Solar-activated tandem thin films based on CuInS2, TiO2 and SnO2 in optimized wastewater treatment processes

• Higher absorbed energy compared to the effective band gap.
• The surface energy outlines a good hydrophilic behavior.
• Photocatalysts will generate charge carriers, both under UV and Vis irradiation.
• TOC removal efficiencies higher than 90%.
• Long term runs proved the stability of the photocatalytic activity after 36 h.

Tandem structures involving CuInS2, TiO2 and SnO2 thin films were prepared by Robotic spray pyrolysis and were optimized in terms of composition and deposition temperatures. The compounds have structural compatibility (crystalline structures and crystallites sizes) and good homogeneity. The crystalline structures were investigated by X-ray diffraction and the morphology by atomic force microscopy and scanning electron microscopy. The wettability properties and the surface energy were calculated based on contact angle measurements using two liquids with different polarities (glycerol and ethylene glycol). The components in the tandem systems have suitably positioned the energy of the conduction and valance bands, limiting charge recombination, therefore, the sample containing all three semiconductors has the highest mineralization efficiency (95.3% removal of total organic carbon) under UV + Vis irradiation, mimicking solar radiation. Long term use of the photocatalytic films (up to 36 h) proved the need to design the process with interim washing steps for preserving the photocatalytic efficiency by avoiding surface clogging.

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