Enhanced photocatalytic activities of LaCoO3 perovskite nanostructures for degrading cationic and anionic dyes

Sustainable and cost-effective technologies are crucial for addressing environmental pollution issues. Perovskite photocatalysts have gained increased attention as a potential solution which could be accounted to their tuneable structural characteristics, flexible bandgap, and superior catalytic properties. In this study, we synthesized Lanthanum Cobaltite (LaCoO3) perovskite nanostructures via co-precipitation for photocatalytic purposes. Morphological analysis revealed LaCoO3 nanoparticles with an average size of ∼33 nm whose crystalline characteristics were investigated as a function of annealing temperatures. We found that the pure and crystalline LaCoO3 phase formed after the post-annealing process at 600°C. Photocatalytic studies showed that the degradation potential of LaCoO3 was significantly improved when hydrogen peroxide (H2O2) was added as co-catalyst system. We achieved degradation efficiencies of up to 91% and 85% for MB and methyl orange (MO), respectively. LaCoO3/H2O2 exhibited excellent photocatalytic potential for degrading both anionic and cationic organic dyes for environmental remediation applications.


Introduction
Advanced environmental protection strategies have gained immense research interest owing to the everlasting hike in global pollution.Pure water is the fundamental requirement for maintaining life on Earth.Unfortunately, freshwater resources are seriously affected by the release of harmful contaminants [1,2].Industrial and other wastewater effluents mainly contain dyes, personal care products, fertilizers, pesticides, and organic additives [3][4][5].Most organic dyes consist of multiple aromatic rings and are characterized by high thermal and chemical stability.The expulsion of these contaminants can cause an enhanced water pollution rate.The stability of the ecosystem can be thereby seriously affected, which necessitates the implementation of appropriate pollution control strategies.
Moreover, exposure to these dyes can drastically affect public health and well-being, as they are highly carcinogenic, mutagenic, and allergenic in nature [6].Even trace levels of dyes can interfere with photosynthesis, thereby adversely influencing the aquatic environment as well.Methylene blue (MB) and methyl orange (MO) are common dyes that are stable and difficult to degrade [7].These dyes are frequently employed in various industrial processes, which can be discharged into water bodies.Therefore, regular monitoring and treatment of wastewater effluents are essential before disposal into the environment.Nevertheless, conventional water treatment techniques are inadequate to address the vast demands for achieving the goals of environmental sustainability, necessitating the implementation of economically viable water remediation tools for a greener environment [8].
Heterogeneous photocatalysis offers tremendous opportunities for eliminating micro-pollutants from wastewater [9].Under UV or visible light irradiation, the catalysis of organic pollutants can be facilitated by the presence of an appropriate photocatalyst material.The absorption capability, charge recombination rate, and efficiency of solar light conversion are crucial in determining the functionality of the photocatalyst system.Semiconductor metal oxides such as titanium dioxide (TiO2) and zinc oxide (ZnO) have been extensively employed for photocatalytic purposes [10,11].However, their higher bandgap restricts solar light capture, preventing their use as visible-light-driven photocatalyst systems.Perovskite materials with the empirical formula ABO3 can be designed with tunable properties for appropriate photocatalytic applications [12].Lanthanum cobaltite (LaCoO3) is an orthorhombic perovskite-like structure, which has been reported to have promising catalytic activities [13,14].Owing to the suitable physiochemical properties, LaCoO3-based photocatalyst systems have attracted much research interest.The large cationic size of Lanthanum has been found to improve the structural stability of the catalyst.It has also been reported that doping strategies can facilitate their absorption in the visible range, thus facilitating solar light-induced photocatalytic potentials [15][16][17][18].
In this study, we evaluated the photocatalytic efficiency of lanthanum cobaltite (LaCoO3) nanoparticles synthesized via the co-precipitation technique.The structural and morphological properties of as-synthesized LaCoO3 were characterized, including the effect of annealing temperature on the crystalline properties of LaCoO3.The photocatalytic performance of LaCoO3 with hydrogen peroxide (H2O2) as a co-catalyst was investigated for both anionic and cationic dyes.The mechanism of the photocatalytic action of LaCoO3/H2O2 was illustrated.Our findings revealed that the LaCoO3/H2O2 system is an excellent candidate for completely removing organic pollutants for environmental remediation applications.

Synthesis and characterization of LaCoO3 nanostructures
Lanthanum cobaltite (LaCoO3) nanostructures were synthesized using the co-precipitation method with equimolar amounts of Cobalt (II) acetate and Lanthanum (III) acetate (0.1 M each).After mixing the acetates, the solution was stirred for five minutes at ambient conditions.Next, sodium hydroxide (NaOH, 0.2 M) was added to precipitate the mixture, and the resultant solution was stirred at 80º C for 3 hours.After completing the reaction, the product was centrifuged and washed thoroughly with deionized water and absolute ethanol several times.Subsequently, the powder was dried at 70º C overnight.To study the effect of annealing on the structural properties of LaCoO3, the powder was subjected to an annealing process from 300 to 600º C. The synthesized LaCoO3 nanoparticles were analyzed for morphology and elemental confirmation using the Field Emission SEM/EDS (Thermo Scientific Apreo C).The crystalline properties were studied using an X-ray diffraction technique with an X-ray diffractometer (Bruker, D8 advance).The absorbance spectra of the dye moieties during photocatalysis were captured using the UV Shimadzu (model 2600) spectrometer.

Photocatalysis studies
As-synthesized LaCoO3 nanoparticles were explored for degrading cationic and anionic dyes such as methylene blue and methyl orange, respectively.Degradation of dyes was carried out using a photochemical reactor (Techinstro, India) equipped with a 450 W UV lamp [19,20].About 100 mL of aqueous solutions containing 0.02 mM dye were stirred with ~0.1 g of as-synthesized LaCoO3, while the absorbance of dye solutions was monitored at regular intervals.Before UV-light exposure, the solution was kept in the dark to attain equilibrium between the adsorption and desorption of dye molecules on the catalyst surface.After the dark phase, ~50 µL of hydrogen peroxide was added as a co-catalyst before light exposure.After switching on the UV light, dye solutions were retrieved at 20minute intervals, and absorbance spectra were measured until the completion of the degradation reaction.

Results and Discussion
LaCoO3 nanoparticles have been successfully synthesized using a co-precipitation process whose crystalline properties were investigated as a function of annealing temperatures, as shown in Figure 1.XRD diffractograms revealed the subsequent evolution of crystallinity of as-synthesized nanoparticles by subjecting them to an annealing process from 300 to 600º C. One can see that crystalline peaks of LaCoO3 were fully observed at 600º C. The peaks at 23.2 (012), 32.9 (110), 33.3 (104), 40.6 (202), 41.5 (006), 47.5 (024), 53.6 (122), 53.7 (116), 58.9 (214), 59.7 (018), 68.9 (220), 70.0 (028) were observed which confirms the formation of LaCoO3 perovskite phase as per JCPDS file no.25-1060 and 00-036-1392 [21].The morphology of as-synthesized lanthanum cobaltite nanoparticles has been demonstrated in Figure 2a.Homogenous and compact nanoparticle-sized aggregates could be seen from the FESEM image.Also, analysis using image J has revealed that the sizes of individual nanoparticles range from 20 to 40 nm, with an average size of 33 ± 0.69 nm.Furthermore, the elemental analysis using EDS  The photocatalytic performance of LaCoO3 was evaluated using H2O2 as a co-catalyst for degrading both cationic and anionic dyes.It is clearly visible that the absorbance spectra of methylene blue decrease with increasing irradiation time (Figure 3a).After 120 minutes of light exposure, almost complete degradation of MB occurs.Figure 3b shows the plot of ln(A/A0) versus irradiation time with pseudo-first-order kinetics, where the slope of the linear fit provides the kinetic coefficient of the reaction.A kinetic coefficient (k-value) of 3.7 × 10 -2 was obtained, indicating superior photocatalytic properties of LaCoO3/H2O2 photocatalysts.
The efficacy of photocatalysis was also studied using an anionic dye such as MO (Figure 3c).After 220 minutes of exposure, MO showed considerable decomposition.The degradation efficiencies for MB and MO were almost 91% and 85%, respectively (Figure 3d).Also, we investigated the photocatalytic activities of LaCoO3 and H2O2 alone.We observed that ~20% and 18% degradation was observed within 120 min for LaCoO3 and H2O2 respectively for degrading MB.However, LaCoO3/H2O2 showed synergistic photocatalytic activity (91%) for MB decomposition.The mechanism of photocatalytic degradation of organic pollutants assisted by LaCoO3/H2O2 was also illustrated (Figure 4).Upon UV-light irradiation, photons can be absorbed by LaCoO3, exciting valence band (VB) electrons to the conduction band (CB).The excited electrons can either be captured by H2O2 to form hydroxyl (OH .) radicals or dissolve oxygen to undergo reduction, forming superoxide ( .O2 -) radicals.The holes present in the VB can also react with water to form more hydroxyl radicals.Hydroxyl and superoxide radicals primarily attack the organic dye moieties, leading to their subsequent decomposition into carbon dioxide and water molecules [22,23].
Table 1 shows earlier studies that dealt with LaCoO3-based photocatalysts for degrading MB and MO.In the present work, we attempted to investigate the photocatalytic efficiency of LaCoO3/H2O2 for degrading both cationic and anionic dyes.Our findings showed that the proposed system is highly efficient for being used as a photocatalyst for environmental remediation applications.

Conclusions
Lanthanum cobaltite nanoparticles were synthesized successfully using the co-precipitation technique.
The effect of annealing temperature on as-fabricated LaCoO3 was investigated, which showed that 600º C was the optimum annealing temperature.The morphological analysis has shown that ~33 nm-sized LaCoO3 particles have been formed.LaCoO3/H2O2 system demonstrated 91 and 85% photocatalytic efficiencies for degrading methylene blue and methyl orange.The current investigation demonstrated that LaCoO3/H2O2 could be exploited as an efficient photocatalyst for sustainably degrading organic pollutants.

Figure 1 :
Figure 1: XRD pattern of as-prepared LaCoO3 nanoparticles at different annealing temperatures from 300 to 600º C.
of elements such as La, Co, and O; thereby affirming the absence of any impurities within the as-synthesized powder.Meanwhile, the composition of LaCoO3 is confirmed as 51.7% (La), 35.3% (O), and 13.1% (Co).The EDS mapping images demonstrated the distribution of elements as well (Figures2d & 2e).

Figure 2 :
Figure 2: (a) FESEM image depicting surface morphology of as-synthesized LaCoO3 nanoparticles; (b) corresponding particle size distribution; (c) & (d) EDS spectra and mapping image and (e) the constituent elements in the third row.

Table 1 :
Analysis of photocatalytic efficiencies of LaCoO3-based systems for degrading dyes.