Optical, luminescence and magnetic properties of braunite‒rhodonite nanocomposites synthesized by green aqueous sol‒gel route

Abstract

Braunite (Mn₇SiO₁₂)‒rhodonite (MnSiO₃) nanocomposites have been synthesized utilizing a green aqueous citrate sol‒gel route. The influence of heat-treatment temperature on the structure and properties of these nanocomposites was investigated. X-ray diffraction and high resolution transmission electron microscopy analyses demonstrated that well-crystallized nanoparticles, having average sizes in the range of 18‒42 nm, were produced. MnSiO₃-content increased in the nanocomposites with increasing calcination temperature from 600 to 900 °C. Optical, photoluminescence and magnetic properties were determined. Ultraviolet-visible-near infrared diffuse reflectance spectra were used, applying Kubelka-Munk function, for optical absorbance calculation and determination of the band gap energy (E_g). Optical absorption spectra exhibited bands at 415‒438 nm originated from Mn²⁺ ions, and other bands at 550 and 599 nm due to absorption of Mn³⁺ ions. E_g was found to increase with increasing MnSiO₃-content in the nanocomposites. The obtained nanocomposites gave green fluorescence emissions at 525‒565 nm, a yellow emission at 584 nm and red emissions at 619 nm. All the synthesized nanocomposites exhibited antiferromagnetic properties in which the paramagnetic contribution and magnetization increased as the MnSiO₃-content increased. The present nanocomposites are promising candidates for application as light emitting diodes as well as magnetoelectronic materials which are used in biomedical applications.

Keywords: Antiferromagnetism; Manganese‒silicates; Optical materials; Photoluminescence; Sol‒gel; XRD-HRTEM.

Fig. 1

XRD patterns of the Mn₇SiO₁₂‒MnSiO₃ nanocomposites prepared at 600, 750 and 900 °C.

Fig. 2

(a) TEM image of the MS600 nanocomposite and (b) its HRTEM lattice image indicating d = 0.27 nm.

Fig. 3

IFFT profile of the MS600 nanocomposite.

Fig. 4

(a) TEM image of the MS750 nanocomposite and (b) its HRTEM lattice image indicating d = 0.28 nm.

Fig. 5

IFFT profile of the MS750 nanocomposite.

Fig. 6

(a) TEM image of the MS900 nanocomposite and (b) its HRTEM lattice image indicating d = 0.27 nm.

Fig. 7

IFFT profile of the MS900 nanocomposite.

Fig. 8

(a, b, c) SAED patterns of the MS600, MS750 and MS900 nanocomposites, respectively.

Fig. 9

Absorbance spectra of the MS600, MS750 and MS900 nanocomposites.

Fig. 10

Tauc plots for determination of E_g of the MS600, MS750 and MS900 nanocomposites.

Fig. 11

Room temperature PL emission spectra, excited at 254 nm, of the MS600, MS750 and MS900 nanocomposites.

Fig. 12

Schematic diagram of the energy levels of Mn²⁺ illustrating the present fluorescence emission mechanism.

Fig. 13

Excitation spectra of the MS600, MS750 and MS900 nanocomposites monitored at λ_em = 560, 560 and 533 nm, respectively.

Fig. 14

Room temperature magnetic M‒H hysteresis loops of the MS600, MS750 and MS900 nanocomposites. The inset graph shows their narrow hysteresis loops with non-vanishing coercivity and remanence.