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Optical properties of La0.9Sr0.1Fe1-xMoxO3 (x = 0.1, 0.2, and 0.3) perovskite material prepared by sol-gel method

The perovskite-type oxide Lao.9Sro.1Fe1-xMoxO3 (LSFMO) with x = 0.1, 0.2, and 0.3 were successfully prepared by sol-gel method. The synthesized perovskite has been characterized by Fourier transformed infrared spectroscopy (FTIR) and UV-Vis spectroscopy for further studies. FTIR spectra confirm the presence of Fe/Mo—O stretching and Fe/Mo—O— Fe/Mo bending vibrations in lattice. UV-Vis analysis displayed the optical band gap increase with increasing of Mo-content.


Introduction
Perovskite with general formula ABO3 have been paid attention for twenty century due to the unique dielectric, electronic conductivity, multiferroics behaviour, magnetic ordering, and optical properties [1][2]. LaFeO 3 is the one of the perovskite materials which is widely studied for various potential application such as solid oxide fuel cells (SOFC), batteries, dan photocatalysis [2]. The semiconducting perovskite LaFeO 3 nanostructure has potential in photocatalyst application due to the lower direct bandgap energy in the range of visible light [1]. Based on theoretical model using ab initio pseudopotential method, Shein et al. [3] calculated the band gap energy of LaFeO 3 and found to be 2.52 eV. Meanwhile, Arima et al. [4] calculated the band gap energy of LaFeO 3 based on the atomic ionization potential and found to be 2.10 eV. Subramaniam et al. [5] studied the effect of experimental condition and molar concentration in LaFeO3 and found that these parameters could be used to controlled the morphology, surface area, particle size, and band gap energy of LaFeO3. Murade et al. [6] reported the appearance of Sr−O and Fe−O stretching modes due to the Sr-substitution on La0.7Sr0.3FeO3 samples. Based on several research above, we study the effect Mo-substitution on La0.9Sr0.1Fe1-xMoxO3 (LSFMO) with x = 0.1, 0.2, and 0.3. The samples were prepared by sol-gel method. The LSFMO materials were characterized by Fourier transformed infrared spectroscopy (FTIR) to investigate the Sr/La−O and Mo/Fe−O vibration modes in lattice and UV-Visible spectrophotometer to reveal the bandgap energy. From this, the effect of Mo-doped on LSFMO perovskite materials could be explained.

Experimental Methods
Perovskite LSFMO was synthesized by sol-gel method of La2O3, Sr(NO3)2, Fe(NO3)3.9H2O, MoO3, citric acid, and aqua distillation as a precursor. The solution was converted to gel phase with stirring and heating process. Then, the drying process was applied to remove water, citric acid and another organic compounds. The powder phase was obtained after calcined process. The details of the sample preparation have been explained in previous work [7]. The FTIR spectrum was recorded using PerkinElmer type SpectrumTwo to confirm the structural information. The spectrum was collected in the mid-IR range from 4000 to 250 cm -1 at room temperature. Optical reflectance spectrum was recorded in the range of 200 -800 nm by using UV-VIS (Agilent Cary 100/300).

Fourier transform infrared spectroscopy
Vibrational spectroscopy measurements were performed to investigate further on the structure of different compositions prepared. The FTIR spectra of La0.9Sr0.1Fe1-xMoxO3 (x=0.1, 0.2, 0.3) nanoparticles is shown in figure 1. The small shift is observed to the lower wavenumber due to the Mosubstitution.

UV-Vis analysis
Optical band gap energy of LSFMO was obtained based on diffuse reflectance spectrum mode with UV-Vis spectroscopy. Figure 2  In order to estimate the bandgap energy of LSFMO, the diffuse reflectance data is used to obtain the coefficient absorption based on the Kubelka-Monk (KM) function [10].
where F(R) is the KM function, is absorption coefficient, is the scattering coefficient, and R is the diffuse reflectance recorded. Figure 3 shows the bandgap energy (Eg) which was estimated by using Tauc's plot. The band gap energy is found in the range of 2.0 -2.2 eV. The band gap energy increases with increasing of Mo-doped. It is possible caused by the Mo 6+ substituted in Fe 3+ -site in lattice introducing the mixed valence cation of Mo 4+ /Mo 6+ and/or Fe 2+ /Fe 3+ which has smaller ionic radii that leads to the increase the atomic bonding in lattice. Accordingly, the movement of charger carriers become more difficult, so the energy that needed to facilitate the movement of carriers, i.e., gap energy tends to increase.  Figure 4 shows bandgap energy as a function of Mo-concentration. The increasing of band gap energy indicates the decreasing of conduction response in the samples. Similar trend has been reported by Yutana et al. [11] that substitution of Al on A-sites in LaFeO3 increases band gap energy with rise Alsubstitution.