Introduction

Bismuth borate glass is a promising candidate for applications such as infrared transmission components, ultrafast optical switches, and photonic devices, as well as a variety of other applications such as magneto-optical current transducer (MOCT) for high voltage current systems, optical rotators, and isolates, or circulators, switches, and polarimeters [1,2,3,4].

Boron oxide (B2O3) is the most frequent glass-forming, with trigonal BO3 and tetrahedral BO4 units entering the structure. This material has good transparency, chemical durability, and thermal stability and is created at a low melting temperature [5]. Electrical, optical, and luminescent capabilities are all present in borate glass [6]. The addition of transition metal to borate glass increases their optical properties, which are due to the transition metal’s varied oxidation states [7]. Microelectronics, optical glasses, solid-state lasers [8, 9], and semiconductor characteristics [10] all benefit from it.

Depending on the chemical content and composition, the addition of MoO3 in alkali borate glass [6] produces two oxidation states: Mo5+ and Mo6+. The polarizability of oxygen around magnetic ions, the concentration of NBOs, and the coordination number were all altered by the ratio of different oxidation states of MoO3.

Because it is employed in thermal, mechanical, and optical devices, heavy metal oxide glass receives greater attention. Bi2O3 exists in the valance states Bi6+ and Bi3+, which have low field strength and high polarizability [11].

Bismuth glass also has other benefits, including third-order nonlinear optics, a high refractive index, and an infrared cut-off. Because of its piezoelectric, ferroelectric, and pyroelectric capabilities, the glass with Bi and B is used in more applications.

Kh. S. Shaaban [10] studied the structural, thermal, mechanical, and optical properties of the bismuth borosilicate glass system when Bi2O3 was replaced with MoO3 and discovered that the properties of study glass samples are more susceptible to the MoO3 concentration. Alternatively, the bond strength, ultrasonic velocity, elastic moduli, and thermal stability all increase.

E. S. Moustafa et al. [12] investigated the structural and optical properties of lithium borobismuthate glasses and discovered that as the structure closes due to the transformation of BO3 to BO4, the density and molar volume decrease at the expense of Bi2O3 as the structure becomes closed, as confirmed by FTIR measurements. They discovered that the bond strength of B–O and the reduction in the oxygen environment of bismuth cations were linked to changes in optical characteristics.

Anu Bajaj et al. [13] investigated bismuth borate glass and discovered that increasing the Bi2O3 content increased the molar volume while decreasing the glass transition temperature. A glass sample with 50 mol% Bi2O3 has the most tetra-boron-coordinated atoms (N4).

The goal of this research is to see how adding B2O3, a heavy metal, and MoO3, a transition metal, affects the results. Simultaneously, investigate the implications of substituting transition metals for heavy metals on structural, thermal, and optical properties.

Experimental

As starting materials, reagent grades H3BO3, Bi2O3, and MoO3 are used to make glasses with the composition 50 Bi2O3–(50-x) B2O3–x MoO3mol%, where x = 0, 5, 10, 15, and 20 mol%, and the samples are kept at roughly 300 °C for half an hour to de-carbonize the carbonate.

The temperature was then raised to 1000 °C and held for 1 h. At normal temperature, the molten liquid spilled between two copper plates and a few droplets in the air. The glassy and crystalline character of the samples was investigated and validated using a Philips Analytical System (XRD) of type PW3710 with a Cu tube anode of wavelengths Kα1 = 1.54060 and Kα2 = 1.54439. The generator current was 30 mA, and the generator tension was 40 kV. The starting angle (θ) was 10°, and the ending angle was 70°.

In this study, absorption spectra in the range of 400–4000 cm−1 were analyzed using an infrared spectrometer (FTIR 4100; JASCO, Japan).

The produced samples were subjected to differential thermal analysis (DTA) in the temperature range of 30 to 1200 °C at a heating rate of 30 °C/min using Al2O3 powder as a reference material (Shimadzu DTA-50analyzer).

The density is calculated using the Archimedes method and the following equation:

$$\uprho \left(g{cm}^{-3}\right)=\left[a{\left(a-b\right)}^{-1}\right](density of the Toluene)$$

where a represents the sample weight in air and b represents the sample weight in toluene.

A computerized spectrophotometer was used to measure the optical transmission spectra in the region of 190 to 2500 nm (JASCO, V-570).

X-ray photoelectron spectroscopy (XPS) studies were performed with an ESCALAB 220iXL spectrometer (Fisons Instruments) consisting of two vacuum chambers: the analyzer and the fast entry airlock/preparation chamber. The powdered samples were fixed on carbon conductive tape (Pelco International) at the top of the sample holder and transferred into the ultra-high vacuum. The X-ray source was monochromatic-focused Al Ka radiation (1486.6 eV) with an input power of 150 W. The emerging charge of the sample equalized with the installed charge compensation. The final peak position was determined using the C 1 s peak (shifted to 285.0 eV) corresponding to absorb carbon species. The XPS measurements were carried out at the constant pass energy of 25 eV.

Results and discussion

Structural properties

X-Ray diffraction (XRD)

The XRD for bismuth borate glass samples containing molybdenum oxide from 0 to 20 mol% is shown in Fig. 1a. It was observed that samples containing molybdenum oxide up to 15 mol% MoO3 are amorphous in nature (Fig. 1a). However, a sample having 20 mol% MoO3 is both un-crystalline and crystalline.

Fig. 1
figure 1

The XRD for bismuth borate glass samples containing molybdenum oxide from 0 to 20 mol%

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The sample of 20 mol% MoO3 was poured between two copper plates or poured into the air contained 20 mol% MoO3, according to the analysis.

The creation of the crystalline Bi2MoO6 phase is more intense in the sample poured into the air than in the sample poured between two copper plates, according to the X-ray diffraction patterns of the sample containing 20 mol%. Aurivillius oxides [14], which have perovskite-like layer structures, photo-catalytic activity under visible light irradiation, high optical absorption efficiency, and low photo-induced recombination efficiency, are photosensitive to this phase. And the crystalline phase separated in a sample containing 20 mol% MoO3 compared with XRD patterns of Bi2MoO6, B2O3, Bi2O3, and MoO3 crystalline phases as shown in Fig. 1b.

FTIR

The infrared spectra of glass samples are shown in Fig. 2. The FTIR spectra revealed the structural arrangement of the structural units presents in glass samples with a composition of 50Bi2O3–(50-x) B2O3–xMoO3, where x = 0.0, 5, 10, 15, and 20 mol%. For the Bi and B groups, the FTIR provides information on stretching and bending vibration. It reveals three wide zones [7], all of which clearly illustrate the presence of borate groups.

Fig. 2
figure 2

The infrared spectra of glass samples

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The very first group the asymmetric stretching vibration of B–O bonds of trigonal BO3 units causes the change from 1500 to 1140 cm−1. (ii) Due to the B–O bond stretching of tetrahedral BO4 units, the second band was allocated from 1140 to 750 cm−1. (iii) The third band, ranging from 750 to 650 cm−1, is caused by the borate network’s B–O–B bending vibrations.

Bismuth in borate glass, on the other hand, appears in FTIR spectra as precisely placed bands. There are four infrared active basic vibrations of [BiO3] pyramidal units: an asymmetric stretching vibration (840 cm−1), a doubly degenerate stretching vibration (540–620 cm−1), an asymmetric bending vibration (470 cm−1), and a doubly degenerate bending vibration (540–620 cm−1) (350 cm−1) [15].

When MoO3 is added to bismuth borate glass, it forms Mo6+ and Mo5+, which act as a former when coupled to four oxygen atoms, generating MoO4 tetrahedral structural units, and as a modifier when linked to six oxygen atoms, forming MoO6 octahedral structural units [7].

The emergence of vibration bands around 750–920 cm−1 and 430–410 cm−1 as the concentration of MoO3 increased was attributed to the presence of isolated MoO4 structural groups in the glass network [16].

The symmetric and asymmetric Mo–O–Mo vibrations are responsible for the two bands at 450 and 600 cm−1, respectively. As the symmetric stretching vibration of BiO3 pyramid units [7] overlapped with the Mo–O–Mo band and as the BiO6 structure units [5] overlapped with the MoO4 band [6], the increase in MoO3 led to the appearance of Bi in the former structure. On the other hand, as Mo increases, the coupling between B and Bi in the 968–1025 cm−1 band appears to extend the B–O–Bi vibrations [5].

However, no Mo–O–B bonds were found. The significant tendency for liquid phase separation in compositions comprising B2O3 and MoO3 concurrently is due to the absence of connectivity between MoOn and BOn polyhedral in the amorphous network. It also said that at MoO3 concentrations less than 50 mol%, there is no chance of the MoO6 forming.

XPS measurements

Molybdenum can exist in the glass with four valences (Mo3+, Mo4+, Mo5+, and Mo6+) that result from the loss of two electrons from the 5 s and one or two of their 5d electrons.

In order to identify the valence state of Mo ions in the glass samples under study, high-resolution XPS spectra for the Mo 3d core level were measured, and the Mo 3d spin–orbit doublet spectra for the glass sample containing 15 mol% MoO3 are shown in Fig. 3. For the glass sample, the observed two peaks changed within the BEs range 231.84 eV to 232.4 eV and 234.9 eV to 235.66 eV are assigned to the binding energies (BEs) of Mo 3d5/2 and Mo 3d3/2 (spin–orbit components) of Mo6+ ions, respectively [17]. The DE noticed between the Mo 3d5/2 and Mo 3d3/2 peaks is changed within the range 3.06 eV to 3.22 eV for all the glass compositions. Since no peaks are observed at BEs 230 eV (for Mo5+ state) and 229 eV (for Mo4+ state) [18, 19] in any of the Mo 3d spectra of glass samples, we assume that there are no Mo5+ and Mo4+. Oxidation states are present in our glasses, as the shape of the Mo 3d5/2 peak indicates that most Mo presents in a + 6 charge state. Thus, the analyses of the Mo 3d spectra confirm the existence of Mo ions in the 6 + oxidation states for all the glass compositions from the XPS measurements.

Fig. 3
figure 3

Mo 3d core-level spectra for the glass sample containing 20 mol% MoO3

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Physical parameters

The density and molar volume calculations show the variance that occurs in the structural unit. The density overview is explained by the packing between the ions and ionic groups in the structure. [20]

The following formula is used to compute theoretical density \({\rho }_{th}\):

$${\rho }_{th}= \sum {x}_{i}{\rho }_{x}$$

Table 1 summarizes the experimental and theoretical densities, where \({x}_{i}\) is the molar proportion of oxide and \({\rho }_{x}\) is the density of oxide. There is a small variation between the computed and experimental values that represent the exact measures, as shown in Table 1.

Table 1 Density, molar volume, equivalent crystalline volume, transition temperature Tg, Tc, and Tm, values of glass-forming ability Kgl, ΔT, Hrubý parameter, and the values of the average boron–boron separation
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The molar volume, density, and equivalent crystalline volume as a function of molybdenum concentration are shown in Fig. 4. Figure 4 shows that density rises as the amount of MoO3 (which has a higher molecular weight than B2O3) increases. The addition of MoO3 to the mixture transforms symmetric BO3 triangles to BO4 tetrahedral or asymmetric BO4 triangles in terms of structural units.

Fig. 4
figure 4

The relation between molar volume, density, and the equivalent crystalline volume as a function of molybdenum content

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BO4 tetrahedral and asymmetric BO3 triangles are significantly denser than symmetric BO3 triangles [9]. According to the relationships [21, 22], the average boron–boron separation < dB-B > was determined.

$$<{d}_{B-B}> =(\frac{{V}_{m}^{B}}{N}{)}^{1/3}$$

where \({V}_{m}^{B}\) is the volume containing 1 mol of boron atoms within the given glass structure, and \({V}_{m }^{B}=\frac{{V}_{m}}{2\left(1-{x}_{n}\right)}\), \({x}_{n}\) is the B2O3 molar proportion, and N is the Avogadro’s number.

The estimated numbers in Table 1 show that when MoO3 increases, the values that support the assumption of an increase in glass sample density diminish.

And the molar volume \({V}_{m}\) was determined using the following relation [23]:

$${V}_{m}= \sum {~}^{{x}_{i}{M}_{i}}\!\left/ \!{~}_{\rho }\right.$$

where \(\rho\) is the sample density, \({x}_{i}\) is the molar fraction, and \({M}_{i}\) is the molecular weight of all chemical components. Table 1 lists the density values that were collected. Increases in MoO3 cause a decrease in BO4 concentration, the development of a new link between B and Bi, and the formation of isolated MO4 groups, all of which enhance the sample’s smallness.

The equivalent crystalline volume (\({V}_{c}\)) computed by [23] and given in Table 1 determines whether the sample will become a crystalline or amorphous material:

$${V}_{c}= \sum {x}_{i}{V}_{i}$$

where \({V}_{i}\) is the molar volume of the oxide component in the crystalline phase.

Figure 4 shows that the molar volume (\({V}_{i}\)) is greater than the crystalline volume (\({V}_{c}\)), indicating the presence of surplus structural volume in these samples and indicating that they are amorphous in nature. The crystalline volume increases as the Mo content increases, according to the XRD curve.

Thermal properties

The DTA traces show the temperature range for crystallization and the glass transition temperature. The DTA curves obtained for all samples are shown in Fig. 5.

Fig. 5
figure 5

DTA curves obtained for all the studied samples

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Using Fig. 5, calculate Tg (glass transition temperature), Tc (crystallization temperature), and Tm (melting temperature) for the glass characterization temperatures.

Many factors influence the (Tg) characteristic of structural relaxations in the glass network, including bond strength, cross-linking density, and the nature and connectedness of the structural units that make up the network [4].

Table 1 shows that when the amount of MoO3 increases, the Tg values fall. It can go over the bond dissociation of Bi–O (343 kJ/mol), B–O (806 kJ/mol), and Mo–O (607 kJ/mol) in general. Because the dissociation energy of Mo–O is low, the amount of energy required to break the bond lowers.

The decrease in B–O–B cross-linkages reduces the connectedness of the glass network and results in a looser macromolecular structure with lower internal energy requirements for chain mobility [24].

Increased MoO3 content, on the other hand, degrades the network structure by increasing voids in the glass matrix. The isolated MoO4 was not able to form a bond with borate. These are in agreement with the FTIR readings.

Using the characterization temperatures from DTA curves, the thermal stability, capacity of the system to become glass rather than crystal, and glass-forming ability of the system were determined.

The temperature difference [25] is discovered.

Consider this a useful measure of thermal stability because a higher number suggest a delay in nucleation in the sample.

The results of the samples are listed in Table 1.

It can be seen that glass samples containing 5 and 10 mol% MoO3 have higher thermal stability than glass samples without MoO3 and that the value drops as the amount of MoO3 increases, indicating that the sample is more likely to be a glass crystalline material.

By [26], the Hrub parameter is used to provide information about the sample’s potential to become glass or crystalline:

$${H}_{R}= \frac{({T}_{c}-{T}_{g})}{({T}_{m}- {T}_{c})}$$

The results show that when the molybdenum concentration rises, the system becomes more crystalline. The ability to create glass is determined by the following relationship:

$${T}_{rg}={T}_{g}/{T}_{m}$$

The \({T}_{rg}\) parameter is utilized to discriminate between homogeneous and heterogeneous nucleation, and the resultant value is used to calculate the glass-forming ability. The glass has a value in the range of 2/3, which indicates that it has a glass-forming potential. The crystallization surface or volume is indicated by these numbers. This value exceeds 0.58–0.6 surface crystallization but falls short of 0.58–0.6 considerable volume crystallization [25].

The table shows that as the amount of MoO3 increases, the values fall, indicating that the glass system is prone to volume crystallization.

Optical properties

The UV/Vis/NIR spectra of glass containing various concentrations of MoO3 are shown in Fig. 6. The curve noticed that the samples in the glass composition act as a sharp-cut filter based on the transmission. The glassy state sample cut light in the near-ultraviolet and visible ranges, but the glass crystalline sample’s cut-off shifted to higher wavelengths. The cut-off wavelength determines the optical band gap, according to the following relationship [11]:

Fig. 6
figure 6

The typical transmission spectra of glass samples

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$${E}_{ASF}^{opt}= \frac{hc}{{\lambda }_{cut}}= \frac{1239.38}{{\lambda }_{cut}}$$

With increasing MoO3 content, the cut-off wavelength rises higher, breaking the link between the two cations and forming a distinct BO3, BO4 with several configurations, and isolated MoO4 [27, 28].

With increasing MoO3 concentration, the optical band gap of the current glass system lowers. As a result, the Eopt falls as the number of NBOs grows (the MoO3 content increases).

According to [29], the refractive index n is calculated from the optical band gap:

$$\frac{({n}^{2}- 1)}{({n}^{2}+ 1)}=1- \sqrt{Eg/20}$$

Table 2 shows the results. The linear optical susceptibility X(1) is calculated using the refractive index determined using the following formula:

Table 2 The values of Eopt (optical energy gap), refractive index n, the linear optical susceptibility X(1), the third-order nonlinear susceptibility x.(3), and the nonlinear refractive index for all the studied samples
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$${x}^{\left(1\right)}=\left({n}^{2}-1\right)/4\pi$$

This equation [30] estimates the third-order nonlinear susceptibility x(3) from the value of the linear optical susceptibility X(1):

$${x}^{\left(3\right)}={\left[{x}^{\left(1\right)}\right]}^{4}\times {10}^{-10}$$

Table 2 shows the calculated value of x(3), where x(3) = (n n2) /12 (n2 represents the nonlinear refractive index)[31].

The third-order nonlinear optical susceptibility is in the range [(0.38–0.53) 1012esu] based on X(3) values for glass samples free of MoO3 and containing MoO3 up to 10 mol%. This indicates that these samples could be used in nonlinear optical applications [32].

Conclusion

Glasses studied utilizing structural (XRD, FTIR), thermal (DTA), XPS, and optical (optical absorption) tests after doping molybdenum into bismuth borate with concentrations ranging from 5 to 20 mol%. The sample with 20 mol% MoO3 has Bi2MoO6 crystalline phase, according to XRD measurements. The steady narrowing of band gaps indicates the creation of non-bridging oxygen and the breaking of BO bonds, resulting in a less rigid glass network. Set up different functional units in FTIR glasses. The study of the Mo 3d XPS spectra indicates the existence of Mo ions in the 6 + oxidation state for the glass compositions. Differential thermal analysis studies demonstrate that the research glasses are thermally stable. Observe a rise in the wavelength of the cut with increased MoO3 concentration from optical absorption spectra. Nonlinear optical applications are candidates for samples that include MoO3 up to 10 mol%.