Synthesis of donor-acceptor-type conjugated polymer dots as organic photocatalysts for dye degradation and hydrogen evolution

Article abstract of DOI:10.1016/j.polymer.2021.124004

Conjugated polymers (CPs) having various donor-acceptor units in the main chain were synthesized via Suzuki coupling reaction for an organic photocatalyst that can generate reactive oxygen species (ROS) for dye degradation and H₂ evolution unde

Full text of DOI:10.1016/j.polymer.2021.124004

Polymer 229 (2021) 124004
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of donor-acceptor-type conjugated polymer dots as organic
photocatalysts for dye degradation and hydrogen evolution
,
,
,*
Yougjin Gwona 1, Seonyoung Joa 1, Hyun-Jun Leeb, Soo Young Parkb, Taek Seung Leea
a Organic and Optoelectronic Materials Laboratory, Department of Organic Materials Engineering, Chungnam National University, Daejeon, 34134, South Korea
b Department of Material Science and Engineering, Seoul National University, Seoul, 08826, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords:
Conjugated polymers (CPs) having various donor-acceptor units in the main chain were synthesized via Suzuki
coupling reaction for an organic photocatalyst that can generate reactive oxygen species (ROS) for dye degra-
dation and H2 evolution under visible-light irradiation. To synthesize four kinds of CPs, two donor units (phe-
nylene and fluorene) and two acceptor groups (benzothiadiazole and bisthienylbenzothiadiazole) were cross-
combined. The cyclic voltammograms of the CPs showed their generation ability for ROS, in which the ROS
generation efficiency would depend on the main chain structure. Because the CPs are inherently hydrophobic,
they were fabricated in nanoparticle (NP) form for convenient use in an aqueous solution. The CP NPs showed
excellent dye degradability under visible light. In addition, CPs were incorporated in a filter paper to provide
repeated uses after simple washing. The ROS generation from CPs played an important role in dye degradation,
as well as in H2 evolution of up to 8.4 mmol/g under visible-light irradiation.
Conjugated polymers
Photocatalysts
Visible light
Dye degradation
Hydrogen evolution
1. Introduction
functionalization, and tunable structure and composition [21,22]. By
changing the repeat units of the backbone, the CPs have tunable optical
Photocatalysts are one of the important materials that produce useful
chemicals using light energy. Among the numerous photocatalysts,
various inorganic materials such as TiO2, ZnO, CdS, Fe2O3, and MoO3,
and organic g-C3N4 are generally used for photocatalysis [1–6].
Electron-hole pairs are generated on the surface of inorganic photo-
catalysts by light, and react with water molecules to produce reactive
oxygen species (ROS) [7]. The ROS generated by photocatalysts are used
for dye degradation, organic synthesis, water splitting, and photody-
namic therapy [8–13]. However, inorganic photocatalysts have draw-
backs such as difficulty in tuning their optical properties, resulting in
being activated only by UV light, which limits their versatile use.
Therefore, investigations have been conducted to introduce nano-
materials into inorganic photocatalysts for ROS generation in the visible
region [14–16]. These techniques, however, need additional synthetic
steps and might have a problem at the interface between two materials.
To overcome the disadvantage of inorganic photocatalysts, in-
vestigations on the ROS generation using organic conjugated polymers
(CPs) have been recently attempted [17–22]. CPs have been used as new
types of optoelectronic materials with high light stability, easy surface
absorption, so that absorbance can be obtained in the
visible-wavelength region. Because the energy band gap of the CPs can
be controlled, the effect of the repeat unit structure on the catalytic
activity needs to be understood. Because of the CPs’ extended π-conju-
gation with aromatic groups, CP-based photocatalysts are hydrophobic
in nature, resulting in a difficulty in obtaining uniform dispersion in an
aqueous solution [23,24]. To enhance photocatalytic activity and avoid
inherent hydrophobicity, CPs should be hybridized with inorganic
semiconductors. Such hybridization, however, requires proper choice of
materials with both well-defined energy levels and improved interfacial
interaction for easy carrier transfer [25]. To enhance H2 production CPs
were hybridized with C60, and thus their role in photocatalysis was
limited to light harvesting [26]. CPs have been prepared in nanoparticle
(NP) form with a size less than 100 nm and, finally, they can be finely
dispersed in an aqueous phase, which have been called “polymer dots”
(P-dots) [27,28]. P-dots inherit their optical properties from the pristine
CPs, and show rapid electron transfer and excellent optical stability
[29–33]. In addition, P-dots have been used to generate ROS by light
irradiation, resulting in cancer cell apoptosis via photodynamic therapy
* Corresponding author.
E-mail address: tslee@cnu.ac.kr (T.S. Lee).
1
These authors contributed equally.
https://doi.org/10.1016/j.polymer.2021.124004
Received 14 May 2021; Received in revised form 24 June 2021; Accepted 6 July 2021
Available online 9 July 2021
0032-3861/© 2021 Elsevier Ltd. All rights reserved.

Y. Gwon et al.
Polymer 229 (2021) 124004
[34,35]. Like CPs, P-dots have an absorption wavelength in the visible
region, and their ROS generation under visible-light irradiation is far
less restricted than inorganic photocatalysts. P-dots as well as CPs have
been used as photocatalysts for H2 generation [36–38]. P-dots, however,
were located on the g-C3N4 surface to assist the H2 production of the
g-C3N4. In addition, hollow P-dots have been reported to be more effi-
cient in photocatalytic reaction than their pristine polymer [39].
In this work, we wanted to elucidate CP-based photocatalysis. ROS
generation was investigated by synthesizing four CPs with various
backbone structures, followed by fabricating CPs into P-dots. The two
main structural advantages of the P-dot form are its high surface area
and its good water-dispersibility, resulting from the nanosized spherical
shape. This enables the reactants (here, water molecules) and products
(here, ROS or H2) to easily access and form the catalytic sites on the P-
dots. In addition, by careful combination of the donor and acceptor
units, it was possible to finely tune the P-dots’ optical properties. Most
nanosized photocatalysts are only reused with difficulty after the pho-
tocatalytic reaction because of their small size and particulate shape. To
facilitate reuse after photocatalysis, we used a paper-based photo-
catalyst containing CPs [40,41].
2 M K2CO3 was added slowly, and then Pd(PPh3)4 (0.05 g, 0.04 mmol)
was added and reacted at 80 ◦C for 72 h. After the reaction, the mixture
was poured into methanol to precipitate and the precipitate was isolated
by filtration and purified in a Soxhlet apparatus with methanol and
acetone (yield 360 mg, 77%). 1H NMR (300 MHz, CDCl3) δ = 7.93 (m),
7.46 (m), 4.03 (m), 1.58 (m), 1.25 (m), 0.86 ppm (m). 13C NMR (CDCl3)
δ = 154.39, 150.58, 130.58, 130.13, 127.57, 117.12, 69.52, 31.8, 29.7,
29.37, 29.28,29.25, 29.23, 26, 22.65, 14.11 ppm. Anal. Calcd. for
C28H40N2O2S: C, 71.75%; H, 8.60%; N, 5.98%; O, 6.83%; S, 6.84%;
Found: C, 69.1%; H, 8.50%; N, 4.97%; S, 5.96%.
Poly(2,5-dioctyloxyphenylene-alt-bisthienylbenzithiadiazole)
(P8BTB) Compounds 3 (0.422 g, 1.0 mmol) and 6 (0.458 g, 1.0 mmol)
were dissolved in THF and toluene. The subsequent procedure was the
same as that used for P8BT synthesis (yield 429 mg, 68%). 1H NMR (300
MHz, CDCl3) δ = 8.17 (m), 7.9 (m), 7.6 (m), 7.36 (m), 4.19 (m), 1.61
(m), 1.25 (m), 0.89 ppm (m). 13C NMR (CDCl3) δ = 29.7, 29.29, 22.67,
14.11 ppm. Anal. Calcd. for C36H44N2O2S3: C, 68.31%; H, 7.01%; N,
4.43%; S, 15.20%; Found: C, 69.1%; H, 8.80%; N, 4.14%; S, 13.4%.
Poly(9,9-dihexylfluorene-alt-benzithiadiazole)
(F6BT)
9,9-
Dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester (0.502
g, 1.0 mmol) and compound 4 (0.291 g, 1.0 mmol) were dissolved in
THF and toluene. The subsequent procedure was the same as that used
for P8BT synthesis (yield 0.30 g, 64%). 1H NMR (300 MHz, CDCl3) δ =
8.11-7.96 (m), 2.16 (m), 2.0 (m), 1.54 (m), 1.25 (m), 1.7 (m), 0.80 ppm
(m). 13C NMR (CDCl3) δ = 31.55, 29.7, 22.64, 14.08 ppm. Anal. Calcd.
for C31H34N2S: C, 79.7%; H, 7.34%; N, 6.0%; S, 6.87%; Found: C, 78.1%;
H, 7.90%; N, 5.60%; S, 6.18%.
2. Experimental
2.1. Materials
2,1,3-Benzothiadiazole, 1-bromooctane, 2-(tributylstannyl)thio-
phene, tetrahydrofuran (THF), toluene, hydroquinone, N-bromosucci-
nimide (NBS), N,N-dimethyl-4-nitrosoaniline (RNO), Br2, KOH, K2CO3,
9,9-dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester,
methylene blue (MB), and tetrakis(triphenylphosphine) palladium were
purchased from Sigma-Aldrich Chemicals. All the reagents and solvents
were used without further purification. The compounds 1,4-bis(octy-
loxy) benzene (1), 1,4-dibromo-2,5-bis(octyloxy) benzene (2), 2,5-bis
(octyloxy)phenyldiboronic acid (3), 4,7-dibromo-2,1,3-benzothiadazole
(4), 4,7-di-2-thienyl-2,1,3-benzothiadiazole (5), and 4,7-bis(5-bromo-2-
thienyl)-2,1,3-benzothiadiazole (6) were synthesized according to pre-
viously reported methods that are briefly described in the Supporting
Information [42–44].
Poly(9,9-dihexylfluorene-alt-bisthienylbenzithiadiazole)
(F6BTB) 9,9-Dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol)
ester (0.502 g, 1.0 mmol) and compound 6 (0.458 g, 1.0 mmol) were
dissolved in THF and toluene. The subsequent procedure was the same
as that used for P8BT synthesis (yield 0.49 g, 78%). 1H NMR (300 MHz,
CDCl3) δ = 8.16 (m), 8.14 (m), 7.68 (m), 2.03 (m), 1.56 (m), 1.25 (m),
1.1 (m), 0.80 ppm (m). 13C NMR (CDCl3) δ = 31.81, 29.71, 29.37, 29.25,
22.7, 22.61, 14.08 ppm. Anal. Calcd. for C39H39N2S3: C, 74.1%; H,
6.22%; N, 4.43%; S, 15.2%; Found: C, 72.1%; H, 5.90%; N,4.14%; S,
14.9%.
2.4. Removal of Pd catalyst
2.2. Characterization
CP (150 mg) was dissolved in chloroform (50 mL), followed by
addition of thiourea (600 mg) and the mixture was stirred for 24 h. The
thiourea was removed by filtration and the filtrate was concentrated
under reduced pressure; the polymer was then precipitated in methanol
and dried in a vacuum oven.
1H and 13C nuclear magnetic resonance (NMR) spectra were recor-
ded on a Fourier-300 spectrometer at Korea Basic Science Institute
(Bruker, MA, USA). Ultraviolet and visible (UV–vis) absorption spectra
were recorded on a Lambda 35 spectrometer (PerkinElmer, CT, USA).
Fluorescence spectra were obtained using a Cary Eclipse fluorescence
spectrophotometer (Varian, CA, USA) equipped with a Xe flash-lamp
excitation source. Elemental analysis (EA) was performed on an
Elemental Analyzer EA 1108 (Fisons Instruments, UK). The molecular
weights (MWs) of polymers were determined by gel-permeation chro-
matography (GPC), with THF as an eluent with a polystyrene standard
(Youngin Scientific, Korea). The nanoparticle images were obtained
using a field-emission scanning electron microscopy (FE-SEM, Hitachi S-
4800, Japan), and the size was determined using dynamic light scat-
tering (DLS, Malvern Zetasizer, UK). A solar simulator (PEC-L01, Peccell
Technologies, Japan) was used for visible light. The residual Pd catalyst
in CPs was determined with an inductively coupled plasma mass-
spectrometer (ICP-MS, ELAN DRC II, PerkinElmer). The contact angles
of CPs and their nanoparticles was obtained with a droplet shape
analyzer (DSA 100, KRÜSS, Germany) by coating them on glass slides.
2.5. Preparation of P-dots
CP was dissolved in THF and injected into distilled water under
sonication. After 30 min sonication, N2 gas was blown to remove THF for
30 min. Then, a P-dot solution was prepared after filtration with a 0.45
μ
m syringe filter. Four kinds of P-dots (denoted as P8BT-, P8BTB-, F6BT-,
and F6BTB-dots) were fabricated from four CPs (P8BT, P8BTB, F6BT,
and F6BTB), respectively.
2.6. ROS generation of P-dots
To evaluate the ROS generation of the P-dot solution, an RNO dye
was used as a ROS indicator. The P-dot solution was mixed with RNO
and then irradiated with white light for varying time periods. ROS
generation was determined by investigating the changes in UV–vis ab-
sorption at 430 nm, resulting from the reduced RNO by ROS.
2.3. Polymerization
Poly(2,5-dioctyloxyphenylene-alt-benzithiadiazole)
(P8BT)
2.7. Electrochemical cyclic voltammetry (CV) [45]
Compounds 3 (0.422 g, 1.0 mmol) and 4 (0.291 g, 1.0 mmol) were
dissolved in mixed solvents of THF and toluene. An aqueous solution of
The CV of CPs was investigated on a three-electrode electrochemical
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Y. Gwon et al.
Polymer 229 (2021) 124004
Fig. 1. Synthesis of CPs.
2.8. Photocatalysis by paper-based photocatalyst
system. A 0.1 M solution of Bu4NPF6 in anhydrous acetonitrile was used
as a supporting electrolyte. A Pt wire was used as the counter electrode,
and an Ag/Ag+ electrode (0.01 M AgNO3 and Bu4NBF4 in acetonitrile)
was used as the reference electrode, calibrated with the ferrocene/fer-
rocenium couple (Fc/Fc+). The working electrodes were prepared by
dropping the polymer solution onto an indium tin oxide (ITO) and then
The paper-based photocatalyst was fabricated by introducing P8BT,
P8BTB, F6BT, and F6BTB (0.6 mg) to a filter paper (Whatman No. 2); the
samples were denote as P8BT-, P8BTB-, F6BT-, and F6BTB-paper,
respectively. After this procedure, the filter paper was immersed in an
aqueous RNO solution for various periods of times (0; 30; 60; 90; 120
min). ROS generation was evaluated by the same method used for the P-
dot cases. For reusability of the CP-containing photocatalytic paper,
regeneration of F6BT-paper was accomplished by washing with water
after each photocatalytic cycle.
dried in air. The half-wave potential of the Fc/Fc+ redox couple (E1/2
,
Fc,
Fc+) was estimated from E1/2, Fc, Fc+ = (Eap + Ecp)/2, where Eap and Ecp
are the anodic and cathodic peak potentials, respectively. The redox
potential of Fc/F+c was calibrated under the same conditions, and it was
located at +0.42 V from the Ag/AgCl electrode. For the conversion from
the Ag/Ag+ redox couple to the normal hydrogen electrode (NHE), the
formula ENHE = EAg/Ag+ + 0.26 V was applied.
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Y. Gwon et al.
Polymer 229 (2021) 124004
Fig. 2. UV–vis absorption (black) and fluorescence spectra (blue) of (a) P8BT-dot, (b) P8BTB-dot, (c) F6BT-dot, and (d) F6BTB-dot in aqueous solution. Fluorescence
spectra were obtained by excitation of corresponding UV–vis absorptions. Photographs of P-dot solutions were taken under ambient (left) and 365 nm UV lights
(right). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 3. SEM images of P-dots (scale bars: 500 nm).
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Y. Gwon et al.
Polymer 229 (2021) 124004
Table 1
Optical and electrochemical properties of polymers.
Polymer
Eg,opt
Ered, onset
LUMO
HOMO
[eV]d)
Eg
[eV]a)
[V]b)
[eV]c)
[eV]e)
P8BT
2.39
1.72
2.37
1.95
ꢀ 1.04
ꢀ 1.03
ꢀ 1.20
ꢀ 1.09
ꢀ 3.40
ꢀ 3.41
ꢀ 3.24
ꢀ 3.35
ꢀ 5.79
ꢀ 5.13
ꢀ 5.61
ꢀ 5.30
2.39
1.72
2.37
1.95
P8BTB
F6BT
F6BTB
During the light irradiation of the P-dots, electron-hole pairs were
generated, in which the electrons in the highest occupied molecular
orbital (HOMO) were excited to the lowest unoccupied molecular orbital
(LUMO). The generated electron-hole pairs would react with water
molecules to produce ROS (Fig. 4). The LUMO levels of the polymers
were associated with the photocatalytic activity of P-dots, and to esti-
mate them the reduction potentials were investigated by CV (Fig. S6).
All polymers exhibited well-defined reversible reduction waves. F6BT
showed the lowest reduction potential of ꢀ 1.20 V, and the reduction
potentials of P8BT, P8BTB, and F6BTB were determined to be ꢀ 1.04,
ꢀ 1.03, and ꢀ 1.09 V, respectively. The HOMO and LUMO levels were
determined from the reduction potential and onset absorption wave-
length of each polymer (Table 1). From the viewpoint of narrow band
gap related to visible light absorption, P8BTB (1.72 eV) and F6BTB
(1.95 eV) should be more advantageous than P8BT (2.39 eV) and F6BT
(2.37 eV), in which the BTB units in the backbone mainly affected such a
narrow band gap. In principle, the narrow band gap contributes the
long-wavelength absorption within the visible range, expecting visible-
light-induced generation of electron-hole pairs. By contrast, to achieve
an efficient ROS generation, a wide band gap, in which the HOMO-
LUMO energy level covers the reduction potential for O2⋅- and oxida-
tion potential for ⋅OH, is preferred. The energy levels of the polymers
were determined; in which the reduction potential was higher than
ꢀ 3.4 eV for all polymers and the oxidation potential was lower than
ꢀ 5.6 eV for P8BT and F6BT (Fig. S7). Thus, in terms of the position and
width of the energy level, a more efficient ROS generation was expected
from the P8BT- and F6BT-dots.
Fig. 4. Proposed electron transfer mechanism of P-dot for photocatalytic
ROS generation.
2.9. Photocatalytic H2 evolution [45]
P-dot solution (10 ml) containing 0.2 M ascorbic acid was used as a
sacrificial agent. The mixture solution was placed into a vial, bubbled
with Ar gas, and ultrasonication was performed for 5 min at ambient
temperature. The visible light was generated by a Xe lamp (LSP-X500A,
Zolix, China). The incident light wavelength was controlled using an
appropriate long-pass cut-off filter (>420 nm) to emit visible white light.
The reactant solution was maintained at room temperature by a flow of
cooling water during the reaction. The evolved H2 was analyzed using an
online gas chromatograph (GC 7890A, Agilent Instrument Ltd., CA,
USA), referencing against a standard gas with a known volume of H2.
3. Results and discussion
To synthesize donor-acceptor-type CPs, phenylene and fluorene units
were used as electron donors and benzothiadiazole (BT) and bisthie-
nylbenzothiadiazole (BTB) groups were used as electron-accepting units
(Fig. 1). Thus, four kinds of CPs were obtained by Suzuki coupling re-
action of corresponding monomers. The chemical structures of the
polymers were confirmed with 1H NMR, 13C NMR (Figs. S1 and S2) and
EA. Each polymer had a number-average MW between 8100 and 14,100
(Fig. S3, Table S1). P-dots were obtained from the CPs by the conven-
tional nanoprecipitation technique (Scheme S1) [46]. P-dots fabricated
from BT-containing polymers, including P8BT-dot and F6BT-dot showed
yellow fluorescence with emission at 575 and 545 nm, respectively,
whereas P8BTB- and F6BTB-dot, which contained more extended
conjugation of BTB presented a more red-shifted fluorescence at 720 and
660 nm, respectively. In conjunction with the red-shifted fluorescence of
P8BTB-dot (720 nm), composed of phenylene and BTB units, it showed a
highly red-shifted blue color under ambient light with absorption at 590
nm (Fig. 2 and Table S2). The yellow-colored P-dots (P8BT- and
F6BT-dots) and the purple-colored P-dots (F6BTB-dots) exhibited
different absorption wavelengths, depending on the acceptor units (BT
and BTB) used. The donor moieties (phenylene and fluorene) did not
significantly affect the colors of the P-dots (inset photographs in Fig. 2).
The shapes and sizes of the P-dots were determined by SEM and DLS,
respectively (Fig. 3 and S4), indicating that each P-dot was fabricated
with a spherical shape, 50–68 nm in size.
a) Optical band gap determined by the absorption onset (Eg,opt
=
1240/λonset). b) vs. Ag/Ag+ in acetonitrile solution with 0.1 M n-
Bu4NPF6 as supporting electrolyte at a scan speed of 50 mV sꢀ 1, E1/2,Fc,
Fc+ = 0.36 V. c) The LUMO levels of polymers were calculated according
to the formula: LUMO = ꢀ (Ered, onset ꢀ E1/2,Fc,Fc+ + 4.8) = ꢀ (Ered, onset
+
4.44) eV, where Ered, onset was determined from the onset potentials of
the reduction waves. d) Estimated from EHOMO = ELUMO ꢀ Eg,opt. e)
Nanosized P-dots show superior wettability and dispersibility in
water than hydrophobic CPs. Therefore, P-dots have an advantage in the
catalytic reaction using water as a medium over film shape. To compare
CP’s water wettability, the contact angles of film and P-dots were
investigated (Fig. S5). The contact angle of the F6BT film coated on a
glass was determined to be 77.9◦, whereas the contact angle of the F6BT-
dot was found to be 13.2◦. The result indicates that P-dots were more
hydrophilic and had better water wettability.
Fig. 5. Changes in the absorption of RNO dye at 430 nm in each P-dot solution
upon white-light irradiation. Ao and A correspond to UV–vis absorption of RNO
dye at 430 nm before and after white-light irradiation, respectively.
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Y. Gwon et al.
Polymer 229 (2021) 124004
Table 2
MB degradation in the presence of P-dot after white-light irradiation.
P-dots
MB Degradation (%)a)
10 min
240 min
P8BT-dot
P8BTB-dot
F6BT-dot
F6BTB-dot
22
13
43
27
84
33
90
81
a) MB degradation was determined by following equation.
Energy band gap = ELUMO – EHOMO
The RNO dye has an absorption wavelength at 430 nm, and in the
presence of ROS its absorption decreases. Therefore, the RNO dye is
generally used to investigate ROS generation [47]. The RNO dye was
photostable enough to maintain its chemical structure without degra-
dation during prolonged white-light irradiation up to 240 min (Fig. S8).
The changes in the RNO dye absorption in P-dot solutions were inves-
tigated to estimate the relative amounts of ROS produced, according to
the chemical structure of the P-dots (Fig. S9). After the addition of RNO
dye to each P-dot solution, it was shown that the absorption at 430 nm
was reduced by white-light irradiation. Because we already found that
the RNO absorption was not varied by the light, decrease at 430 nm
clearly indicated RNO degradation by ROS. Also, considering that the
UV–vis spectra of P-dots were not changed by light irradiation (at other
ranges except around 430 nm), the decrease in the spectra around 430
nm clearly resulted from the RNO degradation. The ROS generation was
quantified in terms of RNO decomposition of each P-dot; the RNO
decomposition was determined to be 46.6%, 43.3%, 68.4%, and 68.2%
for P8BT-, P8BTB-, F6BT-, and F6BTB-dots, respectively, after irradia-
tion for 240 min (Fig. 5). The results show that the acceptor unit (BT or
BTB) did not affect ROS generation, while the ROS generation was
highly dependent on the donor group (phenylene or fluorene). In
particular, the F6BT- and F6BTB-dots, which contained fluorene as a
Fig. 7. Light-driven hydrogen evolution of P-dots at ambient temperature.
[ascorbic acid] = 0.2 M, pH 4.0 (adjusted with 1 M NaOH).
more MB was decomposed, but further decrease in the MB absorbance
was limited because of the spectral overlap. The MB degradation by
photocatalytic activity of P-dots was clearly observed by the naked eye
(Fig. S11). The color of each P-dot became a green or dark violet color in
the presence of MB, but the original colors of the P-dot solutions were
recovered as the irradiation time increased (except for the P8BTB-dot),
indicating the MB was degraded and lost its original color.
The nanosized, spherical shape of the P-dots resulted in difficulty of
separation from the solution for further reuse. To achieve a reusable
photocatalyst, the use of a substrate that accommodates CPs was
necessary. Therefore, a paper-based photocatalyst was fabricated by
introducing a CP to a filter paper. P8BT, P8BTB, F6BT, or F6BTB was
incorporated in a conventional filter paper by immersing the paper in
the polymer solution. Then the filter paper was exposed to an aqueous
solution of RNO dye. The light source was the same as that used for the
photodegradation of RNO and MB in the P-dot solution. The RNO
decomposition efficiency of the paper-based photocatalysts was similar
to that of the P-dot solution, in which the F6BT-paper showed better
photocatalytic activity over other papers (Fig. 6a). It was concluded that
the CPs maintained photocatalytic activity, even when they were
located in the substrate. The F6BT-paper-based photocataly showed an
advantage in reusability over the particle-based one in terms of simple
regeneration of the paper by washing and, subsequently, the photo-
catalytic efficiency was maintained even after being used four times.
(Fig. 6b).
donor showed
a
more effective ROS generation than
phenylene-containing P8BT- and P8BTB-dots. Therefore, ROS genera-
tion was strongly affected by the donor unit in the backbone, regardless
of the UV–vis absorption spectra of the polymers.
For practical photocatalysis, the photocatalytic activity of P-dots was
investigated via MB dye degradation. Similar to the RNO dye, MB could
be degraded by ROS generated from P-dots upon light irradiation. The
MB degradation was evaluated by determining changes in the UV–vis
absorption of MB (Fig. S10). For RNO dye cases, a more efficient ROS
generation was observed for the F6BT- and F6BTB-dots that contained
fluorene units. A significant photocatalytic degradation of MB was found
for the F6BT-dot, in which 90% MB degradation was observed (Table 2).
The low MB degradation by the P8BTB-dot (33% after 240 min irradi-
ation) was presumed to result from the overlap of the UV–vis absorption
spectra of the P8BTB-dot and MB at around 665 nm. We believe that
MB degradation (%)= (Ao-A)/Ao × 100
Fig. 6. (a) Changes in the UV–vis absorption of RNO dye at 430 nm by paper-based photocatalysts containing CPs. Ao and A correspond to UV–vis absorption of RNO
dye at 430 nm before and after white-light irradiation, respectively. (b) Repeated uses of F6BT-paper over 4 cycles.
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Y. Gwon et al.
Polymer 229 (2021) 124004
where Ao and A correspond to UV–vis absorbance at 665 nm before and
after white-light irradiation, respectively.
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Declaration of competing interest
The authors declare that they have no known competing financial
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