Introduction

Glass plays a vital role worldwide, especially in science and industry. The amorphous structure of glasses is a unique physical state that differs greatly from crystals and other material forms. Transition metal oxides (TMO) have excellent optical and electronic properties and can be utilized in various scientific and industrials fields. Modified oxide glasses that contain transition metal ions are of great interest as semiconducting materials. They have potential applications such as optoelectronic devices, solid-state batteries, memory switching, and cathode materials.1,2 These glasses are known as mixed ionic-electronic conductors, the ionic conduction depending on the alkali ion concentration. At the same time, the electronic conductivity is owing to the unpaired electron hopping between high- and low-valence states within the polyvalent transition ions.

Glassy structures of different compositions can be synthesized using glass formers and additive materials.3 The characteristic feature of borate glasses is a glass network that mainly consists of BO4 and BO3 units. The presence and conversion between these groups are fundamentally dependent on the glass modifier content as alkali and alkaline earth oxides. The presence of the different fractions of the coordinated boron units provides the possibility of fine physical properties.

The addition of alkali oxides such as lithium oxide (Li2O) to the glassy borate network, modifies the glass structure.4 Non-bridging oxygen (NBO) will be achieved if the Li2O content exceeds 25%.5 At Li2O less than 25%, the triangle units BO3 sp2 are changed to tetra-borate units BO4 sp3. BO4 unit is connected to two other BO4 units and the structure leads to the formation of long chains. Lithium borate glasses are of great technological interest due to their multiple uses in a large range of electrochemical applications, solar energy converters, and high-density energy batteries. Lithium ions within the glass network act as ionic conductors. In the presence of fluoride ions in borate glass, some BO2F and BO3F are formed. These modifications affect the properties of borate glass.6

Vanadium-doped glasses show a semiconducting nature. Vanadium is a strong transition metal that is preferred for use in different applications such as solar cells, optoelectronics, and radiation shielding due to its unpaired electron and the different oxidation states (V3+, V4+, and V5+)7,8 in glass matrix according to the quantity of the modifiers, the structure of the host-glasses, the ions size, and their field strength.9,10,11,12 The presence of V2O5 in these glasses is used as a colorant for semiconductors applied in memory-switching applications,13,14,15 and gamma radiation shielding,10,14,15 where the properties of oxide glasses are mainly related to the state of the vanadium ions, the number of vacancies (defects), and the glass structure.11,16 Various spectral and electrical studies of different glasses doped with relatively high ratios of vanadium have been carried out as phosphate2,17,18,19 borosilicate,1 Sr-borate,16 Bi-borate5,6 and mixed alkali fluoro-borate.14 However, studies on glasses doped with low ratios of vanadium oxide are limited.

Vanadium ions are expected to dissolve easily in the borate network because some of the infrared vibrational bands of these ions lie in the same region as those of the BO4 and BO3 units.20

The present work is concerned with structural studies on two binary glass systems, lithium (oxy and fluoro) borate glass doped with low mol.% (0.05, 0.2, and 0.5) of V2O5. The purpose is to check the effect of the various factors on ac-electrical conductivity, permittivity, and electrical modulus. The Li-borate glass containing LiF instead of Li2O was prepared to study the effect of the presence of fluoride ions on the electrical properties and the effect of different low concentrations of vanadium oxide on both prepared Li-borate glasses. FTIR, optical properties, band gap calculations, and electrical measurements were carried out, achieving the predicted results and confirming the stable vanadium valence states in these glass systems.

Experimental

Materials and Methods

Binary glass systems of (oxy and fluoro) lithium borate of chemical composition (0.25Li2O–0.75B2O3) and (0.25LiF–0.75B2O3):xV2O5, where x is varied at 0.05 mol.%, 0.2 mol.%, and 0.5 mol.%, were prepared using the melt-quenching method. The glass samples with chemical composition were encoded and are listed in Table I. High-purity chemical materials of orthoboric acid (H3BO3, 99% from Nasr Lab, Egypt), lithium carbonate (Li2CO3, 99.9%, Sigma Aldrich), and lithium fluoride were used as the starting materials for B2O3, Li2O, and LiF, respectively. Vanadium oxide (V2O5 99.6%, Sigma Aldrich) was used as a dopant. The weighted fractions of their powder were mixed well before melting in a silica crucible for 30 min at 450°C, then the temperature was raised to 1150°C for 1 h in an electrical furnace with occasional rotating of the crucible, to eliminate the bubbles and ensure that all components were completely homogenized. Then melted glass samples were poured into stainless steel molds of selected dimensions, then immediately transferred to annealing preheated furnace regulated at 300°C and cooled gradually at a rate of 30°C/h. The glass samples were of faint green color that changed with increasing vanadium oxide content, but the base sample was colorless, as displayed in Fig. 1.

Table I The chemical composition of the glass samples
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Fig. 1
figure 1

Photographs of the prepared glass samples.

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Characterization Techniques

X-Ray Diffraction (XRD)

XRD spectra of the glass samples were determined using a Bruker diffractometer (Bruker D8 Advance) with a target Cu Kα radiation source (λ = 1.5405 Ǻ) and a scanning rate 0.2 min−1.

FTIR and UV Analysis

Fourier transform infrared (FTIR) studies were performed on the ground powder of the glass samples through the KBr method in the range 400–4000 cm−1 using a computerized Fourier transform infrared spectrometer (FT/IR-4600, JASCO Corp., Japan).

Ultraviolet–visible absorption (UV–visible) and transmission spectra were acquired for the polished glass samples of thickness 3 mm ± 0.1 mm using a double-beam spectrophotometer (JASCO V-570, Japan) over the range of 200–2500 nm with accuracy of ± 0.002 absorbance and ± 0.3% transmittance.

The optical band gap energies Eg of the prepared glasses were estimated from the Tauc equation.12,21 Absorption coefficient α was calculated from the Beer–Lambert relation:

$$ \propto \left( n \right) = \frac{2.303 A}{t} $$
(1)

where A is absorbance and t is the thickness. Tauc plots were obtained from the relation

$$ (\alpha h\upsilon )^{n } = A\left( {h\upsilon - E_{{\text{g}}} } \right) $$
(2)

where A is the proportionality constant depends on the nature of the material, h is Planck's constant, υ is the frequency, and n is the indirect transition. In the Tauc equation, the plot of (α hυ)n versus photon energy (hυ) for the investigated sample leads to the design of a certain curve, and the intersection of the extrapolated linear portion of this curve with the (hυ) axis is used to obtain Eg.

Electrical Studies

The electrical properties of the polished glass samples were investigated in frequencies ranging from 102 Hz to 8 MHz using an RLC Bridge (HIOKI model 3532, Japan). The prepared samples as a disc were placed between the two electrodes.

It is well known that the ac-electrical conductivity, σac, is frequency-dependent and consists of two terms according to

$$ \sigma_{{{\text{ac}}}} \left( \omega \right) = \sigma_{{{\text{tot}}}} \left( \omega \right) - \sigma_{{{\text{dc}}}} \left( T \right) $$
(3)

where σdc (T) is dc-electrical conductivity and σtot. (ω) is total electrical conductivity. The term σac (ω) can be written in the form of a power law as22

$$ \sigma_{{\text{ac }}} \left( \omega \right) = A(\omega )^{s} $$
(4)

where ω = 2πf is the angular frequency and s is the composition-dependent parameter obtained from the slope of these lines.23

The dielectrics are an important property of semiconductor material. In this study, the real part of the dielectric constant ε′ was calculated from the measured capacitance at all frequencies under consideration according to the following equations24

$$ \varepsilon^{\prime} = C \cdot \frac{t}{{A\varepsilon_{0} }} $$
(5)

where ɛ0 is the permittivity of the free space.

Results and Discussion

X-Ray Diffraction (XRD)

Figure 2 shows XRD patterns of 0.5 mol.% vanadium (Li2O or LiF) borate glasses. The absence of sharp peaks over a wide range of diffraction angles confirms the amorphous nature of the glass samples.

Fig. 2
figure 2

XRD pattern of the prepared glass.

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FTIR Absorption Spectra of the Two Binary Glass Systems

The infrared absorption spectra of the two binary glass systems were recorded to obtain detailed information on the arrangements of the different structural units in the glass network. Figure 3a, b demonstrate the IR spectra of the LiO–B2O3 and LiF–B2O3 glasses with and without V2O5 doping. The IR spectra of the two base glasses reveal a close analogy of several bands and peaks related mainly to the borate groups that constitute the glass network.

Fig. 3
figure 3

FTIR absorption spectra (a) oxy- and (b) fluoro-lithium borate glasses with and without V2O5 doping

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It is accepted by several studies on fluoro-borate glasses that due to the analogous masses and almost equal radii of oxygen and fluorine, major variations are not expected to be identified in the mid-IR spectra.25,26 The fluoride ions in the borate glass network cause some structural modifications where some BO3 and BO4 species are modified to be BO2F and BO3F structural units, and the absorption in the regions 1200–1600 and 800–1200 cm−1 can be associated with their oscillations, respectively.25,27,28 Therefore, the current IR absorption peaks of the undoped glasses (base) can be attributed to their B-O vibration modes as follows:

  1. 1.

    The absorption peaks at 410 cm−1, 460 cm−1, 510 cm−1, and 545 cm−1 within the wavenumber range 400–550 cm−1 can be correlated to vibrations of cation ions (Li+, Na+, Ca2+), as well as the distinct band at 460 cm−1, which is related to the alteration of B–O–B bond angle, while the bands 570 cm−1 is assigned to O–B–O winding oscillations as suggested by several authors.29,30,31,32

  2. 2.

    The bands located at 600–800 cm−1 are due to O4B–O–BO3 twisting oscillations caused by bending oscillation modes of different borate units.13,14,33

  3. 3.

    The mid-IR range 800–1200 cm−1 contains the characteristic vibration modes of the B–O bond of the (BO3–O) units that are represented in the spectra by wave numbers at 896 cm−1, 914 cm−1, 970 cm−1, 1033 cm−1, and 1118 cm−1.25

  4. 4.

    The absorption peaks positioned at 1249 cm−1, 1340 cm−1, 1394 cm−1, and 1431 cm−1 extending into the IR region 1200–1600 cm−1 represent the oscillation modes of B–O linkages in BO3 or (BO2O)− species.32

On doping the V2O5 in increasing amounts (0.05 mol.%, 0.2 mol.%, 0.5 mol.%), the IR spectra were retained in nearly the same number and position of IR absorption vibration bands. It should be mentioned that the recognized bands at 800–1200 cm−1 and 1200–1600 cm−1 became clearer than that in the spectra of base glass. The intensity of peaks centered at 880, 1035, and 1348 cm−1 with respect to the V–O–V links and the vibration of the V=O group in the VO2 polyhedra and the formation of B–O–V units increased, respectively.11,13,15 Thus, this indicated that the vanadium ions in glass lattice create non-bridging oxygen linkages (NBOs) or B–O–V chains which could be found in the V4+ state with vanadyl complexes.

Optical Studies

Ultraviolet and visible transmittance spectra were obtained for finely polished glasses of Li-borate glasses without and with different amounts of V2O5 are given in Fig. 4. It is clear from these spectra that the UV–Visible transmittance of the oxy and fluoro Li-borate glasses decreases with the addition of 0.05 mol.% V2O5, indicating that the V ions change the band structure of the glasses, reducing the optical transition, and then increase again with increasing V2O5. The transparency of 0.5 V gradually increases from ~ 600 nm to 2500 nm.

Fig. 4
figure 4

Optical transmittance spectra of (a) Li2O and (b) LiF borate glasses undoped and doped with different mol.% of Vanadium oxide.

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Figure 5 shows the absorption spectra of the two binary glass systems (Li2O–B2O3) and (LiF–B2O3) as a base and V2O5-doped glasses (0.05 mol.%, 0.2 mol.%, and 0.5 mol.%). The UV absorption spectra of vanadium-free glasses show strong peaks that are attributed to trace iron impurities of the used raw materials.10,13,34 Even small concentrations of vanadium ions cause an electron transfer, including the transition of an electron from the coordinating orbital of an oxygen atom to an orbital of the metal ion. The spectra absorption is very close to undoped glasses in addition to other peaks with high-intensity shifts to longer wavelength (redshift) with increasing vanadium concentration as listed in Table II. This suggests that the non-bridging oxygen is increasing with the increase in vanadium.

Fig. 5
figure 5

The UV–Vis absorption spectra (a) oxy-borate and (b) fluoro-borate undoped and V2O5-doped glasses.

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Table II The cut off wavelength, the optical band gap values of glass samples
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The possible valance states of vanadium ions in glasses are V+5, V4+, and V3+ as confirmed through extended optical studies.10,13,15 The UV absorption is observed to extend to a peak at 350 nm in oxy-borate, while there are two peaks at 350 and 410 nm in the fluoro-borate glasses. It is accepted that V5+ ions provide a charge transfer absorption band around 380 nm where it belongs to the 3d0 configuration and has no free electrons to exhibit visible absorption.2,15 This means that the energy gaps are impacted by the proportion of vanadium ions and their possibility to form non-bridging oxygen (NBO) within the network. The similar performance of the non appearance of the visible peaks was detected when lower fractions of transition metal oxide as cupric oxide (0.04% and 0.1% CuO) doped lithium phosphate glass2 and (0.5–2 mol.% vanadium oxide) doped mixed alkali borate glass.13

The band gap of the prepared binary glass was determined by Tauc’s equation as mentioned in the experimental section. Figure 6 represents the Tauc plot (αhυ)1/2 versus (hυ) and Eg. The band gap energy was determined by extrapolating the linear portion of the curve intersecting the hυ axis at (αhυ)1/2 = 0. Optical transition glasses are associated with absorbing phonons such as electronic behavior between the conduction and valence bands.35 The energy band gap is also calculated from the cut-off wavelength. The calculated Eg listed in Table II lies in the same range as reported for semiconductors.36 The Eg value decreases with increasing doped V2O5 for all the glasses. This means that there are several changes in the network structure due to intercalation of the doped V2O5 in the glass samples. The vanadium ions proceed as the defect centers close to the Fermi level. The transition occurs from the valence band to the vanadium sites and finally from the site of the defect to the conduction band. This performance may be attributed to the structural modifications that lead to increasing the NBO due to the presence of vanadium ions in the glass matrix.37

Fig. 6
figure 6

Tauc plots for (a) Li2O and (b) LiF glasses undoped and doped with different concentrations of vanadium oxide.

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The values of the optical band gap vary between 2.1 eV and 3.2 eV13 as listed in Table II. Figure 7 displays the relationship between the optical band gap and the vanadium content. The values decreased with the increase in vanadium content from 3.2 eV to 2.07 eV and from 3.1 eV to 2.16 eV for oxy and fluoro Li borate glasses, respectively, which is related to the intercalation of vanadium‏ ions in the glass matrix that form NBO bonds and introduce a new level of impurities between the conduction and valence bands, as reported previously.38,39 On increasing V2O5 content, the Eg decreases due to the structural changes by conversion of BO3 to BO4 with increasing NBOs.

Fig. 7
figure 7

The dependence of Eg on V2O5 content.

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Electrical Properties

The electrical properties of the prepared glass samples were investigated by keeping the molar ratio of the network (B2O3) constant while increasing the V2O5 in the presence of an ionically active network (Li2O or LiF).

The electrical properties including ac electrical conductivity, dielectric constant, and electrical modulus of the prepared Li2O or LiF glasses undoped and doped with V2O5 were studied at room temperature and a frequency range from 102 Hz to 8 MHz.

Figure 8 shows the frequency dependence of the ac-electrical conductivity for the undoped and vanadium oxide-doped glasses. The conductivity increases with frequency, indicating that the glass samples have a semiconducting nature. It can be concluded that samples doped with vanadium can be used as energy storage material in electronic devices. This relation is characterized by a plateau region at low frequencies matching the dc-conductivity17 and the dispersion region at high frequencies (frequency-dependent conductivity), showing that the conductivity increases with frequency implying conductivity relaxation.17,40 Borate glass with a structure composed of B2O3 is an insulator and insensitive to ionic migration because it requires high energy to produce B3+ ions. In the base Li2O glass samples free of vanadium, the conduction process is mainly due to the contribution of Li+ ions.41 The ionic conduction takes place by the mobility of Li ions in all samples. This structural change may convert BO3 units into BO4 structural units by creating more bridging oxygen networks and enhancing the compactness of the glass system leading to a conductivity decrease. It is found that the beginning of LiF instead of Li2O decrease ionic conductivity due to the formation of local coulombic traps of fluorine ions, which delay the motion of Li‏ ions.18 Doping transition metal ions in lithium borate glasses enhances their electrical properties due to hopping of a mobile electron from a low- to a high-valance state. Therefore, the addition of V ions creates paths for the movement of the charge carriers (Li ions) increasing the ac-conductivity. As a result, the electrical parameters were changed, which involved two different mechanisms.42 One is associated with ionic conduction due to the motion of lithium ions. On the other hand, with increased vanadium content, the electronic conduction increased due conversion of V4+ into V5+ ions.43 The conductivity results support the IR spectra and indicate the presence of V5+. The conducting path could be recognized by the NBO allowing the disruption of Li ions through the glass matrix and another way is the electronic transfer.

Fig. 8
figure 8

Ac-electrical conductivity of (a) Li2O and (b) LiF borate glasses doped with vanadium oxide.

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The conductivity increases slightly with increasing vanadium content in Li2O at low and high frequency. But in LiF, with increasing vanadium content the conductivity increases at a low frequency and increases slightly at a high frequency. As shown in Fig. 9, the conductivity of Li2O borate glass is higher than LiF borate glass doped with 0.5 mol.% V2O5. This increase is related to the increase in the mobility of lithium ions and the electron transfer from the lower to higher valence state. The presence of non-bridging oxygen leads to expansion in the glass structure and facilitates the Li ion mobility, and subsequently the ionic conductivity increases.44,45 The slight increase in conductivity can be taken as a basis to rule out the role of F− ions as the charge carriers in LiF–B2O3 glasses. Therefore, the electronic conduction in these glasses is mainly due to the transport of Li ions rather than F− ions.

Fig. 9
figure 9

The ac-conductivity of Li2O and LiF doped with 0.5 mol.% V2O5.

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Figure 8 shows that the ac-electrical conductivity increases with two different rates; it increases at a slow rate in the low-frequency region, followed by a remarkable increase at high frequency, i.e., it follows a power law relation Aωs17 The behavior of ac-electrical conductivity obeys a power law relation22 which gives the relevant hopping mechanism.24 The exponent ''s calculated from the slope of Fig. 8 using Eq. 11 measures the degree of interaction with the surrounding and also depends on the glass composition. Figure 10 shows the variation of calculated s values with mol.% of V2O5. Exponent s decreases with increasing V content due to the formation of NBO atoms disrupted in the glasses.40 The reduced exponent seen for mixed glasses could be coupled with a reduction of the pathways over that in the single oxide glasses.

Fig. 10
figure 10

Variation of S with V2O5 content.

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Figure 11 shows the frequency dependence of the real part of dielectric constant ɛ′ for Li2O and LiF borate glass with and without different concentrations of vanadium oxide. For all glasses, ɛ′ decreases with frequency. The decreasing numbers of dipoles cause a decrease in dielectric constant. At low frequency, ɛ′ is high and falls with increasing frequency and then becomes constant above 104 Hz. The increase of V2O5 forms NBO and prolongs the structure of the glass network, which increases the dielectric values.

Fig. 11
figure 11

The variation of permittivity with frequency of undoped and doped (a) Li2O and (b) LiF borate glasses with vanadium oxide.

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The high value of the dielectric constant at a low frequency decreases rapidly with increasing frequency. The high value is due to the presence of an electric field which helps the electrons jump between filled and unfilled sites.46 At high frequency, minor decreases are observed because the dipoles cannot rotate, so the oscillation starts to lie behind the field. It has been shown that the dielectric constant at 102 Hz is found to be 37.04, 32.43, 19.57, and 43.51274, 95, 121, 117 for base glass and 0.05 mol.%, 0.025 mol.%, and 0.5 mol.% V2O5, respectively, for Li–O borate glasses. However, for Li-F borate glasses the dielectric constants are 9.29, 7.98, 11.65, and 17.05. It is seen that the values of ɛ′ increased with the addition of vanadium oxide.

The dielectric constant is affected by polarizations (electronic, ionic, dipolar, and space charge). The space charge depends on the purity of the glasses. Its action is noticeable in the low-frequency region. The vanadium ions present as modifiers weaken the network and construct pathways,47 and higher ɛ' due to the lithium ions hop easily48,49 Under the electric field, lithium ions move through the network. However, at high frequencies, mobility is impeded.

The electrical modulus of conducting materials was used to study electrical relaxation.50 The benefit of this illustration is the effects of electrode polarization are minimized. The M′ and M″ real and imaginary parts of the electric modulus as a function of frequency are shown in Figs. 12 and 13. Figure 12 shows that the electric modulus M′ goes to zero at low frequencies signifying a slight polarization,19 while M′ becomes constant at high frequencies due to the relaxation processes. Figure 13 shows that M″ curves are asymmetric in nature. The maximum of the modulus shifted to a higher frequency indicating the increase of charge carrier. At low frequency, the value of M″ is smaller than the maximum one because the charge carrier moves over long distances indicating the negligible contribution of electrode polarization. But at high frequency, the charge carrier moves at a short distance. The mobility of ions in amorphous glasses is in random distribution, and the effect of inter-ionic interaction gives the non-bridging oxygen.

Fig. 12
figure 12

Variation of real part (M') of electrical modulus with frequency for of undoped and doped (a) Li2O and (b) LiF borate glasses with vanadium oxide.

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Fig. 13
figure 13

Variation of imaginary part (M'') of electrical modulus with frequency for of undoped and doped (a) Li2O and (b) LiF borate glasses with vanadium oxide.

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Conclusion

Binary glass systems of the chemical composition 0.25LiO–0.75B2O3 and 0.25LiF–0.75B2O3 with low mol.% V2O5 were prepared using the melt-quenching method. The glassy systems were characterized by FTIR. From the results obtained it can be concluded that the vanadium ions cause small variations in the intensities of IR bands due to an increase in the glass network stability. Optical properties were measured as transmittance and absorbance. Optical band gap energies were calculated from absorbance using cut-off wavelength and the Tauc equation; the small values indicate semiconducting properties. The band gap energy of oxy-borate is lower than fluoro-borate.

The ac-electrical conductivity σac and the dielectric constant of the prepared glassy systems were studied in the range of frequency. The dielectric constant ɛ' decreased with increasing frequency and increased with the addition of vanadium. The main mechanism occurring in the base glasses is ionic conduction. In addition, in the presence of vanadium ions the conductivity increases to higher values owing to the electronic hopping in the conduction mechanism. Some vanadium ions existing in the V5+ valance state strengthen the structure of the glass. The ac-electrical conductivity values of the glass samples increase with frequency, indicating the semiconducting nature. This increase in oxy-lithium borate is higher than in lithium fluoro-borate and this matches with the values of band gap energy.

This suggests that borate glasses containing V2O5 oxides are promising candidates to be applied in electronic devices.