Physical parameters such as vanadium ion concentration Ni, mean vanadium ion separation Ri, polaron radius Rp for the present samples were evaluated from the measured values of the density and presented in Table 2. These observations clearly indicate that the density of the samples increases lightly with the concentration of vanadium ions.

      Fig. 1 shows the XRD patterns of zinc oxy fluro borate glasses doped with different concentrations of V2O5. These X-ray diffraction patterns confirm the amorphous nature in the samples.

      Fig. 2 represents the DSC traces of the present studied glass samples recorded from 300 to 850 K. The traces have exhibited an endothermic effect due to glass transition temperature Tg followed by a distinct exothermic effect due to crystallization temperature Tc. The values of Tg and Tc acquired for the present studied glasses are furnished in Table 3. The value (Tc-Tg) is found to increase with the

      Glass

      Glass composition

      increase in the concentration of vanadium ions in

      Sample Vo 10.0ZnO30ZnF260B2O3

      V2 9.8ZnO30ZnF260B2O30.2V2O5

      V4 9.6ZnO30ZnF260B2O3 0.4V2O5

      V6 9.4ZnO30ZnF260B2O3 0.6V2O5

      V8 9.2ZnO30ZnF260B2O3 0.8V2O5 V10 9.0ZnO30ZnF260B2O31.0V2O5

      the glass network, points out that the increasing resistance of the glass samples.

      Table 2 Physical Parameters of ZnO-ZnF2-B2O3 glasses doped with different concentration of V2O5.

      Sample	Density(g/cm3)	Dopant ion conc. Ni (-1020 ions/cm3)	Inter ionic distance(Ã…)	Polaron radius(Ã…)	Refractive index(n)
      Vo	3.518	—-	—-	—-	1.648
      V2	3.512	0.52	1.24	0.50	1.645
      V4	3.506	1.04	0.99	0.40	1.641
      V6	3.501	1.55	0.86	0.35	1.637
      V8	3.497	2.06	0.79	0.32	1.632
      V10	3.492	2.56	0.73	0.29	1.628

      Fig. 3 shows the optical absorption spectra of zinc oxy fluro borate glasses doped with vanadium ions. For the pure glass sample V0, the absorption edge observed at 330 nm. This absorption edge is found to be shifted gradually towards higher wavelength with an increase of the content of V2O5.

      V5

      Intensity( arb. Units)

      V4

      V3

      V2

      V1

      0 10 20 30 40 50 60 70 80

      2 (degrees)

      Fig. 1. XRD patterns of ZnOZnF2B2O3 glasses doped with different concentrations of V2O5

      Additionally, the spectra of V2 glass (doped with 0.2 mol % of V2O5) have exhibited two wide absorption bands at 640 and 1070 nm, in which these bands are attributed to 2B22B1 and 2B22E transitions of VO2+ ions [31]; with an increase in the concentration of V2O5 up to 1.0 mol %, the half width and intensity of these bands are observed to increase and shifted slightly towards higher wavelength.

      Table 3

      Sample	Tg(K)	Tc(K)	Tc-Tg
      V2	651	720	69
      V4	644	725	81
      V6	638	732	94
      V8	632	745	113
      V10	628	768	140
      	Fig. 4 shows	the Urbach	plots of the present

      DSC data of ZnOZnF2B2O3: V2O5 glasses.

      studied glasses. From these plots, the optical band gap is obtained by extrapolating the linear region of the curve to the h axis. The obtained optical band along with cutoff wavelengths and band positions for the present glass samples are given Table 4. From this data, it is observed that the value of optical band gap Eo is found to decrease with the increase in concentration of vanadium.

      Exo

      V10

      V8

      Endo

      V6

      V4

      V2

      400 500 600 700 800

      Temperature (K)

      Fig. 2. DSC traces of ZnOZnF2B2O3 glasses doped with different concentrations of V2O5.

      2B2 2B1

      Absorption coefficient (cm-1)

      2B2 2E

      V10

      V8 V6

      V4 V2

      V0

      300 400 500 600 700 800 900 1000 1100 1200

      Wavenumber (nm)

      Fig. 3. Optical absorption spectrum of ZnOZnF2B2O3 glasses doped with different concentrations of V2O5.

      Table 4

      Data on optical absorption spectra of ZnOZnF2B2O3: V2O5 glasses.

      Sample	Cut-off wavelength (nm)	Optical band gap Eo (eV)	Position of 2B22B1 band (nm)	Position of 2B22E band (nm)
      Vo	330	2.72
      V2	340	2.58	642	1068
      V4	351	2.48	649	1077
      V6	357	2.41	655	1081
      V8	366	2.36	665	1099
      V10	375	2.29	668	1104

      Fig. 5 shows the experimental FT-IR spectra of V2O5 doped Zinc oxy fluro borate glasses.

      12

      gradual increasing in the content of V2O5 up to 1.0 mol %. The pertinent data related to FT-IR spectra of these glasses are given in Table 5.

      V10

      10

      (h)1/2 (eV- cm)1/2

      8

      6

      V V4 V2 V0

      6

      V8

      BO3 units

      V=0 / BO4 units

      B-O-B linkages

      V-O-V

      bendings

      ZnO4 units

      V10

      V8

      V6

      V4

      V2

      Transmittance %

      V0

      4

      2

      0

      2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8

      h (eV)

      1600

      1400

      1200

      1000

      800

      600

      400

      Fig. 4. Tauc plots of ZnOZnF B O glasses doped with different concentrations of V O .

      Wavenumber (cm-1)

      2 2 3 2 5

      The IR spectrum of V2O5 free ZnO-ZnF2-B2O3 glass show the presence of the following bands at ~460,

      ~700, ~1026, ~1397 cm-1. The band at ~460 cm-1 was ascribed to the vibrations of ZnO4 tetrahedral units [32 34]. At 700 cm-1 appears a band due to the bending vibrations of B-O-B linkage in the borate network. Absorption from ~1026 cm-1 cab be due to B-O stretching vibrations in BO4 units while the band at ~ 1397 cm-1 was assigned to B-O stretching vibrations of in BO3 units. The addition of V2O5 in the ZnO-ZnF2-B2O3 glass matrix, ~ 600 cm-1 appears a new band, which can be due to V-O-V bending vibrations. In the region of BO4 units at about 1030 cm-1 band due to vibrations of isolated VO groups in V2O5 trigonal bipyramids is also reported [35]. The intensity of BO3 structural units is observed to increase, whereas that of the bands due to BO4 is observed to decrease with a

      Fig. 5 IR spectrum of ZnOZnF2B2O3 glasses doped with different concentrations of V2O5.

      The ESR spectra of zinc oxy fluro borate glasses doped with different concentrations of V2O5 are presented in Fig. 6 recorded at room temperature. It is observed to be complex made up of resolving hyperfine components arising from unpaired (3d1) electron with 51V isotope whose spin is 7/2 in an axially symmetric crystal field. From these spectra, g and g values are evaluated and presented in Table 6. From this data, this is observed that an increasing degree of resolution and the intensity of signal with the increased content of V2O5.

      Table 5

      Data on various band positions (cm1) of IR spectra of ZnOZnF2B2O3: V2O5 glasses.

      Sample	Borate groups			ZnO4	V-O-V units
      	BO3	V=O/BO4	B-O-B
      Vo	1397	1026	701	460
      V2	1384	1030	700	462	600
      V4	1377	1036	700	464	606
      V6	1370	1044	704	460	610
      V8	1365	1051	700	458	614
      V10	1358	1059	701	460	621

   2. DISCUSSION

      B2O3 is a well-known strong glass former oxide, the structure of vitreous B2O3 is composed essentially of BO3 triangles forming three- member boroxol rings connected by B-O-B linkages. Moreover, the addition of a modifier oxide causes a progressive change of BO3 triangles to BO4 tetrahedra and results in the formation of various cyclic units like biborate, triborate, tetraborate or pentaborate groups.

      ZnO can exist either as in octahedral modifying positions or as a network forming

      ZnO4 tetrahedral. When glass modifier ZnO

      higher wavelength with increase in the concentration of V2O5 up to 1.0 mol%. V4+ ion belongs to d1 configuration with 2D as the ground state. In the presence of pure octahedral crystal field, the 2D state splits into 2T2 and 2E, while an octahedral field with tetragonal distortion further splits the 2T2 level into 2E and 2B2; among these, the 2B2 will be the ground state. Further 2E level splits into 2A1 |3z2 r2 and 2B1|x2y2 where as 2B2 splits into three |xy, |yz and |zx states. Thus, for the Vanadyl ions we can expect 3 bands

      z

      corresponding to the transitions 2B22B1 (dxy d 2 2), 2B 2E (d d ) and 2B 2A (d

      x y 2

      xy zx,yz

      2 1 xy

      enters the glass, network break the B-O-B, Zn-O-

      Zn bonds and form easily BOZn bridges, this can be related to the reaction

      + = 2( ).

      The B-O-Zn linkages is more stable relative to the mixture of BOB and ZnOZn linkages [36, 37] and introduces co-ordinated defects known as dangling bonds along with non- bridging oxygen ions; in this case Zn2+ is octahedrally co-ordinated . However, ZnO may also participate in the glass network with ZnO4 structural units when zinc is linked to four oxygen ions in a covalency bond configuration.

      Vanadium ions are expected to be present mainly in V5+ states in the Zinc oxy fluro borate glass matrix. However, there is a chance to exist in V4+ state with the following redox equilibrium during the melting of the glasses at higher temperatures:

      2

      25+ + 2 24+ + 1 ;

      2

      d 2 ). However, in the spectra of the present

      glasses, only the first two bands are observed.

      First derivative of absorption ( arb. Units)

      V5

      V4

      V3

      V2

      V1

      200 250 300 350 400 450

      Magnetic field (mT)

      Fig. 6. ESR spectrum of ZnOZnF2B2O3 glasses doped with different concentrations of V2O5.

      The V5+ ions take part network forming positions with VO5 structural units where as V4+ ions may distort the glass network.

      The optical absorption spectrum of vanadium doped glass has exhibited two wide absorption bands at 640 and 1070 nm. The bands are attributed due to 2B2 2B1 and 2B2 2Eg transitions of 3d1 electron in the V4+ state; the assignment of these bands has been made on the basis of an energy level scheme for molecular orbitals of VO2+ ion in a ligand field of C4v symmetry provided by Bullhausen and Gray [31]. The half width and peak height of these bands are observed to increase and shifted slightly towards

      We can understand the observed decrease in the optical band gap with an increase in the concentration of V2O5 is as follows: The gradual increase in the concentration of Vanadyl ions, causes a creation of a large number of donor centers; subsequently, the excited states of localized electrons originally trapped on VO2+ sites begin to overlap with the empty 3d states on the neighboring V5+ sites. As a result, the impurity band becomes more extended into the main band gap. This development might have shifted the absorption edge to the lower energy which leads up to a significant contraction in the band gap.

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      From the IR spectra, it is noticed that with the gradual increase in the concentration of V2O5, the intensity of BO3 structural units is observed to increase, whereas that of the band due to BO4 structural units is decreased. Such observation clearly indicates an increase in the concentration of Vanadyl ions that induce structural disorder in the glass network.

      The ESR spectra of the present studied

      obviously due to the variation in the concentration of V4+ ions and also due to structural and microstructural modifications, that can produce fluctuations of the degree of distortion or even of the coordination geometry of V4+ sites. The observed broadening of the ESR signal with the concentration of V2O5 indicates the growing presence of V4+ ions [38]. The quantitative analysis of ESR results indicates that

      glass samples consist of the well-resolved hyperfine structure with eight components of the

      the ratio

      g g

      ||

      is observed to decrease

      electron-nuclear interactions corresponding to eight values of nuclear magnetic quantum numbers: MI = 7/2, 5/2, . . . , +7/2 in accordance with selection rules MI = 0 and Ms

      = ±1.The variation of ESR line width and the

      resolution with the concentration of V2O5 is

      Table 6

      (Table 6) gradually with a concentration of V2O5,

      indicating an increasing degree of distortion (elongation) of the VO6 octahedron.

      Data on ESR spectra of ZnOZnF2B2O3: V2O5 glasses.

      Sample	g	g	g	g	g / g
      V2	1.837	1.940	0.159	0.060	2.656
      V4	1.833	1.937	0.164	0.062	2.645
      V6	1.829	1.935	0.166	0.065	2.554
      V8	1.827	1.934	0.168	0.066	2.545
      V10	1.824	1.932	0.169	0.068	2.485

   3. CONCLUSIONS

Zinc oxy fluro borate glasses doped with different concentration of V2O5 have been investigated. The DSC studies have indicated that the glass forming ability increases with an increase in the concentration of V2O5. The IR spectral studies indicated that the glass samples contain various structural units with the linkages of the type BOB, VOV; the increasing content of V2O5 in the glass samples seemed to have weakened such linkages. The Optical absorption and ESR studies have indicated that a considerable proportion of vanadium ions do exist in V4+ state in addition to V5+ state with an increase in the concentration of V2O5.

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