Synproportionation Reaction for the Fabrication of Sn²⁺Self-Doped SnO_2-xNanocrystals with Tunable Band Structure and Highly Efficient Visible Light Photocatalytic Activity

Tailored fabrication of nonstoichiometric semiconductor nanocrystals with tunable electronic structures has attracted considerable attention owing to their scientific and technological importance. In this work, we have developed a novel and facile approach to prepare stable Sn2+ self-doped SnO2-x nanocrystals with a large surface area via a synproportionation reaction of Sn4+ with metal tin under mild conditions. The effects of Sn2+ doping concentration in SnO2-x lattice on the nanoparticle size, band structure, and photodegradation of methyl orange (MO) were investigated in detail. It is found that the obtained deep-yellow colored Sn2+ self-doped SnO2-x sample shows exceptionally higher visible-light photocatalytic performance than stoichiometric SnO2, which is only sensitive to UV light due to its intrinsic large band gap. To the best of our knowledge, this is the first experimental example that self-doped metal oxide nanocrystals have been utilized as an effective photocatalyst for the degradation of pollutants within 15 min under visible-light irradiation (λ ≥ 400 nm). The superior photodegradation activity of the Sn2+ self-doped SnO2-x can be ascribed to the incorporation of Sn2+ into the lattice matrix and accompanying oxygen vacancies, which can result in significant narrowing of the band gap and enhancement in the visible-light absorption capability, notably, the efficient separation of the photogenerated electron–hole pairs in SnO2-x, which has been further confirmed by remarkable enhancement of the photocurrent response. Moreover, strong photo-oxidation capability for high content •OH radical formation over SnO2-x (ca. 25 times higher than SnO2 sample) also contributes to the improvement of photocatalytic performance. Our synthetic approach could be extended to design other nonstoichiometric semiconductor nanostructures with tunable band structure, highly efficient visible-light photocatalytic activity, and enhanced photoelectric conversion properties in the future.