Modification of the structural and optical properties of tin oxide nanoparticles by Co doping and thermal annealing

Enhancement of the UV photoluminescence emission of sol–gel synthesized tin oxide nanoparticles is achieved by a combination of thermal annealing and Co doping. The UV as well as the defect-related visible photoluminescence are correlated to the structural characteristics and surface Sn(OH)₂ content. The nanoparticle structure, size, crystallinity, and Sn(OH)₂ content are monitored by a combination of X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. In the undoped powders, a suitable annealing leads to a significant UV luminescence at around 365 nm. After doping with Co and annealing, the UV emission is further enhanced. The improvement in the UV emission intensity following annealing and Co doping of SnO₂ is demonstrated to be due to a reduction in the hydroxyl content. The defect-related broad visible photoluminescence (～400–650 nm) can be deconvoluted into three bands at around 440 nm (blue), 510 nm (green), and 600 nm (orange). The green emission is related to Sn(OH)₂ determined by Raman spectroscopy. The blue and orange emissions are attributed to oxygen vacancies.