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PFAS Photodegradation Very Important Paper
Multiphoton-Driven Photocatalytic Defluorination of Persistent
Perfluoroalkyl Substances and Polymers by Visible Light
Yuzo Arima, Yoshinori Okayasu, Daisuke Yoshioka, Yuki Nagai, and Yoichi Kobayashi*
Abstract: Perfluoroalkyl substances (PFASs) and fluori-
nated polymers (FPs) have been extensively utilized in
various industries, whereas their extremely high stability
poses environmental persistence and difficulty in waste
treatment. Current decomposition approaches of PFASs
and FPs typically require harsh conditions such as
heating over 400 °C. Thus, there is a pressing need to
develop a new technique capable of decomposing them
under mild conditions. Here, we demonstrated that
perfluorooctanesulfonate (PFOS), known as a “persis-
tent chemical,” and Nafion, a widely utilized sulfonated
FP for ion-exchange membranes, can be efficiently
decomposed into fluorine ions under ambient conditions
via the irradiation of visible LED light onto semi-
conductor nanocrystals (NCs). PFOS was completely
defluorinated within 8-h irradiation of 405-nm LED
light, and the turnover number of the CF bond
dissociation per NC was 17200. Furthermore, 81 %
defluorination of Nafion was achieved for 24-h light
irradiation, demonstrating the efficient photocatalytic
properties under visible light. We revealed that this
decomposition is driven by cooperative mechanisms
involving light-induced ligand displacements and Auger-
induced electron injections via hydrated electrons and
higher excited states. This study not only demonstrates
the feasibility of efficiently breaking down various
PFASs and FPs under mild conditions but also paves the
way for advancing toward a sustainable fluorine-recy-
cling society.
Introduction
Perfluoroalkyl substances (PFASs) and fluorinated polymers
(FPs) exhibit excellent heat resistance, chemical resistance,
insulating properties, and interfacial characteristics, render-
ing them indispensable in various industrial fields. However,
the extremely strong carbon–fluorine (CF) bond in these
compounds leads to various environment-related challenges.
For example, PFASs typically exhibit notable environmental
persistence, and certain PFASs such as perfluorooctane
sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are
highly bioaccumulative. Moreover, waste treatment of
fluorinated compounds is challenging because the hydrogen
fluoride generated by the combustion of fluorinated com-
pounds deteriorates incinerators.[1,2]
Several techniques have been reported to decompose
PFAS into recyclable fluoride ions, including oxidation using
strong oxidants such as peroxydisulfuric acid and ultraviolet-
C (UVC) irradiation (wavelength below 260 nm) using a
mercury lamp under high temperature and pressure.[3–13]
Fluorine ions can be converted to calcium fluoride, which is
a natural source of fluorine, and therefore, fluorine-recycling
can be achieved. However, the use of mercury lamps as a
light source is becoming less feasible owing to regulatory
constraints imposed by the Minamata Convention on
Mercury. A recent study demonstrates that PFOA can be
mineralized under strong basic conditions at 100 °C.[13]
However, such mild conditions are not applicable to the
decomposition of more stable PFASs, such as PFOS and
FPs. Despite the widespread use of FPs in diverse industrial
applications, their recycling rate remains low, and many of
them are subjected to landfill burial.[14] Therefore, it is
necessary to develop a method to decompose extremely
stable PFAS under mild conditions to address the multi-
faceted social issues linked with PFAS and to contribute to
the realization of a sustainable fluorine-recycling society.
In recent years, nonlinear optical processes that could be
induced by weak incoherent light have been extensively
investigated,[15–22] and these have been applied to various
organic photocatalytic reactions.[20,23–27] Specifically, long-
lived transient states of organic molecules and metal
complexes generated by LED light are further photoexcited
to proceed with chemical reactions requiring higher photon
energy than incident light energy. In addition to organic
compounds, advanced photocatalytic reactions have also
been reported using inorganic nanostructures. In particular,
semiconductor nanocrystals (NCs) have a high absorption
coefficient, and multiple excitons can be generated relatively
easily within a NC. When multiple excitons interact, a
process by which one exciton uses its recombination energy
to transition to a higher excited state, called Auger
recombination, occurs, and a highly excited electron or hole
is produced.[28,29] The highly excited electronic states are
generally thermally deactivated quickly. On the other hand,
[*] Y. Arima, Dr. Y. Okayasu, D. Yoshioka, Dr. Y. Nagai,
Prof. Y. Kobayashi
Department of Applied Chemistry, College of Life Sciences
Ritsumeikan University
1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577 (Japan)
E-mail: ykobayas@fc.ritsumei.ac.jp
© 2024 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
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if the highly excited electrons can be extracted efficiently,
this can be an extremely effective upconversion reaction
system.
Klimov and Oron groups have achieved upconversion
using Auger recombination from CdSe to CdS moieties in
these CdS/CdSe heteronanostructures.[30–32] Son and Game-
lin groups have achieved energy transfer upconversion
similar to Auger recombination using Mn-doped CdS
NCs.[33–36] More recently, Krauss and Weix et al.[37,38] demon-
strated elimination reactions of halogens of aryl halides
using CdS NCs. On the other hand, their reactions are
limited to the decomposition of allyl halide and chloroacetic
acid, which are relatively easy to reduce. Moreover, most
systems are constrained to photoredox reactions in organic
solvents, and there are few examples in aqueous solutions
except for research on hydrogen evolutions probably
because water induces various side reactions including the
decomposition of catalysts.
In this study, we demonstrate that PFOS and FPs can be
efficiently decomposed to fluorine ions in aqueous solutions
at room temperature and atmospheric pressure through the
irradiation of visible LED light onto CdS and copper-doped
CdS (CuCdS) NCs. While visible light energy is typically
insufficient to break the strong CF bonds, we overcome
this limitation by Auger recombination. Typically, electrons
in the higher state tend to relax to the lowest excited state
via nonradiative relaxation. However, a fraction of these
electrons is either directly transferred to PFASs when
adsorbed onto the NC surface or is released into the
medium, forming hydrated electrons. Hydrated electrons
possess a reduction potential (2.9 V vs standard hydrogen
electrode (SHE)) exceeding that of metallic sodium,[39,40] and
therefore can be used to decompose PFASs through
reduction reactions. In colloidal NC systems, trapping of a
photogenerated hole by hole scavengers likely extends the
lifetime of the conduction band electron (Figure 1), which
facilitates the absorption of another photon to produce a
negative trion (a three-carrier state composed of two
electrons and a hole). Simultaneously, photoirradiation
promotes the desorption of organic ligands from the NC
surface, which in turn enhances the adsorption of PFAS
onto the NC surface. Consequently, the efficient decom-
position of PFAS occurs through cooperative multiphoton
processes involving light-induced ligand displacements and
Auger-induced electron injections via hydrated electrons
and the higher excited states of NCs. If a negative trion is
replaced with a positive trion (a three-carrier state com-
posed of one electron and two holes), highly oxidative states
can also be produced. This advanced decomposition techni-
que can be applied to decompose other persistent chemicals
under mild conditions, contributing to the realization of a
sustainable fluorine-recycling society.
Results and Discussion
CdS and CuCdS NCs were synthesized in aqueous
solutions using 3-mercaptopropionic acid (MPA), following
a reported protocol.[41] X-ray diffraction (XRD) measure-
ments showed that the crystal structures of CdS and
CuCdS NCs corresponded to zincblende structures, regard-
less of the amount of Cu (Figure S1 in the Supporting
Information (SI)). The average diameters of CdS NCs were
estimated based on the excitonic absorption peak in the
steady-state absorption spectra,[42] while those of CuCdS
NCs were estimated by the peak width of XRD patterns.
The atomic ratio of Cu per Cd ranged from 0–12 %, as
confirmed by X-ray fluorescence spectroscopy. Cu doping
did not introduce XRD peaks related to impurities or
optical absorption tails in the longer wavelength, indicating
the absence of the CuxS domain. The aqueous solutions of
MPA-capped CdS and CuCdS NCs exhibited absorption
below 450–500 nm, depending on the size and composition
(Figure S3). Cu doping slightly shifted the absorption
spectra to a longer wavelength, probably because of the
formation of midgap states dominated by Cu(3d) above the
valence band.[43]
To investigate the photogenerated transient species, we
conducted laser flash photolysis measurements using a 355-
nm nanosecond laser pulse. As an example, the transient
absorption spectra and dynamics of the aqueous solution of
12 % CuCdS NCs are shown in Figure 2. CuCdS NCs do
Figure 1. Plausible reaction mechanism of visible-light-induced defluorination of PFAS by semiconductor NCs.
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not show the defect emission of CdS NCs on microsecond
timescales, which interferes with the observation of the
transient species, whereas similar photocatalytic reactions
were observed in CuCdS and CdS NCs. Results of CdS
NCs measured by the randomly interleaved pulse train
(RIPT) method, which removes the emission component by
taking the difference for each laser shot, are shown in
Figure S4. Immediately after the excitation, a broad positive
transient absorption band was observed at 640 nm (Fig-
ure 2a). The transient species decayed with a time constant
of 0.90 μs (Figure 2b). Moreover, the signal increased non-
linearly with increase in the excitation intensity (Figure 2c).
The spectral shape, lifetime, and nonlinear dependence
indicated that the short-lived transient species originated
from a hydrated electron generated by Auger
recombination.[44] Because the photon flux density was
insufficient for a simultaneous two-photon process, the
hydrated electron was produced by the stepwise two-photon
process. The quantum yield for hydrated electron generation
was estimated to be 1.5×102when the excitation intensity
was 4.0 mJ pulse1(details can be found in the SI). The
transient signal associated with the hydrated electron
decayed faster (0.46 μs) upon the addition of PFOS to the
solution (~6.2×104M, Figure 2d). On the other hand, the
emission decay of CdS NCs did not change at all by the
addition of PFOS to the solution (Figure S10). These results
suggest that PFOS reacted with the hydrated electron
generated by Auger recombination.
Solutions for photocatalytic reactions were prepared by
adding CdS NCs, PFOS, and triethanolamine (TEOA, as a
hole scavenger) to milli-Q water and N2bubbling (see
Methods and Table S1 for details). In a typical photo-
catalytic reaction, 0.8 mg of CdS NCs (equivalent to
1.0×108mol for 3.4-nm CdS NCs), PFOS (0.65 mg,
1.2×106mol), and TEOA (20 mg, 1.3×104mol) were added
to 1.0 mL of milli-Q water. Then the sample was mixed by
sonication and stirring for 10 min. After N2bubbling for
30 min in the 10-mm quartz cuvette, 405-nm LED light was
irradiated into the solution to investigate the decomposition
of PFOS (Figure S5). After the light irradiation, the solution
was centrifuged (15000 rpm, 5 min) and decantated. The
decomposition process of PFOS was monitored by 19-
fluorine nuclear magnetic resonance (19F NMR) spectro-
scopy, ion chromatography, and liquid chromatography-
mass spectrometry (LC–MS) measurements. Based on the
concentration of fluorine ions in the aqueous solution, the
defluorination efficiency (overall deF %) was calculated by
the following equation,
overall deF% ¼F
½
nPFAS½ 0100 %ð Þ (1)
where F
½ ,n, and PFAS½ 0are the concentration of fluorine
ions, the number of CF bonds per molecule (17 and 3460
for PFOS and Nafion, respectively), and the initial concen-
tration of PFOS and the sulfonated polymer, respectively.
The peaks of the 19F NMR spectrum of the reaction
solution (deuterated water was used only for NMR measure-
ments) before light irradiation originate from PFOS (Fig-
ure 3a). After 405-nm light irradiation (830 mW cm2) for
24 h, only an intense sharp peak at 121 ppm was observed,
attributable to the fluorine ion. This observation shows that
PFOS can be decomposed to fluorine ions by visible-light
irradiation.
The defluorination efficiency (overall deF %) depends
on the amounts of NCs and TEOA (Figure S8), and the
overall deF % under the optimized condition were 55 %, 70–
80 %, and 100% for 1-, 2-, and 8-h light irradiation,
respectively (Figure 3b). We have confirmed that when the
experiment was repeated multiple times under the same
conditions at the same time, the variation in defluorination
efficiency was approximately within ~5 %. The overall
defluorination efficiency gradually increased with the in-
crease in the irradiation period. On the other hand, the
concentration of PFOS in the solution quickly dropped to
almost zero only after 1-h light irradiation. The concen-
tration of PFOS was <0.1 % after 8-h light irradiation. It
indicates that PFOS is quickly converted to other PFASs by
defluorination and/or adsorbed on the surface of NCs (Vide
infra). The quantum efficiency of CF bond dissociations
was determined to be 2.0×103using the overall deF % after
1-h irradiation of 405-nm LED light (see the Supporting
Information for details). Considering that the quantum
efficiency for the generation of hydrated electrons was
1.5×102under the nanosecond laser pulse excitation, the
observed quantum efficiency for CF bond dissociations
using LED light appears remarkably high. This is probably
due to the contribution of the direct electron transfer from
higher excited states to the decomposition of PFOS in
addition to the reaction with hydrated electrons. Although
lifetimes of higher excited states are generally extremely
Figure 2. (a) Transient absorption spectra of 12% CuCdS NCs excited
with a 355-nm nanosecond laser pulse (4.0 mJ pulse1). (b) Transient
absorption dynamics at 700 nm under different excitation intensities.
(c) Amplitude of the submicrosecond decay component (ΔΔOD) as a
function of excitation intensity. (d) Reaction of the hydrated electron
with PFOS in the presence of triethanolamine (TEOA).
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short, the electron transfer from the higher excited states is
possible if the substrate is in close proximity. In addition,
the higher excited state generated by Auger recombination
in the present CdS NCs (~3.5 V vs SHE) is even more
reductive than those of hydrated electrons. Therefore,
reactions of higher excited states with PFOS are plausible if
PFOS is directly adsorbed on the NC surface. Similar
photocatalytic behaviors were observed with CuCdS NCs
(Figure S9). Although Cu doping may help extend the
lifetime of the excited state, it might also increase the
deactivation pathways for the excited electrons.
The overall deF% after 4-h light irradiation exhibited a
nonlinear dependence on the excitation intensity, even with
LED light (Figure 3c, with a slope of 2.5). This result
indicates that multiphoton processes are involved in the
PFOS decomposition. The reduction potential of the con-
duction band of CdS (0.9 V vs. SHE)[45] is lower than that
of PFOS (1.3 V vs. SHE)[5] even taking the pH and
quantum size effect into account (1.19 V vs. SHE).[45,46]
Moreover, the emission decay of CdS NCs was not affected
at all by the presence of PFOS (Figure S10). These results
strongly suggest that hydrated electrons and higher excited
states generated by Auger recombination are involved in
PFOS decomposition. The defluorination efficiency in-
creased with decreasing NC diameter (Figure 3d), and the
defluorination efficiency substantially decreased in commer-
cial bulk CdS although the primary particle size estimated
by the linewidth of X-ray diffractions was 7.4 nm. The
efficient decomposition reaction in smaller NCs is consistent
with the fact that Auger recombination is enhanced in a
strong confinement regime and the rate of Auger recombi-
nation increases with smaller NC size in addition to the
larger surface-to-volume ratio in smaller NCs.[28,47] More-
over, the slope of the power dependence in this study
exceeded 2, whereas the experimentally observed slope of
the power dependence of two-photon processes is typically
below 2 owing to the involvement of other optical processes
and reabsorption by photogenerated species. It suggests that
another photochemical process, i.e, three-photon processes
in total, is involved in PFOS decomposition, as discussed
later.
The defluorination efficiency substantially decreased
without hole scavengers (TEOA), owing to the photo-
corrosion of CdS NCs (Figure S11). In addition, the
presence of molecular oxygen resulted in low defluorination
efficiency (Figure S11). Several researchers have suggested
that hydroxyl radicals generated by photogenerated holes
are involved in PFAS decomposition.[48] Although we
detected hydroxyl radicals with a hydroxyl radical probe
(terephthalate anion) upon light irradiation, the generation
Figure 3. (a) 19F NMR spectra of the aqueous reaction solution before and after 405-nm light irradiation (830 mWcm2). (b) Time profiles of PFOS
concentration in the reaction solution and overall defluorination efficiency of the reaction solution. Dependence of the overall defluorination
efficiency on (c) irradiation power, and (d) NC diameter. (e) Repeated decomposition experiments using the same CdS NCs. One cycle includes
(1) N2bubbling of the reaction solution for 30 min, (2) irradiation of 405-nm LED light for 12 h, (3) centrifugation and removal of the supernatant,
and (4) addition of 1.0 mL of the aqueous solution containing PFOS (1.2×106mol) and TEOA (1.3×104mol). The defluorination efficiency was
measured for the solution subjected to centrifugation. Details of the experimental conditions can be found in the Materials and Methods section in
the Supporting Information.
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of hydroxyl radicals was suppressed by the addition of
TEOA (Figure S12). These results suggest that the photo-
generated holes mainly lead to photocorrosion and do not
contribute to the decomposition of PFOS in this system.
During the photocatalytic reactions, the solution color
changed from yellow to brown or dark brown by light
irradiation (Figure S13), which could be ascribed to the
photocharging and the generation of metal cadmium (dis-
cussed in detail later). Moreover, the solution gradually
became turbid and the precipitate was observed within 2-h
light irradiation, suggesting that the initial surface ligands of
the NCs were desorbed or deteriorated by light irradiation.
The XRD pattern and peak width of the precipitates
indicated that the diameter of CdS NCs remained constant
despite the photochemical reactions (Figure S14b). Fourier
transform infrared (FTIR) spectra show that MPA was
replaced with other organic molecules probably ascribed to
TEOA and its photoproducts (Figure S14a). These results
indicate that organic molecules on the surface of NCs
dynamically change upon visible light, whereas NCs more or
less retain their sizes.
PFOS can coordinate with the surface Cd atoms by its
sulfonate group. Because the NC surface covered with
TEOA and its decomposed products is presumably posi-
tively charged, the ligand exchange upon light irradiation
may facilitate the adsorption of negatively charged PFOS
through electrostatic interactions. On the other hand, we did
not observe induction periods in the time profiles of the
defluorination efficiency of the reaction solution (Figure 3b).
It may indicate that the exchange of surface molecules
progresses relatively quickly.
The precipitated CdS NCs could be reused to decompose
PFOS over more than 12 cycles (Figure 3e), although the
overall deF % gradually decreased. Each cycle involved (1)
N2bubbling of the reaction solution for 30 min, (2)
irradiation of 405-nm LED light for 12 h, (3) centrifugation
and decantation of the supernatant, and (4) addition of
1.0 mL of the aqueous solution containing PFOS
(1.2×106mol) and TEOA (1.3×104mol). The ratio of CdS
NC and PFOS per cycle was 1 : 122.2, with each PFOS
molecule containing 17 CF bonds. The turnover number of
CF bond dissociation per CdS NC was calculated to be
17200 from the line exploration. The concentration of
cadmium (II) ions dissolved in the solution was 3 ppm after
the first cycle, and varied between 6–46 ppm (the average
was 13 ppm except for outliers) depending on the exper-
imental conditions (Figure S15), which corresponds to 1–7 %
of the remained CdS NCs. The large deviation of the Cd
concentration may be due to insufficient removal of
molecular oxygen because molecular oxygen promotes
photocorrosion. The slight photocorrosion is consistent with
the gradual decrease in the overall deF % by repeating the
photocatalytic reactions.
Scanning transmission electron microscopy (STEM)
measurements revealed that the shape and crystallinity were
preserved after 1-h light irradiation (Figure S16), which is
consistent with the results of XRD measurements (Fig-
ure S14b). Moreover, elemental mapping revealed that F
atoms were observed around CdS NCs, whereas they were
hardly observed in the sulfur layers probably generated by
the dissolution of CdS NCs (Figure S17). It indicates that
the decomposition of PFOS mainly proceeds on the surface
of CdS NCs.
Previous reports on PFOS decomposition by UVC light
using a mercury lamp highlighted two potential mechanisms
for PFOS decomposition: H/F exchange and chain-short-
ening (Figure 4).[3,6] The latter mechanism is known as
decarboxylation (desulfonation as a first step in this case)-
hydroxylation-elimination-hydrolysis (DHEH) mechanism.
In the hydroxylation of fluoroalkyl radicals during PFOS
decomposition, Bentel et al. reported that water is a reactant
for the hydroxylation.[6] On the other hand, it should be
noted that another mechanism proposed by Tagami et al. is
also plausible for the hydroxylation: they reported that
molecular oxygen is a reactant for the hydroxylation of
fluoroalkyl radicals during photoconversion of bromodi-
fluoroacetate esters to oxamate esters.[49] Trifluoroacetic
acids generated by the chain-shortening further decompose
to CO2and H2O.
LC–MS analyses before light irradiation suggested that
the commercial linear PFOS used in this study contained
several branched PFASs, such as perfluoroheptanesulfonate
(PFHpS, m/z=449), perfluorohexanesulfonate (PFHxS, m/
z=399), and a small amount of PFOA (m/z=413, Fig-
ure S18–S30). After light irradiation for 1 h, the signals
associated with PFOS substantially decreased, and multiple
Figure 4. Plausible decomposition mechanism of PFOS by visible light irradiation to CdS NCs.
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peaks originating from molecules with one or two F atoms
of PFOS replaced with H atoms were observed (m/z=481
and 463, Figure S31–S57). In addition, several peaks asso-
ciated with molecules, where several even numbers of F
atoms were abstracted and some of CC bonds were
converted to double or triple bonds, were observed (such as
m/z=461, 443, 425, 407, 405). These results suggested that
the H/F exchange reaction occurred during the decomposi-
tion of PFOS.
On the other hand, only products with fewer than three
substituted hydrogen atoms were observed by H/F ex-
change, and the most defluorinated anion detected even
after prolonged light irradiation was C8F10H3SO3(m/z=
369). Moreover, the signals associated with the anions
generated by H/F exchange reactions decayed with increas-
ing irradiation time. Considering the near-unity defluorina-
tion efficiency in this system, another defluorination mecha-
nism, i.e., the chain-shortening reaction likely occurred
through visible-light irradiation. However, peaks derived
from shorter-chain carboxylate anions, including trifluoro-
acetic acetate, were not observed even in the matrix-assisted
laser desorption/ionization-time-of-flight mass spectrometry
(MALDI-TOF MS) measurements of the solution after the
reaction (Figure S105–S108). Shorter-chain carboxylate
PFASs are usually more reactive than PFOS and could be
readsorbed on the NC surface, where further defluorination
reactions would proceed. Therefore, the absence of these
small PFASs may indicate the efficient progression of
DHEH reactions, resulting in the complete decomposition
of PFOS to fluorine ions.
Because hydrated electrons and higher excited states are
short-lived, PFOS and NCs being in close proximity, i.e., the
adsorption of PFOS onto the surface of NCs, is crucial for
an efficient decomposition reaction. The decrease in the
19F NMR signals of PFOS dissolved in the aqueous solution
by the addition of CdS NCs indicates that PFOS was
promptly adsorbed onto the surface of NCs in the solution
(Figure 5a). LC–MS analyses revealed that 21 % of PFOS
were adsorbed on the surface of NCs 5 min after the
addition of CdS NCs to the solution for photocatalytic
reactions (Figure S109). Prior studies have shown that
coordinated ligands are displaced under light irradiation,
and the readsorption of displaced ligands requires more
than several seconds.[50,51] The origin of the photoinduced
ligand displacement was revealed to be the dramatic
decrease in the Coulomb force between the NC moiety and
the ligand by photoinduced electron transfer.[50] In the
present experiment, despite MPA being coordinated to the
NC surface before the reaction, TEOA and its likely
decomposed products were primarily observed on the NC
Figure 5. (a) 19F NMR spectra of the aqueous solution of PFOS (1.2×103M) before and after the addition of CdS NCs (2.7×104M). (b) 1H NMR
spectra of CdS NCs before and after light irradiation (405 nm, 830 mW cm2) for 5 min. Broad peaks indicate MPA ligands coordinated to the
surface of NCs, while sharp peaks correspond to free MPA. (c) Schematics of light irradiation conditions, along with overall defluorination
efficiency (overall deF%) and temperature of the reaction solution upon completion of light irradiation. (d) Plausible mechanism underlying the
relationship between ligand displacement and PFOS decomposition.
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surface after the reaction (Figure S14a). It suggests that the
organic molecules on the NC surface dynamically transform
under light irradiation. The photoinduced ligand displace-
ment was observed through proton NMR (1H NMR) spec-
troscopy of the deuterated solution of MPA-capped CdS
NCs (Figure 5b). Broad peaks at 2.2–3.5 ppm before light
irradiation were attributable to the MPA coordinated on the
surface of NCs, while several sharp peaks were ascribed to
weakly bound or free MPA. The sharp peaks associated
with free MPA increased upon irradiation of 405-nm LED
light (830 mW cm2) for 5 min. Moreover, the sharp peaks
decreased again 1 h after light irradiation. These results
indicated that MPA ligands were displaced by light irradi-
ation, and the displaced MPA gradually readsorbed onto the
NCs. This ligand displacement most probably facilitates the
adsorption of PFOS onto the surface of NCs and accelerates
the photodecomposition of PFOS. The timescale of the
observed reaction was considerably slower than that re-
ported previously, possibly because the extended duration
of light irradiation promoted the persistence of displaced
ligands.
The reaction between PFOS and hydrated electrons, as
well as higher excited states, should be completed within
several microseconds. If the ligand displacement reaction
plays a pivotal role in driving PFOS decomposition, the
decomposition efficiency is expected to be affected by the
non-light irradiation periods during the light irradiation
even when the total irradiation period is the same. To
elucidate the relationship between ligand displacement on
the NC surface and PFOS decomposition, we measured the
overall deF% under different irradiation conditions. Specif-
ically, the light was irradiated for 10 s and paused for
specified intervals (10 s, 1 min, and 2 min, as illustrated in
Figure 5c). This cycle was repeated to achieve an equivalent
total irradiation duration as that of continuous irradiation
for 1 h. Moreover, we monitored the solution temperature
because it changes based on the duration of irradiation.
Following continuous light irradiation for 1 h, the overall
deF% was 56% and the solution temperature after the light
irradiation was 311 K. The decrease in the solution temper-
ature to 296 K under the same irradiation condition led to a
decrease in the overall deF % to 41 %. In contrast, 10-second
irradiation intervals resulted in a decrease in the overall
deF % to 32 %, even though the solution temperature was
300 K. As the interval increased to 1 min, the overall deF %
further decreased to 11 % at 298 K, whereas the overall
deF% maintained at 296 K with a 2-min interval. These
observations indicate the involvement of another chemical
reaction occurring on timescales of seconds to tens of
seconds in PFOS decomposition. Moreover, the observed
adsorption of displaced ligands, requiring several seconds to
tens of seconds, and the slope of excitation dependence
exceeding 2 highlight the significance of light-induced ligand
desorption/adsorption reactions in PFOS decomposition.
Once a ligand is desorbed, PFOS can coordinate with the
NC surface, facilitating its efficient degradation. These
results collectively suggest that the interplay between ligand
desorption, PFOS adsorption, and nonlinear photoreaction
processes plays a pivotal role in the efficient decomposition
of PFOS under visible light (Figure 5d).
Metal cadmium is expected to be generated upon light
irradiation. Thus, its role in the defluorination reaction was
experimentally studied. The generation of the metal cadmi-
um on the NC surface was confirmed by a change in solution
color from yellow to dark brown upon light irradiation.[52]
The dark brown color persisted throughout the reaction and
gradually faded over a day after the termination of light
irradiation. This long lifetime of metal cadmium is incon-
sistent with the fact that the transient species contributing to
the defluorination reaction are generated within a few
seconds or tens of seconds. Notably, although the formation
of metal cadmium was promoted by the addition of
cadmium precursors (CdCl2or Cd(CH3COO)2), the overall
deF% decreased (remaining nearly constant for Cd-
(CH3COO)2) with the addition of these precursors (Fig-
ure S110 and S111). These results indicate that metal
cadmium was not involved in the main pathways of
defluorination reactions. Although sulfide ions and their
byproducts may be potential photodegradation products of
CdS, the reaction efficiency did not improve even after the
addition of sulfide ions (Figure S112). This result suggests
that sulfur ions and their byproducts were not likely to be
involved in the reaction. This result is consistent with the
results of STEM measurements that the F atoms were hardly
observed in the photogenerated sulfur layers.
This defluorination method can be applied to perfluori-
nated alkyl polymers such as Nafion (Figure 1). Nafion is
widely used as an ion-exchange membrane in electrolysis
and batteries. For the powder form of Nafion, where the
sulfonate groups are -SO2F, the overall deF % under the
same experimental condition with PFOS was at most 1.8%
after irradiation for 24 h (Figure S112). When the same
reaction was performed with Nafion dispersed in a water/
ethanol mixture solution by saponification, where the
sulfonate groups are -SO3, the overall deF % dramatically
increased to 81 % over 24-h light irradiation (Figure S113
and S114). These results demonstrated that nonlinear photo-
reactions of semiconductor NCs can efficiently decompose
perfluoroalkyl polymers, emphasizing the significance of
proximity between the NCs and the substrate for the
reaction. Furthermore, polytetrafluoroethylene (PTFE)
powder can also be defluorinated, although the overall
deF% was at most 5% after irradiation for 48 h. Images
captured before and after the reactions demonstrated that
the surface of the PTFE powder, which is strongly hydro-
phobic, became hydrophilic and the powder sank into the
aqueous solution (Supplementary Figure S115). In other
words, PTFE can be defluorinated by visible LED light
although the reaction efficiency was relatively low.
Conclusion
This research showcased the efficient defluorination of
PFOS and Nafion using visible LED light irradiation onto
semiconductor NCs at room temperature under atmospheric
pressure. Because CdS is a typical compound semiconductor
Angewandte
Chemie
Research Article
Angew. Chem. Int. Ed. 2024,63, e202408687 (7 of 9) © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
and nonlinear reactions are prevalent across various compo-
sitions, this demonstrated concept has the potential for
broader applicability to other low-toxicity materials. The
proposed methodology is promising for the effective decom-
position of diverse perfluoroalkyl substances under gentle
conditions, thereby significantly contributing towards the
establishment of a sustainable fluorine-recycling society.
Supporting Information
The authors have cited an additional reference within the
Supporting Information.[53]
Acknowledgements
This work was supported by JST, PRESTO Grant Numbers
JPMJPR22N6, JSPS KAKENHI Grant Numbers
JP21K05012, JP24K01460, and Advanced Research Infra-
structure for Materials and Nanotechnology, The Ultra-
microscopy Research Center, Kyushu University
(JPMXP1223KU0055). The authors express their gratitude
to Dr. Tatsuo Nakagawa from UNISOKU Co., Ltd., for
helping nanosecond to microsecond transient absorption
measurements.
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
Keywords: semiconductor nanocrystals ·nonlinear optical
processes ·photocatalysis ·PFAS ·hydrated electron
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Manuscript received: May 8, 2024
Accepted manuscript online: June 19, 2024
Version of record online: August 4, 2024
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Angew. Chem. Int. Ed. 2024,63, e202408687 (9 of 9) © 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
