Preparation and magneto-optical properties of stable bismuth phosphate nanoparticles in phosphate glass

Abstract

Synthesis of bismuth phosphate (BiPO₄) nanocrystals in low melting phosphate glass and its optical and magneto-optical properties were investigated. The presence of BiPO₄ nanoparticles was confirmed by X-ray diffractometry and Raman spectroscopy. The optical transmission spectra of the glass samples showed that the red shift in the absorption edge is due to the presence of BiPO₄ nanoparticles in the glass. Faraday rotation tests on the glass nanocomposite showed a large increase in the Verdet constant caused by BiPO₄ nanoparticles in the phosphate glass. The BiPO₄-glass nanosystem showed a higher Verdet constant (21.2° T⁻¹ cm⁻¹), which was increased 3-fold compared with the Verdet constant of BK-7 glass (7.52° T⁻¹ cm⁻¹). The BiPO₄ glass nanocomposite may have potential applications in magneto-optical devices.

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Introduction

Glasses are amorphous supercooled liquids that are often transparent. Glasses can consist of fused inorganic compounds, which are cooled to a rigid state without crystallizing. The main distinction between crystalline solids and glasses is the presence of a long-range ordered crystal structure in crystals, and random short-range order in glasses.^1a The optimization of properties, which are a function of composition and other processing parameters, requires good knowledge of the microscopic glassy structure.^1b

In recent years, glasses containing nanoparticle dispersed systems (NDS) and optically functional nanoparticles embedded in glass matrices have attracted much attention in the field of applied optics, in applications such as light-emitting diodes, optical switches, and optical circuits.^1c,2 NDS can be used in magneto-optical devices, including optical isolators and optical modulators.^3,4,5a,5b,5c Magneto-optic glasses have applications in magneto-optic current transformers (MOCTs), optical fibre sensors and highly sensitive magnetic field detectors.⁶ Optical current sensors have important uses in high-voltage environments due to their immunity to electromagnetic induction (EMI) and high bandwidth capabilities.⁷ Highly sensitive magnetic field sensors can be useful in detecting currents of less than 1 mA. The sensitivity and linearity of existing magneto-optic devices are the limiting parameters, which necessitate further research to enhance the magneto-optic effect in glasses.

Recently, the Faraday rotation (FR) properties of glasses have been extensively studied, focusing on the magneto-optical properties of binary and ternary glass systems activated by doped rare earth ions.⁸ The FR properties of silica-based glasses have also been the subject of great interest.^9–11a However, very little work has been carried out on the FR properties of phosphate glasses.^11b The melting temperature of silica-based glasses is in the range of 1300–1500 °C, which is energy intensive and critical for casting blanks. To overcome these problems, researchers have focused their attention on the synthesis and characterization of low-temperature melting glasses such as phosphate glasses. Phosphate glasses are known for their unique properties including high thermal expansion, low melting temperature, low refractive index and high ultraviolet transparency. Owing to these properties, phosphate glasses are used in glass-to-metals seals and various optical devices.^12–16 However, phosphate glasses are not widely used commercially due to their poor chemical solubility.^17,18 In view of this, we have investigated the fabrication of stable phosphate glass. The present paper reports the synthesis of BiPO₄ phosphate glasses and measurement of its magneto-optical properties to determine the Faraday rotation. The Verdet constant is calculated from the Faraday rotation. The Faraday effect, namely the diamagnetic and paramagnetic effect, is already well-known in glasses. A theoretical explanation of the Faraday effect with respect to the birefringence is provided.

Faraday rotation measurements by photoelastic modulation

If x is the direction assigned to the polarization of incident light, then the wave exiting the sample after rotation by angle θ can be expressed as

Ē = E₀(cos θx̂ + sin θŷ (1)

The light travels through the photoelastic modulator (PEM), which imposes a sinusoidal oscillation in the polarization angle. If the PEM modulation amplitude is A₀ and frequency is Ω, then the resultant wave field is

where A = A₀ cos Ωt.

The polarization oriented at 45° to the x-axis allows the measurement of the original polarization angle.

Power at the detector is given by

This expression can be expanded in the Bessel function coefficient as

By choosing a modulation amplitude that is the zero of the lowest order Bessel functions, and measuring both the static signal and second harmonic of the modulation frequency, we can extract the polarization angle. The zero of the Bessel function is 0.383 waves (2.405 rad), which eliminates the second term of the above equation. Signals from the detector can be written as the DC term and the term that oscillates at the second harmonics of the PEM modulation, with an amplitude of V_2f. In volts, this signal is expressed as

We can deduct the Faraday angle by measuring each of these signals independently. It is assumed that the Faraday angle is very small and a factor of is introduced to account for the RMS read by the lock-in amplifier.

In the present communication, considering the potential applications of nanocrystalline glasses, we developed quantum dots of bismuth phosphate in stable phosphate glasses. This nanocrystalline glass was characterized to investigate its structural and optical properties. The Faraday rotations were measured with a magnetic field to investigate the magneto-optical properties of the glass. The glass containing BiPO₄ nanocrystals showed good magneto-optical properties.

Experimental

Preparation of glasses

The raw materials for the host glass and BiPO₄ glass nanocomposites were prepared with compositions of 66 wt% P₂O₅, 20 wt% K₂O, 10 wt% B₂O₃, and 4 wt% Ta₂O₅; and 66 wt% P₂O₅, 20 wt% Bi₂O₃, 8 wt% K₂O, 5 wt% B₂O₃, and 1 wt% Ta₂O₅, respectively. Materials with 99–99.9% purity were procured from reputable companies such as Sigma-Aldrich. All chemicals were used as received.

The host glass and BiPO₄ nanoparticles in phosphate glasses were prepared by thoroughly mixing and melting appropriate amounts of analytical reagent grade chemicals. The mixtures were preheated slowly to 450 °C, prior to melting in an alumina crucible to remove H₂O, NH₃, and CO₂. The preheated mixture was melted at 800–850 °C. The melt was soaked for 2 h, the crucible was removed from the furnace, and the melt was air quenched on a highly polished stainless steel mould. The mould was preheated at ∼400 °C to prevent cracking of the glass because of thermal shock. The glasses were annealed to room temperature at a cooling rate 0.5 °C min⁻¹.

Characterization

The glass samples were characterized by various techniques. The crystalline phase and crystallite size was determined by X-ray powder diffraction (XRD; Advance D8, Bruker-AXS). Room temperature micro-Raman scattering (RS) was performed by using a HR 800-Raman spectrometer (Horiba Jobin Yvon), with excitation at 632.81 nm by a coherent He–Ne ion laser and a liquid nitrogen-cooled CCD detector to collect and process the backscattered data. Optical characterization of the glass composites was performed with a UV-Vis-NIR spectrophotometer (λ-950, PerkinElmer). The surface characterization of the glasses was performed by high-resolution transmission electron microscopy (HRTEM; 2010F, JEOL). For HRTEM studies, the glass samples were prepared by dispersing the glass powder in ethanol, sonicating it in an ultrasonic bath for 10 min, drop casting the sample on a carbon-coated copper grid, and then drying it.

Experimental setup for measurement of faraday rotation

The experimental setup is shown in Fig. 1. Diode lasers (λ₀ = 405, 532, 635, 670, 980, 1310 nm) were used as the light source. Details of the experimental setup for the measurement of the Verdet constant at relatively low fields of 2–4 mT rms have been reported previously^19,20. The present experimental setup was based on these reports. Use of AC magnetic fields meant low fields could be generated at convenient levels to avoid excess heat from bulky coils. The AC magnetic field method is an efficient way of calculating the Verdet constant by using phase sensitive detection with the lock-in amplifier and a Fourier transform method to calculate the rotation and hence the Verdet constant. A Helmholtz coil capable of generating a magnetic field of up to 7 mT rms was used. This ensured uniformity of the field. The coil was driven at 60 Hz. The optical chopper wheel pulsed light at a set frequency equal to that of the magnetic field frequency.

Fig. 1 Experimental setup for measuring the Faraday rotation angle.

A high-speed photodetector (DET 100, Thorlabs) was used to measure small changes in intensity. The photodetector had a spectral response from 350–1100 nm and was used to convert the light signal into an equivalent voltage output. A Phywe Tesla meter was used for the measurement of magnetic field. A National Instruments data-logger data acquisition device was used in conjunction with LabVIEW software to transfer the output of the lock-in amplifier to a computer to record and maintain data accurately. Initial measurements of the Verdet constant of known magneto-optic glasses, such as BK-7 and SF-57, in our setup yielded an accuracy of 6% compared with published values.^19,21

Results and discussion

X-ray diffraction analysis

X-ray diffractometry of glass samples (Fig. 2) showed growth of hexagonal bismuth phosphate (BiPO₄) nanocrystals in the matrix (JCPDS data card no.15-0766). The broadening of peaks was induced by the small size of the nanocrystals embedded in the glass matrix. The average crystallite size was determined as 9–10 nm from XRD data by using the Scherrer equation. The nanocrystal size calculated from the XRD was slightly higher than the observed size, which may be because the nanoparticles were embedded in a glassy matrix. The peaks were noisy because of the insulating amorphous glass matrix. XRD of the host glass also revealed the amorphous structure of the host glass.

Fig. 2 XRD of BiPO₄ nanoparticle glass and host glass.

Raman spectroscopy

The BiPO₄ nanoparticle glass nanocomposite was characterized by Raman spectroscopy in the 100–1200 cm⁻¹ region (Fig. 3). All assigned peaks are summarized in Table 1. The absorption bands in these spectra are in good agreement with the literature.^22a The intense peak at 202 cm⁻¹ may be assigned to the symmetric bending vibration of Bi–O. A main feature of Raman spectroscopy is that bands below 400 cm⁻¹ can be easily assigned. Intense bands attributable to M–O vibrations were observed. Thus, the Raman bands between 450 and 600 cm⁻¹ (Fig. 3) were attributed to the v₄ bending modes of the PO₄ units, and the two intense bands between 950 and 1062 cm⁻¹ were assigned to the v₁ symmetric and v₃ asymmetric stretching modes of the PO₄ tetrahedron. The additional peaks at 712 and 1130 cm⁻¹ were from the M–O–P symmetric stretching vibration and P–O stretching vibration (Fig. 3).^22b,c These results showed the presence of BiPO₄ nanoparticles in the glass matrix.

Fig. 3 Raman spectra of BiPO₄ glass nanocomposite and host glass.

Table 1 Summary of assigned Raman peaks

No.	Peak frequency (cm^−1)	Peak assignments
1	202	O–Bi–O symmetric bending mode
2	400	B–O vibrations and v2 bending modes of PO4 units
3	446 and 600	v4 bending modes of PO4
4	712, 730	M–O–P symmetric stretching vibration
5	966	v1 symmetric stretching modes of PO4 tetrahedron
6	1062 & 1130	v3 antisymmetric stretching modes of PO4 tetrahedron and P–O stretching vibration

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Fig. S1 (see ESI†) shows the FT-IR spectrum of the BiPO₄ glass nanocomposite in the range of 600 to 2000 cm⁻¹. The broad peak observed at 1164 cm⁻¹ is from O–P–O asymmetric stretching, and those at 722 and 845 cm⁻¹ are from symmetric and asymmetric stretching of bridging oxygen (P–O–P).

Transmission electron microscopy

Glasses containing BiPO₄ nanoparticles were characterized by using TEM to determine the morphology of the BiPO₄ nanoparticles. Fig. 4 shows TEM images and the electron diffraction (ED) pattern of BiPO₄ nanoparticles in phosphate glass. The TEM images clearly show the nanocrystalline phase in the glass.

Fig. 4 (a–c) TEM images and (d) electron diffraction (ED) pattern of BiPO₄ nanoparticles embedded in phosphate glass.

From the TEM images (Fig. 4a and b), the particle size of BiPO₄ was in the range of 5 to 6 nm for the BiPO₄ glass nanocomposite. Fig. 4c shows the high-resolution transmission electron microscopy (HRTEM) image. HRTEM reveals that the interplanar spacing is about 0.186 nm, which corresponds to the (212) lattice planes of hexagonal BiPO₄ and it is consistent with the XRD results. The electron diffraction pattern shows the polycrystalline nature of the BiPO₄ nanoparticles. The XRD and ED patterns clearly indicate the presence of BiPO₄ nanocrystals.

Optical characterization

The UV-vis spectrum shows the optical transmission spectra of the glass nanosystem (Fig. 5).

Fig. 5 UV-Vis. Transmittance spectra of BiPO₄ glass nanocomposite.

The transmittance spectra show that the introduction of BiPO₄ nanoparticles in the glass matrix causes a strong red shift in the absorption edge. The host glass shows a band gap of 3.51 eV, whereas the BiPO₄ glass nanocomposite shows a band gap of 2.04 eV. Fig. 6 shows photographs of glasses with and without BiPO₄ nanoparticles in glass; the nanoparticles change the glass from colourless to dark orange.

Fig. 6 Photographs of the glasses. (a) BiPO₄ glass nanocomposite, and (b) host glass.

In the present study, FR was measured for the BiPO₄-phosphate glass nanocomposite at room temperature in magnetic fields of 0–214 mT with a diode laser of different wavelengths. The photodetector intensities were averaged at each value of the magnetic field for all nanocomposites. Normalization of the relative change in intensities was performed with respect to the intensity of the blank host sample. The FR angles were calculated from²³

where V_ref is the reference signal from lockin amplifier and V_DC is the signals from the detector in terms of DC voltage. Fig. 7 shows the magnetic field-dependent Faraday rotation (A–C) and wavelength-dependent Verdet constant of the BiPO₄-glass nanocomposite, host glass, and BK-7 glass.

Fig. 7 Magnetic field-dependent Faraday rotation of (A) BiPO₄-glass nanocomposite, (B) host glass and (C) BK-7 glass. (D) Wavelength-dependent Verdet constant of BiPO₄-glass nanocomposite, host glass and BK-7 glass.

The quantum dot-glass nanocomposite exhibited a size-related FR magnetic field strength. Fig. 7 shows the expected linear increase of FR with the magnetic field, and the variation in FR for the BiPO₄-glass nanocomposite, host glass, and BK-7 glass. The variation of the Verdet constant is shown in Table 2. There is a significant increase in the FR in the BiPO₄-glass nanocomposite with respect to the host glass and standard BK-7 glass. The BiPO₄-glass nanocomposite showed a high Verdet constant (21.2° T⁻¹ cm⁻¹) at 405 nm, which was 2 and 3 times higher than the host glass and BK-7 glass, respectively. Surprisingly, the host glass showed a high Verdet constant (9.64° T⁻¹ cm⁻¹) compared with the previously reported value. The Verdet constant for the BiPO₄-glass nanocomposite was 68 times larger than the previous reported value (0.312° T⁻¹ cm⁻¹).²⁴ This may be because of the higher refractive index of the glass owing to the BiPO₄. However, this unique observation is not fully understood yet, and requires further discussion and study. We intend to investigate the magnetic properties of the BiPO₄-glass nanocomposites further, and to perform femtosecond time-resolved photoluminescence studies to elucidate the origin of the large Faraday effect in these materials.

Table 2 Verdet constant of BiPO₄ glass, host glass and BK-7 glass^a

Wavelength (nm)	Verdet constant (° T^−1 cm^−1)
	BiPO4-glass nanocomposite	Host glass	BK-7 glass
a Values in square brackets indicate thickness of the sample.
Thickness (cm)	0.27	0.31	1.80
405	21.2	9.64	7.52
532	8.7	4.96	3.65
635	6.4	1.01	2.61
670	7.1	1.56	2.54
980	2.3	0.75	1.13
1310	1.4	0.24	0.73

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Conclusions

A new, chemically stable host phosphate glass with a low melting temperature of 800–850 °C was investigated. Bismuth phosphate (BiPO₄) nanoparticles were embedded in the phosphate glass. The effect of the BiPO₄ content on the optical and magneto-optical properties was investigated. The UV-Vis spectra showed that the optical cut-off of the glasses was shifted to a higher wavelength in the presence of BiPO₄ nanoparticles. The BiPO₄ nanoparticles were uniformly distributed in the glass matrix with an average particle size of 5–6 nm. The highest Verdet constant (21.2° T⁻¹ cm⁻¹) was observed for the glass nanocomposite with BiPO₄ nanoparticles, and was higher than the host glass and standard BK-7 glass. This increase of the Verdet constant can be attributed to the increased excitonic confinement by the quantum dots in the glass matrix. This is the first report of this type of increase in the Verdet constant for BiPO₄ phosphate glass nanocomposites. The BiPO₄ quantum dot-glass nanocomposite may have potential applications in magneto-optical devices.

Acknowledgements

The authors would like to thank the Executive Director of C-MET for technical support and DeitY, New Delhi for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c5ra05483d

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