Effect of Variable Oxidation States of Vanadium on the Structural

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Effect of Variable Oxidation States of Vanadium on Structural, Optical and Dielectric Properties of BO-LiO-ZnO-VO Glasses 2

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Sunil Kumar Arya, Satwinder Singh Danewalia, Manju Arora, and Kulvir Singh J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b08285 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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The Journal of Physical Chemistry

Effect of Variable Oxidation States of Vanadium on Structural, Optical and Dielectric Properties of B2O3-Li2O-ZnO-V2O5 Glasses S.K. Aryaa, S.S. Danewaliab, Manju Arorac and K. Singh*,b a

Department of Applied Sciences, ABES Institute of Technology, Ghaziabad-201009, UP, India b

School of Physics & Materials Science, Thapar University, Patiala-147004, Punjab, India c

National Physical Laboratory, New Delhi-110012, India Abstract

*Corresponding author Tel.: +91-1752393130; Fax No.: +911752393005 E-mail address: [email protected]

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Effect of Variable Oxidation States of Vanadium on Structural, Optical and Dielectric Properties of B2O3-Li2O-ZnO-V2O5 Glasses S.K. Aryaa, S.S. Danewaliab, Manju Arorac and K. Singh*,b a

Department of Applied Sciences, ABES Institute of Technology, Ghaziabad-201009, UP, India b

School of Physics & Materials Science, Thapar University, Patiala-147004, Punjab, India c

National Physical Laboratory, New Delhi-110012, India Abstract

In the present study, the effect of variable vanadium oxidation states on structural, optical and dielectric properties of vanadium oxide containing lithium borate glasses has been investigated. The electron paramagnetic resonance study indicates that vanadium in these glasses is in mostly in V4+ state having tetragonal symmetry. As V2O5 increases in glass composition, the tetragonality also increases in the cost of octahedral symmetry. The photoluminescence (PL) spectra of these glasses are dominated by zinc oxide transition; whereas, the peaks pertaining to vanadyl group are not visible in the PL spectra. The optical absorption spectra show a single wide absorption band, which is attributed to V4+ ions in these glasses. The ac conductivity of the glasses increases with increase in vanadium content. The highest electrical conductivity is observed ~10-5 S-cm-1 at 250 °C for glass with 2.5 mol% V2O5. Electrical conductivity is dominated by the electron conduction as indicated by activation energy calculation.

*Corresponding author Tel.: +91-1752393130; Fax No.: +911752393005 E-mail address: [email protected]


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1. Introduction Glasses have wide range of technological applications in electronic devices, optical switches, optical filters, reflecting windows, ray absorbers, mechanical sensors, electro-optic devices and sealants.1–4 The use of alkali and transition metal (TM) ions in borate glass compositions has been the potential area of research in nonlinear optics.5 The incorporation of TM ions in glass host has been used in developing very efficient luminescent devices owing to their different valence states. It is also helpful to understand the local structure of glass and then to correlate with different properties due to their variable oxidation states. Moreover, lithium vanadate glasses have successfully been used as cathode active materials in lithium ions batteries. Mostly, vanadium ion acquires +4 and +5 oxidation states and the transport mechanism involves exchange of electrons between the two states.6 V4+ ion has coordination number six and generally forms a distorted octahedron with oxygen. Thus, the doping of vanadium into glass introduces vanadyl ion (VO2+) causing change in the local structure of glasses. The bond length between vanadium and the vanadyl oxygen is very small as compared with its bond length with other ligands. Vanadyl ion in glass matrix generally acquires threefold or fourfold symmetry.7–9 The occurrence of vanadium in different oxidation states influences the overall electrical conductivity of the vanadium containing glasses. Vanadate glasses have conductivity of the order of 10-7-10-5 S-cm-1.10 The polaron hopping between vanadium ions in different oxidation states is responsible for conduction in such glasses. However, the presence of other components of the glasses such as alkali metal cations such as lithium, sodium etc. also contributes significantly to the electrical conductivity of glasses due to their lower charge and smaller sizes.11 Presence of ZnO in the glasses is favorable to control their optical absorption, refractive index and optical band gap etc. Zinc oxide may act as intermediate oxide and influences the local structure of the 3 ACS Paragon Plus Environment


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glass. Zinc cations can take both six and four coordination in oxide glasses and can be used to modify their structure sensitive properties. EPR, photoluminescence and optical absorption spectra have been extensively used to obtain detailed information about such structural changes and other dynamic phenomena of the material.12,13 As pairing of electrons in d-orbital of transition elements changes in different oxidation states, these studies are much helpful by identifying the site symmetry around the paramagnetic ions. Additional information about conductivity and transport mechanism can be obtained by impedance spectroscopy studies. With the increase in temperature, the conductivity and dielectric properties of the glasses drastically alter, which are directly linked to the change in network structure and mobility of the modifying ions. Thus, temperature and frequency dependence of conductivity as well as dielectric parameters of glasses can be seen along with spectroscopic measurements to get a clearer approximation of structure of the glasses and their relationship with optical and dielectric properties. Based on above motivation, lithium borate composition is selected, wherein ZnO is systematically replaced with V2O5. The structural, optical and dielectric properties of these glasses are investigated and correlated with the variable oxidation states of vanadium. 2. Experimental methods The glass compositions 55B2O3-30Li2O-(15-x)ZnO-xV2O5 (0≤x≤7.5) was chosen for the present study. The variation of x=0.0, 2.5, 5.0 and 7.5 hereafter is called ZV-00, ZV-2.5, ZV-5.0 and ZV-7.5, respectively. The glasses were prepared by taking required stoichiometric amounts of B2O3, ZnO, V2O5 and Li2CO3 of high purity (99.9%). These constituents were first mixed together in agate mortar–pestle in acetone medium for 1 h. The powder obtained after grinding was melted at 1200 °C in high resistance furnace. The detailed sample preparation is given elsewhere.14 EPR spectrum of each sample was recorded on X-band Bruker-Biospin EPR


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spectrometer at ambient temperature in 3400 + 1000 G region, 1x6 modulation amplitude and 23 dB microwave power. 1,1 Diphenyl 2-Picryl hydrazyl (DPPH) has been used as magnetic field marker. DPPH g-value taken for calculations was 2.0036. UV-visible absorption spectra of the glasses were recorded at room temperature using a double beam UV–Vis spectrophotometer (Hitachi- U3900 H) in the wavelength range of 200–800 nm on powder glass samples. Photoluminescence spectra of as quenched glasses in powdered form were recorded using Edinburgh Instruments FS920 spectrometer. The spectrometer was equipped with 450W Xenon Arc Lamp along with a cooled single photon counting photomultiplier (Hamamatsu R2658P). For measurements of electrical parameters, the as-quenched ingots were uniformly sliced using a diamond cutter (Buehler, IsoMet). After ultrasonic cleaning, both sides of the slices were coated with platinum using JEOL Auto Fine Coater (JEC-3000FC) operative at 20 mA for 160 sec. Temperature dependent capacitance measurements were taken on Solartron Impedance Analyzer (SI 1260) with frequency varying from 100 to 106 Hz. Temperature was increased in steps of 10 °C each from ambient temperature up to 400 °C. The least count of the temperature controller was ± 1°C. To achieve sufficient temperature stability the furnace was held at fixed temperature before taking any measurements. 3. Results and discussion 3.1 EPR analysis Fig. 1 presents EPR spectra of powdered glasses taken in transition metal ions free quartz tubes. For ZV-0.0 glass, no EPR signal has been observed. It confirms the absence of paramagnetic impurities in this glass. While in V2O5 containing glasses, EPR spectra have similar hyperfine pattern pertaining to

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V nucleus with 7/2 nuclear spin.15 EPR spectra show that the peak

intensity increases with the increasing content of V2O5. These spectra contain both parallel and



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perpendicular components of hyperfine lines. EPR spectra were characterized for vanadium as vanadyl (VO2+) ion radical of C4v symmetry16,17 by the Spin-Hamiltonian.18 The solutions of magnetic field components for parallel and perpendicular operations are given below18,19:

[

]

B|| (m ) = B|| (0) − mA|| − (63 / 4) − m 2 A⊥2 / 2 B|| (0)

[

](

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B⊥ (m ) = B⊥ (0) − mA⊥ − (63 / 4) − m 2 A||2 + A⊥2 / 4 B⊥ (0)

(1) (2)

Where, m is the magnetic quantum number of the vanadium nucleus having values 7/2, 5/2 and 3/2; B|| (0 ) = hν / g || β and B⊥ (0 ) = hν / g ⊥ β . B|| position corresponds to maximum in absorption curve of the parallel hyperfine structure component and B⊥ position is measured from the derivative peak maximum at zero.20 The spin Hamiltonian parameters of VO2+ ion are determined from the observed positions of the lines in EPR spectra. The obtained results are summarized in Table 1. The uncertainty in g-tensor values is ±0.001 and for A-tensors is ± 1×104

cm-1. The A-tensors are presented in the following equations as follows: A|| = − P [K + (4 / 7 ) − ∆g || − (3 / 7 )∆g ⊥ ]

(3)

A⊥ = − P[K − (2 / 7 ) − (11 / 14 )∆g ⊥ ]

(4)

where P = 2γββN〈r-3〉 is bipolar hyperfine coupling parameter and K is the Fermi contact interaction term which represents the value of unpaired electron density, ∆g || = g || − g e and

∆g ⊥ = g ⊥ − g e ( g e = 2.0023). As per molecular orbital theory, A|| and A⊥ consists of contribution from A||' and A⊥' of the 3dxy electrons and the term PK arises from the anomalous contribution of the s electrons.21 This s-character appears from the partial polarization or weak unpairing of the inner s-electrons resulted from the interaction with unpaired d-electrons. Equations (3) and (4) can also be written as follows: 6 ACS Paragon Plus Environment

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A|| = − P[K + (4 / 7 ) − ∆g || − (3 / 7 )∆g ⊥ ] = − PK − A||'

(5)

A⊥ = − P[K − (2 / 7 ) − (11 / 14)∆g ⊥ ] = − PK + A⊥'

(6)

By substituting the values of A|| , A⊥ , g || and g ⊥ in these expressions, A||' and A⊥' values are evaluated and tabulated in Table 1. The evaluated values for g || , g ⊥ , A|| and A⊥ increases from (1.9365-1.9372), (1.9732-1.9755), (164.2-166.5) and (62.3 – 62.8) on increasing V2O5 content in the present glasses. The intensity of EPR signal increases with V2O5 content. It is attributed to the presence of more concentration of V4+ as compared to V5+ in the present glasses. In V2O5 containing glasses, when spin Hamiltonian parameters follow the following trend g || < g ⊥ < g e and | A|| | > | A⊥ |, then V4+ ions exist as VO2+ ions with octahedral coordination and belongs to C4V symmetry.18 However, in the present glasses, the calculated parameter values follows reverse trend than above mentioned trend. It indicates that the octahedral symmetry decreases, while tetragonal symmetry increases with V2O5 content. The values ∆ g || |/∆ g ⊥ ratio are also given in Table 1, which measures the tetragonality of the vanadium site.22 The increase in the anisotropic contribution (i.e. | A⊥' |) of 3dxy electron to the hyperfine splitting with V2O5 concentration is attributed to decrease in screening of VO2+ 3dxy orbital from vanadium nucleus by the overlap of electron or from the surrounding oxygen ligand. The increase in P value confirms the compression of 3dxy orbital with increase in V2O5 content in these glasses. The increase in K and ∆ g || /∆ g ⊥ values on increasing V2O5 content in present glasses further confirms increase in the tetragonal symmetry with reduced octahedral symmetry. 3.2 Photoluminescence spectroscopic study PL spectrum of transition metal oxides consist of two parts (i) an excitation emission arises from the transitions of electrons from the conduction band to the valence band with the wavelength 7 ACS Paragon Plus Environment


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located in the ultraviolet light region and (ii) transitions take place between electrons from the defect energy level to the valence band, generally, in the visible light region. The oxygen vacancies, vanadium ion, vanadium/substitution or deep level impurities are some of the common defects present in the glasses. Among these, oxygen vacancies are the major source of emission in oxide glasses. Oxygen vacancies exist in two configurations as F and F+ centres in glass matrix. The neutral oxygen vacancy and electron deficient oxygen vacancy is assigned as F centre and paramagnetic F+ centres, respectively. The schematic of sub-bands contributed from the two kinds of oxygen vacancies at ~ 465 nm and 520 nm in the PL spectrum, is presented in Fig. 2. The V5+ ions are readily reduced to lower valence such as V4+ state during glass formation. V4+ ions are present in glass matrix as vanadyl [VO2+] ions and give three characteristic bands at about 420 nm, 760 - 860 nm and 1000 nm.23,24 The broad band at 1000 nm arises from 2B2g → 2Eg transition and weak feature at 612 nm has been observed due to 2B2g → 2

B1g terminal states transitions.25 While V5+ ion does not give d–d transitions and emits only

charge transfer band results from relaxation of ≡V-O- to ≡V = O at ~ 380 nm and gives a broad band in 400 to 700 nm.26,27 The presence of each valence state of vanadium ion in glass matrix depends upon the type, composition and synthesis parameters. The strong UV absorption has been attributed to 1A1 → 1T1 charge transfer transition from 2p O2- to 3d V5+ state.28,29 Fig. 3 presents the room temperature PL emission spectra in vanadium containing lithium zinc borate glasses, in 450-700 nm region. PL spectra of theses glasses exhibit number of strong, medium and weak intensity peaks with minor effect on their intensity on variation of vanadium ions in glass matrix. In 450-700 nm region, these emission spectra are dominated by ZnO peaks appearing at 466, (474, 476), 488, 497, 528, 540, 551, 563, 571, 582, 590, (602, 606), (610, 612), (625, 630), (640, 645), (658, 660) and (664, 668) nm. The peak at 358 nm is observed from near-


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band-edge (NBE) ultra-violet (UV) emission. The emission peaks in the visible region are observed from oxygen vacancy, extrinsic and intrinsic deep level defects (DLDs) of zinc oxide. The ‘native’ intrinsic deep levels in ZnO are oxygen vacancy (VO), zinc vacancy (VZn), oxygen interstitial (Oi), zinc interstitial (Zni), oxygen anti-site (OZn), and zinc anti-site (ZnO). In addition to native defects, the clusters are usually formed by the combination of two point defects or one point defect and one extrinsic element, e.g. (VO.Zni) cluster formed by Zni and VO. This VOZni cluster is one of the clusters appears at 574 nm (2.16 eV) below the conduction band. These defects control doping, compensation, minority carrier lifetime and luminescence efficiency in semiconductor devices and imparts n-type conductivity in ZnO. The compensation effect pertaining to intrinsic defects present in the forbidden gap (deep centres) is the major problem in obtaining stable p-type ZnO. The orange and red emissions in ZnO takes place from transitions related to oxygen and zinc interstitials, respectively. The proposed schematic of ZnO transitions are shown in Fig. 4 and their probable transition wavelengths are listed in Table 2. ZnO indicates three peaks at 474 nm (blue), 528 nm (green), 590 nm (yellow) and 660 nm (red).30 Several groups have reported that different defect centres in ZnO are responsible for the blue, green and red emissions. The green emission at 528 nm is obtained from oxygen vacancies of VO, and the red emission at 660 nm appears to zinc vacancies (VZn) or excess oxygen. The violet emission at 433 nm is arising from the interstitial zinc (Zni).31 It was recently reported that the violet emission corresponds to Zni and the transition involving VZn would result in blue emission.32–34 It has also been reported that in wide band gap semiconductors, the broad band luminescence is related to transitions from donor states to deep acceptor states.30,35–38 These spectra are dominated by zinc oxide transition and the peaks pertaining to vanadyl group are not visible in these glasses whose transitions also appear at 420 nm, 750 -850 nm and 1000 nm due to very low



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concentration of vanadium content in lithium zinc borate glass i.e. below the detection limit of PL spectrometer. This is confirmed by the appearance of vanadyl ion hyperfine structure in EPR spectra. 3.3 Optical absorption spectra The optical absorption spectra of all the prepared glasses in the wavelength region of 300–800 nm are shown in Fig. 5. The absorption spectra exhibit single absorption band. The absorption spectra of glasses having vanadium are very complex nature due to variable oxidation states of vanadium i.e. V3+, V4+ and V5+. Most probably, it exists in varying proportions depending mainly on the nature of the host glass and synthesis conditions. V4+ invariably exists as VO2+ ion with a single d-electron and gives rise to the free ion term 2D. Such electron, in octahedral symmetry, occupies the orbital and causes the ground state term 2T2g. Electron is excited to the orbital by absorbing energy and consequently only one band corresponding to 2T2g → 2Eg transition is anticipated in octahedral geometry. Generally, due to non-symmetrical alignment of the V=O bond along the axis, the site symmetry is lowered to tetragonal C4V symmetry where 2

T2g splits into 2B2g and 2Eg, whereas 2Eg splits into 2B1g, 2A1g. Thus, three bands in C4V

symmetry are expected to appear. It might be expected that during the melting process of preparing the glass samples, there are chances that V5+ converts to V4+ due to depleted presence of oxygen inside the furnace at 1200° C. On the other hand, there are very negligible chances that V5+ converts to V3+ states due to its instability. The broad band observed at 303 nm has been attributed to the V4+ ion, and this band is assigned to the 2B2g → 2Eg transition.39 At higher concentration of vanadium, a very slight blue shift from 303 nm to 305 nm is observed. No other, even weak absorption band is observed for V3+ states corresponding to 3T1g (F) → 3T2g (F)


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transition. These observations are consistent with EPR and photoluminescence emission spectra of the present glasses. 3.4 Electrical conductivity analysis 3.4.1 Effect of composition and temperature

The conductivity of the present samples is accessed through Nyquist plots between real and imaginary parts of the impedance of each sample. The representative Nyquist plot of ZV-5.0 is shown in Fig. 6. The experimental data is fitted with an equivalent circuit (diagram shown in inset) consisting of a resistor and a constant phase element both connected in parallel to each other. The constant phase element is introduced in place of capacitor because in practice the system does not behave as ideal capacitor. Instead a constant phase element defined as = 1⁄ is used, where, the exponent, α has value 1 for ideal capacitor, and

Effect of Variable Oxidation States of Vanadium on the Structural

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