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O. Yanushevska et al.
synthesis, were investigated. In the first stage of this synthesis, 3D urchin-like TiO 2
nanospheres were grown, in the second stage 1D ZnO structures were synthesized.
According to the BET, the surface area of the ZnO–TiO 2 composite was 187.3 m
2 /g,
a pore diameter of about 8 nm. The photocatalysis results showed a high catalytic
activity of ZnO–TiO 2 compared to the pure ZnO (NPs), TiO 2 (P25) and urchinlike structure (TiO 2 ). Moreover, the photodegradation of both pollutants improves
with increasing irradiation time, which indicates their complete decomposition. The
authors explain this synergy by the fact that, unlike the short «life» time of the photoinduced charge carrier in a single semiconductor, rapid separation of photogenerated
electrons and holes in the ZnO–TiO 2 composite occurs due to the high recombination rate of the «electron–hole» pairs. This caused by close phase interaction in
the ZnO/TiO 2 surface heterostructure. Such heterostructure also contributes to the
broadening of the incident light spectrum, which is able to intensify photocatalysis.
Thus, the above studies prove that the use of nanocomposite photocatalysts based
on binary TiO 2 –ZnO systems can be an effective method for removing hazardous
organic toxicants from wastewater. This process is facilitated by the features of the
surface structure of TiO 2 –ZnO systems and the synergistic effect resulting from the
activation of the adsorption process of organic molecules and increasing in the rate
of charge transfer in «electron–hole» systems.
The synergistic effect in the photocatalytic decomposition of dyes on binary
ZnO–TiO 2 systems observed in the above works is also confirmed by the study
[21]. According to which, the highest photodegradation efficiency of methylene blue
and naproxen (NPX) is inherent in zinc oxide. It was found that the photocatalytic
activity of ZnO–TiO 2 depends on the type of zinc titanate molecule (Zn 2 TiO 4 ) but
is insignificantly higher than the photoactivity of TiO 2 nanoparticles.
In [22], commercial ZnO and TiO 2 nanopowders and a mixed ZnO–TiO 2
nanopowder (molar ratio of 2:1) were compared. Specific surface measurements
showed that TiO 2 has a much larger surface area (9.3 m
2 /g) than ZnO (5.1 m
2 /g), but
the structural features of the ZnO nanopowder have a dominant effect on the specific
surface area of the ZnO–TiO 2 mixture (4.7 m
2 /g). This fact confirms the absence
of a clear correlation between the photocatalytic activity and the specific surface
area of the metal oxides but leads to the conclusion about the significant influence
of the ZnO nanoparticles geometry (nanorod structure) on the selective and active
properties of the photocatalyst. The X-ray diffraction method showed the presence
of the new phase of zinc titanate (Zn 2 TiO 4 ) corresponds to the cubic spinel structure,
in addition to the two initial phases of ZnO and TiO 2 metal oxides (hexagonal ZnO
structure and tetragonal TiO 2 structure anatase). The calculated average values of
crystallites are 83, 74, and 84 nm for ZnO, Zn 2 TiO 4, and TiO 2 , respectively. This
fact also testifies to the dominant influence of the ZnO nanorod structure rather than
its particle size.
According to a literature review of scientific articles [23–25], the authors observed
a similar behavior of ZnO in comparison with TiO 2 , which is explained by the
higher efficiency of generation, mobility, and separation of photoinduced electrons
and «holes» in ZnO. However, the authors also cited the facts [26] that such ZnO
properties are not a general rule and TiO 2 was more active during the degradation of
O. Yanushevska et al.
synthesis, were investigated. In the first stage of this synthesis, 3D urchin-like TiO 2
nanospheres were grown, in the second stage 1D ZnO structures were synthesized.
According to the BET, the surface area of the ZnO–TiO 2 composite was 187.3 m
2 /g,
a pore diameter of about 8 nm. The photocatalysis results showed a high catalytic
activity of ZnO–TiO 2 compared to the pure ZnO (NPs), TiO 2 (P25) and urchinlike structure (TiO 2 ). Moreover, the photodegradation of both pollutants improves
with increasing irradiation time, which indicates their complete decomposition. The
authors explain this synergy by the fact that, unlike the short «life» time of the photoinduced charge carrier in a single semiconductor, rapid separation of photogenerated
electrons and holes in the ZnO–TiO 2 composite occurs due to the high recombination rate of the «electron–hole» pairs. This caused by close phase interaction in
the ZnO/TiO 2 surface heterostructure. Such heterostructure also contributes to the
broadening of the incident light spectrum, which is able to intensify photocatalysis.
Thus, the above studies prove that the use of nanocomposite photocatalysts based
on binary TiO 2 –ZnO systems can be an effective method for removing hazardous
organic toxicants from wastewater. This process is facilitated by the features of the
surface structure of TiO 2 –ZnO systems and the synergistic effect resulting from the
activation of the adsorption process of organic molecules and increasing in the rate
of charge transfer in «electron–hole» systems.
The synergistic effect in the photocatalytic decomposition of dyes on binary
ZnO–TiO 2 systems observed in the above works is also confirmed by the study
[21]. According to which, the highest photodegradation efficiency of methylene blue
and naproxen (NPX) is inherent in zinc oxide. It was found that the photocatalytic
activity of ZnO–TiO 2 depends on the type of zinc titanate molecule (Zn 2 TiO 4 ) but
is insignificantly higher than the photoactivity of TiO 2 nanoparticles.
In [22], commercial ZnO and TiO 2 nanopowders and a mixed ZnO–TiO 2
nanopowder (molar ratio of 2:1) were compared. Specific surface measurements
showed that TiO 2 has a much larger surface area (9.3 m
2 /g) than ZnO (5.1 m
2 /g), but
the structural features of the ZnO nanopowder have a dominant effect on the specific
surface area of the ZnO–TiO 2 mixture (4.7 m
2 /g). This fact confirms the absence
of a clear correlation between the photocatalytic activity and the specific surface
area of the metal oxides but leads to the conclusion about the significant influence
of the ZnO nanoparticles geometry (nanorod structure) on the selective and active
properties of the photocatalyst. The X-ray diffraction method showed the presence
of the new phase of zinc titanate (Zn 2 TiO 4 ) corresponds to the cubic spinel structure,
in addition to the two initial phases of ZnO and TiO 2 metal oxides (hexagonal ZnO
structure and tetragonal TiO 2 structure anatase). The calculated average values of
crystallites are 83, 74, and 84 nm for ZnO, Zn 2 TiO 4, and TiO 2 , respectively. This
fact also testifies to the dominant influence of the ZnO nanorod structure rather than
its particle size.
According to a literature review of scientific articles [23–25], the authors observed
a similar behavior of ZnO in comparison with TiO 2 , which is explained by the
higher efficiency of generation, mobility, and separation of photoinduced electrons
and «holes» in ZnO. However, the authors also cited the facts [26] that such ZnO
properties are not a general rule and TiO 2 was more active during the degradation of