Structural and optical properties of (Zn, Co) co-doped SnO2 nano particles

Optical bandgap narrowing is observed in Sn-0.94 Zn0.05Co0.01O2, Sn-0.92 Zn0.05Co0.03O2 and Sn-0.90 Zn0.05Co0.05O2 nanoparticles synthesized by the chemical co- precipitation method. The estimated particle size and optical band gap of Sn-0.94 Zn0.05Co0.01O2, Sn-0.92 Zn0.05Co0.03O2 and Sn-0.90 Zn0.05Co0.05O2 nanoparticles are 16.51, 11.75 and 6.6 nm and 1.45, 1.36 and 0.93 eV, respectively. A red shift is noticed for all samples as compared to band gap value (3.6 eV) for bulk SnO2. Increasing Co content results in decreased particle size, narrowed band gap and enhanced emission intensities in visible range of light. Small sized nano particles have larger surface areas and these large surface areas lead to more defects (oxygen vacancies, tin interstitials). The increasing defects concentration (especially oxygen vacancies, tin interstitials) results to narrow the band gap. With increasing Co content, the particles are in oxygen-poor state indicating the presence of many O vacancies and Sn interstitials. The oxygen content decreases steadily with increasing Co content, with the Sn:O atomic ratio increases suggesting the high density of defects. Large number of defects (oxygen vacancies) creates several donor levels within the forbidden energy gap to narrow the band gap and these defects in the band gap act as luminescent centers to enhance the emission intensities in visible spectrum.