Enhanced magnetic and optical properties of oxygen deficient TiO$_{2-δ}$ nanoparticles synthesized by environment-friendly green route using whole plant extract of Phyllanthus niruri

Abstract

We report magnetic, optical and oxidation states of oxygen deficient TiO$_{2-δ}$ nanoparticles (NPs) synthesized by environment-friendly green route using Phyllanthus niruri (PN) whole plant extract instead of leaf extract. Rietveld refinement of room temperature XRD pattern confirms the formation of pure phase anatase TiO$_2$ crystals in a tetragonal structure with space group I41/amd. TEM and SEM microstructure shows agglomerated spherical shape NPs exhibiting average particle size $\sim$ 35~nm. FTIR result confirms the presence of biomolecules and functional group attached to the surface of TiO$_2$ NPs. The core level XPS of O-1s and Ti-2p confirms the presence of oxygen vacancies that leads to the mixed oxidation states of Ti (Ti$^{4+}$ and Ti$^{3+}$). UV-vis result shows a strong absorption peak ($\sim$ 250~nm) along with reduced optical band gap energy E$_g$ $\sim$ 2.75~eV, possibly arises due to the surface plasmon resonance (SPR) caused by lower band gap energy emerging from oxygen vacancies. Magnetization as a function of applied magnetic field shows ferromagnetic nature at room temperature [M$_S$ $\sim$ 0.029~emu/g and H$_C$ $\sim$ 0.0143~T]. The observed ferromagnetic behaviour can be understood by virtual hopping of electrons from Ti$^{3+}$(3d$^1$) to Ti$^{4+}$(3d$^0$)-sites, however, vice versa is prohibited.