Chemosphere 351 (2024) 141206

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Study on triphase of polymorphs TiO2 (anatase/rutile/brookite) for
boosting photocatalytic activity of metformin degradation
Diana Rakhmawaty Eddy a, *, Geometry Amal Nur Sheha a, Muhamad Diki Permana a, b, c,
Norio Saito c, Takahiro Takei c, Nobuhiro Kumada c, Irkham a, Iman Rahayu a, Ikki Abe d,
Yuta Sekine d, Tomoki Oyumi d, Yasuo Izumi daDepartment of Chemistry, Faculty Mathematics and Natural Sciences, Universitas Padjadjaran, Sumedang, 45363, Indonesia
Special Educational Program for Green Energy Conversion Science and Technology, Integrated Graduate School of Medicine, Engineering, and Agricultural Sciences,
University of Yamanashi, Kofu, 400-8511, Japanc
Center for Crystal Science and Technology, University of Yamanashi, Kofu, 400-8511, Japand
Department of Chemistry, Graduate School of Science, Chiba University, Chiba, 263-8522, Japanb

H I G H L I G H T S

G R A P H I C A L A B S T R A C T• The triphasic polymorph TiO2 was suc­cessfully prepared by ultrasound assis­
ted sol-gel method.
• The ratio of anatase-rutile-brookite is
tuned by changing the volume of acidand base condition.
• Triphasic TiO2 has a highest photo­catalytic activity than single andbiphasic phase.
• The increased photoactivity of triphasic
TiO2 is attributed to the heterojunction
effect.

A R T I C L E I N F O

A B S T R A C T

Handling editor: E. Brillas

The elution of pharmaceutical products such as metformin at higher concentrations than the safe level in aquatic
systems is a serious threat to human health and the ecosystem. Photocatalytic technology using TiO2 semi­
conductors potentially fixes this problem. This study aims to synthesize triphasic anatase–rutile–brookite TiO2
using ultrasound assisted sol–gel technique in the presence of acid and its application to photodegradation of
metformin under UV light irradiation. Based on X-ray diffraction analysis, a TiO2 sample consisted of anatase(76%), rutile (7%), and brookite (17%) polymorph (A76R7B17) that was fully crystallized. Scanning electron
microscopy (EM)–energy dispersive X-ray spectra results showed agglomerated triphasic A76R7B17 with irregular

Keywords:
Heterophase polymorph
Heterojunction
Metformin degradation
Photocatalysis
Titanium dioxide* Corresponding author.
E-mail address: diana.rahmawati@unpad.ac.id (D.R. Eddy).
https://doi.org/10.1016/j.chemosphere.2024.141206
Received 13 October 2023; Received in revised form 10 January 2024; Accepted 12 January 2024
Available online 12 January 20240045-6535/© 2024 Elsevier Ltd. All rights reserved.

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Chemosphere 351 (2024) 141206

spherical clusters. Transmission EM results revealed that the crystal size of A76R7B17 was 4–14 nm. The Bru­
nauer–Emmett–Teller analysis showed the sample’s specific surface area of 149 m2 g− 1. The degradation test of
metformin demonstrated that the A76R7B17 exhibited a 75.4% degradation efficiency after 120 min under UVlight irradiation, significantly higher than using biphasic and single-phase TiO2 photocatalysts. This difference
could be attributed to the heterojunction effect of triphasic materials that effectively reduced electron–holerecombination rate as well as the combination of effective electron transfer from conduction band of brookite
and anatase and the utilization of wider range of UV–visible light using rutile.

1. Introduction

assisted sol–gel technique and hydrothermal treatment (Kaplan et al.,2016), low temperature dissolution–precipitation on a porous micro­
filtration membrane (Fischer et al., 2017), electrochemical anodization
technique (Preethi et al., 2017), hydrothermal synthesis (Can­o-Casanova et al., 2018, 2020), and hydrothermal methods using poly­mer (Xiong et al., 2019).
Herein, we report a simple method to synthesize anata­se–rutile–brookite (triphasic phase TiO2) via ultrasound-assisted sol–gel
technique using Ti(IV) tetraisopropoxide (TTIP) as a precursor and acidor base as a phase control reagent. This method has been chosen because
inexpensive equipment used, high homogeneity, and high purity due tothe acoustic cavitation (formation, expansion, and explosion of bubbles)
leading to the formation of materials with higher crystallinity (Jameel
et al., 2020; Eddy et al., 2023a). To the best of our knowledge, there
have been no studies using ultrasound assisted sol–gel technique tosynthesis triphasic TiO2. The unique triphasic TiO2 was applied for the
first time to the metformin degradation under the UV light irradiation.

The most common medicine for type 2 diabetes is metformin, whichis also used as an anticancer agent as well as for a therapy for polycystic
ovarian syndrome (Elizalde-Velázquez and Gómez-Oliván, 2019). Un­
like the most medicines that are digested in the human body, metformin
is difficult to degrade and easy to elute to the environment as unchanged
through urine and feces (Bailey, 2017). Conventional wastewater
treatment cannot completely eliminate metformin because of its physi­
cochemical properties, e.g. high solubility, low octanol–water partition
coefficient, low biodegradation rate, and high mobility in aqueous so­lutions (Parra-Marfil et al., 2023) as proven by Wilkinson et al. (2022);
metformin is one of the most often found organic contaminants in 258
rivers and also identified in 104 nations worldwide. Such substances areseriously harmful to various living things and the environment because
they cause the oxidative stress, mutagenesis, carcinogenesis, reproduc­tive toxicity, medication resistance, and allergic reactions (Parra Marfil
et al., 2023).
In this context, techniques to remove metformin from contaminatedwaters using various processes are attracting much attentions, such as
photocatalysis using titanium dioxide (TiO2) semiconductor (Quintão
et al., 2016; Nezar and Laoufi, 2018; Chinnaiyan et al., 2019; Carbuloni
et al., 2020; Prashanth et al., 2021). TiO2 is widely used due to its stablephysical and chemical properties, strong oxidizing capacity, high pho­tocatalytic activity, and excellent biocompatibility compared to theother semiconductor materials such as SrTiO3, ZnO, WO3, ZrO2, andgraphitic-C3N4 (Li et al., 2020).
Three distinct polymorphs of TiO2 are anatase, rutile, and brookite(Wang et al., 2017). All three crystal structures comprise of distorted
TiO6 octahedra but connected in different ways. Anatase adopts atetragonal form in which each octahedron has four common edges but
no shared corners. A similar tetragonal structure exists in rutile, where
two opposing edges of an octahedron are shared to form a linear chain in
the [0 0 1] direction. Furthermore, chains are additionally connected attheir corners. In the orthorhombic crystal structure of brookite, octa­
hedrons share three edges as well as three corners (Liao et al., 2012).
Thermodynamically, the most stable phase is rutile, while anatase andbrookite are metastable phases which transform to rutile at high tem­
peratures (Ma et al., 2014).
However, the low quantum efficiency of TiO2 photocatalysts causedby the rapid recombination of excited electron–hole pairs seriously
limits the photocatalytic activity of the material (Chen and Mao, 2007).
The presence of more than one TiO2 polymorph is expected to exhibit abeneficial effect for improving electron–hole separation efficiency by
reducing the recombination probability to enhance the photocatalytic
performance (Mutuma et al., 2015). A typical example is commercial
Evonik P25 containing anatase–rutile mixed phase exhibits a good
photocatalytic activity than single phase TiO2 due to electron–holeseparation (Wang et al., 2015). On the other hand, the investigation of
TiO2 containing all three phases (anatase–rutile–brookite) is extremely
limited. Furthermore, the combination effects of advantages of eachpolymorph (band energy level v.s. redox potential, light utilization
range) were also discussed.
Moreover, reports on the preparation of triphasic TiO2 are limited,
but several are known using solvothermal methods (Liao et al., 2012),
sol–gel methods (Mutuma et al., 2015), combination of ultrasound

2. Methods
2.1. Materials
The chemicals used for the synthesis were TTIP (97%, Sigma­–Aldrich, USA), isopropyl alcohol (IPA, 99%, Sigma–Aldrich), anataseand rutile (TiO2, 99%, Kanto Chemical Co., Japan), nitric acid (HNO3,
65%, Merck, USA), sodium hydroxide (NaOH, 98%, Merck), ethanol(C2H5OH, 100%, Merck), metformin⋅HCl (C4H11N5⋅HCl, HexpharmJaya, Indonesia), ammonium oxalate monohydrate ((NH4)2C2O4⋅H2O),
99%, Sigma–Aldrich), and silver nitrate (AgNO3, 99% Sigma–Aldrich).
All chemicals were used without further purification.
2.2. Synthesis of heterophase TiO2
Heterophase TiO2 was prepared using ultrasound assisted sol–gel
technique methods by varying the added volume of acid and/or base
based on the previous reports (Quintero et al., 2020; Luthfiah et al.,2021). The acidic conditions were provided by the addition of 1 M HNO3
(A3 and A6), while the basic conditions used 1 M NaOH (B3 and B6).
Table S1 shows the various synthesis conditions of TiO2 samples. In atypical procedure, 2 mL of TTIP was slowly added into 48 mL of IPA
under vigorous, constant stirring. The solution was ultrasonicated for 20
min and 20 mL of deionized water was added. Then, acid or base solu­
tion was added dropwise to the suspension. The mixture was continu­
ously ultrasonicated for 3 h to complete the reaction. The obtained solid
was collected by centrifugation, washed with ethanol and distilled water
three times, followed by drying at 343 K for 90 min. The resulting
product was obtained, and calcined at 673 K for 2 h.
2.3. Characterizations
The crystal structure, phase composition, and crystallite size wereanalyzed by powder X-ray diffraction (XRD, Rigaku/MiniFlex 600,
Tokyo, Japan) at 295 K using Cu Kα emission (wavelength λ = 0.15418
nm) and scans were performed in the range of 20–70◦ (2θBragg) with thescan speed of 10◦ min− 1. The resulting data were refined by the Rietveld
method using a software HighScore Plus version 3.0.5 (Marvern
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Chemosphere 351 (2024) 141206

PANalytical, UK) to determine the crystal structure and phase compo­
sition (Permana et al., 2022). A software, Visualization for Electronic
and Structural Analysis (VESTA) version 3.5.7 (Momma and Izumi,2011) was used to visualize the crystal structure. The Debye–Scherrer
Eq. (1) was used to determine the crystallite size (Patterson, 1939):

bration following the Lambert–Beer’s law. The photodegradation rate
was calculated using the 1st-order Langmuir–Hinshelwood kinetic
model (Eq. (3)).

(1)

Where C0 dan Ct are the concentrations before and after irradiation at a
certain time, t represents the time, and k is the photodegradation rate
constant. The degradation efficiency (%) was determined by Eq. (4).
This photocatalytic activity test was also carried out with irradiation andthe absence of catalyst (photolysis).

B = (K λ) / (D cos θ)–ln(Ct / C0 ) = kt

where B is the crystallite size (nm), K is the Scherrer constant (0.89), λ is
the wavelength of X-ray (nm), D is the full width at half maximum ofpeak evaluated using Origin software version 8.5.1 (rad), and θ isdiffraction angle (rad).
The surface morphology was characterized by scanning electron
microscopy–energy dispersive X-ray spectroscopy (SEM–EDS, Tabletop
Microscope-1000, Hitachi, Tokyo, Japan). Several milligrams of thesample were fixed in a sample holder. The morphology and size were
examined at an acceleration voltage of 15.0 kV and magnification of
200–2000 times. Then, the EDS was measured for elemental analysis.
Transmission electron microscopy (TEM; JEM-1400, JEOL Ltd, Tokyo,
Japan) images were obtained at an acceleration voltage of 100 kV to
examine the particle size and morphology of samples.
The specific surface area (SA) of the samples were measured based ona multipoint Brunauer–Emmett–Teller (BET) method while the pores
size and pore volume were determined based on Bar­
rett–Joyner–Halanda (BJH) method by nitrogen adsorption–desorption
isotherm measurements at 77 K, using a BELSORP-mini analyzer (BEL
JAPAN Inc, Tokyo, Japan).
The light absorption properties were characterized by diffuse
reflectance ultraviolet–visible spectroscopy (DR UV–Vis, Jasco V-550,
Tokyo, Japan) to determine the band gap (Eg) through the Davis–Mott
function (Eq. (2)) (Makuła et al., 2018):)((2)(F(R∞ )⋅hv)1/γ = B hv – Eg

Degradation efficiency (%) = (1 – (Ct / C0 ) × 100

(3)

(4)

For the optimization of reaction parameters, the catalyst dosage (0.5,
1.0, and 1.5 g L− 1), pH value (3, 5, 7, 9, and 11), and initial concen­
tration of metformin (10, 15, and 20 mg L− 1) were varied using the best
catalyst materials based on the first experiments. Furthermore, toinvestigate the photocatalytic mechanism, experiments of active species
trapping were carried out by adding 100 mM of IPA (a quencher of•OH), ammonium oxalate (OA, a quencher of h+), AgNO3 (a quencher ofe− ), and bubbling N2 (99.9%, 20 mL min− 1, remove dissolved oxygen).
The detailed experimental conditions were similar to those for the otherphotocatalytic experiments.
3. Results and discussion
3.1. Structural Analysis
The crystallographic structures of the samples characterized bypowder XRD patterns are depicted in Fig. 1a. The characteristic peak ofanatase commercial appears at 25.4◦ (0 1 1), 37.1◦ (0 1 3), 37.9◦ (0 0 4),
38.7◦ (1 1 2), 48.2◦ (0 2 0), 54.1◦ (0 1 5), 55.2◦ (1 2 1), and 62.9◦ (0 2 4)
diffractions. It can be referred to the Inorganic Crystal Structure Data­base (ICSD) 98-002-4276 for tetragonal anatase (Wagemaker et al.,2003). On the other hand, the diffraction peaks for rutile commercial
appears at 27.4◦ (1 1 0), 36.1◦ (0 1 1), 39.1◦ (0 2 0), 41.2◦ (1 1 1), 44.0◦(1 2 0), 54.3◦ (12 1), 56.5◦ (2 2 0), 62.7◦ (0 0 2), and 63.94◦ (1 3 0)
diffractions that can be indexed from ICSD 98-016-5920 for tetragonal
rutile (Tobaldi et al., 2010). The diffractogram for P25 Evonik showed
the presence both of anatase and rutile phase. Meanwhile, the peak
pattern for sample A3 with the addition of acid was consisted of major
peaks owing to anatase, small peaks of rutile, and overlapping peaks ofbrookite. Brookite phase in sample A3 appeared at 25.4◦ (0 2 1), 48.1◦ (13 2), and 55.4◦ (1 4 2) diffractions that can be indexed from ICSD
98-007-7693 for orthorhombic brookite (Meagher and Lager, 1979).
To investigate the effect of acid and/or base addition during syn­thesis on the crystal phase composition, all diffractograms of the TiO2
samples at different conditions are summarized in Fig. S1. At neutral
condition, anatase was a dominant phase with a minor brookite.
Furthermore, in acidic condition (A3 and A6) shows three phases(anatase, rutile, and brookite). Then, the excess amount of acid insample A6 increased the ratio of brookite and decreased anatase phase.
On the other hand, the addition of base (3 mL; Fig. S2a) resulted in peaks
that can be indexed to brookite phase as the 2θBragg angles at 25.4◦ (0 21), 30.8◦ (1 2 1), and 48.0◦ (1 3 2) diffractions. Meanwhile, further
addition of base (6 mL; Fig. S2b) led to triclinic or anorthic TiO2 crystal
system corresponding to peaks appeared at 31.5◦ (2 2 2). The details canbe seen in references ICSD 98-003-5364 (Le Page and Strobel, 1982). It
has been confirmed that the addition of HNO3 and NaOH for synthesis
played a critical role in tailoring the composition of anata­se–rutile–brookite mixed phase TiO2 samples. The phase ratio in the
XRD data was summarized in Table 1.
Fig. 2 schematically shows the formation process of heterophase of
TiO2 polymorphs. In this research, we found that the acid or base con­ditions during synthesis played an important role in polymorph
composition. The formation of TiO2 under neutral and acidic conditions

in which F(R∞ ) is the Kubelka-Munk function, ℎ is the Planck’s constant,v is the photon’s frequency, B is a constant, γ is 2 for allowed indirecttransition, and Eg denotes the band gap energy.
The absorption–fluorescence spectra were recorded on an FP-8600(JASCO; Chiba Iodine Resource Innovation Center, Chiba, Japan)
equipped with a 150 W Xe arc lamp (UXL-159, Ushio, Japan) and aphotomultiplier tube for excitation at 300 nm within a fluorescencerange of 310–800 nm. The incident excitation light from the Xe lamp
was monitored by a Si photodiode, and the monitored fluorescence lightemitted from the sample was normalized based on the incident lightintensity at each wavelength. The photocatalyst powder (2.0 mg) was
mixed with purified water (3.0 mL) and ultrasonicated (430 W, 38 kHz)for 20 min. All spectra were recorded for the suspensions in a quartz cellat 295 K.
2.4. Photocatalytic activity of metformin degradation under the UV light
The photocatalytic activity was evaluated for photodegradation of
metformin under the UV light irradiation. Photocatalytic tests were
performed with a 10 mg L− 1 metformin solution at the pH condition of
7.0. First, reactions were performed using 1.0 g L− 1 catalyst. Before
illumination, the suspension was stirred for 30 min in dark to attain theadsorption–desorption equilibrium. Then, the suspension was placed on
top of the photoreactor and irradiated by the UVB lamp (Exo Terra, Type
Reptile UVB 150, 25 W) at the distance of 5 cm, under continuous
stirring for 120 min. The reaction temperature was controlled using a
blower so that the reactants remained at 295 K. To determine the met­
formin concentration during reaction tests, 3 mL solution were taken
every 30 min, filtered using a 0.22-μm polycarbonate filter membrane,
and then the absorbance of the metformin solution was measured at
λmax = 233 nm using a UV–vis spectrophotometer (Shimadzu, 1800;
Tokyo, Japan).
The concentration of metformin was determined based on the cali­3

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Chemosphere 351 (2024) 141206

Fig. 1. XRD pattern, (b) N2 adsorption–desorption isotherms, (c) UV–vis absorption spectra, and (d) band-gap plot using Davis–Mott equation of the samples.

Furthermore, the alkaline condition that used NaOH led to a plausible
formation of sodium titanate before forming brookite by releasing Na+from the surface accompanied by oxidation of Ti in the structure, anddue to the incomplete transformation, sodium titanate was still detectedbased on the EDS (Figs. S10 and S11). This result was consistent withprevious report by Tay et al. (2013).
Table S4 and Fig. S7 present the crystallite size of the samples esti­
mated using Scherer’s equation. The crystal size of synthesized mixed
phase of TiO2 was smaller than those of single-phase reference com­
mercial anatase and rutile. In acidic media, especially for higher content
of HNO3, the crystallite size was significantly reduced. It might beconcluded that crystal growth was inhibited when three phases areformed simultaneously during synthesis. Meanwhile, in neutral condi­
tions, the crystallite size was the smallest for anatase TiO2. On thecontrary, in alkaline conditions it produced the smallest crystallite sizeof brookite and for higher content of NaOH led to amorphous structure.

Table 1
Phase ratio percentage of the samples used in this study.
SampleAnatase
RutileP25
EvonikA6

Phase ratio (%)aRutileBrookite

Triclinic TiO2
(Anorthic)

100.0 ±
0.3
–

–

–

–

A100

100.0 ±
0.3
14.2 ±
0.3
7.1 ±
0.1
7.0 ±
0.1
–

–

–

R100

–

–

A86R1425.4 ±
0.5
17.1 ±
0.4
0.8 ± 0.1–

A68R7B25–

A76R7B17

–

A99B1

–

100.0 ±
1.1
–

–

B100

100.0 ± 1.6

T100

B3

85.8 ±
0.4
67.5 ±
0.3
76.0 ±
0.3
99.2 ±
0.3
–

B6

–

A3
N

a

SampleCodeAnatase–

3.2. Morphological analysis

Standard deviation values are also noted.

The morphologies of single phase, biphasic, and triphasic phase wereobserved by SEM. Fig. 3a and b shows that A100 and R100 have a ho­mogeneous smaller particle shape. Furthermore, for biphasic samples(A86R14) in Fig. 3c consist of smaller aggregated particles. Meanwhile,
triphasic phase samples (A76R7B17) depicted in Fig. 3d shows the for­mation of irregular clusters composed of bulk primary particles.
Furthermore, EDS mappings (Fig. S8) confirmed the high purity of
A76R7B17 sample, containing only titanium and oxygen atoms. Then, the
EDS confirmed the atomic ratio of titanium and oxygen in A76R7B17
sample (Table S5). The atomic ratio of the EDS results demonstrated
oxygen vacancy sites in A76R7B17 sample and the empirical formula was

led to a high crystallinity which is later helpful for good electron transfer
in view of photocatalysis. In alkaline conditions, TiO2 led to lower
crystallinity even after calcination at 673 K for 2 h. Despite the poor
crystallinity under alkaline conditions, brookite peaks for B3 sample canstill be identified (Pauling and Sturdivant, 1928). Under more alkalineconditions (B6), some TiO2 was identified in the triclinic (anorthic) andorthorhombic forms. Under these conditions, TiO2 cannot form atetragonal structure as in anatase and rutile but undergoes distortion toform an anorthic and orthorhombic TiO2 (Katyal et al., 1999).
4

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Chemosphere 351 (2024) 141206

Fig. 2. Schematic representation of the formation process of TiO2 polymorphs and the effect of acid and base volumes to control the crystal and phase compositions.

evaluated to TiO1.98. These oxygen vacancies facilitate the charge sep­aration at the semiconductor interface, significantly reducing therecombination of electrons and hole, thus greatly enhancing the pho­tocatalytic activity (Chen et al., 2018).
Further characterization was performed by using TEM (Fig. 3e) toobserve a closer look of A76R7B17 that showed the presence of sphericalstructures. The size distribution of A76R7B17 was shown in Fig. S9. It is
known that the crystal size ranges between 4 and 14 nm. This isessentially in accordance with the calculation of the crystallite size fromthe XRD pattern: anatase sizes of 7–9 nm, 11–16 nm rutile, and 6–9 nm
brookite for A76R7B17 sample.

specific SA should enhance the possibility interaction and adsorption
between pollutant and catalyst (Amano et al., 2010).
3.4. Optical properties
The optical properties of the samples were investigated by diffuse
reflectance UV–vis spectroscopy. Fig. 1c shows the absorption spectrumof single phase, biphasic, and triphasic TiO2 samples observed in therange of 220–800 nm. Davis–Mott plot was obtained (Fig. 1d) todetermine bandgap energy value (Eg) based on Eq. (2). The absorption
spectrum and bandgap value of all variant TiO2 phases are shown in
Figs. S12–S14.
The obtained Eg values for anatase (3.3 eV) are greater than rutile
phase (3.1 eV), in accord with reported Eg values of TiO2 polymorphs(Luttrell et al., 2014). On the other hand, pure brookite was reported toshow a greater band gap than single phase anatase and rutile (Reyes-­Coronado et al., 2008; Zhang et al., 2014). It should be noted that thevalues in Table 2 are slightly greater than generally reported values (3.2
for anatase, 3.0 for rutile and 3.3 eV for brookite). Meanwhile, band gap
value for A86R14 have a similar value to that in our previous report (Eddyet al., 2021) while have a smaller band gap than single phase anatase.
Furthermore, triphasic phase with a higher ratio of brookite phase havea Eg value of 3.1 eV. The decrease in the band gap value in heterophase iscaused by charge transfer between phases of TiO2. This band gap value
strongly affects the light absorption capacity because the lower band gap
broadens the absorption range (Chen et al., 2020).

3.3. Surface analysis
Comparison of the specific SA for various TiO2 samples is importantbecause the morphology will affect the photocatalytic activity. Fig. 1b
represents the BET plots of the TiO2 samples. The specific SA, BJH poreradius, and BJH pore volume were evaluated (Table 2). Based on thegraph, it has been observed that all samples show a type of IV isotherm
according to the international union of pure and applied chemistry(IUPAC) classification. The type IV reveals that the samples have mes­
oporous structure (IUPAC et al., 1985). The triphasic phase (A76R7B17)
sample exhibited a larger specific SA 149.04 m2 g− 1 than biphasic phase(49.9 m2 g− 1) and single-phase samples (9.33 and 5.13 m2 g− 1). The
detailed results of all samples were summarized in Table S8. Photo­catalyst materials with greater amount of pore volume, porosity, and5

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Chemosphere 351 (2024) 141206

Fig. 3. SEM image of (a) A100, (b) R100, (c) A86R14, (d) A76R7B17, and (e) TEM image of A76R7B17.

The mixed phase TiO2 potentially transfers photogenerated charges
between different TiO2 polymorphs that have different band energy
levels, leading to charge carriers’ separations and suppressing electro­
n–hole recombination. However, the fluorescence spectra (Fig. S15) on
the time scale of 50 ms were similar for anatase, rutile, biphase (A86R14),
and triphase (A76R7B17). In a closer look, the energy positions of major
fluorescence peak reflected direct band-gap de-excitation: 370 nm forbrookite, 395 nm for anatase, and 415 nm for rutile. Due to the major
peak energy shift as well as the difference of reflection/diffraction of
excitation light (λ = 300 nm) to draw increasing background toward

Table 2Band-gap value and surface properties of samples.
Samples

Band-gap
Energy (eV)Specific surface
area (m2 g− 1)

Pore size(nm)

Pore volume(cm3 g− 1)

A100
R100
A86R14
A76R7B17

3.31
3.05
3.08
3.07

9.33
5.13
49.95
149.04

1.21
2.71
1.21
2.38

0.04
0.01
0.16
0.28

6

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Chemosphere 351 (2024) 141206

300 nm, the fluorescence from A76R7B17 sample looked more intense,but the intensity of effective fluorescence (the band gap de-excitation
major peak and peaks related to interband impurity level at 450 and
465 nm) was similar for A76R7B17, A86R14, and rutile. The fluorescenceintensity for anatase was weaker by ~30% compared to the others. This
fact suggested that charge separation owing to UV irradiation andoxidation/reduction reactions by the charges proceeded faster than 50
ms. As a result, the multiple-phase materials showed higher photo­catalytic activity than single phase TiO2 (Khedr et al., 2021).
As an alternative interpretation, based on the VB and CB energy

levels for these crystalline phases (see the last part of 3.5 Photocatalytic
Activity section), charges are not necessarily separated via heterophasejunction but rather tend to be collected at rutile VB (h+) and CB (e− )(Fig. 1c). Therefore, the charge separation effects at the heterophasejunction were marginal, and the energy level of CB minimum above thereduction potential of O2/⋅O−2 (− 0.046 eV @NHE) for brookite andanatase was the most critical. Secondly, oxidation ability was in theorder brookite > anatase > rutile based on the energy level of VB
maximum. The brookite (and anatase) should comprise key reactive
species for both reduction and oxidation reactions, and combined rutile

Fig. 4. (a) Photocatalytic activity, (b) reaction rate (volume = 60 mL, [metformin] = 10 mg L− 1, catalyst dose = 1 g L− 1, pH = 7), (c) photocatalytic activity of
A76R7B17 with effect of pH condition ([metformin] = 10 mg L− 1, catalyst dose = 1 g L− 1), (d) catalyst dose ([metformin] = 10 mg L− 1, pH = 7), (e) metformin
concentration (catalyst dose = 1 g L− 1, pH = 7), and (f) effects of trapping agent for metformin photodegradation.
7

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Chemosphere 351 (2024) 141206(and anatase) worked to widen the photon absorption range between
370 and 415 nm (Fig. 1c and S15) for charge separation in photocatalyst.

with value 75.6%. Meanwhile, increasing the initial pH metformin until
11 conversely inactivated the photocatalyst. This could be explained by
the properties of both the metformin and triphasic TiO2. The point ofzero charge (pHPZC) for the photocatalyst TiO2 is 6.25 (Chinnaiyan et al.,2019). Therefore, its surface is predominately positively charged below
pHPZC (i.e. TiO2 + H+ ↔ TiOH+2 ) and negatively charged above pHPZC (i.
e., TiOH + OH− ↔ TiO− + H2O) (Mezenner and Hamadi, 2012).
Furthermore, a pKa of 12.4 indicates metformin will exist almost entirelyin the cationic form at pH values of 5–9 (Rao, 2014). In acidic pH,
metformin and triphasic TiO2 exhibit a repulsive effect as both are
positive charged (pH < pHPZC). Hence, the adsorption on the surface ofTiO2 might be limited. At pH 7, metformin is positive and triphasic TiO2
is negative (pH > pHPZC), thus favoring adsorption ability. Furthermore,in basic condition metformin and triphasic TiO2 exhibit a repulsive ef­
fect as both are negative charged (pH > pHPZC) that caused inactive
adsorption ability. Thus, the trend of pH value dependence was inaccord with the other reports (Chinnaiyan et al., 2019).
The effects of initial metformin concentration was also evaluatedwith 1.0 g L− 1 catalyst concentration at pH 7. Fig. 4e shows that thecondition of 10 g L− 1 was the most favorable than the other concen­
trations. When the initial metformin concentration exceeded 15–20 mg
L− 1, the degradation rate decreased. This was due to the effect of active
site catalyst are covered by metformin molecules to prevent UV lightreaching the catalyst surface, resulting in less radical generated thatsignificantly reducing photocatalytic activity (Wang et al., 2023).
Therefore, the optimized condition for degradation metformin using
triphasic phase A76R7B17 were determined at 1.0 g L− 1 of catalyst dose,pH 7, and 10 mg L− 1 initial metformin concentration.
To validate the major active species during the photocatalytic pro­cess, experiments using a series of scavengers have been performed for
A76R7B17 photocatalyst. Different scavengers with 100 mM of IPA,
ammonium oxalate (OA), AgNO3, and N2 gas (99.9%, 20 mL min− 1) astrapping/preventing agents for ⋅OH, h+, e− , and ⋅O−2 , respectively, wasadded to reaction system. The obtained data in Fig. 4f shows that theactivity was significantly decreased in the presence of all scavengers.
These results indicated that ⋅OH, h+, e− , and ⋅O−2 are all the key species
for metformin degradation reaction.
The plausible mechanism for the photocatalytic activity of triphasic
phase A76R7B17 can be evaluated theoretically by determining the po­sitions of VB and CB using equations Eq. (5) and (6):3.5. Photocatalytic activity
The photocatalytic properties of the various TiO2 samples were
evaluated by the photodegradation reaction of metformin under 25-W
UVB lamp irradiation. Prior to the test, the direct photolysis, e.g. in
the absence of photocatalyst, was investigated. As shown in Fig. S16a,
no activity was observed for the photolysis for metformin degradation.
When we start the UV–visible light irradiation, triphasic phase A76R7B17
TiO2 exhibited the higher activity than that using biphasic A86R14 andsingle phase of anatase (A100) and rutile (R100) as illustrated in Fig. 4a. It
is believed that the presence of more than one polymorph suppresses therecombination of electron–hole (e− –h+) pairs, since photo-excitedelectrons can migrate from one phase to the other, and thus leading toan enhancement on the photocatalytic performance (Zhou et al., 2017).
However, not all the heterophase have a good photocatalytic activity
because it depends on the phase composition and mostly on the con­duction band (CB) minimum and valence band (VB) minimum levels(Liu et al., 2019; Chalastara et al., 2020; Eddy et al., 2023b). In thisstudy, it can be concluded that a large proportion of anatase significantly
affected the photocatalytic activity. The reason why indirect band-gap
anatase exhibits a longer lifetime of photoexcited electrons and holes
than direct band gap of rutile and brookite is that the direct transitions ofphotogenerated electrons from CB to VB of anatase TiO2 is impossible(Zhang et al., 2014). Furthermore, triphasic phase A76R7B17 has thehigher specific SA than single phase and biphasic phase (Table 2).
The kinetic analysis (ln Ct/C0 versus the irradiation time) is shown in
Fig. 4b and for all samples in Fig. S16b. Triphasic A76R7B17 showed thegreatest rate constant (k = 0.0108 min− 1), which was 1.6, 1.9, and 6.0
times greater than biphasic A86R14 (k = 0.0068 min− 1), single phaseanatase A100 (k = 0.0058 min− 1), and single-phase rutile R100 (k =0.0018 min− 1), respectively. The degradation efficiency of A100, R100,
A86R14, and A76R7B17 are 54.3, 20.2, 60.7, and 75.4%, respectively. The
data for all samples are summarized in Table S9.
In order to verify that the major effect of photocatalytic activity isphase composition rather than the specific SA, we further designed thephotocatalytic activity tests using high specific SA of anatase with 270
m2 g− 1 (Sakai Chemical Industry Co. Ltd.) and rutile with 100 m2 g− 1
(Sakai Chemical Industry Co. Ltd.) comparing with biphasic phase
A86R14 with SSA 49.95 m2 g− 1 and triphasic phase A76R7B17 with SSA
149.04 m2 g− 1. The results in Fig. S17 showed that triphasic phase
A76R7B17 has the highest photocatalytic activity than pure materials
with high specific SA. It is confirmed that in this experiment phasecomposition has the main effect of photocatalytic activity. Meanwhile,high specific surface area of triphasic phase also enhance the adsorptionof metformin that will improving photocatalytic activity.
Analyzing the optimization parameters, e.g. pH, catalyst concentra­tion, substrate concentration and the presence of oxidants will affectphotocatalytic activity (Reza et al., 2017). In this study, we investigatedthe effects of catalyst concentration, pH, and initial metformin concen­
tration. The results in Fig. 4c showed that optimal dose for the metfor­
min photodegradation test is 1.0 g L− 1 because it performs better than
0.5 g L− 1 and 1.5 g L− 1 of catalyst concentration. This is due to thedecrease of active site number to interact with metformin in 0.5 g L− 1 ofcatalyst concentration (Mezenner and Hamadi, 2012). Meanwhile, 1.5 g
L− 1 was too high concentration that raised the metformin solution
turbidity and light scattering, which prevented effective photon reach­ing the catalyst surface and thus further decreasing the degradation rate
(Sun et al., 2008; Wang et al., 2023), in consistent with report by Car­buloni et al. (2020).
Then, the evaluation at different pH is shown in Fig. 4d. It was foundthat at lower initial pH of 3 and 5 the degradation efficiency of met­
formin was 2.9 and 33.6%, respectively. Further, as the pH increases,
degradation efficiency of metformin increases until the initial pH of 7.0

EVB = χ – EC + 0.5 Eg(5)ECB = EVB – Eg

(6)The χ represents the absolute electronegativity of semiconductors,
which is defined as the geometric mean of the absolute electronegativityof the constituent atoms. χ value for TiO2 anatase, rutile, and brookite is
5.81 eV (Usgodaarachchi et al., 2022). EC is the energy of the freeelectrons on the normal hydrogen electrode scale (4.50 eV) and Eg isband gap energy of the material. The band gap of TiO2 anatase, rutile,and brookite is 3.31, 3.05, and 3.47 eV, respectively, were obtained fromDR UV–vis analyses. The parameter toward the calculation of VB and CB
energy position has been tabulated in Table S10. Based on the calculatedband gap energy of the VB and CB position of anatase, brookite, andrutile, a possible schematic illustration of the possible heterojunction
mechanism for photocatalytic activity of triphasic phase A76R7B17 wasshown in Fig. 5.
When the material was exposed to UV light, the electrons were
excited from VB to CB and leave holes in VB. The excited electron will
migrate from brookite to anatase to rutile and the holes transfer in thesame direction. This assumption is accord with reports by Kaplan et al.
(2016) and Preethi et al. (2017). The accumulated electron in rutile
reduced O2 into ⋅O−2 because VB of brookite is more negative than
O2/⋅O−2 potential (− 0.046 eV vs. NHE) (Rosman et al., 2020). Moreover,
the accumulated holes in rutile capable to develop ⋅OH due to the VB
position more positive than OH− /⋅OH potential (+2.38 eV vs. NHE)
8

D.R. Eddy et al.

Chemosphere 351 (2024) 141206

Fig. 5. Proposed mechanism of A76R7B17 for metformin photodegradation.

(Rosman et al., 2020). Furthermore, based on Fig. 4f, holes also oxidized
metformin directly. Both of these radical will oxidized metformin into
non-toxic product such as guanyl urea (C2H6N4O) (Quintão et al., 2016;
Carbuloni et al., 2020). On the basis of the aforementioned experimental
outcomes, two junction interfaces highly facilitate interfacial chargetransfer compared to single junction anatase-rutile or pure phaseanatase or rutile. The performance of the photocatalytic system will be
improved by this interfacial charge transfer process, which enhanced thelife of charge carriers.

Conceptualization, Data curation, Investigation, Supervision, Valida­tion, Writing - review & editing.
Declaration of competing interest
The authors declare the following financial interests/personal re­lationships which may be considered as potential competing inter­
estsDiana Rakhmawaty Eddy reports was provided by Padjadjaran
University Faculty of Mathematics and Natural Sciences. Yasuo Izumi
reports a relationship with Chiba University that includes: non-financial
support.

4. Conclusions

Data availability

The triphasic polymorph of TiO2 was successfully prepared by ul­
trasound assisted sol-gel procedure in the presence of acid condition.
The ratio of anatase-rutile-brookite is easily tuned by changing thevolume of acid and base. The efficiency of triphasic TiO2 for metformin
photodegradation is compared with pure anatase, pure rutile, andbiphasic TiO2 (anatase–rutile). It is found that triphasic TiO2 with 76%
anatase, 7% rutile, and 17% brookite has a highest photocatalytic ac­
tivity than single and biphasic phase. This could be the result of syner­gistic effects in polymorph that suppress electron-hole recombination.
Furthermore, the high specific SA also give an impact but did not crucial
since pure anatase and pure rutile with larger specific SA have lower
photocatalytic activity.

No data was used for the research described in the article.
Acknowledgements
Authors are grateful for the facilities from Universitas Padjadjaran,
Indonesia, by Academic Leadership Grant (ALG), Prof. Iman Rahayu (ID:1549/UN6.3.1/PT.00/2023), Hibah Riset Unpad (HRU) by Riset Kom­petensi Dosen Unpad (RKDU), Diana Rakhmawaty Eddy (ID: 1549/
UN6.3.1/PT.00/2023), and World Class Professor (WCP) Program
Directorate of Resources, Directorate General of Education and Tech­
nology, Ministry of Education and Culture in 2023 grant.

CRediT authorship contribution statement

Appendix A. Supplementary data

Diana Rakhmawaty Eddy: Conceptualization, Funding acquisition,
Project administration, Supervision, Validation. Geometry Amal Nur
Sheha: Data curation, Formal analysis, Methodology, Resources,
Writing - original draft. Muhamad Diki Permana: Data curation,
Formal analysis, Methodology, Software, Writing - original draft,
Writing - review & editing. Norio Saito: Data curation, Formal analysis.
Takahiro Takei: Data curation, Formal analysis. Nobuhiro Kumada:
Data curation, Formal analysis, Supervision. Irkham: Data curation,
Formal analysis, Software, Data curation, Formal analysis, Software.
Iman Rahayu: Funding acquisition, Supervision. Ikki Abe: Data cura­tion, Formal analysis. Yuta Sekine: Data curation, Formal analysis.
Tomoki Oyumi: Data curation, Formal analysis. Yasuo Izumi:

Supplementary data to this article can be found online at https://doi.
org/10.1016/j.chemosphere.2024.141206.
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