1 Introduction

Borate glasses have attracted significant research attention owing to their unique physical and chemical properties that make them suitable across diverse applications, ranging from optics and biology to water treatment and radiation shielding. The distinct properties of borate glasses arise from their ability to accommodate different impurities and modifiers, allowing the development of novel glass compositions tailored for specific uses [1,2,3,4,5,6]. Researchers have explored borate glasses extensively due to their strong covalent bonding, low melting temperatures, high thermal stability, and outstanding solubility for rare-earth ions and other oxide modifiers. In particular, the structural anomaly exhibited by borate glasses, wherein trigonal [BO3] and tetrahedral [BO4] units can interconvert, imparts these glasses with exceptional characteristics that can be harnessed for advanced applications. Owing to this versatility, borate glasses continue to be an important platform for developing new functional materials to meet emerging technological needs. Both BO3 triangle and BO4 tetrahedral units are the foremost simple structural units of borate glasses which are sensitive to any compositional changes especially when any type of modifier is added to the B2O3 network [7,8,9], addition of a second type of glass former owe an effective influence on BO3 transformation [10].

The addition of alkali modifier oxides causes changes in the coordination number of boron, shifting it from three to four coordination. Furthermore, as the alkali content exceeds 33 mol%, non-bridging oxygen atoms (NBO) are formed. When vanadium (V2O5) was introduced to boron (B2O3), vanadium borate glass consisting of mixed network formers could be formed. V2O5 is a conditional glass former that can form glass with the addition of an additional compartment under the conventional method. The impact of vanadium varies depending on its concentration [11, 12]. Higher concentrations can be seen as an intermediary in the glass structure, while lower concentrations of vanadium act as a modifier to the network [13,14,15]. The phenomenon of the mixed glass former effect has harvested research attention due to the notable structural and physical properties exhibited by these mixed glass systems. Furthermore, previous research has demonstrated that vanadium-containing borate glasses degrade rapidly [16]. Materials containing V+5 ions can generate hydroxyapatite, making them promising candidates for bone tissue applications. Vanadium compounds, as a medicinal element, reflect the effects of insulin and growth hormones [17]. Previously reported work [18] has found that a vanadium–ascorbic acid complex can stimulate osteoblastic differentiation and mineralization, suggesting it has bone-forming effects. Vanadium exists in trace amounts in the body, primarily as tetravalent and pentavalent ions in bone. Vanadium appears to play a function in bone metabolism, growth, and generation in vivo. Vanadate (V) and vanadyl (IV) compounds are biocompatible and have osteogenic potential, which can help to speed up bone regeneration and fracture recovery [19]. In addition, the release of vanadium ions from vanadium-incorporated bioactive glasses in physiological conditions may promote bone formation.

In terms of novelty, it was found, in the previously published work [20,21,22], that the presence of even a small concentration from Nd2O3 as an intermediate oxide leads to a high boron back conversion leading to the reduction of BO4 tetrahedral units. In this work, it is useful to study the process of back conversion under the addition of small and high V2O5 content. The presented work aims to investigate the "back conversion" process in borate glasses with the addition of vanadium oxide (V2O5), where trigonal BO3 units convert back to tetrahedral BO4 units. Moreover, the work extended to examine the dual role of V2O5 as both a network former and modifier in the glass structure depending on its concentration providing insights into composition–structure–property relationships in V2O5-doped borate glasses.

2 Experimental procedure

2.1 Sample preparation

Glasses from the system, xV2O5–(45 − x)B2O3–24.5CaO–24.5Na2O–6P2O5 wt%, (x varying from 0 to 25 wt%) were successfully prepared using a traditional melting route. All chemicals used for the preparation are of analytical grade, vanadium pentoxide purchased from Sigma-Aldrich Co. and used as received. Calcium oxide and sodium oxide are obtained from their carbonate partner, Boric acid (H3BO3) was used as a source of boron oxide and supplied by ElNasr Pharmaceuticals. Ammonium dihydrogen orthophosphate (NH4H2PO4) was used as a source of P2O5. Glass name abbreviations along with their composition are listed in Table 1.

Table 1 Glass name abbreviations along with their composition in wt%
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2.2 Sample characterization

X-ray diffraction patterns (XRD) plotted as Bragg angle (2ϴ) (10°–70°) versus intensity were recorded via PANalyticalXPert PRO XRD system for all samples before and after being heat treated [23]. The density of the glass was measured for triplicate samples at room temperature adopting the Archimedes principle via a sensitive 4-digit balance with pure Xylene as an immersion fluid [24].

The density is calculated using the formula:

$$D = \frac{{W_{A} }}{{W_{A} - W_{B} }} \times d$$
(1)

where WA is the weight of the sample in air, WB is the weight of the sample in benzene and d is the density of the benzene. FTIR spectral data were recorded for the studied samples adopting 32 scans through the spectral range extended between 4000 and 400 cm−1 adopting Nicolet is10 single beam spectrometer. 1:100 mg sample: KBr mixed and subjected to high pressure until a clear homogenous disc was formed.

Deconvolution analysis technique (DAT) or peak fitting deconvolution of FTIR spectral data was performed using the Peakfit 4.12 program utilizing Gaussian peaks at positions that were determined using the first derivative and previously reported peaks from the literature and controlling their numbers. Peak parameters like height, width, and position for each constituent peak were supposed to be iteratively adjusted via algorithms like Levenberg–Marquardt to optimize the fit between modeled and measured data and to observe the residual variance to assess the quality of fit. Obtained data including peak position, relative area, and full width at half maximum was also recorded and used to calculate the four coordinated atom percent N4 and B4.

3 Results and discussion

3.1 X-ray diffraction

Figure 1 depicts the X-ray diffraction of selected samples of as-prepared vanadium borate glasses. Selected samples cover the borders of preparation only to approve the amorphous nature without any evidence for crystallization or vetrification results from vanadium addition. Even with increasing V2O5 content, the diffraction patterns revealed that all as-prepared glasses had an amorphous structure. The presence of a broad hump indicates the amorphous, non-crystalline nature of the glass structure. As a test of the ability for crystallization, samples of the glass having different concentrations of V2O5 were subjected to a thermal heat treatment process at different temperatures for a fixed treating time, of 6 h. Figure 2 presents XRD patterns of samples treated at 520 °C, 560 °C, and 580 °C for a duration time of 6 h. It was observed that the increase in annealing temperature was combined with an increase in the intensity of diffraction peaks and with a decrease of full width at half maximum (FWHM) pointing to the increase in crystalline nature of the studied samples.

Fig. 1
figure 1

X-ray diffraction pattern of as-prepared glasses

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Fig. 2
figure 2

X-ray diffraction pattern (A, B, and C) of prepared glasses treated at 520 °C, 560 °C, and 580 °C, respectively, for 6 h

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The plots also revealed that there are some types of crystalline species formed by the effect of thermal heat treatment. Crystalline phases of Na3Ca6(PO4)5 [PDF nr.11–236], CaB2O4 [PDF nr.73–804], and (VO)2P4O12 [PDF nr.34–1040] are the main crystalline phases. These phosphate- and borate-based phases are crucial for imparting key biological, bonding, bone growth, and control properties to bioglass implants to aid medical applications [25] for several reasons:

  1. 1.

    Improves biocompatibility: The phosphate groups in Na3Ca6(PO4)5 and the Ca and B ions released from CaB2O4 regulate cellular behaviors at the material–tissue interface by binding proteins and stimulating osteoblast differentiation and growth. This improves the biocompatibility of the bioglass and its ability to integrate with living tissue.

  2. 2.

    Supports bone growth: Na3Ca6(PO4)5 has a composition very close to hydroxyapatite, which is the main mineral component of natural bone. When implanted, Na3Ca6(PO4)5 and CaB2O4.5 dissolve and release ions that stimulate the deposition of new bone matrix by osteoblasts, supporting new bone growth.

  3. 3.

    Creates bioactive bond: Dissolving Na3Ca6(PO4)5 and CaB2O4 phases allows functional groups to form on the bioglass surface that readily bonds with tissue proteins. This creates a bioactive bond between the implant and surrounding tissue which is important for integration.

  4. 4.

    Controls degradation rate: The amount and makeup of Na3Ca6(PO4)5 and CaB2O4 phases influence the dissolution rate and bioactive properties of the bioglass. Controlling this helps tailor biodegradation and tissue bonding to match the tissue regeneration rate.

3.2 Fourier transform infrared spectroscopy

It is widely known that modified vanadium borate glasses are mostly composed of two structure-forming groups (BO3, BO4). The presence of BO3, BO4, and VO4 structural units can be correlated with the primary spectral bands found in their FTIR spectra. The type of units and their assignment are listed in Table 2.

Table 2 Band positions of FTIR spectra of the glasses V2O5–B2O3–Na2O–CaO–P2O5
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The absence of the band at 1219 cm−1 in the V1, V2, and V3 samples has been attributed to B–O stretching vibration in BO4. As a result of the addition of vanadium oxide with different concentrations, a new band around 871 cm−1 is resolved and attributed to the formation of the VO4 group. This band is shifted from 854 to 871 cm−1 and its intensity increases with increasing V2O5 content. At the same time, the intensity of the band at 1420 cm−1 was decreased with increasing V2O5 concentration. These changes confirmed that V2O5 can enter as a network former up to a specific content, resulting in the conversion of BO3 into BO4 group. At higher V2O5 concentrations, vanadium is suggested to play the role of a network former [26,27,28].

Deconvolution analysis is a useful technique that can provide insights into the structural changes occurring in borate glass since the deconvolution discerns more structural details from spectroscopic data through selective separation of the overlapping spectral contributions, generating valuable insights into borate glass structure–composition inter-relationships. The FTIR spectrum of borate glass contains multiple overlapping peaks representing various borate structural units. Deconvolution mathematically separates the complex spectrum into individual component bands, allowing precise assignment of structures and enabling quantification of the relative fractions of trigonal, tetragonal, pentaborate, and orthoborate groups in the borate glass network. Tracking changes in these fractions sheds light on the structural reorganization happening. Moreover, the changes in the BO3 to BO4 ratio determined after deconvolution indicate the extent of change in boron coordination and thus the connectivity of the borate glass network when compositional parameters are varied. The deconvolution unambiguously identifies bands due to modifier cations (Na, Ca, etc.) attached to BO3 and BO4 units. Analyzing changes in the relevant band areas helps reveal the different modifier roles in facilitating structural change (Fig. 3).

Fig. 3
figure 3

FTIR spectra of V2O5 doped borate glasses

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The determination of different structural groups in the host glass network is based on a quantitative analysis of the fractions of the total types of four different tetrahedral units (B4) and the fraction of four coordinated boron units (N4).

Data obtained from the FTIR technique is combined with the deconvolution analysis technique (DAT) shown in Fig. 4:

$${\text{B}}_{{4}} { = }\frac{{\left( {{\text{BO}}_{{4}} {\text{ + VO}}_{{4}} } \right)}}{{\left( {{\text{BO}}_{{4}} {\text{ + VO}}_{{4}} {\text{ + BO}}_{{3}} } \right)}}$$
(2)
Fig. 4
figure 4

Deconvoluted FT-IR spectra of selected samples (V1, V8) doped prepared glasses

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Since (BO4 + VO4) is represented by a spectral region that is resolved between 800 cm−1 and 1200 cm−1, the proportion of boron in tetrahedral coordination can be calculated using the aforementioned equation [29]:

$${\text{N}}_{{4}} { = }\frac{{\left( {{\text{BO}}_{{4}} } \right)}}{{\left( {{\text{BO}}_{{4}} {\text{ + BO}}_{{3}} } \right)}}$$
(3)

Subtracting the numerical data determined from Eq. (3) from Eq. (2), the fraction of V2O5 as a former VO4 unit can be obtained, i.e.,

Figure 5 represents the change of B4 and N4 fractions determined from Eqs. (2) and (3), respectively. As shown in this figure the values of N4 and B4 are initially decreased to reach a minimum at about 9 mol% V2O5 and then increased.

Fig. 5
figure 5

Variation of N4&B4 fraction of glass containing V2O5 with different concentrations

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This behavior led one to suggest that V2O5 could play a dual role in the investigated glasses. At lower concentrations (up to 9 wt%) it acts as a network modifier which explains the decrease in N4 and B4 values but higher concentration from V2O5 content forces V2O5 to play the role of a glass former. The difference between the B4 and N4 values gives V4(VO4) represented in Fig. 6. It can be shown from this figure that the tendency of V2O5 to form a glass increases with increases in its content [30]. For more observations, we followed the distribution of the borate in terms of structural factors named R and K where,

Fig. 6
figure 6

Variation of V4 fraction of glass containing V2O5 with different concentrations

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R = modifier/B2O3 and K = V2O5/B2O3.

In normal cases the value of K increases, the value of R decreases and this is not what is demonstrated in our work. As shown in Fig. 7. at the start of the curve, N4 and B4 decreased as R values increased, and then N4 and B4 increased with increasing R values. These results indicated that vanadium oxide acts as a network modifier at lower concentrations, and vice versa at higher concentrations most of V2O5 played the role of a glass former in the investigated glasses. As well as in the relation between N4 and B4 as a function of structure factor K as shown in Fig. 8.

Fig. 7
figure 7

Variation of N4&B4 fraction of glass containing V2O5 with structure factor R

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Fig. 8
figure 8

Variation of N4&B4 fraction of glass containing V2O5 with structure factor K

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3.3 Density measurements

Density measurements were employed to determine the volumes of various structural units in different types of glass. This technique was effective in discerning any modifications occurring within the glass network. By examining density, it was possible to track changes in dimensions, coordination number, and atomic geometrical configuration within the glass network. Specifically, when comparing the density of as prepared glasses, it was observed that the density of the glass system increased with the introduction of V2O5 in place of B2O3. This can be attributed to the greater availability of oxygen from V2O5, which led to a shift in the coordination of BO3 to BO4 [31, 32].

The incorporation of V2O5 leads to the formation of VO4 groups, which contribute to higher density due to their stronger bonding compared to the triangular BO3 groups. This results in a more compact structure overall. The increased density is also attributed to the higher field intensity values exhibited by the VO4 and BO4 groups. In the study, density (D), molar volume (Vm), and Oxygen Packing Density (OPD) of the as-prepared samples were measured or calculated using Eqs. (4) and (5). The variation of density and molar volume with varying amounts of V2O5 is depicted in Fig. 9. This information is important for understanding the structural changes in the glass network and how they affect the density and molar volume of the glass system where it can provide valuable insights into the composition–property relationship of the studied glasses.

Fig. 9
figure 9

Dependence of density (D) and molar volume (Vm) of prepared glasses on V2O5 content

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The increase in Vm and D with the increase in vanadium oxide concentration indicates a higher degree of structure compactness in the glasses. Molar volume is a measure of the space occupied by each mole of the material, while density is a measure of mass per unit volume. The compactness of the glass structure is influenced by the presence of bridging oxygen atoms (BOs) and non-bridging oxygen atoms (NBOs). Bridging oxygen atoms connect two or more network-forming cations, contributing to the stability and rigidity of the glass structure. Non-bridging oxygen atoms, on the other hand, are not bonded to other network-forming cations and tend to introduce structural disorder and flexibility. In this case, the incorporation of vanadium oxide, which forms VO4 groups, increases the number of bridging oxygen atoms in the glass network. This increased connectivity leads to a more compact and denser structure, as reflected in the higher values of Vm and D. Overall, the observed trends in Vm and D provide insights into how the addition of vanadium oxide affects the structural characteristics and compactness of the glass system.

The density can be used to calculate the molar volume of glass [23], where

$$Vm \, = \, M/D$$
(4)

where M is the total molecular weight of a particular composition. The Oxygen Packing Density (OPD) is also exclusively related to the density by the following relation [33]:

$${\text{ OPD = }}\frac{D}{M} \times {\text{N}}_{{\text{O}}}$$
(5)

where NO represents the number of oxygen atoms per formula unit.

It has been shown that the decrease in the volume of BO3 and BO4 units is compensated by the increase in VO4 units. These changes produce an increase in density (D) and molar volume (Vm).

Theoretical calculations were used to determine the density based on the following equation [34]:

$$D_{{{\text{Theo}}}} = \sum\limits_{i} {x_{i} D_{i} }$$
(6)

where xi is the molar fraction and Di is the density for each component. The values were obtained by considering the crystalline densities of 2.46, 3.34, 2.27, 2.39, and 3.36 gm/cm3 for B2O3, CaO, Na2O, P2O5, and V2O5, respectively, see Table 3.

Table 3 Some physical parameters of the prepared glasses V2O5–B2O3–CaO–Na2O–P2O5
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4 Conclusion

The base glass composition of 45B2O3–24.5Na2O–24.5CaO–2.6P2O5 was prepared using the standard melt quenching technique, along with additional samples, where B2O3 was partially substituted with 5 at concentrations ranging from 0 to 25 wt%. The incorporation of V2O5 was found to alter the borate glass structure through the conversion of trigonal BO3 units into tetrahedral BO4 units. This structural change led to an increase in glass density with increasing V2O5 content. Fourier transform infrared spectroscopy revealed the presence of BO3, BO4, and V–O stretching vibrations associated with VO4 structural groups (Fig. 10). As the V2O5 content was raised to 10 mol%, the fraction of BO4 units was observed to decrease. This indicates that, depending on its concentration, V2O5 plays a dual role, as a network former and modifier in the borate glass structure. The emergence of VO4 and BO4 vibrational modes at higher V2O5 concentrations suggests a network-forming behavior of vanadium. The amorphous, glassy nature of the samples was evidenced by the broad humps rather than sharp peaks in the XRD patterns. However, heat treatment of the glasses resulted in crystallization and conversion to an opaque white color. Overall, the incorporation of V2O5 significantly modifies the borate glass structure and properties, in addition to a network former at varying concentrations.

Fig. 10
figure 10

Dependence of OPD and free volume (Vf) of prepared glasses on V2O5 content

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