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

TL;DR

This work demonstrates a green synthesis route for oxygen-deficient TiO2-δ nanoparticles using Phyllanthus niruri whole-plant extract. Structural analysis confirms anatase TiO2 with ~35 nm crystallites and surface oxygen vacancies leading to Ti3+/Ti4+ mixed valence, a reduced Eg ≈ 2.75 eV, and a strong 250 nm absorption. Magnetic measurements show room-temperature ferromagnetism with Ms ≈ 0.029 emu/g, explained by double-exchange hopping between Ti3+ and Ti4+ at oxygen-vacant sites. Compared with PN leaf extract syntheses, the whole-plant route yields two orders of magnitude higher magnetization and notable optical-tuning, underscoring the potential of PN-derived green routes for functional TiO2-δ NPs in photocatalysis and spintronic applications.

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.

Figures (9)

• Figure 1: Left panel - Rietveld refined XRD pattern of TiO$_2$ NPs; inset shows broadness of most intense peaks of TiO$_2$ NPs (red line) and reference sample polycrystalline TiO$_2$ (blue line) at normalized intensity scale. Right panel - crystal structure of anatase TiO$_2$ NPs obtained by VESTA software. The smaller atom oxygen O and larger atom titanium Ti is shown by red and blue solid sphere, respectively.
• Figure 2: (a) SEM micrograph of green synthesized TiO$_{2-\delta}$ NPs, (b) Energy dispersive X-ray (EDX) spectra of TiO$_{2-\delta}$ NPs; inset shows titanium (golden color) and oxygen (red color) elemental color mapping and (c) Selected area at which EDX elemental color mapping were measured.
• Figure 3: (a) Transmission electron microscopy (TEM) image, and (b) selected area electron diffraction pattern of TiO$_{2-\delta}$ NPs synthesized by green route.
• Figure 4: Fourier-transform infrared spectroscopy (FTIR) spectrum of green synthesized TiO$_{2-\delta}$ NPs. Inset I and II show enlarged image of the same.
• Figure 5: Raman spectrum of green synthesized TiO$_{2-\delta}$ NPs measured in the range 104-10000 cm$^{-1}$ using excitation wavelength of laser $\lambda$ = 532 nm.