Article

From Anatase TiO2 Nano-Cuboids to Nano-Bipyramids:

Influence of Particle Shape on the TiO2 Photocatalytic
Degradation of Emerging Contaminants in Contrasted
Water Matrices
Humaira Asghar 1 , Daphne Hermosilla 2 , Francesco Pellegrino 1 , Virginia Muelas-Ramos 2 ,
Christian de los Ríos 2 , Antonio Gascó 2 , Valter Maurino 1, * and Muhammad Ahsan Iqbal 3

1 Department of Chemistry and UNITO-ITT Joint Lab, University of Torino, Via Giuria 7, 10125 Torino, Italy;
humaira.asghar@unito.it (H.A.); francesco.pellegrino@unito.it (F.P.)
2 G-Aqua Research Group, Departamento de Ingeniería y Gestión Forestal y Ambiental, Escuela Técnica
Superior de Ingeniería de Montes, Forestal y del Medio Natural, Universidad Politécnica de Madrid, C/José
Antonio Novais 10, 28040 Madrid, Spain; daphne.hermosilla@upm.es (D.H.);
virginia.muelas@upm.es (V.M.-R.); christian.delosrios.quinones@alumnos.upm.es (C.d.l.R.);
antonio.gasco@upm.es (A.G.)
3 Departamento de Ingeniería Química y de Materiales, Facultad de Ciencias Químicas, Universidad
Complutense de Madrid, 28040 Madrid, Spain; miqbal@ucm.es
* Correspondence: valter.maurino@unito.it

Abstract: Water pollution, resulting from industrial effluents, agricultural runoff, and
pharmaceutical residues, poses serious threats to ecosystems and human health, high-
lighting the need for innovative approaches to effective remediation, particularly for
non-biodegradable emerging pollutants. This research work explores the influence of
shape-controlled nanocrystalline titanium dioxide (TiO2 NC), synthesized by a simple
hydrothermal method, on the photodegradation efficiency of three different classes of
emerging environmental pollutants: phenol, pesticides (methomyl), and drugs (sodium
Academic Editors: Liangliang Feng
and Yipu Liu diclofenac). Experiments were conducted to assess the influence of the water matrix on
treatment efficiency by using ultrapure water and stormwater (basic) collected from an
Received: 24 November 2024
Revised: 11 January 2025 urban drainage system as matrices. The size and shape of the nano-cuboids were accurately
Accepted: 13 January 2025 controlled during synthesis to assess their impact on photoactivity and selectivity. Re-
Published: 20 January 2025 garding total organic carbon removal using TiO2 nano-cuboids in basic environments, the
Citation: Asghar, H.; Hermosilla, D.; results were particularly remarkable. TiO2 nano-cuboids and truncated bipyramids synthe-
Pellegrino, F.; Muelas-Ramos, V.; de sized in the 200–250 ◦ C temperature range showed an enhanced photocatalytic efficiency
los Ríos, C.; Gascó, A.; Maurino, V.; when compared to alternative formulations. Diclofenac, methomyl, and phenol were fully
Iqbal, M.A. From Anatase TiO2
mineralized from ultrapure water and basic stormwater. The TiO2 nano-cuboids/nano-
Nano-Cuboids to Nano-Bipyramids:
Influence of Particle Shape on the
bipyramids demonstrated better selectivity and photoactivity in comparison to irregular
TiO2 Photocatalytic Degradation of TiO2 nanoparticles. The differences in photoactivity and selectivity are explained in terms
Emerging Contaminants in of charge carrier separation and trapping on the different crystal facets. Their performance
Contrasted Water Matrices. Molecules demonstrates their potential as sustainable materials for the photodegradation of emerging
2025, 30, 424. https://doi.org/ pollutants in various water matrices.
10.3390/molecules30020424

Copyright: © 2025 by the authors. Keywords: shape-controlled TiO2 ; nano-cuboids; photocatalysis; dyes; diclofenac;
Licensee MDPI, Basel, Switzerland. methomyl; stormwater
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license
(https://creativecommons.org/
licenses/by/4.0/).

Molecules 2025, 30, 424 https://doi.org/10.3390/molecules30020424

Molecules 2025, 30, 424 2 of 17

1. Introduction
Human activities have resulted in the widespread presence of organic contaminants,
such as pesticides, dyes, and drug residues, in soils and waters. New water contaminants
classified as emerging concerns are difficult to remove using conventional biological treat-
ment technologies and can be hazardous even at low concentrations [1–3]. Therefore, new
and more effective tertiary and quaternary water treatment strategies that guarantee the re-
moval and mineralization of these non-biodegradable and polar contaminants, preserving
aquatic ecosystems as well as a safe drinking water supply, are urgently needed.
Photocatalysis has emerged as a viable method to treat emerging organic pollutants
in recent years. This advanced water treatment technology leverages the power of light-
activated catalysts to break down complex and persistent contaminants, offering several
advantages, namely low cost, mild operational conditions, no need for chemicals other
than oxygen, the possibility to use solar light to activate the catalysts, and the absence of
byproducts [4–7]. Its main drawbacks are a low photonic efficiency caused by fast charge
carrier recombination rates and, in liquid systems, the low mass transfer rates [8,9].
The photocatalytic treatment of various contaminants has already been reported in
studies considering the effect of different water matrices and the shape of the photocatalyst
nanoparticles on the efficiency of several pollutant classes [10–15]. It was established that
the exposed facets of TiO2 have significantly influenced photocatalytic activity and the
spatial separation of charge carriers. Different crystal faces can be exposed by control-
ling the size and shape of photocatalytic materials at the nanoscale, each with distinct
surface states and catalytic properties. This nano-structuring can enhance the photocat-
alytic efficiency of TiO2 by improving charge carrier separation and transport across the
liquid–solid interface. Several controlled synthesis methods have been attempted in the last
several decades to develop TiO2 nanostructures with various morphologies, such as sol-gel,
hydrothermal, microwave-assisted, and chemical vapor deposition [16,17]. In particular,
the cost-effective hydrothermal process produces nanomaterials with a large surface area
and a high degree of crystallinity, in contrast to other methods. In particular, TiO2 can
be successfully produced in a range of morphologies using the hydrothermal approach,
including nanowires, nanotubes, nanobelts, nanorods, nanosheets, nanoflowers, and hi-
erarchical microspheres [17–22]. The control of the shape and size of TiO2 nanoparticles
allows the study of structure/photoactivity relationships, unravelling the understanding
of various insights affecting the process (e.g., photogenerated charge carrier separation and
trapping, and the role of the adsorption of ions and substrates) and enabling the tuning
and optimization of the photocatalytic activity of the material, and, possibly, its selectiv-
ity [23–27]. It is worth noting that the shape and exposed facets of TiO2 crystals play a
crucial role in determining their photoelectrochemical and photocatalytic performance due
to differences in surface energy, bandgap alignment, and electronic band topologies [28].
The exposed high-energy facets of TiO2 crystals can enhance photocatalytic activity by
promoting surface adsorption and interfacial electron transfer. In this regard, nanomaterials
with controlled distribution of facets have been recognized as very active in photocatalytic
applications spurring the development of new nanomaterials optimized for photocatalytic
conversion of new pollutants in diverse water matrices [28].
In this study, we developed a cost-effective hydrothermal synthesis method to produce
TiO2 nano-cuboids with varied morphologies and high crystallinity. The one-pot hydrother-
mal approach utilized titanium tetraisopropoxide (TTIP) as the titanium precursor and
acetic acid as a capping agent to control the shape of the nanoparticles. Hydrothermal
conditions, such as temperature and treatment time, were optimized to tailor the surface
morphologies and achieve distinct structural features. To evaluate the photocatalytic effi-
ciency, the shape-controlled catalysts were evaluated in different water matrices (ultrapure
Molecules 2025, 30, 424 3 of 17

and basic stormwater). The diclofenac, methomyl, and phenol were selected as model
pollutants in this study due to their prevalence in water systems, diverse chemical struc-
tures, and hazardous impacts (Table S1). Diclofenac, a widely used anti-inflammatory
drug, is increasingly detected in freshwater systems, where it poses toxicity risks to aquatic
organisms. Its aromatic structure and functional groups, as well as the presence of chlorine,
contribute to its resistance to conventional treatment methods, often resulting in removal
efficiencies ranging from 30% to 70% [29,30]. Methomyl, a carbamate-based insecticide, is
highly soluble in water, leading to contamination of both ground and surface waters. The
thiol ester group in its structure plays a key role in its chemical stability and degradation
pathways, while its low sorption affinity to soils ensures its high mobility in aquifers, rais-
ing concerns about its long-term environmental effects [31,32]. Phenol and its derivatives
are well known for their bio-recalcitrant and acute toxicity and are commonly found in
industrial wastewater [33]. These pollutants were chosen to represent a broad spectrum of
chemical complexities and environmental concerns, providing a diverse set of candidates
to evaluate the performance of the synthesized photocatalyst. TiO2 nano-cuboids with
varying morphologies were characterized using X-ray diffraction (XRD), field emission
scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), Fourier
transform infrared spectroscopy (FTIR) and UV–visible diffuse reflectance spectroscopy
(UV-Vis-DRS), and their photocatalytic performance was evaluated by degrading phenol,
methomyl, and diclofenac in stormwater and Milli-Q water under UV light. The correlation
between controlled morphology and pollutant degradation provides valuable insights
for designing next-generation photocatalysts, highlighting the noteworthy potential of
nano-cuboid/nano-bipyramid-based systems for efficient pollutant removal.

2. Results and Discussion

2.1. Structural Analysis
Figure 1 shows field emission scanning electron microscopy (FESEM) images of synthe-
sized TiO2 nanoparticles, where the reaction temperature exhibited a considerable impact
on the shape of the particles and the developed TiO2 nanostructures transformed from
nano-cuboid-shaped particles to truncated bipyramids. Specifically, the particles trans-
formed from larger nano-cuboids at a temperature of 175 ◦ C into well-defined cuboidal
shapes with good size uniformity at 200 ◦ C (TN-200 ◦ C). This suggests that the reaction
temperature plays a critical role in determining the shape of the TiO2 particles and that a
200 ◦ C temperature leads to a more uniform and defined cuboidal shape. When TiO2 is
hydrothermally synthesized at 225 ◦ C (TN-250 ◦ C), the crystallite shapes start to distort,
and at 250 ◦ C, they transform into truncated bipyramids. This suggests that the TiO2
crystals are undergoing a morphological transition, resulting in a change in their shape and
structure. It is worth noting that such changes in the morphology of TiO2 can significantly
affect its properties and, therefore, can be considered a potential photocatalyst (discussed
in a later section). The high-resolution FESEM images highlight evidence that the tetrag-
onal nano-cuboids have characteristic square-like top facets and bar-like lateral facets. A
[001]-elongated anatase single crystal with four {100} lateral facets and two {001} top facets
can be seen if we concentrate on the image of a single nano-cuboid lying on its one lateral
facet, which shows sharp diffraction spots attributed to the [010] zone axis of the tetragonal
anatase TiO2 crystal. The two flat square surfaces are designated as the {001} facets, while
the remaining four rectangular surfaces are referred to as the {100} facets, based on the
symmetry of the well-defined anatase TiO2 crystals.
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Figure
Figure1.1.
FESEM
FESEM
FESEM images
images ofofTiO
images TiO
of 2 2nano-cuboids
TiO nano-cuboids prepared
preparedunder
underdifferent
differentreaction
reactiontemperature
temperaturecon-
con-
Figure 1. 2 nano-cuboids prepared under different reaction temperature
ditions
ditionsatat24
24h:
h:(a,e)
(a,e)TN-175
TN-175 °C;
°C;(b,f)
◦ TN-200
(b,f) TN-200°C;
°C;(c,g)
◦
(c,g)TN-225
TN-225°C;
°C;and
◦
and(d,h)
(d,h)TN-250
TN-250
conditions at 24 h: (a,e) TN-175 C; (b,f) TN-200 C; (c,g) TN-225 C; and (d,h) TN-250 C. °C.
°C. ◦

The
TheX-ray
The X-raydiffraction
X-ray diffraction(XRD)
diffraction (XRD)patterns
(XRD) patternsof
patterns ofthe
of theTiO
the TiO222NCs
TiO NCsdeveloped
developedunder undervarious
varioustem-tem-
perature
peratureconditions
perature conditionsare
conditions arereported
are reportedin
reported inFigure
in Figure2.
Figure 2.2.The
Thenano-cuboid
The nano-cuboidTiO
nano-cuboid TiO222displays
TiO displayscharacteristic
displays characteristic
characteristic
◦ , 37.8 ◦ , 48.0 ◦ , 53.9 ◦ , 55.0 ◦ , 62.7 ◦ , 68.7 ◦ , 70.3
diffraction
diffractionpeaks
diffraction peaksatat at2θ2θvalues
2θ values
values ofofof
25.3°,
25.337.8°,
25.3°, 37.8°,48.0°,
48.0°, 53.9°,
53.9°, 55.0°,
55.0°, 62.7°,
62.7°, 68.7°,
68.7°, 70.3°,
70.3°, and◦75.0°,
and ,75.0°,
and
◦
corresponding totothe
75.0 , corresponding
corresponding the(101),
to the(004),
(101), (101),
(004), (200),
(200), (105),
(004), (200),
(105), (211), (204),
(105),
(211), (116),
(211),
(204), (220),
(204),
(116), and
(116),
(220), (215)
(215)planes
and(220), and ofof
(215)
planes
tetragonal
planes
tetragonal anatase
of tetragonal TiO
anatase TiO (JCPDS
anatase
2 2 (JCPDS card
TiOcard no.
2 (JCPDS 21-1272). Comparatively,
card no.Comparatively,
no. 21-1272). the
21-1272). Comparatively, TiO
the TiO2 NCs
2 NCs developed
the developed
TiO2 NCs
under
undercertain
developed reaction
under
certain temperatures
certain
reaction exhibited
exhibitedcomparable
reaction temperatures
temperatures diffraction
diffractionpatterns,
exhibited comparable
comparable patterns,with
diffraction slight
patterns,
with slight
variations
with in peak
slight variations
variations intensity for
in peak for
in peak intensity all
intensityplanes. With
for allWith
all planes. the
planes. increase
the With in reaction
the increase
increase in reaction temperature,
in reaction the
tempera-
temperature, the
(101) ◦
(101)peak
ture, the
peak intensity
(101) varies,
varies,where
peak intensity
intensity varies,
where the TN-250
where
the TN-250the°C °Cspecimen
TN-250
specimen shows
showsaashows
C specimen relatively
relatively less
lessintense
a relatively less
intense
broader
intense peak
peakintensity,
broaderbroader possibly
peak intensity,
intensity, possibly due
duetotothe
possibly thepresence
due ofofmixed
to the presence
presence mixed of morphologies
mixed such
morphologies
morphologies suchas nano-
such
as nano-as
cuboids
cuboidsand
nano-cuboidsandtruncated
and truncated
truncated pyramids. Furthermore,
pyramids.
pyramids. Furthermore,
Furthermore, for
forcomparatively
for comparatively
comparatively small nanocrystals,
smallsmall dif-
nanocrystals,
nanocrystals, dif-
fraction
diffractionpeaks
peakscan canbroaden
broaden and and shift due
shift due
fraction peaks can broaden and shift due to slight changes to slight
to slightchanges
changes in
in the
the interatomic
interatomic distances
distances
interatomic distances
(TN-225 ◦ C).However,
(TN-225°C).
(TN-225 However,no
°C). However, noother
no othersignificant
other significantchanges
significant changesare
changes areobserved.
are observed.

Figure
Figure2.2.XRD
Figure XRDpatterns
XRD patternsofof
patterns ofTiO2 2nano-cuboids
TiO
TiO2 nano-cuboidsprepared
nano-cuboids preparedunder
prepared underdifferent
under differentconditions.
different conditions.
conditions.
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Figure
Figure33displays
displaysthetheFourier
Fouriertransform
transforminfrared
infraredspectroscopy
spectroscopy(FTIR)
(FTIR)spectra
spectraofofTiO
TiO22
NCs synthesized at various temperatures for 24 h. Ti-O stretching
NCs synthesized at various temperatures for 24 h. Ti-O stretching and Ti-O-Ti bridgingand Ti-O-Ti bridging
stretching −1−1
stretchingmodes
modesare areresponsible
responsiblefor forthethebands
bandsseen seenbetween
between400 400andand1000
1000cmcm . .The
The
stretching
stretching of the O-H mode of the hydroxyl group causes the broadband between and
of the O-H mode of the hydroxyl group causes the broadband between 3000 3000
3600 −1 The cm−1cm
and cm
3600 .cm −1. band at 1659
The band at 1659 for−1the
for TiO
the2TiOnanoparticles is attributable
2 nanoparticles to thetoOthe
is attributable C-O-Ti
O C-
link of the carbonate structure [28]. The 1535 cm −1 band represents N-H in-plane bending
O-Ti link of the carbonate structure [28]. The 1535 cm band represents N-H in-plane
−1

and C-N stretching vibrations, while the while

1179 cm −1 band is caused by C-N stretching [34].
bending and C-N stretching vibrations, the 1179 cm−1 band is caused by C-N stretch-
There are no nitrogen-related bands
ing [34]. There are no nitrogen-related bands in TiO , implying that the N
2 in TiO2, implying species
that the Nare interstitial.
species are in-
As the reaction temperature rises from 175 ◦ C to 250 ◦ C, the O-H stretching mode of the
terstitial. As the reaction temperature rises from 175 °C to 250 °C, the O-H stretching mode
hydroxyl group becomes weaker.
of the hydroxyl group becomes weaker.

Figure 3.
Figure 3. FTIR spectra of developed nano-cuboid TiO
TiO22 under different
different reaction conditions.

For
Forbetter
bettercomparison,
comparison,X-ray X-rayphotoelectron
photoelectronspectroscopy
spectroscopy(XPS) (XPS)analysis
analysisisisconducted
conducted
on ◦ ◦
onthe
thespecimens
specimensdeveloped
developed at at
200200C °C andand250250 C and
°C andthe specimen
the specimen without post-treatment
without post-treat-
(see Section 3.2) to understand the role of the selected synthesis solution (TN-200 ◦ C,
ment (see Section 3.2) to understand the role of the selected synthesis solution (TN-200 °C,
TN-250 ◦
TN-250 C) °C)on ontheir
theirstructural
structural compositions
compositions (Figure
(Figure4). 4).
TheThebroad
broadscan survey
scan surveyspectrum
spectrum in-
dicated that TiO
indicated that TiO 2 particles include C, N, Ti, and O elements, with photoelectron
2 particles include C, N, Ti, and O elements, with photoelectron peaks peaks with
binding energies
with binding of 284.8
energies of (C 1s),(C
284.8 400.09 (N 1s),(N
1s), 400.09 458.8
1s), (Ti 2p),
458.8 (Tiand
2p),527.5
and (O527.51s),(Orespectively.
1s), respec-
The
tively. The untreated material showed the 683.88 eV (F 1s) peak, demonstrating the of
untreated material showed the 683.88 eV (F 1s) peak, demonstrating the removal re-
surface
moval fluorine
of surface and other residual
fluorine and other organic
residualcompounds following post-treatment.
organic compounds following post-treat- Ionic
liquids are anliquids
ment. Ionic environmentally friendly and friendly
are an environmentally operationally safe alternative
and operationally safetoalternative
other corro- to
sive fluoride sources, such as HFsuch [35]. as
The +
other corrosive fluoride sources, HF[bmim]
[35]. Theion adsorbed
[bmim] on the surface
+ ion adsorbed on theofsurface
TiO2
nanocrystals is responsible
of TiO2 nanocrystals for the Nfor
is responsible 1s the
peak Nat 1s401.1
peakeV. The nitrogen
at 401.1 eV. The atoms
nitrogen in the
atoms C-Nin
bonds
the C-N may comemay
bonds from the capping
come from the agent capping or the
agent precursor. Moreover,
or the precursor. the C–O bond
Moreover, the C–O is
responsible for the weak peak at 284.8 eV (Figure 4b) [36,37].
bond is responsible for the weak peak at 284.8 eV (Figure 4b) [36,37]. Numerous studies Numerous studies have
connected
have connected carbonate species,
carbonate namely
species, Ti-O-C,
namely on theon
Ti-O-C, TiO
the2 surface to C dopants.
TiO2 surface to C dopants. On the On
other
the other hand, substitutional C-doping can be excluded, due to the absence of a peakatat
hand, substitutional C-doping can be excluded, due to the absence of a peak
281–282
281–282eV, eV,which
whichis is characteristic
characteristic of of
Ti-C
Ti-Cbonding
bonding (O-Ti-C
(O-Ti-Clinkage).
linkage).According
According to Figure
to Figure4c,
the binding energies of
4c, the binding energies of Ti3/2 Ti 2p and Ti 2p
2p3/2 and Ti 1/2 are 458.8 and 464.4 eV, respectively.
2p1/2 are 458.8 and 464.4 eV, respectively. This This
suggests
suggeststhat thatthethesurface
surfaceTiTioxidation
oxidationstate stateisisidentical
identicaltotothat
thatofofbulk
bulkTiOTiO2 2[37–39].
[37–39].As Asseen
seen
by the approximately 5.7 eV split between the Ti 2p and Ti 2p core levels, the Ti 4+ in
by the approximately 5.7 eV split between the Ti 2p1/2 and Ti 2p3/2 core levels, the Ti4+ in
1/2 3/2
the
theTiO
TiO2 2isisin
inaanormal
normalstate.
state.ItItisisimportant
importantto tonote
notethat
thatalthough
althoughthe thedifferent
differentcalcination
calcination
temperatures
temperatures lead to the significant surface morphological refinement of TiO2particles,
lead to the significant surface morphological refinement of TiO 2 particles,
from
fromnano-cuboids
nano-cuboidstoto nano-bipyramids,
nano-bipyramids, their chemical
their chemicalcomposition
composition is comparable
is comparable for bothfor
types of surface morphologies.
both types of surface morphologies.
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Figure
Figure4.
Figure 4.4.XPS
XPS
XPSspectra
spectra of TiO
ofof
spectra TiO 22 nano-cuboids
TiO nano-cuboids and
and nano-bipyramids:
nano-bipyramids: (a)
(a) full
full scan,
scan, (b)
(b) C
C 1s,
1s, (c)
(c) Ti
Ti 2p,
2p, (d)
2 nano-cuboids and nano-bipyramids: (a) full scan, (b) C 1s, (c) Ti 2p,
(d)
O
O 1s.
(d)1s.
O 1s.

2.2. UV–Visible
2.2.
2.2. UV–VisibleDRS
UV–Visible DRSSpectra
DRS Spectra
Spectra
The diffuse
The
The diffuse reflectance
diffuse reflectance spectrum
reflectance spectrum
spectrum of of TiO
of TiO nano-cuboids is
TiO222 nano-cuboids
nano-cuboids is shown
is shown in
shown in Figure
in Figure 5a,
Figure 5a, with
5a, with
with
the fundamental absorption edge located at 385–400 nm. For the TN-200 ◦ C, there is aaa
the
the fundamental
fundamental absorption
absorption edgeedge located
located atat 385–400
385–400 nm.nm. For
For the
the TN-200
TN-200 °C,
°C, there
there is
is
noticeable
noticeable shift
noticeable shift in
shift in the
in the absorption
the absorption
absorption edge edge into
edge into the
into the visible
the visible light
visible light area
light area (wavelengths
area (wavelengths longer
(wavelengths longer than
longer than
than
400
400 nm).
400 nm). This
nm). This shift
This shift could
shift could be
could be explained
be explained
explained by by the
by the existence
the existence
existence ofof oxygen
of oxygen vacancy
oxygen vacancy states,
vacancy states, which
states, which
which
result from
from the production of Ti 3+ species inside the TiO lattice.
result
result from the
the production
production of of Ti
Ti3+3+ species
species inside
inside the
the TiO
TiO222lattice.
lattice.

Figure
Figure 5.
5. (a)
(a) UV-Vis
UV-Vis absorption
absorption spectra,
spectra, (b)
(b) bandgap
bandgap of
of developed
developed TiO
TiO222 nano-cuboids.
nano-cuboids.
Molecules 2025, 30, 424 7 of 17

The bandgap energies (Eg ) of synthesized TiO2 NCs may be determined using Tauc
plots, taking into account that TiO2 is an indirect bandgap material. Figure 5b depicts a
plot of (αhv)2 against hν, where α is the absorption coefficient and hν is the photon energy.
The bandgaps of the TiO2 NCs synthesized at 175 ◦ C, 200 ◦ C, 225 ◦ C, and 250 ◦ C were 3.16,
3.05, 3.14, and 3.12 eV, respectively. It was found that surface carbonaceous and carbonate
species, as well as interstitial nitrogen species (Ti-O-N), did not significantly alter the TiO2
bandgap. What should be underlined is that, in comparison to structural studies (XRD),
the analysis of the pure anatase form demonstrates a smaller bandgap, which can lead to
absorption in the visible energy range, thereby increasing TiO2 photoconversion efficiency.

2.3. Photoactivity of Anatase Nano-Cuboids and Nano-Bipyramids

The photoactivity assessments of the different samples were carried out measuring
the photoconversion rates of three distinct pollutants under UV irradiation (385 nm). The
measurements were performed over an irradiation time of 120 min, and TOC measurements
were conducted before and after the end of the degradation experiment. Preliminary
adsorption tests were carried out in dark conditions for 30 min; these experiments showed
a negligible amount of adsorption on these catalysts (<1%). The reaction kinetics in the
photodegradation (first-order kinetics) was also calculated (Tables S2–S4) by the Langmuir–
Hinshelwood model (low coverage case), fitting the time profile data with Equation (1).
 
C
− ln = kr k ad t (1)
C0

where kr = intrinsic rate constant, kad = adsorption equilibrium constant, t = time, and
C0 represents the initial pollutant concentration in the system. The obtained starting
rate values were utilized to compare the photocatalytic process’s efficiency across varied
reaction circumstances.

2.3.1. Photodegradation of Phenol

TiO2 photocatalysis is able to convert phenol into carbon dioxide and water. The
primary photoconversion intermediates are di-hydroxylated derivatives and quinones
(e.g., catechol and benzoquinone) and low amounts of condensation products (e.g., phe-
noxyphenols). Orto dihydroxy derivatives then undergo ring opening with the formation
of dicarboxylic acids. In the mineralization phase, these compounds are mineralized to CO2
and H2 O [40], as confirmed by total organic carbon (TOC) measurements. To explore its
effectiveness, reactions were carried out with developed morphologies of nano-cuboids, in
two different water matrices (ultrapure water (UW) and highly basic stormwater (SW)) and
at the 0.1 mM fixed concentrations of phenol. The catalytic performance of the cuboids and
truncated bipyramids obtained at 200 ◦ C and 250 ◦ C, respectively, was much better with
respect to the corresponding nanomaterials synthesized at a lower temperature (Figure 6).
Both catalysts showed the best photodegradation efficiency within 20 min (UW), as shown
in Figure 6a, and complete degradation was achieved in 30 min in the case of rainwater,
which might have been due to the presence of salts; however, it was still significant. All
the selected catalysts demonstrated substantial degradation of phenol in just 20–45 min in
both water matrices. The higher surface area of the nanoparticles and active sites is what
causes the efficient degrading behavior. Better degradation could be due to the larger levels
of reactive hydroxyl radicals caused by an increase in electron–hole pairs on the catalyst
surface. A slower rate of photogenerated charge recombination, improved light absorption
by the materials, and improved interfacial charge transfer are some of the possible causes
of this increased activity [41,42]. First-order kinetics concerning phenol concentrations
were found to fit all the experimental data, and first-order rate constants were estimated.
Molecules 2025,
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The
wererate of degradation
estimated. The rate of
of phenol was also
degradation calculated
of phenol by the
was also Langmuir–Hinshelwood
calculated by the Langmuir–
model. Table S2 (Supplementary data) illustrates the results obtained
Hinshelwood model. Table S2 (Supplementary data) illustrates the results for the photocatalytic
obtained for
treatment of the target
the photocatalytic pollutant
treatment in UW
of the targetaspollutant
well as ininSW.
UW as well as in SW.

Figure 6.6. Phenol

Figure Phenolphotodegradation
photodegradationofofdeveloped
developed TiO2 and
TiO pseudo-first-order
2 and kinetics
pseudo-first-order plots:
kinetics (a,b)
plots: ul-
(a,b)
trapure water (UW), (c,d) harvested stormwater (SW).
ultrapure water (UW), (c,d) harvested stormwater (SW).

2.3.2. Photodegradation of Methomyl

2.3.2. Photodegradation of Methomyl
The influence of the water matrix on pesticide (methomyl) photodegradation in the
The influence of the water matrix on pesticide (methomyl) photodegradation in the
presence of TiO2 nano-cuboids under UV irradiation is shown in Figure 7. Some important
presence of TiO2 nano-cuboids under UV irradiation is shown in Figure 7. Some important
differences can be observed. Almost all the pesticide was completely degraded in ultrapure
differences can be observed. Almost all the pesticide was completely degraded in ul-
water after 90 min of irradiation for TN-200 ◦ C and for TN-250 ◦ C (Figure 7). Conversely,
trapure water after 90 min of irradiation for TN-200 °C and for TN-250 °C (Figure 7). Con-
comparatively slower pesticide degradation was always addressed (120 min) when rain-
versely, comparatively slower pesticide degradation was always addressed (120 min)
water was used. In terms of mineralization, sufficient TOC conversions were obtained
when rainwater was used. In terms of mineralization, sufficient TOC conversions were
for TN-200 ◦ C and TN-250 ◦ C, with values of 41% and 55% for ultrapure water and 25%
obtained for TN-200 °C and TN-250 °C, with values of 41% and 55% for ultrapure water
and 39% for rainwater after 120 min, respectively; this was most likely due to the pres-
and 25% and 39% for rainwater after 120 min, respectively; this was most likely due to the
ence of refractory byproducts in aqueous solution. The corresponding degradation curves
presence of refractory byproducts in aqueous solution. The corresponding degradation
were fitted to pseudo-first-order kinetic constants, assuming that a modified Langmuir–
curves were fitted to pseudo-first-order kinetic constants, assuming that a modified Lang-
Hinshelwood kinetic model adequately described the kinetics of this photocatalytic process,
muir–Hinshelwood kinetic model adequately described the kinetics of this photocatalytic
allowing the apparent kinetic constant, kpesticide , to be calculated for each catalyst. Table S3
process, allowing the apparent kinetic constant, kpesticide, to be calculated for each catalyst.
(Supplementary data) shows that all the nanoparticles had high coefficients of determina-
Table S3 (Supplementary data) shows that all the nanoparticles had high coefficients of
tion (r2 = 0.9726–0.9995). The acquired results were comparably significant when compared
determination (r2 = 0.9726–0.9995). The acquired results were comparably significant
to comparable findings in previous literature investigations [43–45].
when compared to comparable findings in previous literature investigations [43–45].
Molecules 2025, 30,
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Figure 7.
Figure 7. Methomyl
Methomylphotodegradation
photodegradation of of
developed TiOTiO
developed 2 andand
2
pseudo-first-order kinetics
pseudo-first-order plots:plots:
kinetics (a,b)
ultrapure
(a,b) waterwater
ultrapure (UW), (c,d) (c,d)
(UW), stormwater (SW).(SW).
stormwater

2.3.3. Photodegradation of
2.3.3. Photodegradation of Sodium
Sodium Diclofenac
Diclofenac
As was previously discussed, diclofenac
As was previously discussed, diclofenac breaks down
breaks downunder photocatalytic
under treatment
photocatalytic treat-
into a variety of species, resulting in the identification of many hydroxy-
ment into a variety of species, resulting in the identification of many hydroxy- and dihy- and dihydroxy-
diclofenac derivatives,
droxy-diclofenac which were
derivatives, whichthenwerefurther
thenconverted into derivatives
further converted of chloro- or
into derivatives of
hydroxyl-phenol. Complete mineralization is eventually attained
chloro- or hydroxyl-phenol. Complete mineralization is eventually attained throughthrough the ring opening,
the
which leads towhich
ring opening, the production
leads to theof dicarboxylic
production acids [46–48].
of dicarboxylic Figure
acids 8 depicts
[46–48]. Figure 8the time
depicts
profiles of the DCF concentration in ultrapure water and rainfall exposed
the time profiles of the DCF concentration in ultrapure water and rainfall exposed to UV to UV radiation.
During
radiation. theDuring
experiment, it was observed
the experiment, it wasthat the degradation
observed of diclofenac
that the degradation ofwas minimal
diclofenac in
was
the absence of a photocatalyst. The degradation rates measured in
minimal in the absence of a photocatalyst. The degradation rates measured in ultrapureultrapure water were
higher than those
water were higherobtained
than thosewithobtained
stormwater,withwhich might bewhich
stormwater, attributable
might beto the presence of
attributable to
salts that can adsorb the degradation process. As shown in Figure 8,
the presence of salts that can adsorb the degradation process. As shown in Figure 8, andand as summarized
in ◦ C and TN-225 ◦ C had approximately 99% degradation in 120 min,
as Table S5, TN-175
summarized in Table S5, TN-175 °C and TN-225 °C had approximately 99% degrada-
whereas TN-200 ◦ C and TN-250 ◦ had remarkably fast degradation in just 45 min in UW,
tion in 120 min, whereas TN-200C°C and TN-250 °C had remarkably fast degradation in
while it was 45 and 60 min in rainwater,
just 45 min in UW, while it was 45 and 60 respectively. The maximum
min in rainwater, TOC removal
respectively. of 79%
The maximum
was achieved with catalyst TN-250 ◦ C in UW, and it was 67% for rainwater, as shown in
TOC removal of 79% was achieved with catalyst TN-250 °C in UW, and it was 67% for
Table S4 (Supplementary
rainwater, as shown in Table data).
S4 (Supplementary data).
Molecules 2025, 30,
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Figure 8.
Figure 8. Diclofenac
Diclofenacphotodegradation
photodegradation of of
developed TiOTiO
developed 2 andand
2
pseudo-first-order kinetics
pseudo-first-order plots:plots:
kinetics (a,b)
ultrapure
(a,b) waterwater
ultrapure (UW), (c,d) (c,d)
(UW), harvested stormwater
harvested (SW).(SW).
stormwater

2.3.4.
2.3.4. Comparative
Comparative Analysis
Analysis of of the
the Photodegradation
Photodegradation Efficiency
Efficiency
TiO
TiO2 has potential as a photocatalyst for degrading organic
2 has potential as a photocatalyst for degrading organic pollutants;
pollutants; however,
however, it it
faces challenges due to limited solar light absorption, low quantum
faces challenges due to limited solar light absorption, low quantum efficiency, and efficiency, andinade-
inad-
equate
quate massmass transfer
transfer in in liquid
liquid environments.
environments. These These limitations
limitations stemstem from
from undesirable
undesirable
properties, fast recombination rates, and colloidal instability of nanosized
properties, fast recombination rates, and colloidal instability of nanosized particles, particles, lead-
lead-
ing
ing to
to aggregation
aggregation and and reduced
reduced lightlight absorption.
absorption. However,
However, shape-controlled
shape-controlled anatase anatase
nano-cuboids
nano-cuboids (main planes exposed (001) and (100)) and nano-bipyramids (plane
(main planes exposed (001) and (100)) and nano-bipyramids (plane exposed
exposed
(101)
(101) and
and (001))
(001)) possibly
possibly enhanced
enhanced charge
charge carrier
carrier separation,
separation, suppressing
suppressing electron–hole
electron–hole
recombination, and showed relatively good photodegradation
recombination, and showed relatively good photodegradation efficiency efficiency andand different
different se-
selectivity for three different types of pollutants. The nano-cuboids
lectivity for three different types of pollutants. The nano-cuboids were synthesized via a were synthesized via
acost-effective,
cost-effective, scalable
scalable hydrothermal
hydrothermal method,
method, ensuring
ensuring feasibility
feasibility forfor large-scale
large-scale appli-
applica-
cations.
tions. A Asurfactant-free
surfactant-freesynthesis
synthesisapproach
approachminimized
minimized harmful
harmful additives,
additives, emphasizing
emphasizing
eco-friendliness. Their compact size, compared to commercial
eco-friendliness. Their compact size, compared to commercial TiO22 (Degussa TiO (Degussa P25),P25), facili-
facili-
tates
tates efficient
efficient regeneration
regeneration and and reuse,
reuse, overcoming
overcoming challenges
challenges like
like particle
particle agglomeration
agglomeration
and recovery issues associated with P25’s small size (10–50
and recovery issues associated with P25’s small size (10–50 nm). Thermal nm). Thermal treatment demon-
treatment
strated a morphology
demonstrated transition,
a morphology combining
transition, features features
combining of nano-cuboids and bipyramidal
of nano-cuboids and bi-
structures, which formed heterostructures that enhance charge
pyramidal structures, which formed heterostructures that enhance charge separation separation and improve and
photocatalytic activity. These materials provide a sustainable, efficient
improve photocatalytic activity. These materials provide a sustainable, efficient alterna- alternative to P25
for large-scale water treatment technologies.
tive to P25 for large-scale water treatment technologies.
In
In the
the treatment
treatment of of organic
organic pollutants
pollutants through
through photocatalysis,
photocatalysis, the the catalyst’s
catalyst’s perfor-
perfor-
mance varied significantly depending on whether UW or SW effluents
mance varied significantly depending on whether UW or SW effluents were used to pre- were used to prepare
the
parepollutant mixture.
the pollutant The use
mixture. TheofuseUW ofwater generally
UW water resultsresults
generally in higher photo-efficiency,
in higher photo-effi-
ciency, with a higher TOC removal rate (approximately 79%) and complete degradation
Molecules 2025, 30, 424
Molecules 2025, 30, x FOR PEER REVIEW 1111ofof17
17

with a higher TOC removal rate (approximately 79%) and complete degradation of pollu-
of pollutants in just 20–60 min of UV irradiation. The results showed the faster photodeg-
tants in just 20–60 min of UV irradiation. The results showed the faster photodegradation
radation of phenol and sodium diclofenac, while the degradation kinetics of methomyl
of phenol and sodium diclofenac, while the degradation kinetics of methomyl were slightly
were slightly slower. On the other hand, when the SW matrix was used in the experiment,
slower. On the other hand, when the SW matrix was used in the experiment, the TOC
the TOC removal of all the model pollutants was relatively low, but the photoconversions
removal of all the model pollutants was relatively low, but the photoconversions were still
were still significant compared to the literature studies and thus demonstrated the signif-
significant compared to the literature studies and thus demonstrated the significance of
icance of shape-controlled TiO2 nanomaterials (Figure 9).
shape-controlled TiO2 nanomaterials (Figure 9).

Figure
Figure 9.9. Photodegradation efficiency of
Photodegradation efficiency of pollutants
pollutants in
in the
the presence
presence of
of anatase
anatase nano-cuboids
nano-cuboids(TN-
(TN-
200 ◦ C) and nano-bipyramids (TN-250 ◦ C) and no shape-controlled nanoparticles (TN-175 ◦ C and
200 °C) and nano-bipyramids (TN-250 °C) and no shape-controlled nanoparticles (TN-175 °C and
225 ◦ C) under UV irradiation in ultrapure water (UW) and stormwater (SW).
225 °C) under UV irradiation in ultrapure water (UW) and stormwater (SW).

Two critical factors that greatly influence a catalyst’s photocatalytic efficiency are its
Two critical factors that greatly influence a catalyst’s photocatalytic efficiency are its
specific surface area and crystallinity. More active sites are available on a surface with
specific surface area and crystallinity. More active sites are available on a surface with a
a larger specific surface area, which allows electrons and holes to go further without
larger specific surface area, which allows electrons and holes to go further without recom-
recombining. TN-200 ◦ C and TN-250 ◦ C have higher photocatalytic performance than
bining. TN-200 °C and TN-250 °C have higher photocatalytic performance than TN-175
TN-175 ◦ C and TN-225 ◦ C because of their lower dispersity in shape and their improved
°C and TN-225 °C because of their lower dispersity in shape and their improved crystal-
crystallinity. Improved crystallinity facilitates the charge carriers’ migration to the surface
linity. Improved crystallinity facilitates the charge carriers’ migration to the surface free
free from defects. These act as recombination sites for carriers produced by photolysis
from defects. These act as recombination sites for carriers produced by photolysis and
and boost photocatalytic activity. Under UV light, titania absorbs photons (Equation (2))
boost photocatalytic activity. Under UV light, titania absorbs photons (Equation (2)) and
and ionizes oxygen (Equation (3)). The photo-holes neutralized the (OH) groups, produc-
ionizes oxygen (Equation (3)).• The photo-holes neutralized the (OH) groups, producing
ing the hydroxyl radical (OH ), as shown in the equation (Equation (4)). The resulting
the hydroxyl radical (OH•), as shown in the equation (Equation (4)). The resulting OH•
•
OH radials can oxidize organic contaminants (Equation (5)) or react directly with holes
radials can oxidize organic contaminants (Equation (5)) or react directly with holes (Equa-
(Equation (6)) [16,20,21,49]. Nanostructured materials have a high surface area-to-volume
tion (6)) [16,20,21,49]. Nanostructured materials have a high surface area-to-volume ratio
ratio and unique properties that enhance photocatalytic treatment and catalytic efficacy.
and unique properties that enhance photocatalytic treatment and catalytic efficacy.

𝑇𝑖𝑂 −
TiO 2 ++
hvℎ𝑣→→eCB
𝑒 ++h+
ℎ
vB
(2)
(2)

O2𝑂++eCB
−𝑒
O2•−
→→++𝑂 (3)
(3)
•
h+
( H2𝐻O)𝑂ads + + +
vbℎ →→H𝐻 ++OH
𝑂𝐻  (4)
(4)
RH + OH • → R• + H2 O (5)
𝑅𝐻 + 𝑂𝐻 → 𝑅 + 𝐻 𝑂 (5)
RH + h+ → R• + H + (6)
Molecules 2025, 30, 424 12 of 17

Previous studies have shown that fine-tuning the particle morphology, specifically the
exposed specific crystal faces, can enhance the photoactivity of TiO2 nanocrystals [50,51].
The reactivity of different faces is related to the type, amount, and location of electronic
defects, e− and h+ . These initial findings suggest promising potential for the obtained
anatase TiO2 nano-cuboids as photocatalysts. However, a comprehensive examination
of the photo-reactivity of these nano-cuboids, considering systematically varied aspect
ratios and sizes, is essential to thoroughly assess the facet reactivity order for various
photocatalytic reactions. Phenol, with its simple structure and high reactivity, shows the
fastest degradation rate (30 min) and nearly complete mineralization under UV light with
TiO2 nano-cuboids, resulting in highly efficient water purification in both Milli-Q and
rainwater systems. Diclofenac, an anti-inflammatory drug, degrades at a moderate but
promising rate (45 min), achieving effective mineralization while potentially minimizing
harmful byproducts, such as hydroxy-diclofenac and dichlorodiclofenac [52], as confirmed
by TOC analysis. Methomyl, a pesticide, follows a more complex degradation pathway [53]
and achieves complete mineralization within 90 min under similar conditions. TiO2 nano-
cuboids demonstrate excellent photocatalytic performance across various pollutants, with
photodegradation occurring within 30–90 min in both Milli-Q and rainwater. Although
a comprehensive evaluation of TiO2 nano-cuboids versus P25 requires that factors such
as byproduct formation, irradiation intensity, regeneration, and reaction conditions are
accounted for, both materials effectively degrade phenol. P25 (C0 = 0.53 mM, 0.5 g/L TiO2 )
achieves full degradation in 180 min [54], whereas nano-cuboids (C0 = 0.1 mM, 1 g/L
TiO2 ) achieve this in 30 min. The nano-cuboids show superior total organic carbon (TOC)
removal [55], indicating more complete mineralization. Their unique shape-controlled
morphology (size > 40 nm) increases light harvesting, improves charge separation, and
enhances electron transfer, significantly reducing electron–hole recombination. These prop-
erties make TiO2 nano-cuboids potentially sustainable alternatives for high-performance
environmental remediation. However, further studies are required to evaluate additional
parameters, such as byproduct formation, regeneration efficiency, and long-term stability,
to fully assess the potential of TiO2 nano-cuboids as a viable alternative to commercial TiO2
for environmental remediation applications.

3. Materials and Methods

3.1. Materials
Alfa Aesar (Ward Hill, Massachusetts, U.S) provided the butyl-3-methylimidazolium
tetrafluoroborate ([bmim][BF4]) ionic liquid. Acros provided titanium tetraisopropoxide
(TTIP), while Beijing Chemical Works (Gongye Dongqu, Anding Town, Daxing Dist.,
Beijng, China) supplied acetic acid (HAc) and hydrofluoric acid (40%). Scharlab (Barcelona,
Spain) provided methanol and orthophosphoric acid of HPLC quality for conducting
chromatographic investigations. Every solution was made using Milli-Q Type 1 water.
The pesticide used for treatment was Aragonesas Agro S.A. (Madrid, Spain) 99.5% pure
methomyl. The selected drug was sodium diclofenac (DCF, >99% purity), and phenol (99%
purity) were supplied by Aldrich (Darmstadt, Germany). Table S1 (Supplementary data)
lists the investigated pollutants’ general characteristics.

3.2. Synthesis of Titanium Dioxide Nano-Cuboids

Anatase TiO2 nano-cuboids/nano-bipyramids of varying sizes were developed using
a quaternary solution system made up of TTIP, deionized water, [bmim][BF4 ], and HAc. To
synthesize, 40 mL of Hac, 100 µL of water, and 1.2 mL of [bmim][BF4 ] were combined in a
100 mL Teflon autoclave and then mixed with 1 mL of TTIP. Reaction time and temperature
were shown to have a substantial influence on the controlled shape structure when the
Molecules 2025, 30, 424 13 of 17

mixture was held for different times (temperatures) for microstructure refinement. The
reaction time and temperature were initially methodically adjusted during the synthesis
to achieve anatase TiO2 nano-cuboids of distinct sizes and shapes (controlled facets). This
work concludes the four unique shapes, and their photoactivity, obtained at reaction tem-
peratures of 175 ◦ C, 200 ◦ C, 225 ◦ C, and 250 ◦ C over a constant period of 24 h. The materials
are termed TN-175 ◦ C, TN-200 ◦ C, TN-225 ◦ C, and TN-250 ◦ C, respectively. The resultant
white powder was separated by centrifugation and then rinsed with ethanol and water
before being freeze-dried for a whole night. To produce pure, fluorine-free anatase nano-
cuboids, surface fluorine and other leftover organic compounds were removed using a heat
treatment procedure that required a time period of three hours at 800 ◦ C. As a reference, the
specimen without calcination (800 ◦ C) was also analyzed. Figure S1 (Supplementary data)
illustrates the developed nano-cuboids’ synthesis process.

3.3. Characterization
The morphology and microstructural characteristics were analyzed using a SEM
(JEOL-IT300 microscope coupled with an EDS detector, JEOL Ldt, Tokyo, Japan). The
XRD diffractograms were recorded using the X’Pert High Score diffractometer (Rigaku,
Tokyo, Japan), utilizing a copper K-α (λ = 1.54 Å−1) emission source at 10 mA and 30 kV.
Diffractograms were recorded in the 2θ range of 10–90◦ with a step size of 0.017◦ . Fourier
transform infrared spectroscopy (FTIR) (Excalibur Series instrument, Agilent Technologies,
Santa Clara CA, USA) in the ATR mode was used to analyze the surface functional group
and chemical bonding of the samples in the range of 550 to 4000 cm−1 with a 4 cm−1
resolution and 32 scans using a diamond crystal as an internal reflective element (IRE).
Thermo Scientific (Waltam MA, USA) ESCALAB 250Xi X-ray spectrometer was utilized to
conduct XPS using Al Kα radiation at 1486.6 eV. A Shimadzu (Kyoto, Japan) UV-Vis-NIR
spectrophotometer (Model UV-3600) was used to collect UV-Vis diffuse reflectance spectra
from 200 to 800 nm at ambient temperature.

3.4. Photocatalytic Efficiency Assessment

The catalyst’s photocatalytic activity was measured by exposing suspensions (with a
catalyst loading of 1 g L−1 ) in the photoreactor. The irradiation source used was a 385 nm
UVA lamp with 10 LEDs (Seoul Viosys, Seoul, Republic of Korea) scattered to equally cover
the reactor surface. Each LED lamp setup used a current intensity of 250 mA, which is
equivalent to the consumption of 8.38 W of electrical power by 385 nm LED lights. The
lamp was placed 4.5 cm away from the water surface. The actual radiant power under
this experimental condition was determined using the potassium ferrioxalate actinometry
method. The results showed that the 385 nm LEDs emitted 1682.8 ± 77.1 µmol m−2 s−1
photons. With a starting pollutant concentration of 0.1 mM, the photocatalytic activity was
determined by fitting disappearance curves to an exponential decay. The pH was measured
at its normal level. After the reaction mixture was mechanically stirred and left in the dark
for half an hour, the pollutant adsorption in the material was assessed. Then, 0.8 mL aliquots
of the irradiation solution were taken with a syringe fitted with a nylon filter (0.45 µm pore
size) to prevent suspended photocatalyst particles from monitoring the reaction’s progress.
The contaminant’s content was evaluated using an Agilent Technologies (Santa Clara, CA,
USA) HPLC chromatograph 1200 Series, which included a diode array detector, a binary
gradient high-pressure pump, and an automated sampler. Isocratic elution was carried
out using a 20/80 acetonitrile/formic acid aqueous solution (0.05% w/v) at a flow rate
of 0.5 mL/min and injection volume of 20 µL. The column used was Kinetex C18 150-2
(150 mm length, 2 mm I.D., 2.6 µm core–shell particles, Phenomenex, Torrance, CA, USA).
Molecules 2025, 30, 424 14 of 17

For diclofenac, elution was performed using an 80/20 0.1% orthophosphoric

acid/acetonitrile aqueous solution. The detection was carried out at 220 nm for phe-
nol and 222 nm for sodium diclofenac. For methomyl, the column used was the Ultrasep
ES Pest with dimensions of 250 × 3 mm; the particle size was 5 µm, and the pressure was
400 bar. Elution was performed using 10% acetonitrile/0.1% orthophosphoric acid, and
detection was carried out at 231 nm.
Following complete irradiation, the samples were taken out to measure the TOC con-
tent in the reaction medium. These samples were then examined using a TOC-VCSH/CSN
Shimadzu analyzer. The water sources used for the studies were Milli-Q ultrapure water
(pH = 6.5) and stormwater (pH = 8.1–8.6), which was collected from the retention tank of a
sustainable urban drainage system (SUDS) established in Madrid, Spain. Table S5 of the
Supplementary data provides an analysis of the physicochemical characteristics of these
water samples. Stormwater has a more basic pH range of 8.1–8.6, greater conductivity,
and total dissolved solids due to its much higher ion concentration. Rainwater is a hetero-
geneous electrolyte that contains nitrogen and other nitrogenous compounds, as well as
ammonia, nitrate, nitrite, magnesium, calcium, sodium, potassium, chloride, bicarbonate,
and sulphate. The catalyst optimization was first conducted on phenol, which served as
a model pollutant, to determine the optimal catalyst concentration (1 g/L). This concen-
tration was then applied to the degradation of all three pollutants—phenol, methomyl,
and diclofenac—each possessing unique and complex molecular structures. This approach
allowed a comprehensive comparison, ensuring the effectiveness of the TiO2 nano-cuboids
across different pollutants. All the studies in this work were conducted in duplicate, and
the observed deviations were consistently less than 3%, indicating the high reproducibility
and reliability of the results; the single photolysis experiments conducted without any
catalyst demonstrated no degradation under the tested conditions, further underscoring
the improvement in degradation efficiency achieved with the TiO2 nano-cuboids.

4. Conclusions
This study highlights the potential of shape-controlled TiO2 , in which nanoparticles
with low shape dispersity (nano-cuboids and nano-bipyramids) showed greater photoac-
tivity due to fewer defects, fewer recombination sites, and higher crystallinity. The photoac-
tivity was about twice as high in the samples with uncontrolled TiO2 shapes. Notably, the
TN-250 ◦ C and TN-200 ◦ C catalysts exhibited superior performance in photodegradation
and total organic carbon (TOC) abatement compared to TN-175 ◦ C and TN-225 ◦ C. Among
the shape-controlled TN-250 (truncated bipyramids) and TN-200 (cuboids) nanoparticles,
the TN-250 exhibited higher activity. This behavior confirms that the facet couple {101} and
{001} leads to improved charge separation with respect to the couple {010} and {001}. Due
to the superior photocatalytic properties of anatase, the nano-cuboids/nano-bipyramids
offer a distinct advantage, successfully mineralizing three emerging pollutants—phenol,
methomyl, and diclofenac—in two different water systems: harvested stormwater and
ultrapure water. This study emphasizes the effectiveness of shape-controlled nanoparticles
to increase photo-efficiency in the conversion of complex contaminants in water systems,
including harvested stormwater for reuse applications. From a technical perspective, op-
timizing catalyst loading to improve photocatalytic efficiency for individual pollutants,
coupled with assessing the catalyst’s reusability, is a crucial step toward the further devel-
opment of nano-cuboids/nano-bipyramids as efficient solutions for pollutant degradation
in various environmental applications.
Molecules 2025, 30, 424 15 of 17

Supplementary Materials: The following supporting information can be downloaded at: https:
//www.mdpi.com/article/10.3390/molecules30020424/s1, Figure S1: Schematic route of the prepa-
ration of the TiO2 nanocuboids (TN-200 ◦ C); Table S1: General characteristics of the pollutants
investigated in this work; Table S2: Constant rate (k) and decomposition rate (χ) of phenol in the pres-
ence of TiO2 -based nanocuboids; Table S3: Constant rate (k) and decomposition rate (χ) of Methomyl
in the presence of TiO2 -based nanomaterials; Table S4: Constant rate (k) and decomposition rate (χ)
of Diclofenac in the presence of TiO2 -based nanomaterials; Table S5: Physicochemical parameters
of the two assessed water matrices. The supplementary file is included to address the details of
experimental additional information and photodegradation data of emerging pollutants.

Author Contributions: H.A.: conceptualization, formal analysis, methodology, investigation,

writing—original draft; D.H.: conceptualization, methodology, resources, review—original draft; F.P.:
formal analysis, investigation; V.M.-R.: formal analysis, methodology, investigation; C.d.l.R.: formal
analysis, methodology; A.G.: conceptualization, methodology, review—original draft, resources;
V.M.: conceptualization, formal analysis, methodology, writing, review and editing—original draft,
project administration, funding acquisition; M.A.I.: conceptualization, formal analysis, resources,
writing and editing. All authors have read and agreed to the published version of the manuscript.

Funding: V. Maurino kindly acknowledges funding by Regione Piemonte, Italy, through the project
ECOBRAKE “Studio e Sviluppo di materiali frenanti ecologici e a bassa emissione di particolato per
applicazioni automotive”—L.R. 34/2004—D.D. n◦ 409 del 02/11/2021. D. Hermosilla is grateful to
Grant PID2020-114918RB-I00 (Project PHOTOPREBIO) funded by MCIN/AEI/10.13039/501100011033.
MA Iqbal is grateful for the support of the European project UNA4CAREER (Marie Skłodowska
Curie grant No 847635).

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Data are contained within the article and Supplementary Materials.

Conflicts of Interest: The authors declare no conflicts of interest.

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