In recent years, semiconductor photocatalysis has become more
and more attractive and important since it has a great potential in
solving environmental problems. One of the most important
aspects of environmental photocatalysis is the selection of photoactive
materials. Semiconductor materials like ZnO and TiO2 are
two of the ideal photocatalysts in several respects. Both provide
photo-generated holes with high oxidizing power due to their wide
band gap energy and especially zinc oxide is relatively inexpensiveAs well-known photocatalyst, nanostructured ZnO has received
much attention in the degradation and complete mineralization of
environmental pollutants [1–4]. ZnO and TiO2 are the widely
studied metal-oxide semiconductors with a wide band gap of
3.2 eV and 3.3 eV, respectively [5]. Doping or dispersing it withheteroatoms are paths often used to reduce the metal oxide semiconductor
band gap [6].
According to literature data, the mixed oxides and composite
materials could increase the interface charge transfer thus improving
the response of nanostructure under the UV light irradiation
[7]. It was reported that ZnO–TiO2 mixed oxides featured very good
photocatalytic activity for technological application in photocatalysis,
gas sensing and as bactericidal agent [8]. Several authors have
reported that silver added to TiO2, CuO or ZnO semiconductors
leads to improved photocatalytic properties [9–11].
Photocatalytic activity can be investigated following the degradation
of different dyes such as methylene blue (MB), methyl
orange and astrazon red in water under exposure to UV or visible
light. In the case of aqueous solutions of MB, the photocatalytic
decolorization of MB is initiated by the formation of electron–hole
pair on the surface of the catalyst according to the reaction (1). The
high oxidative potential of the hole ðhþ
VBÞ in the catalyst allows for
the direct oxidation of the dye to reactive intermediates, by
chemical reaction (2) [11]: