Thermodynamic and dynamic dual regulation Bi2O2CO3/Bi5O7I enabling high-flux photogenerated charge migration for enhanced visible-light-driven photocatalysis

Article abstract of DOI:10.1039/d0ta02588g

Herein, the essence of high-flux photogenerated charge migration for Bi₂O₂CO₃/Bi₅O₇I heterojunction photocatalysts is revealed from both thermodynamic and dynamic perspectives. The appropriate energy band offset provides the thermodynamic conditions of photogenerated electrons and holes migration in opposite directions, which is derived from the lower valence and conduction band positions of Bi₅O₇I than Bi₂O₂CO₃. Meanwhile, the dynamic conditions are provided by the interface electric field generated via the work function difference between Bi₅O₇I and Bi₂O₂CO₃, which affords a driving force for the above-mentioned reverse migration of photogenerated electrons and holes. Moreover, the Bi-O chemical bond on the interface creates a charge transport bridge between Bi₅O₇I and Bi₂O₂CO₃. By benefiting from the dual regulation of thermodynamics and dynamics and the charge transport bridge, the photocatalytic activity of the Bi₂O₂CO₃/Bi₅O₇I heterojunction photocatalyst has significantly increased at least 23 times under visible light irradiation. This finding could also be applied to other fields to tune the optimal charge transport performance of heterojunctions.

Full text of DOI:10.1039/d0ta02588g

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March 2018
Pages 4883-5230
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DOI: 10.1039/D0TA02588G
ARTICLE
Thermodynamic and Dynamic Dual Regulation Bi O CO / Bi O I
2
2
3
5
7
Enabling High-flux Photogenerated Charge Migration for
Enhanced Visible-light-driven Photocatalysis
Received 00th January 20xx,
Accepted 00th January 20xx
Yan Guo, Jun Nan , Yongpeng Xu , Fuyi Cui , Wenxin Shi, * and Yongfa Zhu*c
a
a
a
b
a,b
DOI: 10.1039/x0xx00000x
2 2 3 5 7
Herein, the essence of high-flux photogenerated charge migration for Bi O CO /Bi O I heterojunction photocatalysts is
revealed from both thermodynamic and dynamic perspectives. The appropriate energy band offset provides the
thermodynamic condition of photogenerated electrons and holes migration in the opposite direction, which is derived from
the lower valence and conduction band positions of Bi
by the interface electric field generated via the work function difference between Bi
force for the above-mentioned reverse migration of photogenerated electrons and holes. Moreover, the Bi-O chemical bond
on the interface construct the charge transport bridge between Bi I and Bi CO . Benefit from the dual regulation of
thermodynamics and dynamics and the charge transport bridge, the photocatalytic activity of Bi CO /Bi
5
O
7
I than Bi
2
O
2
CO
3
. Meanwhile, the dynamic condition are supplied
5
O
7
I and Bi CO , which afford driving
2
O
2
3
5
O
7
2
O
2
3
2
O
2
3
5 7
O I
heterojunction photocatalyst has significantly increased at least 23 times under visible light irradition. This finding could be
also applied to other fields to tune the optimal charge transport performance of the heterojunctions.
photogenerated charge transfer is lacking to be considered
further, that is, the dynamic condition is not used as a measure
of the heterojunction material. As we all know,
thermodynamics and dynamics are complementary and are
two indispensable branches for studying a reaction.
Therefore, there is an urgent need to achieve dual regulation
of heterojunction photocatalysts from the perspectives of
thermodynamics and dynamics.
More importantly, the charge transfer performance of
heterojunctions is limited by the migration of random charge
accumulation and depletion layer formation of different
materials, while a large number of migration channels for
interfacial charges are able to effectively reduce this
limitation. 10 The 2D-2D heterojunction connected by van der
Waals force is widely reported in the literature. 11, 12 However,
in view of Van der Waals force is an instantaneous induction
of electric dipole moment, it decreases rapidly with
increasing distance lead to the weak interface interaction
force. In contrast, the interface combined with chemical
bonds form a much stronger interaction force between the
atoms at the contact interface, accompanied by changes in
atomic electronic structure. 13, 14 Therefore, the interface
connected by chemical bonds provides stable migration
bridges for the continuous charge transfer and meanwhile
effectively avoid the random accumulation of charges. 15, 16
Zhang's group found that the photocatalytic degradation of
contaminants can act as carbon source lead to one-unit-cell
Introduction
The separation and migration of photogenerated carriers are
a key factor determining the photocatalytic performance. 1-3
Semiconductor heterojunction is an effective way to promote
4
the separation of photogenerated charges. Type II junctions
have been widely investigated as an effective strategy for
photocatalysis applications due to their interface interaction,
allowing the directional electron transport. 5, 6 However, the
ambiguity of the effect of interface interaction on
photoinduced charge transfer severely limits the further
design and development of the heterojunction photocatalyst.
Most of the work reported in the literature on heterojunction
photocatalysts elaborates the migration direction of
photogenerated charges at the interface from the perspective
of band position matching. There have also been more or less
reports that the interfacial electric field improve the
separation effect of photogenerated electron-hole pairs but
still lack of in-depth research to quantify the built-in electric
field. 7-9 In other words, the adjustment of photogenerated
charge by heterojunction has been explained the possibility of
this phenomenon from the perspective of thermodynamics.
However, the influence of the driving force on the speed of
a. State Key Laboratory of Urban Water Resource and Environment, School of
Environment, Harbin Institute of Technology, Harbin 150090, China. E-mail:
swx@hit.edu.cn
School of Environment and Ecology, Chongqing University, Chongqing, 400044,
b.
Bi
photocatalyst during the progress of photodegradation, and
finally forming a heterojunction Bi /Bi CO
group constructed Bi Ti /γ-Bi by in-situ fabrication
where the γ-Bi as a support and tertbutyl titanite as
2 2 3 2 4
O CO layers in-situ grow from the surface of Bi O
China
c.
Department of Chemistry, Tsinghua University, Beijing, 100084, China. E-mail:
. 17 Li' s
3
2
O
4
2
O
2
zhuyf@tsinghua.edu.cn
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
2
2
O
7
2 3
O
2 3
O
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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titanium source. The co-sharing of Bi-O structure help to
ARTICLE
stirring 30min, followed by addition of 5ml deiVoienwizAretidclewOanltiener
DOI: 10.1039/D0TA02588G
dissolved with 1mmol KI and 0.24g KOH. Then the resulting
1
8
fabricate a tightly connected interface. Hou's group reported
the synthesis of mesoporous heterostructures In –x/In
through the in-situ oxidation of In atomic layers by an
2
O
3
S
2 3
solution was transferred to a 100 ml polytetrafluoroethylene-
lined autoclave and heated at 160°C for 6 h. The final product
was washed 6 times with deionized water and absolute
ethanol and dried at 60 °C. Marked as: BOC/BOI-1,
BOC/BOI-2 and BOC/BOI-3 (where 1, 2, 3 respectively
represent the ratio of glycerol in the solvent is 0.2, 0.5, 0.9).
For the synthesis of Bi O CO , 25 ml of glycerol and 25 ml
2 2 3
of water were used as the solvent, no KI was added, and the
rest were the same as above.
2 3
S
oxygen plasma-induced strategy.19 Moreover, they also
emphasis on an in-situ phase-induced etching chemical
strategy to prepare Ta N @NaTaON heterojunction with the
3 5
strong interfacial Ta-O-N bonding connection. 10Without
exception, these heterojunctions are formed by producing
another substance in situ on one substance. Inspired by above
mentioned, we employed in-situ conversion of Bi
fabricate Bi I / Bi CO for the following reasons: 1) In-
situ conversion is conducive to build an interface contacted
with Bi-O bonds between Bi I and Bi CO , which is
5 7
O I to
5
O
7
2
O
2
3
5 7
For the synthesis of Bi O I, 50 ml of water was used as a
solvent, and the rest were the same as above.
5
O
7
2
O
2
3
expected to provide a smooth charge separation pathway; 2)
It is easy to realize the controllable preparation of the
structure through adjusting the reaction conditions of in-situ
conversion; 3) Due to the Bi source control, the interface
Sample characterization
Powder X-ray diffraction (XRD) patterns of the samples were
obtained on a Bruker D8 X-ray diffractometer with Cu Ka
radiation (λ=0.15418 nm). The morphologies of the samples
were survived by SEM on ZEISS Sigma 500, TEM on JEOL
JEM-1400 TEM at an accelerating voltage of 100 kV and
HRTEM on Tecnai F20 operated at an accelerating voltage of
5 7 2 2 3
between Bi O I and Bi O CO is effectively regulated
meanwhile forming nanostructure; 4) Carbonate is derived
from the oxidation of organic matter during the hydrothermal
reaction, which is beneficial to control the thickness and scale
2
2
00 kV. XPS were performed using a Thermo ESCALAB
50 with Al Kα X-ray (hν=1486.6 eV) radiation. UV-Vis
of Bi
Under the guidance of this mentality and based on our
previous research on bismuth oxyiodide, Bi I (BOI), which
has the most photocatalytic potential in bismuth oxyiodide, 20
and Bi CO (BOC), which has regular layered structure, is
2 2 3
O CO .
diffuse reflection spectra (DRS) were obtained on a
spectrophotometer (Shimadzu, UV2550) with BaSO4 as
reference. The BET surface area measurements were
recorded on an ASAP2020 instrument. The steady-state
photoluminescence (PL) spectra was detected by using a
Perkline LS55 fluorescence spectrophotometer with λex=250
nm. The surface photovoltage (SPV) spectra were recorded
with a home-built apparatus, equipped with a lock-in
amplifier (SR830, USA) synchronized with a light chopper
5
O
7
2
O
2
3
selected to form an interface. This work prepared BOC/BOI
type Ⅱ heterojunction photocatalyst by means of in-situ
conversion via a simple one-pot hydrothermal method. The
Bi-O chemical bond at the heterojunction interface provides
paths for the continuous transmission of photogenerated
charges, thereby realizing high-flux photogeneration charge
transfer. More importantly, from the perspective of
thermodynamics, the possibility of the direction of
photogenerated charge transfer is explained by the theory of
interfacial band offset. Further from the perspective of
dynamics, the interfacial electric field provides the driving
force for high-speed migration of photogenerated charges.
(SR540, USA). For transient-state surface photovoltage (TS-
SPV) responses of the samples, the samples were excited by
a radiation pulse of 355 nm with 10 ns width from the second
harmonic of a neodymium-doped yttrium aluminum garnet
(Nd:YAG) laser (Lab-130-10H, Newport, Co.) and the
signals were amplified with a pre-amplier and registered with
a 1GHz digital phosphor oscilloscope (DPO 4104B,
Tektronix).
Photoelectrochemical measurements were performed on a
CHI-660 E (China) electrochemical workstation with a
standard three-electrode cell, including counter electrode (Pt
wire), reference electrode (saturated calomel electrode) and
working electrodes (as-prepared samples covered ITO glass).
Thus, the nature of Bi
separation with minimal recombination was revealed. As a
direct envidence, the photocatalytic activity of Bi CO
Bi I heterojunction has significantly increased at least 23
2 2 3 5 7
O CO /Bi O I achieved efficient
2
O
2
3
/
5
O
7
times under visible light.
2 4
Na SO (0.1 M) was used as electrolyte solution. For the
Experimental
Sample preparation
working electrodes, a sample (2 mg) was dispersed in Nafion
solution (Naphthol: ethanol=1:9) to obtain a slurry. Then the
slurry was coated onto the ITO glass and dried in an oven
overnight. Electrochemical impedance spectroscopy spectra
were recorded under an AC perturbation signal of 10 mV over
the frequency range from 100 KHz to 1 Hz.
2 2 3 5 7
In this paper, different ratios of Bi O CO / Bi O I
(BOC/BOI) heterojunctions were prepared by adjusting the
ratio of glycerol as a carbon source. First, BOI is formed at
room temperature, then glycerol is oxidized to carbonate in a
solvothermal reaction, and BOI is converted in situ to BOC.
Photocatalytic performance evaluation
For Bi
Bi(NO
2
O
2
CO
·5 H
3
/ Bi
5 7
O I heterojunctions, 0.486 g (1 mmol) of
)
3 3
2
O was dissolved in 40, 25 or 5 ml of deionized
The photocatalytic activity was estimated by measuring the
degradation of Bisphenol A (BPA), 4-chlorophenol (4-CP),
water, 10, 25 or 45 ml of glycerol was added with vigorous
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,4-dichlorophenol (2,4-DCP) and phenol. In a static
ARTICLE
2
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DOI: 10.1039/D0TA02588G
photocatalytic system, 50 mg of samples was dispersed in 100
mL of BPA (4-CP, 2,4-DCP and phenol) solution with 10
ppm under λ≥420 nm irradiation. Before the light irradiation,
the mixture was stirred for 30 min to reach the adsorption
equilibrium in the dark. The temperature of the reactor was
maintained at 20°C by continuous circulating water. The
continuous flow reaction was conducted in a fixed bed reactor
with 50 mg of BOC/BOI-2 photocatalyst loaded in the groove
(40 mm × 20 mm × 2 mm) and the two opposite sides serve
as import and export. A 300 W xenon lamp equip with λ≥ 420
nm cut-off filter were placed above the reactor as the light
source. The 10 mg / L BPA solution was fed by a peristaltic
pump with a constant flow rate of 1 ml / min. At given
irradiation time intervals, about 2 mL of the solution was
taken out to analyse the residual contaminant concentration.
The concentration was detected by the HPLC system (Waters
Baseline 810) with a Waters 486 tunable UV absorbance
detector and a Supelco LC-18-DB column (250mm×4.6mm).
Total organic carbon (TOC) analyser (Multi N/C 2100s) was
employed for mineralization degree analysis of pollutants.
Figure 1 Schematic thermodynamic condition: ΔEc (ΔEv) (a), dynamic condition: qV barrier
(b) and dual regulation of thermodynamic and dynamic for BOC/BOI heterojunction (c).
Theoretical calculations
All theoretical calculations were performed through the
generalized gradient approximation (GGA) within the
Perdew−Burke−Emzerhof (PBE) exchange-correlation
functional implemented in the CASTEP code. The ultrasoft
pseudopential in the Vanderbilt form was used for description
of ion-electron interaction. The plane-wave function was set
with the cut-off energy of 420 eV. Geometry optimizations
were done before single point energy calculation with the
self-consistent convergence accuracy of 1×10-5 eV atom-1.
The convergence criterion for the maximal force between
atoms is 0.03 eV Å-1. The maximum displacement is 1×10-3
Å, and the stress is less than 0.05 GPa.
that the positions of the VB and the CB of BOC are higher
than those of BOI. Therefore, the thermodynamic conditions
provided by the energy band offset enable holes to migrate
from BOI to BOC and converse for electrons.
According to the work function difference, when BOC and
BOI are in contact, the electron would flow from the BOI with
low work function (2.4 eV) to the BOC with high work
function (2.6 eV) until the Fermi energy level balanced (Fig.
S3). 18, 23 A depletion layer and an accumulation layer of
electrons are respectively formed at BOI and BOC
meanwhile, and an energy barrier (qVbarrier) about 0.2 eV is
generated, thereby forming band bend and interfacial electric
field (IEF) directed from the BOI to the BOC (Fig. 1b). 24-26
It is well known that the electric field influences the migration
of interface charges. Electric field force accelerates the
forward electric field movement of positive charges and the
reverse electric field movement of negative charges.
Fortunately, the direction of the IEF at the BOC/BOI interface
is the same (opposite) to the direction of movement of the
photogenerated holes (electrons) determined by the above
thermodynamic condition. In fact, for BOC/BOI, it can be
predicted that the dynamic result must accelerate the direction
Results and Discussion
Dual regulation of thermodynamic and dynamic for
BOC/BOI heterojunction with charge transfer bridges
Firstly, first-principle calculations were conducted to
examine the thermodynamic and dynamic conditions for
BOC/BOI heterojunction. The DFT calculations show that
the inconsistent width of the forbidden band between BOC
and BOI (Fig. S1), which means there exits band offset at the
interface. 21The relative positions of the energy bands of the
two semiconductors determine the migration direction of the
photogenerated electrons and holes at the interface, that is, the
thermodynamic condition. To clear the direction of the
interface charge migration determined by the band offset, the
energy band alignment structure is extracted by experimental
of
photogenerated
charge
that
determined
by
thermodynamics. This is because the direction of the IEF
must be from a material with a small work function to a large
work function. According to the above DFT calculation, it is
known that the work function of BOI is smaller than BOC.
The thermodynamic conditions provided by the energy band
position allow photogenerated holes to be transferred from
BOI to BOC. It cannot be neglected that the IEF directed from
BOI to BOC provides a driving force for the transfer motion
of photogenerated charges to achieve a dynamic acceleration
process. The photogenerated holes of BOI arrive the VB of
BOC and then migrate to the surface of BOC. The motion
characterization (Fig. S2a, b, c). The CB offset (ΔE
produced at the bottom of the CB is +0.12 eV and VB offset
ΔE ) generated at the bottom of the VB is +0.37 eV (Fig. 1a
c
)
(
v
&
Fig. S2d, the specific calculation see support information).
The calculation of the energy band step directly indicates
2
2
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DOI: 10.1039/D0TA02588G
Figure 2 (a) Schematic illustration of the preparation of BOC/BOI-2. (b) TEM (c) HRTEM, (d) AFM image (e) and the corresponding height profile of BOC/BOI-2.
direction of photogenerated electrons is opposite. In this
process, thanks to the kinetic acceleration process offered by
IEF, the photogenerated electron-hole pairs are effectively
separated. Eventually, photogenerated holes and electrons
arrived at the surfaces of BOC and BOI participate in
oxidation and reduction reactions, respectively. At this point,
the thermodynamic condition indicates the possibility of
directional migration of photogenerated charges at the
interface, and the dynamic condition demonstrates this
movement is an accelerated process. As a result, BOC / BOI
achieves both thermodynamic and dynamic regulation--
depletion at the interface (Fig. S7). The bond levels further
illustrate the electron coupling originates from the presence
of Bi-O bonds at interface (see Table S2 for specific analysis).
These theoretical results confirm the constructing carrier
migration channels of Bi-O bond is a candidate for weak
recombination effect.
In-situ conversion to prepare BOC/BOI heterojunction
Motivated by these theoretical calculations, we synthesized
BOC/BOI heterojunction by one-pot in-situ conversion.
XRD, FT-IR, XPS were employed to prove heterojunctions
successfully obtained by in-situ conversion (see Fig. S8 for
specific analysis). Fig. 2a displays the in-situ conversion
process of BOC on BOI by replacement reaction. Firstly,
v c
qVbarrier + ΔE (ΔE ) (Fig. 1c).
The interface effect plays an important role during the
photogenerated charge transfer process. 27-30 Because the
interface with no tight connection is not support to the charge
transmission at the interface. Providing efficient transport
pathways has been applied to maximize the efficiency of
photogenerated charge utilization. 19,31 Compared with pure
material, the partial projected density of states (PDOS) are
higher coincidence for heterojunction (Fig. S4), which shows
their electronic states highly hybrid and means more likely of
charge transfer at the interface. 32 To reveal the presence of
Bi-O chemical bonds between BOC and BOI, the static
charge changes of the interface atoms were analysed firstly.
Compared to pure substances, the Bi static charge at the
interface is significantly changed, which indicates that charge
transfer after the formation of heterojunction, resulting in
strong Bi-O electron coupling (see Fig. S5 & Table S1 for
specific analysis). Moreover, taking the charge density map
containing Bi1-O1 as an example (Fig. S6), the electron
density at the interface is strong around the O1 of the BOI,
but weak for the Bi1 of the BOC, which make clear close
electronic coupling between BOI and BOI. Then, after
avoiding the error of the atomic static charge value inherent
and express the net change in the spatial distribution of
charge, the differential charge density difference also
suggested that Bi-O has obvious electron accumulation and
thick board BOI produced at room temperature (Fig. S9a and
2-
Fig. S10a), then CO
3
favorably bind with the surface of BOI
under the electrostatic force during the hydrothermal reaction,
next BOI surface begin to transform into BOC piece
2-
meanwhile eroded into block itself. Enough CO
3
continue to
corrode the surface of the BOI to nanorods, and BOC become
thinner into two-dimensional (2D) nanosheets in subsequent
reactions. Finally, a nano heterojunction BOC/BOI-2 with
large contact area was obtained (Fig. S9c and Fig. S10c).
However, too little carbon source causes the reaction to stay
in step 3, forming a micron-sized heterojunction (Fig. S9b
and Fig. S10b) while excessive carbon source resulting
serious agglomeration (Fig. S9d and Fig. S10d). The Fig. 2b
illustration shows the two-phase boundary of BOC/BOI-2
and a large contact area benefiting from the flakes and the
small size of the nanorods. Further, the lattice fringes of 0.273
nm and 0.283 nm correspond to the (0 10 0) crystal plane of
BOC and (0 0 4) of BOI, respectively (Fig. 2c). AFM shows
that the thickness of these sheets is ~5 nm (Fig. 2d-e), which
directly confirm that BOC is a 2D sheet. Furthermore,
electron paramagnetic resonance (EPR) has been performed
on the prepared samples (Fig. 3a), which is a sensitive and
direct technique for monitoring the behavior of material
surface defects. The peaks at g = ~ 2.1 33,34 and g = ~ 2.0 35
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represent bismuth defects and oxygen defects, respectively.
Interestingly, compared to other synthetic samples,
The realization of BOC/BOI-2 allow to applyVieXwPArSticlteeOsntilifnye
DOI: 10.1039/D0TA02588G
DFT about Bi-O bonds at interface. Generally, the XPS
binding energy is negatively correlated with the surface
electron density. 38 The 288.3 eV and 285.7 eV characterizing
C = O and C - O in BOC increase to 288.7 eV and 285.9 eV,
3
9
which attributed to the strong interaction between BOC and
BOI affecting the electrons of C in CO 2
(Fig. 3d). The
-
3
binding energy of 530.7 eV in BOI belongs to Bi-O, which
shifted down to 530.6 eV after forming a heterojunction, 40
because Bi electron of BOC at the interface transfer to O atom
of BOI to form strong Bi-O chemical bonds (Fig. 3e).
Meanwhile, higher binding energies of 166.7 and 161.5 eV
appeared in BOC (Fig. 3f), which is also consistent with its
strongest Bi defect signal shown in Fig. 3a. As mentioned
earlier, theoretical calculations show that strong Bi-O
chemical bonds tightly connect BOC and BOI to form a
strong interaction interface. Experimental characterization
confirms that among the BOC/BOI heterojunctions prepared
by different carbon source dosing ratios, the interface owned
by BOC/BOI-2 is closest to the result of theoretical
settlement.
Dual regulation and charge transfer bridges promote
photogenerated charge separation
Figure 3 (a) ESR spectra, (b) UV–vis diffuse reflection spectra, (c) N2 adsorption–desorption
isotherms of as-prepared samples. High resolution XPS spectra of (d) C 1s, (e) O 1s (f) Bi 4f.
BOC/BOI-2 has the least bismuth defects and the EPR signal
of BOC/BOI-2 oxygen deficiency was hardly detected.
Because of the less defects of BOC/BOI-2, the more regular
of lattice, the local state proportion caused by defects in the
lattice is reduced. On the contrary, the degree of
delocalization is increased, and the degree of commonality is
increased. 36, 37 The results of energy band broadening,
reducing the band gap, and red-shifting the absorbed light are
obtained by BOC/BOI-2 (Fig. 3b). Note that the specific
surface area of nano heterojunction BOC/BOI-2 (up to 126.93
2
-1
2
-1
m g ) is significantly larger than that of BOI (4.62 m g )
2
-1
and BOC (27.51 m g ), which provide efficient diffusion
and transportation in degradation (Fig. 3c). Moreover, 2D
with high specific surface area helps to form a large contact
area at the interface, thereby making it possible for the
interface to dominate. Although BOC/BOI heterojunctions
can be prepared with different proportions of carbon source
dosing, there is an optimal amount of carbon source dosing to
make the prepared heterojunction a neat morphology and
larger contact interface. Considering that the morphology
affects the transfer of charges on the semiconductor, the large
contact interface determines the amount of separated
photogenerated charges, obviously BOC/BOI-2 has more
advantages.
Figure 4 (a) Electrochemical impedance spectroscopy (EIS) (b) photoluminescence
spectra (PL) at 250 nm excitation wavelength, (c) Photocurrent under λ≥420 nm, (d)
surface photovoltage irradiation (e) normalized open-circuit potential (OCP)
attenuation curves after turning off the λ≥420 nm irradiation for samples prepared
(f) the apparent reaction rate constants for BPA under visible light (λ≥420 nm)
irradiation.
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Next, the effect of dual regulation and bridges at the interface
on photogenerated charge separation were explored. The
charge transfer resistance of the photocatalyst is simulated by
using the equivalent circuit R(C(R(QR))) (CR) of Fig. 4a
according to the electrochemical impedance spectroscopy
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DOI: 10.1039/D0TA02588G
(
EIS). The analog charge transfer resistance of BOC/BOI-2
-2
-2
(724.9 Ω cm ) is far less than that of the BOI (1032 Ω cm )
-2
and BOC (2913 Ω cm ), exhibiting that the lots bridges
reduce the charge transfer resistance between the BOC and
BOI. The BOC/BOI-2 quenches the steady-state
photoluminescence (PL) intensity of BOI and BOC by 76.3%
and 67.2%, respectively (Fig. 4b), meaning the decrease of
the recombination of photogenerated electron-hole pairs. The
excellent
separation
effect
promotes
extensive
photogenerated charges to reach the surface of the
photocatalyst, which proved by strongest photocurrent
density (Fig. 4c) and SPV signal (Fig. 4d) of BOC/BOI-2.
Subsequently, the charge recombination efficiencies of
BOC/BOI-2 were quantitatively measured with the open
circuit potential (OCP) transients. Fig. 4e compares the
attenuation curve of OCP after turning off the light among as-
prepared samples without externally applied bias. The
average recombination constant is estimated by normalizing
the OCP signal and fitting the data to a first-order dynamic
model (Equation 1): 41, 42
퐸푡 ― 퐸푝ℎ
Figure
5
Proposed degradation pathways of BPA during BOC/BOI-2
=
1 ― 푒푥푝 ―푘푡
(1)
photodegradation under λ≥420 nm irradiation.
퐸푑푎 ― 퐸푝ℎ
Where E
t
is the OCP over time, Eda is the static OCP value in
recycling performance of BOC/BOI-2 heterojunction has
been shown in Fig. S14. Moreover, the EPR shows that ·O −
the dark, Eph is the steady OCP value under visible light
illumination, and k is the pseudo first order recombination
rate constant. The OCP decay dynamics reflect charge
recombination and show how the heterojunction alters
2
is the main active species, while hole has little effect on it,
and ·OH radical has no effect (Fig. S15).
In order to approximate the effect of photocatalysis applied to
the purification of actual water treatment project, besides
static tests, continuous flow experiments for BOC/BOI-2
photocatalytic degradation was carried. Phenol which is more
difficult to degrade was employed as a probe to flow through
the reaction system at 1ml/min. After continuous operation
for 45 hours, the removal rate of phenol retains about 70%
(Fig. S16), which not only shows that BOC/BOI-2 has
efficient photodegradation activity and once again proves that
the excellent stability for BOC/BOI-2.
To fully understand the process of BPA oxidative degradation
by photogenerated active species in the BOC/BOI-2 system
under λ≥420nm irradiation, the main degradation
intermediates identified with UPLC/MS/MS further the
possible degradation path was analysed. A total of 10
intermediate products were inferred during BOC/BOI-2
photodegradation BPA. The molecular structure and MS/MS
fragmentation information of the oxidation products are
displayed in Fig. S17. Based on these degradation
intermediates, the possible degradation pathway of BPA is
proposed in Fig. 5. Due to the two-electron donating hydroxyl
group increased the electron density of each benzene ring, the
C-C bond connecting the two benzene rings in BPA is more
vulnerable.43 The β-cleavage of the isopropyl group between
the two phenyl groups is generated, forming phenol (m/z=93),
-4
-1
recombination. The fitted k of BOC/BOI-2 (2.06×10 s ) is
reduced by 90% and 96% compared to BOI (2.01×10-3 s-1)
and BOC (3.16×10-3 s-1), respectively. The OCP decay is
markedly retarded significant for BOC/BOI-2 due to lots of
channel at the interface avoids the random accumulation and
the recombination of photogenerated charges. These results
powerfully suggest constructing dual regulation and generous
bridges at the interface is a promising approach to improve
photogenerated charge transfer by inhibiting the
recombination.
The photoactivity is highly correlated with the charge
separation efficiency, and the ameliorative photogenerated
charge transfer endowed BOC/BOI-2 with visible
photoactivity for degradation. The apparent rate constants of
BOC/BOI-2 with outstanding activity are 23, 17, 15, 60 times
that of BOI, BOC/BOI-1, BOC/BOI-3 and BOC (Fig. 4f &
S11).
Moreover,
BOC/BOI-2
shows
remarkable
photodegradation BPA activity under visible light compared
to some other reported photocatalysts as presented in Table
S5.Then, the results in Fig. S12 demonstrate the universality
of BOC/BOI-2 for photodegradation of organic pollutants.
Next, the TOC removal results show that BOC/BOI-2 has
acceptable salinity for different pollutants due to its nice
oxidizing ability (Fig. S13). In addition, the excellent
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J. Name., 2013, 00, 1-3 | 6
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Page 7 of 9
J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
Journal Name
ARTICLE
charges access to the surface of the photocataVlieywstA,rtitchleeOrenlbinye
DOI: 10.1039/D0TA02588G
obtaining excellent photodegradation activity (Fig. S19).
Conclusions
Through the interfacial band offset and the IEF, the dual
regulation of the thermodynamic and dynamic conditions of
the heterojunction is achieved, which promotes the high-
speed directional migration of photogenerated charges. And
BOC/BOI heterojunction photocatalyst was constructed with
Bi-O chemical bonds as interface charge transfer bridge for
high-flux charge migrate. The visible photocatalytic activity
of BOC/BOI heterojunction has increased above 23 times.
This strategy is applicable to the interface structure
photocatalysts for high performances.
Figure
6 (a) The Time-resolved fluorescence spectrum (b) The lifetime of
photogenerated holes detected by time-resolved SPV of the as-prepared samples.
4
-isopropylphenol (m/z=133) and 1-(4-hydroxyphenyl)-2-
propanol (m/z=151). Phenol is generally oxidized to
hydroquinone (m/z=109) further oxidized to p-benzoquinone
(
m/z=107) that cannot be detected as not easily ionized.44
Meanwhile the active oxygen component further oxidizes 4-
isopropylphenol to 4-hydroxycetophenone (m/z=135). Then,
the intermediate products were subsequently attacked
yielding small organic acids such as maleic acid (m/z=115),
glycolic acid (m/z=75) acetic acid (m/z=59) and 2-
gydroxypropionic acid (m/z=89), which were eventually
mineralized.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was partly supported by the National Natural
Science Foundation of China (No. 51978098, 21872077,
Dual regulation and charge transfer bridges extend
photogenerated charge lifetime
21673126, 21761142017, 21621003) and Collaborative
Innovation Center for Regional Environmental Quality.
Inspired by the better photogenerated charge transfer and
remarkable photodegradation achieved via the dual regulation
and charge transfer bridges at the interface, the carrier
lifetime was investigated. First, the lifetime of the
photogenerated charge in the excited state was quantified by
the time-responsive PL spectrum (Fig. 6a & Table S3).
Compared with BOI and BOC with decay lifetimes of 0.58 ns
and 0.68 ns, respectively, the decay lifetime of the BOC/BOI-
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Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/D0TA02588G
Dual regulation of thermodynamic and dynamic of Bi O CO /Bi O I heterojunction enabling
2
2
3
5
7
visible-light-driven high-flux photogenerated charge migration.