Aderbal C. Oliveira

The Journal retracts the article titled, “Synthesis of CaFe₂O₄-NGO Nanocomposite for Effective Removal of Heavy Metal Ion and Photocatalytic Degradation of Organic Pollutants”

The journal retracts the article titled “Structural and Photocatalytic Studies on Oxygen Hyperstoichiometric Titanium-Substituted Strontium Ferrite Nanoparticles”

In the original publication [...].

Doping of ferrites is an important domain of research for their application as photocatalysts. In the present work, the effect of Ti⁴⁺ substitution on the structural and photocatalytic properties of strontium ferrite nanoparticles (NPs) is studied. Ternary doped Sr_1−xTi_xFe₂O_4+δ ferrite NPs (x = 0.0–1.0) were synthesized by sol–gel methodology. Tetravalent Ti⁴⁺ ions caused oxygen hyperstoichiometry and enhancement in the surface area from 44.3 m²/g for SrFe₂O₄ NPs to 77.6 m²/g for Sr_0.4Ti_0.6Fe₂O_4+δ NPs. The average diameter of NPs ranged between 25–35 nm as revealed by TEM analysis. The presence of two sextets in the Mössbauer spectrum of pristine SrFe₂O₄ and Ti⁴⁺-substituted ferrite NPs and a paramagnetic doublet in the TiFe₂O₅ confirmed their phase purity. The photocatalytic potential of pure and Ti⁴⁺-substituted ferrite NPs was studied using nitroaromatic compounds, viz. pendimethalin, p-nitrophenol and Martius yellow, as model pollutants. Doped ferrite NPs with a composition of Sr_0.4Ti_0.6Fe₂O_4+δ NPs showed the highest degradation efficiency ranging from 87.2% to 94.4%. The increased photocatalytic potential was ascribed to the lowering of band gap (E_g) from 2.45 eV to 2.18 eV, a fourfold decrease in photoluminescence intensity, increased charge carrier concentration (4.90 × 10¹⁵ cm⁻³ to 6.96 × 10¹⁵ cm⁻³), and decreased barrier height from 1.20 to 1.02 eV. O₂^●− radicals appeared to be the main reactive oxygen species involved in photodegradation. The apparent rate constant values using the Langmuir–Hinshelwood kinetic model were 1.9 × 10⁻² min⁻¹, 2.3 × 10⁻² min⁻¹ and 1.3 × 10⁻² min⁻¹ for p-nitrophenol, pendimethalin and Martius yellow, respectively. Thus, tuning the Ti⁴⁺ content in strontium ferrite NPs proved to be an effective strategy in improving their photocatalytic potential for the degradation of nitroaromatic pollutants.

In the present study, nanocomposites having hierarchical nanoflowers (HNFs) -like morphology were synthesized by ultra-sonication approach. HNFs were ternary composite of MgFe₂O₄ and bentonite with boron-, phosphorous- co-doped graphene oxide (BPGO). The HNFs were fully characterized using different analytical tools viz. X-ray photoelectron spectroscopy, scanning electron microscopy, energy dispersion spectroscopy, transmission electron microscopy, X-ray diffraction, vibrating sample magnetometry and Mössbauer analysis. Transmission electron micrographs showed that chiffon-like BPGO nanosheets were wrapped on the MgFe₂O₄-bentonite surface, resulting in a porous flower-like morphology. The red-shift in XPS binding energies of HNFs as compared to MgFe₂O₄-bentoniteand BPGO revealed the presence of strong interactions between the two materials. Box–Behnken statistical methodology was employed to optimize adsorptive and photocatalytic parameters using Pb(II) and malathion as model pollutants, respectively. HNFs exhibited excellent adsorption ability for Pb(II) ions, with the Langmuir adsorption capacity of 654 mg g⁻¹ at optimized pH 6.0 and 96% photocatalytic degradation of malathion at pH 9.0 as compared to MgFe₂O₄-bentonite and BPGO. Results obtained in this study clearly indicate that HNFs are promising nanocomposite for the removal of inorganic and organic contaminants from the aqueous solutions.

This paper reports the successful synthesis of magnetic nanocomposite of calcium ferrite with nitrogen doped graphene oxide (CaFe₂O₄-NGO) for the effective removal of Pb(II) ions and photocatalytic degradation of congo red and p-nitrophenol. X-ray diffraction (XRD), Fourier transform infrared (FT-IR), transmission electron microscopy (TEM), and scanning electron microscopy-energy dispersive X-ray (SEM-EDX) techniques confirmed the presence of NGO and CaFe₂O₄ in the nanocomposite. The Mössbauer studies depicted the presence of paramagnetic doublet and sextet due to presence of CaFe₂O₄ NPs in the nanocomposite. The higher BET surface area in case of CaFe₂O₄-NGO (52.86 m²/g) as compared to CaFe₂O₄ NPs (23.45 m²/g) was ascribed to the effective modulation of surface in the presence of NGO. Adsorption followed the Langmuir model with maximum adsorption capacity of 780.5 mg/g for Pb(II) ions. Photoluminescence spectrum of nanocomposite displayed four-fold decrease in the intensity, as compared to ferrite NPs, thus confirming its high light capturing potential and enhanced photocatalytic activity. The presence of NGO in nanocomposite offered an excellent visible light driven photocatalytic performance. The quenching experiments supported ^●OH and O₂^●− radicals as the main reactive species involved in carrying out the catalytic system. The presence of Pb(II) had synergistic effect on photocatalytic degradation of pollutants. This study highlights the synthesis of CaFe₂O₄-NGO nanocomposite as an efficient adsorbent and photocatalyst for remediating pollutants.

We have prepared nano-structured In-doped (1 mol %) LiFePO₄/C samples by sol–gel method followed by a selective high temperature (600 and 700 °C) annealing in a reducing environment of flowing Ar/H₂ atmosphere. The crystal structure, particle size, morphology, and magnetic properties of nano-composites were characterized by X-ray diffraction (XRD), scanning electron microsopy (SEM), transmission electron microscopy (TEM), and ⁵⁷Fe Mössbauer spectroscopy. The Rietveld refinement of XRD patterns of the nano-composites were indexed to the olivine crystal structure of LiFePO₄ with space group Pnma, showing minor impurities of Fe₂P and Li₃PO₄ due to decomposition of LiFePO₄. We found that the doping of In in LiFePO₄/C nanocomposites affects the amount of decomposed products, when compared to the un-doped ones treated under similar conditions. An optimum amount of Fe₂P present in the In-doped samples enhances the electronic conductivity to achieve a much improved electrochemical performance. The galvanostatic charge/discharge curves show a significant improvement in the electrochemical performance of 700 °C annealed In-doped-LiFePO₄/C sample with a discharge capacity of 142 mAh·g⁻¹ at 1 C rate, better rate capability (~128 mAh·g⁻¹ at 10 C rate, ~75% of the theoretical capacity) and excellent cyclic stability (96% retention after 250 cycles) compared to other samples. This enhancement in electrochemical performance is consistent with the results of our electrochemical impedance spectroscopy measurements showing decreased charge-transfer resistance and high exchange current density.