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O. Yanushevska et al.
or reflect certain wavelengths when light is incident on them. They are used together
for the manufacture of dyes [10].
Most organic dyes and pigments that are used in the industry, getting into the
body in the form of dust, lead to a general toxic effect. They cause conjunctivitis, skin
diseases of open body parts mainly, in severe cases spread throughout the body, sometimes because of contact with colored products. Numerous occupational diseases can
arise not from the organic dyes and pigments themselves, but from the toxic impurities of intermediates or from dangerous metabolites formed from organics in the
body. Wastewater treatment, which simultaneously contains more than one category of pollutants, i.e., pharmaceuticals and dyes, is a serious problem of modern
pharmacological, food, and textile production. Preparation of photocatalysts that are
effective for solving these problems is the goal of many scientific studies [9].
Such methods as adsorption, membrane filtration, coagulation/flocculation, ion
exchange, microbial degradation, and advanced oxidation processes are successfully
used in modern technologies to remove dyes from wastewater. In comparison with
most of the above methods, adsorption has advantages because of its high efficiency,
selectivity, ease of operation, and design of equipment. Instead, when using sorbents,
there is a problem of regeneration of waste sorption materials, which only accumulate
fluid or gas-phase pollutants but do not expose them to degradation, eliminating
toxicity [10].
Methods based on oxidation reactions, such as heterogeneous photocatalysis, are
considered to be the best among many methods of water purifying from organic
compounds (petroleum products, dyes, bacteria, etc.). The variety of photocatalysts
application brings this method to the level of a separate scientific direction in the
processes of neutralization of most organic pollutants. Several semiconductors (e.g.,
TiO 2 , ZnO, Fe 2 O 3 , CdS, GaP, and ZnS) have been successfully used as advanced
oxidation photocatalysts. Nanostructured TiO 2 is the most studied material among
the above-mentioned so far. In the recent researches, however, ZnO emerges as a
strong competitor to TiO 2 in wastewater treatment technologies.
2.2 Prerequisites for Creating of TiO 2 –ZnO Composites
Titanium oxide is mechanically strong, chemically resistant to dilute solutions of
acids and alkalis, its own solubility is low, it does not give secondary contaminants
(unlike the used natural minerals). The sorption activity of TiO 2 to various naturally
occurring impurities is high due to the presence on its surface of both basic (TiO
δ− )
and acid (Ti
δ+ ) centers.
Zinc oxide is a semiconductor and promising material due to its favorable piezoelectric and pyroelectric properties, which are used in optoelectronics, thin-film transistors, biosensors, gas sensors, bactericidal agents, and photocatalysis. Chemical
stability, low toxicity, and resistance to oxidation contribute to these applications. It
is noted that the morphology, crystal size, and orientation of ZnO nanocrystals play
a key role in achieving the desired response [11]. This is due to the fact that, despite
O. Yanushevska et al.
or reflect certain wavelengths when light is incident on them. They are used together
for the manufacture of dyes [10].
Most organic dyes and pigments that are used in the industry, getting into the
body in the form of dust, lead to a general toxic effect. They cause conjunctivitis, skin
diseases of open body parts mainly, in severe cases spread throughout the body, sometimes because of contact with colored products. Numerous occupational diseases can
arise not from the organic dyes and pigments themselves, but from the toxic impurities of intermediates or from dangerous metabolites formed from organics in the
body. Wastewater treatment, which simultaneously contains more than one category of pollutants, i.e., pharmaceuticals and dyes, is a serious problem of modern
pharmacological, food, and textile production. Preparation of photocatalysts that are
effective for solving these problems is the goal of many scientific studies [9].
Such methods as adsorption, membrane filtration, coagulation/flocculation, ion
exchange, microbial degradation, and advanced oxidation processes are successfully
used in modern technologies to remove dyes from wastewater. In comparison with
most of the above methods, adsorption has advantages because of its high efficiency,
selectivity, ease of operation, and design of equipment. Instead, when using sorbents,
there is a problem of regeneration of waste sorption materials, which only accumulate
fluid or gas-phase pollutants but do not expose them to degradation, eliminating
toxicity [10].
Methods based on oxidation reactions, such as heterogeneous photocatalysis, are
considered to be the best among many methods of water purifying from organic
compounds (petroleum products, dyes, bacteria, etc.). The variety of photocatalysts
application brings this method to the level of a separate scientific direction in the
processes of neutralization of most organic pollutants. Several semiconductors (e.g.,
TiO 2 , ZnO, Fe 2 O 3 , CdS, GaP, and ZnS) have been successfully used as advanced
oxidation photocatalysts. Nanostructured TiO 2 is the most studied material among
the above-mentioned so far. In the recent researches, however, ZnO emerges as a
strong competitor to TiO 2 in wastewater treatment technologies.
2.2 Prerequisites for Creating of TiO 2 –ZnO Composites
Titanium oxide is mechanically strong, chemically resistant to dilute solutions of
acids and alkalis, its own solubility is low, it does not give secondary contaminants
(unlike the used natural minerals). The sorption activity of TiO 2 to various naturally
occurring impurities is high due to the presence on its surface of both basic (TiO
δ− )
and acid (Ti
δ+ ) centers.
Zinc oxide is a semiconductor and promising material due to its favorable piezoelectric and pyroelectric properties, which are used in optoelectronics, thin-film transistors, biosensors, gas sensors, bactericidal agents, and photocatalysis. Chemical
stability, low toxicity, and resistance to oxidation contribute to these applications. It
is noted that the morphology, crystal size, and orientation of ZnO nanocrystals play
a key role in achieving the desired response [11]. This is due to the fact that, despite