2. Zinc oxide

Zinc oxide (ZnO) is a II-VI group semiconductor with wurtzite structure with lattice parameters a = 0.3296 nm and c = 0.52065 nm. Zn2+ atoms are tetrahedrally coordinated with O2 atoms stacked alternately along the c axis so that d-electrons of zinc are hybridized with 2p-electrons of oxygen [2]. ZnO has the potential to become an important material for photocatalysis because it is nontoxic (it is often included in pharmaceuticals), photostable, and low cost.

We consider at least three windows of opportunity to boost ZnO as photocatalyst in academy and industry. The first is related to the quantum yield in visible light rather than UV for the efficient utilization of the electron-hole (e/h<sup>+</sup> ) pairs during decomposition of endocrine disruptors and emergent contaminants in water. A second opportunity is the immobilization of the photocatalyst that could make possible its utilization on continuous mode and toward implementation at industrial scale. The third challenge deals with nonlinear mathematical modeling of the factors that rule the photocatalytic kinetics of the metallic-modified ZnO for optimization purposes. This is not a trivial problem in photocatalysis and artificial neural network tools (ANNs) do not demand of academic expertise for a fast implementation; for example, in the industry sector. The reader can consult [3] for the approach that Meimaroglou and collaborators follow to assess ZnO photocatalyst structure-photocatalytic performance associations toward the maximization of the photocatalytic efficiency. The first two challenges are addressed in the present chapter.
