Electronic structure and chemical states of green synthesized silica nanoparticles from biomasses

Silica nanoparticles (SiO2 NPs) were synthesized from the sugarcane bagasse (SB), pinewood stem (PS), walnut shell (WS), and rice husk (RH) using ambient burning, hydro-ball-milling, filtration, and post-annealing at 650 °C. The broadened XRD peak pattern (2θ = 22°) indicates the amorphous nature of all four samples, further confirmed by the circles of the SAED pattern. Microscopic images (snap-shots, SEM, TEM) revealed the agglomeration of SiO2 NPs with an average grain size of 40–80 nm. All samples' bandgap (Eg) was found in the 5.69–5.78 eV range determined from the Tauc's plot. The symmetric stretching of O–Si–O at 440 cm−1, Si–O at 880 cm−1, and asymmetric stretching of Si–O–Si peaks at 1062 cm−1 indicate the formation of polymorphic SiO2 NPs. Electronic and chemical states of SiO2 NPs were investigated by X-ray photoelectron spectroscopy (XPS), and the chemical bonding of SiOx and Si–O–Si occurred at 99.5 eV and 528.6 eV, respectively. Silicon (Si) concentration was observed to be ∼14.35 %, ∼18.88 %, ∼21.25 %, and ∼9.5 %, estimated from core-level XPS analysis. The pinewood stem extracted SiO2 NPs were observed to have a regular spherical shape with a size of 40 nm. The different atomic concentration of Si in all samples establishes their utilization in medical and industrial applications.