Modulating the photocatalytic activity of Ag nanoparticles-titanate nanotubes heterojunctions through control of microwave-assisted synthesis conditions

Abstract

Protonic titanate nanotubes (HTiNTs) have been widely used as photocatalyst for dye degradation, however, due to its high band-gap energy (∼3.2 eV), it can only absorb UV light, which is mainly disadvantageous when sunlight radiation is used as light source. The significant portion of the fallout from the Sun to the Earth corresponds to visible light, whereas the UV is only around 4 %. Thus, it is critical to develop strategies to sensitize this kind of photocatalyst to visible radiation. In this work, silver nanoparticles (AgNP)-HTiNTs heterojunctions were prepared, aiming to produce a visible light sensitive catalyst. The AgNPs were prepared by simple microwave-assisted routes, using water (Ag(H2O)), ethanol (Ag(EtOH)) and ethylene glycol (Ag(EG))as solvents, and sodium borohydride as reducing agent. The obtained AgNPs exhibit different physical-chemical properties, such as shapes, sizes and crystalline structures. As a result, the heterojunctions have different photocatalytic efficiencies for rhodamine B degradation under simulated sunlight irradiation: 72 % using Ag(EG)-HTiNT, 46 % using Ag(EtOH)-HTiNT and 33 % using Ag(H2O)-HTiNT as catalysts. The changes on the catalytic activities were ascribed to the plasmonic effect, lowering of surface area and more negative zeta potentials caused by the presence AgNPs. To better understand the photocatalytic response, the amount of radical species generated by the materials was determined by using spectroscopic methods.