1.2 Research question:
1. What are the parameters that affect the morphology of TiO2/ZnO composite ?
2. What are the techniques available to synthesize TiO2/ZnO composite?
3. What are the factors that influence the efficiency of photocatalytic activity?
2.3 Effect of morphology on TiO2/ZnO composite as photocatalyst
Morphology plays an important role in improving photocatalytic performance as it
influences its activity. Therefore, in order to achieve optimum photocatalytic activity,
the
morphology of the catalyst was studied according to a number of parameters. Effect on
morphology based on parameters namely presence of acid, pre-treatment with different
concentration of NaOH, molar ratio of catalyst, and calcination temperature. The synthesis of the
composite is prepared either via Sol-gel technique or hydrothermal method in order to study the
effect of morphology in affecting the photocatalytic activity of TiO2/ZnO.
2.3.1 Presence of acid
HCl plays its role as a catalyst in forming brookite and rutile phase in Sol gel method. The
hydrochloric acid solution could greatly influence the properties of the composites and it could
also enhance the crystallinity of the photocatalyst.
According to (Alkaim et al.,2014), the preparation of TiO2 was prepared with sol-gel and
hydrothermal. The two series of methods are divided by 5 samples each. To distinguish the sample
prepared, the synthesis materials are labeled as TiO2-XM-SG and TiO2-XM-HT to indicate that
one of the samples is made by using hydrothermal and sol gel method. Both of the method samples
were treated with different amounts of molar concentration of HCL (0M, 0.8M, 1M, 5M and 12M).
The XM labels indicate the molar concentration of HCL.
Figure 1 shows the distribution of crystalline and amorphous phase in TiO2 synthesis with
hydrothermal and sol gel method with different amounts of molar concentration of acid. Based on
the result, anatase phase only appears to the sample TiO2 0M and 12M by sol gel method while
other samples show that anatase phase is decreasing and brookite and rutile phase start to appear.
For the hydrothermal method, only sample TiO2 with 0M contains only anatase phase while others
sample when treated with HCL the amount of anatase decreases but brookite and rutile starts to
appear. The sample that in contrast between sol-gel and hydrothermal is the TiO2 with 12M
contains mixture of anatase and brookite while the 12M TiO2 sol-gel only anatase appears.
Figure 1. Distribution of crystalline and amorphous phases in TiO -SG and TiO -HT
samples.
(Alkaim et al.,2014), stated that a high anatase content, low brookite and rutile content will
result in the increase of surface area and when this happens, the photocatalytic activity improves.
Figure 2 illustrates the photocatalytic activity of each sample made by using Sol gel and
hydrothermal method. It is proven from the result in Figure 2 that sample 5M TiO2 with sol gel
and 0.8M TiO2 with hydrothermal shows that the photocatalytic improves where there is a higher
anatase content with lower brookite and rutile content.
Figure 2 photocatalytic activity of TiO2 samples with different concentration of acid
From the result, both samples crystalline phases increase with increasing the HCL
concentration. To conclude, the presence of HCl is necessary in the sol-gel one to enhance
crystallization. However, under hydrothermal conditions the crystallization process is more
controlled and the absence of HCl is less significant. In general, the hydrothermal method leads to
more crystalline samples, with a slightly larger average crystal size than the sol-gel method. It can
be derived that anatase and rutile mixture crystals help to increase the surface area of the catalyst.
In general, both Sol-Gel and hydrothermal methods are ideal in obtaining photocatalysts with large
surface area. However, the hydrothermal method has larger surface areas as compared to the Sol
gel method.
In another study conducted by (Kwiatkowski et al., 2017), HCI was also used in Sol Gel
method however, ZnO/TiO2 composite was further synthesised in this study. It is further observed
that such treatment of the sample with HCI would lead to the shift of the spectrum back towards
the UV range where it results in the composite to increase in their UV-vis absorption which
enhances the photocatalytic activity. Figure 3 indicated that the decolorization rate of MB
significantly decreased after the treatment of ZnO/TiO2 photocatalyst with HCI.
Based on these two authors, it can be deduced that HCI helps in transforming the
morphology of the catalyst and it also improves the absorption spectra of the catalyst.
Figure 3. Photo decomposition of MB on catalyst ZnO/TiO2 with HCL and ZnO/TiO2
2.3.2 Varying temperature
Calcination temperature plays an important role in controlling the particle size, which in
turn helps to modify structural and size of the samples. Calcination temperature also influences
the photocatalytic activity of a catalyst, since it affects the content in the morphology.
The TiO2/ZnO composite powder with the same molar ratio was prepared by sol- gel
method. The composite was treated with different thermal calcination temperatures at 500, 600
and 700°C in air. Based on the SEM result both microscale and nanoscale particles were observed.
The result shows that when calcination temperature increased from 500°C to 600 °C the particle
surface of the composite had a smooth surface, while the sample treated with 700°C the surface
had a rough surface. (Wang et al., 2013) reports that large surface area will help in enhancing
photocatalytic activity. During this process, TiO2 reacted with ZnO to form h-ZnTiO3 crystallites
via an extensive chemical reaction. The derived powder shows a rather rough surface. As a result,
enhanced photocatalytic efficiency was achieved for the powder.
Figure 4 SEM morphologies at low magnification and at high magnification of the TiO2/ZnO
composite powder at various temperatures of 500, 600, and 700 °C.
A further reference could be made in (Mazinani, Zalani, Sakaki and Yanagisawa, 2018)
study, where ZnO/TiO2 composite were synthesized via one step hydrothermal. The samples were
heat treated at 400, 600, 800 and 1000 °C. Based on the XRD results in figure 5, samples with 0,
400 and 600 exhibit similar patterns which are anatase crystalline phases of TiO2. At higher
calcination temperature 800°C the result shows that a new diffraction peak appear which indexed
to TiO2·2ZnO besides anatase and wurtzite crystals. Below 800 °C only anatase can be seen, this
result indicates that adding ZnO can inhibit the anatase and rutile phase transformation. However
when the sample is treated at 1000°C transformation of anatase to rutile begins to take place. We
can conclude that higher calcination temperature can improve the crystallinity of a catalyst.
Despite using different method which is via sol-gel method and one step hydrothermal, a
similar and one conclusion could be inferred in which when observed, according to (Mazinani,
Zalani, Sakaki and Yanagisawa, 2018), increasing the calcination temperature also leads to
improve the degree of crystallinity and effectively eliminating the bulk defect positions for the
recombination of the photoinduced electron–hole pairs. Author also mentions that increasing the
calcination temperature would also increase the surface area and pore volume of the catalyst.
Therefore, increase in surface area can improve photocatalytic activity.
Figure 5 XRD pattern of catalyst composite at different temperatures.(Mazinani, Zalani, Sakaki
and Yanagisawa, 2018)
According to (Wang et al., 2013), via sol-gel process, the photocatalytic behaviour of
TiO2/ZnO composite powder is related to the crystallinity and particle morphology of the
composite powder. Photodegradation of methyl orange was also used in evaluating its
photocatalytic performance and figure 6 illustrates the rate of MO decomposition by TiO2/ZnO at
different calcination temperatures. It is clear from the result that the decomposition for MO
solution catalyzed by the composite powder was definitely lower than that without the catalyst.
Specifically, the decomposition MO decreases when the calcining temperature increases and this
indicates that the photocatalytic efficiency of the composite powder has been improved from the
increase of the calcining temperature.
It is put out that the photocatalytic activity of TiO2 catalyst is dependent on both the
crystallinity and specific surface area of the material therefore, it could be observed that large
specific surface area is helpful to the improvement of photocatalytic efficiency of TiO2 catalyst.
In conclusion, the crystallinity of the composite powder was visibly enhanced with the increase of
the calcining temperature from 500 degree celsius to 700 degree celsius. The particles calcined at
higher temperature also showed a rough surface and as a result, the photocatalytic efficiency of
the powder was notably intensified.
Figure 6. Decomposition of MO at different calcination temperatures.
However, referring to (Mazinani, Zalani, Sakaki and Yanagisawa, 2018), via hydrothermal
method, the photocatalytic efficiency was measured as according to figure 7 where increasing the
calcination temperature until 800 degree C has promoted the reaction rate of the samples. Higher
calcination temperatures could improve the degree of crystallinity and at a very high calcination
temperature (1000degree C), the reaction rate constant of the sample has dropped dramatically. In
addition, based on this article, it is observed that a higher surface area, pore sizes, and pore volume
can improve the photocatalytic performance by creating more active sites for the photochemical
process.
Figure 7. Photocatalytic activity of TiO2/ZnO under different temperature
2.3.3 Pre-treatment with NaOH
The role of Sodium hydroxide (NaOH) is used to introduce the hydroxyl radical (OH-) in
solutions and the function of OH- is to help in shaping the morphology of the catalyst in terms of
size. In addition, hydroxyl radical (OH) also plays a function in improving photocatalytic
degradation.
For TiO2, it was treated with different amounts of NaOH concentrations (2, 5, 10 and 15).
The morphology and the crystal structure of TiO2 was observed. Based on the XRD result by
(Hou, Hu and Zhu, 2018), only high purity of anatase appeared when TiO2 is not treated with
NaOH. However, increasing the concentration of NaOH to 10M a presence of rutile is seen but
when increasing the concentration until 15M the sample crystallinity becomes poor. The author
mentions that at low concentrations of NaOH (2M and 5M) the surface area of the catalyst is large
but the pore size is small therefore, the crystallinity of the 2M and 5M sample is observed to be
decent. Meanwhile for 10M samples the surface area is moderate but the pore size is larger than
the low concentration samples hence, the 10M sample is noted to be better than both the 2M and
the 5M sample. For the 15M sample the surface area is small and the pore size is moderate. As it
has been reported that large surface area and pore size of a catalyst can help in improving
photocatalytic activity. From the result we can draw a conclusion that as the concentration of
sodium hydroxide increases, the morphology changes. Introducing NaOH in the solution proved
it can improve the morphology of TiO2. In the result, sample 10M is the best amount of NaOH to
improve the morphology because in the sample anatase and rutile mixture are present and help to
increase the surface area (Hou, Hu and Zhu, 2018).
Figure 8, shows the photocatalytic performance of MB samples with and without the
presence of NaOH solution. From the result the degradation of MB is improved when NaOH was
introduced. Sample 10M shows the best performance followed by 2M, 5M and 15M. However,
when there was no photocatalyst without light, only light with a catalyst, and when there was no
catalyst, the concentration of methylene blue almost did not change.
Figure 8.photocatalytic performance of MB samples;
Based on this article, it is also observed that an experiment on the ideal amount of catalyst
for 10M was also conducted hence when altering the amount of 10 M catalyst, 20 mg of 10M
shows the best amount that can degrade MB when compared with 5mg, 15mg, 20mg, 30mg, and
50mg of 10 M catalyst in figure 9.
Figure 9.The effect of the different amount of 10 M samples on methylene blue
The best performance that can degrade methylene blue is the 10M sample. This is mainly
because the 10M consists of rutile-mixed and anatase crystals that result in the best degradation of
methylene blue. In the entire process of degrading methylene blue, the amount of catalyst also
plays an important role. Introducing NaOH inside the catalyst solution is important because it
changes the morphology of the crystal where it can introduce another type of crystal phase that
helps in improving the photocatalytic properties.
2.3.4 Molar Ratio of TiO2/ZnO
The preparation of the TiO2/ZnO composite is done by electrospinning method. The
surface morphology of the composite is observed by using Field-emission Scanning Electron
Microscopy (FESEM). For the fabrication of TiO2/ZnO composite nanofibers, different
proportions of TiO2/ZnO solutions (1:1, 1:2, 2:1 and 1:3) are used. Based on the result of FESEM
in figure 10, when the TiO2/ZnO at (1:1) the surface morphology is smooth and the nanofibers are
aligned. When the ratio is at (1:2) TiO2/ZnO nanofibers were noticed to be thinner and nonuniform. For sample TiO2/ZnO (2:1) the morphology is smoother compared to previous results.
However for sample TiO2/ZnO (1:3) the formation of rough and well-aligned nanofibers is
observed. It means the roughness of the fibers is enhanced with the increased concentration of ZnO
solution. (Someswararao, Dubey and Subbarao, 2021) also stated that doubling the ZnO in the
TiO2/ZnO will decrease the diameter of the fiber and result in a rough morphology. The author
also mentions that TiO2/ZnO (1:3) show the highest degradation of dye compared to other
samples. From this result we can conclude that rough morphology can enhance the photocatalytic
activity of a catalyst.
Figure 10. Surface morphology of TiO2/ZnO molar ratio (a) TiO2/ZnO (1:1), (b) TiO2/ZnO
(2:1), (c) TiO2/ZnO (1:2) and (d) TiO2/ZnO (1:3)
A different approach was adopted in (Moradi et al., 2016) where a different molar ratio of
ZnO by way of Sol-gel method was used in preparing the TiO2/ZnO nanocomposite as a
photocatalyst where the composite is further divided into five different molar ratio of 0:50, 1:50,
3:50, 5:50, 10:50 and 15:50. The photocatalytic activity of the synthesized nanocomposites was
investigated for decolorization of Methylene blue (MB) in the water and under the UV irradiation.
The article mentioned that Anatase can be seen in all samples and after increasing the molar ratio
of the catalyst, many other phases can be seen namely Rutile, Zincite, Zinc, Titanium Zinc Oxide
and Titanium Zinc Phase.
Next, reference to the samples could be made in which for Sample 1 (0:50), only anatase
phase of TiO2 was formed. Sample 2 (1:50), shows that there were three phases including Anatase,
Rutile and Zincite. In Sample 3 (3:50) and 4 (5:50), a new phase called Titanium Zinc was formed
after increasing the molar ratio of zinc oxide. Next in Sample 5 (10:50), separate crystallization of
TiO2 and ZnO was observed and with increasing the percentage of the Titanium and Zincite
phases. Lastly in Sample 6 (15:50), where a new phase Zn2TiO4 was formed after further increase
in the molar ratio of zinc oxide. It was clear from the observation that the addition of ZnO affects
the particle morphology of TiO2 and the size distribution of TiO2/ZnO nanocomposite was
improved with increasing ZnO molar ratio except in Sample 6 (Moradi et al., 2016).
According to (Moradi et al., 2016), he states that the regular morphology could enhance
the photocatalytic activity as it could also improve the surface area. However, this regular
morphology could only be seen in Sample 5. While in Sample 6, increases in ZnO molar ratio until
(50:15), shows some large aggregation of particles that increase the particle size and the formation
of Zn2TiO4 somehow causes photo inactive that decreases the photocatalytic activity. Figure 11
shows that decolorization of MB solution proves that sample 5 shows an ideal improvement of
photocatalytic activity due to the surface area being increased while the particle size is smaller.
Sample 6 on the other hand, has a smaller surface area due to the increasing particle size.
This was shown in the result after increasing the molar ratio of Zinc Oxide until (50/10) where the
photocatalytic activity is seen to be enhanced gradually from Sample 1-5. Moreover, the formation
of photo inactive Zn2TiO4 at high ZnO ratio slows down the photocatalytic activity. Therefore,
all in all, Sample 5 has the ideal photocatalytic activity in comparison to other samples.
In comparison, both authors relate to increasing the ratio of ZnO as a factor in improving
the photocatalytic activity. However, according to the experiment conducted in (Someswararao,
Dubey and Subbarao, 2021), it is observed that the increased amount of ratio of ZnO leads to a
rough morphology and this could enhance the photocatalytic performance of a catalyst. (Moradi
et al., 2016) however, specifically observed that excessive amounts of ZnO would result in a large
aggregation of particle which brings to the formation of Zn2Tio4 and this would leads to a
decreasing photocatalytic performance.
Figure 11. Decolorization of MB solution for 5 different sample of TiO2/ZnO
2.3.5 pH value of dye
In this study by (Alkaim et al., 2014), methylene blue was utilized to study the adsorption
capacity and photocatalytic degradation of methylene blue dye by two types of semiconductors
TiO2 and ZnO. At different pH solutions of 4,6,9, and 11, the photocatalytic experiments were
conducted under ambient atmospheric conditions. Before the beginning of irradiation, the mixture
was allowed to equilibrate in the dark while stirring for 60 min. The result for adsorption capacity
of methylene blue for each value of pH solution could be observed in figure 12 where the result
shows that the adsorption capacity increases at higher (pH 4) for the TiO2 and at (pH 9) for ZnO.
Figure 13 shows the result of photocatalytic degradation of methylene blue for both
catalysts where in the presence of TiO2 the photodegradation was significantly enhanced at
medium pH (pH = 6). Additionally, when ZnO is present, the photodegradation was notably
intensified at high pH (pH = 11). It is observed that since the dye molecule is of negative charge
and when the particle surface of the catalyst on the other hand is positively charged, it will result
in the increase of absorption dye that would further lead to the increase of the photocatalytic
degradation. The reason that TiO2 increases in adsorption at pH 6 is due to TiO2 surface charge
being positively charged in pH less than 6 and it is negatively charged in alkaline conditions.
However, for ZnO the surface charge is negatively charged at low pH.
Figure 12. Effect of pH on adsorption capacity of methylene blue on catalysts.
Figure 13. Photocatalytic degradation of methylene blue in presence of different pH
solution; (a) tio2, (b) zno
CONCLUSION
In conclusion, the synthesis of TiO2 / ZnO composites could be conducted via sol gel
method or hydrothermal. Both techniques are suitable in synthesizing the photocatalyst. In
addition, it is both easy, simple and cost effective to be conducted. Morphology plays an important
role in photocatalytic performance. The morphology of the photocatalyst composite can be
controlled by varying their parameters, namely, introduction of acid, varying the calcination
temperature, pre treatment with NaOH, altering the molar ratio of catalyst and treatment with
different PH value of dye.
The result shown for the presence of acid of 5M TiO2 with sol gel and 0.8M TiO2 with
hydrothermal method shows that the photocatalytic improves when there is a higher anatase
content with lower brookite and rutile content. Next, in varying the calcination temperature, it is
observed that the increase of temperature will help to modify the surface structure which also leads
to the increase of surface area and pore volume. The optimum calcination temperature as observed
would be 800 ℃. Moving on to the pre-treatment of NaOH, it is further observed that a new crystal
phase is introduced. NaOH inside the catalyst solution is important because it changes the
morphology of the crystal where it can introduce another type of crystal phase that helps in
improving the photocatalytic properties. In regards to the alteration of molar ratio, it is studied that
doubling the ZnO in the TiO2/ZnO will decrease the diameter of the fiber and result in a rough
morphology and this could enhance the photocatalytic performance of a catalyst. For the last
parameter that affects the photocatalytic activity which is the treatment for different PH value of
dye, the journal observed that dye molecule is of negative charge and when the particle surface of
the catalyst on the other hand is positively charged, it will result in the increase of absorption dye
that would further lead to the increase of the photocatalytic degradation.
These parameters discussed above are required in improving photocatalytic performance
especially from the perspective of sol gel and hydrothermal methods.