Influence of Silver and Copper Substitution on Structural, Dielectric, Magnetic, and Catalytic Properties of Nano-Lanthanum Ferrites

INTRODUCTION
Perovskite oxides ABO3, composed of rare earth metal A ion with a radius larger than 1.0 Å and transition metal B ion with radius in the order of 0.6 – 0.8 Å, is attained great interest in the modern chemical industry due to its unique properties like high conductivity as well as excellent thermal and chemical stability [1, 2]. Among various ABO3 type perovskite oxides, the orthorhombic distorted perovskite structured lanthanum ferrite with formula LaFeO3 has gained immense interest under its wide applications in the areas of catalysis as well as electronic and magnetic materials [3, 4].
In recent years, the substitution of metal ions into A- or B-sites of LaFeO3 has been adopted by researchers to provide exciting prospects in improving the dielectric, magnetic and catalytic properties of LaFeO3 [1], [5 – 7]. For instance, Kundu et al. demonstrated that the enhancement of multiferroic and magnetoelectric coupling in LaFeO3 nano-ferrites of samarium substitution [8]. The authors reported a significant change in the magnetic properties in samarium and holmium doped LaFeO3 as compared to the pure LaFeO3 nano-ferrites [8, 9]. In particular, several attempts have been reported in the literature to enhance the catalytic performance of LaFeO3 by substituting the other metals in place of La or Fe of LaFeO3 (1), (5), [10, 11]. The strontium substituted LaFeO3 has shown a significant increase in the catalytic reactions towards the reduction of NO by hydrogen gas [12], methane combustion [13], and N2O decomposition [14]. The calcium substituted LaFeO3 is also showed excellent catalytic performance for the combustion of methane [15]. Although, many reactions are catalyzed by substituted LaFeO3 nano-ferrites; still, there is a high demand to explore the vast catalytic applicability of substituted LaFeO3 nano-ferrites. This can be achieved by the utilization of substituted LaFeO3 nano-ferrites as a catalyst for other reactions. 
The present work was designed to understand the influence of silver and copper doping in the LaFeO3 nano-ferrite. The synthesis was achieved by the sol-gel technique and the structural confirmation can be done by different techniques. To understand the influence of silver and copper on electric and magnetic, catalytic properties were discussed in detail. However, the results are preliminary and a detailed investigation has to be done to understand the mechanism. 

MATERIALS AND METHODS 
Materials and instruments
Ferric nitrate (Fe(NO3)3.9H2O), silver nitrate (AgNO3), cupric nitrate (Cu(NO3)2.3H2O), lanthanum nitrate (La(NO3)3.6H2O), citric acid (C6H8O7), ammonia solution (25% LR) were procured from Merck (India). 
The obtained powder samples were characterized by Brukers X-Ray diffraction (XRD) (Brukers AXS D8, USA), Fourier transform infrared (FT-IR) spectroscopy (PerkinElmer/ Spectrum 65), Fourier electron scanning emission microscope (FE-SEM) (NOVA-230, JEOL JSM-7600F FEG-SEM), LCR meter (HIOKI 3532-50, Japan) (vibrating sample magnetometer (VSM) (Lake-shore 7407, USA) and UV-visible Spectroscopy (Lab Man LMSP-UV 1200, India). The energy dispersive X-ray (EDX) data were measured on the FE-SEM coupled with the EDX detector. 

Method
In the present work, the LaFeO3 and silver, copper doped LaFeO3 nano-ferrites were prepared by sol-gel technique employing citric acid as stabilizing agent.

Preparation of LaFeO3
To prepare the lanthanum ferrite (LaFeO3) nano-ferrite, the stoichiometric amounts of 4.33 gm of La(NO3)3.6H2O, 4.03 gm of Fe(NO3)3.9H2O were dissolved in 100 ml distilled water.  The 1.92 gm of citric acid was dissolved in 100 ml of doubled distilled water. In this reaction, citric acid is used as a capping agent to reduce the agglomeration of the particles. These solutions were mixed and stirred vigorously under 80 to 90 0C followed by the addition of ammonia to maintain the pH 7. The resultant mixture was then heated up to 150 0C with constant stirring until the formation of a viscous gel. The heating was continued till the formation of a brown colour ash powder. The powder was then calcinated in the air around 900 0C for 6 hours, subsequently cooled and ground to get a fine powder. The calcinated powder sample was used for characterization.

Preparation of doped LaFeO3 nano-ferrites: 
To prepare the silver doped LaFeO3 (La0.5Ag0.5FeO3) nano-ferrite, the stoichiometric amounts of 2.16 gm of La(NO3)3.6H2O, 4.03 gm of Fe(NO3)3.9H2O, 0.84 gm of AgNO3 were dissolved in 100 ml of double-distilled water. The 1.92 gm of citric acid was dissolved in 100 ml of double-distilled water. Now, the molar ratio of metal nitrates and citric acid solutions was controlled at the ratio of 1:1. Then, these solutions were mixed and stirred vigorously under 80 to 900C followed by the addition of ammonia solution to maintain the pH 7. The resultant mixture is heated up to 1500C with constant stirring until the formation of a viscous gel. The heating is continued up to the consumption of the whole citrate complex to turn into brown color ash powder. The prepared powder was calcinated in the air around 9000C for 6 hours, subsequently cooled and ground into fine powder. The sample copper doped LaFeO3 (La0.5Cu0.5FeO3) nano-ferrite was prepared by adding the stoichiometric amount of 1.2gm (Cu (NO3)2.3H2O) in place of AgNO3. The other sample silver and copper doped LaFeO3 (La0.5Ag0.25Cu0.25FeO3) nano-ferrite was prepared by varying stoichiometric amount of AgNO3 into 0.42 gm instead of 0.84 gm (in case of La0.5Ag0.5FeO3) and adding a stoichiometric amount of 0.6gm (Cu (NO3)2.3H2O). 
Catalysis of p-Nitroaniline reduction 
 0.6906 gm of p-nitroaniline and 0.189 gm of NaBH4 were dissolved in 50 ml of deionized water. Then from this 400 µl of each NaBH4 and p-nitroaniline solutions were dissolved in 3200 µl of deionized water. Take 1500 µl of this reaction mixture into the cuvette which was dissolved in the 1500 µl of deionized water. To this add 0.05 gm of the prepared LaFeO3 and also the doped LaFeO3 powders as the catalysts to the reaction. Now observe the change in the reaction after 10 min employing UV-Visible Spectroscopy.

RESULTS AND DISCUSSION
The XRD pattern of the synthesized ferrites was presented in Fig. 1. The XRD patterns of LaFeO3, La0.5Ag0.5FeO3, La0.5Cu0.5FeO3, and La0.5Ag0.25Cu0.25FeO3 revealed that the formation of a single-phase distorted rhombohedral structure in the samples. The patterns of all the four nano ferrites diffraction peaks were matched with the LaFeO3 of JCPDS Card No: 37-1493. Additionally, the La0.5Ag0.5FeO3 nano ferrite showed other peaks related to Ag with miller indices of (111), (220), and (311) planes and matched with the JCPDS Card No: 04-0783 too [16]. La0.5Cu0.5FeO3 nano ferrite showed additional peaks of (210), (112). La0.5Ag0.25Cu0.25FeO3 nano ferrite showed additional peaks of both La0.5Ag0.5FeO3, La0.5Cu0.5FeO3 and this confirmed the presence of doped metals of both Ag and Cu in La0.5Ag0.25Cu0.25FeO3 nano ferrite. These doped ferrite samples had the same symmetry as LaFeO3 with the Pdnm space group. It can be observed from the figure that some peaks of doped ferrite samples become slightly shifted the two theta values in comparison to the LaFeO3 sample. This may be due to the slight variation in the atomic radius of La3+, Ag2+, and Cu2+ ions. The size of the synthesized nano ferrites was calculated from the Debye Scherrer equation and all the nano ferrites were in the range of 13 to 22 nm. Individually the size of LaFeO3 nano ferrites was 22 nm, La0.5Ag0.5FeO3 was 15 nm, La0.5Cu0.5FeO3 was 13.2 nm and La0.5Ag0.25Cu0.25FeO3 was 13.6  nm.  
The surface morphology of the synthesized nano ferrites was carried out by SEM analysis and presented in Fig. 2. From the figure, the synthesized LaFeO3 and the doped LaFeO3 nano ferrites of La0.5Ag0.5FeO3, La0.5Cu0.5FeO3, and La0.5Ag0.25Cu0.25FeO3 were little agglomerated and showed a flake-like structure which may be due to the orthorhombic crystal structure. Furthermore, the EDX spectra of all the LaFeO3 nano ferrites were shown in Fig. 2 (a1) – (d1). From the EDX spectrum, each sample of the synthesized LaFeO3 nano ferrites stoichiometric ratio of the constituent chemical composition (La, Ag, Cu, Fe, O) was confirmed and its elemental composition was presented. It was observed that in all the samples La rich environment was observed.These results are in good agreement with the XRD results. 
Fig. 3 represents the FTIR spectra of all the four LaFeO3 nano ferrites and its wavenumber ranges from 500 - 4000 cm-1. FT-IR spectroscopy was used for the control of the reaction process and material properties. Therefore, FTIR spectroscopy was performed to know the chemical bonding nature of the constituent elements present in the nano ferrites. The FTIR spectra of LaFeO3 showed two bands correspond to the perovskite phase. In that, the vibrational band around 570 cm-1 corresponds to Fe-O stretching vibration (υ1 mode) and another sharp band around 420 cm-1 corresponds to O-Fe-O bending vibrational mode (υ2 mode). By doping the metals Ag, Cu separately and both Ag, Cu into the LaFeO3 these bands were slightly shifted to the higher wavenumber side [17]. The peaks at 1630 cm-1 was attributed to H-O stretching. The existence of sharp peaks in the spectrum indicates the formation of single-phase crystalline compounds. However, the FTIR spectrum of nano ferrites was calcined at 900 0C has been confirmed with the orthorhombic structure of LaFeO3. These results were in good agreement with the XRD results.

Dielectric properties
Variation of dielectric constant(ε’) with temperature and frequency
The dielectric constant (εʹ) and dielectric loss (εʺ) plots were studied over the range of 100 Hz to 5000 Hz. The change in dielectric constant and loss were shown in Fig. 4 and Fig. 5. It is noticed that at lower frequencies all the prepared samples showed higher dielectric constants, particularly LaFeO3 showed the highest value than doped composites. On increasing frequency, the dielectric constant decreases and attains a constant value. This type of dispersion is called Debye type dispersion, which was a common feature in ferrites. This dispersion was closely related to Maxwell Wagner and two-layer Koop’s theory. Due to the effect of space charge polarization the dielectric dispersion occurs. In detail at low frequency, there was the maximum effectiveness of all types of polarizations like interfacial, dipolar, electronic, and ionic which leads to the high dielectric constant value formation. Whereas on increasing the frequency at a certain point the number of dipoles contributing to the net polarization gets decreased. Because of the assembly of space charge carriers, it requires a certain time to line up their axes parallel to an alternating electric field [18, 19]. This leads to the ineffectiveness of some polarization which shows the direct effect on the net polarization value, then decreased the dielectric constant value. According to Koop’s theory [20] at the low-frequency range, the origin of grain boundaries leads to the formation of high resistivity in the sample. Therefore, the value of the dielectric constant increases at this range. In the direction of an applied field, the displacement of an electron occurs on the B site due to the hopping of Fe+2 –Fe+3ions. But at a high-frequency range, the low dielectric constant value was observed. Here the dielectric values were reported from the grains of small value with low resistivity which was typical behavior of ferrites [18].
Another reason for the high dielectric constant is due to the presence of oxygen vacancies, defects, and dislocations of the sample [19]. Around 1500 Hz all the samples almost reached a constant value.  In the prepared samples the LaFeO3 shows high dielectric constant value than the remaining samples. When the LaFeO3 is doped with silver the value decreased. This may be due to the presence of silver ions which may not be effective for dielectric polarization as the silver has conductance. Surprisingly, the dielectric value of copper doped LaFeO3 is also decreased. This may be due to the influence of the magnetic nature of copper on the surface of LaFeO3. The LaFeO3 is doped with both the metals of silver and copper then the value increased but not more than the LaFeO3. In the doped particles, La0.5Ag0.25Cu0.25FeO3 showed a high dielectric constant value. This implies that in the prepared sample LaFeO3 may work as an efficient device for telecommunication than the doped LaFeO3 [21].

Variation of ε” with temperature and frequency
The dielectric loss (ε”) also shows the same trend as dielectric constant. On increasing temperature with frequency, the dielectric loss increases. At low frequencies due to the influence of both charge carriers jump and conduction loss high value of the dielectric loss is observed [22]. On increasing the frequency dielectric loss value decreases up to a certain frequency and then attains almost a constant value.

Magnetic properties 
The room temperature magnetic response of the synthesized nano ferrites is depicted in Fig. 6. The evaluation of magnetization in Ag and Cu doped LaFeO3 was achieved with an applied magnetic field of 75 kOe at room temperature. From the figure it was evident that the hysteresis of La0.5Cu0.5FeO3 nano ferrites showed the highest saturation magnetization, La0.5Ag0.5FeO3 nano ferrites showed the least saturation magnetization among the doped nano ferrites. Whereas the La0.5Ag0.25Cu0.25FeO3 nano ferrites exhibited the saturation magnetization value in between the two individual composites, technically it was nearer to La0.5Cu0.5FeO3. The same phenomenon is observed in the case of remanence but the coercivity of the La0.5Cu0.5FeO3 nanoparticles is high. It was also noticed that the magnetism of bare LaFeO3 is very low and the magnetism of La0.5Ag0.25Cu0.25FeO3 and La0.5Cu0.5FeO3 has increased almost 100 times. This may be attributed to the size and shape of the nano ferrites. According to Brown’s relation, the coercivity is inversely proportional to the magnetic saturation which is obeyed by the samples [23]. Also, it is a well-known fact that the change in the coercivity of the nano ferrites can be explained by grain sizes. As the size is small, the magnetic anisotropy of the hard grains is averaged because of exchange coupling. This coupling strengthens the ferromagnetic component and the presence of oxygen vacancies can also disturb the exchange coupling interactions thus increases the ferromagnetism. From the structural characterization, it was noticed that the doping of copper in lanthanum ferrite, the magnetic core from copper is more on the surface of the particles which may cause the uncompensated surface spins which contribute to the increase in magnetization. On the other hand, the M-H curve of La0.5Ag0.5FeO3 composite exhibited an almost paramagnetic nature due to the presence of electrically conductive silver is present in it which causes a decrease in coercivity and an increase in remnant magnetization.   

Catalytic properties 
The reaction in the Fig. 7 displays the reduction of p-nitroaniline into p-phenylenediamine in the presence of NaBH4 in aqueous solutions and with LaFeO3, La0.5Ag0.5FeO3, La0.5Cu0.5FeO3 and La0.5Ag0.25Cu0.25FeO3 samples.
The formation of p-phenylenediamine in the presence of LaFeO3, La0.5Ag0.5FeO3, La0.5Cu0.5FeO3 and La0.5Ag0.25Cu0.25FeO3 samples were studied by UV-visible spectroscopy (Fig. 8). Initially, a blank reaction was carried out without the developed nano ferrites to understand the influence of NaBH4. It was noticed that even after 60 mins there was no change in the reactant concentration and no new peak observed in the UV Vis spectra. Later the reaction was performed with the addition of synthesized nanoparticles. Since there was no observable change noticed in the reaction up to 10 min, it was taken as the optimum time to study. The UV–visible spectrum of the reaction mixture without nano ferrites (curve i) showed two absorption peaks at 383nm and 226 nm that correspond to p-nitroaniline. While observing Fig. 8 (a) and (b) there was no observable change in the UV–visible spectrum of LaFeO3, La0.5Ag0.5FeO3. But in the case of La0.5Cu0.5FeO3 and La0.5Ag0.25Cu0.25FeO3 samples, there was a notable change in the UV–visible spectrum of the reaction mixture. From Fig. 8 (c) and (d), the reaction mixture containing nano ferrites (curve ii) with a reaction period of 10 minutes showed the decrease in the characteristic peak of p-nitro aniline at 383 nm [24] shift in the absorbance peak at 226 nm and appearance of a new peak at 305 nm. This result indicated that the La0.5Cu0.5FeO3and La0.5Ag0.25Cu0.25FeO3 nanomaterials accelerated the reduction reaction of p-nitroaniline with NaBH4 in aqueous solutions by the formation of p-phenylenediamine. The percentage conversion of p-nitroaniline to p-phenylenediamine is 86% in the presence of La0.5Cu0.5FeO3 and 100 % in the presence of La0.5Ag0.25Cu0.25FeO3. This result suggests that as prepared La0.5Ag0.25Cu0.25FeO3 sample showed better catalytic properties than the La0.5Cu0.5FeO3 sample. However further studies are required to understand the kinetics of the reaction. 

CONCLUSIONS
Cu and Ag doped nano size lanthanum ferrites were synthesized effectively by using the sol-gel method. The XRD pattern confirmed orthorhombic crystal structure and size of all the synthesized nano ferrites were in the range of 13 to 22 nm. There was a good correlation between the XRD, SEM/EDX and FT-IR data. The prepared nano ferrites showed good results of magnetic, dieelectric properties which can be useful in the application of telecommunication devices, optical materials. Furthermore, these developed nano ferrites were studied their catalytic reduction of p-nitroaniline and among the synthesized nanoferrites La0.5Ag0.25Cu0.25FeO3 exhibited prominent catalytic activity to reduce p-nitroaniline into p-phenylenediamine. However, further studies are required to explore the samples.

ACKNOWLEDGMENTS
One of the authors (Lalitha Ammadu Kolahalam) is thankful to Koneru Lakshmaiah Education Foundation, Vaddeswaram (A.P) India, for providing necessary laboratory facilities and financial support to carry out this work.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.