LaCoxFe1-XO3 (0≤x≤1) spherical nanostructures prepared via ultrasonic approach as photocatalysts.

To harvest the photon energy, a sequenceof perovskite-type oxides of LaCoxFe1-xO3 (0 ≤x≤1) nanostructures with distinct 'Cobalt' doping at the position of B-site are successfully prepared via a simple ultrasonic approach as photocatalyst. The crystallinity, phase identification, microstructure, and morphology of perovskite nanocomposites were analyzed to better understand their physicochemical properties. The catalytic efficiency was assessedusing Congo Red (CR) dye by visible light irradiation for 30 min. Applying terephthalic acid as a probe molecule, the formation of hydroxyl radicals during the processes was investigated. The photocatalytic efficacy was measured by varying different Co/Fe stoichiometric molar ratios and noticed the order of sequence is 0.2 > 0.6 > 0.4 > 0.8 > 0.5 > 0 > 1 after 30 min of reaction time. Finally using LaCo0.2Fe0.8O3 nanostructures, cycling studies (n = 3) were performed to determine its photostability and reusability. The photocatalytic methodology proposed in this study was discussed extensively.

Introduction

Perovskite oxides with general chemical formula ABO3, have more potential towards transducers, [1], [2], [3], [4] magneto resistance devices, [5], [6] and in catalytic reactions because of its chemical stability, crystal structure, electrical, and piezoelectric capabilities. [7], [8], [9] Furthermore, the perovskite having corner-shared BO6 octahedral coordination network, which enables electron transfer, [10], [11] and greatly improved partial replacement on A and/or B-sites cations are attributed to better photocatalytic activity. [12], [13], [14] In this regard, LaFeO3 perovskite has received great attention with extraordinary performance, towards inventive applications, including solid oxide fuel cells, [15], [16] sensors, [17], [18] magnetic materials, [19] and environmental remediation. [20], [21], [22], [23] This is due to the adequate bandgap (∼2.1 eV) of LaFeO3, which plays a major role in catalytic activity and has received much attention for its applicability in the solar energy sensitive characteristic. [24], [25], [26] The iron mixed-oxide perovskite (LaFeO3) doped with cobalt in B-site (for ex. LaCoxFe1-xO3) regulates the electronic structure, bandgap, and oxygen vacancy properties, to realize the versatile characteristics and enhanced photocatalytic performance. [27], [28] LaCoxFe1-xO3 is shown to be suitable for a variety of applications, including water purification, methane oxidation, sodium-ion batteries, and catalyst for hydrogen generation by water splitting and degradation of dye pollutants. [29], [30], [31] Azo (-N = N-) dyes (Congo Red, Acid Orange, Methyl red, Tartrazine, Sudan red, etc.) are the most frequently used dyes in textiles and certain other associated industries such as paper, plastic, pharmaceutical, leather, and so on. [10], [32], [33], [34], [35] Congo red (CR) is a sodium salt of the benzidinediazo-bis-1-naphthylamine-4-sulfonic acid [36] is chosen for this study due to its unusual characteristics like hydrophilic, non-biodegradable, anionic nature, and severe environmental concerns. [37], [38] Such effluent combines directly with rivers and other nearby water bodies, causing water pollution. The CR dye effluent is highly carcinogenic, it poses a serious threat to aquatic animals and human life. [39] Several physicochemical approaches such as biodegradation, [40] microbial/enzymatic treatment, [41] adsorption, [42] coagulation [43], and flocculation are employed to eliminate CR pollutants. These processes produce huge volumes of sludge, which causes disposal issues. Unfortunately, the conventional techniques described above are ineffective in removing azo dyes. [44], [45] Hence, in recent decades, many researchers have extensively focused on advanced oxidation processes (AOPs) as a significant technique for effective decomposition of dangerous toxic chemicals contained in wastewater into less hazardous CO2 and water, while also being a less expensive and low-temperature process. [46], [47] Semiconductor photocatalysts absorbed sunlight, enable to generate hole (h+) and electron (e-) pairs, which initiate oxidation reactions with strong reactive oxidizing radicals (e.g., OH•) generated in these reactions for conversion of organic contaminants to innocuous products. [48], [49], [50], [51], [52], [53] The earlier reported photocatalytic approaches with metal oxides have some drawbacks, which could be overcome by developing a novel photocatalyst for efficient CR dye degradation under visible photon irradiation.

In this context, following a simple ultrasonic technique to synthesize LaCoxFe1-xO3 perovskite nanomaterials and investigate their catalytic performance in the presence of visible photons using CR dye as a pollutant. Furthermore, the benefits and drawbacks of the current approach are thoroughly discussed.

Experimental section

Materials and equipment used

All precursor chemicals utilized in the present study were of analytical reagent grade and were utilized as purchased without further additional purification. Lanthanum nitrate hexahydrate [La(NO3)3·6H2O, 99.9 %, CAS no. 10277–43-7], was procured from Alfa Aesar, India. Cobalt nitrate hexahydrate [Co(NO3)3·6H2O, ≥98%, CAS no. 10026–22-9], and Potassium hydroxide [KOH, ≥85%, CAS no. 1310–58-3] purchased from Sigma-Aldrich, India. Iron (III) nitrate nonahydrate [Fe (NO3)3·9H2O, ≥98%, CAS no. 7782–61-8] was obtained from Merck Life, India. Congo red (CR) [C22H22N6Na2O6S2, product no. 22120, CAS no. 573–58-0], was received from Loba Chemie Pvt. Ltd., Mumbai, India. De-ionized (D.I.) water was utilized for the preparation of an aqueous solution throughout the work. Fabricated a photocatalytic chamber (The Revathi Enterprises, Chennai, Tamil Nadu, India) of dimensions (W × H × L = 47x 30 × 32 cm3) with cooling fans to maintain a steady temperature within the chamber throughout the experiment.

LaCoxFe1-xO3 perovskites synthesis

The catalyst with the formula LaCoxFe1-xO3 (with x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0) was synthesized by a single step ultrasonication process in a typical experiment. Initially, a stoichiometric amount of (0.05 mol) La(NO3)3·6H2O (0.05 mol; 1.082 g) and Fe(NO3)3·9H2O (0.05 mol, 1.01 g) was added to 100 mL of distilled water, stirring constantly, and then adding an aqueous KOH solution to the precursor mixed solution until pH value of 12 was attained, stirring and dissolving till the clear solution is obtained. Employing, a direct immersion of titanium horn into 100 mL of the precursor solution was irradiated with highly intensive ultrasonic radiation for 2 h (Sonics, 750 W, 20 kHz,). A homogenous precipitate was produced, after washing with double distilled water followed by acetone, and dried at 80 °C in a vacuum oven overnight, which was then sintered at 800 °C for 6 hr in a tubular furnace to generate the perovskite-type LaFeO3 nanostructures. According to the above synthesis techniques, upon altering the amount of Fe(NO3)3·9H2O and Co(NO3)3·6H2O, a sequence of many Cobalt doped LaFeO3 (LaCo0.2Fe0.8O3, LaCo0.4Fe0.6O3, LaCo0.5Fe0.5O3, LaCo0.6Fe0.4O3, LaCo0.8Fe0.2O3, and LaCoO3) perovskites were obtained. An ultrasonic preparation reaction mechanism is provided in supporting information.

Characterization analysis details

The wide-angle Rigaku D/max-2500 X-ray diffractometer with a Cu-Kα radiation (λ = 1.54 A0) source with 40 kV accelerating voltage and an intensity of 200 mA with Ni filtered was utilized to investigate the phase identification and crystalline structure of the LaCoxFe1-xO3 samples. The spectra were taken at a 2°/min scanning speed in the 2-theta degree range from 10° to 90°. Field emission scanning electron microscopy (FESEM model JEOL, JSM-6700F), high-resolution transmission electron microscopy (HR-TEM model FEI Tecnai G2 F20 microscope piloted at 300 kV), and selected area electron diffraction (SAED) were used to examine the surface morphology and microstructural properties of the synthesized LaCoxFe1-xO3 samples. An energy-dispersive analyzed X-ray spectrometer (EDAX, Oxford Inc coupled to FESEM) and elemental color mapping was done to identify the elemental composition. Using Physical Electronics PHI 5600 X-ray photoelectron spectroscopy (XPS) quantification study was performed on the chemical states of the elements survey with monochromatic Al Kα (photon energy − 1361 eV) excitation source. The optical properties of the as-synthesized material were explored using a UV–vis spectrophotometer (T90 + double beam, PG Instruments, UK) with BaSO4 as reference material and the photoluminescence spectra of the synthesized samples were obtained by a Shimadzu (RF5301PC) spectrofluorometer.

Photocatalytic experimental setup for Congo red (CR) dye degradation

The degradation of CR dye (λmax≈ 498 nm) was conducted under visible light irradiation at natural pH (8.5) and under the ambient condition to assess the catalytic activity of the synthesized LaCoxFe1-xO3 perovskite samples. The photoreactor consists of three Osram Low-voltage halogen lamps without reflector (400 nm; 24Volts/150 Watts, Nominal luminous flux − 5000 lm, radiation intensity − 80,600 lx,) that emit light which is equivalent to natural visible source radiation. In a photoreactor, 100 mL of 3 x10-5 mol/L of CR dye solution were taken in a 125 mL borosilicate glass bottle. Around 20 mg of catalyst was added and a chemically inert magnetic stir bar covered with polytetrafluoroethylene (PTFE) was inserted in the aforementioned solution to produce stirring action. The solution was kept for 10 min in the dark to achieve adsorption and desorption equilibrium between catalyst surface and dye molecules. Following this procedure, to reach photocatalytic degradation, the suspension was subjected to light illumination for 30 min. At regular sampling intervals, 4 mL of the aqueous suspension from the reactor was sampled and filtered through a 0.45 μm pore size PVDF membrane syringe filter to remove the photocatalyst particles, owing supernatant solution. Following that, a UV–vis spectrophotometer (T90 Double beam UV–Vis spectrophotometer) was utilized to evaluate the concentration of a supernatant solution by measuring the absorption at its characteristic wavelength (200 nm–800 nm) using 1 cm optical route quartz cuvettes with a 4 mL volume. Calculating the changes in the absorbance of the reaction solution revealed the degradation efficacy [η, %] of CR dye using equation (1).

where, A0 and At are the concentration and absorbance of the CR dye solutions before degradation and after degradation treatment, respectively. Moreover, the rate constant (k) of photocatalytic performance was obtained by utilizing the first-order rate equation (2).

Analysis of hydroxyl (OH●) radical generation

A simple photoluminescence (PL) approach was used to analyze hydroxyl (OH●) radicals. 20 mg of the photocatalyst was introduced to the 100 mL of 2 x10-3 mol/L terephthalic acid in an alkaline solution (Conc. 1 × 10-3 mol/L NaOH). At sampling intervals, 4 mL of aliquots were procured and the catalyst was filtered using a PVDF membrane syringe filter (0.45 μm). The PL analysis was then performed with an excitation wavelength at 315 nm.

Results and discussion

Several characterization techniques were utilized to validate the prepared perovskite-type LaCoxFe1-xO3 nanoparticles. The XRD studies were utilized to certify the phase and crystalline structure of the as-prepared nanomaterials. The XRD pattern (range 10°-90°) of as-prepared LaCoxFe1-xO3 (0 perovskite-type oxides sintered at 800 °C is shown in Fig. 1. The sharpness of the diffraction patterns reveals the prepared samples are highly crystalline, and all of the XRD peaks were so well correlated with the standard JCPDS No.37–1493 data of LaFeO3, and JCPDS No. 48–0123 data of LaCoO3 indicating the sample's purity. [54], [55] The diffraction patterns at 2θ = 22.60°, 32.19°, 39.67°, 46.14°, 51.99°, 57.39°, 67.34° and 76.64° are indexed to (1 0 1), (1 2 1), (2 2 0), (2 0 2), (1 4 1), (2 4 0), (2 4 2), and (2 0 4) dhkl planes of orthorhombic phase with Pn*a space group (a = 5.56 Å, b = 7.85 Å, c = 5.53 Å and α = β = γ = 90) of pristine LaFeO3 (JCPDS No.37–1493). Whereas the diffraction peaks at 2θ = 23.22°, 32.88°, 41.33°, 47.49°, 53.79°, 58.95°, 68.92° and 79.43°, are indexed to (0 1 2), (1 1 0), (0 0 6), (0 2 4), (1 1 6), (2 1 4), (2 2 0), and (1 2 8) dhkl planes of pristine LaCoO3 respectively. [56], [57] The detected XRD peaks for LaCoO3 correspond to a perovskite phase with rhombohedral structure (a = b = 5.44 Å, c = 13.09 Å, and α = β = 90, γ = 120) and R-3c space group with a lattice constant as per standard JCPDS Card No: 48–0123. No additional diffraction patterns indicative of La, Fe, Co, or their oxides were identified in the diffraction pattern, and all peaks may be strongly correlated to that of the perovskite crystalline phase. The two theta values of the prepared LaCoxFe1-xO3 oxides shifted to higher angles (right side) as the Co doping concentration increased, implying that Co doping causes lattice contraction and structural distortion because of the lesser Co3+ ionic radius than Fe3+ ions and/or the stronger Fe-O bonding than Co-O interaction. [6], [58] Further, the XRD patterns of as-synthesized LaCo0.2Fe0.8O3 and LaCo0.4Fe0.6O3 were well matched with earlier published reference articles and JCPDS card nos. (88–0641 and 40–0224), respectively [59], [60].

Fig. 1

XRD patterns of as-synthesized LaCoxFe1-xO3 samples.

Fig. 2 (a-g) depicts the morphologic features of the as-synthesized LaCoxFe1-xO3 (0 samples. The FE-SEM images of as-synthesized LaCoxFe1-xO3 at low magnification show an irregular granular structure that is more or less equally distributed and contains a substantial number of micro or nanoparticles. When these micrographs are compared to those of pristine LaFeO3, subtle changes in particle morphogenesis emerge. Fig. 2 (b–d) showed progressively rounded and evenly distributed particles as the amount of cobalt doping was increased (0.2, 0.4, and 0.5). Whereas cobalt doping is increased to 0.6, 0.8, and 1, aggregation among the particles is increased which is readily seen in Fig. 2. (e-g), indicating that catalytic activity will be reduced due to bigger size particles. Thus, the results revealed that molar stoichiometric ratios of 0.2, 0.4, and 0.6 for ‘Co’ to ‘Fe’ provide the best morphology for better catalytic efficacy. The TEM, HR-TEM, and SAED pictures are provided in Fig. 3 (a-d) results in more information on the morphology of LaCo0.2Fe0.8O3. That is smaller nanoparticles with an average particle size (90 5 nm) which leads to largely agglomerated nanospheres, as seen in Fig. 3 (a-b). These findings were in accordance with those of the SEM results. In addition, the HR-TEM picture in Fig. 3c reveals that the well crystalline LaCo0.2Fe0.8O3 nanoparticles have distinct lattice spacing (d values). The interplanar (1 2 1) spacing of nanoparticles was evaluated to be 0.28 nm, which correlates to the orthorhombic phase. The polycrystalline system and the lattice hkl planes of LaCo0.2Fe0.8O3 assigned by XRD analysis are congruent with the few notable diffraction peaks indexed in related SAED patterns as shown in Fig. 3d.

Fig. 2

Typical SEM images of as-synthesized (a) LaFeO3, (b) LaCo0.2Fe0.8O3, (c) LaCo0.4Fe0.6O3 (d) LaCo0.5Fe0.5O3, (e) LaCo0.6Fe0.4O3, (f) LaCo0.8Fe0.2O3 and (g) LaCoO3 samples.

Fig. 3

(a and b) TEM and (c) HR-TEM images and (d) SAED patterns inset of as-synthesized LaCo0.2Fe0.8O3 perovskite sample.

Additional information was gathered from SEM-EDS analysis and mapping, as shown in Fig. 4 (a-f), to authenticate the elemental composition of cobalt in LaCo2Fe0.8O3. Finally, the spectra validate the chemical elements that correspond to catalyst compounds, with no additional elements found. Fig. 4 (b-f) shows that the SEM-EDS color mapping performed in a selected section of LaCo0.2Fe0.8O3 revealed a uniform distribution of La, Fe, Co, and O elements over the nanocomposites, as well as homogeneity. The SEM-EDS spectra of other LaCoxFe1-xO3 (x = 0, 0.4, 0.5, 0.6, and 1) are provided in Supporting Information (Fig. S1).

Fig. 4

(a) EDS spectrum, (b) elemental color mapping of LaCo0.2Fe0.8O3, (c) La, (d) O, (e) Fe, and (f) Co elements.

The surface species and chemical valence state of the as-prepared LaCo0.2Fe0.8O3 perovskite sample were investigated using a high-resolution XPS and the results are presented in Fig. 5 (a-e). The survey spectrum shown in Fig. 5a affirmed purity of the LaCo0.2Fe0.8O3 with the presence of La, Fe, Co and O only and no other corresponding elements identified. The survey results are well matched with SEM-EDS analysis and the C 1 s peak at 288.5 eV is assigned as a reference peak. Four peaks 835.4 eV- 838.1 eV and 852.3 eV- 854.9 eV were signifying La 3d5/2 and La 3d3/2, respectively (Fig. 5b). [61], [62] The observed XPS spectra results of La 3d5/2 at 835.4 eV and La 3d3/2 at 852.3 eV are closely matched with the standard XPS values of La 3d5/2 at 836.0 eV and La 3d3/2 at 853.0 eV. The 16.9 eV peak gap between La 3d5/2 and La 3d3/2 implies the presence of La3+ valence state in perovskite form. [63]

Fig. 5

XPS spectra of LaCo0.2Fe0.8O3 perovskite sample, (a) survey spectra, (b) La, (c) Fe, (d) Co and (e) O elements in composite.

The Fe 2p peak presented in Fig. 5c can be split into two peaks, Fe 2p3/2 and Fe 2p1/2, with binding energy values of 711.1 eV and 724.9 eV, respectively, corresponding to the Fe3+ ion in its oxide structure. [64] The perovskite's Co 2p spectrum shown in Fig. 5d was fitted by two prominent peaks at 781.1 eV, which is comparable to Co2O3 at 779.2 eV, and 797.8 eV, which were assigned to the existence of Co3+ with the basic satellite peak at 787.2 eV. [65] Fig. 5e represents three peaks at 528.9, 530.9 eV, and 533.2 eV that have indexed the lattice, adsorbed, and hydroxyl oxygen, respectively. That 530.9 eV peak was linked to asymmetric and wide O 1 s spectra, confirming the existence of an oxygen (O2−) network in the LaCo0.2Fe0.8O3 perovskite lattice. [66] The above valuable information from the XPS surveys, confirms the chemical framework of LaCo0.2Fe0.8O3 perovskite.

The optical absorption and energy band characteristics of LaCoxFe1-xO3 nanoparticles may have a significant impact on their catalytic performance. Herein, Fig. 6a displays the diffused reflectance spectra of LaCoxFe1-xO3 (0 perovskites produced at various Cobalt doping stoichiometric ratios. The optical absorption spectra of the samples result from the entire absorption edges that extend beyond 580 nm, indicating that they can respond well in the visible range. As a result, the materials' characteristics can be investigated utilizing visible photon-driven methodologies (photocatalysis). Using the equation (3), evaluated the optical energy bandgap (Eg) of the prepared samples:

Fig. 6

(a) Absorbance spectra and (b) Tauc plot of synthesized LaCoxFe1-xO3 perovskite samples.

where the absorption coefficient is referred to as 'α', photon energy is represented by 'h', ν is the frequency of the stimulated photon, C is the constant, and Egbulk is the sample's bandgap energy. The m value is based on the kind of transitions in semiconductors. According to theoretical results, for direct transition (m = 2) and indirect transition (m = 1/2). Plotting (αhν) against the light energy (hν) and extending the plotline, as shown in Fig. 6b, provides the semiconductor's energy bandgap.

The bandgap energies obtained for the LaCoxFe1-xO3 materials (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1) which were evaluated to be 1.89, 1.83, 1.94, 2.15, 2.08, 1.98 and 2.31 eV, respectively. The sample with Cobalt doping, (x = 0.2) had the lowest bandgap (1.83 eV), whereas the sample with x = 1 had the highest energy bandgap (2.31 eV). These results support future photocatalytic experiments by demonstrating the ability to regulate the optical response of LaCoxFe1-xO3 perovskites by modifying the bandgap. The LaCo0.2Fe0.8O3 shows improved photocatalysis performance by synergic effect and narrow bandgap energy allowing more visible light to be absorbed.

The following Mott-Schottky relation is also used to generate an energy level diagram (ELD) for the theoretical evaluation of the relative band edge position of the photocatalysts. Moreover, identifying the conduction band (ECB) and valence band (EVB), potential edge values of the LaCoxFe1-xO3 nanoparticle samples were evaluated approximately (Table 1) by utilizing the following equation (4), (5):

Table 1

Bandgap energies with potential edge positions of VB and CB of LaCoxFe1-xO3 samples in different Cobalt rates from 0 to 1.

Catalyst sample	Bandgap (eV)	EVB (eV)	ECB (eV)
LaFeO3	1.89	2.015	0.125
LaCo0.2Fe0.8O3	1.83	1.605	−0.225
LaCo0.4Fe0.6O3	1.94	2.095	0.195
LaCo0.5Fe0.5O3	2.15	2.245	0.095
LaCo0.6Fe0.4O3	2.08	2.165	0.085
LaCo0.8Fe0.2O3	1.98	1.68	−0.3
LaCoO3	2.31	2.295	−0.015

where, ECB, EVB, Eg, and X are represented the CB potential edge, VB potential edge, energy bandgap, and absolute electronegativity of the as-synthesized LaCoxFe1-xO3 samples, respectively. E0 stands for free-electron energy on the H2 scale (4.5 eV vs NHE). The X value of the LaFeO3 and LaCoO3 is calculated to be 5.58 eV and 5.64 eV, respectively [67], [68] with the reference absolute electronegativities values of La, Fe, Co, and O were 3.1, 4.06, 4.3, and 7.54 eV, respectively as mentioned in ‘Pearson Absolute Electronegativity table’. The VB potential value for the LaCo0.2Fe0.8O3 sample (EVB = 1.605 eV) was determined to be the lowest in the sequence, with a CB potential of −0.225 eV leading to a narrow bandgap (1.83 eV) that absorbs optimum visible photons. Photocatalytic activity necessitates the excitation of VB electrons towards the high-energy transition levels in the CB, which resulted in the production of electron-hole pairs. A lower potential of the VB position facilitates this excitation and increases the redox potential of photocatalytic efficacy.

The photocatalytic performance of the LaCoxFe1-xO3 (x = 0, 0.2, 0.4,0.5, 0.6, 0.8, 1) perovskite toward photodegradation of aqueous CR dye solution at natural pH (8.5) was monitored by UV − visible absorption spectroscopy under simulated visible photon illumination. Fig. S2 in supporting information depicts the structure and absorption spectra of CR dye. The π- π* transition is assigned to the high energy absorption signal at 343 nm, another low energy peak around 498 nm might be attributed with n- π* electron transition existent in the nitrogen of an azo chromophore (-N = N-) group. The photocatalytic effectiveness at various Cobalt doping rates was determined under optimal circumstances (Fig. 7 (a-b)) using visible photon irradiation for 30 min. The significant catalytic efficacy changed with Cobalt doping at the B-site when compared with pristine LaFeO3 perovskite. The photocatalytic efficacy varying with different Co/Fe stoichiometric molar ratios was noticed in the sequence of 0.2 > 0.6 > 0.4 > 0.8 > 0.5 > 0 > 1 after 30 min of reaction time. Among all synthesized samples, LaCo0.2Fe0.8O3 perovskite revealed 81 % of photocatalytic efficacy of CR dye degradations after 30 min under the visible photon illumination shown in Fig. 7a. In the absence of a photocatalyst, no significant reduction (only 3.1 % degradation) in CR dye concentration is reported under visible light is presented in Fig. 7b. The first-order kinetic model was employed to estimate the photocatalytic degradation of CR dye after 30 min irradiation on LaCoxFe1-xO3 nanoparticles (the calculated rate constant values were 0.01118, 0.0461, 0.02308, 0.01642, 0.02516, 0.01706 and 0.00947 min−1, respectively min−1, for x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1 respectively) displayed in Fig. 8(a-b) and Table 2. The LaCo0.2Fe0.8O3 perovskite nanocomposite sample exhibited the maximum photocatalytic performance with a rate constant of 0.0461 min−1 in 30 min. According to the observations, the rate constant for LaCo0.2Fe0.8O3 is 5-fold more than that of pure LaCoO3 and LaFeO3 samples. As compared the catalytic efficacy of as-prepared LaCoxFe1-xO3 samples with previously reported CR dye degradation using various nanocatalysts were presented in supporting information Table S1. It is found that as-prepared photocatalyst showed superior catalytic ability towards CR dye (3x10-5 M) degradation in 30 min at natural pH.

Fig. 7

(a) Time-dependence UV–Visible absorption spectra of Congo red dye without catalyst and (b) with LaCo0.2Fe0.8O3 catalyst (Dye conc = 3 x10-5 M; Catalyst amount = 20 mg/100 mL; pH-8.5) and inset (a and b) shows the color change of dye during photodegradation.

Fig. 8

(a) Photodegradation efficiency and (b) pseudo-first-order rate kinetics curve of CR dye with LaCoxFe1-xO3 photocatalyst samples under the visible light illumination.

Table 2

Evaluation of pseudo-first-order rate constant (k) for photocatalytic degradation of CR dye with LaCoxFe1-xO3 photocatalyst samples under the visible photon illumination (Dye conc = 3 x10-5 M; Catalyst amount = 20 mg/100 mL; pH-8.5).

Sl. no	Sample	Rate Constant (min−1)	R2value
1	Light only	0.00093	0.973
2	Dark condition	0.00576	0.987
3	LaFeO3	0.01118	0.981
4	LaCo0.2Fe0.8O3	0.04610	0.961
5	LaCo0.4Fe0.6O3	0.02516	0.975
6	LaCo0.5Fe0.5O3	0.01642	0.944
7	LaCo0.6Fe0.4O3	0.03016	0.910
8	LaCo0.8Fe0.2O3	0.01706	0.988
9	LaCoO3	0.00947	0.933

Further investigations on the LaCoxFe1-xO3 perovskite samples revealed that the degradation of CR dye was dependent on the initial dye concentration in the aqueous solution. Fig. 9a illustrates the variation of CR dye content in solutions as a function of light stimulation duration. Photocatalytic degradation efficacy was investigated using a defined concentration of LaCo0.2Fe0.8O3 perovskite samples (20 mg) with constant adsorption sites, on the variation of initial CR dye concentrations ranging from 2 x10-5 mol/L to 5 x10-5 mol/L at natural pH-8.5 as revealed in Fig. 9a and supporting information Fig. S3. The rate of photodegradation was enhanced initially up to a CR dye concentration of 3 x10-5 mol/L, afterwards the rate diminished as the CR dye concentration was increased further. As a consequence, at optimum CR dye concentrations, a significant number of catalyst molecules are accessible to adsorb the majority of the molecules in suspension, whereas, at higher CR dye concentrations, a significant number of CR dye molecules compete for active sites (screening effect), [69] which leads to diminishing the degradation rate. [70]

Fig. 9

The influence of (a) initial dye concentration, (b) photocatalyst loading on the CR photocatalytic degradation efficiency.

The effects of adjusting the dose of LaCo0.2Fe0.8O3 perovskite samples on CR decolorization proficiency are shown in Fig. 9b and supporting information Fig. S3. The initial CR dye concentration was fixed (3 x10-5 mol/L) at natural pH (8.5), while adding LaCo0.2Fe0.8O3 samples of about 10, 20, 30, and 40 mg/100 mL, the photocatalytic rate altered. In the absence of LaCo0.2Fe0.8O3, the CR dye remains unaffected after 30 min in the visible photon. The maximal photocatalytic efficacy of CR dye was achieved by increasing the dose of LaCo0.2Fe0.8O3. Under visible photons, 81 % of color pollutants degraded after 30 min with LaCo0.2Fe0.8O3 loading of 20 mg/100 mL, may be ascribed to the abundance of surface-active sites of photocatalyst for degradation. However, further elevating the LaCo0.2Fe0.8O3 content did not result in a considerable improvement in degradation rate, due to the increased particles' opacity and light scattering.

Finally, the catalytic activity mechanism of LaCoxFe1-xO3 nanoparticles was studied to be coherent with the different analytical results. Based on the following facts, we can postulate that the contributing factors on catalytic activity might be associated with crystallite size, energy bandgap, and reactive species. Initially, the small crystallite size of the material shows the fast photon-generated charge carriers migrate from the bulk to the active surface, enhancing the catalytic efficacy on the surface before recombination. The bandgaps of the Cobalt doped LaFeO3 samples were varied and the positions of the VB and CB edges were regulated to accelerate CR dye degradation.

Generally, the possible mechanism of photodegradation of dyes pollutants on catalyst surface is caused by either photocatalytic oxidation or photosensitization of dye displayed in Fig. 10. [71] The dye is activated in the photosensitization mechanism by capturing visible source photons and then transferring electrons to the catalyst's conduction band, which combines with the oxygen in the surface to generate a superoxide oxidant to induce the degradation as presented in equation (6–9) [72] :

Fig. 10

Schematic illustration of visible-light-induced photocatalytic degradation of CR dye over LaCo0.2Fe0.8O3 sample.

Moreover, in the instance of photocatalytic degradation, electron-hole charge pairs produced on the surface of LaCo0.2Fe0.8O3 catalyst combine with water and oxygen molecule to produce hydroxyl (OH•) and superoxide (O2•–) radicals respectively as shown in equation (10), (11), (12), (13). The oxidation process is considered to be carried out by photon-generated OH• radicals. The azo chromophore group of the dye is initially degraded by these extremely reactive oxidants. [73], [74] The photoluminescence approach, which used terephthalic acid (TA) is reacted with OH• radical to produce 2-hydroxyterephthalic acid (HTA) which exhibits strong fluorescence and validate the origin of the OH• radical. The PL emission spectra of TA in a NaOH solution with a LaCo0.2Fe0.8O3 catalyst material during visible photon stimulation are displayed in Fig. 11.

Fig. 11

PL emission spectra of terephthalic acid in alkaline (NaOH) solution under photocatalytic irradiation with LaCo0.2Fe0.8O3 sample.

The reusability of the perovskite catalyst in photocatalysis is essential because it reduces the cost expenses of dye pollutant treatment. After every single reaction, the insoluble LaCoxFe1-xO3 catalyst was centrifuged, extensively washed with deionized water to eliminate the adsorbed dye moieties, and dried before being employed in further photoreactions. To accomplish this, three repeated cycles of photocatalytic CR dye degradation were carried out, employing the same catalyst with a fresh dye solution. The catalytic efficiency was reduced by a very less percentage (4%) throughout each cycle, as illustrated in Fig. 12a. The XRD findings of reused sample in Fig. 12b further demonstrate that the manufactured catalyst would not degrade itself and can be easily recycled for extended uses.

Fig. 12

(a) Reusability of LaCo0.2Fe0.8O3 sample and (b) corresponding XRD spectra of LaCo0.2Fe0.8O3 before and after photodegradation process.

Conclusions

In conclusion, Cobalt doped perovskite-type oxides LaFeO3 nanocomposites with varying doping concentrations were successfully manufactured via a single-step ultrasonic approach. The existence of the doped 'Cobalt' metals was confirmed by XRD measurements and EDS spectra. XPS survey was used to identify the chemical oxidation states of the elements in the photocatalyst. Under photocatalytic conditions, Cobalt doped catalysts lead to better CR dye degradation efficacy than LaFeO3. The electronic band structure of a material can be altered by doping with Cobalt on B-site, as is widely known. When exposed to incident radiation, a narrower bandgap of doped catalyst displayed considerable changes in the valence and conduction band positions and provided facile electron accessibility, resulting in a greater synergistic effect and improved catalytic activity. The developed LaCo0.2Fe0.8O3 catalyst displayed high photocatalytic activity for CR dye, accelerating>81 % dye degradation and exhibiting a first-order rate constant of 0.0461 min−1 upon visible photon illumination for 30 min. This study shows a facile approach to tailoring the essential features of ferrite-based perovskite catalysts for visible range photocatalysis.

CRediT authorship contribution statement

Madappa C. Maridevaru: Conceptualization, Data curation, Writing. Sambandam Anandan: Conceptualization, Data curation, Supervision, Writing – review & editing. Belqasem Aljafari: Software, Writing. Jerry J. Wu: Formal analysis, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.