Bio-synthesised black α-Cr2O3 nanoparticles; experimental analysis and density function theory calculations

Highlights: • Bio-synthesis of black α-Cr₂O₃ nanoparticles using extracts of sweet potatoes peels. • UV–Vis absorbance peaks of extracts confirmed presence of active compounds. • Quantitative analyze of XRD data fit well with rhombohedral structure of Eskolaite Cr₂O_3. • Magnetic study confirms that synthesised Cr₂O₃ nanoparticles are antiferromagnetic. • DFT analysis applies a +U coulomb interaction to agree with experimental results. -- Abstract: A selective single phase black α-Cr₂O₃ nanoparticles was bio-synthesised via simple straight-forward green synthesis approach. The process involves extraction of phytochemicals contained in peels of sweet potatoes. Extraction was done in distilled water under constant magnetic stirring at a temperature of 70–80 °C resulting in a dusty yellow colour aqueous extracts. Afterwards, chromic nitrate salt was added to extracts resulting in reduction of metal salt to metal nanoparticles. Obtained precipitates were dried and annealed in the air for 2 h ready to be applied without further post synthesis modifications. SEM and EDS analysis of annealed precipitates reveal distinct shapes and high purity of nanoparticles. The effects of the annealing temperature are evident in the nanoparticle sizes. SAED and XRD patterns expose bright diffraction peaks which are harmonized to the rhombohedral structure of pure Eskolaiteα-Cr₂O_3. By quantitative analysis of XRD data, it was noted that lattice parameters and crystal sizes slightly decrease w.r.t increase annealing temperature. Raman spectra recorded peaks ascribe to vibrations in A_1g and E_g mode whereas FTIR analysis show absorption bands at 641 and 632 cm⁻¹ which evidence the presence of α-Cr₂O₃ nanoparticles. UV–Vis absorbance peak generated Cr₂O₃ nanoparticles are observed at 402 nm yielding a band gap of 3.08eV. Magnetism results of α-Cr₂O₃ nanoparticles shows linear increase upon field increasing, which can be elucidated by the existing of uncompensated spins at the surface of the nanoparticles that may lead to nonmagnetic or antiferromagnetic state. Zero field cooling (ZFC) results of α-Cr₂O₃ nanoparticles were analysed based on Curie-Wien relation which yield values of magnetic moment (μ_eff) of the synthesised Cr₂O₃ nanoparticles close to the value that was assigned for Cr^2+.The density functional theory (DFT) with the PW91, PBE, PBESOL and a Hubbard U Coulomb interactionwas utilized to study the optimum structure, electronic and magnetic properties of antiferromagnetically ordered Cr₂O₃. The computed results are consistent with the experimental measurements.