1. is coupling the TiO2 with wide band

1.
Introduction

Environmental pollution is one
of the major global challenges faced by human beings. Semiconductor
photocatalysis is the most efficient method for decomposing organic pollutants
in aqueous media 1. Among various
semiconductor photocatalysts that have been studied, TiO2
has attracted much attention as a photocatalyst due to its desirable properties
such as low
cost, strong oxidizing power, nontoxicity, photostability, and chemical
inertness 2. However, practical application of TiO2 in
photocatalytic reactions is obstructed
by two essential drawbacks: wide energy gap (3–3.2 eV) that limits its
application to ultraviolet region and fast recombination of electron–hole
pairs, which are generated after photon absorption when the TiO2 is
irradiated with energy equal to or higher than its band gap 3. One of the approaches
applied to solve these problems is to change the energy structure of TiO2,
i.e. to extend optical absorption range from ultraviolet to visible region and decrease
the electron/hole recombination rate 4.

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An
effective method for improved photocatalytic efficiency of TiO2 is coupling the TiO2 with wide band gap semiconductors such
as WO3, V2O5, SnO2, CdS, CdSe, etc.
5. CdS has relatively low band gap energy (~2.3 eV) and its mixing with TiO2
enhances the photocatalytic activity of TiO2/CdS system not only
because of promotion of visible light absorption, but it also features better
separation of photogenerated electron–hole pairs 6-8. The position of CdS
conduction and valence band gap edges enables the injection of photoexcited
electrons from conduction band of CdS into the low-lying conduction band of TiO2.
On the other hand, the holes generated in CdS valence band cannot be transferred
to valence band of TiO2 because CdS valence band is more cathodic
than that of TiO2. The recombination between photogenerated electrons
and holes is suppressed as a result of the separation effect and overall photocatalytic
activity TiO2/CdS system is improved.

Hydrothermal method 9, liquid ion-exchange technique 10, sol–gel
method 11, solvothermal method 12, etc. have been used to prepare TiO2/CdS photocatalysts. In this
work, we applied high-voltage plasma electrolytic oxidation process 13,14 of
titanium in alkaline electrolyte containing CdS particles for the formation of TiO2/CdS
photocatalyst. Generally,
PEO is considered a valuable pathway for the formation of mixed oxide coatings.
High temperatures and pressures present inside of the micro-discharging
channels cause the melting of the substrate material which reacts with
electrolyte (which is much cooler), thus solidifying and crystallizing quickly
upon being ejected from the micro-discharge channel. This process repeats
randomly over the substrate surface, resulting in the formation of relatively
uniform oxide coating 14. In-situ incorporation of particles into the PEO coatings
has been explored as new strategies to provide the coatings with a wider range
of compositions and functionalities 15. CdS particles have negative zeta
potential in alkaline media 16 which promotes their movement toward the
titanium anode. CdS particles have melting point around 1750 °C, hence locally high temperature induced
at the micro-discharging sites (~5000 °C) during PEO of titanium 17 should result in deposition
of CdS particles on the surface of coatings.