Highly Efficient Degradation of Persistent Pollutants with 3D Nanocone TiO2-Based Photoelectrocatalysis

Article abstract of DOI:10.1021/jacs.1c05008

Photoelectrocatalytic (PEC) degradation of organic pollutants into CO2 and H2O is a promising strategy for addressing ever-growing environmental problems. Titanium dioxide (TiO2) has been widely studied because of its good performance and environmental benignancy; however, the PEC activity of TiO2 catalyst is substantially limited due to its fast electron-hole recombination. Herein, we report a TiO2 nanocone-based photoelectrocatalyst with superior degradation performance and outstanding durability. The unique conical catalyst can boost the PEC degradation of 4-chlorophenol (4-CP) with 99% degradation efficiency and higher than 55% mineralization efficiency at a concentration of 20 ppm. The normalized apparent rate constant of a nanocone catalyst is 5.05 h-1 g-1 m2, which is 3 times that of a nanorod catalyst and 6 times that of an aggregated particle catalyst, respectively. Further characterizations reveal that the conical morphology of TiO2 can make photogenerated charges separate and transfer more efficiently, resulting in outstanding PEC activity. Moreover, computational fluid dynamics simulations indicate that a three-dimensional conical structure is beneficial for mass transfer. This work highlights that tuning the morphology of a photoelectrocatalyst at the nanometer scale not only promotes the charge transfer but also facilitates the mass transportation, which jointly enhance the PEC performance in the degradation of persistent pollutants.

Full text of DOI:10.1021/jacs.1c05008

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Article
Highly Efficient Degradation of Persistent Pollutants with 3D
Nanocone TiO ‑Based Photoelectrocatalysis
2
#
#
Rui Song, Haibo Chi, Qiuling Ma, Dongfeng Li, Xiaomei Wang, Wensheng Gao, Hao Wang,
Xiuli Wang, Zelong Li,* and Can Li*
Cite This: J. Am. Chem. Soc. 2021, 143, 13664−13674
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sı Supporting Information
ABSTRACT: Photoelectrocatalytic (PEC) degradation of organic
pollutants into CO and H O is a promising strategy for addressing
2
2
ever-growing environmental problems. Titanium dioxide (TiO ) has
2
been widely studied because of its good performance and environ-
mental benignancy; however, the PEC activity of TiO catalyst is
2
substantially limited due to its fast electron−hole recombination.
Herein, we report a TiO nanocone-based photoelectrocatalyst with
2
superior degradation performance and outstanding durability. The
unique conical catalyst can boost the PEC degradation of 4-
chlorophenol (4-CP) with 99% degradation efficiency and higher
than 55% mineralization efficiency at a concentration of 20 ppm. The normalized apparent rate constant of a nanocone catalyst is
−
1
−1
2
5
.05 h
g
m , which is 3 times that of a nanorod catalyst and 6 times that of an aggregated particle catalyst, respectively. Further
characterizations reveal that the conical morphology of TiO can make photogenerated charges separate and transfer more
2
efficiently, resulting in outstanding PEC activity. Moreover, computational fluid dynamics simulations indicate that a three-
dimensional conical structure is beneficial for mass transfer. This work highlights that tuning the morphology of a
photoelectrocatalyst at the nanometer scale not only promotes the charge transfer but also facilitates the mass transportation,
which jointly enhance the PEC performance in the degradation of persistent pollutants.
7
INTRODUCTION
crucial factor that mainly determines the PEC performance.
Many strategies, such as heterostructure construction,
nanostructure engineering, and cocatalyst modification, have
been devoted to enhancing the PEC performance of the
photoelectrode via improving the efficiency of charge
■
Water pollution is threatening human beings and receiving
increasing attention all around the world. Organic pollutants
1
from wastewater are recalcitrant and toxic, so its disposal is one
of the most urgent topics in environmental research. Phenolic
compounds are common persistent organic contaminants,
which show non-biodegradability, posing serious risks to the
environment. Some of the most toxic phenolic organics are
those of chlorinated phenols, which are used as crucial
intermediates in the synthesis of disinfectants, dyes, and
7
separation in the field of photoelectrocatalysis. Among
them, building specific nanoarchitectures such as one-dimen-
sional (1D) nanostructures (nanorods, nanotubes, nanocones,
etc.) and their 3D arrays, which can dramatically reduce the
recombination of electron−hole pairs through shortening the
8
,9
2
,3
transport distance of minority charge carriers to the surface,
has been proved to be an effective approach to design efficient
pesticides. Among them, 4-chlorophenol (4-CP) is a typical
chlorinated phenol because of its widespread applications in
the chlorination of organic material or the production of
10
PEC systems.
4
,5
Besides the common advantages of the spatial configuration
and charge separation in the 1D nanostructure, nanocone
arrays possess a built-in electric field along the axial direction,
which can further drive the separation of photogenerated
drugs.
Meanwhile, conventional biological or physical−
chemical processes are ineffective in eliminating 4-CP due to
6
its chemical stability and thermal stability. Therefore, it is
highly desirable to develop an advanced strategy for efficiently
breaking down such persistent pollutants, based on the
essential understanding of the degradation mechanism of
pollutants.
Photoelectrocatalytic (PEC) pollutant degradation, which
potentially combines the merits of both electrocatalysis and
photocatalysis, is becoming one of the most promising yet
challenging approaches for practical applications. Fundamen-
tally, the separation efficiency of photogenerated charges is a
1
1
carriers. In addition, nanocone has more active sites
Received: May 21, 2021
Published: August 19, 2021
©
2021 American Chemical Society
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Scheme 1. Synthesis Scheme of TiO Catalysts with Different Structures by a Na EDTA-Assisted Hydrothermal Approach
2
2
compared to other 1D nanostructure with the same surface
area due to more side surface of the conical structure, which
will collect minority charges, thereby promoting PEC perform-
1
2
ance. Therefore, it is important to fabricate the conical
morphology of photoelectrocatalysts for efficient separation
and transport of photogenerated charges.
Herein, we choose titanium dioxide (TiO ), a typical
2
13,14
photoelectrocatalyst that has been widely investigated
and substantially limited by its fast electron−hole recombina-
1
5
tion, to design a photoelectrocatalyst with a conical structure
and explore how different morphologies influence the PEC
performance. A TiO photoelectrocatalyst with a 3D conical
2
structure was prepared by a disodium ethylenediamine
11
tetraacetate (Na EDTA)-assisted hydrothermal approach to
2
eliminate persistent organic pollutants. It was found that
benefiting from the significant improvement in the efficiency of
charge separation and mass transport, as well as the great
increase in the amount of photogenerated active species, the
conical TiO catalyst shows superior PEC performance on
2
degradation and mineralization of 4-CP. This work offers an
effective strategy to rationally design and fabricate photo-
electrodes for efficient pollutant degradation.
RESULTS AND DISCUSSION
■
The TiO photoelectrocatalysts with different structure were
2
prepared via a Na EDTA-assisted hydrothermal approach
2
(Scheme 1). Ti mesh was used as the substrate, which is
favorable to solve the problem of poor mass transfer and
16,17
Figure 1. Structure characterizations of photoelectrocatalysts. SEM
images of (a) aggregated particles, (b) nanorods, and (c) nanocones
of TiO . (d) SEM side views of nanocones of TiO . (e) XRD patterns
surface sluggish reaction kinetics for planar devices.
a shows that the TiO2 synthesized with acetylacetone
ACAC) alone results in aggregated particles. In contrast,
Figure
1
2
2
(
and (f) Raman spectra for TiO photoelectrocatalysts with different
2
the sample fabricated in the presence of Na EDTA displays
2
structures.
nanorod arrays. These nanorods are well aligned and
perpendicular to the substrate with diameters and lengths of
1
00 and 450 nm, respectively (Figure 1b). Conical structures
Figure 1e shows the X-ray diffraction (XRD) patterns of the
are obtained when both ACAC and Na EDTA are present in
three different TiO samples. The XRD peaks at 35.1°, 38.4°,
2
2
the hydrothermal process (Figure S1). Well-oriented TiO2
nanocones with a diameter of 50 nm and a length of 1 μm are
formed and distributed uniformly and densely on a Ti mesh, in
a 3D structure with a highly exposed surface area (Figure 1c
and d).
and 40.1° are assigned to the (100), (002), and (101) plane of
Ti mesh (JCPDS No. 44-1294). The diffraction peaks of the
nanocone are indexed in accordance with the rutile structure
(JCPDS No. 21-1276), and the nanorod sample exhibits
typical rutile patterns as well. Apart from the diffraction peaks
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Figure 2. PEC performance of TiO photoelectrocatalysts with different nanostructures. (a) Photocurrent densities of TiO catalysts with different
2
2
nanostructures, measured under chopped light. The percentage of (b) 4-CP degradation, (c) TOC removal, and (d) dechlorination for the PEC
degradation process on TiO catalysts with different nanostructures. (e) Kinetic plots of TiO catalysts with different nanostructures for the 4-CP
2
2
degradation reaction at 20 ppm. (f) Degradation of different concentrations of 4-CP on TiO catalysts with different nanostructures.
2
of Ti mesh, the intensity of XRD patterns for aggregated
NCs for the degradation reaction. The transient photocurrent
particles is weak, and only the peak at 25.3° is visible, which
response to Xe lamp light of different TiO catalysts indicates
2
corresponds to the (101) plane of anatase TiO (JCPDS No.
the largest amount of photogenerated carriers in TiO NCs
2
2
2
1-1272). This result demonstrates that aggregated particles
(Figure S6). In addition, no photocurrent decrease is observed
mainly show anatase phase with poor crystallinity.
for TiO NCs in 3 h of illumination at 2.22 V
(Figure S6),
2
RHE
Raman spectroscopy was then employed to further
whereas TiO NRs and TiO NPs show a dramatic decrease in
2 2
distinguish the anatase from rutile TiO , and the results are
performance. The decrease of photocurrent suggests that the
formation and progressive accumulation of intermediates may
occur on the surface of photoelectrodes.
2
shown in Figure 1f. The Raman bands of the nanocone at 445
−
1
−1
20
cm (E ) and 610 cm (A ) are well-resolved, confirming
g
1g
18
the rutile phase of the nanocone obtained in this work.
Moreover, the nanorod catalyst also exhibits characteristic
Raman modes of rutile phase at 144 cm (B ), 446 cm
The PEC activities of TiO catalysts with different structures
2
were evaluated by monitoring the degradation of 4-CP. In
order to achieve high efficiency of degradation and
mineralization, the reaction potential of 2.22 VRHE was
employed, even if oxygen evolution reaction (OER) occurs
at such high potential (Figure S7). As shown in Figure 2b, all
three catalysts are active in the degradation of 4-CP under PEC
−
1
−1
1g
−
1
18
(
Eg), and 608 cm (A ). The enhanced Raman spectra of
1g
aggregated particles show peaks located at 145, 399, 517, and
−
1
6
39 cm , which are in accordance with characteristic anatase
phase with vibrational modes of E , B , A , and E ,
g
1g
1g
g
1
9
respectively. Moreover, no obvious impurity was detected
conditions. The TiO catalysts with low surface area (0.31,
2
2
in all of the TiO catalysts (Figures S2−S4).
0.36, 0.39 m /g for TiO NCs, NRs and NPs, respectively,
2
2
The PEC performance of TiO catalysts with different
Table S1) show poor ability for 4-CP adsorption (Figure S10).
However, the concentration of 4-CP is decreased rapidly for
2
structures was investigated in 0.1 M Na SO electrolyte with
2
4
2
0 ppm 4-CP under xenon lamp illumination in a conventional
the TiO NCs-based PEC degradation process, and it takes 32
2
three-electrode cell. The photocurrents of these catalysts were
recorded in the dark and under irradiation with a set of
chopped linear sweeps. As shown in Figure 2a, the three TiO2
catalysts show a very low dark current with respect to their
photocurrents. Among the three photoelectrocatalysts, the
highest anodic photocurrent was generated for the TiO2
nanocone catalyst under illumination and increased steeply
with increasing bias. The photocurrent of the nanocone
min to degrade 50% 4-CP, which is much faster than TiO2
NRs (67 min) and TiO NPs (125 min). Moreover, the
2
degradation efficiency is up to 99.3% in 180 min for TiO NCs,
2
which is higher than TiO NRs (82.8%) and TiO NPs
2
2
(62.7%) in the same degradation time. The results clearly
indicate that TiO NCs show an outstanding PEC activity in
2
the degradation reaction.
To study the degradation process of 4-CP in detail, we
conducted experiments to monitor the percentage of total
organic carbon (TOC) removal and dechlorination during the
reaction process. We first measured the change of TOC
concentration versus time during the degradation, as it can be
used to describe the degree of mineralization. As shown in
catalyst (denoted as TiO NCs) reaches a saturated current
2
−
2
density of 1.68 mA cm , which is 2.4- and 12-fold of the
photocurrent density of TiO nanorods (denoted as TiO
2
2
NRs) and TiO nanoparticles (denoted as TiO NPs). The
2
2
results clearly indicate the excellent PEC performance of TiO2
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Figure 3. PEC performance of TiO NCs. (a) Photoelectrocatalytic (PEC), electrocatalytic (EC), photocatalytic (PC), and direct photolysis
2
process of 4-CP degradation on TiO NCs. (b) Trapping experiments of active species during PEC degradation of 4-CP. (c) DMPO spin-trapping
2
•
•−
ESR spectra of OH and O in the dark and on Xe lamp light irradiation. (d) Scheme of the degradation process for the nanocone-based PEC
2
process. (e) Cycle runs of TiO NCs for PEC degradation of 4-CP. (f) Degradation performance of TiO NCs with a 7 h reaction time.
2
2
Figure 2c, TiO NCs demonstrate the efficiency of TOC
understand how the mass diffusion varies with the 3D
2
removal, 54.9% in 180 min, while TiO NRs and TiO NPs
morphology of the TiO catalysts. Figure 2f shows that as
2
2
2
show efficiencies of only 19.5% and 11.3%, respectively,
the initial concentration of 4-CP increased from 20 ppm to 100
ppm, the degradation efficiencies decreased continuously.
Since the mass transfer changes with increased initial
concentration of pollutant, the decrease of degradation
efficiency demonstrates the crucial role of mass transfer for
pollutant degradation. Meanwhile, active sites of catalysts and
generated active species are exhausted by excess 4-CP; thus the
degradation kinetics becomes sluggish and the amount of 4-CP
removed in unit time reaches an upper limit. In this case, the
indicating superior mineralization performance of TiO NCs.
2
−
The chlorine atom in 4-CP is turned into inorganic ions Cl by
−
dechlorination; thus the concentration of the Cl generated
during the 4-CP degradation process was then determined. We
used the practical concentration of 4-CP as a standard example
to estimate the theoretical concentration of Cl− and the
percentage of dechlorination. Figure 2d shows the percentage
of dechlorination in 180 min is 95.6% for TiO NCs, which is
2
close to complete dechlorination, while the percentages of
dechlorination are 57.3% and 42.8% for TiO NRs and TiO
TiO NCs catalyst, which shows the best degradation efficiency
2
at high concentrations of 4-CP, implies the highest reaction
rate of the surface reaction. The above results indicate that
tuning the morphology of a photoelectrocatalyst at the
nanoscale can greatly influence the PEC activities through
partially affecting the active sites of the catalysts, and the
nanocone morphology is the most adapted to the surface
reaction kinetics.
2
2
NPs, respectively. The extremely low rates of dechlorination of
TiO NRs and TiO NPs suggest that TiO NRs and TiO
2
2
2
2
NPs catalysts are not active enough to mineralize such
persistent pollutants, similar to what has been previously
21
reported in the literature. These results may also imply an
accumulation of recalcitrant intermediates on the surface of
TiO NRs and TiO NPs, namely, sluggish surface reaction
As described above, the TiO catalyst with a nanocone
2
2
2
kinetics, which usually also affects the mineralization efficiency
of the catalyst.
morphology exhibits the highest PEC performance on the
degradation reaction among the three TiO samples with
2
Furthermore, the intrinsic activity of the catalysts was
determined by measuring the 4-CP removal kinetics. Figure 2e
shows that the data are fitted to the pseudo-first-order kinetics
model. On using the specific area to normalize the apparent
different structures. To further ascertain the superior perform-
ance of TiO NCs, more experiments were carried out. The
2
electrocatalytic (EC), photocatalytic (PC), and PEC process
for 4-CP degradation were conducted for TiO NCs, as shown
2
−
1
−1
2
rate constant, the normalized constant is 5.05 h
g
m for
in Figure 3a. The EC process shows an insignificant
degradation efficiency of 4-CP (<1%, 180 min), and the
degradation efficiency of the PC process is quite low (36.1%,
180 min), while the PEC degradation efficiency (>99%, 180
TiO NCs, which is about 3 and 6 times that of TiO NRs
2
2
−
1
−1
2
−1
−1
2
(
1.71 h
g
m ) and TiO2 NPs (0.79 h
g
m ),
respectively. The above results clearly reveal that TiO NCs
2
show the highest intrinsic activity among the three catalysts in
min) of TiO NCs is much higher than the sum of EC and PC
2
the PEC degradation of 4-CP.
processes. This result unambiguously suggests a synergetic or
We also investigated the PEC performance of catalysts when
the initial concentration of 4-CP is increased in order to
cooperative effect in the TiO NCs catalyst between the
electrocatalysis and photocatalysis, and it is responsible for
2
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Figure 4. Mechanism. (a) Concentration of generated 4-CC during the degradation process. (b) Proposed degradation pathway of 4-CP during the
PEC degradation on TiO catalysts. (c) Tested (dot) and simulated (line) photo EIS spectra of different catalysts under Xe lamp illumination at 0.1
2
V vs SCE in a 0.1 M Na SO aqueous solution. The equivalent circuit is in the inset. (d) Mott−Schottky plots of TiO catalysts with the AC
2
4
2
potential frequency of 1000 Hz. Mott−Schottky plots for TiO NRs and TiO NPs are shown in the inset. (e) PL intensity of TiO catalysts with
2
2
2
different structures. (f) TAS spectra at 508.5 nm for TiO photoanodes at 1.6 V
under illumination. (g) Injection efficiencies of TiO catalysts
2
RHE
2
with different nanostructures that are determined with and without Na SO (h) PL intensity of TAOH generated on TiO catalysts with different
2
3
2
structures. The excitation wavelength is 310 nm. (i) Kinetic plots of TiO catalysts with different structures as the initial concentration of 4-CP is
2
1
00 ppm.
increasing the PEC performance. The synergetic effect could
be explained in terms of the photogenerated charge separation
structure in the nanocone catalyst, resulting in the highest
PEC degradation performance.
It has been proposed that photoinduced reactive species play
(
(
from the PC process) promoted by the applied potential
from the EC process). Similar synergetic effects are also
key roles in the oxidation of organic contaminants for organic
22
pollutant degradation, so the active radical species generated
observed in the other catalysts (Figure S12). However, their
promoting effect in PEC degradation efficiency is not as
obvious as for TiO2 NCs. Obviously, the nanocone
morphology is special in charge separation because the
photogenerated charges are separated effectively when a strong
built-in electric field is provided by the unique conical
during 4-CP degradation were identified by quenching
experiments. We used methanol (MeOH), Na EDTA, and 4-
2
hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) as
•
2
3
24
quenchers for hydroxyl radical ( OH), holes and super-
oxide radical (O2• ), respectively. As shown in Figure 3b,
when MeOH was added into the reaction system, the
−
25
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efficiency of 4-CP degradation was decreased to 53.9% in 180
morphologies of TiO catalysts, we conducted more experi-
2
•
min, suggesting the critical role of OH. However, if Na EDTA
ments. According to the UHPLC-MS and HPLC results
(Figures S17 and S18), three different intermediates, 4-
chlorocatechol (4-CC), hydroquinone (HQ), and benzoqui-
none (BQ), are detected and determined. The results show
that all three key intermediates are generated as soon as light
illumination occurs, and the quantities of each intermediate are
2
was added into the solution, the degradation efficiency was
reduced to 77.1%, demonstrating the presence of holes during
the degradation process. Furthermore, on adding TEMPOL
into the degradation system, the efficiency was suppressed
slightly to 83.5%, indicating the formation of O2• . These
−
observations reveal that the PEC degradation process on TiO
quite different on the three TiO catalysts. For HQ and BQ
2
2
•
NCs is dominated by the OH radical together with other
(Figure S19), no accumulation is observed on TiO NCs, and
2
•
−
oxidative species, holes, and O2 . Furthermore, the direct
both of them are destructed ultimately. However, those species
are accumulated on TiO2 NRs and TiO2 NPs, and the
concentration of such intermediates is higher than those of
reason for the different degradation efficiencies of the three
TiO catalysts with different structures is the different amount
2
of reactive species on the surface of the catalysts, which will be
further discussed later.
TiO NCs. Besides, another detected important intermediate is
2
4-CC, and its concentration is much higher than those of BQ
and HQ. Figure 4a shows that the concentration of 4-CC
increases to 2 ppm first and then decreases to 0.6 ppm on TiO2
NCs, whereas it increases continuously to 2.7 and 1.9 ppm on
TiO NRs and TiO NPs. TiO NCs show adequate PEC
To provide direct evidence for the existence of reactive
species, 5,5-dimethyl-1-pyrroline N-oxide (DMPO)-trapped
electron paramagnetic resonance (EPR) experiments were
conducted. Figure 3c displays that no characteristic EPR
signals were detected in the dark, while the characteristic peaks
2
2
2
degradation activities to those recalcitrant intermediates, which
can be attributed to the built-in electric field provided by the
unique conical morphology, resulting in efficient charge
separation. Accordingly, inefficient charge separation results
•
of DMPO− OH (α = α = 14.9 G) were observed under Xe
H
N
2
3
•−
lamp illumination. Furthermore, O2 was captured and
verified by characteristic peaks (α = 14.3 G, αβ = 11.2 G,
N
H
26
αγH1 = 1.3 G) in MeOH solution under PEC conditions.
in poor PEC performance of the other two TiO photo-
2
•
−
The formation of O2 radical can also be confirmed by the
EPR signals of TEMPOL (Figure S14). The weakened
intensity of characteristic peaks of TEMPOL in the EPR
spectra implies that the concentration of TEMPOL is
catalysts. In addition, the total concentration of detected
−
chlorine species (4-CC, Cl , and remaining 4-CP) after the
degradation reaction is consistent with that of the initial 4-CP
for TiO NCs but not in accordance for TiO NRs and NPs.
2
2
•−
27
decreased after reacting with O2
.
These EPR results clearly
These results indicate that there is no other chloride
intermediates on TiO2 NCs, whereas still some chlorine
intermediates are not detected on TiO NRs and TiO NPs.
•
•−
indicate the generation of OH and O2 on TiO NCs during
2
the degradation reaction.
2
2
Figure 3d illustrates the degradation process on TiO NCs.
Thus, we propose the degradation pathway of 4-CP (Figure
2
6,29
The nanocone TiO catalyst is activated by light irradiation to
4b) according to the reported literature
and the above
2
generate photoinduced hole and electron pairs. The generated
charges are separated by the applied bias. The holes on the
anode catalyst directly oxidize water and surface hydroxyl
results. Two main reaction approaches are present during the
degradation process of 4-CP. Hydroxyl radicals can attack the
para-position of the hydroxyl group of 4-CP and HQ is formed
•
28
30,31
groups to produce OH, which then reacts with pollutants.
Meanwhile, the electrons in the cathode can reduce the
with a following dechlorination.
BQ is produced via fast
31
electron shuttle mechanism existence between HQ and BQ.
•
−
dissolved oxygen molecules in the solution to generate O
The other approach is that hydroxyl radicals attack the ortho-
2
2
4
32
radicals, which are confirmed to be also involved in the
degradation process. These active species can effectively
degrade and mineralize the persistent pollutants. In short,
the photogenerated holes and electrons are separated
effectively in space and improve the PEC degradation
performance of TiO2 NCs. The highly efficient charge
separation on the nanocone catalyst should be the reason
position of the hydroxyl group of 4-CP to form 4-CC. The
generated intermediates can be mineralized to CO and H O
2
2
by deep oxidation. Nevertheless, there are still some unknown
intermediates that are not detected.
TiO NCs show superior degradation performance not only
2
for the initial pollutant but also for the generated
intermediates. Thus, it is necessary and worthwhile to
investigate the difference of catalytic activity of different
morphologies of catalysts. An electrochemical impedance
spectroscopy (EIS) test was carried out to study the charge
transfer and recombination process at semiconductor−electro-
that TiO NCs outperform the other different morphologies of
2
TiO catalysts in PEC performance.
2
We also investigated the stability of TiO NCs. Figure 3e
2
shows that no noticeable variation on the degradation
efficiency of 4-CP is observed in five cycling tests under
PEC conditions. Further XRD patterns and SEM images
33
lyte interfaces. Figure 4c shows a typical Nyquist plot
obtained from the illuminated different catalysts. Compared to
the dark conditions (Figure S20), the diameters of the
semicircle loop on the Nyquist plot of different catalysts
were decreased prominently, indicating a large number of
(
Figure S15) of post-TiO NCs after 5 cycles show no distinct
2
alteration in conical structure. These results demonstrate
robust durability and resistant to light corrosion of the TiO2
NCs electrode, suggesting its potential for practical applica-
tions. Moreover, after prolonging the reaction period in 20
charges generated under illumination. TiO NCs show a 35.9
2
Ω charge transfer impedance under illumination, which is
much smaller than that of TiO NRs (137 Ω) and TiO NPs
ppm 4-CP, TiO NCs exhibit a mineralization efficiency of
2
2
2
9
3.8% in 7 h (Figure 3f). On further delaying the reaction
period to 12 h, the degree of mineralization can reach over
9% (Figure S14). These results suggest TiO2 NCs can
(442.3 Ω), suggesting more efficient separation of photo-
generated electron−hole pairs and faster charge transfer
9
9
kinetics at the interface between electrolyte and catalyst.
degrade persistent pollutants completely.
To study the mechanism of the 4-CP degradation reaction
and the difference in PEC performance for different
We also conducted electrochemical measurements on TiO2
catalysts to determine the nature of the space charge region.
Figure 4d shows that all three TiO catalysts with different
2
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Figure 5. CFD simulations. The volume fraction magnitude field of water flow on (a) TiO NCs and (b) TiO NRs after different times of
2
2
simulation. The velocity magnitude contours of water flow on (c) TiO NCs and (d) TiO NRs.
2
2
3
9
structures display positive slopes in Mott−Schottky plots,
recombination of free carriers, the low PL intensity of
which indicates that the obtained TiO catalysts are n-type
TiO NCs implies a reduced recombination of carriers, which
2
2
semiconductors. Based on the Mott−Schottky relation (eqs 1
is ascribed to efficient charge separation, beneficial for
achieving high PEC activity.
Figure 4f shows the normalized in situ transient absorption
and 2), we can calculate the thickness of the depletion layer to
34
describe the space charge region:
spectra (TAS) at 508.5 nm to investigate photoinduced hole
2
−1
N = (2/e εε )[d(1/C )/dV]
(1)
(2)
40
D
0
0
dynamics in the three TiO catalysts with different structures.
2
We find that the decay dynamics of TiO NCs is slightly faster
2
1
/2
W = [2εε (E − E )/e N ]
than TiO2 NRs and TiO2 NPs. The decay half-time of
0
FB
0 D
photogenerated holes (τ50%, the time when the TAS amplitude
where e is the electron charge, ε is the dielectric constant of
= 170, εanatase = 48 ), ε is the permittivity of a
vacuum, N is the donor density, E is the electrode potential,
and EFB is the flat band potential. When TiO NCs contact the
electrolyte, the calculated thickness is 343.23 nm (Table S1),
which is much larger than that of TiO NRs (47.66 nm) and
TiO NPs (8.86 nm), indicating the formation of the strongest
41
0
decreases to half of its initial value at 10 μs ) is estimated to
3
5
36
TiO (ε
2
rutile
0
be 0.321, 0.432, and 1.266 ms for TiO NCs, TiO NRs, and
2
2
D
TiO NPs, respectively (Table S1). It clearly demonstrates that
2
2
the photogenerated holes for TiO NCs survive the shortest
2
lifetimes under PEC conditions, which is directly related to
2
42
efficient charge separation and transfer processes. Therefore,
2
37
TAS results further reveal that the efficiency of charge
separation and transfer is enhanced by the built-in electric
field provided from the conical structure, resulting in fast
transfer and efficient utilization of holes under PEC conditions,
which thereby promotes the PEC performance significantly.
We used Na SO as a hole scavenger to determine the
built-in electric field in TiO NCs. In a typical n-type surface
2
depletion layer of a photocatalyst, photogenerated minority
carriers (holes) are driven to the surface and majority carriers
electrons) cannot migrate to the surface reaction centers.
(
Therefore, electrons are scarce in the depletion region and the
38
recombination of holes is greatly reduced. In the case of the
wide space charge region, the recombination of photo-
generated charges can be reduced dramatically and the surface
oxidation reaction could be greatly promoted thereby.
To reveal the dynamics on migration and separation of
photogenerated electrons and holes in TiO catalysts, photo-
luminescence (PL) emission spectroscopy was employed.
Figure 4e shows the PL spectra of three TiO catalysts in
the wavelength range of 350−500 nm with the excitation light
at 270 nm. It clearly demonstrates that the PL peak intensity of
TiO NCs is the lower than that of the other two TiO
2
3
efficiency of the injection of charges to the electrolyte, which is
another important factor influencing the PEC performance.
Figure 4g shows that at the reaction potential of 2.22 VRHE, the
injection efficiency of TiO NCs is 85%, which is 3 and 7 times
2
that of TiO NRs (28%) and TiO NPs (5%). This result
2
2
2
demonstrates that the largest proportion of holes in TiO
NCs
2
is injected into the electrolyte, which may be derived from
2
43
different morphology of catalysts.
Terephthalic acid (TA) was used to quantitatively evaluate
•
44
the concentration of generated OH radicals in the solution,
2
2
catalysts. Since the intensity of PL is related to the
which is related to efficient utilization of holes. TA can react
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•
with OH to generate the fluorescent product 2-hydroxyter-
Scheme 2. Schematic Diagram for PEC Degradation of 4-
ephthalic acid (TAOH), which can be detected by its
fluorescence at 425 nm when excited with light at 310 nm.
CP on TiO NCs under Illumination
2
45
Strong fluorescence spectra associated with TAOH are
generated upon irradiation within 300−600 nm of all TiO
2
catalysts (Figure S23). Figure 4h shows that the fluorescence
intensity of TAOH for all catalysts increases continuously,
•
indicating OH is generated constantly under light illumina-
tion. Meanwhile, TiO NCs, which are the most favorable for
2
efficient charge separation and transfer, exhibits the highest
fluorescence intensity of TAOH, demonstrating the largest
amount of generated OH radicals on the surface. This result
certifies that efficient charge separation and transfer are
beneficial to the formation of active species, thereby enhancing
the PEC degradation performance.
•
derived from distinct conical structure, the efficiency of
photogenerated charge separation and transport is enhanced
significantly, thus facilitating the generation of reactive species
for degradation reaction. Altogether, the PEC process based on
a nanocone catalyst shows outstanding performance in the
degradation of persistent pollutants (Table S2).
In order to compare the kinetics of surface reactions, we
conducted experiments at a high initial concentration of 4-CP
to exclude the possible influence of mass transport on the
degradation kinetics. Figure 4i shows that the kinetics model is
zero-order when the initial concentration of 4-CP is 100 ppm.
In this case, the normalized rate constant has excluded the
influence of mass transfer and the difference in surface area.
CONCLUSIONS
In summary, we have found that TiO photocatalysts with
■
2
conical arrays in PEC show outstanding performance in the
degradation of persistent organic pollutants, with more than
99% degradation efficiency and over 55% mineralization
efficiency for 4-CP. Characterization results indicate that the
superior PEC performance is attributed to efficient charge
separation and mass transfer. The formation of active species
for the degradation reaction is facilitated by efficient charge
separation, transfer, and injection. Meanwhile, the high volume
distribution fraction of water and faster water flow in space of
the conical structure are beneficial to mass transfer, further
facilitating the degradation process of 4-CP. Building such
three-dimensional structures provides an effective strategy to
achieve highly efficient PEC degradation of persistent
pollutants.
−
1
−2
TiO NCs exhibit a constant of 44.2 h
m
ppm g, which is
2
−
1
−2
1
.4 and 2.9 times that of TiO NRs (30.6 h
m
ppm g) and
2
−
1
−2
TiO NPs (15.3 h
m
ppm g), respectively, demonstrating
2
that TiO NCs have the highest intrinsic activity among the
2
three catalysts. Based on the above results, the origin of
intrinsic activity can be attributed to efficient charge transport
and separation, which is related to the distinct morphological
structure of the catalyst.
Besides efficient charge transfer and separation, mass
transport may play an important role in the degradation
reaction, particularly at low concentration of the substrate
molecule. Thus, we carried out computational fluid dynamics
(
CFD) simulations to evaluate the impact of the 3D structure
EXPERIMENTAL SECTION
Fabrication of Photoelectrocatalysts. Different TiO catalysts
on the mass transfer in the degradation process. Figure 5a and
b show the volume fraction magnitude contours of water flow.
It exhibits that the volume distribution of water flow in the
interspace of conical structures gradually increases and the
volume fraction of water in every space between neighbor
cones is close to 100% finally (Figure 5a), indicating water
■
2
were synthesized by using a Na
EDTA-assisted hydrothermal
2
approach. Ti mesh was used as the substrate metal with dimensions
2
of 2.5 × 3.5 cm and a thickness of 0.1 cm. They were washed
ultrasonically in acetone, isopropanol, ethanol, and deionized water
for 30 min, respectively. Briefly, a Ti precursor containing 0.3 mL of
titanium isopropoxide and 2 mL of ACAC was added into 18 mL of a
contacts fully with the surface of TiO NCs. However, the
2
volume fraction of water in nanorod arrays is lower than
nanocones, and the water flow cannot fill up the interspace no
matter how long the time is (Figure 5b). This result clearly
demonstrates that the conical structure favors the rapid
diffusion of water. Meanwhile, Figure 5c and d show the
velocity magnitude contours of the fluence. The velocity of
water in the interstices of nanocone arrays is 0.45 μm/s, which
is 1.3 times that of the rod structure (0.34 μm/s). Faster
0
.075 M Na EDTA aqueous solution. After stirring for 30 min, the
2
solution turned transparent and was then transferred into a Teflon-
lined stainless steel autoclave. The TiO2 nanocone catalyst was
fabricated by hydrothermal treatment in the above solution at 200 °C
for 12 h. The obtained sample was washed extensively with ethanol
and deionized water and then annealed in air atmosphere at 500 °C
for 1 h. The TiO catalysts with nanoparticle and nanorod structures
were prepared through similar methods without Na EDTA and
ACAC, respectively.
2
2
microfluidic flow of TiO NCs indicates that the nanocone
2
Structure Characterization of Catalysts. The crystal structure
structure is beneficial for the mass transport for the
degradation process. Since TiO2 NCs show 3 times the
and morphology of the TiO catalysts were identified by XRD and
2
scanning electron microscopy (SEM). Raman spectra of TiO
catalysts obtained with an excitation source of 532 nm laser light
were used to distinguish the anatase phase from rutile TiO . The
2
normalized degradation rate constant than that of TiO NRs, it
2
implies again that, besides mass transfer, charge transfer and
separation have a great impact on the reaction. These
observations elucidate that the 3D conical structure is
beneficial to enhance mass transfer.
2
surface area of TiO catalysts was obtained from Brunauer−Emmett−
2
Teller surface area analysis. X-ray photoelectron spectra and energy
dispersive spectroscopy were used to provide information about the
surface and bulk composition of the materials. Mott−Schottky
analysis was investigated at a DC potential range with an AC
potential frequency of 1 kHz under dark conditions. The amplitude of
On the basis of these results, a schematic diagram for PEC
degradation of 4-CP on TiO NCs is proposed (Scheme 2).
2
The nanocone structure is demonstrated to facilitate mass
transfer between the interspace of the catalysts arrays.
Meanwhile, benefiting from the strong built-in electric field
5
the AC potential was 10 mV. EIS was conducted from 0.1 to 10 Hz
at an alternating current signal of 10 mV. The dynamics of charge
migration and separation was determined by photoluminescence (PL)
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spectra and TAS. The concentration of hydroxyl radicals was
investigated through the fluorescent product which was generated
from TA and hydroxyl radicals. The intensity of PL of this fluorescent
product was recorded by a fluorescence spectrophotometer with the
Lanzhou University, Lanzhou, Gansu 730000, China;
Email: lizl@lzu.edu.cn
Can Li − Key Laboratory of Advanced Catalysis, Gansu
Province, State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, Gansu 730000, China; School
of Chemical and Materials Science, University of Science and
Technology of China, Hefei, Anhui 230026, China; State Key
Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian National Laboratory
for Clean Energy, The Collaborative Innovation Center of
Chemistry for Energy Materials (iChEM), Dalian 116023,
China; orcid.org/0000-0002-9301-7850; Email: canli@
dicp.ac.cn
•
excitation wavelength of 310 nm. Reactive oxygen radicals ( OH and
•−
O2 ) were captured using DMPO under illumination and then was
analyzed by EPR spectra.
PEC Performance and Catalytic Analysis for Degradation
Reaction. PEC performance was evaluated in a conventional three-
electrode system with a 0.1 M Na SO aqueous solution (100 mL)
2
4
containing 20 ppm of 4-CP (pH = 5.67), under irradiation of a 300 W
xenon lamp (Perfectlight PLS-SXE 300D, China). The system was
controlled by an electrochemical workstation (Chenhua CHI760E,
China), and the scan rate was 50 mV/s. The fabricated photoanode
2
2
(
7.5 cm ), a pure graphite plate (3 × 5 cm ), and saturated calomel
electrode (SCE) were used as the working electrode, counter
electrode, and reference electrode, respectively. The measured
potentials were converted to V vs RHE (ERHE = ESCE + 0.241 V +
Authors
0
.059 V × pH). The distance between the working electrode and the
Rui Song − Key Laboratory of Advanced Catalysis, Gansu
Province, State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, Gansu 730000, China
Haibo Chi − School of Chemical and Materials Science,
University of Science and Technology of China, Hefei, Anhui
counter electrode is 1.5 cm, and the light source is 10 cm away from
the quartz cell. The practical intensity of the light source is 200 mW/
cm .
2
In a typical degradation experiment, electrodes were immersed into
the above solution, and the solution was stirred for 30 min to establish
the adsorption−desorption equilibrium. At each time intervals, 0.7
mL of the reaction solution was withdrawn and analyzed
subsequently. The potential of the degradation reaction is 2.22
VRHE. For the cycling tests, the catalyst was recycled after each run of
the experiment by washing thoroughly with ethanol and deionized
water.
2
30026, China; State Key Laboratory of Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian National Laboratory for Clean Energy, The
Collaborative Innovation Center of Chemistry for Energy
Materials (iChEM), Dalian 116023, China
The concentration of 4-CP was analyzed by high-performance
liquid chromatography (HPLC, Aglient, 1260-Infinity) with a
Poroshell 120 EC-C18 column and detected at a wavelength of 280
nm using a VWD detector. The mobile phase was a mixture of
acetonitrile/water (30:70, v/v) at a flow rate of 1 mL/min. The
concentration of intermediates was analyzed at dual detection
wavelengths of 280 and 254 nm with a methanol/water mixture
Qiuling Ma − Key Laboratory of Advanced Catalysis, Gansu
Province, State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, Gansu 730000, China
Dongfeng Li − State Key Laboratory of Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian National Laboratory for Clean Energy, The
Collaborative Innovation Center of Chemistry for Energy
Materials (iChEM), Dalian 116023, China; University of
Chinese Academy of Sciences, Beijing 100049, China
Xiaomei Wang − Key Laboratory of Advanced Catalysis,
Gansu Province, State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, Gansu 730000, China
Wensheng Gao − Key Laboratory of Advanced Catalysis,
Gansu Province, State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, Gansu 730000, China;
orcid.org/0000-0002-8700-3831
Hao Wang − Key Laboratory of Advanced Catalysis, Gansu
Province, State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou, Gansu 730000, China
Xiuli Wang − State Key Laboratory of Catalysis, Dalian
Institute of Chemical Physics, Chinese Academy of Sciences,
Dalian National Laboratory for Clean Energy, The
Collaborative Innovation Center of Chemistry for Energy
Materials (iChEM), Dalian 116023, China
(
50:50, v/v) as the mobile phase. Equipped with the above
chromatographic column, the combination of HPLC (UltiMate
000, ThermoFisher Scientific) and mass spectrometry (QExactive,
3
ThermoFisher Scientific) was used to validate the intermediates. The
concentration of total organic carbon was measured in a TOC
analyzer (Shimadzu TOC-L). The concentration of generated
chloride ion was quantified by ion chromatography (YC 7000).
Computational Fluid Dynamics Simulation. The flow volume
fraction and velocity distribution in the electrodes were modeled by
laminar equations and mixture models in CFD. For the theoretical
models, the diameter of conical structures and rod arrays is consistent
with the SEM results, and the distance between two neighboring
nanocones or nanorods is 1.0 nm, respectively. Considering the water
flow for the mesh electrode, the direction of water flow was set
parallel to the substrate and perpendicular to it. To be consistent with
the size of catalysts, the magnitude of the velocity was set to 1 μm/s.
ASSOCIATED CONTENT
sı Supporting Information
■
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c05008.
Detailed experiments section, characterization methods,
and additional PEC test methods (PDF)
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c05008
AUTHOR INFORMATION
■
Author Contributions
Corresponding Authors
#R.S. and H.B.C. contributed equally.
Zelong Li − Key Laboratory of Advanced Catalysis, Gansu
Province, State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Notes
The authors declare no competing financial interest.
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(
20) Liu, C.; Zhang, A. Y.; Si, Y.; Pei, D. N.; Yu, H. Q.
ACKNOWLEDGMENTS
■
Photochemical Anti-Fouling Approach for Electrochemical Pollutant
Degradation on Facet-Tailored TiO2 Single Crystals. Environ. Sci.
Technol. 2017, 51, 11326−11335.
This work was supported by the Fundamental Research Funds
for the Central Universities (Lzujbky-2019-60).
(
21) Lin, C. F.; Wu, C. H.; Onn, Z. N. Degradation of 4-
chlorophenol in TiO2, WO3, SnO2, TiO2/WO3 and TiO2/SnO2
REFERENCES
■
systems. J. Hazard. Mater. 2008, 154, 1033−1039.
(
1) Bai, J.; Wang, R.; Li, Y.; Tang, Y.; Zeng, Q.; Xia, L.; Li, X.; Li, J.;
(22) Xu, J.; Zheng, X.; Feng, Z.; Lu, Z.; Zhang, Z.; Huang, W.; Li, Y.;
Li, C.; Zhou, B. A solar light driven dual photoelectrode photo-
catalytic fuel cell (PFC) for simultaneous wastewater treatment and
electricity generation. J. Hazard. Mater. 2016, 311, 51−62.
Vuckovic, D.; Li, Y.; Dai, S.; Chen, G.; Wang, K.; Wang, H.; Chen, J.
K.; Mitch, W.; Cui, Y. Organic wastewater treatment by a single-atom
catalyst and electrolytically produced H2O2. Nat. Sustain. 2021, 4,
(
2) Du, Y.; Zhou, M.; Lei, L. Kinetic model of 4-CP degradation by
2
(
33−241.
Fenton/O2 system. Water Res. 2007, 41, 1121−1133.
3) Gimeno, O.; Carbajo, M.; Beltran, F. J.; Rivas, F. J. Phenol and
substituted phenols AOPs remediation. J. Hazard. Mater. 2005, 119,
9−108.
4) Wang, N.; Li, X.; Wang, Y.; Quan, X.; Chen, G. Evaluation of
23) Li, X.; Huang, X.; Xi, S.; Miao, S.; Ding, J.; Cai, W.; Liu, S.;
(
Yang, X.; Yang, H.; Gao, J.; Wang, J.; Huang, Y.; Zhang, T.; Liu, B.
Single Cobalt Atoms Anchored on Porous N-Doped Graphene with
Dual Reaction Sites for Efficient Fenton-like Catalysis. J. Am. Chem.
Soc. 2018, 140, 12469−12475.
9
(
bias potential enhanced photocatalytic degradation of 4-chlorophenol
with TiO2 nanotube fabricated by anodic oxidation method. Chem.
Eng. J. 2009, 146, 30−35.
(24) Tang, H.; Shang, Q.; Tang, Y.; Yi, X.; Wei, Y.; Yin, K.; Liu, M.;
Liu, C. Static and continuous flow photoelectrocatalytic treatment of
antibiotic wastewater over mesh of TiO2 nanotubes implanted with g-
C3N4 nanosheets. J. Hazard. Mater. 2020, 384, 121248.
(
5) Oudjehani, K.; Boule, P. Photoreactivity of 4-chlorophenol in
aqueous solution. J. Photochem. Photobiol., A 1992, 68, 363−373.
6) Fu, T.; Gong, X.; Guo, J.; Yang, Z.; Liu, Y. Zn-CNTs-Cu
catalytic in-situ generation of H2O2 for efficient catalytic wet
peroxide oxidation of high-concentration 4-chlorophenol. J. Hazard.
Mater. 2021, 401, 123392.
7) Chen, H. M.; Chen, C. K.; Liu, R. S.; Zhang, L.; Zhang, J.;
Wilkinson, D. P. Nano-architecture and material designs for water
splitting photoelectrodes. Chem. Soc. Rev. 2012, 41, 5654−5671.
8) Hahn, N. T.; Ye, H.; Flaherty, D. W.; Bard, A. J.; Mullins, C. B.
(25) Zheng, J.; Wang, Z.; Ma, J.; Xu, S.; Wu, Z. Development of an
(
Electrochemical Ceramic Membrane Filtration System for Efficient
Contaminant Removal from Waters. Environ. Sci. Technol. 2018, 52,
4
(
117−4126.
26) Wang, L.; Lan, X.; Peng, W.; Wang, Z. Uncertainty and
(
misinterpretation over identification, quantification and transforma-
tion of reactive species generated in catalytic oxidation processes: A
review. J. Hazard. Mater. 2021, 408, 124436.
(
(27) Scheinok, S.; Leveque, P.; Sonveaux, P.; Driesschaert, B.;
Deposition of α-Fe2O3 Thin Films for Photoelectrochemical Water
Oxidation. ACS Nano 2010, 4, 1977−1986.
9) Wang, H.; Liang, Y.; Liu, L.; Hu, J.; Cui, W. Highly ordered
Gallez, B. Comparison of different methods for measuring the
superoxide radical by EPR spectroscopy in buffer, cell lysates and
cells. Free Radical Res. 2018, 52, 1182−1196.
(
TiO nanotube arrays wrapped with g-C N nanoparticles for efficient
2
3
4
(28) Wang, Y.; Hong, C.-S. Effect of hydrogen peroxide, periodate
charge separation and increased photoelectrocatalytic degradation of
phenol. J. Hazard. Mater. 2018, 344, 369−380.
10) Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y.; Zhang,
J. Z. Photoelectrochemical water splitting using dense and aligned
TiO2 nanorod arrays. Small 2009, 5, 104−111.
11) Zhong, D.; Cai, B.; Wang, X.; Yang, Z.; Xing, Y.; Miao, S.;
and persulfate on photocatalysis of 2-chlorobiphenyl in aqueous TiO2
suspensions. Water Res. 1999, 33, 2031−2036.
(
(29) Theurich, J.; Lindner, M.; Bahnemann, D. W. Photocatalytic
Degradation of 4-Chlorophenol in Aerated Aqueous Titanium
Dioxide Suspensions: A Kinetic and Mechanistic Study. Langmuir
(
1
(
996, 12, 6368−6376.
Zhang, W.-H.; Li, C. Synthesis of oriented TiO2 nanocones with fast
charge transfer for perovskite solar cells. Nano Energy 2015, 11, 409−
30) Xu, L.; Wang, J. Magnetic nanoscaled Fe3O4/CeO2 composite
as an efficient Fenton-like heterogeneous catalyst for degradation of 4-
4
18.
chlorophenol. Environ. Sci. Technol. 2012, 46, 10145−10153.
(
12) Zhang, P.; Gao, L.; Song, X.; Sun, J. Micro- and nanostructures
of photoelectrodes for solar-driven water splitting. Adv. Mater. 2015,
7, 562−568.
13) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W.
Environmental Applications of Semiconductor Photocatalysis. Chem.
Rev. 1995, 95, 69−96.
14) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi,
(31) Xu, M.; Chen, Y.; Qin, J.; Feng, Y.; Li, W.; Chen, W.; Zhu, J.;
Li, H.; Bian, Z. Unveiling the Role of Defects on Oxygen Activation
and Photodegradation of Organic Pollutants. Environ. Sci. Technol.
2
(
2
(
018, 52, 13879−13886.
32) Lukes, P.; Locke, B. R. Degradation of Substituted Phenols in a
Hybrid Gas−Liquid Electrical Discharge Reactor. Ind. Eng. Chem. Res.
(
Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 photo-
2005, 44, 2921−2930.
(33) Kang, Q.; Cao, J.; Zhang, Y.; Liu, L.; Xu, H.; Ye, J. Reduced
catalysis: mechanisms and materials. Chem. Rev. 2014, 114, 9919−
9
(
986.
TiO2 nanotube arrays for photoelectrochemical water splitting. J.
Mater. Chem. A 2013, 1, 5766−5774.
15) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.;
Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem.
Rev. 2010, 110, 6446−6473.
(34) Qi, X.; She, G.; Huang, X.; Zhang, T.; Wang, H.; Mu, L.; Shi,
W. High-performance n-Si/alpha-Fe2O3 core/shell nanowire array
photoanode towards photoelectrochemical water splitting. Nanoscale
2014, 6, 3182−3189.
(
16) Huang, L.; Li, D.; Liu, J.; Yang, L.; Dai, C.; Ren, N.; Feng, Y.
Construction of TiO2 nanotube clusters on Ti mesh for immobilizing
Sb-SnO2 to boost electrocatalytic phenol degradation. J. Hazard.
Mater. 2020, 393, 122329.
(35) Parker, R. A. Static Dielectric Constant of Rutile (TiO2), 1.6−
1060 K. Phys. Rev. 1961, 124, 1719−1722.
(
17) Hankin, A.; Bedoya-Lora, F. E.; Ong, C. K.; Alexander, J. C.;
(36) Ohno, T.; Tokieda, K.; Higashida, S.; Matsumura, M.
Synergism between rutile and anatase TiO2 particles in photocatalytic
oxidation of naphthalene. Appl. Catal., A 2003, 244, 383−391.
(37) Zhang, Z.; Yates, J. T., Jr. Band bending in semiconductors:
chemical and physical consequences at surfaces and interfaces. Chem.
Rev. 2012, 112, 5520−5551.
(38) Smith, W. A.; Sharp, I. D.; Strandwitz, N. C.; Bisquert, J.
Interfacial band-edge energetics for solar fuels production. Energy
Environ. Sci. 2015, 8, 2851−2862.
Petter, F.; Kelsall, G. H. From millimetres to metres: the critical role
of current density distributions in photo-electrochemical reactor
design. Energy Environ. Sci. 2017, 10, 346−360.
(
18) Zhang, J.; Li, M.; Feng, Z.; Chen, J.; Li, C. UV Raman
Spectroscopic Study on TiO2. I. Phase Transformation at the Surface
and in the Bulk. J. Phys. Chem. B 2006, 110, 927−935.
(
19) Ohsaka, T.; Izumi, F.; Fujiki, Y. Raman spectrum of anatase,
TiO2. J. Raman Spectrosc. 1978, 7, 321−324.
1
3673
https://doi.org/10.1021/jacs.1c05008
J. Am. Chem. Soc. 2021, 143, 13664−13674

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
(
39) Dozzi, M. V.; D’Andrea, C.; Ohtani, B.; Valentini, G.; Selli, E.
Fluorine-Doped TiO2 Materials: Photocatalytic Activity vs Time-
Resolved Photoluminescence. J. Phys. Chem. C 2013, 117, 25586−
2
(
5595.
40) Kafizas, A.; Ma, Y.; Pastor, E.; Pendlebury, S. R.; Mesa, C.;
Francas, L.; Le Formal, F.; Noor, N.; Ling, M.; Sotelo-Vazquez, C.;
̀
Carmalt, C. J.; Parkin, I. P.; Durrant, J. R. Water Oxidation Kinetics of
Accumulated Holes on the Surface of a TiO2 Photoanode: A Rate
Law Analysis. ACS Catal. 2017, 7, 4896−4903.
(
41) Jing, L.; Zhou, J.; Durrant, J. R.; Tang, J.; Liu, D.; Fu, H.
Dynamics of photogenerated charges in the phosphate modified TiO2
and the enhanced activity for photoelectrochemical water splitting.
Energy Environ. Sci. 2012, 5, 6552−6558.
(
42) Corby, S.; Francas, L.; Selim, S.; Sachs, M.; Blackman, C.;
Kafizas, A.; Durrant, J. R. Water Oxidation and Electron Extraction
Kinetics in Nanostructured Tungsten Trioxide Photoanodes. J. Am.
Chem. Soc. 2018, 140, 16168−16177.
(
43) Dotan, H.; Sivula, K.; Grätzel, M.; Rothschild, A.; Warren, S. C.
Probing the photoelectrochemical properties of hematite (α-Fe2O3)
electrodes using hydrogen peroxide as a hole scavenger. Energy
Environ. Sci. 2011, 4, 958−964.
(
44) Nosaka, Y.; Nosaka, A. Y. Generation and Detection of
Reactive Oxygen Species in Photocatalysis. Chem. Rev. 2017, 117,
1302−11336.
45) Wu, J. M.; Sun, Y.-G.; Chang, W.-E.; Lee, J.-T. Piezoelectricity
1
(
induced water splitting and formation of hydroxyl radical from active
edge sites of MoS2 nanoflowers. Nano Energy 2018, 46, 372−382.
1
3674
https://doi.org/10.1021/jacs.1c05008
J. Am. Chem. Soc. 2021, 143, 13664−13674