Floating ZnO QDs-modified TiO2/LLDPE hybrid polymer film for the effective photodegradation of tetracycline under fluorescent light irradiation: Synthesis and characterisation

Article abstract of DOI:10.3390/molecules26092509

In this work, mesoporous TiO₂-modified ZnO quantum dots (QDs) were immobilised on a linear low-density polyethylene (LLDPE) polymer using a solution casting method for the photodegradation of tetracycline (TC) antibiotics under fluorescent light irradiation. Various spectroscopic and microscopic techniques were used to investigate the physicochemical properties of the floating hybrid polymer film catalyst (8%-ZT@LLDPE). The highest removal (89.5%) of TC (40 mg/L) was achieved within 90 min at pH 9 due to enhanced water uptake by the LDDPE film and the surface roughness of the hybrid film. The formation of heterojunctions increased the separation of photogenerated electron-hole pairs. The QDs size-dependent quantum confinement effect leads to the displacement of the conduction band potential of ZnO QDs to more negative energy values than TiO₂. The displacement generates more reactive species with higher oxidation ability. The highly stable film photocatalyst can be separated easily and can be repeatedly used up to 8 cycles without significant loss in the photocatalytic ability. The scavenging test indicates that the main species responsible for the photodegradation was O₂ ^?-. The proposed photodegradation mechanism of TC was demonstrated in further detail based on the intermediates detected by LC-time-of-flight/mass spectrometry (LC/TOF-MS).

Full text of DOI:10.3390/molecules26092509

molecules
Article
Floating ZnO QDs-Modified TiO2/LLDPE Hybrid Polymer Film
for the Effective Photodegradation of Tetracycline under
Fluorescent Light Irradiation: Synthesis and Characterisation
Anwar Iqbal 1,
* ,
, Usman Saidu 1, Farook Adam 1, Srimala Sreekantan 2 , Noorfatimah Yahaya 3
Mohammad Norazmi Ahmad 4, Rajabathar Jothi Ramalingam 5,
*
and Lee D. Wilson 6,
*
1
School of Chemical Sciences, Universiti Sains Malaysia, Gelugor, Penang 11800, Malaysia;
usmaniyya2000@gmail.com (U.S.); farook@usm.my (F.A.)
2
3
4
5
6
School of Materials & Mineral Resources Engineering, Universiti Sains Malaysia, Nibong Tebal,
Penang 14300, Malaysia; srimala@usm.my
Integrative Medicine Cluster, Advanced Medical and Dental Institute, Universiti Sains Malaysia, Bertam,
Kepala Batas, Penang 13200, Malaysia; noorfatimah@usm.my
Experimental and Theoretical Research Lab, Department of Chemistry, Kulliyyah of Science,
Kuantan, Pahang 25200, Malaysia; mnorazmi@iium.edu.my
Surfactant Research Chair, Chemistry Department, College of Science, King Saud University, P.O. Box 2455,
Riyadh 11451, Saudi Arabia
Department of Chemistry, University of Saskatchewan, 110 Science Place, Room 165 Thorvaldson Building,
Saskatoon, SK S7N 5C9, Canada
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
*
Correspondence: anwariqbal@usm.my (A.I.); jrajabathar@ksu.edu.sa (R.J.R.); lee.wilson@usask.ca (L.D.W.);
Tel.: +60-4653-3565 (A.I.); +1-306-966-2961 (L.D.W.)
Citation: Iqbal, A.; Saidu, U.; Adam,
F.; Sreekantan, S.; Yahaya, N.; Ahmad,
M.N.; Ramalingam, R.J.; Wilson, L.D.
Floating ZnO QDs-Modified
Abstract: In this work, mesoporous TiO2-modified ZnO quantum dots (QDs) were immobilised
on a linear low-density polyethylene (LLDPE) polymer using a solution casting method for the
photodegradation of tetracycline (TC) antibiotics under fluorescent light irradiation. Various spectro-
scopic and microscopic techniques were used to investigate the physicochemical properties of the
floating hybrid polymer film catalyst (8%-ZT@LLDPE). The highest removal (89.5%) of TC (40 mg/L)
was achieved within 90 min at pH 9 due to enhanced water uptake by the LDDPE film and the
surface roughness of the hybrid film. The formation of heterojunctions increased the separation of
photogenerated electron-hole pairs. The QDs size-dependent quantum confinement effect leads to
the displacement of the conduction band potential of ZnO QDs to more negative energy values than
TiO2. The displacement generates more reactive species with higher oxidation ability. The highly
stable film photocatalyst can be separated easily and can be repeatedly used up to 8 cycles without
significant loss in the photocatalytic ability.•−The scavenging test indicates that the main species
responsible for the photodegradation was O2 . The proposed photodegradation mechanism of TC
was demonstrated in further detail based on the intermediates detected by LC-time-of-flight/mass
spectrometry (LC/TOF-MS).
TiO2/LLDPE Hybrid Polymer Film
for the Effective Photodegradation of
Tetracycline under Fluorescent Light
Irradiation: Synthesis and
Characterisation. Molecules 2021, 26,
2509. https://doi.org/10.3390/
molecules26092509
Academic Editors: Wanhong Ma and
Marcello Brigante
Received: 15 March 2021
Accepted: 20 April 2021
Published: 25 April 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Keywords: LLDPE; zinc oxide; titanium dioxide; quantum dots; tetracycline; photodegradation
1. Introduction
The efficiency of conventional wastewater treatment plants in removing antibiotics
lies in the range of 12% to 80%. The annual consumption of antibiotics in the world is
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
estimated to be around 200,000 tons [
metabolised in the body, which results in ca. 30% to 90% of the remaining antibiotic entering
the ecosystem either as the parent compound or as by-product metabolites [ ]. Current
conventional wastewater treatment plants are ineffective in the removal of antibiotics at
trace levels [ ]. Hence, there is an urgent need to develop wastewater treatment systems to
tackle this global water security issue.
1]. Much of the consumed antibiotics are incompletely
2
3
Molecules 2021, 26, 2509. https://doi.org/10.3390/molecules26092509
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 2509
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The utilisation of direct solar light or solar simulation by photocatalysis can serve
as an alternative to promote the degradation of antibiotics in the aquatic environment
to harmless by-products [
commonly used photocatalyst due to its low cost, high activity, excellent photostability,
chemical inertness, and nontoxicity [ ]. However, the high electron-hole recombination
1]. Among the semiconductor photocatalysts, TiO2 is the most
4
rate makes TiO2 a relatively inefficient photocatalyst. Heterojunction formation between
the TiO2 and semiconductor quantum dots (QDs) has proven to be one of the most effective
approaches to overcome this drawback. QDs’ modified titania catalyst promotes a charge
separation and minimises or inhibits electron/hole recombination for improved photocat-
alytic activity [5–7]. The semiconductor QDs act as sensitisers or separation centres when
deposited onto the surface of TiO2. Fabrication of bulk semiconductor with QDs may also
maximise the specific surface area of the photocatalyst due to their reduced particle sizes,
and the migration distance required for electron/hole pairs to reach the surface is min-
imised [
showed improved photocatalytic activity when compared with pristine TiO2. For instance,
WO3-x QDs [ ], SnO2 QDs [ ], CdSe QDs [ ], and PbS QDs [ ] have been reported to exhibit
5,6]. Many semiconductor QDs have been successfully deposited on TiO2 that all
7
8
6
9
enhanced photocatalytic properties. However, the photocatalysts’ main drawback is the
difficult separation from the water system for their reusability. This relates to the common
use of a powdered form of the photocatalysts, where they function as suspended colloidal
particles in wastewater to degrade the organic contaminants. Other drawbacks include low
light utilisation efficiency of the suspended photocatalyst; aggregation of catalytic particles,
especially at higher concentrations, and potential human health problems associated with
the fate and transport of dispersed powder and its mobility [10,11]. These drawbacks
limit their application in large-scale commercial production. An efficient alternative to
address this issue relates to the immobilisation of the nanoparticles onto various substrates,
such as glass fibres, clay beads, ceramics, and polymers [12–15]. Moreover, most of the
supports are either expensive, contain potential impurities, or display reduced durability
and flexibility [16]. Polymer supports have proved to be among the best candidates for
immobilising photocatalyst nanoparticles due to their numerous advantages: low cost,
ready accessibility, chemical inertness, and mechanical stability with high durability. In
addition, polymer supports have relatively low density, allowing for the design of floating
photocatalysts, especially in aqueous media [11,17–19].
Linear low-density polyethylene (LLDPE) exhibits excellent chemical, mechanical, and
thermal stability [17]. It can also be used to fabricate thinner films with better environmental
stress cracking resistance. In addition, its low-density property offer the option to produce
floatable-type photocatalyst systems. Some of the most important advantages of such
photocatalysts relate to their maximum utilisation of solar energy, where they can be
directly applied into the contaminated wastewater treatment efficiently under non-stirred
conditions to achieve cost reduction for photocatalysis [20,21].
In this work, floating ZnO QDs-modified TiO2 was supported on an LLDPE hybrid
film prepared by a facile and eco-friendly solution-casting method. The photocatalytic
activity and proposed mechanisms of the prepared hybrid film were evaluated for the
photodegradation of tetracycline (TC) in an aqueous medium under fluorescent light
irradiation. The effect of various parameters and a kinetic study of the photodegradation
degradation rate for TC was investigated using the modified Langmuir–Hinshelwood
method. The intermediates formed during the photodegradation process were identified
using liquid chromatography/time-of-flight/mass spectrometry (LC/TOF/MS) to propose
possible degradation pathways.
2. Results and Discussion
2.1. Characterisation
2.1.1. X-ray Diffraction (XRD) Analysis
The XRD profiles of ZnO QDs, ZT, bare LLDPE, and the 8%-ZT@LLDPE are presented
in Figure 1a–d, respectively. The XRD diffraction planes observed in the diffractogram of

Molecules 2021, 26, 2509
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ZnO QDs (Figure 1a) are indexed to the hexagonal Wurtzite phase (JCPDS 01-071-6424).
The size of the ZnO QDs (4.4 nm) was determined using the Scherrer equation, according
to Equation (1).
kλ
D =
(1)
βcosθ
is the wavelength equal to 0.154 nm, k is a constant taken as 0.94,
is the full width at half-maximum intensity (FWHM) of the XRD band, and is the Bragg
-values of 25.4◦, 37.9◦, 48.0◦,
D is the crystal size,
λ
β
θ
angle of XRD band. The diffraction bands at the following 2
θ
54.1◦, 55.1◦, and 62.9◦ (Figure 1b) correspond to the (101), (004), (200), (105), (211), and
(204) planes of anatase TiO2 phase (JCPDS 00-021-1272) [22]. The average crystallite size
of the ZT determined from equation (1) was 9.33 nm. The diffraction peaks related to the
ZnO QDs were not observed in the XRD profile of ZT. Several reasons can account for the
absence of the peaks: (1) The concentration of the ZnO QDs is low, where Han et al. [23
]
reported a similar observation upon incorporation of CdSe QDs into the framework of
ZnO flowers. (2) The effect may relate to the level of the homogeneity of the mixing of
Ti and Zn components in ZT [24]. This means that anatase TiO2 and ZnO QDs did not
exist as a separate phase. (3) Incorporation of ZnO QDs into the lattice of TiO2 led to a
change in the pH of the media, where the high calcination pH may have induced a strain
on the crystal framework of the ZnO QDs. The induced strains can distort the crystal
framework, hence favouring an amorphous product. In turn, the XRD bands related to
ZnO crystal structure were not detected in XRD profile. From the diffractogram of the bare
LLDPE film (Figure 1c), XRD bands at 2θ
values of 21.3◦, 23.6◦, and 36.0◦ were detected
that correspond to (110), (200), and (020) planes of an orthorhombic polyethylene crystal
phase, respectively [25]. The presence of ZT within the matrix of LLDPE is evident with
the emergence of an anatase peak (101) at a 2θ
= 25.4◦. The diffraction peaks related to the
LLDPE and anatase TiO2 were observed to be broadened and attenuated. The broadening
of the XRD lines and their weaker intensity indicates that LLDPE became less crystalline
after ZT underwent immobilisation.
2.1.2. Field Emission Scanning Electron Microscopy (FESEM) Analysis
The FESEM analysis was carried out to understand the morphology of the ZT and its
influence on the LLDPE film’s morphology. The FESEM image of ZT (Figure 2a) shows
the presence of pores on the surface of ZT. The average pore diameter was measured to be
~ 1.17 µm. The pores were formed due to the use of starch as a template in the synthesis.
Hydrogen bonds in the starch granules are weakened to allow more sorption of water
molecules upon swelling of the starch granules. Based on the concept of the partial charge
model, the hydrolysis of a titanium cation that forms a stable [Ti(OH)(OH2)5]3+. As the
pH increases, deprotonation of [Ti(OH)(OH2)5]3+ forms [Ti(OH)2(OH2)5]2+. Condensation
takes places when [Ti(OH)2(OH2)5]2+ is further deprotonated to [TiO(OH)(OH2)4]+, which
undergoes intramolecular deoxolation to [TiO(OH)3(OH2)3]+ [26
,27]. As the condensation
takes place, the species is drawn to the starch granules by van der Waals attraction. A
three-dimensional oxide framework incorporating the ZnO QDs forms around the swollen
starch granules. The starch granules are removed during calcination, leaving behind a
TiO2/ZnO QDs embedded within a porous nanocomposite oxide framework.

Molecules 2021, 26, 2509
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Figure 1. XRD diffractogram of materials: (a) ZnO QDs, (b) ZT, (c) bare LLDPE, and (d) 8%-
ZT@LLDPE.
Figure 2. FESEM image of (
a) ZT, (b) bare LLDPE, (c) 8%-ZT@LLDPE, and (d) cross-section of 8%-ZT@LLDPE. The green
arrows indicate the ZnO QDs.

Molecules 2021, 26, 2509
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From the FESEM image of bare LLDPE film (Figure 2b), the surface of the film ap-
peared to be virtually smooth. The molten LLDPE was observed to have higher viscosity
in the presence of ZT. As it solidified, the surface of the film became uneven (Figure 2c).
The bright spots on the SEM image (shown using green arrows) were probably related
to ZnO QDs [28]. A cross-section of the 8%-ZT@LLDPE (Figure 2d) indicates that the ZT
nanocomposite was positioned on the film and free from any voids or microcracks. The
average thickness of the film was measured as 120 µm.
2.1.3. Field Emission Scanning Electron Microscope (FESEM) Analysis
The TEM images of the ZnO QDs and ZT are shown in Figure 3. The histograms
are given as an insert therein. The particles were irregular and spherically shaped with
evidence of agglomeration. The average size of ZT was 12.5
size of the ZnO QDs were 4.12 ± 0.23 nm.
±
2.05 nm, whereas the average
Figure 3. TEM image of (a) ZT and (b) ZnO QDs. The average particle size is shown in the insert.
2.1.4. Atomic Force Microscope (AFM) Analysis
Surface roughness is a vital aspect to be considered to afford enhanced photocatalytic
activity of a film-based photocatalyst. A higher degree of surface roughness would improve
the hydrophilicity of the films to enhance the interaction of the film with oxygen, water,
and pollutants for the reaction to occur [29
view of the AFM images (5 m) for the bare LLDPE film and the 8%-ZT@LLDPE.
The surface mean roughness (Ra) was 103 nm, and 273 nm for both the bare LLDPE and
8%-ZT@LLDPE, in agreement with trends reported from other studies. Hir et al. [27
,30]. Figure 4 shows the three-dimensional
µm
×
5 µ
]
reported the mean surface roughness of a polyethersulfone-TiO2 (PT) film increased from
2.19 nm to 4.46 nm for neat PES to PES immobilised with 13 wt.% TiO2 (PT-13). Similarly,
Mohamed et al. [30] reported an increase in the mean surface roughness of regenerated
cellulose/N-doped TiO2 (RC/TiO2) membrane from 29.53 nm to 79.11 nm for bare RC and
RC/TiO2-0.7, respectively.
2.1.5. Contact Angle Results
A liquid in contact with a solid will exhibit a contact angle (
mined by the interaction of the three interfaces (solid, liquid, and gas). A
than 90◦ indicates an attraction, where the liquid will spread to some extent over the solid
surface. A
-value above 90◦ indicates less attraction, where the liquid will minimise its
contact area with the solid surface and form a droplet. Figure 5 displayed variable -values
θ
). The
θ
-value is deter-
θ
-value smaller
θ
θ
for water onto bare LLDPE and 8%-ZT@LLDPE. The increase in the hydrophilicity was
due to the decrease in the composite films’ crystallinity as the ZT was added. It has been
reported that a lower degree of crystallinity allows a polymer to absorb more water [31],
which is beneficial for the photocatalytic activity.

Molecules 2021, 26, 2509
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(a)
(b)
Figure 4. Three-dimensional AFM images of (a) bare LLDPE and (b) 8%-ZT@LLDPE.
Figure 5. Water contact angle (θ) measurements: (a) bare LLDPE and (b) 8%-ZT@LLDPE.
2.1.6. UV–Visible Diffuse Reflectance Analysis
The absorption spectra of ZT, TiO2, ZnO QDs, bare LLDPE, and 8%-ZT@LLDPE are
shown in Figure 6. The bare LLDPE shows a broad absorption spectrum. The sensitivity
of 8%-ZT@LLDPE towards the UV light region was higher due to the presence of the ZT
nanocomposite. The increase in UV light sensitivity is caused by ZnO and TiO2, which
are known to be good UV absorber materials. The band edge of ZnO QDs, TiO2, and
8%-ZT@LLDPE was determined to be 377–380 nm, which is in the UV–Vis region.
The bandgap energies of the ZnO QDs and TiO2 were estimated using Tauc’s equation,
shown in Equation (2).
αhν = A(hν − Eg)n/2
(2)
α
is the absorption coefficient, h is Planck’s constant,
ν
is the light frequency, A is the
proportionality constant, and Eg is the bandgap energy. The bandgap energy of the
1/2
semiconductors was estimated from a Tauc plot of (
α
h
ν
)
versus hν (Figure 6b). The
estimated bandgap was 3.10 eV and 3.50 eV for TiO2 and ZnO QDs, respectively.
2.2. Photocatalytic Activity of the LLDPE Hybrid Films
2.2.1. Effect of Various Photocatalysts Immobilised on LLDPE Film
The effect of various nanocomposites immobilised onto a LLDPE film for photodegra-
dation of TC is presented in Figure 7a. Tetracycline was not photodegraded without
any catalyst (blank) or in the presence of bare LLDPE film. The TiO2@LLDPE removed
42.7% of TC, whereas ZnO QDs@LLDPE removed 53.4% of TC. Commercial TiO2 P25
(Degussa) immobilised on the LLDPE (P25@LLDPE) removed 20.5% of the TC, which is
lower compared to TiO2@LLDPE. The presence of pores on the TiO2 surface resulted in
attenuation of the recombination rate of photogenerated e-/h+ pairs, hence increasing the
photocatalytic activity.

Molecules 2021, 26, 2509
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Figure 6.
(
a) Diffuse reflectance UV–Vis spectra of ZT, TiO2, ZnO QDs, bare LLDPE, and 8%-
ZT@LLDPE; and (b) the Tauc plot of ZnO QDs and TiO2.
Figure 7.
(
a) Photocatalytic degradation of TC with various LLDPE hybrid films ([TC] = 40 mg/L; pH 9; lamp power: 48
W); ( ) the rate curve based on the Langmuir–Hinshelwood kinetic model; (
b
c) UV spectral changes during photocatalytic
degradation of TC using 8%-ZT@LLDPE composite ([TC] = 40 mg/L; pH 9; lamp power: 48 W); and (
of the solution ([TC] = 40 mg/L; lamp power: 48 W).
d) effect of initial pH
The highest removal (85.4%) of TC was obtained in the presence of 8%-ZT@LLDPE.
Previously, it has been discussed that ZT distorted the crystalline framework of the LLDPE,
and increased the water uptake and surface roughness. In addition, such enhancement was

Molecules 2021, 26, 2509
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also attributed to the formation of a heterojunction between ZnO QDs and TiO2, favouring
the separation of photogenerated e−/h+ pairs. The structural defects of the Ti species and
−
oxygen vacancies can also suppress the recombination rate of photogenerated e /h+ pairs.
The size-dependent quantum confinement effect displaces the conduction band potentials
of ZnO QDs to more negative energy values than TiO2, leading to the generation of reactive
species with higher oxidation ability.
The TC degradation rate plot was obtained using the modified Langmuir–Hinshelwood
equation (cf. Figure 7b), whereas the calculated rate constants and the corresponding corre-
lation coefficients (R2) for the films are presented in Table 1. The 8%-ZT@LLDPE has the
highest rate constant (0.01077 min−1). The progressive degradation of TC was confirmed
by the steady decrease in the peak intensity at 358 nm, as illustrated by the UV–vis spectra
in Figure 7c.
Table 1. Kinetic data for the photodegradation of TC using LLDPE hybrid films.
R2
−1
Catalyst
Efficiency (%)
k (min
)
Bare LLDPE
P25@LLDPE
TiO2@LLDPE
ZnO@LLDPE
8%-ZT@LLDPE
4
0.0002
0.00151
0.00334
0.00480
0.01077
0.99724
0.96392
0.99503
0.99690
0.99897
20.5
42.7
53.4
85.4
2.2.2. Effect of the Initial pH of the Solution
The efficiency of the 8%-ZT@LLDPE in the photodegradation of TC at different pH
values that range from pH 3 to pH 11 is shown in Figure 7d. After 180 min of irradiation, a
variable TC photodegradation efficiency was observed, as follows: 40.76% (pH 3), 73.45%
(pH 5.8), 85.43% (pH 9), and 56.42% (pH 11). The pKa values of TC were reported to be
3.30, 7.68, and 9.69 [32]. Hence, TC will be positively charged in solution at pH
≤
3, neutral
in solution with pH from 3.3 to 7.7, and negatively charged in solution with pH > 7.7. The
PZC of ZT was determined to be 7.1, as shown in Figure S3 (cf. Supplementary Materials).
Therefore, in the acidic medium, the positively charged surface of 8%-ZT@LLDPE will repel
the positively charged TC. At pH 6, a reduced electrostatic attraction between the neutral
TC molecule and the positively charged surface of the catalyst. Hence, an increase in the TC
adsorption and photodegradation rate was observed. However, at pH 9, an opposite trend
was observed. At alkaline pH, negatively charged TC species tend to attract reactive groups
•
such as OH due to their high electronic density on the ring system, which would result in
the enhancement of removal efficiency of TC [33,34]. However, the degradation rate of TC
is inhibited when the pH > 10, since the •OH will compete with TC to be adsorbed on the
photocatalyst surface [35–37].
2.2.3. Effect of Natural Organic Matter (NOM)
The concentration of NOM in surface waters typically varies from about 2 mg/L
to more than 50 mg/L, depending on the origin [38], and about 90% of NOM is humic
acid (HA) [39]. Thus, it was used as a NOM model in the present study. As shown in
Figure 8a, the addition of 5 mg/L of HA did not significantly affect the TC degradation.
However, when the HA concentration was increased to 10, 20, and 50 mg/L, the TC
removal efficiency decreased to 76.91%, 66.07%, and 52.86%, respectively. The inhibitory
effect of HA is ascribed to the competitive adsorption of the HA onto the active sites of the
photocatalyst, and reduced light penetration through the solution [40]. In addition, HA can
also act as a reactive species quencher, which decreases the availability of reactive species
for TC degradation.

Molecules 2021, 26, 2509
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Figure 8. (a) Effect of HA on the TC removal; (b) the effect of radical scavengers on the photodegradation of TC using
8%-ZT@LLDPE film ([TC] = 40 mg/L; pH 9; lamp power: 48 W).
2.2.4. Scavenging Test
To identify the types of photogenerated oxidative species and to propose a possible
photocatalytic mechanism, scavenging tests were conducted. Triethanolamine (TEA),
ascorbic acid (AA), and isopropanol (IPA) were used as scavengers for photogenerated
•−
•
holes (h+), superoxide radicals (O2 ), and hydroxyl radicals ( OH) [41], respectively. The
experimental reaction conditions for scavenging were similar to the photodegradation
experiment, except with the addition of 5 mM of scavengers prior to the light irradiation.
As depicted in Figure 8b, the addition of AA resulted in the drastic decrease (68.61%) in TC
•−
photodegradation efficiency, which implied that the O2 radicals (which were trapped by
AA in solution) play a significant role in the degradation of TC. When the IPA was added
to the solution, about 26% of TC was degraded, whereas only about 20% decrease in TC
degradation efficiency was obtained when TEA was added. Hence, it is concluded that
the main photogenerated oxidative species responsible for the photodegradation of TC by
8%-ZT@LLDPE hybrid film is O2•−, followed by •OH and h+.
2.2.5. Mechanism of TC Degradation Using 8%-ZT@LLDPE
The possible photodegradation mechanism of TC degradation using 8%-ZT@LLDPE
is shown in Figure 9. The potentials of the valence band (VB) and the conduction band
(CB) edges of the catalyst were calculated using Mulliken electronegativity theory, using
Equations (3) and (4):
EVB = X − Ee + 0.5Eg
ECB = EVB − Eg
(3)
(4)
EVB is the valence band edge potential, ECB is the conduction band potential, Eg is the
bandgap of the semiconductor, Ee is the energy of free electrons on the hydrogen scale
(~4.5 eV), and X is the electronegativity of the semiconductor, which is 5.62 eV and 5.81 eV
for ZnO and TiO2, respectively. The calculated VB and CB potentials of ZnO QDs in
8%-ZT@LLDPE composite film are 2.87 and
VB and CB potentials for the TiO2 in 8%-ZT@LLDPE are 2.95 and
−
0.46 eV, respectively; whereas the calculated
0.15 eV, respectively.
−
Since the CB potential of the mesoporous TiO2 is less negative than the reduction potential
•−
of oxygen (E0(O2/ O2
=
−
0.33 eV/NHE), the TiO2 does not have sufficient reduction
potential to generate O2•−. Based on the above experimental results and analysis, a direct
Z-scheme mechanism in the photodegradation of TC using 8%-ZT@LLDPE is proposed.
When irradiated with light, both electrons in the VB of ZnO QDs and mesoporous TiO2

Molecules 2021, 26, 2509
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will be excited to the CB, leaving behind photogenerated holes (h+). The photogenerated
electrons (e-) in the CB of the mesoporous TiO2 has less redox potential compared to the
ZnO QDs, hence they combine with the h+ in the VB of ZnO QDs. Meanwhile, the e- in
the CB of ZnO QDs have more negative potential than the standard redox potential of O2,
E0(O2/ O2
=
−
0.33 eV vs. NHE). Thus, there is a reduction in the adsorbed oxygen to
•−
O2•−. The h+ that remained in the VB of the TiO2 has more positive potential than the
standard redox potential of OH−, (E0(OH−/•OH) = 2.4 eV); hence, TiO2 can oxidise OH−
to produce •OH. The h+ can also directly oxidise the adsorbed TC to intermediates and
even CO2 and H2O. The directional migration of photogenerated electrons and holes in
the 8%-ZT@LLDPE hybrid film prevented the recombination of charge carriers, leading to
photocatalytic activity improvement. The proposed mechanism is further supported by
the scavenging tests reported herein.
Figure 9. Proposed mechanism for the photocatalytic degradation of TC using 8%-ZT@LLDPE under visible light irradiation.
The LC/TOF/MS analysis detected a number of intermediate products formed during
the degradation process, and the mass spectra are shown Figure S4 (cf. Supplementary
Materials). The samples collected at 30 min intervals were analysed and photodegra-
dation products were determined. Based on the detected photodegradation products,
plausible photodegradation pathways are proposed, as shown in Figure 10. Generally, in
the photodegradation process, the intermediates were mainly generated by three routes:
the loss of functional group(s), the ring-opening reactions, and the complete oxidation
process to harmless compounds [42
study was justified as follows. Dominant radicals such as O2•− and
,
43]. The formation of intermediates in the present
OH firstly oxidise
•
the TC (m/z = 445), which undergoes a series of reactions such as deamination, dihydroxy-
lation, and demethylation, as suggested by Cao et al. [44] and Zhu et al. [45], to generate
the intermediate with m/z = 396. The species with m/z = 396 undergoes demethylation
followed by decarbonylation to give an intermediate with m/z = 340, which undergoes
further ring opening to yield an intermediate with m/z = 205. Subsequent degradation of
the intermediates result in compounds with variable molecular ions (m/z = 197, 181, and
106). Finally, small molecular weight species were completely decomposed into terminal
products, CO2 and H2O.
2.2.6. Mineralisation Efficiency
In order to determine the mineralisation ability of the hybrid film, total organic carbon
(TOC) analysis was conducted after photodegradation of TC under visible light irradiation.
As shown in Figure 11a, the mineralisation efficiency of TC was observed to increase as
the irradiation time increased. Moreover, the 8%-ZT@LLDPE hybrid film possessed higher
mineralisation efficiency than Degussa P25@LLDPE. This is due to the effective formation
of a heterojunction formation between the two semiconductors like ZnO QDs and TiO2,

Molecules 2021, 26, 2509
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−
which favour the separation of photogenerated e /h+ pairs and enhance the recombination
rate for the photocatalytic process.
Figure 10. Proposed possible pathways for the photodegradation of TC in the presence of 8%-
ZT@LLDPE.
Figure 11. (a) Photocatalytic mineralisation of TC in the presence of bare LLDPE and LLDPE hybrid films, and (b) recycle
experiment for the photodegradation of TC using 8%-ZT@LLDPE.
2.2.7. Reusability
Reusability studies are of paramount importance in photocatalysis, as it provides
insight on the photostability and robustness of the photocatalyst [46]. Reusability of 8%-
ZT@LLDPE for the photodegradation of TC was tested under the optimum conditions.
After each cycle, the film was removed and washed with deionised water and dried
before being placed into another fresh solution of TC for another cycle. The process was
repeated eight times, where the results are presented in Figure 11b. The prepared film
with photocatalyst exhibited comparable degradation performance, with a slight reduction
in performance (<5%) after 8 cycles of the photodegradation process. This remarkable
stability provides support for the potential utility of this photocatalyst film for practical
applications.

Molecules 2021, 26, 2509
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Table 2 presents the TC removal (%) and the required conditions of several reported
immobilised photocatalyst systems in the literature. Based on the results in Table 2, it is
noted that 8%-ZT@LLDPE requires a simple set up such as a common household fluorescent
lamps and very mild conditions. By comparison, Shams et al. [47] recently reported the
adsorption-based removal of TC (81.1%) from hospital wastewater using a vanadium
oxide nano cuboid adsorbent material (cf. Table 8 in [47]). This level of physical (non-
degradative) removal corresponds to a maximum adsorption capacity of 126.8 mg/g for
TC under optimised experimental conditions: 20.5 ◦C, pH of 4.1, mixing time of 44.5 min,
adsorbent dosage of 0.49 g/L, and initial concentration of TC = 30.7 mg/L. This is in
contrast to the relatively efficient chemical transformation of TC (89.45%) reported in
Table 2 by photocatalysis with 8%-ZT@LLDPE.
Table 2. Comparison of recently reported studies on immobilised photocatalysts with the present study on the degradation
of TC.
Photocatalyst
TC Conc.
5 mg/L
Light Source
Results
Ref.
[48]
[49]
Xe lamp
(λ > 420 nm)
77.4% after 120 min
(k = 0.01276 min−1).
60% after 120 min
(k = 0.0085 min
44% after 60 min
(k = 0.0009 min
Au/Ag/TiO2 (cellulose acetate)
TiO2
(alumina pellets)
35 mg/L
UV light
−1
)
−1
TiO2 (chitosan)
40 mg/L
UV light
)
[50]
Reusability = 4 cycles
63.8% after 120 min
Reusability = 3 cycles
92.4% after 120 min
Reusability = 4 cycles
91.59% after 90 min
Reusability = 4 cycles
90% after 120 min
H4SiW12O40 (cellulose acetate)
MoS2/BiOBr (carbon fibres)
10 mg/L
20 mg/L
50 mg/L
20 mg/L
300W Hg lamp
300 W Xe lamp
500 W Xe lamp
300W Xe lamp
[51]
[52]
[53]
[54]
Bi2WO6/ZnFe2O4
(polyurethane)
g-C3N4/rGO
(3D Ni foam)
Reusability = 4 cycles
89.45% after 180 min
−1
8%-ZT@LLDPE
40 mg/L
48W fluorescent lamp
(k = 0.01312 min
)
Present study
Reusability = 8 cycles
3. Materials and Methods
3.1. Materials
The LLDPE (LL1001 XV) was purchased from ExxonMobil Chemical (Singapore),
with a melt flow index (MFI) of 1.0 g/10 min and a density of 0.918 g/cm3. Titanium (IV)
tetraisopropoxide (TTIP, >98%), tetracycline (TC, >99%), and tetraethylorthosilicate (TEOS,
98%) were purchased from Acros Organics (Fair Lawn, NJ, USA). Zinc acetate dihydrate
(99.8%), and soluble starch were from Fisher Scientific (Leicestershire, UK); whereas toluene
(99.5%), ethanol (>99.7%), methanol, and potassium hydroxide (>95%) were purchased
from QReC (Bangkok, Thailand), Sdn Bhd. All chemicals and reagents were of analytical
grade and were used without further purification.
3.2. Synthesis of ZnO Quantum Dots (ZnO QDs)
The synthesis of ZnO QDs was carried out according to a reported method [55], with
some modifications. A zinc acetate solution was prepared by dissolving 1.1 g of zinc acetate
dihydrate in 100 mL of methanol under vigorous stirring. The pH of the solution was
raised to pH 14 using KOH solution (1 M). The resulting solution was stirred continuously
for 45 min using a magnetic stirrer. TEOS (0.25 mL) was added to the solution to stop the
ZnO nucleation process, followed by the addition of 0.5 mL of distilled water to initiate the
sol gel reaction between the silica surface and the ZnO QDs. The ZnO QDs precipitate
−
was recovered by centrifugation and washing thrice with methanol and distilled water to
remove the unreacted precursors. The ZnO QDs powder was dried at 100 ◦C for 24 h.

Molecules 2021, 26, 2509
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3.3. Synthesis of ZnO QDs Modified TiO2
The ZT was synthesised according to the procedure reported by [56
,
57], with some
modifications. A starch solution was prepared by dissolving 2 g of soluble starch in
200 mL of hot boiling water. Into the starch solution, 5.90 mL of TTIP was added and
continuously stirred for 10 min at 65 ◦C. The pH of the solution was adjusted to pH 9 by
adding ammonium hydroxide solution dropwise with constant stirring for 30 min. At this
stage, 0.26 g of ZnO QDs was added to the TiO2 sol and stirred continuously for another
1 h at 85 ◦C. Finally, the mixture was centrifuged, and the product obtained was washed
several times with ethanol and distilled water to remove any unreacted precursors. The
product was dried at 100 ◦C for 12 h, and then calcined at 500 ◦C for 2 h with a heating
rate of 3 ◦C min−1. The synthesised nanocomposite was labelled as ZT. Pristine TiO2 was
synthesised without the ZnO QDs for comparison.
3.4. Synthesis of ZnO QDs Modified TiO2/LLDPE Floating Hybrid Polymer Film
The hybrid film was prepared by using a simple solution casting method, as reported
by Saharudin et al. [31]. In a typical synthesis, 1.0 g LLDPE pellets were melted in 20 mL of
toluene at 90 ◦C under continuous stirring for 20 min. An 8 wt.% (based on LLDPE mass)
of ZT- toluene mixture was prepared by dispersing an appropriate amount of ZT in 10 mL
of toluene, and sonicated for 5 min. The ZT- toluene mixture was added dropwise into
the melted LLDPE and stirred for 5 min. Finally, the mixture was poured into a 90 mm
◦
diameter petri dish and oven-dried at 60 C for 12 h. The TGA analysis indicated that
the weight loss for the composite film was 8%. The value indicates that all the ZT added
during the preparation was incorporated into the polymer matrix. Hence, the synthesised
hybrid film was denoted as 8%-ZT@LLDPE. For comparison, bare LLDPE film, TiO2, and
ZnO QDs immobilised on LLDPE were prepared in the same way and labelled as bare
LLDPE, TiO2@LLDPE, and ZnO@LLDPE, respectively. The photographs of the films are
shown in Figure S1 (cf. Supplementary Materials).
3.5. Characterisation of the Hybrid Film Photocatalyst
The structural properties of the synthesised ZT and the hybrid film were investigated
using various characterisation techniques. The X-ray diffraction (XRD) patterns were
recorded in the 2
diffractometer (BRUKER AXS D8) with Cu–K
θ
range of 20◦–70◦ with a scanning speed of 0.02◦ min−1 using an X-ray
α
radiation (λ = 0.15478 nm). The morphology
was determined by high-resolution field emission scanning electron microscopy (FESEM),
while the surface roughness was analysed using atomic force microscopy (AFM), Dimen-
sion EDGE, BRUKER model. The wettability or water contact angle (CA) of the hybrid
films surface was measured using Data Physics OCA 20 Optical Contact Angle Goniometer,
at ambient temperature, based on the Sessile Method. The probe liquid was distilled water,
and a drop of 0.6
µL was used for every measurement. The lower values of contact angle
(
θ
) represent a more hydrophilic surface. The average θ
-value was averaged by measuring
five different positions measured on a sample. The optical properties of the photocatalysts
were measured using solid-state UV/vis diffuse reflectance spectroscopy (PerkinElmer
Lambda 35 UV/VIS Spectrometer) method by scanning the reflectance of the samples in
the range of 200 nm to 650 nm.
3.6. Photocatalytic Reactions
The photocatalytic activity of the hybrid film photocatalyst was investigated by moni-
toring the photodegradation of TC in aqueous solution using a homemade photoreactor
equipped with two fluorescent lamps (24 W each). The light intensity was determined
to be 104.4 W/m2, whereas the residual UV leakage was detected to be 0.40 W/m2. The
measurement was done using Dual-Input Data Logging Radiometer (model PMA2100)
equipped with visible, UVA, and UVB detectors. The schematic diagram of the photoreac-
tor is shown in Figure S2 (cf. Supplementary Materials). In a typical process, 5
the hybrid film was immersed in a 100 mL of 40 ppm TC solution. Before light irradiation,
×
5 cm of

Molecules 2021, 26, 2509
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the mixture was kept in the dark for 30 min to reach adsorption–desorption equilibrium.
An aliquot was withdrawn at regular time intervals. The change in TC concentration was
determined using UV–Vis spectrophotometer (UV2600, Shimadzu, Duisburg, Germany),
and the removal efficiency of TC was calculated using the following Equation (5),
Ct
C0
R = 1 −
(5)
where C0 represents the initial concentration before fluorescent light irradiation, Ct the
concentration at a time interval, and R is the TC removal efficiency.
The mineralisation of TC was studied by monitoring the decrease in total organic
carbon (TOC) content. The TOC content was measured with a Shimadzu 5000 TOC
Analyzer equipped with an autosampler. The degree of mineralisation of TC was calculated
using Equation (6):
TOC0 − TOCt
T(%) =
× 100
(6)
TOC0
where TOC0 represents the initial TOC concentration before oxidation reaction, and TOCt
is the concentration after oxidation reaction.
The reaction rate of photocatalytic degradation of TC was investigated using the
modified Langmuir–Hinshelwood model (Equation (7)). The rate constant was obtained by
plotting ln(C0/C) against time t, and the negative of the slope (k) represents the apparent
reaction rate constant.
ꢀ
ꢁ
C
C0
ln
= −kt
(7)
where k is the reaction rate (min−1), t is the irradiation time, and C0 and C are the initial
and final concentrations of the antibiotic, respectively.
The intermediates formed during the photodegradation process at every 30 min in-
terval were determined using liquid chromatography/time-of-flight/mass spectrometer
(LC/TOF/MS) reverse-phase Agilent Poroshell 120 (2.7
by Waters Acquity UPLC System, coupled with LCT Premier XE mass spectrometer system
with a binary pump and autosampler. The mobile phase consisted of 0.1% formic acid in
µ
m particle size, 4 µm id × 100 mm
)
water (mobile phase A) and methanol (mobile phase B), following isocratic elution tech-
nique at ratio 35:65, flow rate 0.2 µL/min, injection volume 5 µL, and column temperature
27 ◦C, for 15 min. The flow from the LC column was transferred to a mass spectrometer
equipped with an ESI source. The analysis was performed in positive polarity mode for all
samples. Instrument control, data acquisition, and evaluation were done with MassLynxTM
software, Version 4.1.
4. Conclusions
A floating TiO2-modified ZnO QDs/LLDPE hybrid polymer film photocatalyst (8%-
ZT@LLDPE) was successfully prepared via a solution casting method. The AFM and
water contact analyses indicate that the immobilisation resulted in an increase in the water
uptake of LDDPE due to its greater surface roughness, which indirectly increased the
TC adsorption properties. The formation of a heterojunction between ZnO QDs and the
mesoporous TiO2 favours the separation of photogenerated electron-hole pairs. On the
other hand, the QDs’ size-dependent quantum confinement effect leads to the displacement
of the conduction band potentials of ZnO QDs to more negative energy values, as compared
with mesoporous TiO2. The above-noted displacement leads to the generation of reactive
species with higher oxidation ability. Under the optimum conditions (pH 9, 90 min), TC
was successfully photodegraded up to 89.45%. The scavenging experiments reveal that
superoxide radicals were the main active species responsible for the TC photodegradation.
Moreover, the hybrid film retained its high photodegradation efficiency even after eight
cycles. Hence, the prepared 8%-ZT@LLDPE has potential for the photocatalytic treatment
of the toxic antibiotics present in industrial wastewater systems [51,58].

Molecules 2021, 26, 2509
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Supplementary Materials: The following are available online. Figure S1: The photographs of (
bare LLDPE and ( ) 8%-ZT@LLDPE hybrid films, Figure S2: Schematic diagram of the photocatalytic
reactor for the degradation of TC using the hybrid LLDPE film, Figure S3: The pH point of zero
a)
b
charge (pHPZC) of ZT nanocomposite, Figure S4: Mass spectra of (
during degradation in the presence of 8%-ZT@LLDPE.
a) TC and (b) its intermediates
Author Contributions: Conceptualisation, A.I., U.S., F.A., and S.S. Methodology, A.I., U.S., and S.S.
Validation, A.I., M.N.A., R.J.R., N.Y., and S.S. Formal analysis, A.I., U.S., M.N.A., R.J.R., N.Y., and S.S.
Investigation, A.I., U.S., M.N.A., R.J.R., N.Y., and S.S. Resources, A.I., U.S., F.A., M.N.A., R.J.R., N.Y.,
S.S., and L.D.W. Data curation, A.I., U.S., M.N.A., R.J.R., N.Y., S.S., and L.D.W. Writing—original
draft preparation, A.I., U.S., M.N.A., R.J.R., N.Y., S.S., F.A., and L.D.W. Writing—review and editing,
A.I., U.S., F.A., M.N.A., R.J.R., N.Y., S.S., and L.D.W. Visualization, A.I. and U.S. Supervision, A.I.,
N.Y., and S.S. Project administration, A.I. Funding acquisition, A.I., F.A., and S.S. All authors have
read and agreed to the published version of the manuscript.
Funding: This research was funded by the Ministry of Education Malaysia (Higher Education)
through the Fundamental Research Grant Scheme (FRGS/1/2019/STG01/USM/02/7).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: Usman Saidu acknowledges Sule Lamido University Kafin Hausa, Nigeria, for
the PhD sponsorship awarded to him through the Tertiary Education Trust Fund (TETFund). The
author R.J. is grateful to the Deanship of Scientific Research, King Saud University, for financial
support through the Vice Deanship of Research Chairs.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
Sample Availability: Not applicable.
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