TiO 2 –ZnO Nanocomposites for Photodegradation of Dyes …
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methylene blue along with the inactivation of certain bacteria. The authors attribute
this effect to the higher concentration of active Oxygen forms induced on TiO 2 .
In addition to the high photodegradation activity of methylene blue ZnO
nanopowder also demonstrates greater efficiency to the destruction of naproxen
among the tested samples, but with a significantly smaller advantage over TiO 2 .
It should be noted that no information on the ZnO usage as photocatalyst in the
degradation reaction of naproxen has been found in the literature. At the same time,
information on the use of TiO 2 for the destruction process of naproxen is present in the
literature [27–29]. The analysis of the literature search shows that ZnO demonstrated
greater efficiency in the reaction of degradation of naproxen (the rate of photocatalytic degradation is 0.00764 min compared with the rate of 0.006 min for TiO 2 in
the similar reaction).
The authors of the following work [30] prefer the finding a scientific approach
to explain the synergism and efficiency of photocatalytic activity of ZnO/TiO 2 catalysts precisely in the methods of catalyst synthesis (sol–gel method and deposition
method) and in the mass ratio of ZnO to TiO 2 (ZnO:TiO 2 = 0.1…0.9). The authors
argue that the crystalline structure, morphological, optical, catalytic properties of
the composites are directly dependent on the mass ratio of the components and the
synthesis method of ZnO–TiO 2 systems.
The study of the photocatalytic properties of Zn(SG)–Ti–R (sol–gel method) and
Zn(Pr)–Ti–R (deposition method) was conducted using degradation of Methyl orange
dye in aqueous solution under the action of UV irradiation (λ = 365 nm). In their
previous studies [31], the authors reported the influence of methods and the nature
of precursors on the morphology, structure, optical properties, and photocatalytic
properties of synthesized ZnO. It is known [32–34] that the addition of ZnO to TiO 2
leads to the formation of the crystalline phase of zinc titanate (ZnTiO 3 , Zn 2 TiO 4 ).
X-ray studies showed that crystalline anatase phase is present in pure TiO 2 , but with
an increase in the ZnO mass percentage in the sample, the intensity of the anatase
peaks markedly decreases and, at mass ratio of 0.5, the Zn(SG)–Ti–R sample becomes
amorphous. At a mass ratio of 0.67–0.9, the ZnO phase (wurtzite) appears. In contrast,
the Zn(Pr)–Ti–R sample with ZnO content of 0.33% is characterized by the reflexes
inherent the Zn 2 TiO 4 titanate phase, and the wurtzite peaks are observed when the
ZnO content is increased to 0.8…0.9%.
It should be noted that in no case crystalline phases of TiO 2 and ZnO–wurtzite
are presented simultaneously in the samples obtained by different methods (sol–gel
method and deposition method). This fact can be explained by the presence of these
phases in the amorphous state, which is correlated with the results of studies [35].
The study of photocatalytic activity led to the conclusion that the activity of the
Zn(SG)–Ti–R sample is better with a mass content of ZnO ≥ 0.67 (MO conversion ~
97%). Whereas in the case of the Zn(Pr)–Ti–R sample, the methyl orange conversion
decreases with a mass content of ZnO = 0.2 and gradually increases to 90% with an
increase in the ratio of components to 0.9 (Fig. 5).
Increasing the initial concentration of the dye in all cases showed a decrease in
photocatalytic activity in the process of its photodegradation. A similar effect has
been claimed previously by many authors [36–38] and is explained by the fact that,
727
methylene blue along with the inactivation of certain bacteria. The authors attribute
this effect to the higher concentration of active Oxygen forms induced on TiO 2 .
In addition to the high photodegradation activity of methylene blue ZnO
nanopowder also demonstrates greater efficiency to the destruction of naproxen
among the tested samples, but with a significantly smaller advantage over TiO 2 .
It should be noted that no information on the ZnO usage as photocatalyst in the
degradation reaction of naproxen has been found in the literature. At the same time,
information on the use of TiO 2 for the destruction process of naproxen is present in the
literature [27–29]. The analysis of the literature search shows that ZnO demonstrated
greater efficiency in the reaction of degradation of naproxen (the rate of photocatalytic degradation is 0.00764 min compared with the rate of 0.006 min for TiO 2 in
the similar reaction).
The authors of the following work [30] prefer the finding a scientific approach
to explain the synergism and efficiency of photocatalytic activity of ZnO/TiO 2 catalysts precisely in the methods of catalyst synthesis (sol–gel method and deposition
method) and in the mass ratio of ZnO to TiO 2 (ZnO:TiO 2 = 0.1…0.9). The authors
argue that the crystalline structure, morphological, optical, catalytic properties of
the composites are directly dependent on the mass ratio of the components and the
synthesis method of ZnO–TiO 2 systems.
The study of the photocatalytic properties of Zn(SG)–Ti–R (sol–gel method) and
Zn(Pr)–Ti–R (deposition method) was conducted using degradation of Methyl orange
dye in aqueous solution under the action of UV irradiation (λ = 365 nm). In their
previous studies [31], the authors reported the influence of methods and the nature
of precursors on the morphology, structure, optical properties, and photocatalytic
properties of synthesized ZnO. It is known [32–34] that the addition of ZnO to TiO 2
leads to the formation of the crystalline phase of zinc titanate (ZnTiO 3 , Zn 2 TiO 4 ).
X-ray studies showed that crystalline anatase phase is present in pure TiO 2 , but with
an increase in the ZnO mass percentage in the sample, the intensity of the anatase
peaks markedly decreases and, at mass ratio of 0.5, the Zn(SG)–Ti–R sample becomes
amorphous. At a mass ratio of 0.67–0.9, the ZnO phase (wurtzite) appears. In contrast,
the Zn(Pr)–Ti–R sample with ZnO content of 0.33% is characterized by the reflexes
inherent the Zn 2 TiO 4 titanate phase, and the wurtzite peaks are observed when the
ZnO content is increased to 0.8…0.9%.
It should be noted that in no case crystalline phases of TiO 2 and ZnO–wurtzite
are presented simultaneously in the samples obtained by different methods (sol–gel
method and deposition method). This fact can be explained by the presence of these
phases in the amorphous state, which is correlated with the results of studies [35].
The study of photocatalytic activity led to the conclusion that the activity of the
Zn(SG)–Ti–R sample is better with a mass content of ZnO ≥ 0.67 (MO conversion ~
97%). Whereas in the case of the Zn(Pr)–Ti–R sample, the methyl orange conversion
decreases with a mass content of ZnO = 0.2 and gradually increases to 90% with an
increase in the ratio of components to 0.9 (Fig. 5).
Increasing the initial concentration of the dye in all cases showed a decrease in
photocatalytic activity in the process of its photodegradation. A similar effect has
been claimed previously by many authors [36–38] and is explained by the fact that,