Barium and strontium doped La-based perovskite synthesis via sol-gel route and photocatalytic activity evaluation for methylene blue

In the current work, a La-based nanocomposite (La x Ba 1 (cid:0) x Sr y Cd 1 (cid:0) y O 3 ± δ ) has been synthesized via sol-gel method. The influence of Ba and Sr doping on the structural, dielectric and photocatalytic characteristics of La-based perovskite nanocomposite has been investigated. The synthesized material was characterized employing different techniques such as FTIR, XRD, which confirmed the formation of nanocomposite. Representative peaks of metal oxygen bonds (M – O), in FTIR spectra confirmed the formation of the product. Orthorhombic crystalline structures having particle size 89.92 nm was confirmed by XRD analysis. Dielectric studies were conducted for the synthesized nanocomposite. The real and imaginary dielectric constant exhibited conservation and consumption of energy and AC conductivity analysis elaborated the effect of frequency on conductivity of the synthesized sample. Doping of nanomaterial with Ba and Sr caused inhomogeneity in the structure that caused reduction in number of moving charges due to which dielectric constant was found maximum at lower frequency. Enhanced dielectric properties caused a decrement in energy losses with increasing frequency. The results of AC conductivity were found directly related to frequency that may be due to enhanced charge storage capacity of perovskite after doping. In addition, synthesized nanoparticles exhibited excellent photocatalytic activity for the degradation of methylene blue (5 ppm). Under optimized conditions of pH, catalyst dose and UV light, 89.92% degradation of dye was achieved in 65 min. It is concluded that the synthesized nanomaterial i.e. Ba and Sr doped Lanthanum Perovskites can be a potent candidate for photocatalytic applications.


Introduction
Increase of environmental pollution at massive level due to effluents containing toxic pollutants, discharged by different industries, may pose multiple hazardous health effects [1,2].Among these pollutants, organic compounds such as azo dyes (methylene blue and methylene orange) have now become very crucial due to their deleterious effects on environment and human health [3].Aromatic azo dyes constitute half of total amount of the dyes produced and used in world.These dyes are being used in different industries and 15% of the used dyes are being discharged in waterbodies [4].Azo dyes are potential carcinogenic [5], allergic to eyes and skin and may also cause respiratory problems [6].These contaminants are being generally removed employing different methods such as biodegradation, chlorination, flocculation, ozonation, photochemical oxidation and researchers are still focusing in developing environment friendly and economical methods for the removal of dyes [7].Photocatalytic degradation methodology is an environment friendly method that produces less harmful products.This technique relies on the interaction of radiation with a surface of nanocatalyst and oxidative reagents are formed as a result degradation of many contaminants take place [8,9].Recently, photocatalytic activity of perovskite-type oxides has gotten appreciable attention due to their economical, have high catalytic ability, and environmental adaptability [10].
Huge consumption of non-renewable energy resources has become a serious threat for the environment, being against the sustainable development goals set by united nation.There is a dire need to develop low cost, efficient, less hazardous materials for energy storage devices [11].Supercapacitors have received much attention of researchers due to the advantages including long life-cycle, high power density, fast charge and discharge speed.Low energy density is a problem that has restricted their commercial application.Thus, development of materials with improved energy density of supercapacitor has become an important issue [12].Electrode material is the basic component that responsible for electrochemical performance of a supercapacitor.Researchers are now focusing on the development of high performance electrode materials and perovskites found to be a promising candidate [13].
General formula of perovskites is ABX 3 , where A or B site carry metal ions.Substitution of selected metal at position A or B has been reported and such substitutions can strongly influence the properties of perovskites [14].Modifications in structural, catalytic, electrochemical activities of perovskite materials have been achieved by introducing different metals at position A and B [15,16].In the light of previous reports, it is observed that the doping is a useful tool to enhanced the activity of the doped material, which is because of reduction in the bandgap as well as the optical and physicochemical properties of the doped materials [17].The Mn and Co-based perovskites showed promising photocatalytic activity for oxidation reaction [18][19][20].Mostly Lanthanum based perovskites such as LaMnO 3 have been reported for their promising photocatalytic applications.Ghiasi, M. et al. ( 2014) reported synthesis of LaSrMnO 3 [21] and Esmaeili et al. (2020) reported the formation of LaBaMnO 3 for the degradation of methyl orange in UV and sunlight [22].Advancements in designing and synthesis of novel nanocomposites especially perovskite are being reported.Scientists are making efforts to synthesize nanocomposite with appropriate composition with maximum photocatalytic and charge storage potential.Still, there is need to synthesize perovskite based nanomaterials with novel composition.
The La-based perovskites nanomaterials can be synthesized by different methods such as hydrothermal [23], reverse-phase hydrolysis [24], co-precipitation [25], microwave assisted methods [26] and solidstate reaction [27].Sol-gel is one of the easiest methods that can be used for the manufacturing of high quality nanoparticles [28].This method is easy, cost effective and helpful in inducing certain properties such as control over size, morphology and many electrochemical properties [29].In this paper, we have reported the synthesis of Ba and Sr doped La-based perovskite nanocomposite LaBaSrCdO 3 (La x Ba 1− x Sr y Cd 1− y O 3 ±δ ) by sol-gel method.Photocatalytic degradation of methylene blue using synthesized nanocatalyst has also been achieved under UV light employing different conditions of pH and catalyst dose.In addition, dielectric studies have been carried out to determine its potential for supercapacitor applications.Current study will be helpful in designing and synthesis of perovskite type nanomaterials with enhaced properteis.

Experimental
, ammonium hydroxide (NH 4 OH) % w/w sodium hydroxide (NaOH), hydrochloric acid (HCl), ethanol, Methylene blue and distilled water were used and all the chemical reagents are of analytical grade and purchased from Sigma Aldrich and were consumed without any further purification.

Synthesis of LaBaSrCdO 3
Synthesis of Ba and Sr doped La-based perovskite nanocomposite was done following an already reported sol-gel method (Fig. 1) after some modifications [30].
The beaker filled with 50 mL distilled water and was placed on hot plate having magnetic stirrer in it.Solutions (0.05 M) of lanthanum nitrate (La(NO 3 ) 3 ⋅6H 2 O), barium chloride (BaCl 2 ⋅2H 2 O), strontium chloride (SrCl 2 ⋅6H 2 O) and cadmium nitrate (Cd(NO 3 ) 2 were added to beaker.Then whole solution was stirred for 8-10 min and then 0.125 M of citric acid and NH 4 OH solution (10 % w/w) was poured drop wise to maintain pH 10. Temperature of the resulting mixture was allowed to increase up to 180 • C. Then xerogel was obtained after about 1 h under continuously heating and stirring accompanied by dried in oven at • C for 2 h.The dried, powdered sample was washed with distilled water twice in centrifuge machine and 1 time with ethanol.Then sample was taken in china dish and again dried for 2 h at 100 • C. The synthesized sample was calcined in muffle furnace at 300 • C for 3 h to remove volatile substances and impurities.

Characterization
The functionalities of the prepared materials were monitored using FTIR analysis, which was performed in 4000-400 cm − 1 range at room temperature and KBr pellets (IR-TRACER-100) [31].Synthesized nanocomposites were analyzed using XRD (Bruker D8 advance X-ray Diffractometer) with CuKα (0.154 nm) as a radiation source in 2θ range of 20 • -60 • .Automatic JCPDS library was used identification purposes [32].

Dielectric studies
The prepared materials dielectric permittivity and electrical resistivity was appraised using Keysight impedance analyzer (E4990A) in 100 Hz-5 MHz and 20-200 • C range.For electrical measurements, the pallets of ~10 mm and ~2 mm by applying 5000 kg/cm2 uniaxial pressure by hydraulic press, which were sintered for 6 h at 1300 • C and cooled down 25 • C. The pellets are coated with silver paste and kept at for 30 min at 500 • C and then, subjected to dielectric studies [33,34].

Photocatalytic degradation
Dye degradation studies under UV light were conducted following already reported methods [8,35].Solutions of Methylene blue (0.05 mM), NaBH 4 (20 mM) and nanocomposite (10, 15, 20 mg/L) were prepared.In a test tube, 3 mL methylene blue (substrate), 0.3 mL NaBH 4 (reducing agent) were mixed.In this resulting solution, 0.5 mL of nanocomposite solution (catalyst) was added.All the observations were recorded at 665 nm (λ max MB).Effect of catalyst dose on dye degradation was determined using different concentration (10 mg, 15 mg and 20 mg) solutions of nanocatalyst.Percentage degradation of MB was calculated and pH optimization was carried out by adjusting the pH of the reaction mixture at 3, 6, and 9.

FT-IR analysis
Vibrational band analysis of synthesized perovskites sample was done via FT-IR as shown in Fig. 2. The characteristics peaks observe in FT-IR spectrum at 510-850 cm − 1 , due to stretching vibrations of M-O bond.Therefore, it has been confirmed that prepared sample contain metal oxygen linkages (La-O, Ba-O, Cd-O and Sr-O).Moreover, it is also noted that the bending and stretching mode of the OH bond of water has been showed in the FT-IR spectrum around 1500 and 3485 cm − 1 , respectively.Thus, FT-IR analysis has confirmed the presence of metal oxygen linkage formed in sample [36].

XRD studies
The XRD spectrum of the synthesized perovskites was recorded using Bruker D8 advanced diffractometer of Cu-Kα (0.154 nm) having the range of 2θ = 20-60 • to calculate the average crystallite basal planes (Fig. 3) [37].The spectrum indicates that diffraction peaks at 2θ = 27.9 • , 35.8 • , 39.5 • , 44.3 • , 48.7, 55.4 • and 58.3 • correspond to the planes (1 1 0), (2 0 0), (1 1 1), (2 1 1), (2 2 0), (2 1 0), and (0 1 3) which confirmed the formation of perovskite and also verify its orthorhombic structure and all the peaks were found perfectly match with JCPDS-00-15-1691.Diffraction peak appeared at 27.9 has confirmed the structure of perovskites.The diffraction peak at 35.8 • has confirmed the presence of Sr in sample.The substitution of Ba decreases the crystallinity and the main reflection of the (1 1 0) plane would have been moved toward lower angle.This is because when larger Ba 2+ (radii 0.135 nm) substitute smaller La 3+ (0.106 nm) in the cubic perovskite structure and thus help in increasing volume of the unit cell [38].Thus, with the help of XRD analysis, structure of sample and orientation of basal planes has been explained.
The average crystallites size of synthesized perovskites has been measured by Scherrer's formula; D = Kλ/βcosθ, Where D is particle size in nanometer, K is Scherrer's constant whose value is 0.9, λ is X ray wavelength (1.541), θ = Half of diffraction angle and β is Full width half maxima (FWHM) [39].Thus, average crystallites size of prepared sample has calculated to be 89.21nm using Scherrer's formula.

Dielectric studies
The dielectric property of the synthesize perovskite sample was calculated.Alterations in real part of dielectric constant Ɛ' with respect to frequency has been studied [40,41].Dielectric constant was measured using the given formula: ε ′ = Cd/Aε0, where C donates the capacitance of the material, A shows an area of pallets, width of pallets is represented by d and ε o is constant which is free space permittivity.The effect of frequency on dielectric constant is shown in Fig. 4a and it reveals that dielectric constant decreases with increasing frequency.The substitution of Ba and Sr introduced inhomogeneity in the structure of perovskite and also reduced the number of moving charges resulting in maximum dielectric constant value at lower frequency.The results of current study were found in agreement with already reported studies [42,43].
The imaginary part of dielectric constant i.e.Ɛ ′′ of synthesized perovskite has been presented in Fig. 4b.The capacitance of the perovskite is elaborated by ε′ and ε ′′ which gives information regarding dissipation of energy.Imaginary part of dielectric constant i.e. ε ′′ depends on tangent losses and it is calculated using given formula: ε ′′ = ε ′ × tanδ, where real part of dielectric constant is expressed by the ε′.It is explained that dielectric losses are inversely related to frequency and dielectric losses express the amount of dissipated energy.In current study results confirmed that substitution has increased dielectric  properties that reduced the losses with an increase in frequency.Researchers are designing different material to reduce dielectric losses in terms of dissipated energy.Current study, doping of perovskite favored reduction in losses with respect of increase in frequency and the same already reported in different studies [44,45].

Tangent loss
Tangent loss can be evaluated by using formula tanδ = 1/2πfRC, where f represents applied frequency, resistance of the material is represented by R and C shows the capacitance of the material.The results of tangential loss (Fig. 5) were found inversely related to f and become constant at highest frequency.This process can be elaborated by Koopmans' theory that may help in elucidating the structure of dielectric sample.A net polarization occurred at lower frequency because charges easily align themselves in direction of applied electric field.But at higher frequency tangent loss decrease due to low polarization [46] and it is singn that favors the doping of Ba and Sr in La-based perovskite.

AC conductivity
AC conductivity of the synthesized perovskite sample is calculated by the given equation.ρ ac = 1/2πf∈ ′′ ε 0 where 'f' represents frequency, 'ε o ' shows permittivity of free space and dielectric loss constant is represented by 'ε ′′ '.The results revealed (Fig. 6) that the conductivity of the material is directly related to frequency.This is because substitution of Sr and Ba enhance the charge storage capacity of perovskite due to which its conductivity increased.Results are comparable with the already reported many studies that is confirmed positive impact of doping on perovskite structure and properties [47,48].

Photocatalytic degradation potential
Photocatalytic potential of synthesized nanocomposite was evaluated in terms of dye degradation potential [49,50].For the purpose, photocatalytic degradation of methylene blue (MB) was achieved.Degradation of dye was carried out using different concentration of perovskite sample (10 mg, 15 mg and 20 mg).Significant effect of catalyst dose was recorded in degradation of dyes as shown in Fig. 7a.It was observed that 20 mg catalyst exhibited maximum 47% degradation of dye (MB) while 10 mg and 15 mg degrade dye by 45.29% and 46.47%.Therefore, it was confirmed that percentage degradation increase with increase in amount of catalyst [51].In current study La-based pervoskite were doped with two different metals i.e.Ba and Sr. Work function values for Ba and Sr are 2.5 eV and 2.1 eV respectively and this  difference in work function will allow the movment of electron from lower work function metal to high work function metal.Doping with metals having different work functions will serve as Schottky barrier that will result in movement of electrons necessary for photocatalyti degradation [52].Carrier diffusion length is another parameter that determine the efficiency of perovskite material.Keeping in veiw the results of photocatalytic degradation, synthesized pervoskite material is expected to have suitable career diffusion length suitable and required for catalytic activity [53].Effect of pH on photocatalytic degradation of MB was studied and change in pH affected the rate of dye degradation.Maximum photocatalytic degradation was observed in basic pH and lesser in acidic medium as shown in Fig. 7b.It is the charge on catalyst that vary due to change in pH and ultimately the degradation of dye [50,54].The maximum percentage degradation up to 89.92 % was observed at pH 9 and least (29.41%) was observed at pH 3. Maximum degradation has occurred in basic medium because of greater OH radicals' generation that convert methylene blue into less harmful product [55].

Conclusion
Doping is a strategy that is helpful to modify the properties of any material according to need.In current study, Ba and Sr doped La-based perovskites nanocomposite (LaBaSrCdO 3 ) were fabricated by sol-gel method.Photocatalytic and dielectric properties were demonstrated as a function of doping.Dielectric studies including real/ imaginary dielectric constant, tangent loss and AC conductivity of the synthesis material was determined.Photocatalytic potential of the nancomposite was tested against MB dye and degradation of the dye was achieved under different conditions of catalyst dose and pH.All the results revealed that doping can enhance the charge storage as well as photocatalytic potential of the nanocomposite.Authors strongly recommend that La x Ba 1− x Sr y Cd 1− y O 3±δ like nanocomposites should be designed and doping strategy should be adopted to modify the properties of the materials according to the requirement of specific application.In future, different metals, instead of Ba and Sr, can be used for doping to achieve desired characteristics.

Fig. 4 .Fig. 5 .
Fig. 4. Effect of frequency on Real dielectric constant (a) and dielectric loss constant (b) of Ba and Sr doped La-based perovskite nanocomposite.

Fig. 6 .
Fig. 6.Effect of frequency on AC conductivity of Ba and Sr doped La-based perovskite nanocomposite.

Fig. 7 .
Fig. 7. Effect of catalyst dose (a) and pH (b) on dye degradation potential of Ba and Sr doped La-based perovskite nanocomposite.