Abstract
Highly porous heterojunction films of SnO2/TiO2 were prepared using gas-flow thermal evaporation followed by atomic layer deposition (ALD). Highly porous SnO2 was fabricated by introducing an inert gas, Ar, during thermal evaporation. To build heterogeneous structures, the TiO2 layers were conformally deposited on porous SnO2 with a range of 10 to 100 cycles by means of ALD. The photocatalytic properties for different TiO2 thicknesses on the porous SnO2 were compared using the degradation of methylene blue (MB) under UV irradiation. The comparisons showed that the SnO2/TiO2-50 heterostructures had the highest photocatalytic efficiency. It removed 99% of the MB concentration, and the decomposition rate constant (K) was 0.013 min−1, which was approximately ten times that of the porous SnO2. On the other hand, SnO2/TiO2-100 exhibited a lower photocatalytic efficiency despite having a TiO2 layer thicker than SnO2/TiO2-50. After 100 cycles of TiO2 ALD deposition, the structure was transferred from the heterojunction to the core–sell structure covered with TiO2 on the porous SnO2, which was confirmed by TEM analysis. Since the electrons photogenerated by light irradiation were separated into SnO2 and produced reactive oxygen, O2−, the heterojunction structure, in which SnO2 was exposed to the surface, contributed to the high performance of the photocatalyst.
Highlights
Water contamination with dye discharges from various industries is considered a significant threat to the environment and public health [1,2]
The highly porous heterojunction photocatalyst SnO2/TiO2 was fabricated by gasflow-modified thermal evaporation followed by atomic layer deposition (ALD)
Transmission electron microscopy (TEM) analysis with electron energy loss spectroscopy showed that the TiO2 layers were well deposited on the surface of the porous SnO2 by ALD (Figure 2)
Summary
Water contamination with dye discharges from various industries is considered a significant threat to the environment and public health [1,2]. The heterojunction of a metal oxide semiconductor facilitates the separation of the generated electron–hole (e−/h+) pairs These generated excitons produce reactive oxygen species in aqueous media, such as hydroxyl radicals (·OH) and superoxides (·O2−), which are known to be photocatalytically active for the decomposition of organic contaminants, thereby purifying dye-contaminated water [7–9]. Since nanoparticle-type photocatalysts enlarge the active surface area and enhance light absorption, the synthesis of metal oxide nanoparticles has been widely investigated as a high-performance photocatalyst. Despite their high performance, particle-type photocatalysts require additional separation processes in the suspension system, leading to high costs. The results reveal that the heterostructure, not the core-shell structure of SnO2/TiO2, induces high photocatalytic activity, improves charge separation, and utilizes separated electrons and holes for photocatalysis in TiO2 and SnO2, respectively
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