Hydrophilic-amorphous versus hydrophobic-crystalline Sb2S3: The battle for higher photocatalytic efficiency

The dependence of crystallinity and hydrophilicity of the material on its photocatalytic activity is rarely explored and understood. In this work, the influence of crystallinity and surface wettability is demonstrated through the photocatalytic activity of Sb2S3 thin films. Amorphous and crystalline thin films of Sb2S3 having bandgaps of 2.36 and 1.89 eV respectively are prepared using thermal evaporation. Photocatalytic activity of both the films are evaluated in the degradation of methylene blue dye under visible light. Amorphous sample degraded 88 % of dye in 180 min of irradiation, with a reaction rate constant of 0.01/min. The crystalline sample removed only 64 % of the dye under the same condition. The analysis of the surface nature of the film using contact angle measurement revealed that, amorphous sample is hydrophilic and crystalline sample hydrophobic with water contact angles of 68° and 92° respectively. The role of surface wettability on photocatalysis is also analysed by modifying the film surface with the help of plasma treatment. The treatment reduced the hydrophilicity of the amorphous surface and at the same time transformed the crystalline surface from hydrophobic to hydrophilic. The change in the surface morphology such as formation of micro/nanostructures and variation in roughness, which contribute to the hydrophobicity is observed through AFM. The surface changes are reflected in the dye degradation efficiencies as well. The dye degradation efficiency reduced to 79 % in the amorphous sample and increased to 72 % in the crystalline sample. The study unveils the exciting efficiency of the hydrophilic, amorphous Sb2S3 films without any thermal or surface treatments as photocatalyst.