The fabrication of TiO2-supported clinoptilolite: via F-contained hydrothermal etching and a resultant highly energetic {001} facet for the enhancement of its photocatalytic activity

Article abstract of DOI:10.1039/d1ra02269e

TiO2-supported clinoptilolite (TiO2/CP) was synthesized in the presence of F- ions. Various characterizations demonstrated that the particle size of loaded TiO2 increased linearly with an increase in the temperature and concentration of F- ions. In particular, the additive F- ions were favored to produce the mutually independent co-exposed {001} and {101} facets of loaded TiO2, while TiO2/CPs synthesized in the absence of F- ions were dominated by the thermodynamically stable {101} facet. As photocatalysts for the removal of crystal violet or methyl orange dyes under UV-irradiation in aqueous solutions, TiO2/CPs (ACP6) synthesized in the presence of F- ions significantly improved the degradation efficiency, as compared to ACP3 obtained in the absence of F- ions. These results elucidated that the highly energetic {001} exposed facet, large particle size and fine dispersion of loaded TiO2 in TiO2/CP accounts for its best photocatalytic performance. The effected mechanism of operational parameters on the degradation performances is proposed.

Full text of DOI:10.1039/d1ra02269e

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The fabrication of TiO2-supported clinoptilolite via
Fꢀ contained hydrothermal etching and a resultant
highly energetic {001} facet for the enhancement
of its photocatalytic activity†
Cite this: RSC Adv., 2021, 11, 17849
*
*
Tallat Munir and Shiyang Bai
Anadil Gul,‡ Raza Ullah,‡ Jihong Sun,
TiO2-supported clinoptilolite (TiO2/CP) was synthesized in the presence of Fꢀ ions. Various
characterizations demonstrated that the particle size of loaded TiO2 increased linearly with an increase in
the temperature and concentration of Fꢀ ions. In particular, the additive Fꢀ ions were favored to produce
the mutually independent co-exposed {001} and {101} facets of loaded TiO2, while TiO2/CPs synthesized
in the absence of Fꢀ ions were dominated by the thermodynamically stable {101} facet. As photocatalysts
for the removal of crystal violet or methyl orange dyes under UV-irradiation in aqueous solutions, TiO2/
CPs (ACP6) synthesized in the presence of Fꢀ ions significantly improved the degradation efficiency, as
compared to ACP3 obtained in the absence of Fꢀ ions. These results elucidated that the highly energetic
{001} exposed facet, large particle size and fine dispersion of loaded TiO2 in TiO2/CP accounts for its
best photocatalytic performance. The effected mechanism of operational parameters on the degradation
performances is proposed.
Received 22nd March 2021
Accepted 3rd May 2021
DOI: 10.1039/d1ra02269e
rsc.li/rsc-advances
composite conguration tailors the photocatalytic properties of
loaded TiO2 by controlling the particle size, bandgap, surface
1. Introduction
area, loading content and porosity of loaded TiO2. Moreover,
the physical and electronic properties of the composite photo-
catalysts can be regulated by interfacial interactions between
TiO2 and zeolite. The use of zeolite as a supporting matrix for
TiO2 nanoparticles has also been found to enhance the
adsorption capacity10 and dispersion of titania,11 which in turn
improve the photoactivity of the system. Generally, the immo-
bilization of TiO2 onto the surface of zeolite can be achieved by
numerous methods including sol–gel,10 hydrothermal,12
impregnation,11 and solid-state dispersion methods.13 Among
the aforementioned methods, the hydrothermal method is an
effective route for immobilizing TiO2 onto the surface of zeolite
with high surface area, good crystallinity and variant particle
size,14 which offer several advantages such as the formation of
the anatase phase at relatively low temperature, low agglomer-
ation between particles, defect-free nano-crystals with high
surface area, and narrow particle size distribution.15,16
Clinoptilolite (CP), being one of the most abundantly
occurring zeolites in nature, is regarded as the best candidate as
a support for TiO2.17–19 Though signicant research is available
on the synthesis and application of TiO2/CP composites, the
literature still lacks data relating to the loading of anatase TiO2
onto CP with controlled particle size and highly reactive
exposed crystal facets. Recently, intensive research interest has
been focused on controlling the crystal particle size and
exposed facets of TiO2 to enhance its photocatalytic
With the increasing demands of energy- and environment-
related problems, photocatalysis, as a light-driven photochem-
ical process over the surface of a photocatalyst, has attracted
increasing attention over the last few decades due to its
potential applications in the elds of wastewater treatment and
the generation of renewable energy.1,2 Although TiO2-based
photocatalysis has been over-exploited in the past few decades,
there are still some challenges related to the reaction efficiency,
complicated treatment of catalyst recovery step, electron–hole
recombination rate and electron–hole transport mechanism of
TiO2, which restrain the large-scale practical application of the
process.3–5 To achieve higher efficiency in the practical appli-
cation of TiO2, the tailoring of its physical properties, such as
phase composition, exposed crystal facets, average particle size,
surface area, crystallinity and porosity, are of key importance.6–9
The immobilization of TiO2 onto the surface of some suitable
support is an encouraging alternative to the inherent limita-
tions of unsupported TiO2.
The preparation of the TiO2/zeolite composite photocatalyst
has attracted increasing attention over recent years, as the
Beijing Key Laboratory for Green Catalysis and Separation, Department of
Environmental and Chemical Engineering, Beijing University of Technology, Beijing
100124, P. R. China. E-mail: jhsun@bjut.edu.cn; sybai@bjut.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra02269e
‡ Equal contribution.
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performance.20,21 There exists an optimum particle size for purity), NH4F (96.0%), CV dye (high purity biological stain), and
excellent photocatalytic performance of TiO2 due to the MO dye were obtained from J & K Co. Ltd. All solutions were
competing effects of the effective particle size on light absorp- made in deionized water with a resistivity of 18.25 MU cm.
tion and scattering efficiency. Almquist and Biswas22 synthe-
sized anatase TiO2 particles in the range of 5 to 165 nm and
found that an optimum particle size of 25 to 40 nm exhibited
excellent photoactivity. The tailoring of anatase TiO2 crystals
with the termination of specic facets has received great
interest for many years. As reported by Liu et al.,9 the anatase
phase {001} facet was found to be the most reactive with an
average surface energy of 0.90 J molꢀ2; therefore, the photo-
catalytic performance of anatase TiO2 depends not only on its
particle size, but also on the type of exposed crystal facets.
However, unfortunately, most available anatase TiO2 is domi-
nated by the thermodynamically stable {101} facet during the
2.2. Synthesis of TiO2/CP catalysts
The nely divided powdered natural CP was rst puried by
heating its suspension in deionized water (10 g Lꢀ1) at 70 C
ꢁ
under continuous vigorous stirring for 8 hours. The resulting
suspension was then allowed to settle down prior to ltration
and then washed thoroughly with deionized water. TiO2/CP
photocatalysts were synthesized using a procedure similar to
that reported by Ullah et al.14 In a typical procedure, 1.00 g of CP
was dispersed in 25 mL of 0.25 M TiCl4 solution made in ice-
cold water and the suspension was then autoclaved on oil
baths under continuous vigorous stirring for 5 hours at 100,
crystallization process due to its relatively low surface energy,
ꢁ
150, and 200 C. The precipitate was then separated by ltra-
i.e. 0.44 J molꢀ2
.
The breakthrough in controlling the
23
tion, washed thoroughly with deionized water and dried in an
oven at 80 ꢁC overnight. The obtained products were ground to
a ne powder and named ACP1, ACP2, and ACP3, correspond-
ing to the hydrothermal temperature at 100, 150, and 200 C,
respectively.
Keeping the other conditions constant as used for the rst
set of three catalysts, the second set of four catalysts were
synthesized at 200 ꢁC by adding 0.08, 0.15, 0.25, and 0.35 M
NH4F to 0.25 M TiCl4 solution, and named ACP4, ACP5, ACP6,
and ACP7, respectively. Pure TiO2 was also made by the same
procedure using 0.25 M Fꢀ ions without adding CP.
A representative scheme showing the preparation proce-
dures and the structure of TiO2/CPs is illustrated in Fig. S1 of
the ESI† section.
synthesis of crystals with a high percentage of reactive high
energy {001} facets was not made until 2008.24 Roy et al.25
demonstrated that among the various facets, {101} was the least
active towards methyl orange (MO) degradation, whilst an
optimum {001}/{101} ratio resulted in excellent performance
because of the reduced electron–hole recombination rate. In
order to enhance the photocatalytic performance of TiO2,
a large group of researchers are making efforts to synthesize
anatase TiO2 with highly reactive exposed facets, i.e. {001}.
Hence, TiO2/CPs with highly reactive exposed facets i.e. {001}
are still highly desired and pursued in the eld of heteroge-
neous photocatalysis. Numerous methods have been developed
to fabricate anatase TiO2 nanocrystals with {001} exposed facets,
in which surface uorination is the most effective in stabilizing
{001} facets based on rst-principles calculations.23,24
ꢁ
2.3. Characterizations
Herein, TiO2/CP photocatalysts with controlled anatase TiO2
particle size and exposed highly reactive crystal facets were The phase compositions and crystalline structures of the TiO2/
synthesized under different hydrothermal treatment tempera- CP catalysts were analyzed by X-ray diffractometer (Beijing
tures and concentrations of uoride ions (Fꢀ ions) in an Purkinje General Instrument Co. Ltd), having a CuKa irradia-
aqueous solution of TiCl4. The effect of hydrothermal treatment tion source, and 2q (diffraction angle) ranging from 5–75ꢁ. The
temperature and additive amount of Fꢀ ions on the photo- supported TiO2, crystallite size was calculated with the help of
catalytic activity of TiO2/CP was investigated in detail via the Scherrer's equation (D ¼ kl/b cos q), where D is the estimated
degradation of crystal violet (CV) and methyl orange (MO) dyes crystallite size (nm), k is a constant (0.9), l is the X-ray wave-
under UV-irradiation in aqueous media. Fꢀ ions were used as length (0.15432 nm), b is the peak width at half of the maximum
tailoring agents to expose the highly energetic {001} facet of the height, q is the diffraction angle. ICP-OES (Perkin Elmer Optima
loaded TiO2 in order to enhance its photocatalytic activity. The 2000-DV) was used to analyze the elemental composition and
as-prepared TiO2/CP catalysts were characterized via X-ray weight % of TiO2 in the synthesized samples (TiO2/CP). The
diffraction (XRD), scanning electron microscopy (SEM), trans- chemical structures of the samples were examined via FT-IR
mission electron microscopy (TEM), inductively coupled spectrophotometer in the range of 400–4000 cmꢀ1. N2 adsorp-
plasma-optical emission spectrometry (ICP-OES), X-ray Photo- tion–desorption isotherms were used to determine the specic
electron Spectroscopy (XPS), Fourier Transform Infrared (FT- surface area of synthesized samples at ꢀ196 ꢁC, with the help of
IR), UV-visible spectroscopy and BET-isotherm. Finally, the a JW-BK300 (Beijꢁing Sci. & Techno. Co. Ltd). The samples were
degradation kinetics of CV and MO dye in the aqueous solutions degassed at 120 C for 6 hours prior to each measurement in
were elucidated.
order to remove all the adsorbed gas molecules in the samples.
The structural morphology and particle size of the synthesized
samples (TiO2/CP) were examined using scanning and trans-
mission electron microscopy (JEOLJEM-200 and JEOL-2010,
respectively). The operating voltage was 15.0 kV for SEM and
2. Experimental
2.1. Materials
Natural CP with a Si/Al ratio of 10.63 was supplied by Guo- 200 kV for TEM. The optical bandgaps of the samples were
TouShengShi Science and Technology Co. Ltd. TiCl4 (99% determined using a Shimadzu UV-2600 spectrophotometer. The
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pHpzc (point of zero charge) was measured with the help of
a Zeta-sizer (Malvern Instrument Ltd, UK) through light scat-
tering at different pH values. Surface chemical analysis of the
synthesized samples (TiO2/CP) was performed by XPS spectra
(ESCALAB 250Xi, Germany). The total organic carbon (TOC)
removal efficiencies of CV and MO dye via photocatalytic
degradation were investigated via vario TOC (UK).
2.4. Photocatalytic degradation experiments
The photocatalytic evaluation of the obtained TiO2/CPs for the
degradation of CV and MO dyes was carried out in a 250 mL
glass beaker containing 100 mL aqueous solution at room
temperature. A set of three 20 W high-pressure UVC Hg lamps
enclosed in a rectangular steel box, was used as the irradiation
source. Before being subjected to the UV-light source, the
reaction mixture was stirred for 30 min continuously in the dark
to reach the adsorption–desorption equilibrium. The amount of
both the dyes adsorbed was less than 5% for all the photo-
catalysts synthesized, which indicates that adsorption has
a negligible role in the removal process. Therefore, the degra-
dation process was mainly focused in the removal of these dyes
from the reaction mixture. At a set time, the aliquots of used
samples were withdrawn, centrifuged prior to the analysis to
remove the suspended catalyst particles and then analyzed
using a UV-vis spectrophotometer. Each experiment was carried
out in duplicate in order to validate the results.
Fig. 1 XRD patterns of (a) natural CP, (b) ACP1, (c) ACP3, (d) ACP4, (e)
ACP6, and (f) pure TiO2.
form 0.08 M for ACP4 (Fig. 1d) to 0.25 M for ACP6 (Fig. 1e), the
decline in peak intensity of CP characteristic phases, but the
enhancement in the intensity of anatase peaks may be because
TiO2 particles effectively covered the surfaces of CP during
composite formation and hindered peaks of CP from being
detected by XRD patterns. Another reason may be related to the
etching effect of used Fꢀ ions at a high hydrothermal temper-
ature on the excessive dealumination of CP structures.26
Furthermore, the crystallite size of loaded TiO2 calculated using
Scherrer's equation, as shown in Table 1, presented the
increasing tendency from 7.8 to 17.7 nm along with enhance-
ments of temperature and concentrations of used Fꢀ ions. As
can be seen in Fig. 1, the intensity of the (004) diffraction peak,
being used (I004/I101) to represent the highly energetic {001}
facet in TiO2/CP,9 remarkably improved with enhancements of
temperature and concentrations of used Fꢀ ions, and was also
higher than that of pure anatase TiO2 (ca. 0.30, as shown in
Table S1 of the ESI† section, corresponding to the reference
standard (JCPDS. 21-1272) which presented I004/I101 of (0.2)9).
These observations suggest the preferential orientation growth
along the {001} facet direction for TiO2/CPs (ACP4-ACP7); in
other words, the existence of both Fꢀ ions and CPs plays
a signicant role in exposing the highly energetic {001} facet of
loaded TiO2. However, in the absence of CP, the hydrothermal
treatment of the TiO2 precursor in HF solution easily leads to
the formation of pure anatase particles with a relatively low
intensity of {004} diffraction peaks (as seen from the I004/I101
values in Table S1 of the ESI† section). This unique co-existence
of highly energetic {001} and thermodynamically stable {101}
facets in TiO2/CP (ACP6) is responsible for the synergistic
removal of CV and MO dyes from aqueous solution under UV-
irradiation.
The following eqn (1) was used for the percentage degrada-
tion (X%) calculation of CV and MO dyes, as follows:
100
X% ¼ ðC0 ꢀ CeÞ$
(1)
C0
where C0 and Ce are the initial and nal concentrations (mM) of
the dye at equilibrium.
The TOC removal percentage (%) was estimated using the
following expression (2):
ꢀ
ꢁ
TOC
TOC removal percentage ð%Þ ¼ 1 ꢀ
ꢂ 100% (2)
TOC0
where TOC corresponds to time t, and TOC0 corresponds to the
initial conditions.
3. Results and discussion
3.1. Characterization of TiO2/CPs
The XRD patterns of parent natural CP, TiO2, and TiO2/CPs
(ACP1, ACP3, ACP4, and ACP6) are shown in Fig. 1. The
heulandite (HEU) structure of CP and the crystal phase of TiO2
were conrmed by comparison with the Joint Committee on
Powder Diffraction Standards (JCPDSs), in which, the charac-
teristic diffraction peaks at 2q values of 9.98, 11.27, 22.29, 26.82,
30.07, and 32.23ꢁ belonged to the major components of the CP
(as shown in Fig. 1a), and those at 25.5, 38.2, 48.3, 54.2, 55.4,
and 62.5ꢁ corresponded to anatase phase TiO2 (as shown in
Fig. 1f), which are indexed as (101), (001), (200), (105), (211), and
(204) crystal planes, respectively. With the improved hydro-
thermal temperatures from 100 ꢁC for ACP1 (Fig. 1b) to 200 ꢁC
for ACP3 (Fig. 1c), or increased concentrations of used Fꢀ ions
All the TiO2/CP samples formed in the presence of Fꢀ ions
exhibited much sharper and higher characteristic peaks of
loaded TiO2, which may be ascribed to their highly crystalline
characteristics. Table S1† also shows that the crystallinity
degrees of CP support decreased, while that of loaded TiO2
increased along with the enhancement of the temperature and
concentration of Fꢀ ions during hydrothermal treatments. The
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Table 1 Collections of various parameters of CP and TiO2/CPs
TiO2
particle size
(nm)
Bandgap (eV)
Sample Formula
TiO2 amounta (wt%) TiO2 phase XRDb SEMc Si/Ald (molar ratio) SBETe (m2 gꢀ1
)
n ¼ 2 n ¼ 1/2
CP
Na0.75K1.89Al8.06Si85.70O184.81
—
Ti21.57Na1.36K1.23Al6.12Si88.24O230.10 27.31
Ti23.14Na1.45K1.12Al5.07Si101.47O258.11 29.29
Ti23.85Na1.44K0.27Al3.31Si138.35O130.22 30.20
Ti24.49Na1.59K0.61Al4.80Si132.93O330.22 31.01
Ti24.29Na1.47K0.35Al2.86Si108.83O271.44 30.76
Ti33.93Na1.64K0.21Al2.79Si129.96O332.96 42.95
—
—
—
10.63
14.41
20.02
15.8
193.7
116.3
50.1
51.3
44.8
—
—
ACP1
ACP2
ACP3
ACP4
ACP5
ACP6
ACP7
Anatase
Anatase
Anatase
Anatase
Anatase
Anatase
Anatase
7.80 4.7
8.77 6.2
2.73 3.33
2.72 3.33
2.81 3.32
2.91 3.43
2.85 3.22
2.80 3.28
2.50 3.57
9.25 10.4 41.79
10.01 14.5 27.68
14.92 19.0 38.11
17.36 16.0 46.48
17.72 20.0 90.62
43.6
35.6
Ti36.87Na1.57Al1.52Si137.46O351.80
46.68
a
b
c
d
Determined from ICP data. Determined from XRD patterns according to Schererr's equation. Determined from SEM images. Determined
from ICP data. e BET surface area determined from N2 adsorption–desorption isotherms.
XRD patterns of ACP2, ACP5 and ACP7 displayed similar These nano-scale TiO2 particles loaded onto CP support are
observations as shown in Fig. S2A of the ESI† section.
benecial to provide more active sites for synergistic effects in
The SEM images of natural CP, TiO2 and TiO2/CPs (ACP1, the photocatalytic degradation of CV and MO dyes. As can be
ACP3, ACP4, and ACP6) are illustrated in Fig. 2. As can be seen, seen in Fig. 2b and c, both ACP1 and ACP3 prepared in the
the pure natural CP (Fig. 2a) revealed smooth sheet-like struc- absence of Fꢀ ions maintained the sheet-like structure of the CP
tures, while that of TiO2/CPs (Fig. 2 from b to e) presented rough support. However, Fig. 2d and e indicated that the TiO2 loadings
surfaces with nanoscale particles of TiO2 with a size of around in ACP4 and ACP6 were relatively uniform but there were no
4.7–20.0 nm determined from ImageJ soware27 (as shown in apparent sites of unloaded CP. Thermal shocks at high ꢀhydro-
Table 1), which were dispersed on the surface of the CP support. thermal temperature (200 ꢁC) and the existence of F ions
would cause the excessive dealumination and etching
phenomena of the CP supports in ACP3-ACP7, leading to the
partial dissolution of the CP framework and good dispersions of
the loaded TiO2, similar to the XRD results. The SEM image of
bare TiO2 (Fig. 2f) exhibited obvious aggregation and therefore
indicated that CP supports could act as dispersants to avoid the
typical tendency of TiO2 to form agglomerates.
The SEM images of ACP2, ACP5 and ACP7 as shown in Fig. S3
of the ESI† section exhibited almost similar information to that
of ACP1, ACP4 and pure TiO2, respectively. The use of relatively
high concentrations of Fꢀ ions (up to 0.35 M) easily led to the
severe aggregation of loaded TiO2 as seen in ACP7 (Fig. S3c†),
which looks like pure TiO2 (Fig. 2f).
The micro/nanostructures of the ACP3 and ACP6 were
further characterized by TEM images. As manifested in Fig. 3a
and b, the grain micro-photos of ACP3 and ACP6 were
composed of a large amount of TiO2 nanocrystals with an
average size of about 3 and 2.7 mm, respectively. The high
magnication TEM image of ACP6 exhibited relatively large and
highly crystalline TiO2 particles with more uniform and inde-
pendent distributions on the surface of the CP support (Fig. 3d),
which is very useful for improving its photocatalytic properties,
as compared to that of ACP3 (Fig. 3c), in which the severe
aggregation of loaded TiO2 appeared. The mean sizes of the
loaded TiO2 nano-particles in ACP3 and ACP6 determined from
high magnication TEM image using ImageJ soware11,28 were
found to be around 10.4 nm (Fig. 3c (inset)) and 19 nm (Fig. 3d
(inset)), respectively, consistent with that calculated from SEM
Fig. 2 SEM images of (a) natural CP, (b) ACP1, (c) ACP3, (d) ACP4, (e) images (as shown in Fig. 2c and e) and XRD patterns (as shown
ACP6, and (f) pure TiO2.
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results in preserving the highly reactive facets. The selected
areas in the elliptical shape represent {101} facets, while that in
the box shape represents the {001} facets. According to Wolff's
construction principle,9 more than 90% of the exposed facets
consist of the thermodynamically stable {101} facet during the
crystal growth process of TiO2. The co-existence of the reactive
high energy {001} facet (0.90 J mꢀ2) and the thermodynamically
stable {101} facet (0.44 J mꢀ2) may be benecial to produce
a number of thermal intermediate interfaces with a reduction of
their Gibbs free energy,30 and thereby enhance the synergistic
removal of CV and MO dyes from aqueous solution.
The corresponding SAED patterns of ACP3 and ACP6, as
shown in Fig. 3g and h, respectively, further demonstrated the
existence of the anatase phase of loaded TiO2. The SAED pattern
of ACP6 (Fig. 3h) exhibited more bright circular elds, indi-
cating its high crystalline characteristics, as compared to that of
ACP3 (Fig. 3g).
Fig. 4A illustrates the FT-IR spectra of parent CP and TiO2/
CPs (ACP3 and ACP6) in the wavenumber range of 400–
4000 cmꢀ1. As can be seen, all samples exhibited almost the
same peak proles, suggesting that the synthesis method did
not cause signicant destruction of the HEU microstructure.
However, the intensity of all peaks in TiO2/CPs related to CP
support became weak, consistent with that of XRD analysis. The
most intense peak at around 1030 cmꢀ1 was associated with the
asymmetric stretching vibration of O–Si(Al)–O, showing high
sensitivity to the degree of dealumination.31 This band became
Fig. 3 (a) and (b) TEM images of ACP3 and APC6, (c) and (d) high-
magnification TEM images of ACP3 and APC6, (e) and (f) high-reso-
lution TEM images of ACP3 and APC6, (the selected areas in elliptical
shapes represent {101} facets, while those in box shapes represent
{001} facets) and (g) and (h) the SAED pattern of ACP3 and APC6. The
histograms of TiO2 particle size distribution determined from the high-
magnification TEM images of ACP3 and ACP6 are given in the inset of
(c) and (d).
in Fig. 1c and e). The high-resolution TEM image of ACP3, as
shown in Fig. 3e, revealed that the lattice spaces of 0.35 nm
supporting the exposed facets are {101} of the anatase TiO2. The
high-resolution TEM image of ACP6 (Fig. 3f) presented a clear
lattice with well-dened lattice fringes of 0.24 and 0.35 nm,
corresponding to {001} and {101} exposed facets, respectively,
which conrmed the anatase phase of TiO2.9 These observations
further proved that the existence of Fꢀ ions could be benecial
for the etching effect and formation of the co-exposed {001} and
{101} facets of TiO2 in TiO2/CPs.9
The different crystallographic exposed facets of TiO2 nano-
crystals are controlled by altering their relative stability during
the crystal growth, which is intrinsically determined by their
surface energies. The capping agents (Fꢀ ions) adsorbed on the
surface of TiO2 interact differently with different crystalline
facets leading to various dominant reactive facets.29 The
capping agents (Fꢀ ions) play a crucial role of selectively
adsorbing on the surface of TiO2 and reducing the surface free
energy with more active {001} facets of loaded TiO2, which
Fig. 4 (A) FT-IR spectra of (a) CP, (b) ACP3, (c) ACP6. (B) Zeta potential
of (a) CP, (b) ACP6, and (c) TiO2.
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weak and shied to a relatively higher wavenumber (1100 cmꢀ1
)
in TiO2/CPs (Fig. 4A-b and c), as compared to that of parent CP
(as shown in Fig. 4A-a). This decrease in intensity and the slight
shi in the position of the absorption band around 1030 cmꢀ1
may be due to the elution of a portion of the Al3+/Si4+ intra-
framework on hydrothermal treatment.32 The peaks that
appeared at around 465, 605, and 795 cmꢀ1 were attributed to
the stretching vibrations of SiO4 and AlO4 tetrahedral atoms
present in the zeolite structure.33 The absorption band at
around 1640 cmꢀ1 in CP (Fig. 4A-a) was assigned to the –OH
bending vibration of physically adsorbed water. This band
became very weak in TiO2/CPs, which was due to the desorption
of physically adsorbed water during hydrothermal treatment.
The intensity of all peaks below 1000 cmꢀ1 belonging to CP
features decreased aer loading TiO2 in the HEU structures,
which may be due to the overlapping of the Si–O–Si band of CP
and Ti–O band of TiO2.34 However, the existence of peaks
between 400 and 1200 cmꢀ1 in all the TiO2/CPs reected that
the skeleton structure of CP was not destroyed aer hydro-
thermal treatment, thus supporting XRD analysis.
Fig. 5 XPS spectra of the survey scan (A), Ti2p (B), O1s (C) and F1s (D).
(a) ACP3 and (b) ACP6.
The FT-IR spectra of ACP1, ACP2, ACP4, ACP5 and ACP7 are
shown in Fig. S2B of the ESI† section, exhibiting almost the
same information as that of ACP3 and ACP6. The absorption
bands centered at 430 and 740 cmꢀ1 in TiO2 (Fig. S2B-f†) may be
attributed to the bending vibration of the Ti–O–Ti bonds.34
The zeta potentials of the parent CP, bare TiO2 and TiO2/CP
(ACP6) as a function of equilibrium pH of the solution in the
range of 2–10 are shown in Fig. 4B. The observed variation in
the zeta potentials of all samples with different pH values is due
to the acid and base used to adjust the pH values of the media.
The zeta potential of bare CP was found to be highly negative in
the pH range studied, which is consistent with that reported by
Ullah et al.14 The surface charges of TiO2 and ACP6 were found
to be positive at low pH value due to the effect of highly acidic
TiCl4 aqueous solution during their synthesis. The pH values of
point of zero charge (pHpzc) for CP (Fig. 4B-a), bare TiO2 (Fig. 4B-
c), and ACP6 (Fig. 4B-b) were found to be 2.3, 5.9, and 6.9,
respectively.
The surfaces of these samples at a pH value above their pHpzc
were negatively charged, whereas they were positively charged at
pH below pHpzc. Therefore, it was concluded that the surface of
bare CP is highly negatively charged, while the hydrothermal
treatment of CP under highly acidic TiCl4 aqueous solution
leads to a relatively higher value of pHpzc, i.e. 6.9 of the TiO2/CP.
The XPS spectra of ACP3 and ACP6 are shown in Fig. 5. In
Fig. 5A, the XPS spectra of the survey scan exhibited that the
peaks of Ti, O, C and Si were common in both ACP3 and ACP6
samples, whereas a weak extra peak of F appeared in the spec-
trum of ACP6. The two strong peaks at 458.22 and 463.96 eV in
the high-resolution XPS spectra of Ti2p (Fig. 5B) correspond to
Ti4+2p3/2 and Ti4+2p1/2, respectively, consistent with the values
of Ti4+ in the TiO2 lattice. The high-resolution XPS spectra of
O1s (Fig. 5C) presented two clear peaks at 529.55 and 532.96 eV,
which were assigned to the metal–oxygen bond (lattice oxygen)
and oxygen defects sites, respectively.35 The peak at 532.96 eV
representing oxygen vacancy was stronger in ACP6 (Fig. 5C-b) as
compared to ACP3 (Fig. 5C-a), indicating the signicant role of
Fꢀ ions in creating oxygen vacancies. Additionally, the C1s peak
(Fig. 5A) arises from the adventitious hydrocarbon present in
the high vacuum of the XPS instrument. The ACP6 sample
(Fig. 5D-b) clearly revealed two distinct peaks at 683.96 and
687.36 eV, which correspond to the F1s level, whereas ACP3
(Fig. 5D-a) contained no such peaks. The F1s binding energy of
684 eV in Fig. 5D-b corresponds to Fꢀ adsorbed on TiO2 in
ACP6, whereas the binding energy of 687 eV showed signs of F
ions in the lattice of TiO2 in ACP6.36
UV-vis absorption spectra of the TiO2/CPs were obtained to
estimate the optical bandgaps, which were determined using
the Tauc relation given in eqn (3).
(Ahn)n ¼ C(hn ꢀ Eg)
(3)
where A is the absorbance, h is Planck's constant, n is the
frequency and Eg is the bandgap energy. The integral n has
values of 2 and 1/2 for the allowed direct and allowed indirect
transitions, respectively. The optical band gaps were deter-
mined by extrapolating the linear portion of the (Ahn)n vs. hn
plot to the y-axis ¼ 0, which gives the bandgap energy (Eg). The
obtained optical band gap energy values of the TiO2/CPs, as
listed in Table 1, indicate their semiconducting properties.
The chemical formulas of TiO2/CPs and the TiO2-loaded
amount determined from ICP are summarized in Table 1. The
increased Si/Al ratios along with the enhancement of the
hydrothermal treatment temperature and concentrations of
used Fꢀ ions are due to the excessive dealumination of the CP
supports. As can be seen, although the prepared TiO2/CPs had
a higher surface area than the bare CP, their BET surface areas
decreased gradually as the particle size of TiO2 loaded on the
surface of CPs increased from 7.8 to 17.7 nm.
Pure CP has a relatively low BET surface area of around 15.8
m2 gꢀ1. The BET surface area was rst increased in TiO2/CPs
formed at a relatively low hydrothermal temperature, i.e. ACP1
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and ACP2 with 193.7 and 116.3 m2 gꢀ1, due to the relatively low
particle size of loaded TiO2 (around 4.7 and 6.2 nm) in these
samples. As the particle size of loaded TiO2 increased, the BET
surface area decreased abruptly due to the fact that surface area
and particle size were inversely related.
The pore size distribution curves of pure CP and TiO2/CPs
are shown in Fig. S4 of the ESI† section. Pure CP bears pores of
both micro- (pore size < 2 nm) and meso-dimensions (pore size
> 2 nm), while all of TiO2/CPs were mesoporous in nature. This
is due to the fact that the micropores of CP are lled by the
small loaded-TiO2 nanoparticles. The broadly distributed mes-
opores in TiO2/CPs are due to aggregations of intra-particles
during the hydrothermal procedures.
3.2. Photodegradation performances
To demonstrate the application of fabricated samples, their
photocatalytic activities for the synergistic removal of CV or MO
dyes from aqueous solutions were investigated under UV-
irradiation. Prior to photocatalytic degradation, the dye solu-
tions containing photocatalysts were stirred continuously for 30
minutes in the dark to reach the adsorption–desorption equi-
librium. The amount of each dye adsorbed was found to be less
than 5% for all the synthesized TiO2/CPs. However, it was
observed that bare CP exhibited good adsorption properties for
CV dye, with 51% removal, while there was poor adsorption with
2% removal for the MO dye. The effect of UV-light on the pho-
todegradation behavior of CV and MO dyes was investigated by
carrying out a blank experiment under UV-irradiation in the
absence of photocatalyst. Only about 5% degradation of each
dye was obtained under UV-irradiation within 180 minutes. The
efficiency of the photocatalytic reaction was affected by the
characteristic properties of the synthesized TiO2/CPs, such as
crystal structure, particle size, surface charge, surface area, and
reaction conditions like the pH of the solution, dose of catalyst,
and concentration of pollutant, etc. Therefore, to elucidate the
effect of operational parameters on the photodegradation
performance, the reaction conditions were kept constant. The
inuences of various parameters on the photocatalytic activity
of CV and MO dyes are thoroughly investigated and given in the
following results and discussion section.
3.2.1. The effect of the type of catalyst. Prior to photo-
catalytic degradation, the adsorption–desorption equilibrium
was satised under continuous stirring for 30 minutes in the
dark. Using TiO2/CPs as the photocatalyst, their photocatalytic
activities in the degradation of CV and MO dyes were assessed.
The photocatalytic activities of ACP1, ACP3, ACP4, ACP6 and
pure TiO2 are shown in Fig. 6, while those of ACP2, ACP5 and
ACP7 are shown in Fig. S5 of the ESI† section. As can be seen,
the degradation efficiencies were higher using TiO2/CPs as
photocatalysts, as compared with the photolysis alone and bare
CP (as shown in Fig. S6†). The photocatalytic activity increased
as the hydrothermal temperature increased, in the following
order: ACP1 < ACP2 < ACP3, or ACP4 < ACP5 < ACP7 < ACP6
when the concentration of Fꢀ ions (as shown in Fig. 6 and S5†)
was enhanced, which was also evidenced by the values of the
pseudo-rst-order rate constants calculated for these catalysts
Fig. 6 The effects of various catalysts on the adsorption equilibrium
and photocatalytic degradation of CV (A) and MO (B) dyes: (a) ACP1, (b)
ACP3, (c) ACP4, (d) ACP6, (e) pure TiO2, and (f) pure CP. Conditions:
initial concentration of dye ¼ 0.0245 mM, catalyst dose ¼ 0.5 g Lꢀ1, pH
¼ 6.0, room temperature.
(as shown in Table S2 of the ESI† section). The possible expla-
nation for the marked differences in their photocatalytic
behaviors is related to the particle size of the loaded-TiO2 and
their exposed crystal facets. In detail, the ACP1, ACP2, and ACP3
exhibited relatively low photocatalytic activities, which may be
due to the smaller particle size of loaded TiO2 (as shown in
Table 1, calculated from XRD patterns and SEM images).
Furthermore, the high-resolution TEM image of ACP3 (Fig. 3e)
depicted that the exposed surfaces were dominated by the
thermodynamically stable low energy {101} facet, which also
accounts for their low photocatalytic activity. ACP4–ACP6
showed a linear increase in photocatalytic activity with the
increase in the particle size of loaded TiO2 (as shown in Table 1).
Although ACP7 had a high particle size of loaded-TiO2, as
compared to ACP6, it exhibited relatively low photocatalytic
activity, mainly due to the severe aggregation of loaded TiO2, as
seen in SEM images (Fig. S3c†). ACP6 showed the best photo-
catalytic performance among all the synthesized photocatalysts
with 98.7 and 96.8% degradation of CV and MO dyes, respec-
tively. The probable reasons for the best photocatalytic perfor-
mance of ACP6 are due to the more uniform distributions,
suitable size of the loaded-TiO2 particles, and particularly the
dominant reactive {001} exposed facet of the nanocrystals,
which effectively enhanced the charge separation of the pho-
tocatalyst. Moreover, the relatively high number of oxygen
vacancies in ACP6 produced by the use of Fꢀ ions, as
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demonstrated in XPS results (Fig. 5C-b), is another important Based on the best degradation properties, the dosage of 0.75 g
factor for its enhanced photocatalytic performance. Therefore, Lꢀ1 was selected as the optimum amount and therefore all other
ACP6 was selected as the optimum photocatalyst for further investigations were performed with this dose.
exploration.
The pH of the reaction medium is also an important factor
3.2.2. The effects of the dose of the catalyst and the initial due to its strong inuences on the charge of the catalyst.39 While
pH of solution. Different doses of ACP6, i.e. 0.15, 0.25, 0.5, 0.75, keeping other conditions constant, the effect of the pH value on
1.00, and 1.25 g Lꢀ1, were used to determine the optimum the photocatalytic degradation of CV and MO dyes was inves-
amount of photocatalyst for the degradation of CV and MO tigated. As shown in Fig. 7B, the degradation efficiency of both
dyes. Before the irradiation experiments, the suspensions were dyes rst increased by raising the solution pH from 2 to 8 and
shaken for 30 min in dark conditions to reach the adsorption– thereaer decreased. In detail, the photocatalytic degradation
desorption equilibrium. As can be seen in Fig. 7A, the degra- was around 57 and 48% for CV and MO, respectively, at a low pH
dation rates of CV (Fig. 7A-a) and MO (Fig. 7A-b) dyes rstly value of around 2.0, but gradually increased to 85 and 79% for
increased to 80.0 and 71.7%, respectively, with increasing the CV and MO with increasing the initial pH (up to 8) of the
dose up to 0.75 g Lꢀ1, and then decreased to 73.5 and 66.7% for solution. As shown in Fig. 7B-b, the pHpzc value of 6.9 deter-
CV and MO dyes, using a dose of 1.25 g Lꢀ1. Their relatively low mined for ACP6 had a direct inuence on the photocatalytic
degradation rates with lower doses, i.e. 0.15, 0.25, and 0.5 g Lꢀ1
,
degradation of dyes. The ACP6 surface at pH < pHpzc is posi-
were because fewer catalyst particles interacted with UV light to tively charged, which hinders the adsorption of cationic CV and
produce the reactive radicals, i.e. cOH, O2cꢀ etc.37 The decrease MO dyes, leading to the low degradation rate.40 Furthermore, at
in degradation using larger doses, i.e. 1.00, and 1.25 g Lꢀ1
,
pH < pHpzc, there is competition between the cationic dyes and
might be due to the scattering effect of the excess particles, H+ ions for the adsorption sites of ACP6; therefore, small
which masks some parts of the photosensitive surface, making amounts of CV and MO dyes with relatively low positive charges
it difficult for UV-light to pass through the solution. Hence, the were adsorbed on the surface of the ACP6, consequently
penetration depth of photons decreases and fewer catalyst reducing the degradation efficiency.
particles can be activated. Aggregation of the catalyst particles
Both dyes presented the highest degradation efficiency at pH
at larger doses also reduces the effective surface area of the used of 8 and then a small decrease in degradation efficiency
catalyst,38 which in turn reduces the degradation efficiency. occurred at pH 10. These results are similar to those reported by
Liu et al.41 The decreased degradation efficiency at pH > 8
should be due to the appearance of repulsive forces between the
highly negatively charged surface of ACP6 and free electron
pairs of the CV and MO dyes. Therefore, relatively fewer mole-
cules of dyes can reach the ACP6 surface where the highly
reactive cOH radicals are generated. Furthermore, two types of
reactions might occur, such as (i) the reaction of cOH with cOH
due to the presence of greater amount of cOH radicals;41 (ii) the
reaction of cOH with ꢀOH, at high pH values producing rela-
tively less reactive species,42 and consequently reducing the
percentage degradation.
3.2.3. The effect of irradiation time. The effect of UV-
irradiation time on the degradation of CV and MO dyes is
shown in Fig. S7A.† The degradation rate of both dyes increased
gradually by increasing irradiation time. At the irradiation time
of 120 min, the degradation percentages of CV and MO dyes
were 98.7 and 97.0%, respectively. The UV-visible absorption
spectra of CV and MO dyes aer different intervals of photo-
catalytic degradation are illustrated in Fig. S7B and C,†
respectively. The diminishing intensity of the peaks at 582 and
465 nm for CV and MO dyes, respectively, revealed their removal
by photocatalytic degradation.
The extent of mineralization of dyes can be also determined
by its TOC removal efficiency analysis because complete
decolorization does not mean the complete mineralization of
the dye. However, on using ACP6 as a catalyst, the relatively low
TOC removal (81.1%) of the CV dye aer 180 minutes as
compared to the decolorization percentage (98.7%) indicated
that the complete mineralization of CV dye needed a longer
time. Similarly, the TOC removal percentage (78.4%) of MO dye
aer 180 minutes was also lower than that its decolorization
Fig. 7 The effects of the dose of ACP6 catalyst (A) and the initial pH of
the solution (B) on the photocatalytic degradation of CV (a) and MO
dyes (b). Conditions: initial concentration of dye ¼ 0.0245 mM, time ¼
60 min, pH in dose study ¼ 6.0, or catalyst dose in pH study ¼ 0.75 g
L
ꢀ1, and room temperature.
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percentage (96.8%). The rapid decrease in the colour removal of
the dye might be due to the cleavage of the chromophores, while
the low percentage of TOC removal might be due to the
formation of intermediates due to the presence of N atoms in
both CV and MO dyes.43
3.2.4. The effects of initial dye concentration. In order to
investigate the effects of the initial concentration of CV and MO
dyes on the photocatalytic degradation performance, four
different concentrations, i.e. 0.0122, 0.0245, 0.0368, and (d)
0.0490 mM of the dyes were used, as shown in Fig. 8. The results
showed that the initial concentrations of both the dyes had an
obvious inuence on the photocatalytic degradation rate. The
lower the initial concentration, the faster the degradation rate
and vice versa. In detail, the percentage degradation was found
to be 93.6 and 76.1% for CV and MO, respectively, at low
concentration (0.0122 mM), but reduced to 50.9 and 34.5% at
higher concentration (0.0490 mM), which may be because the
different initial concentrations of the dyes were made to react
with a xed number of cOH radicals produced from a given
amount of photocatalyst and UV-light. Furthermore, the reac-
tive solution became more opaque with increasing the initial
concentration of the dyes, and penetration of UV-light through
the solution to reach the catalyst surface became more difficult;
consequently, relatively fewer highly reactive cOH radicals were
produced.
The kinetic behavior of heterogeneous photocatalytic reac-
tions is generally described by the Langmuir Hinshelwood
equation, which states that the initial degradation rate (r) of
organic compounds is proportional to the surface coverage (q),44
as follows:
r ¼ ꢀdc/dt ¼ kq ¼ k(KC/1 + KC)
(4)
where r is the rate of reaction, k is the rate constant, K is the
Langmuir constant, C is the initial concentration of the dye
solution (mM), and c is the concentration of dye at time t (min).
For concentrated solutions where C > 5.0 mM, then KC [ 1,
and the reaction becomes zero order. For dilute solutions where
C < 1.0 mM, KC ꢃ 1, and the reaction becomes pseudo-rst-
order. Hence, the diluted solutions of CV and MO were used
for photocatalytic degradation experiments. Their kinetics were
found to t the pseudo-rst-order kinetic rate expression (5):
ln C0/C ¼ kapp$t
(5)
where C0 and C are the concentrations (ppm) of the dyes at
times 0 and t (min), respectively, kapp is the apparent rate
constant and t is the time interval. A linear correlation was
observed between ln(C0/C) vs. irradiation time using different
initial concentrations of dyes, as shown in Fig. 8C and D. A
decrease in kapp was observed with the increasing C0 values of
both dyes (seen in Table S3†), suggesting a low degradation rate
for a high initial concentration of the dyes.
3.2.5. Degradation pathways of CV and MO dyes. The role
of active species in the photocatalytic degradation of CV and
MO dyes was determined by adding 0.0926 mM of benzoqui-
none and 0.1667 mM of isopropyl alcohol as scavengers for O2cꢀ
Fig. 8 The effects of initial concentration on the photocatalytic
degradation of the CV dye (A) and its corresponding kinetic behavior
(C), as well as MO dye (B) and its corresponding kinetic behavior (D). (a)
0.0122, (b) 0.0245, (c) 0.0368, and (d) 0.0490 mM. Conditions: catalyst
dose ¼ 1.0 g Lꢀ1, pH ¼ 10, room temperature.
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and cOH radicals, respectively. Fig. S8† showed the variation in with deionized water, acetone and ethanol, respectively, and
the degradation of CV and MO dyes as a function of irradiation reused in the next cycle. As shown in Table 2, the decreases in
time before and aer scavengers were added to the photo- photocatalytic degradation rates of the CV and MO were
catalytic system. As can be seen in Fig. S8,† using ACP6 as the observed with the repeated use of the catalyst, which were found
catalyst, the degradation efficiency of CV was reduced from 96 to decrease from 96% for CV and 93% for MO in the rst cycle to
to 27% in the presence of benzoquinine, and reduced to 71% in 78% and 76% in the h cycle, respectively. One of the main
the presence of isopropyl alcohol. Similarly, the degradation reasons for activity reduction may be the adsorption of reaction
efficiency of MO dye decreased from 79 to 25% in the presence products on the surface of the catalyst, leading to the blockage
of benzoquinone and to 56% in the presence of isopropyl of active sites; besides that, the loss of some active sites during
alcohol. Thus, it can be concluded that the photocatalytic the recycling caused a decrease in the degradation rate of the
degradation efficiency mainly depends upon the cOH radicals. dye. Kanakaraju et al.10 also observed a decrease in the photo-
The photocatalytic degradation process is initiated by the catalytic activity for the degradation of amoxicillin due to the
generation of cHO and O2cꢀ radicals. When the reaction mixture blockage of pores on the surface of the catalyst by the pollutant
is illuminated by UV-visible irradiation, the electrons in the and its degradation byproducts. In order to generate the activity
conduction band become excited and shi to the valance band. of the catalyst, they calcined the recovered catalyst. Ullah et al.14
Thus, created holes in the valance band react with water also observed a reduction in the activity of the TiO2/clinoptilo-
molecules and form cHO radicals, while the electrons from the lite catalyst aer repetitive usage. Because the calcination
conduction band react with absorbed oxygen and generate O2cꢀ procedure has a great inuence on the properties of the loaded-
radicals.45 These generated reactive species (cHO and O2cꢀ) are TiO2 particles, all fresh catalysts in the present study were not
responsible for the degradation of CV dye. According to the calcined during the synthesis process.
literature,46 the degradation of CV dye mainly follows two
processes, as shown in Fig. S9a,† namely (i) the demethylation
pathway, and (ii) chromophore cleavage. The CV molecule 4. Conclusions
undergoes demethylation and chromophores cleavage and
forms DLPM (bis(4-(dimethylamino)phenyl)methanone), the
main intermediate product of CV dye degradation, due to the
destruction of the p–p conjugate structure of CV.46 The dihy-
The effects of the hydrothermal treatment temperature and
concentration of Fꢀ ions on the structure, morphology, particle
size, exposed crystal facets and photocatalytic activity of TiO2-
supported clinoptilolite (TiO2/CP) were assessed. The various
droxylation of DLPM generates aromatic compounds, such as
characterizations of obtained TiO2/CPs demonstrated that
benzoic acid, 3,6 dimethyl benzoic acid, and hydroxybenzoic
increasing temperature and concentration of Fꢀ ions caused
acid, which undergo a ring-opening reaction and generate
a net decrease in their surface areas and an increase in the
aliphatic compounds such as ethylene glycol, carbonic acid,
particle size of the loaded TiO2 along with their enhanced
and oxalic acid. The nal degradation products (CO2 and H2O)
crystallinity degree. The presence of Fꢀ ions plays an important
were formed via the mineralization of aliphatic compounds.
role in hydrothermal etching and exposing highly energetic
According to the literature,47,48 the proposed degradation
{001} facets of the supported TiO2. The photocatalytic activity of
TiO2/CPs was evaluated for the synergistic removal of CV and
MO dyes under UV-irradiation in aqueous media. The high
degree of crystallinity, exposed highly energetic {001} facets,
pathway of MO dye is shown in Fig. S9b.† MO dye degradation is
initiated with the attack of reactive species on the chromophore
(–N]N–), and the cleavage of aromatic rings occurs. The
aromatic rings undergo ring-opening reaction and form
uniform and ne dispersion of TiO2 in TiO2/CP (ACP6) caused
aliphatic ꢀsmall compounds that further mineralize into CO2,
ꢀ
by the use of Fꢀ ions reected the higher surface density of the
active sites and the separation efficiency of electron–hole pairs,
which account for its best photocatalytic performance. Mutually
independent exposed {001} and {101} facets of TiO2 loaded onto
the CP supports provide new insight into the design and
fabrication of advanced photocatalytic materials.
H2O, SO3 and NO3
.
3.2.6. Catalyst recycling. Photocatalyst recycling for reuse is
an important factor that can evaluate its stability and economy
for practical applications. The recycling studies in this work
were carried out using ACP6 as a selective photocatalyst for the
degradation of CV and MO over ve successive cycles. At the end
of each cycle, the catalyst was centrifuged, washed thoroughly
Author contributions
Table 2 ACP6 reusability over five successive cycles in the degrada-
tion of CV and MO. Conditions: dose of catalyst ¼ 0.75 g Lꢀ1, pH ¼ 8,
room temperature, time ¼ 90 min, initial dye concentration ¼ 10 ppm
Anadil Gul and Raza Ullah: equal contribution, investigation,
writing original dra preparation; Tallat Munir: data curation;
Jihong Sun: supervision, conceptualization, methodology;
Shiyang Bai: formal analysis, validation.
% Degradation
Dye
1st cycle
2nd cycle
3rd cycle
4th cycle
5th cycle
Conflicts of interest
CV
MO
96
93
92
90
85
84
81
79
78
76
There are no conicts to declare.
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21878006).
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© 2021 The Author(s). Published by the Royal Society of Chemistry
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