An Effectual Role of Molybdenum Ions on the Semiconducting Temperament of Na2O-PbO-Bi2O3-SiO2 Glasses

doi:10.21203/rs.3.rs-661619/v1

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An Effectual Role of Molybdenum Ions on the Semiconducting Temperament of Na2O-PbO-Bi2O3-SiO2 Glasses

https://doi.org/10.21203/rs.3.rs-661619/v1

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Melt quenching method is adopted to synthesize multi-component glasses 10Na₂O –30PbO –10Bi₂O₃ – (50-x)SiO₂ – xMoO₃ (where x = 0 to 5 mol%). These samples are characterized by XRD as well as analysed by optical absorption, EPR, and FTIR spectroscopic techniques in order to discuss the structure of the glass network. The proportionate change of Mo⁶⁺ ions into Mo⁵⁺ local states, which depolymerize the glass network, is publicized by optical absorption studies. EPR spectral analysis of the samples revealed C_4v symmetry in a square pyramidal with a Mo=O bond. FTIR spectra of doped glasses report the presence of MoO₄ and MoO₆ structural units. All spectroscopic studies propped up the semi-conducting nature of sodium lead bismuth silicate glasses doped with the higher content of MoO₃.

Biophysics

Multi-component glasses

XRD

Optical absorption

EPR

FTIR spectra

Amid all oxide glasses silica glasses are widely explored [1, 2] owing to their expansive domestic, mechanical and dental applications. Yet if the silicate glasses are added with heavy metal oxides like PbO and Bi₂O₃ their thermal, mechanical and electrical properties get enhanced due to the raised density, refractive index. Further these glasses display non–linear optical behaviour which promotes them as advanced materials applicable in ultrafast optical switching and photonic devices [3]. In association with heavy metal oxides the Modifier oxide Na₂O originate bonding distortions, which improves the ionic conductivity. Moreover, sodium silicate glasses are thermally as well as mechanically stable and chemically durable applicable in water treatment, paper industry and construction [4]. MoO₃ is catalytic in nature and an intermediate glass forming oxides. Also, it initiates the glass network on accompanying with the modifier oxide PbO with MoO₄²⁻ structural elements and exhibits modifying action through MoO₆ and Mo⁵⁺O³⁻ structural entities [5]. Further, the molybdenum oxide doped glasses grab potential applications in light modulation, smart windows and electro-chromic devices [6].

Glass samples with composition 10Na₂O–30PbO–10Bi₂O₃–(50-x)SiO₂: xMoO₃ (0 ≤ x ≤ 5) were prepared by the melt quenching technique. The compositional (all in mol %) details and their nomenclature of the samples are as follows:

M0: 10Na₂O–30PbO–10Bi₂O₃–50-SiO₂

M1: 10Na₂O–30PbO–10Bi₂O₃–49SiO₂: 1MoO₃

M2: 10Na₂O–30PbO–10Bi₂O₃–48SiO₂: 2MoO₃

M3: 10Na₂O–30PbO–10Bi₂O₃–47SiO₂: 3MoO₃

M4: 10Na₂O–30PbO–10Bi₂O₃–46SiO₂: 4MoO₃

M5: 10Na₂O–30PbO–10Bi₂O₃–45SiO₂: 5MoO₃

All chemicals Na₂Co₃, PbO, Bi₂O₃, SiO₂, and MoO₃ utilized in this investigation are analytical grade reagents (99.9 % pure). Above discussed quantities of compounds were amalgamated in an agate mortar and thoroughly mixed until the homogeneous mixture was obtained. This mixture is transferred into a silica crucible. A programmable high temperature furnace was employed to heat up at 1200 ^oC for 20 minutes. The bubble free liquid obtained was discharged onto a spotless and well-designed rectangular brass plate to quench it at ambient temperature. It was immediately transferred into annealing furnace maintained at 350^oC. Later the samples were let to cool gradually to ambient temperature. The density of the synthesized samples was appraised by using VIBRA HT density measurement kit. The glasses were then cut, grounded and optically polished for optical absorption study. The leftover parts of the samples were pulverized into the fine powder for XRD, EPR and FTIR spectral investigations. The X-ray diffraction patterns of M0 and M5 samples were recorded with the SHIMADZU XRD 7000 Maxima system. A JASCO Model V-670 UV–vis–NIR spectroscope with a spectral resolution of 0.1 nm was employed to have optical absorption spectra in between 200–1000 nm at room temperature. The EPR spectra of 10Na₂O–30PbO–10Bi₂O₃–(50-x)SiO₂: xMoO₃ (1 ≤ x ≤ 5) glasses were traced in the frequency range 8.8 to 9.6 GHz at ambient conditions by employing an X-band microwave spectrometer Model JEOL–FE–IX EPR with 100 kHz field modulation. The magnetic field was varied from 0 to 500 mT at the rate of 25 mT/min. SHIMADZU FTIR- 8400S spectrophotometer was employed to obtain FTIR spectra of glasses under investigation in the range 400–1200 cm^− 1.

SiO₂ is a familiar glass former amongst all components of the glasses under study, with tetrahedral [SiO_4/2]⁰ units. The following chemical equilibrium, de-polymerization, arises upon the addition of modifiers like Na₂O, PbO [7]:

[SiO_4/2]⁰ + Na₂O → [SiO_3/2O]⁻ + 2Na⁺

[SiO_4/2]⁰ + Na₂O → [SiO_2/2O₂]²⁻ + 2Na⁺

[SiO_4/2]⁰ + Na₂O → [SiO_1/2O₃]³⁻ + 2Na⁺

Further Bi₂O₃ exhibits the weak glass forming nature with BiO₃ structural units when associated with SiO₂ in addition to modifying action with BiO₆ octahedral units [8]. Molybdenum ions may occur in two valance states viz., Mo⁶⁺ and Mo⁵⁺ in the glass network. There is a possibility for the reduction of Mo⁶⁺ ions to Mo⁵⁺ state, which take the position at octahedral sites with distortions due to John-Teller effect [9].

3.1 Physical parameters

Evaluation of density ρ, molar volume V_m and average molecular weight M of the prepared samples guided to calculate various physical parameters such as Mo ion concentration N_i, Mo mean ion separation R_i and polaron radius R_p. All the computed parameters are furnished in Table 1. The gradual increment in the values of density from M0 (4.338 g/cm³) to M5 (4.633 g/cm³) is owing to the structural modification and the continuous substitution of relatively insubstantial silicon ions by the weighty molybdenum ions.

3.2. XRD

The XRD patterns of M0 and M5 glasses are shown in Fig. 1. The noisy broad Bragg peaks reported endorse the glassy arrangement of the investigated specimens.

3.3 Optical absorption spectra

Optical absorption spectra, of the specimens prepared in this study, recorded in the wavelength region 200 to 1000 nm are represented in Fig. 2. The cut off wavelength of Mo free sample is observed at 367 nm. Where the optical absorption edge of M1 sample with 1 mol% of MoO₃ is significantly increased to 425 nm and it has red shifted with the amalgamation of MoO₃ up to 5.0 mol%. This abrupt escalation of the cut-off wavelength is a sign of the depolymerization of the network. The spectra also exhibited a wide absorption band between 550 to 800 nm which is ascribed to the excited Mo⁵⁺ (4d¹) ion [10]. With the increase of MoO₃ content from 1 to 5 mol % the intensity as well as half width of the spectra are noticed to increase with a red shift of peak position. This increase of intensity proposes the gradual conversion of Mo⁶⁺ ions into Mo⁵⁺ ions during melting and it suggests the existence of the highest concentration of MoO⁵⁺ ions in the glass network of the sample with 5 mol % MoO₃. Such Mo⁵⁺ ions cause added bonding imperfections and NBOs by forming Mo⁵⁺O³⁻ molecular orbitals leading to depolymerization of the glass network.

An optical band gap (E_g) of glasses under study are estimated from the extrapolated curves between (αhν)^1/2and hν as presented in Fig. 3 and values are furnish in Table 2. As an outcome the optical band gap is found to decrease from 3.38 eV to 3.03 eV with the rise of MoO₃ Concentration. Further, Urbach energy (ΔE) explores the information about the breadth of the extensions of localized states in the band gap, values are assessed by plotting the Urbach’s relation [11].

ln(α) = (hυ/ΔE) + const (4)

Fig. 4 represents these Urbach curves between ln(α) and hν of the glasses taken for study. The Urbach energy values are also presented in Table 2. It is identified that the Urbach energy ΔE of M0 (0.14 eV) is minimum and maximum for M5 (0.35 eV) sample. This type of spectroscopic analysis indicates that growth in the density of Mo⁵⁺ (O_h) ions in the samples from lower to higher concentrations of MoO₃. The development of donor centres and the ensuing overlap of excited states of localized electrons entrapped on Mo⁵⁺ sites with the vacant 4d Mo states on the nearest Mo⁶⁺ sites lead to more invade of polaron band into the main band gap. The red shift of absorption edge, significant fall in the band gap and enhancement in Urbach energy can be attributed to this polaronic development [9].

3.4. EPR spectra

Fig. 5 depicts the EPR signals of MoO₃ contained Na₂O−PbO−Bi₂O₃−SiO₂ glasses traced at ambient temperature. A main central line, aroused due to molybdenum isotopes (I = 0), bounded by satellite lines, corresponding to the hyperfine structure of odd ⁹⁵Mo and ⁹⁷Mo (I = 5/2) isotopes [12], with wee intensity is discernible in the EPR spectra. From these spectra the peak height and half width of the central line is observed to amplify with the rise of MoO₃ content in the glass. It noticeably indicates the enhancement of Mo⁵⁺ ions with the increasing content of MoO₃ in the glass composition. The maximum intensity observed in the spectrum of M5 sample shows the existence of higher proportions of Mo⁵⁺O^3- ions compared to other ones. It is well known that molybdenum take part in the glass network as MoO³⁺ complex. The high charge lingering on the central molybdenum in the interim the drop of the net charge on the neighbouring oxygen ions and accordingly reduced the ability to donate an electron (σ–bonds) stimulates the strong π-bond with two axial oxygens of the compressed octahedron. As a result, MoO₆ transforms into complex Mo⁵⁺O^3- molybdenum ion. In other words, in the axially distorted environment Mo(V) is strongly displaced in its octahedron so that it adopts a pyramidal MoO₅ configuration with a very short Mo–O bond length matching to the molybdenyl ion. This endorses the coordination of Mo⁵⁺ ions with five oxygen ions by C_4v symmetry through Mo=O in studied glasses.

The spin Hamiltonian function [12] for the illustration of Mo⁵⁺ spectrum is

H = βSgB + SAI (5)

Here the symbols have the standard meaning, i.e., the Bohr magneton β, electron spin S = ½, g-tensor g, the nuclear spin of molybdenum ‘I’, hyperfine structure tensor ‘A’ and the applied field B. The degree of axial distortion α of the coordination polyhedron is estimated by (g_e−g_||)/4(g_e− g_^). The values of g_||, g_^, A_||, A_^ and a evaluated from the spectra are presented in Table 3 and also observed g_||< g_^< g_e (2.0023) together with A_||> A_^, demonstrating the octahedral site with tetragonal compression. The observed trends of these parameters with the rise of MoO₃ content support the above viewpoints.

3.5. FTIR spectra

FTIR spectra of glasses considered for the present study are portrayed in Fig. 6. The bands noticed at about 941, 781 and 486 cm^-1 are assigned to the asymmetric, symmetric and bending vibrations of Si−O−Si linkages of silicate structural units, respectively. The weak band reported at about 415 cm^-1 is attributed to the vibrations of Bi−O bonds in BiO₆ units. Another band observed at about 1112 cm^-1 is assigned to O−Si−O asymmetric stretching mode of vibrations of SiO₄ structural entities linked with NBOs [13]. Another two bands at about 843 and 877 cm^-1 are detected which are due to the vibrations of MoO₄^2- and MoO₆ structural units [14]. The intensity of the vibrational band due to MoO₄^2- tetrahedral units is found to reduce and shifted towards greater wave number side whereas the peak height of the band due to MoO₆ structural units is found to increase with a red shift, with the increase of MoO₃ content [14-16]. Thus it is confirmed that the conversion of molybdenum ions from MoO₄ to MoO₆ state with the rise of concentration of MoO₃. As well the intensity of the bands pertinent to asymmetric stretching vibrational modes (Si-O-Si, O-Si-O^-) is noticed to rise at the cost of intensity of symmetric stretching vibrations of Si-O-Si with the rise of MoO₃ content. The above spectral analysis strongly evidences the modifier role of molybdenum ions at higher concentrations of MoO₃ and consequent depolymerisation of the present glass network.

Na₂O − PbO − Bi₂O₃ − SiO₂ glasses blended with diverse concentrations (0 to 5 mol % in steps of 1 mol %) of MoO₃ were made by using melt quenching technique. Glassy nature is confirmed by XRD and Physical parameters were also measured. The conventional spectroscopic techniques viz., optical absorption, EPR and FTIR were conducted. Optical absorption spectra elucidated the existence of molybdenum ions in Mo⁵⁺ and Mo⁶⁺ localized states and also revealed the gradual enhancement in the redox ratio (Mo⁵⁺/ Mo⁶⁺) with the hike in the concentration of MoO₃. The EPR spectroscopic investigation of the glasses supported the enrichment of Mo⁵⁺ ions and their coordination with five oxygen ligands in a square pyramidal, C_4v symmetry, with a Mo = O bond. These Mo⁵⁺ ions produce non bridging oxygens by breaking the Si-O-Si linkages in SiO₄ structural units which enhances the discreteness and consequent depolymerisation in the glass network. The increase in the intensity of asymmetrical vibrations at the expense of symmetrical vibrations of various structural units with a rise in the concentration of MoO₃ in the FTIR spectra also supports the increasing degree of disorder in the glass network which causes depolymerisation. As a whole, the investigations carried out strongly suggest the semiconducting nature of samples with the high concentration of MoO₃ and also strengthen the proposal of usefulness of MoO₃ doped sodium lead bismuth silicate glasses as electronic devices, such as non-crystalline semiconducting materials, IR transmission modules, thermal and mechanical sensors, and scintillators in nuclear instruments.

Acknowledgements

The authors are grateful to managements of BEC and VFSTRU for giving support and encouragement in carrying out this research work.

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Table 1 Physical parameters of MoO3 doped Na2O−PbO−Bi2O3−SiO2:MoO3 glasses.

Sample	Density r (g/cm³) (±0.001)	Molar Volume V_m (cm³) (±0.01)	Conc. of Mo ions N_i(x10²¹ ions/cm³) (±0.01)	Inter ionic distance of Mo ions R_i (Å) (±0.01)	Polaron radius R_p (Å) (±0.01)
M0	4.338	24.95	---	----	----
M1	4.414	24.54	24.88	3.42	2.19
M2	4.468	24.46	49.57	2.72	1.74
M3	4.508	24.42	73.86	2.38	1.52
M4	4.578	24.29	98.47	2.16	1.38
M5	4.633	24.20	122.68	2.01	1.28

Table 2 Data on optical absorption spectra of MoO₃ doped Na₂O–PbO–Bi₂O₃–SiO₂ glasses.

Sample	Cut-off wavelength λ_c (nm) (± 1nm)	Band position	Optical band gap E_g (eV) (± 0.01eV)	Urbach energy ΔE (eV) (± 0.01 eV)
M0	367	---	3.38	0.14
M1	425	683	3.30	0.20
M2	430	686	3.20	0.24
M3	445	688	3.12	0.27
M4	451	691	3.06	0.29
M5	462	694	3.03	0.35

Table 3 Spin – Hamiltonian parameters of MO⁵⁺ ions in Na₂O−PbO−Bi₂O₃− SiO₂: MoO₃glasses.

Sample	g_\|\|		A_\|\| (x10^-4 cm^-1)	A_⊥ (x10^-4cm^-1)	a
M1	1.912	1.941	81.34	46.53	0.368
M2	1.915	1.943	89.46	52.37	0.374
M3	1.917	1.946	94.34	56.82	0.379
M4	1.920	1.949	99.25	62.75	0.386
M5	1.924	1.952	106.3	70.52	0.389

Table 4 Data on FTIR spectra (cm^-1) of Na₂O−PbO−Bi₂O₃− SiO₂:MoO₃glasses

M0	M1	M2	M3	M4	M5	Assignment
422	421	419	417	416	415	Bi-O bonds in BiO₆ units.
474	476	479	482	484	486	Rocking vibrations of Si-O-Si linkages
772	774	776	778	780	781	Si-O-Si symmetric stretching vibrations
-	828	830	835	839	843	Vibrations of MoO₄ units
-	889	886	883	880	877	Vibrations of MoO₆ units
951	949	947	944	943	941	Si-O-Si asymmetric stretch vibrations
1122	1120	1117	1115	1113	1112	O-Si –O asymmetric stretching vibrations in SiO₄ units associated with NBOs

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An Effectual Role of Molybdenum Ions on the Semiconducting Temperament of Na2O-PbO-Bi2O3-SiO2 Glasses

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1. Introduction

2. Experimental

3. Results And Discussion

4. Conclusions

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Acknowledgements

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