CHEMICAL ENGINEERING TRANSACTIONS
VOL. 78, 2020
A publication of
The Italian Association
of Chemical Engineering
Online at www.cetjournal.it
Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš
Copyright © 2020, AIDIC Servizi S.r.l.
ISBN 978-88-95608-76-1; ISSN 2283-9216
Fabrication of TiO2 Monolithic Photocatalyst and Evaluation of
its Antibacterial Activity under Simulated Solar Irradiation
Diep N. Pham, Minh-Vien Le*, Hoang Anh Hoang, Xuan T.T. Tran, Tuan-Anh
Nguyen
Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, VNU-HCM, Ho Chi Minh City 700000, Vietnam
lmvien@hcmut.edu.vn
In this study, the TiO2 nanoparticles were coated on the monolithic surface by a dip-coating technique.
Characterization of synthesized samples was determined by X-ray diffraction. Antibacterial application of the
samples was investigated under simulated solar irradiation on Escherichia coli (E. coli) bacteria. A 5.7 log
CFU mL-1 decrease of E. coli was observed with TiO2 nanoparticles after 3 h irradiation, whereas the number
of surviving E. coli cells decreased by 2.7 log CFU mL-1 with monolithic TiO2 at the same irradiating condition.
Despite the reduction of photocatalytic antibacterial effect, the results confirmed the photocatalytic
antibacterial activity of monolithic TiO2 under simulated sunlight irradiation as well as revealed its potential in
practical water treatment applications.
1. Introduction
The rapidly accelerating urbanization-industrialization process in the 21st Century has helped the economy of
some countries develop notably. On the other hand, its parallel serious consequences are climate change as
well as emerging issues such as degradation and pollution of environment (Liu and Bae, 2018). Water
polluting agents includes harmful microorganisms, heavy metals, organic and inorganic chemicals (Zeliger,
2008). The harmful microorganisms cause really dangerous human infections and inflammations in blood,
skin, eyes; gastrointestinal tract, brain, etc. (Baldursson and Karanis, 2011). The clean water shortage for over
2 billion people on the world leads to at least half a million deaths from diarrhoeal each year (Pichel et al.,
2019). As a result, more difficulty in approaching clean water sources has received more concerns, especially
in developing countries. That is why the topic of polluted water treatment, especially drinking water disinfection
always is cared for and researched urgently.
Current water disinfection methods are limited in worldwide applications and scale-up due to some
obstructions. For instance, disinfection using antiseptics (Hrudey, 2009) and ozonation (Yang et al., 2012)
damages human health due to toxic by-product. Reverse osmosis and nano-filtration membranes remove not
only whole microorganisms and contaminants but also minerals which are important essential for human
health (Ismail et al., 2019). Disinfection using UV light (Lui et al., 2016) and a thermal treatment process (Feng
et al., 2004) always requires high energy as well as accommodates re-infecting risks. Alternative methods
have been developed constantly to overcome these obstructions. Photocatalysis techniques were applied for
an advanced oxidation technology which disinfects with free radicals and reactive oxygen species (Lee and
Park, 2013). The most important benefits of photocatalysts are operability at low concentration, reusability for
a long time and non-existence of toxic by-products. If the photocatalysis is activated by solar light as a
renewable source, it will obviously become a green solution in water treatment.
TiO2 nanomaterial is known as a popular photocatalyst because of its efficiency in degradation of organic
chemicals (Gaya and Abdullah, 2008) and antibacterial. In particular, the photocatalytic antibacterial activity of
TiO2 is effective on many types of microorganism such as algae, viruses, fungi, and bacteria (Laxma Reddy et
al., 2017). A pure anatase TiO2 nanomaterial with particle size of 5.3 nm shown its antibacterial activity
noticeably after 12 h visible irradiation due to a 1 log CFU mL-1 decrease of E. coli (Cao et al., 2013). Another
single-phase anatase TiO2 prepared by a sol-gel method decreased by approximately 3.4 log CFU mL-1 of E.
coli after 1 h UVC irradiation (Moongraksathum and Chen, 2018). As evidence, the commercial Degussa P25
DOI: 10.3303/CET2078060
Paper Received: 27/04/2019; Revised: 20/08/2019; Accepted: 31/10/2019
Please cite this article as: Pham D.N., Le M.-V., Hoang H.A., Tran X.T., Nguyen T.-A., 2020, Fabrication of TiO2 Monolithic Photocatalyst and
Evaluation of its Antibacterial Activity under Simulated Solar Irradiation, Chemical Engineering Transactions, 78, 355-360
DOI:10.3303/CET2078060
355
TiO2 was confirmed to cause a 4 log CFU mL-1 reduction of E. coli after 1 h simulated solar irradiation (Gumy
et al., 2006). In addition, TiO2 nanomaterials also indicate more dominance, for example, non-toxicity(Nguyen
et al., 2019), lower cost than other photo-catalysts (Makhdoomi et al., 2015), inert nature (Gaya and Abdullah,
2008) and high biological stability (Lee and Park, 2013). All the advantages show that TiO2 is capable of
disinfection treatment for drinking water.
In contrast, nano-size of TiO2 cause limitation in practical applications on account of separating nanoparticles
out of water and reuse. Reusing TiO2 nanoparticles, which requires lots of complex recovery equipment, leads
to high cost of antibacterial process. To improve TiO2 reusability, TiO2 nanopowders have been gradually
replaced by nanosized TiO2 coated on inert substrates. TiO2 was deposited on cotton fiber to obtain
cotton/TiO2 composites which indicated antibacterial activity against E. coli bacteria under short UV irradiation
after 5 washing cycles (Galkina et al., 2014). A sol-gel method combined with a dip-coating technique was
used to fabricate TiO2 thin films (Sasani Ghamsari and Bahramian, 2008). TiO2 coated on glass was also
prepared with the corresponding way (Hakki et al., 2018). These types of substrate are seemingly ill-suited to
take a key role in drinking water treatment equipment. Another prefer substrate towards water treatment
applications is a monolith. Ceramic monoliths are lighter than metallic monoliths and variable in size as well as
morphology such as spherical, cubic or cylindrical shape. Cylindrical honeycomb monoliths with lots of
channels on the inside lead to larger surface area (Heck et al., 2001) which enhances contacts between TiO2
and bacteria. The purpose of this study is surveying influence factors to find a good condition for fabricating a
photocatalytic-antibacterial material which is able to withdraw and reuse as well as evaluating initially
antibacterial activity of this material.
In this study, single-phase anatase TiO2 was coated on channel surface of honeycomb monoliths. The
synthesized TiO2 was characterized by using X-ray diffraction (XRD) and its photocatalytic antibacterial
activity was evaluated on E. coli bacteria by spread plate method. Monolith photo-reactor design in this study
was referred from a previous study in gas treatment application (Tahir, 2018). These results are initial
investigation for monolithic TiO2 application in further water treatment studies.
2. Experimental
2.1 Materials and reagents
All materials and reagents were used without purification. Titanium n-butoxide (TNB, 99%) as titania precursor
was obtained from Across Organics. Tetraethyl orthosilicate (TEOS, 99 %) as a silicon source was purchased
from Merck Chemical Company. Acetylacetone (AcAc, 99%), polyethylene glycol (PEG, MW = 20,000),
sodium chloride (99%), poly peptone, bacto yeast extract, and agar were all supplied by Merck Chemical
Company. Nitric acid (HNO3, 65%) was purchased from Scharlab in Spain. Deionized water was used in all
experiments. Commercial honeycomb monoliths (4 x 10 cm) including 177 channels per 2 mm x 2 mm were
supplied by Chauger Honeycomb Ceramics Company in Taiwan.
2.2 Preparation of nano-material
2.2.1 Synthesis of TiO2 nanopowders
TiO2 nanopowders were synthesized according to a sol-gel method from solutions A and B as follow. EtOH,
AcAc, and PEG were added sequentially to TNB with the TNB : EtOH : AcAc molar ratio of 1 : 10 : 1 and the
PEG : TiO2 weight ratio of 1 : 2, then this mixture was dissolved at 40 C for 30 min to obtain the solution A.
H2O was added to EtOH with the H2O : EtOH molar ratios of 2 : 10, 4 : 10 or 6 : 10 to obtain the solution B. pH
value of the solution B was adjusted to 2 by using HNO3. The solution B was added dropwise to the solution A
for 2.5 h at room temperature. This mixture was heated at 80 C for 2 h afterwards to get a TiO2 solution. In
the next step, the TiO2 solution was dried at 120 C for 2.5 h before calcination at 500 C or 550 C for 2 h
with a heating rate of 1 C/min to obtain TiO2 powder samples. These powder samples were named according
to the TNB : water molar ratios of 1 : 2, 1 : 4, 1 : 6 and the calcined temperature at 500 C, 550 C followed
by P1:2-500, P1:4-500, P1:4-550, and P1:6-500.
2.2.2 Fabrication of nano TiO2/monoliths
Fabrication of monolithic TiO2 by a sol-gel dip-coating technique was performed with commercial honeycomb
monoliths, SiO2 and TiO2 solutions. The commercial monoliths had been cleaned by an ultrasonic bath in
acetone media for 30 min before being dried at 100 C for 24 h to obtain cleaned monolith. The TiO2 solutions
in monolith fabrication and nanopowder synthesis were identical. The SiO2 solution was prepared by adding
dropwise an EtOH/H2O solution with EtOH : H2O molar ratio of 1 : 1 to a EtOH/TEOS solution with EtOH :
TEOS molar ratio of 1 : 2. This mixture was adjusted pH value to 3 by using HNO3 before stirred constantly for
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12 h. PEG was added to the mixture with PEG : SiO2 weight ratio of 1 : 2 afterwards. Stirring the mixture
lasted another 6 h to acquire SiO2 solution.
Monolithic TiO2 samples were prepared by repeating a following dip-coating cycle. This cycle consisted of
steps as follows. To begin dip-coating, the cleaned monoliths were immersed in the SiO2 solution at a
constant rate of 0.5 mm / s, then they were soaked in SiO2 solution for 30 min and withdrawn at the same rate
of immersion. The first cycle of SiO2 coating was dried at 180 C for 1 h before the monoliths were dip-coated
with the SiO2 solution one more time. The second cycle of SiO2 coating was dried at 180 C for 1 h before the
monoliths were calcined at 600 C or 800 C for 3 h with a heating rate of 1 C / min to obtain SiO2 coated
monolithic samples named SiO2/M600 or SiO2/M800. The SiO2/M600 and SiO2/M800 were performed
sequentially dip-coating cycles into the SiO2 solution two more times and into the TiO2 solution two more times
with the same speed as the previous SiO2 coating cycles. The monolithic samples were dried at 180 C after
each cycle of SiO2 coating and were dried at 120 C after each cycle of TiO2 coating. The TiO2 coated
monolithic samples were calcined at 500 C for 2 h with a heating rate of 1 C/min to complete the fabrication
of monolithic samples named TiO2/SiO2/M600 and TiO2/SiO2/M800.
2.3 Characterization
Crystal phase of powder samples was determined by XRD using a D2 Phaser (Bruker) with Cu-Kα radiation
(λ=0.15418 nm) at 0.02° step size and 0.2 s step time from 20° to 80°. Crystalline size was calculated by the
Scherrer's equation (Eq(1)) at (101) peak, where β is the full width at half-maximum of the diffraction peak
(FWHM, radian), k = 0.9 is a shape constant, D is the crystalline size and θ is the Bragg angle.
𝐷 =
𝑘. 𝜆
𝛽.𝑐𝑜𝑠𝜑
(1)
2.4 Antibacterial activity tests
A bacterial strain used in antibacterial tests was Escherichia coli K12. The bacterial culture in Luria-Bertani
(LB) Broth was shaken with a shaking rate of 150 rpm at 30 C for 18 h. The culture was centrifuged at 10,000
× g at 4 C for 5 min to obtain a pellet. The pellet was suspended in the same volume of sterilized NaCl 0.8 %
solution. The centrifugation and suspension were repeated to discard all residual LB Broth. The final pellet
was suspended and serially diluted in sterilized NaCl 0.8 % solution to obtain a bacterial concentration of ~106
CFU mL-1. The bacteria suspension containing ~106 CFU mL-1 was used in antibacterial activity tests for both
the powder samples and the monolithic samples.
Figure 1: Illustration of photo-reactor used in the antibacterial activity test of (a) powder and (b) monolith
samples and (c) the simulated solar light spectra of the lamp in the photo-reactor
In the case of the powder samples, the antibacterial activity test was performed in a photo-reactor illustrated in
Figure 1a with a lamp as a simulated solar light source. The powder sample was added to the bacteria
suspension to obtain a concentration of 1,000 ppm. Distance from the lamp to the suspension surface is 5 cm.
The bacteria suspension was stirred constantly and its temperature was maintained at 30 ± 2 C. A photolytic
antibacterial test with simulated solar irradiation but without TiO2 was performed to evaluate effect of the light
source on bacteria.
In the case of the monolith samples, the antibacterial activity test was performed in a photo-reactor which
contains a monolith and a simulated solar lamp (Figure 1b). The bacteria suspension was pumped circularly
into the reactor at a flow rate of 70 mL / min by a peristaltic pump. The bacteria suspension was stirred
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constantly and its temperature was maintained at 30 ± 2 C. A “control” sample containing only bacteria
suspension without TiO2 and irradiation was always conducted simultaneously for each test in antibacterial
activity of TiO2. Conditions of the control test were the same as those of samples with presence of TiO2. In all
cases, the irradiation was maintained for 3 h and samples were taken every 30 min. They were serially diluted
with 10, 100 or 1000-fold dilution and spread onto LB Agar plates. Bacteria concentration was estimated
based on the number of colonies in the plates and the respective dilution levels.
3. Result and Discussion
Powder X-ray diffraction patterns of the powder samples are shown in Figure 2. It can be seen that all peaks
of the powder samples calcined at 500 °C at 2θ diffraction angles of 25.2° (101), 36.9° (004), 48.24° (200),
54.86° (211) and 63.01° (204) are well-indexed with the JCPDS card No 21-1272. No diffraction peaks
representing rutile phase as well as an impurity are detected in any samples calcined at 500 °C. It revealed
that all of the samples including P1:2-500, P1:4-500 and P1:6-500 are pure anatase structures. According to
the XRD results, crystalline size of the samples was calculated and displayed in Table 1. Among samples
calcined at 500 °C, the crystalline size increased significantly from 13.2 nm to 18.4 nm according to
decreasing of the TNB : water molar ratio. The TNB : H2O molar ratio of 1 : 2 corresponding to the low amount
of water not only causes a low rate of both hydrolysis and gelation (Bessekhouad et al., 2003) but also leads
to producing small crystalline size. Because of lack of water, the probability of condensation progress in the
oxolation manner is higher than the deoxolation one. Increasing the amount of water which supports formation
of anatase structures also increases the hydrolysis rate. As a result, excess water promotes the deoxolation to
create a large quantity of Ti–OH. They make primary particles be loosely packed lead to incomplete
development of three-dimensional polymeric bonds (Bessekhouad et al., 2003). This explains the decrease of
crystallization according to the rise of TNB : H2O molar ratio from 1 : 4 to 1 : 6.
Figure 2: XRD patterns of powder TiO2 samples: P1:2-500, P1:4-500, P1:6-500, and P1:4-550
Table 1: Crystalline size of powder samples
Sample P1:2-500 P1:4-500 P1:6-500 P1:4-550
Average crystalline size (nm) 13.2 18.4 16.2 19.0
The P1:4-550 sample also indicated the peak at 2θ diffraction angles which are well-indexed with the JCPDS
card No 21-1272. It disclosed that the anatase structure is still preserved according to the increment of
calcined temperature from 500 C to 550 C. A less intense peak of rutile structure at a diffraction angle of
27.49 seen in the XRD result of P1:4-550 signed a transformation from anatase to rutile of TiO2 which
reduces crystallization of the anatase structure. Increasing the calcined temperature is accompanied by an
increase of crystalline size clearly (Allen et al., 2018). This revealed that the calcined temperature is also an
important factor which affects the crystal structure of TiO2.
Antibacterial efficiency of the powder samples was displayed in Figure 3a. In the photolytic antibacterial with
the lamp but without TiO2 nanopowders, bacterial concentration was slightly decreased during 3 h, indicating
simulated solar light source had almost no effect on the bacteria. In contrast, the bacterial concentration
dramatically decreased by at least 3 log CFU mL-1 of E. coli after 3 h due to the presence of the TiO2
nanopowder under irradiation. The ascending order of antibacterial activity is P1:6-500, P1:4-550, P1:2-500,
P1:4-500, especially, P1:2-500 and P1:4-500. P1:4-500 demonstrated the highest antibacterial activity with a
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3.2 log CFU mL-1 decrease of E. coli after only 1 h irradiation. In general, antibacterial activities depend on the
crystallization of the anatase structure. The higher the crystallization of the anatase structure is, the higher the
antibacterial activity of TiO2 nanomaterial is. In contrast to this trend, the P1:2-500 sample is an exception. In
spite of lower crystallization, its antibacterial activity was still higher than that of P1:6-500 or P1:4-550 sample.
This result was probably caused by much smaller crystalline size, which increases the contact between TiO2
nanoparticles and bacteria. Hence, crystalline size is also an important factor influencing the antibacterial
activity of TiO2 nanoparticles. The TiO2 nanopowder sample synthesized with TNB : H2O molar ratio of 1 : 4
and calcined temperature at 500 C (P1:4-500) showed the highest antibacterial activity.
Figure 3: Antibacterial activity of (a) powder samples and (b) TiO2-coated monolith samples according to
irradiation time
The conditions to prepare the P1:4-500 sample were applied to the TiO2 solution preparation that used for
monolith coating. Optical fibers which support light transmission from the lamp to the narrow channels inside
of monoliths are always required in monolithic photo-reactors to activate monolithic photocatalysts (Yu et al.,
2011). Figure 3b described antibacterial activity of monolithic samples as a function of irradiation time. No
antibacterial activity was shown in the SiO2/M600 sample (without TiO2). After 3 h irradiation, a 1.6 log CFU
mL-1 decrease of E. coli at the TiO2/SiO2/M600 was shown, while the greater decrease of 2.7 log CFU mL-1 of
E. coli was shown at the TiO2/SiO2/M800. It demonstrated that the nanoparticles were coated successfully on
the channel monolith surface. The difference in the antibacterial activity of TiO2/SiO2/M600 and
TiO2/SiO2/M800 was caused by SiO2 background layer with various calcined temperature. SiO2 in the
background layer leads to improving smoothness of the inner monolithic surface and increases adhesion of
the following TiO2 layers. The higher calcined temperature (800 C) caused the more adhesion between SiO2
material and the monolith surface. It results in mechanical strengthen between TiO2, SiO2, and monolith that
lead to elevate the photocatalytic activity of the sample.
4. Conclusions
The TiO2 crystal structure was controlled by molar ratio of TNB: H2O and calcined temperature. The TiO2
nanoparticles were synthesized in single anatase phase. It was coated successfully on surface of monolith
substrate by sol-gel method combined with dip-coating technique and using SiO2 as a background layer which
elevates adhesion of TiO2. TiO2/SiO2/M800 sample reduced by 2.7 log CFU mL-1 of E. coli while
TiO2/SiO2/M600 sample shown a 1.6 log CFU mL-1 decrease of E. coli after 3 h simulated solar irradiation.
This study revealed that TiO2/SiO2/monolith is a potential photocatalyst in drinking water treatment under
sunlight irradiation. The antibacterial activity of this material needs further enhancing in future studies.
Acknowledgments
This research is funded by Ho Chi Minh City University of Technology, VNU-HCM, under grant number BK-
SDH-2020-1770444.
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