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Chemical Papers
https://doi.org/10.1007/s11696-024-03686-x
ORIGINAL PAPER
Sunlight‑driven charge separation foraheterojunction
ofnano‑pyramidal CuWO4‑MOF modified TiO2 nanoflakes
forphotocatalytic degradation ofciprofloxacin
KgaugeloS.Mabape1· ShivaniB.Mishra2· AjayK.Mishra3· MakwenaJ.Moloto1
Received: 12 March 2024 / Accepted: 8 September 2024
© The Author(s) 2024
Abstract
The study presents a breakthrough of a balanced charge separation for heterojunction CuWO4-TiO2 cocatalyst to efficiently
enhance visible light photocatalytic degradation of ciprofloxacin (CIP). A solvothermal-synthesized nanopyramid-like
CuWO4 semiconductor was assembled before sol–gel treatment with TiO2 precursors to generate CuWO4-TiO2 nanocom-
posites. The optical, structural, and morphological properties of CuWO4-TiO 2 were elucidated using UV–Vis DRS, XRD,
FTIR, Raman spectroscopy, and TEM/SEM techniques. The UV–Vis DRS spectroscopy of as-synthesized CuWO4-TiO2
cocatalyst demonstrated enhanced visible light absorbance. The XRD patterns of CuWO4-TiO2 revealed a triclinic phase
nanocrystal. The O-Ti–O functionality was confirmed by FTIR spectroscopy. The photoactive bands corresponding to anatase
redshift were observed from Raman spectroscopy of CuWO4-Ti O2 nanocomposite. The PL studies attributed this redshift
to the elevated extra energy bands that aid electron/hole pair charge separation in a co-catalyst heterojunction CuWO4-TiO2
nanocomposite afforded by embedding CuWO4-MOF within TiO2 crystalline. The TEM showed that un-sintered CuWO4.
MOF mimicked a pyramidal shape and converted to nanoflakes upon sintering, while TiO2 and CuWO4-TiO2 retained a
tetragonal shape. The photocatalytic activity of CuWO4-Ti O2 cocatalyst was studied using CIP, as a model pollutant. The
innovative design of 5CuWO4-TiO2 charge separation nanocomposite completely degraded 10mg L−1 CIP solution at
pH = 6.31 (natural pH) and 9 under 120min of sunlight irradiation.
Keywords CuWO4-TiO2 nanocomposite· Ciprofloxacin photodegradation· CuWO4 semiconductor· TiO2 nanoparticles·
Electron/hole charge pair separation
Introduction
Titanium dioxide (TiO2) photoactivity was discovered in
1972 by Fujishima and Honda during their work on photo-
electrochemical water splitting (Barde etal. 2022; Honda
1972). The photo-functionality of TiO2 made it a research
benchmark for photocatalytic advanced oxidative reac-
tions (AOR) (Xue etal. 2021). This photoactivity is due to
the TiO2 light absorbing O2− → Ti4+ band with an energy
bandgap (Eg) of 3.0–3.21eV (Kaur etal. 2021; Bharagav
etal. 2022). The dominance of ultraviolet (UV) active
O2− → Ti4+ large Eg favors the rapid charge recombination
rate of photo-excitons and opposes the much-required net
chemical reaction of TiO2 and O2. High recombination limits
the generation of free radicals responsible for initiating the
degradation of pharmaceuticals (Luo etal. 2016). The UV-
restricted TiO2 photoactivity prompted a search for other
oxygen evolution photocatalysts (OEP) with a visible light
response. Copper tungstate (CuWO4) emerged as a favorable
OEP due to visible light indirect narrow Eg of 2.2–2.4eV
(Raizada etal. 2020). A narrow bandgap makes CuWO4
suffer from electron (e−)/(h+) hole pair separation. The TiO2
rapid charge recombination and CuWO4 low charge sepa-
ration are limitations at opposite ends. This gave rise to a
* Makwena J. Moloto
makwena.moloto@outlook.com
1 Institute forNanotechnology andWater Sustainability,
College ofScience, Engineering andTechnology, University
ofSouth Africa, Florida Campus, Johannesburg1709,
SouthAfrica
2 College ofPharmaceutical andChemical Engineering, Hebei
University ofScience andTechnology, Shijiazhuang050018,
China
3 Department ofChemistry, University oftheWestern Cape,
Robert Sobukwe Road, Bellville7535, SouthAfrica
Chemical Papers
global curiosity aimed at designing viable charge manage-
ment nanotechnology by pairing TiO2 and CuWO4.
Central to the charge balance management quest is the
fabrication protocols to afford a stable visible light-respon-
sive CuWO4-TiO2 (CWT) (Bharagav etal. 2022). This is
premised on three realities—(1) CuWO4 bandgap is intrinsi-
cally dependent on the structural framework and coupling
of octahedral CuO6 and WO6 (Raizada etal. 2020; Hu etal.
2019). This reality infers that the material design sequence
also controls the redshift from TiO2 UV-active Eg = 3.21eV
to a visible light-driven CWT photocatalyst. Secondly,
CuWO4 crystals are susceptible to photo-corrosion distor-
tion when exposed to alkaline photo-oxidation reactions
(Raizada etal. 2020). Thirdly, TiO2 has a tunable Eg and a
high photostability (Xue etal. 2021; Mmelesi etal. 2023).
The latter realities imply that embedding CuWO4 into TiO2
is an architectural viability for a photostable visible light
active CWT (Bharagav etal. 2022).
CWT nanomaterials were previously prepared by copre-
cipitation, microwave-assisted, alcoholysis sol–gel-assisted,
hydrothermal, and solvothermal techniques (Liu etal. 2019).
Some found that TiO2 transfers electrons into the solid
CuWO4 structure than Cu2+ ion (Luo etal. 2016). Contrary
findings argued that the narrow Eg of 2.22eV for CuWO4
was a motive force behind the excitation of valence band
(VB) electrons and their subsequent photoexcitation transfer
through Cu2+ channels onto TiO2 lattice (Kaur etal. 2021).
These varying photocatalytic mechanisms are a testament
that the photocatalytic strengths of CWT are controlled by
synthesis protocols, structural geometry, and catalyst load-
ing (Bharagav etal. 2022). A recent study proved that a Cu-
MOF (copper metal–organic framework) gives photocata-
lysts a highly defined structural geometry, particle sizes, and
porous crystallinity that stimulates light penetration (Xue
etal. 2021).
Therefore, the current study aims to authenticate
that an architectural nanotechnology marriage between
CuWO4-MOF and TiO2 can yield CuWO4-TiO2 (CWT)
cocatalyst that allows visible light penetration for sufficient
facilitation of e−/h+ pair charge separation. The photocata-
lytic oxidative performance of CWT was evaluated using
ciprofloxacin (CIP) as a model recalcitrant pharmaceutical
contaminant of emerging concern (CEC).
Experimental section
Materials andreagents
The reagents used for material synthesis were titanium
isopropoxide (TTIP) (97%), 2-propanol (99.5%), ammo-
nium hydroxide solution (25%), trimesic acid (1, 3,
5-benzene tricarboxylic acid, H3BTC) (95%), copper
nitrate—Cu(NO3)2.3H2O (99%), tungsten oxide (> 99%),
ethanol (96%) and dimethyl sulfoxide (DMSO) (98%).
Hexacyanoferrate(III) ([Fe(CN)6]3−/4−) was used as an
electrolyte for electrochemical analysis of EIS and CV.
Ciprofloxacin (> 99%), NaOH (99%), and HCl (36%)
were used for photocatalytic evaluation solutions. All rea-
gents and chemicals were of analytical grade and used as
acquired from Merck Group, South Africa. The deionized
water (H2O) used for all experiments was provided by the
Millipore-Q water system.
Synthesis ofnanoparticles andthenanocomposites
TiO2 nanoparticles
Sol–gel method was adopted for TiO2 nanoparticle synthesis
(Wu and Chen 2004; MacWan etal. 2011; Restrepo etal.
2010). A solution of 10ml of TTIP was dissolved into a
magnetically stirred 20ml of 2-propanol. Then, 50ml of
deionized water was added to the solution. The mixture was
then activated dropwise by adding 6ml of NH4OH. The
mixture was allowed to age for 0.5h before raising the tem-
perature to 60°C for 1h. The temperature was then lowered
to room temperature for 2h with continuous stirring. The
contents were cooled and centrifuged at 900rpm for a third
of an hour. The supernatant and filtrates were left in contact
overnight to allow further intercalation of N-atoms from
NH4OH onto TiO2 surfaces. The resulting mixture was re-
centrifuged after 16h, and the supernatant was discarded.
The white solids were sequentially cleaned with deionized
water and ethanol. The cleaning was repeated twice before
drying at 90°C overnight. The white TiO2 crystals with a
faint yellow tinge were calcined at 400°C for 2h.
CuWO4‑MOF nanocomposites
Equimolar solutions of 4mmol were prepared by dissolving
Cu(NO3)2.3H2O and WO3 in separate ethanol solutions and
sonicated for 0.5h. The two solutions were then combined
and sonicated for a further 0.5h. CuWO4-MOF nanopyra-
mids were prepared using a previous solvothermal proce-
dure with significant modifications (Liu etal. 2019; Wu and
Chen 2004; MacWan etal. 2011). A 1.0g (0.00476mol) of
H3BTC was dissolved in a solvent solution mixture made of
20ml DMSO, 20ml of 2-propanol, and 20ml of deionized
water (v/v 1; 1; 1). Then, the sonicated tungstate copper
oxide solution (CuWO4) was added to the resulting H3BTC
mixture. The mixture was further homogenized for 0.5h at
room temperature before being transferred into a 250-ml
thermal autoclave and heated at 160°C for 16h. The result-
ant blueish crystals were centrifuged and washed twice with
deionized water and ethanol. The solids were dried at 90°C
for 24h before sintering at 400°C for 2h.
Chemical Papers
CuWO4‑TiO2 (CWT) nanocomposites
A series of xCuWO4-TiO2 (CWT) samples were prepared
(Bai etal. 2022; Xiong etal. 2015), where x refers to the
mass of 0, 5, 100, and 500mg added as the as-prepared
CuWO4-MOF nanocomposite. A desired mass of as-pre-
pared CuWO4-MOF was sonicated into 20ml of ethanol. In
a separate beaker, 10ml of TTIP was dissolved into 20ml
of 2-propanol. Then, 50ml of deionized water was added
under constant magnetic stirring. The solution was activated
dropwise by adding 4ml of NH4OH and agitated further
for 0.5h. The resulting titania solution was dropwise trans-
ferred into the CuWO4 solution while stirring. The resultant
mixture was agitated for 0.5h. The blueish-white mixture
was further turned basic by a dropwise addition of 2ml
of NH4OH. The solution was then evaporated at 60°C for
1h before cooling to room temperature and centrifuging
at 900rpm for 0.5h. Both the precipitates and supernatant
were left in contact for 16h to allow nitrogen/titania inter-
calations which leads to oxygen deficiencies. The mixture
was then centrifuged again the next day, and the precipi-
tate was collected. All the resulting CuWO4-TiO 2 (CWT)
nanocomposites were triple-cleaned using the centrifugation
method by alternating distilled water and ethanol as cleaning
solvents. The white solids were dried at 90°C for 24h before
sintering at 400°C for 2.5h.
Materials characterization
The absorbance of nanosolids was analyzed directly with-
out any further sample treatment using ultraviolet–visible
diffuse reflectance spectroscopy (UV–vis DRS) measured
between 200nm and 800nm range by PerkinElmer UV/
Vis/NIR spectrometer Lambda 1050. Horiba Jobin Yvo
Fluorolog-3 was used to acquire the photoluminescence
(PL) spectra for the nanosolids dissolved in ethanol. The
charged nature was measured from cyclic voltammetry (CV)
and impedance (EIS) using Biologic EC-Lab. The nanosol-
ids were first dissolved in deionized water before dipping
the cleaned electrodes into each of the prepared paste-like
sample solutions. The surface micromorphological nature
and elemental composition of samples coated with carbon
tape were visualized and recorded using the SEM-JOEL
JSM-IT300 series equipped with EDS (Oxford X-MAXN).
Microstructures and shapes were resolved using TEM-Tecnai
G2F2O X-Twin MAT (Eindhoven, Netherlands) operating at
an accelerating voltage of 100kV. Functional groups of each
nanosolid were recorded from 32 scans acquired from the
PerkinElmer FTIR spectrometer coupled with Frontier-Spec-
trum 100 spectrometer as Fourier transform-infrared (FTIR)
spectra of nanosolid and KBr pellet mixtures converted into
a disk. The solid samples were assessed for X-ray diffraction
(XRD) without any further sample treatment by exposing
each sample to a laser from a Rigaku SmartLab X-ray dif-
fractometer with Cu Kα radiation (λmax = 0.154059nm)
operated at 40kV. Raman spectra of untreated solid samples
were acquired using Witec Raman spectrometer Alpha 300,
TS 150 (Germany), coupled with an operating laser power
source of 532nm.
Electrode preparation
A glassy carbon electrode (GCE) was modified following
the procedure adopted by previous study (Nepfumbada etal.
2024). The GCE working electrode was sequentially cleaned
by applying alumina slurry solutions of concentrations 1.0,
0.3 0.05μm and polishing with polishing pads. The working
electrode was then dipped into a mixture of 1:1 v/v of water/
ethanol under sonication. In a separate experiment, 5mg was
taken from each of the samples TiO2, CuWO4, and CWT and
tranerred into independent sample vials containing 5ml of
deionized water. The resulting solutions were independently
sonicated for an hour. Exactly 8μL of each of the three
solutions was independently drop casted on separate clean
GCE working electrodes. The electrodes were air dried for
16h. Biologic potentiostatic mode technique was used to
analyze voltammetry (CV) and electrochemical impedance
spectroscopy (EIS) with the aid of instrument AUD83909
(Autolab, South Africa). The electrolyte used for all analysis
was hexacyanoferrate(III).
Photocatalytic degradation evaluations
CuWO4-TiO2 (CWT) nanocomposite was examined for pho-
tocatalytic degradation of ciprofloxacin (CIP) under visible
light and then natural sunlight. Four separate 10mg L−1 of
CIP solutions of 120ml each at pH = 3, 5, 7, or 9 adjusted
using 0.1mol L−1 of HCl and 0.1mol L−1 of NaOH while
stirring were prepared. An aliquot of 2.5ml was sampled
from each of the four solutions. A 50mg of CuWO4-TiO2
nanoflakes was dispersed in each of the solutions. The solu-
tions were equilibrated in the dark while stirring for 20min.
Another 2.5ml aliquot was sampled from each of the solu-
tions. The solutions were transferred into a 1000-ml pho-
toreactor equipped with 250watts of visible light, a three-
neck jacketed reactor, an operating magnetic stirrer, and a
circulating water chiller system. An aliquot of 2.5ml of CIP
solution was sampled for each 20min for the 120min of
the reaction under visible light lamb operation. The sam-
pled aliquots were passed through a 0.1µm Simplepure™
syringe filter. The PerkinElmer UV/Vis/NIR spectrometer
Lambda 1050 hosting two cuvette spots was utilized to mon-
itor the absorbance spectra of CIP residual concentration.
One cuvette served as background and the other as a sample
holder for an analyte. The photodegradation rate was com-
puted from Eq.1 (Pei etal. 2021; Bibi etal. 2021).
Chemical Papers
where [CIP]0 and [CIP]t are CIP initial concentration and
CIP concentration at time (t). The kinetic model of best fit
was also evaluated from Eq.2 (Bibi etal. 2021; Bahramian
etal. 2023; Sarafraz etal. 2020) and 3.
where Kapp denotes the rate constant at time (t) during pho-
tocatalytic irradiation (Bahramian etal. 2023).
Results anddiscussion
The nanocomposites prepared from the titanium and tung-
sten materials are explored in terms of their fabrication,
optimization and use for the photocatalytic degradation of
ciprofloxacin. Various parameters were investigated, and
their characterization and observations discussed based on
the techniques used to explore their properties.
Optoelectrical properties
In Figure1a, UV–Vis DRS for CWT samples showed
an apparent Urbach tail that increased with increas-
ing CuWO4-MOF dosages. This redshift suggests that
CuWO4-MOF introduced elevated electronic states above
the valence band of TiO2 forming 5CWT, 100CWT, and
500CWT samples (Tan etal. 2019). Figure1b shows Tauc
plots and Eg for samples as calculated from De Broglie
wavelength relationship—Eq.4 (Yuguru 2022; Meenakshi
etal. 2023).
where the parameters
𝜆,h
, and n represented the junction
wavelength, Planck constant, and some linear constant fac-
tor for bandgap energy changes. The reduced Eg for TiO2
from the reported UV active 3.21eV to 2.99eV symbol-
izes the effect of NH4OH nitriding (Pei etal. 2021). The
measured Eg of 2.32eV for CuWO4 agrees with litera-
ture recorded characteristic Eg range of 2.22eV to 2.4eV
(Raizada etal. 2020; Hu etal. 2019; Meenakshi etal. 2023).
The Tauc plots further showed that increasing CuWO4-MOF
dosages inside CWT samples decreased the bandgap
(1)
Degradation rate
∕efficiency (%) =
([CIP]
(0)
−[CIP]
(t)
[CIP]
(0))
×
100
(2)
Pseudo
−first order =ln(
[CIP
](0)
[CIP]
(t)
)vs−Kappt(in minutes
)
(3)
Pseudo
−second order =
(
1
[CIP]
(t))
=−kappt(in minutes
)
(4)
E
g=hv =nhv =
1240
𝜆
trend as—TiO2 (2.99 eV) > 5CWT (2.90eV) > 100CW T
(2.78eV) > 500CWT (2.65eV) > CuWO4 (2.32eV). The
trend shows that the Eg of CWT samples is sandwiched
between TiO2 and CuWO4 bandgaps despite the observable
Urbach tail redshift for CWT samples. This suggests that
the elevated electronic state introduced by CuWO4-MOF is
responsible for e−/h+ pair charge recombination and separa-
tion management (Tan etal. 2019). Photoluminescent spec-
tra of photogenerated e−/h+ charge pair excited at 353nm
for TiO2 and 302nm for CuWO4 and CuWO4-Ti O2 samples
are shown in Fig.1c. The largest intensity reflects that TiO2
reflects the highest e−/h+ pair recombination. The decreased
intensity for CWT implies a reduced e−/h+ charge pair
recombination in comparison with TiO2. This ties in with
the suggestion that CuWO4 introduced extra energy bandgap
bridges causing Urbach redshift toward visible light edges
under UV-DRS analysis. These heterogenous bridges aid
the separation of e−/h+ charge pair by accepting and linearly
migrating electrons away from the positively charged holes
of O-Ti–O excited valence band atoms across to conduction
bands of CWT through an electric field.
A pH drift method was employed to evaluate the point
of zero charge (pHPZC) of samples between pH = 3 and
pH = 11, Fig. 1d samples (Bibi etal. 2021). The pHpzc
was determined as; TiO2 = 3.2, CuWO4 = 4.8, 5CWT = 3,
100CWT = 4.15 and 500CWT = 4.2. The pHpzc = 3.2
for TiO2 deviated from the literature-reported (Pei etal.
2021) pHpzc = 6.5. The decrease in pHpzc for TiO2 reflects
the dominance of electronegative N-atoms introduced by
NH4OH dropwise nitriding. The pHpzc for TiO2-containing
samples convergence toward a zero as pH = 6.5. This
suggests that the literature reported pHpzc = 6.5, which
may also be TiO2 isoelectric point (IEP). The increase to
pHpzc = 4.15 for 100CWT and pHpzc = 4.2 for 5 00 CWT
shows that increasing CuWO4 dosages affects the nature of
CWT electrical charge. Figure1e shows interfacial charge
transfer resistance for samples TiO2, CuWO4-MOF, and
500CWT acquired from electrochemical impedance spec-
troscopy (EIS) using EIS Nyquist plots. The arc radius
of the samples in the EIS curves increased in the order of
TiO2 > 500CWT > CuWO4-MOF. Since a bigger arc radius
for TiO2 implies a larger charge transfer resistance, it is
prudent to suggest that charge migration increases in the
order of CuWO4-MOF > 500CWT > TiO2 (Vinesh etal.
2022). The smaller radius of CuWO4 implied that it has
a high propensity to enhance interfacial charge migration
for 500CWT (CuWO4-TiO2). The superior charge migra-
tion shown by CuWO4-MOF cannot singularly enhance
photocatalytic reactions. CuWO4-MOF photocatalysts also
needs to be complimented by a suitable conduction band
(CB) position with minimum electrochemical potentials of
− 0,41V for H/H+ conversion, − 0.33V for O2/O2− con-
version, -0.24V for CO2/CO2− conversion, and 2.73V for
Chemical Papers
OH,H+/H2O conversion (Marschall 2014). The cyclic vol-
tammetry curves in Fig.1f(Inset) showed that CuWO4-MOF
has a pronounced CV curve in comparison with seemingly
flat curves for TiO2, 5CWT and 500CWT. Further analysis
(Fig.1f main curves) revealed that the flat curves are also
cyclic with increased edges for 5CWT and 500CWT com-
pared to TiO2 sample TiO2. These findings support the EIS
suggestion of a highly conducting CuWO4-MOF in 500CWT
samples promoted electron flow. By extension, this means
CuWO4-MOF enhances separation of e−/h+ in the TiO2 lat-
tice of CWT nanocomposite.
Functional andcrystallographic properties
Figure2a provides FT-IR spectra for synthesized pale-
yellow TiO2, pale-yellow CuWO4-TiO2 (CWT), black
CuWO4 calcined at 400°C and uncalcined yellowish green
CuWO4. TiO2 anatase characteristic peaks were confirmed at
Fig. 1 UV–Vis DRS spectra a Tauc plots b pHPZCc of TiO2/CuWO4/CuWO4@400 °C/CuWO4-TiO2, EIS spectra d of TiO 2 /CuWO 4
/5CWT/100CWT /500CWT, PL spectrae of TiO2/CuWO4/5CWT, and f cyclic voltammograms f of TiO2/5&500CuWO4 samples
Chemical Papers
469 cm−1 as Ti–O, 794 as Ti–O-Ti linkages, and 1623 cm−1
as Ti–OH vibrations. The wavenumber difference of 70 cm−1
calculated between asymmetric peaks of CuWO4 sample at
1582 cm−1 and 1652 cm−1 is below 200 cm−1. This indi-
cated that trimeric acid organic ligand has two coordination
modes bridging two dentates (Ramezanalizadeh and Man-
teghi 2016). This suggests that CuWO4 is a metallic organic
framework complex with different energy levels. The prob-
able two dentate interaction was evidenced at 1114 cm−1
as a Cun+-O-Wn+ band for both sintered and non-sintered
CuWO4. Previous work also identified this Cun+-O-Wn+
band around 800 cm−1 to 900 cm−1 (Anucha etal. 2021).
For CWT samples, the Ti–O band broadens and shifts to
environments below 800 cm−1. This observation indicates a
possible co-existence of Ti–O bonding interaction, organic
ligands known to absorb at 730 cm−1, deformed W–O band
of tetrahedral WO4 peaks at 486 and 945–1 and stretching
bands of Cu–O peaks at 614 and 728 cm−1. Other research-
ers identified the Cu–O bond at 745 cm−1 and the W–O
bond at 910 cm−1 (Sarwar etal. 2023). The emerging bond
at around 2066 cm−1 for 100CWT may be linked to W = O
or W–OH bond anchored onto a surface of metal oxide. A
recent study made a revelation that oligomerization effect of
WO3 may lead to an introduction of a new FTIR vibrations
around 2000–2100 cm−1 region when reacted with metal
oxides (Jaegers etal. 2019). A previous study showed that
this tungsten (W) atoms bands appear around this region
but if bonded with atoms such as sulfur it appears as W-S
band at higher wavenumbers range of around 2100 cm−1
(Haghighi etal. 2021). The reduced stretching of the Ti–OH
band for CWT samples is suggestive of a different structural
environment. This may also be due to O-W–O band capable
of titrating surface acidic and basic Ti–OH in the region
2200–2900 and 3400 to 3700 (Jaegers etal. 2019). This led
Fig. 2 FTIR spectra a XRD patterns b and (c and d) Raman spectra of TiO2/CuWO4/CuWO4-TiO2 c and CuWO2/5CWT/100CWT/500CWT d
samples
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to a flattened FTIR spectra in this region leaving a significant
appearance of Ti–OH and W-OH existing around broad peak
region of 2066 cm−1.
Figure2b shows XRD phase properties for TiO2, CuWO4,
and xCuWO4-TiO2 (CWT) indexed to 8.8nm tetragonal
planar anatase (JCPDS No. 00–064-0863) (Nagakawa and
Nagata 2021), 9.1nm triclinic phase (ICSD no. 16009), and
8.3nm tetragonal phase, respectively. The CuWO4-TiO2
samples with CuWO4 dosages below 100mg showed an
insignificant shift of 0.01°. This implies that these samples
retained the desired photoactive anatase patterns of TiO2.
There is an emerging peak at 22.80° along 110 planes for
100CWT and 500CWT. The emergence of this peak at lower
values than the intense peak at 25.31° characteristic of TiO2
anatase suggests that CuWO4 was embedded inside the
TiO2 crystal lattice. This arrangement suggests that CuWO4
has been architecturally embedded within TiO2 as per the
desired design to avoid photo-corrosion against oxidative
reactions even under a basic medium. The emerging peak
is also consistent with PL and UV–Vis DRS suggestion
that CuWO4 introduces electronic bands within CWT. The
reduction of tetragonal phase crystal interplanar size from
8.8nm to 8.3nm indicated that 0.074nm Ti4+ ionic radius
was substituted by 0.087nm Cu2+. ionic radius. This substi-
tution reduces size by allowing O-Ti–O to donate electrons
to a larger Cu-atom capable of inwardly mobilizing suba-
tomic particles better than TiO2. charge difference between
Ti4+ and Cu2+ triggers the creation of oxygen vacancy for
maintaining charge neutrality through Jahn–Teller effects
(Bahramian etal. 2023).
Raman spectra displayed in Fig.2c–d represent the phase
purity of TiO2, CuWO4, and CuWO4-TiO2. TiO2 samples in
Fig.3c showed the Raman modes: Eg1 (at 146.9 cm−1), B1g
(at 399.99 cm−1), A1g (at 519 cm−1) and Eg (at 640.12 cm−1).
These TiO2 bands were previously identified at lower wave-
numbers of 141, 390, 512, and 634 cm−1 (Luo etal. 2016).
The redshift to a higher wavelength in the current work
is due to nitriding. The Eg1 band identified at 146.9 cm−1
symbolizes a photoactive TiO2 anatase (Bahramian etal.
2023). For the CuWO4 sample, the peaks at 275 cm−1 and
328 cm−1 confirmed the triclinic phase of the bending
δ(O–W–O) identified at FTIR wavenumber of 146.9 cm−1
(Loka etal. 2023). For the CWT sample, the emerged peaks
at 88.99 cm−1 and 201.61 cm−1 confirmed the interactions
between CuWO4 and TiO2. As per De Broglie’s law from
Eq.1, Moloto and coworkers ascribed Eg1 band redshift from
higher energy frequency (v) wavelength (UV) to lower fre-
quency wavelength (visible light) to electron migration from
O-Ti–O donor bond categorized by reduced band strength
(Moloto etal. 2021).
Morphological surface properties
Morphological investigations of TiO2, CuWO4, and
CuWO4-TiO2 are given from SEM in Fig.3ai–aiii and
TEM in Fig.3bi–biii. Figure 3bi shows agglomerated
cubic-shaped structures consistent with a tetragonal phase
revealed from XRD phase analysis. The SEM images from
Fig.3ai and aii revealed that CuWO4-MOFs formed a
pyramid-like structure. In our research institute, a previ-
ous study by Abera etal. (Ambaye etal. 2022) showed
that a pyramid-like shape emanates from solvothermal
synthesis using Cu precursors and trimeric acid which
forms ultimately octahedral Cu-MOF with a flake-like/
Fig. 3 SEM images of
TiO2 ai, CuWO4 aii and
CuWO4@400°C aiii; and TEM
of TiO2 bi, CuWO4@400°C bii
and CuWO4-TiO2 biii nano-
flakes
ai aiiaiii
bi bii biii
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paddle-wheel-like geometry. The current study reveals
that the flake-like/paddles represent a collapse of nano-
pyramidal Cu-MOF at elevated temperatures as indicated
in Fig.3bii TEM after sintering CuWO4 at 400°C for
2h. The pyramid-like shape concurs with XRD analysis
that CuWO4 is a triclinic phase. XRD Gaussian model
correlation to the triclinic phase suggests that even the
stacked flower-like structures from Fig.3ai inset represent
a triclinic phase in a bipyramid crystal. This is further
supported by previous revelations (Feng etal. 2017) that
hydrothermal synthesis of MOF performed below 18h
usually produces a mixture of pyramid-like particles and
other shapes such as nanotubes. Hence, CuWO4-MOF
crystals earned the name CuWO4 nanopyramids in this
current work. TEM micrographs of CuWO4-TiO2 com-
posite in Fig.3biii taken at a 20nm and 10nm showered
TiO2 nanocomposite image mimicry of a shape between
a distorted CuWO4 nanopyramid and a distorted TiO2
nanoparticle tetragonal planar. These CuWO4-TiO2 mix
shapes in Fig.3biii affirm TiO2 and CuWO4 interactions
to form Ti–O-Cu–O-W–O- arrangement. Therefore, CWT
is bonded as TiO2-CuWO4.
Photocatalytic evaluation ofCIP degradation using
CWT
Photoactive CuWO4-TiO2 (CWT) nanocomposites were
chosen for photocatalytic performance evaluations due
to the observed superior charge separation, improved
redshift, and light penetrable anatase phased nanoflake
geometry as described under the results and discussion
section for characterization. Preliminary studies on photo-
catalytic potential for ciprofloxacin (CIP) were done using
TiO2, and a series of xCWT, namely: 5CWT, 100CWT and
500CWT (where CWT refers to CuWO4-Ti O2 and the pre-
fix x represent either 5mg, 100mg, and 500mg of CuWO4
contained in CWT. As shown in Fig.4a–b, TiO2 photode-
graded just over 70% of CIP, whereas CuWO4-MOF did
not show any significant photocatalytic efficiency for CIP
despite the corresponding EIS spectra having displayed
better electron migration and PL spectra showing reduced
Fig. 4 The photodegradation of 10mg L−1 using a TiO2, b, CuWO4-MOF, effect of CIP initial solution at c pH 3, d pH 5, e pH 7, f pH 9, and g
effect of CuWO4 dosages on the performance of CuWO4-TiO2 catalysts
Chemical Papers
recombination. This suggests that the enhanced electron
migration of CuWO4 has conduction bands (CB) minima
with electrochemical potentials below − 0,41V which lim-
its CuWO4 from converting H into H+ and also unable to
convert O2 to O2•− as this reaction also needs a threshold
of − 0.33V (Marschall 2014). This means CuWO4 sample
is unable to produce necessary reactions to separate e−/h+
pair charge and to sufficiently generate free radicals that
can propel photocatalysis. A 5CWT design was used for
optimization because it showed the highest photocatalytic
efficiency for CIP.
Effect ofpH onphotocatalytic degradation ofCIP using
CWT
Figure4c–f displays the photocatalytic performance of
50mg of 5CWT dissolved into 120ml solutions of 10mg
L−1 CIP monitored at pH = 3, 5, 7, and 9. The dark experi-
ments adsorption conducted for 20min at pH = 3, 5, 7,
and 9 removed 4%, 21%, 9.9%, and 17% of CIP in com-
parison with overall photocatalytic removal efficiency of
35%, 60%, 44%, and 74%, respectively. The variations in
adsorption efficiencies suggest that CIP and 5CWT surface
interactions are pH-dependent. The low removals at pH = 3
may be due to high attraction between –OH radicals and
protons (Bibi etal. 2021), and/or repulsion between h+ and
positively charged CIP species existing at pH < 5.5 (Sara-
fraz etal. 2020; Ngo etal. 2023). The moderate removal
of 60% at pH = 7 is linked to the zwitterionic form as CIP
is amphoteric (Ngo etal. 2023). The highest efficiency
observed at pH = 9 suggests that the addition of –OH
increased CIP speciation density and led CIP to precipitate
near active sites present on the surfaces of 5CWT.
Visible light illumination photocatalytic performance
ofCWT withvarying CuWO4 dosages
Figure4a and f provides UV–Vis DRS that showed that
10mg L−1 of CIP solution at pH = 9 was photodegraded to
80% by TiO2, and 86% by 5CuWO4-TiO2 nanocomposite
under visible light. TiO2 nanoparticles photodegraded 80%
of CIP under visible light because of redshift caused by
the nitriding effect. The high removal efficiency of 86%
shown by 5CWT confirms the propensity of CuWO4-MOF
to enhance photocatalytic charge migration by lowering
electron/hole recombination and increasing charge as per
PL results (Bahramian etal. 2023). The simultaneous
decrease of ciprofloxacin peaks at 271nm and 225nm
further suggests that photodegradation of CIP at pH 9 pro-
ceeds by at least more than one pathway.
The role ofscavengers
The role of 5CWT reactive oxidative species was estab-
lished by quenching superoxide (O2●−), hydroxyl radicals
(●OH), and holes (h+) using p-benzoquinone, 2-propanol,
and EDTA, respectively. As shown in Fig.5f, the addition of
p-benzoquinone completely haltered CIP degradation. This
implied that O2●− was the primary initiator for CIP photo-
degradation. This suggests an effective e−/h+ charge pair
separation that isolated and allowed O2●− to catalyze the
reaction. The unchanged efficiency after adding 2-propanol
implied that ●OH radicals had no significant role. This may
be due to the repulsion between ●OH radicals and nega-
tively charged CIP speciation at pH = 9. The trapping of h+
by EDTA reduced the removal efficiency from 86 to 44%.
This suggests that h+ are involved in at least a single reac-
tion mechanism pathway after degradation is initiated by
O2●−. A photodegradation study for carbamazepine showed
that h+ species dominated when CuWO4 was used (Anucha
etal. 2021).
Effect ofnatural sunlight irradiation
Figure5a and b provides spectra for 120ml of 10mg L−1
CIP solution degraded by 50mg of 5CuWO4-TiO2 (5CWT)
at pH = 9 and 6.3 (natural pH of dissolved CIP) under natu-
ral sunlight, respectively. The results showed that sunlight
aided a ~ 100% photodegradation efficiency of 10mg L−1
CIP at both solutions pHs within 120min. This suggests that
the 4% UV light availed by natural sunlight directly excited
the UV active Ti
→
O2 band of 5CuWO4-TiO2 to generate
additional oxidative species. This implies that CuWO4 aids
5CWT to adsorb 46% visible light flux of sunlight while
the 4% UV light portion directly stimulates more electron
migration from the TiO2 valence band to the conduction
band.
Kinetics ofCIP photodegradation by CuWO4‑TiO2
undersunlight
Kinetic probing experiments for 5CuWO4-TiO2 were con-
ducted over 30min by illuminating 120ml of 10mg L−1 CIP
at pH 9 using natural sunlight as shown in Fig.5c. The pho-
tocatalytic degradation half-life (t1/2) was achieved in 5min.
At least 69% of 10mg L−1 CIP was converted by the end of
a 30-min kinetics experiment. The fitted data showed that
the mechanism preferred the pseudo-second-order model
(r2 = 0.91) over the pseudo-first-order model (r2 = 0.54). This
implied that (i) the reaction rate constant depends (Kapp) on
two factors and (ii) the rate constant for half-life reaction
depends on CIP concentration. This suggests that the rate-
determining step depends on the solution pH and CIP con-
centration. Equation revealed that kapp = 0.116mol.min−1.
Chemical Papers
Chemical Papers
Plausible photocatalytic degradation mechanism
The revelation from XRD spectra that 5CWT has an
increased crystallinity mimicry of TiO2 tetragonal anatase,
the observed reduced PL intensity of 5CWT in compari-
son with TiO2 and red-shift bands from UV–Vis DRS (and
Raman) spectra is a possible indication for a reduced charge
carrier recombination, increased e−/h+ charge pair separa-
tion and enhanced visible light photoactivity of 5CWT,
respectively. These possibilities prompted the calculation of
valence band (VB) position from Eq.4 and conduction band
(CB) positions from Eq.5 using Mulliken electronegativity
theory (Feng etal. 2017) as a strategy to establish charge
pair separation and transfer mechanism involved in photo-
degradation of CIP by CuWO4-Ti O2 (Bharagav etal. 2022).
where EVB and ECB, χ, Eg, and Ee represent VB and CB posi-
tions, electronegativity of the constituent atoms, bandgap,
and energy of free electrons on the hydrogen scale ca. 4.5eV,
respectively. The TiO2 VB position was 2.805eV and the
CB position was–0.185Ev while the CuWO4 VB position
was 2.89eV and the CB position was 0.55eV vs NHE.
Figure6e. Both the CB and VB of TiO2 semiconductor
electrochemical potential rest above their corresponding CB
and VB of CuWO4 (Marschall 2014). This staggered energy
level condition favors type-II heterojunction and Z-scheme.
A possibility for type-I heterojunctions is eliminated because
it requires that both the VB and CB of one semiconductor be
sandwiched in-between the VB and CB energy of the other
semiconductor (Annamalai etal. 2023). A type-III is also
improbable because it requires that both the VB and CB of
either TiO2 or CuWO4-MOF rests above both VB and CB
of the other semiconductor (Marschall 2014). For type-II
heterojunction, the electrons should migrate from a higher
0.395eV vs RHE = TiO2 CB position to a lower 0.55eV vs
RHE = CuWO4 CB. The positively charged CuWO4-MOF
CB position of + 0.55eV vs NHE reveals that CuWO4-MOF
serves as oxygen evolution photocatalyst (OEP) that scaven-
gers the electrons necessary to reduce oxygen to superoxide
anions (O2●−) (Bharagav etal. 2022; Anucha etal. 2021).
If this TiO2 CB electrons migrate to occupy CuWO4-MOF
CB, the new CB energy of 0.55eV vs RHE will inca-
pacitate these electrons from converting O2 to O2●− (O2/
O2 → -0.33eV vs RHE). This means the TiO2 CB electrons
with − 0.395eV vs RHE are responsible for the generation
(4)
EVB =𝜒−Ee+0.5Eg
(5)
ECB=EVB -Eg
of O2●− superoxide scavenged by p-benzoquinone. This sug-
gests that as sunlight irradiates 5CWT nanocomposite, the
presence of CuWO4-MOF with smaller Eg of 2.32eV creates
energy centers between the VB and CB of TiO2 (Kaur etal.
2021). These electrons trapping energy centers cause absorb-
ance edge redshift enabling 5CWT to absorb visible light.
The Cu2+/Cu+ and W6+/W5+ redox half-reactions enhance
the electron trapping by the electronic energy centers (Bai
etal. 2022). This process reduces recombination of elec-
trons and holes from TiO2 VB by promoting migration from
TiO2 VB to TiO2 CB (Anucha etal. 2021). The visible light
irradiation led to a movement of negatively charged excited
electrons occupying CuWO4-MOF CB to recombine with
isolated positively charged holes from TiO2 VB. This has
two effects: Firstly, it suppresses the recombination of TiO2
VB holes and CB electrons; and secondly, this continuous
opposite migration of charge carriers along the heterojunc-
tion of TiO2 and CuWO4 gives rise to localized electric field
that help to transfer these charge carriers across heterojunc-
tion to avoid recombination. The inter-migration of electrons
between CuWO4 CB and TiO2 VB suggests a Z-scheme
heterojunction. Since CUWO4-MOF serves as OEP, then
TiO2 serves as a hydrogen evolution photocatalyst (HEP)
(Bharagav etal. 2022). That means the holes (h+) migrate
from CUWO4-MOF VB to accumulate onto the interface of
TiO2 VB. The interparticle transfer of holes is simplified by
the comparable closeness of the CuWO4-MOF VB position
of 2.89eV and TiO2 VB position of 2.805eV. This close-
ness of CuWO4 VB and TiO2 VB in which the h+ migra-
tion occurs has been reported to increase occurrence of a
heterogenous Z-scheme-like mechanism (Bao etal. 2021).
Since the h+ from TiO2 VB and e− from CuWO4-MOF
recombine in the TiO2 VB, the positively charged h+ from
CuWO4-MOF remains isolated to isolated and accumulate
on the interface of TiO2 VB without occupying TiO2 CB.
The electron potential 2.805eV vs RHE enables these iso-
lated h+ charge carriers from CuWO4-MOF VB to either
directly react with CIP through oxidation or react with
H2O to form –OH hydroxyl radicals (H2O/–OH = 2.8eV vs
RHE) (Annamalai etal. 2023) that can in turn facilitate CIP
degradation provided pH medium is favorable. In the final
analysis, the photodegradation mechanism for CIP by 5CWT
seems to occur via Z-scheme heterojunction with significant
involvement of both the superoxide free radicals and holes.
Reusability andstability tests
The reusability tests and stability of 5CWT were evaluated
by comparing UV–Vis DRS spectra of 120ml containing
10 mg−1 CIP solution post 120min of irradiation under vis-
ible light in the presence of 5CWT. The 5CWT was cleaned
by centrifugation and oven dried at 80°C for 24h after
each reusability test. The UV–Vis DRS analysis from Fig.6a
Fig. 5 UV–Vis DRS spectra for sunlight irradiated at a adjusted pH
9, b natural pH 6.38 (b) and (c) kinetic studies at pH 9, and d mod-
eled kinetics and e scavenger analysis
◂
Chemical Papers
showed that 5CWT retained the photodegradation capacity
for CIP but the efficiency reduced from 92% in cycle −1
to 68% in cycle 4. The reduced photocatalytic performance
can be explained by the presence of C–C vibrations around
2300 cm−1 and C–H faint vibrations at 2931 cm−1 range on
the FTIR spectra of loaded 5CWT in Fig.6c (Janakiraman
and Johnson 2015). The observable organic moiety vibra-
tions are linked to the accumulated residue of degrading
CIP byproducts adsorbed onto the surfaces of 5CWT with
each of the four consecutive photodegradation cycles of CIP
with a repeated 5CWT sample. These organic byproducts
may obstruct a fraction of light from penetrating toward the
5CWT active sites and thereby inhabiting optimum photoac-
tivity of the retained 5CWT photoactive functional groups.
Despite the reduced light absorption intensity and mild
appearance of C–C residue, the structural and functional
changes of 5CWT were negligible. This means that the
removal efficiencies in Fig.6a and b can serve as evidence
that 5CWT is a reusable photocatalyst while in Fig.6c FTIR
spectra serve as an attestation that 5CWT nanocomposite
design is highly stable.
Proposed photodegradation pathways
Studies reported that LC–MS (Sarafraz etal. 2020) and
LC–MS/MS (Zeng etal. 2019) techniques have an appre-
ciable sensitivity to identify photodegraded ciprofloxacin
byproducts. Figure7 shows that CIP photodegradation path-
ways proceeded from four intermediate products: A = m/z
288, B = m/z 313, C = m/z 267, and D = m/z 314. The inter-
mediate m/z 288 from pathway A is ascribed to a loss of
CO2 (m/z 44) from the carboxylic group (decarboxylation)
Fig. 6 a UV–Vis DRS of 5CWT photodegradation efficiency over four cycles, b reusability studies performance of 5CWT in each cycle, and c
FTIR spectra of loaded and unloaded 5CWT
Chemical Papers
(Achad etal. 2018). This was followed by the mineralization
of the piperazine ring, defluorination, and disintegration of
the benzene ring to yield C2H5N, C2H5, F, and m/z 90 from
m/z 197 byproducts. A previous study posted that a route
that initiates piperazine ring cleavage before the defluorina-
tion step to yield intermediate m/z 245 proceeds through
hydroxyl radicals (Zeng etal. 2019). The insignificant role
of negatively charged hydroxyl radicals and the secondary
role of negatively charged superoxide radicals in the cur-
rent study imply that route A cannot be a major degradation
pathway. Pathway B shows a probable degradation process
proceeding in the order: of defluorination, then piperazine
ring cleavage before decarboxylation. This is consistent with
a previous study's argument that pathway B is probable at
high pH (Vasconcelos etal. 2009). Pathway D shows that
CIP m/z 332 is converted to m/z 314 by dehydration (loss of
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
Potential=NHE
-0.395 eV
0.55 eV
2.81 eV
2.90 eV
h+h+h+
.O2
-
O2
h
+
h
+
h
+
e-e-e-
h
+
h
+
eV
e-e-e-
e-e-e-
CuWO4 Piramids
OE semiconductor
TiO
2
nanocubes
HE semiconductor
Hetero-
junction
N
O
HO
N
NH
O
F
200 225 250 275 300 325 350
0.0
0.2
0.4
0.6
0.8
1.0
Absorbance (Abs)
Wavelen
g
th
(
nm
)
10 ppmCIP pH9
CWTCIP AD
20 min
40 min
60 min
80 min
100min
120min
(A)
.O2-
.O2-
.O2-
h+h+h+
UV –Vis DRS
Hetero-
junction
mz=245
-F -Epoxide
-CO
2
H
-C
2
H
5
N
-Fand
-C=O
mz=226
-C
2
H
5
mz=197 mz=90
A
N
HN
NH
O
N
HN
NH
O
NN
NH
O
-C
2
H
4
mz=198
mz=226
-Epoxide
mz=267
-F and
-COOH-H
2
O
NN
NH
O
F
NN
H
O
F
NN
H
O
NNH
O
NNH
O
N
H
O
N
NH
O
N
H
O
N
NH
O
F
N
O
N
NH
O
F
mz=212
mz=231
mz=314
mz=313
-F
N
O
HO
N
NH
O
NNH
NH
2
O
N
O
HO
NH
NH
2
O
-CO
2
H
BCD
Fig. 7 Photocatalytic degradation pathways of 10mg l−1 of ciprofloxacin at pH 9 under visible light for 120min
Chemical Papers
H2O = m/z 18) before it is converted into m/z 267 by losing
epoxide, and to m/z 231 in pathway C by losing both C = O
and fluorine group. Considering that; (i) pathway A—degra-
dation initialized from m/z 288 but requires a significant OH
participation, and (ii) pathway D initial sum loss of C = O
then H2O can yield m/z 314 then m/z 288, it can be hypoth-
esized that pathway D occurs before A. This concurs with
the literature postulation that photolysis of CIP occurs within
2–4min to give m/z 314 and m/z 288 as a major intermedi-
ate (Petrovi etal. 1067). This suggests that pathway D is a
major pathway because initiating defluorination in pathway
B requires more strength than dehydrating or decarboxylat-
ing CIP by removing a good leaving group—CO2H via path-
way D. The LC–MS/MS results for the dissolved CIP by-
products of photodegradation showed that 5CWT is capable
of mineralizing CIP to molecular mass below 90mol. g−1.
Comparative studies
Table 1 provides compares synthesized CuWO4-TiO2
(5CWT) to other TiO2-based nanocomposites applied for
photocatalytic degradation of ciprofloxacin. The 5CWT dis-
played a superior performance for the photocatalytic deg-
radation of CIP using an almost equivalent dosage to the
reported composites. The 5CWT has an advantage because
it performs under both visible light and abundantly avail-
able natural sunlight even at natural CIP solution pH in
comparison with other catalysts that operate near neutral.
The comparative studies complemented the findings that h+
is more involved in the CIP degradation mechanism than
·O2− radicals (Sarafraz etal. 2020; Ngo etal. 2023; Zeng
etal. 2019). The observed large involvement of ·OH radical
in other studies is associated with the near-neutral pHs used
which provide zwitterionic speciation for CIP (Ngo etal.
2023; Zeng etal. 2019). With an exception for TiO2/CDs
(Zeng etal. 2019), the Kapp for 5CWT is at least 40 folds
higher than the other literature-reported catalysts. Even so,
5CWT still holds an advantage over TiO2/CDs since the cur-
rent work rate constant value was determined using natural
sunlight without adding external ozonation additives.
Conclusions
A novel sunlight-driven high charge separation heterogenous
CuWO4-TiO2 (CWT) photocatalyst was sol–gel fabricated by
reacting solvothermal-synthesized CuWO4-MOF nanopyra-
mids and TiO2 nanoparticles precursors. The electron/hole
charge separation was enhanced by rapid electron migration
across heterojunction from TiO2 bands to CuWO4 conduc-
tion band. The enhanced charge separation allowed holes to
dominantly initiate photocatalytic degradation of ciprofloxa-
cin through four probable pathways. CWT loading of 5mg
CuWO4 displayed optimum 10mg l−1 CIP photocatalytic
degradation of 86% under visible light and ~ 100% under
natural sunlight at pH = 9 for 2h reaction time. Compara-
tive studies showed that this recyclable novel CuWO4-TiO2
performed at a rate of 40 folds higher than the majority of
recently manufactured TiO2-based nanocomposites for CIP
degradation. This positions CWT as a promising heterojunc-
tion catalyst for use in the degradation of pharmaceutical
pollutants under visible and natural sunlight conditions.
Acknowledgements This work was funded by National Research
Funding (NRF) of South Africa, grant no. MND190409428558, and
the Institute for Nanotechnology and Water Sustainability under the
Table 1 Comparison of photocatalytic degradation of CIP by TiO2-based composite materials
Catalyst Dose
(g l−1)
CIP
(mol l−1)
Time and pH Light source Kapp
(mol s−1)
Removal (%) Primary radicals Ref
CuWO4-TiO2
nanoflakes
0.42 10 2h pH = 9
and 6.4
Sunlight Pseudo-2st-
order 0.1160
≈100 h+ > ·O2− This work
Black
Ti3+/N-TiO2
0.43 0.5 70min
pH = 6.7
Visible LED
light
unnamed
model
0.0028
≈ 100 h+, ·OH > ·O2− Sarafraz etal.
(2020)
(1 and 3%) Ag-
TiO2/rGO/
HNTs
0.20 20 and 0.5ml
H2O2
5h
pH = 6
7
UV irradiation Pseudo-1st &
2st-order
0.0011
≈ 90 ·OH > h+ > ·O2− Ngo etal.
(2023)
TiO2/Carbon
dots (6%)
1.00 10
(4.7mg l−1
ozone)
30 (16) min
pH = 7
350 W Xenon
simulated
sunlight
Pseudo-2nd-
order
0.129
(0.320)
91
(99.7) ·OH > h+ > ·O2−
Negligible (and
·O3−)
Zeng etal.
(2019)
Cu/TiO20.4 20 4h UV light
15 W
UVC
0.0028
Pseudo-1st-
order
50 N/A Kovačević etal.
(2023)
Chemical Papers
College of Science, Engineering, and Technology at the University
of South Africa.
Funding Open access funding provided by University of South Africa.
National Research Foundation (RSA), MND190409428558, Kgaugelo
S Mabape
Declarations
Conflict of interest The authors declare that there is no known conflict
of interest.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
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permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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