Improved photoactivities for CO2 conversion and phenol degradation of α-Fe2O3 nanoparticles by co-coupling nano-sized BiPO4 and CuO to modulate electrons

Article abstract of DOI:10.1016/j.jallcom.2019.06.032

It's highly desired to design and fabricate effective α-Fe₂O₃-based photocatalysts by increasing the surface area, promoting the charge separation and providing catalytic function. Herein, specific surface area-enlarged α-Fe₂O₃ (SE-FO) nanoparticles have been successfully synthesized by a functional molecule-modulated phase-separated hydrothermal method, with high photocatalytic activities for CO₂ conversion and phenol degradation. The photocatalytic activities of SE-FO could be greatly improved by coupling nano-sized BiPO₄ via an in-situ introduction method. This is attributed to the coupled BiPO₄ as a high-energy platform to accept photogenerated electrons from α-Fe₂O₃ so as to enhance the charge separation mainly by means of surface photovoltage spectra and fluorescence spectra-related to the amount of produced ?OH species. Moreover, the photocatalytic activities are further improved by introducing a proper amount of nanocrystalline CuO via a simple impregnation process. It is confirmed based on the temperature-programmed desorption and electrochemical reduction measurements that the improved photoactivity is attributed to the introduced CuO as the co-catalyst for promoting electron-induced reduction reactions. Remarkably, the optimized α-Fe₂O₃ nanocomposite exhibits about 3-time photoactivity improvement compared with the pristine α-Fe₂O₃. This work would provide a feasible route to fabricate high-activity α-Fe₂O₃-based photocatalysts for CO₂ conversion and phenol degradation.

Full text of DOI:10.1016/j.jallcom.2019.06.032

Accepted Manuscript
Improved photoactivities for CO conversion and phenol degradation of α-Fe O
2 3
2
nanoparticles by co-coupling nano-sized BiPO and CuO to modulate electrons
4
Shuangying Chen, Zhiyuan Mu, Rui Yan, Xuliang Zhang, Iltaf Khan, Zhijun Li, Ning
Sun, Qingyang Zhang, Liqiang Jing
PII:
S0925-8388(19)32097-3
DOI:
https://doi.org/10.1016/j.jallcom.2019.06.032
JALCOM 50930
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 25 April 2019
Revised Date: 3 June 2019
Accepted Date: 4 June 2019
Please cite this article as: S. Chen, Z. Mu, R. Yan, X. Zhang, I. Khan, Z. Li, N. Sun, Q. Zhang, L. Jing,
Improved photoactivities for CO conversion and phenol degradation of α-Fe O nanoparticles by co-
2 3
2
coupling nano-sized BiPO and CuO to modulate electrons, Journal of Alloys and Compounds (2019),
4
doi: https://doi.org/10.1016/j.jallcom.2019.06.032.
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ACCEPTED MANUSCRIPT
Graphical abstract
Photocatalytic activities of α-Fe O nanoparticles are improved after co-modifying BiPO and CuO, attributed to
2
3
4
the promoted photogenerated electron transfer, and the provided catalytic functions.

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Improved photoactivities for CO conversion and phenol degradation of α-Fe O
2
2
3
nanoparticles by co-coupling nano-sized BiPO and CuO to modulate electrons
4
a
a
b
a*
a
a
a
Shuangying Chen , Zhiyuan Mu , Rui Yan , Xuliang Zhang , Iltaf Khan , Zhijun Li , Ning Sun ,
a
a
Qingyang Zhang and Liqiang Jing *
a Key Laboratory of Functional Inorganic Materials Chemistry (Ministry of Education), School of
Chemistry and Materials Science, International Joint Research Center and Lab for Catalytic
Technology, Heilongjiang University, Harbin 150080, P. R. China. E-mail: jinglq@hlju.edu.cn
b Key Laboratory of Photochemical Biomaterials and Energy Storage Materials, Heilongjiang
Province and College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin,
1
50025, PR China.
Abstract
It’s highly desired to design and fabricate effective α-Fe O -based photocatalysts by increasing the
2
3
surface area, promoting the charge separation and providing catalytic function. Herein, specific
surface area-enlarged α-Fe O (SE-FO) nanoparticles have been successfully synthesized by a
2
3
functional molecule-modulated phase-separated hydrothermal method, with high photocatalytic
activities for CO conversion and phenol degradation. The photocatalytic activities of SE-FO could
2
be greatly improved by coupling nano-sized BiPO via an in-situ introduction method. This is
4
attributed to the coupled BiPO as a high-energy platform to accept photogenerated electrons from
4
α-Fe O so as to enhance the charge separation mainly by means of surface photovoltage spectra and
2
3
fluorescence spectra-related to the amount of produced •OH species. Moreover, the photocatalytic
activities are further improved by introducing a proper amount of nanocrystalline CuO via a simple
impregnation process. It is confirmed based on the temperature-programmed desorption and
electrochemical reduction measurements that the improved photoactivity is attributed to the
introduced CuO as the co-catalyst for promoting electron-induced reduction reactions. Remarkably,
the optimized α-Fe O nanocomposite exhibits about 3-time photoactivity improvement compared
2
3

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with the pristine α-Fe O . This work would provide a feasible route to fabricate high-activity
2
3
α-Fe O -based photocatalysts for CO conversion and phenol degradation.
2
3
2
Keywords: α-Fe O , Coupling BiPO , CuO co-catalyst, Charge separation, Photocatalysis
2
3
4
1. Introduction
Massive fossil consumption in the last few decades has caused tremendous concern about the
future energy demand [1]. The emission of huge amount of the global warming carbon dioxide gas as
a result of fossil fuel burning to the atmosphere has attracted vast amount of research to deal with the
prevailing issue [2,3]. Hence, it’s rather meaningful to develop an effcient technique to convert CO2
into useful fuels while also reduce the amount of the greenhouse gas CO [4,5]. Moreover, water
2
body environmental pollution is another challenge being confronted by chemists and technologists in
st
the 21 century. Phenol, as a typical organic pollutant, greatly harms the environment mainly due to
its bio-recalcitrant nature and high toxicity [6,7]. Different methods to deal with these intractable
issues have been investigated, such as thermochemical conversion and electrochemical reduction.
But these techniques are still unsatisfactory due to less stability, low mechanical strength, electrode
corrosion and catalyst poisoning [8]. Hence, it is necessary to develop alternative techniques to
overcome the disadvantages mentioned above.
Noteworthily, photocatalysis with semiconductors is considered as an ideal “green” technology to
convert CO and reduce environmental pollution by utilizing solar light [9,10]. Compared with the
2
traditional techniques, such as thermochemical conversion and electrocatalytic ones, semiconductor
photocatalytic technology has shown the advantages like low cost, sustainability, and
environmentally friendliness [11,12]. Recently, many photocatalysts such as TiO , SnO , ZnO,
2
2
BiVO and α-Fe O , have been largely investigated [13-18]. Since solar light is nearly composed of
4
2
3
46% visible light, development of visible-light-driven photocatalysts become the predominant choice.
Amongst the various visible light responsive photocatalysts, hematite (α-Fe O ) with a narrow band
2
3

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gap of 2.0-2.2 eV displays many desirable properties, such as abundance, stability and non-toxicity
[
19,20]. However, its photocatalytic activities are influenced mainly by the limited specific surface
area, the weak charge separation and the sluggish catalytic function.
Obviously, the large surface area could increase reactant adsorption and offer abundant reactive
sites [21]. Although various strategies such as introducing pores [18] and obtaining two-dimensional
nanostructured materials [20] have been developed to increase the surface area of α-Fe O with some
2
3
success, these techniques display the disadvantages of poor structural stability and complicated
method. So to synthesize small size is a widely accepted strategy. Fortunately, we have synthesized
α-Fe O with small nanoparticle size in our previous study by using a hydrothermal method [22].
2
3
Notably, it is confirmed that the introduced functional molecules in the phase-separated synthetic
process can modulate the specific surface area. Hence, it is expected that specific surface area of
α-Fe O can be further increased by introducing functional molecules to modulate the developed
2
3
hydrothermal synthetic process.
Usually, the sluggish charge separation of α-Fe O are greatly limited by its positive conduction
2
3
band bottom position, indicating that its photogenerated electrons are not energetic enough to induce
reduction reactions thermodynamically [23]. Therefore, it is a great challenge to overcome this
inherited limitation for enhanced photoactivities. So it is meaningful to prolong the lifetime of
excited high-energy-level (HEL) electrons under irradiation for efficient photocatalysis on α-Fe O .
2
3
Interestingly, in our previous studies we have demonstrated that coupling with wide bandgap oxide
nanoparticles such as TiO , SnO as new HEL electrons platforms for accepting the HEL electrons
2
2
can effectively prolong the charge lifetime and promote the charge separation, consequently leading
to the improved photocatalytic activities under visible-light irradiation [23-26]. Compared with the
commonly-used wide bandgap oxides, such as TiO and SnO , BiPO has shown the advantages like
2
2
4
good photocatalytic activities and proper-energy position [27-29]. Hence, BiPO maybe a potential
4
HEL electrons platform, however it is usually neglected since seldom related works have been

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reported to date. In our previous work [30], it has primarily been confirmed that coupling with BiPO4
as a platform for accepting HEL electrons from BiOBr can promote the charge separation,
consequently leading to superior photocatalytic performance. Therefore, it is much feasible to couple
BiPO as a HEL platform for α-Fe O to promote its charge separation. However, related works have
4
2
3
seldom been reported to date.
Besides, the apparent photocatalytic activities of α-Fe O is still affected even if its charge
2
3
separation is enhanced, since it is lacking of catalytic functions for the transferred electrons to induce
reduction reactions. In recent years, some transition metals and their oxides have attracted much
attention due to their excellent properties of catalyzing reactions [31,32]. Compare with noble metals,
nanoparticles composed of metal or inorganic compounds such as copper, stannic oxide and
manganese oxide are therefore more suitable candidates as co-catalysts for CO reduction systems,
2
because they are Earth-abundant and non-toxicity [33]. In particular, copper-containing substances
exhibit high chemisorption of CO [34,35]. Recently, many investigations have confirmed that CuO
2
can act as an excellent co-catalyst with TiO , SnO , WO , which results into the promoted charge
2
2
3
separation and oxygen reduction activation [36]. Ge et al. reported that the grafting of copper oxide
Cu(II)) nanoclusters co-catalysts onto the Nb O nanosheets led to the efficient reduction of CO
2
(
3
8
molecules to CO under UV-light irradiation [34]. Therefore, it is expected that modifying CuO onto
BiPO /α-Fe O hybrid as the co-catalyst would provide catalytic function for the transferred
4
2
3
electrons to induce reduction reactions. However, this complex system has not yet been reported.
Herein, we have successfully synthesized specific surface area-enlarged α-Fe O by a functional
2
3
molecule-modulated phase-separated hydrothermal method, with high photocatalytic activities for
CO conversion and phenol degradation. To improve the photocatalytic activities, the as-prepared
2
SE-FO nanoparticles are coupled with a proper amount of BiPO via an in-situ introduction method.
4
Moreover, the photocatalytic activities are further improved by introducing a proper amount of CuO
via a simple impregnation process. The fabricated amount-optimized CuO modified BiPO /α-Fe O
3
4
2

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nanocomposites have exhibited more favourable photocatalytic activities compared to pristine
α-Fe O . As confirmed by the steady-state surface photovoltage spectra, and fluorescence
2
3
spectra-related to the amount of produced •OH species, the superior photocatalytic activities are
attributed to the facilitated charge separation by coupling a proper amount of BiPO as the
4
high-energy-level electron-accepting platform, and to the provided catalytic function for
electron-induced reduction reactions by introducing CuO as a co-catalyst through the
temperature-programmed desorption and electrochemical reduction measurements. This work
provides a feasible route to improve the photocatalytic activities of α-Fe O for solar fuel production
2
3
and environmental remediation.
2. Experimental
2.1 Reagents
All the reagents were of analytical grade and used as received without further purification.
Deionized water was used throughout.
2
2
.2 Materials synthesis
.2.1 Synthesis of α-Fe O nanoparticles
2
3
Pristine α-Fe O was synthesized by a phase-separated hydrolysis-solvothermal method. n-Butyl
2
3
alcohol as the organic solvent and hydrated ferric nitrate Fe(NO ) ·9H O as the iron source were
3
3
2
used. In a typical procedure [21], 10 mL of 5 wt.% ammonia solution was added into a 50 mL
Teflon-lined autoclave and a 10 mL weighing bottle containing a solution of 0.8 g Fe(NO ) ·9H O in
3
3
2
8
mL n-butyl alcohol was placed in the autoclave with a support between the autoclave bottom and
the weighing bottle. The Teflon-lined autoclave was kept at 140 °C for 6 h. After being cooled
naturally to the room temperature, the sample was washed for several times with ethanol and
deionized water in turn and then dried in an oven at 80 °C to obtain α-Fe O . Finally, the samples
2
3
were calcined in air at 400 °C for 2 h. The pristine α-Fe O is represented by FO. While the specific
2
3
surface area-enlarged α-Fe O was synthesized by same method, only by changing 10 mL of 5 wt.%
2
3

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ammonia solution to 10 mL of 0.4 M carbamide solution. The surface area-enlarged α-Fe O sample
2
3
is coded by SE-FO.
2
.2.2 Synthesis of BiPO /α-Fe O nanocomposite
4 2 3
In a typical experiment, Bi(NO ) ·5H O (0.015 g, 0.045 g, 0.075 g and 0.105 g) was dissolved in
3
3
2
1
1
1
0 mL of ethylene glycol (EG), under vigorous stirring for 10 min, followed by ultrasonication for
0 min, then 0.5 g SE-FO precursor was added into the previously prepared solution with stirring for
0 min, which is labelled as solution A. NaH PO ·2H O (0.005 g, 0.015 g, 0.025 g and 0.035 g) was
2
4
2
dissolved in 10 mL EG, which was kept under stirring for 10 min and labelled as solution B. The
solution B was added dropwise to solution A, which was kept under stirring for 15 min. Then, the
resulting mixture was centrifuged and washed several times with deionized water, followed by
absolute ethanol, and product was dried at 80 °C overnight in oven. Finally, the dry samples were
calcined in air at 400 °C for 2 h. The BiPO /α-Fe O nanocomposite is represented by XBPO/SE-FO
4
2
3
(X in XBPO is the mole ratio percentage of SE-FO used in the composite).
2
.2.3 Synthesis of CuO modified BiPO /α-Fe O nanocomposites
4 2 3
For the preparation of CuO modified nanocomposite samples. In a typical procedure, 0.3 g of
3BPO/SE-FO was mixed with different by different volume ratio (0.094 ml, 0.188 ml and 0.38 ml）
of 0.05 M Cu(NO ) ·H O precursor solution, at the same time deionized water was added up to
3
2
2
constant volume of 10 mL. After vigorous stirring for 3 h, the solution was washed one time with
deionized water. And then the mixtures were dried in oven at 80 °C overnight. Finally, the product
was calcined in air at 300 °C for 0.5 h. The obtained CuO modified samples were denoted by
YCuO-3BPO/SE-FO, Y in YCuO is the mol ratio percentage of CuO to SE-FO. Synthesis of CuO
modified α-Fe O nanoparticle by similar method. 0.3 g SE-FO was mixed with 0.188 mL of 0.05 M
2
3
.
Cu(NO ) H O solution. The obtained CuO modified samples were denoted by ZCuO-SE-FO, Z in
3
2
2
ZCuO is the mole ratio percentage of CuO to SE-FO.

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2
.3. Characterization techniques
The X-ray powder diffraction (XRD) patterns of the prepared samples were measured with a
Bruker D8 advance diffractometer, using CuKα radiation. The UV-vis diffuse reflectance spectra
UV-vis DRS) of the samples were measured with a Model Shimadzu UV2550 spectrophotometer,
(
using BaSO as a reference. The compositions and elemental chemical states of the samples were
4
confirmed through X-ray photoelectron spectroscopy (XPS) using a Kratos-Axis Ultra DLD
apparatus with an Al (mono) X-ray source. Transmission electron microscopy (TEM) and
high-resolution transmission electron microscopy (HRTEM) images of the samples were taken by a
JEOL JEM-2010 electron microscope with a high acceleration voltage of 200 kV.
Brunauer-Emmett-Teller (BET) surface areas of the samples were determined from N adsorption
2
isotherms at 77 K using a Micromeritics Tristar II 3020 specific surface area analyzer after the
samples had been degassed in a flow of N at 180 °C for 3 h. The steady-state surface photovoltage
2
spectroscopy (SS-SPS) measurements of the samples were carried out with a home-built apparatus,
equipped with a lock-in amplier (SR830) synchronized with a light chopper (SR540) at different
atmosphere. The powder sample was sandwiched between two indium-tin-oxide (ITO) glass
electrodes kept in an atmosphere-controlled sealed container. A monochromatic light was obtained
from 500 W Xenon lamp (CHF XQ500W, Global Xenon lamp power) through a double prism
monochromator (SBP300).
2.4. Photoelectrochemical measurements
Firstly, we prepared the electrode films of the samples according to our previous work [22].
Photoelectrochemical (PEC) experiments were performed in a glass cell with 1 M NaOH solution as
the electrolyte using the as-prepared film as the working electrode, Pt wire (99.9 %) as the counter
electrode, and a saturated KCl Ag/AgCl electrode as the reference electrode. High purity nitrogen gas
(99.999 %) was bubbled through the electrolyte before and during the experiments. Applied
potentials were controlled by a commercial computer-controlled potentiostat (AUTOLAB PG

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STAT101). The photocurrent densities at different excitation wavelengths were measured, in which
monochromatic light was obtained from 500 W Xenon lamp (CHF XQ500W, Global Xenon lamp
power) through a double prism monochromator (SBP300).
2.5. Electrochemical reduction measurements
The electrochemical reduction measurement in different gas-bubbled systems was operated using
the above same system with the PEC experiment. high-purity CO , N , O gas was respectively used
2
2
2
to bubble through the electrolyte before and during the experiments. The electrochemical reduction
experiments were measured in dark. Applied potentials were controlled by a commercial
computer-controlled potentiostat (AUTOLABPG STAT 101).
2.6. Temperature-programmed desorption measurements
Temperature-programmed desorption (TPD) of CO was carried out in a conventional apparatus
2
by Chemisorption Analyzer equipped with a TCD detector. About 0.1 g of catalysts were pre-treated
-1
at 300 °C for 1 h under ultra-high-pure He gas flow with the rate of 30 mL min . The highly pure
-1
CO was introduced at a constant temperature of 30 °C under the flow rate of 30 mL min for 60 min.
2
The physically adsorbed CO was removed by being exposed with ultra-high-pure helium at 30 °C
2
-1
for 60 min. Then the temperature was increased to 600 °C with the heating rate of 10 °C min under
ultra-high-pure He. Finally, the desorbed CO was monitored by Chemisorption Analyzer (Tp 5080
2
Chemisorb). While the O -TPD was tested by similar method, only changed CO to O gas.
2
2
2
2.7. Analyses of produced hydroxyl radical amounts
For the hydroxyl radical measurement, 0.05 g powder was dispersed in 40 mL coumarin aqueous
solution (0.001 M) in a beaker. Prior to irradiation, the reactor was magnetically stirred for 10 min to
attain an adsorption-desorption equilibrium. After irradiation for 1 h with a 150 W GYZ220 high
pressure Xenon lamp (made in China), the sample was centrifuged, and then a certain volume of
solution was transferred into a Pyrex glass cell for the fluorescence measurement of
7-hydroxycoumarin at 332 nm excitation wavelength with characteristic emission peak at 460 nm

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through a spectrofluorometer (Perkin-Elmer LS55).
2
.8. Evaluation of photocatalytic activity for CO conversion
2
In the photocatalytic reduction of CO , 0.1 g of the sample was dispersed in 5 mL water and the
2
2
solution was contained in a cylindrical steel reactor with 100 mL volume and 3.5 cm area. Then
high purity CO gas was bubbled in the reactor after passing through water for reaching ambient
2
pressure. Then the CO /H O system was equilibrated for 0.5 h and a 300 W xenon lamp was used as
2
2
the light source for the photocatalytic reaction. During the irradiation, continually 0.25 mL of gas
was taken from the reaction cell at given time interval for the analysis of CO and CH concentration
4
using gas chromatograph (CO and CH by GC-7920 with FID II detector, Ceaulight, Beijing). The
4
stability and recyclability tests were measured for 16 h, with a 4-hour run cycle. For each run, the
sample was dried and irradiated under UV-vis light.
2.9. Evaluation of photocatalytic activity for phenol degradation
The activities of the samples were evaluated by photodegradation of phenol using a 150 W xenon
lamp (GYZ220). the photocatalyst powder (0.1 g) and phenol solution (50 mL,10 mg/L) were mixed
in a photochemical glass reactor under magnetic stirring at the room temperature and standard
pressure. Prior to irradiation, the reactive system was stirred magnetically in dark for 30 min to reach
the adsorption-desorption equilibrium. The solution was kept under stirring and irradiated for 1 h, the
solution was centrifuged and the concentration of phenol measured with a Shimadzu UV-2550
spectrophotometer through the 4-aminoantipyrine spectrophotometric method at the characteristic
optical absorption of 510 nm.
3. Results and Discussion
3.1. Effects of enlarged specific surface area
The XRD measurement is performed to determine the crystalline structure and phase purity. The
corresponding XRD patterns of the specific surface area-enlarged α-Fe O (SE-FO) and the pristine
2
3
α-Fe O (FO) are shown in Fig. S1A. As confirmed by XRD patterns, the SE-FO and FO were both
2
3

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identical to pure hexagonal-phase α-Fe O (JCPDC 33-0664). No characteristic peaks of other phases
2
3
or impurities are observed, indicating the high purity of resulting α-Fe O . In the Fig. S1B, the
2
3
nitrogen adsorption-desorption isotherms were obtained to test the specific surface area, indicating
2
-1
that the surface areas of FO and SE-FO were 37 and 47 m g , respectively. Apparently, SE-FO
exhibits an enlarged specific surface area compared with the FO. The enlarged specific surface of
α-Fe O particles is attributed to the decrease of particles size by introducing functional molecules
2
3
during the hydrolytic process.
The measurements of fluorescence spectra related to the formed hydroxyl radicals (•OH) using the
coumarin fluorescent method was carried out to investigate the properties of photogenerated charges
separation. This method is based on the coumarin easily reacting with •OH to produce luminescent
7-hydroxycoumarin. In general, the stronger is the fluorescent spectra intensity, the larger is the
amount of produced •OH species, and the stronger is the photogenerated charge. The fluorescence
spectra related to the produced •OH species are shown in Fig. S1C. It could be observed that the
fluorescence intensity of SE-FO is much higher than that of FO, indicating that the SE-FO sample
exhibits stronger charge separation. Fig. S1D shows the UV-vis light photocatalytic activities of
SE-FO and FO for CO reduction with H O as the reducing agent. CO was detected as the
2
2
predominant product. Compared with FO, SE-FO exhibits much higher photoactivity, which is in
good agreement with the enhanced charge separation.
3.2. Effects of coupling with BiPO4
In order to further enhance the photogenerated charge separation of SE-FO, BiPO with different
4
amounts is coupled. The XRD patterns of SE-FO and XBPO/SE-FO are shown in Fig. S2A, the
XRD diffraction peaks at around 24.1°, 33.2°, 35.6°, 42.1°, 49.4° and 54.1° are attributed to
hexagonal-phase α-Fe O , respectively corresponding to the (012), (104), (110), (113), (024) and
2
3
(
116) crystal planes [37]. In the 1BPO/SE-FO sample, no characteristic peaks of BiPO are observed,
4
which is possibly because of the tiny amount of formed BiPO . As the content of BiPO increases,
4
4

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the 7BPO/SE-FO shows several characteristic diffraction peaks at around 14.6°, 20.1° and 29.5°,
which are assigned to the BiPO , respectively corresponding to the (100), (101) and (200) crystal
4
planes according to (JPCDS 45-1370) [38]. This implies the successful preparation of BiPO /α-Fe O
3
4
2
nanocomposites. Noticeably, the crystal phase of α-Fe O is not changed after forming the
2
3
nanocomposites. The UV-vis DRS spectra are shown in Fig. S2B, by which the calculated energy
bandgap of α-Fe O is 2.0 eV based on the widely accepted equation (λ = 1240/Eg) [39]. As the
2
3
content of coupled BiPO increases, the optical absorption behavior of α-Fe O is weakened, but the
4
2
3
bandgap does not change.
The photogenerated electron-hole formation and separation of solid semiconductor materials can
be analysed through the steady-state surface photovoltage spectroscopy (SS-SPS) technique. For a
nanosized semiconductor, its SS-SPS response derives from the photogenerated charge separation via
the diffusion fusion process since its built-in electric field is usually neglected. In general, the
stronger is the SS-SPS response, the higher is the photogenerated charge separation [40]. In the Fig.
1A, one can see that the SE-FO sample shows a relatively low SS-SPS response. Interestingly, the
SS-SPS response is enhanced after coupling BiPO , and the 3BPO/SE-FO shows the strongest signal
4
among the nanocomposite samples. Thus, it is confirmed that the photogenerated charge separation
of SE-FO could be enhanced by constructing the nanocomposite with an appropriate amount of
BiPO , and the 3BPO/SE-FO sample displays the highest charge separation. However, the composite
4
would show a decreased SS-SPS response if the amount of coupled BiPO is in excess because it is
4
not beneficial for the light absorption.
To further confirm the enhanced charge separation, the fluorescence spectra related to the amount
of produced •OH are shown in Fig. 1B. One can see that the SE-FO shows a small amount of •OH,
and after coupling with a proper amount of BiPO , the fluorescence intensity become much larger,
4
especially for that of 3BPO/SE-FO. This indicates that the 3BPO/SE-FO displays the most efficient
charge separation. Obviously, this is well in agreement with the above SS-SPS result. Based on the

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photophysical and photochemical results, it is clearly demonstrated that the photogenerated charge
separation of SE-FO could be enhanced by coupling with a proper amount of BiPO . Thus, it is
4
anticipated that coupling with BiPO could improve the photocatalytic activities of SE-FO.
4
The UV-vis photocatalytic activities of SE-FO and XBPO/SE-FO samples were evaluated for CO2
reduction, as shown in Fig. 1C. It is seen that the detected gas products of CO reduction contain CO
2
and CH , especially for CO. The SE-FO exhibits a certain photocatalytic activity for the conversion
4
-1
-1
-1
-1
of CO to produce CO (∼ 1.1 µmol g h ), and CH (∼ 0.1 µmol g h ). Noticeably, the
2
4
photocatalytic activities have been greatly improved after coupling BiPO , and it is much obvious for
4
-1
-1
-1
-1
the amount-optimized 3BPO/SE-FO one (CO∼1.4 µmol g h , and CH ∼0.25 µmol g h ).
4
However, the photoactivity begins to decrease when the amount of BiPO used is in excess, like
4
5BPO/SE-FO.
To further investigate the photoactivities of the as-prepared samples, we also measured the UV-vis
photocatalytic performance of SE-FO and XBPO/SE-FO samples for photocatalytic phenol
degradation, as shown in Fig. 1D. The photocatalytic activities of SE-FO for phenol degradation
have been greatly improved after coupling with appreciate amount of BiPO , consistent with the
4
above photocatalytic activities for CO reduction. Therefore, it is concluded that the improved
2
photocatalytic activity of SE-FO after coupling with BiPO is mainly attributed to the enhanced
4
photogenerated charge separation.
3.3 Effects of modifying CuO
Although the charge separation of α-Fe O has been promoted, it is lacking of catalytic functions
2
3
for the transferred electrons to induce reduction reactions. Thus, CuO modified BiPO /α-Fe O
3
4
2
nanocomposites were synthesized. According to the XRD patterns and UV-vis DRS spectra of
BPO/SE-FO and CuO modified 3BPO/SE-FO nanocomposites, as shown in Fig. S3, no
3
characteristic peaks of BiPO and CuO are observed, which is possibly because of the tiny amount of
4
formed BiPO and CuO. Noticeably, the diffraction peaks of α-Fe O show no obvious changes,
4
2
3

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indicating that its crystallinity has been well maintained in the fabricated nanocomposites. In
addition, as the content of used CuO increases, the optical absorption range of α-Fe O has extended,
2
3
but the bandgap does not change.
TEM and HRTEM images were employed to investigate the morphological and structural features
of 0.5CuO-3BPO/SE-FO sample. In Fig. 2A, one can see that the morphology of CuO, BiPO4,
SE-FO are all nanoparticles. It is also clear that the SE-FO and the BiPO are both spherical with a
4
diameter of ~15 nm, while CuO is spherical with a diameter of ~20 nm. One can see from the
area-selected HRTEM (Fig. 2B) that the lattice spacing of 0.25 nm is due to the (110) facet of
α-Fe O [34], while the lattice spacing of 0.33 and 0.21 nm correspond to the (200) and (111) facets
2
3
of BiPO [41] and CuO [42], respectively. Hence, it is proved that the CuO-3BPO/SE-FO composite
4
has been successfully fabricated.
To investigate the surface composition and chemical states of the elements, XPS measurements of
SE-FO, 3BPO/SE-FO, and 0.5CuO-3BPO/SE-FO samples were performed, as shown in Fig. S4. In
Fig. S4A, one can see that the Fe2p spectra of the three sample are similar. Two distinct peaks at
711.4 eV with a shake-up satellite at 718.9 eV and 725.1 eV with a shake-up satellite at 733.7 eV are
3
+
ascribed to Fe2p3/2 and Fe2p , respectively, and it is characteristic of Fe in α-Fe O . In Fig. S4B,
1/2
2
3
the O1s spectra of all the three samples show the main peaks at around 530.1 eV, which are assigned
to the lattice oxygen. The peaks (Fig. S4C) at 159.2 and 164.6 eV are observed, respectively
3
+
correspond to the Bi4f7/2 and Bi4f5/2 of Bi in BiPO . The XPS spectrum of P2p in 3BPO/SE-FO or
4
0.5CuO-3BPO/SE-FO (Fig. S4D) exhibits a broad band at about 132.9 eV, which is often ascribed to
3−
the PO4 group in the phosphate compounds. Based on the Bi4f and P2p spectra, it is inferred that
the BiPO /α-Fe O nanocomposites has been successfully synthesized. In 0.5CuO-3BPO/SE-FO
4
2
3
sample, the peak at 933 eV is observed (Fig. S4E), corresponding to the Cu2p3/2 in CuO. This means
that CuO has been successfully modified onto the resulting BiPO /α-Fe O nanocomposites.
4
2
3
The SS-SPS responses of 3BPO/SE-FO and YCuO-3BPO/SE-FO samples are shown in Fig. 3A.

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Obviously, the modification with an appropriate amount of CuO greatly enhances the SS-SPS
response of 3BPO/SE-FO. Among the CuO modified 3BPO/SE-FO samples, 0.5CuO-3BPO/SE-FO
sample shows the strongest SS-SPS response, indicating the highest charge separation. However, an
excess amount of CuO modification is unfavourable for the continuously-increased SS-SPS
responses. To further confirm the enhanced charge separation, the fluorescence spectra related to the
amount of produced •OH are shown in Fig. 3B. It could be observed that the amount of produced
•OH is increased with increasing the amount of used CuO, and 0.5CuO-3BPO/SE-FO displays the
strongest FS signal, indicating the most efficient charge separation. This is well consistent with the
SS-SPS responses.
The photocatalytic activities of 3BPO/SE-FO and YCuO-3BPO/SE-FO samples were evaluated
for CO reduction under UV-vis light irradiation as shown in Fig. 3C. Interestingly, the UV-vis
2
photocatalytic activities of YCuO-3BPO/SE-FO nanocomposites for CO conversion are much high
2
compared to that of the non-CuO-modified one (3BPO/SE-FO), and the amount-optimized
0.5CuO-3BPO/SE-FO sample exhibits the highest photocatalytic performance for the reduction of
-1
-1
-1 -1
CO to produce CO (∼ 1.85 µmol g h ) and CH (∼ 0.42 µmol g h ). The photocatalytic activities
2
4
of YCuO-3BPO/SE-FO samples are also measured for the degradation of phenol as shown in Fig. 3D.
The photocatalytic activity of 0.5CuO-3BPO/SE-FO is higher than any other samples. Notably, the
evaluated photocatalytic activities are consistent with the corresponding charge separation situation.
It is concluded that the improved photocatalytic activities of 3BPO/SE-FO after modifying with CuO
is mainly attributed to the enhanced photogenerated charge separation. Since the stability of
photocatalysts for the evaluation of the photocatalytic performance, recyclability of
0
.5CuO-3BPO/SE-FO for CO conversion under UV-vis light irradiation was measured as shown in
2
Fig. S5. It is noticed that no inactivation takes place after 4-run recyclable experiments.
In order to further verify the effects of modified CuO, the photocatalytic activities of CuO
modified SE-FO were also measured. Based on the XRD patterns (Fig. S6A) and UV-vis DRS

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spectra (Fig. S6B), it is suggested that the phase compositions and band edges of SE-FO are
unchanged after introducing CuO. One can see from Fig. S6C that the results of hydroxyl radical of
0.5CuO-SE-FO are similar to that of SE-FO, which is well responsible for the unchanged
photocatalytic activity of phenol degradation as shown in Fig. S6D. This indicates that the modified
CuO does not individually improve the photocatalytic activity of SE-FO. In other words, it is not
feasible to modify SE-FO only by introducing CuO, and there is a good synergistic effect of coupling
with BiPO and CuO. It is expected that the modified CuO could take as a co-catalyst for the
4
transferred electrons to induce reduction reactions.
3.4 Discussion on mechanism
In order to further confirm the enhanced charge separation, we have also carried out the current
density-voltage curves for SE-FO, 3BPO/SE-FO and 0.5CuO-3BPO/SE-FO electrodes in 1 M NaOH
electrolyte under UV-vis light illumination from the back side (Fig. 4A). As expected, the
photocurrent signal of the 3BPO/SE-FO electrode is much larger than that of SE-FO, and the
0
.5CuO-3BPO/SE-FO electrode shows the largest photocurrent density. Meanwhile, to further reveal
the charge transfer mechanism, the photocurrent action spectra of SE-FO, 3BPO/SE-FO, and
.5CuO-BPO/SE-FO with different wavelength excitations were obtained as shown in Fig. 4B. One
0
can see that the photocurrent density of 3BPO/SE-FO gradually increases as the excitation
wavelength decreases from 525 to 400 nm. Noticeably, it begins to sharply increase under 500 nm
excitation, which is the threshold wavelength for facilitating the transfer of excited proper-energy
electrons of α-Fe O to BiPO . In particular, the photocurrent density of 0.5CuO-3BPO/SE-FO
2
3
4
greatly increases after modified CuO. This indicates that it is much favourable for the photogenerated
electron transfer from BiPO to CuO. Moreover, we have measured the SS-SPS responses in N (Fig.
4
2
S7), it is noticed that, bare α-Fe O does not have signal, but the BiPO -coupled sample shows an
2
3
4
obvious SS-SPS response in N , indicating the electron transfer from α-Fe O to BiPO . Interestingly,
2
2
3
4
0.5CuO-3BPO/SE-FO displays much high SS-SPS response compared with 3BPO/SE-FO, meaning

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the higher charge separation. It can be confirmed that the electron transfer from BiPO to CuO
4
happens.
To explore the mechanism of photocatalytic reduction of CO , the electrochemical reduction
2
measurement was used to investigate in different gas-bubbled systems. In the CO -bubbled system
2
(
Fig. 5A), it is noticed that it is much favourable for CO reduction on the 3BPO/SE-FO based on the
2
onset potential, and it is further promoted by modifying CuO. This indicates that it is favoured
thermodynamically to reduce CO on the 0.5CuO-3BPO/SE-FO sample. For comparison, the
2
electrochemical reduction test was also measured in the N -bubbled system (Fig. S8). Similar with
2
the results in the CO -bubbled system, the 0.5CuO-3BPO/SE-FO sample exhibits the most
2
favourable for H O reduction, indicating that it is also favored thermodynamically to evolve H on
2
2
0
.5CuO-3BPO/SE-FO. However, it is found that the onset potentials on different α-Fe O -based
2 3
electrodes for CO reduction shift to the low value compared to the corresponding ones for H2
2
evolution, meaning that it is much easy for CO to be reduced compared to H O reduction. It is
2
2
attributed to the promoted adsorption of CO after coupling CuO by means of the CO TPD curves
2
2
(
Fig. 5A inset). Thus, it is deduced that the modified CuO as the co-catalyst can favor CO reduction.
2
To explore the mechanism of photocatalytic degradation of phenol, the electrochemical reduction
measurements in O bubbled system have also been investigated. It is confirmed from Fig. 5B that
2
the performance for O reduction on 3BPO/SE-FO is better than that on SE-FO, and it is further
2
improved after modifying CuO. As confirmed by the O TPD curves in Fig. 5B as the inset, the
2
adsorption of O could be promoted by coupling BiPO and CuO. This is well responsible for the
2
4
enhanced charge separation, consequently leading to the improved photocatalytic activity for
degrading phenol.
Based on the above results and discussion, a possible mechanism on the transfer and separation of
photogenerated charges in the resulting 0.5CuO-3BPO/SE-FO nanocomposite and the induced
photochemical processes is suggested, as depicted in Fig. 6. It is illustrated that the electrons from

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the VB of α-Fe O could be excited to the different energy-level of CB under light irradiation,
2
3
including high-energy region (above −0.16 eV) and low-energy one (from +0.2 eV to −0.16 eV). As
for the photogenerated electrons at the low-energy level, they would relax rather fast to the CB
bottom and then recombine with holes in the VB. Differently, the partially excited HEL electrons
could thermodynamically transfer to the CB of BiPO , leading to the enhanced charge separation.
4
Moreover, it is confirmed that the decorated CuO could further accept the transferred electrons and
then provide catalytic functions to favor reduction reactions with CO and H O.
2
2
4. Conclusions
In summary, 0.5CuO-3BPO/SE-FO nanocomposites as efficient photocatalysts to reduce CO and
2
degrade phenol have been successfully synthesized. It is clearly demonstrated that the
greatly-improved photocatalytic activities are comprehensively attributed to three factors: the
increased specific surface area by introducing functional molecules in the synthetic process to
decrease the nanoparticle size, the enhanced charge separation by coupling BiPO as a new proper
4
high energy level platform to accept photogenerated electrons from α-Fe O , and the provided
2
3
catalytic functions by modifying CuO as the co-catalyst for the transferred electrons to induce
reduction reactions. This study provides a feasible route to fabricate high-activity α-Fe O -based
2
3
nano photocatalysts for solar fuel production and environmental remediation.
Acknowledgements
We are grateful for financial support from NSFC (U1805255, 91622119), and the Program for
Innovative Research Team in Chinese Universities (IRT1237).
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Figure captions:
Fig. 1. SS-SPS responses in air (A), fluorescence spectra related to the formed •OH amounts after
irradiation for 1 h (B), photocatalytic activities for CO conversion (C) and photocatalytic activities
2
for phenol degradation (D) of SE-FO and XBPO/SE-FO (SE-FO mean specific surface area-enlarged
α-Fe O , BPO means BiPO , respectively. X in XBPO is the mole ratio percentage of specific
2
3
4
surface area-enlarged α-Fe O used in the composite).
2
3
Fig. 2. TEM image of 0.5CuO-3BPO/SE-FO (A) with an area-selected HRTEM (B) (Y in
YCuO-3BPO/SE-FO is the mole ratio percentage of specific surface area-enlarged α-Fe O used in
2
3
the composites).
Fig. 3. SS-SPS responses in air (A), fluorescence spectra related to the formed •OH amounts after
irradiation for 1 h (B), photocatalytic activities for CO conversion (C) and photocatalytic activities
2

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for phenol degradation (D) of 3BPO/SE-FO and YCuO-3BPO/SE-FO.
Fig. 4. Current density-voltage curves of SE-FO, 3BPO/SE-FO, 0.5CuO-3BPO/SE-FO under UV-vis
light (A) and single-wavelength photocurrent action spectra (B) PEC measurements were performed
with a three-electrode system in a 1M NaOH electrolyte, with the tested film as the working
electrode, Ag/AgCl as the reference electrode, and Pt plate as the counter electrode.
Fig. 5. Electrochemical reduction curves in CO2 -bubbled systems (A) and in the O -bubbled
⁠
2
systems (B) of SE-FO, 3BPO/SE-FO, 0.5CuO-3BPO/SE-FO, respectively. The inset in A is
CO -TPD curves and the inset in B is O -TPD curves.
2
2
Fig. 6. Schematic of the photogenerated charge transfer and separation.
Fig. 1.

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Fig. 2.

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Fig. 3.

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Fig. 4.

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Fig. 5.
Fig. 6.

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