Nanostructured SnO2-ZnO Heterojunction Photocatalysts Showing Enhanced Photocatalytic Activity for the Degradation of Organic Dyes

Nanoporous SnO2-ZnO heterojunction nanocatalyst was prepared by a straightforward two-step procedure involving, first, the synthesis of nanosized SnO2 particles by homogeneous precipitation combined with a hydrothermal treatment and, second, the reaction of the as-prepared SnO2 particles with zinc acetate followed by calcination at 500 degrees C. The resulting nanocatalysts were characterized by X-ray diffraction (XRD), FTIR, Raman, X-ray photoelectron spectroscopy (XPS), nitrogen adsorption desorption analyses, transmission electron microscopy (TEM), and UV-vis diffuse reflectance spectroscopy. The SnO2-ZnO photocatalyst was made of a mesoporous network of aggregated wurtzite ZnO and cassiterite SnO2 nanocrystallites, the size of which was estimated to be 27 and 4.5 nm, respectively, after calcination. According to UV visible diffuse reflectance spectroscopy, the evident energy band gap value of the SnO2 ZnO photocatalyst was estimated to be 3.23 eV to be compared with those of pure SnO2, that is, 3.7 eV, and ZnO, that is, 3.2 eV, analogues. The energy band diagram of the SnO2 ZnO heterostructure was directly determined by combining XPS and the energy band gap values. The valence band and conduction band offsets were calculated to be 0.70 0.05 eV and 0.20+/-0.05 eV, respectively, which revealed a type-II band alignment. Moreover, the heterostructure SnO2 ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue than those of individual SnO2 and ZnO nanomaterials. This behavior was rationalized in terms of better charge separation and the suppression of charge recombination in the SnO2 ZnO photocatalyst because of the energy difference between the conduction band edges of SnO2 and ZnO as evidenced by the band alignment determination. Finally, this mesoporous SnO2-ZnO heterojunction nanocatalyst was stable and could be easily recycled several times opening new avenues for potential industrial applications.