Green synthesis using carambola fruit juice and effects of point defects in ZnO nanostructures caused by concentration of Cu2+ or Ti4+ and Cu2+/Ti4+ Co-doping

Abstract

Zinc oxide (ZnO) is an n-type semiconductor with excellent industrial and technological application properties, whose poor optical and electrical properties are attributed to point defects. In an effort to tailor the properties, ZnO was synthesized via a modified oxalate route using carambola fruit juice as a precipitating agent and doped with Ti, Cu, or Cu/Ti to evaluate the influence of the dopant ions and concentration on the ZnO microstructure. The synthesized ZnO NPs adopted a wurtzite hexagonal structure with space group P6₃mc, which changed to p3 as the Cu concentration in the ZnO microstructure increased close to that of Zn. Infrared (IR) spectra of the single-molecule precursor revealed the formation of metallic oxalates. The influences of Cu²⁺, Ti^4+, and Cu²⁺/Ti⁴⁺ doping on the microstructure, morphology, and optical properties of the synthesized ZnO were studied. Scanning electron microscopy (SEM) confirmed the modification of the morphology from cuboidal to porous spherical nanostructures. The PXRD results revealed an insignificant contraction in the cell volume as the dopant elements replaced Zn ions in the microstructure (Ti_0.02Cu_0.02Zn_0.96O, 0.^{5271 nm3} and ZnO, 0.5288 nm^3), but doping improved the mechanical properties, such as the dislocation density, microstrain, and specific surface area, and optical properties, such as the absorbance and optical band gap. Ti_0.02Cu_0.02Zn_0.98O showed a 17.17 % decrease in microstrain, a 9.63 % decrease in dislocation density, and a 5.61 % increase in specific surface area alongside a reduction in the optical band gap within the range of 3.116 eV–2.998 eV. Additionally, the EDS results confirmed the incorporation of various dopant ions within the corresponding microstructures of ZnO. The results also revealed a modification in the crystallite size distributions. These new materials (Cu_0.42Zn_0.58O and Ti_0.02Cu_0.02Zn_0.96O) exhibited greater mechanical stability and a significant propensity for improved microstructural and optical properties, which could potentially modify electrical properties such as conductivity. Therefore, the newly synthesized samples have been confirmed to exhibit improved mechanical and optoelectronic properties compared to those of the parent zinc oxide.