Development of BiOI as an effective photocatalyst for oxygen evolution reaction under simulated solar irradiation

Article abstract of DOI:10.1039/d0cy00266f

In this study, crystalline BiOI powders were prepared for photocatalytic O₂evolution in the presence of NaIO₃as the electron mediator. BiOI with a microspherical morphology, a layered structure composed of [Bi₂O₂]²⁺and intercalated I^?ions, exhibited a suitable valence band level to generate photoexcited holes for O₂evolution. Moreover, ruthenium was loaded using the impregnation or photodeposition method to produce RuO₂as a co-catalyst to improve the photocatalytic activity of BiOI. Photodeposited RuO₂-loaded BiOI showed a high O₂evolution rate of 2730 μmol h^?1and can be reused eight times in the presence of NaIO₃under simulated solar irradiation. The high photocatalytic O₂evolution can be attributed to the highly dispersed RuO₂, which could serve as an effective electron sink, on the surface of BiOI and its enhanced visible light-harvesting ability. Besides, the presence of NaIO₃in the system was effective to receive photoexcited electrons from RuO₂-loaded BiOI for improving charge separation and hence the O₂evolution from RuO₂sites on the BiOI surface. The RuO₂-loaded BiOI with high photocatalytic activity and stability for generating O₂could be a potential candidate for achieving overall water splitting in aZ-scheme system in the presence of NaIO₃for solar utilization in the future.

Full text of DOI:10.1039/d0cy00266f

Catalysis
Science &
Technology
View Article Online
View Journal
PAPER
Development of BiOI as an effective photocatalyst
for oxygen evolution reaction under simulated
solar irradiation†
Cite this: DOI: 10.1039/d0cy00266f
Tzu-Hsin Chen,a Masaaki Yoshida,bc Shun Tsunekawa,b Jia-Hao Wu,a
a
Kun-Yi Andrew Lind and Chechia Hu
*
In this study, crystalline BiOI powders were prepared for photocatalytic O2 evolution in the presence of
NaIO3 as the electron mediator. BiOI with a microspherical morphology, a layered structure composed of
[Bi2O2]2+ and intercalated I− ions, exhibited a suitable valence band level to generate photoexcited holes
for O2 evolution. Moreover, ruthenium was loaded using the impregnation or photodeposition method to
produce RuO2 as a co-catalyst to improve the photocatalytic activity of BiOI. Photodeposited RuO2-loaded
BiOI showed a high O2 evolution rate of 2730 μmol h−1 and can be reused eight times in the presence of
NaIO3 under simulated solar irradiation. The high photocatalytic O2 evolution can be attributed to the
highly dispersed RuO2, which could serve as an effective electron sink, on the surface of BiOI and its
enhanced visible light-harvesting ability. Besides, the presence of NaIO3 in the system was effective to
receive photoexcited electrons from RuO2-loaded BiOI for improving charge separation and hence the O2
evolution from RuO2 sites on the BiOI surface. The RuO2-loaded BiOI with high photocatalytic activity and
stability for generating O2 could be a potential candidate for achieving overall water splitting in a Z-scheme
system in the presence of NaIO3 for solar utilization in the future.
Received 11th February 2020,
Accepted 22nd April 2020
DOI: 10.1039/d0cy00266f
rsc.li/catalysis
process, which mimicked the natural photosynthesis of green
plants, has been extensively studied in past decades.10–16 This
Introduction
Currently, environmental and energy issues are of great
concern. One of the most promising strategies for addressing
these problems is the solar energy conversion for sustainable
and renewable energy sources, as well as the development of
effective and green technologies.1–6 In an attempt to achieve
this aspect, researchers worldwide are making intensive
efforts in photocatalysis, which has been viewed as the most
promising approach for generating hydrogen (H2) and/or
oxygen (O2) and reducing carbon dioxide (CO2). However, very
few examples can achieve overall water splitting under visible
light irradiation using a single photocatalyst.7–9 Therefore, a
Z-scheme system with a two-stage and double excitation
Z-scheme system can be used for overall water splitting by
coupling two semiconductors with a band gap suitable for
visible light-harvesting to respectively act as an O2-evolved
photocatalyst (OEP) and H2-evolved photocatalyst (HEP) in
the presence of an electron mediator. Briefly, photoinduced
electrons and holes at the HEP and OEP sides would reduce
protons and simultaneously oxidize water to produce H2 and
O2, respectively. Meanwhile, photoexcited holes and electrons
at HEP and OEP would oxidize and reduce the electron
mediator for electron–hole transfer to achieve charge
neutrality (Fig. 1). Over the past 20 years, approximately 500
papers on “Z-scheme” and “water splitting” have been
published with more than 23 300 citations (Fig. 1b),
indicating that the Z-scheme system is of particular interest
in photocatalytic water splitting.
In this aspect, both OEP and HEP should be carefully
constructed to strengthen the charge separation to obtain
high solar conversion efficiency and maintain its stability.
Many OEPs and HEPs have been used in the Z-scheme
system, such as Ru/SrTiO3:Rh–BiVO4,17 SrTiO3:Rh–BiVO4,18
Ag3PO4–AgI,19 TiO2–CdS,20 ZnO–CdS,21 Pt/SrTiO3:Cr,Ta–Pt/
WO3,22 CdS–Au–TiO2,23 Ag/AgX/BiOX (X = Cl, Br),24 and
α-Fe2O3–Cu2O.25 The development of HEPs and OEPs is
urgently required to achieve overall water splitting in a
a Department of Chemical Engineering, R&D Center for Membrane Technology and
Luh Hwa Research Center for Circular Economy, Chung Yuan Christian University,
Chungli Dist., Taoyuan City, 32023, Taiwan. E-mail: chechiahu@cycu.edu.tw;
Fax: +886 3 2654199; Tel: +886 3 2654152
b Applied Chemistry, Graduate School of Sciences and Technology for Innovation,
Yamaguchi University, Ube, Yamaguchi, 755-8611, Japan
c Blue Energy Center for SGE Technology (BEST), Yamaguchi University, Ube,
Yamaguchi, 755-8611, Japan
d Department of Environmental Engineering & Innovation and Development Center
of Sustainable Agriculture, National Chung Hsing University, Taichung City, 40227,
Taiwan
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy00266f
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Fig. 1 (a) Schematic diagram of a Z-scheme system containing H2- and O2-evolved photocatalysts in the presence of an electron mediator to
produce H2 and O2. The number of publications searched by keywords (b) “Z scheme” and “water splitting” and (c) BiOI and Photocat* using the
web of science on 12/04/2019.
Z-scheme system. However, materials used for effective
photocatalytic O2 evolution have been rarely considered.
Bismuth-based materials, including BiVO4 and BiOX (X = F,
Cl, Br, and I), have a valence band composed of Bi 6 s and O
2p states that could serve as a potential candidate for
OEPs.26–29 Among these samples, BiOI with a unique layered
structure, band gap energy of ca. 1.8–2.0 eV, and intercalated
I− ions stacked inside the [Bi2O2]2+ layers is considered a
promising photocatalyst. Vacancy and ionic engineering in
BiOI can alternate the light-harvesting ability and enhance
the charge separation to improve the photocatalytic
activity.30–33 However, to the best of our knowledge, only a
few reports studied the photocatalytic H2 or O2 evolution
using BiOI (see Fig. 1c). Zhang reported that BiOI has
negligible photocatalytic activity in the degradation of
rhodamine B and O2 evolution in 2008,34 whereas Wu et al.
produced H2 using BiOI from water splitting with a rate of
about 1316.9 μmol h−1 g−1 at pH 7 under visible light
irradiation in 2018.35 Zhao discussed the band edge
engineering of BiOI through the introduction of vacancies
and defects, showing that pure BiOI has a conduction band
(CB) and a valence band (VB) energy of −5.02 eV and −6.91
eV, respectively.33 This implies that BiOI has a relatively
narrow band gap of ca. 1.9 eV, enabling an enhanced visible
light response and could use more sunlight compared to that
of BiOCl and BiOBr. However, the CB position of BiOI cannot
satisfy the reduction potential of H+/H2 (−4.44 eV). In
contrast, the VB edge of BiOI is much lower than the
oxidation potential of O2/H2O (−5.67 eV).33 This result
indicates that BiOI has a strong potential to produce O2 and
of metallic Ru or metal oxide (RuO2) exhibits remarkable
photocatalytic activity because it can effectively facilitate
charge separation to attract photoexcited holes from the
photocatalyst to Ru/RuO2 sites.36,37 Many methods, including
photodeposition, impregnation, sputtering, direct mixing,
electrodeposition, or reduction, are widely employed to load
nanoparticles onto the surface of the photocatalyst. Among
these methods, the photodeposition method is an alternative
one because the in situ formation of the well-defined particles
on the surface of the photocatalyst as well as the H2
generation could be achieved at the same time during the
photocatalytic reaction.38 However, no studies have been
reported to explore the loading method of Ru on BiOI and
their correlation to the photocatalytic O2 evolution reaction
in the presence of the electron mediator, and this is worth
investigating.
In the present study, BiOI is subjected to a photocatalytic
O2 evolution reaction in the presence of sacrificial reagents
(AgNO3) or electron mediators (NaIO3 or FeCl3) to examine
the O2 production efficiency. In addition, a conventional co-
catalyst, Ru, was used in an attempt to enhance the
photocatalytic activity of BiOI. The preparations of RuO2-
loaded BiOI in two different manners, including
impregnation and photodeposition, have a strong influence
on the photocatalytic O2 evolution activity. The RuO2-loaded
BiOI sample obtained from the photodeposition method
exhibits the highest O2 production amounts (13 650 μmol) in
5 h and can be reused for at least eight cycles under xenon
lamp irradiation. The determination of structural and optical
changes and the effect of electron mediators in a BiOI system
is of vital importance in the photocatalytic water splitting
field, particularly in a Z-scheme system.
serve as
a
promising OEP in
a
Z-scheme system.
Furthermore, the addition of electron mediators, including
solution type (Fe3+/Fe2+ and IO3 /I−) or solid-state type
−
(reduced graphene oxide),17 as
a redox couple could
Materials and methods
significantly assist the electron–hole transfer, thereby
enhancing the photocatalytic activity in a Z-scheme system by
suppressing the backward reaction. Moreover, metal-loading
as a co-catalyst, such as Pt, Pd, Rh, and Ru, could be the
most effective method for further improving the
photocatalytic performance of a catalyst by acting as an
electron sink. In previous works, ruthenium (Ru) in the form
An in situ growth method was conducted to synthesize BiOI
in this study. BiIJNO3)3·5H2O was added to 200 mL of absolute
ethyl alcohol, followed by the addition of a certain amount of
0.1 mol L−1 KI solution gradually. The pH value was adjusted
to 8 using an ammonia solution (NH4OH) to ensure its
stability. The solution was subsequently stirred at 80 °C using
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
a magnetic agitator for 4 h, and, thereafter, filtered and
washed with deionized water three times. The samples were
collected and dried at 60 °C overnight. All chemicals were
purchased from Alfa Aesar and used without any purification.
To improve the photocatalytic activity, Ru was loaded onto
the BiOI surface as a co-catalyst using the impregnation or
photodeposition method. Typically, certain amounts of RuCl3
were added dropwise into aqueous BiOI suspension (1000/50
mg mL−1) with continuous stirring at 80 °C. Until the
evaporation of excess water, the yielding sample was
subjected to a furnace for heat treatment at 300 °C for 1 h
(ramp rate 2 °C min−1), and denoted as xRuim/BiOI, where im
and x indicate the impregnation method and weight ratio of
Ru to BiOI, respectively. For the photodeposition of Ru on
BiOI, RuCl3 was added to aqueous BiOI suspension (1000/50
mg mL−1) with continuous stirring and subjected to light
irradiation using a metal halide lamp for 2 h. Thereafter, the
sample was collected, dried at 60 °C overnight, and denoted
as xRupd/BiOI, where pd refers to photodeposition.
spectra were acquired in fluorescence mode by detecting
fluorescence X-ray from the samples in an Ar atmosphere
using a Lytle detector. The photon energies were calibrated
with the first inflection point of Ru metal (2838 eV) powder
according to previous work.39
Photocatalytic O2 evolution reactions were performed
using 0.25 g of the pristine BiOI, xRupd–BiOI, or xRuim–BiOI
catalysts suspended in 250 mL of an aqueous solution
containing an electron mediator, such as AgNO3 (1 × 10−2, 5 ×
10−3, 1 × 10−3, 5 × 10−4 M), NaIO3 (1 × 10−2, 5 × 10−3, 1 × 10−3,
5 × 10−4 M), or FeCl3 (1 × 10−2 M), in a Pyrex glass reactor cell
with a magnetic stirrer. A 250 W metal halide lamp and a 300
W xenon lamp without any filter were used as the light
source (Fig. S1†). The gas evolutions were determined by gas
chromatography (Shimadzu GC-2014, Japan).
Results and discussion
The crystalline structure of BiOI was examined by XRD
(Fig. 2a), revealing its pure tetragonal system with a space
group of P4/nmm, and its lattice constant along the a- and
c-axes are 3.98 and 9.13 Å (JCPDS #73-2062), respectively,
without any impurity. The sample was investigated by SEM
and TEM, and the images are shown in Fig. 2b–d. It can be
seen that the BiOI sample is composed of a sheet-like
structure forming a flower-shaped microsphere approximately
5–6 μm in size (Fig. 2b and its inset). From the EDS spectra
shown in the inset of Fig. 2b, only the Bi, O, and I elements
could be found. This confirms its pure BiOI configuration. In
Fig. 2c, BiOI has a thin and transparent layered structure,
In this study, we used X-ray diffraction (XRD) to determine
the crystalline structure of the specimens with
a
diffractometer (Bruker D8 advance, USA) at 2θ of 10–80° and
a scan rate of 6° min−1. The morphologies and appearances
of the samples were observed by scanning electron
microscopy (SEM; Hitachi S-4800N, Japan) and high-
resolution transmission electron microscopy (HR-TEM;
Hitachi H-7100, Japan, JEOL JEM-2100, Japan). Elemental
compositions of the specimens were examined by energy-
dispersive X-ray spectroscopy (EDS) equipped with SEM. A
thermogravimetric analyzer (TGA, TA-Q50, DuPont, USA) was
used to determine the thermal behavior of the samples. The
N2 surface area was determined with the Brunauer–Emmett–
Teller (BET) method at −196 °C using an adsorption
apparatus (Micromeritics ASAP 2010, USA). Fourier transform
infrared spectroscopy (FTIR; Tensor 27, Bruker, USA) was
used to investigate the functional groups of the samples. The
light absorption properties of the samples were identified
using an ultraviolet-visible (UV-vis; Hitachi U-3900, Japan)
spectrometer equipped with an integrating sphere. The
oxidation states and chemical binding energy of the samples
were measured using X-ray photoelectron spectroscopy (XPS;
VG Scientific ESCALAB 250, Electron Spectroscopy for
Chemical Analysis, UK) with Al Kα radiation, which was
calibrated using the C 1s peak as the reference energy at
284.8 eV. The VB of the samples were determined by
ultraviolet photoemission spectroscopy (UPS; Thermo VG-
Scientific/Sigma Probe, UK) using He I (0–21.2 eV) as UV
source. The Ru LIII-edge X-ray absorption fine structure
(XAFS) measurements were carried out at BL-9A in KEK-PF
(Ibaraki, Japan) using radiation that has been
monochromatized
with
a
Si
(111)
double-crystal
monochromator. The sample and reference powders were
placed on a sample holder using carbon tape in a small
chamber under He atmosphere. The intensity of the incident
X-ray beam was monitored by an ion chamber filled with He
gas positioned in front of the sample to obtain I0. The XAFS
Fig. 2 (a) XRD, (b) SEM, (c) TEM, and (d) HRTEM images of BiOI. Insets
of (b) show the SEM image with a scale bar of 5 μm and its EDS
spectrum. Insets of (d) show the SAED patterns and schematic
crystalline structure of BiOI.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
which staked into a nanosheet-like configuration that can
easily be observed. In addition, the lattice fringes of the
interplane were estimated to be 0.28 nm, which corresponded
to the d-spacing of (110) in BiOI (Fig. 2d). The selected area
electron diffraction (SAED) patterns indicate the presence of
the (110), (114), and (122) planes, supporting the diffraction
peaks in XRD. According to the crystalline geometry, BiOI
consisted of [Bi2O2]2+ layers containing I atoms along the c
axis, where I atoms were situated within the unit cell without
forming a covalent bond. Instead, they formed an asymmetric
decahedral geometry with electrostatic forces.40 As a result,
the strong intralayer bonding and interlayer forces in the I
atom-intercalated [Bi2O2]2+ layers could construct a platelet
configuration. Fig. S2† shows the N2 adsorption–desorption
isotherm, pore width, and TGA plot of BiOI. It can be noted
that BiOI is a mesoporous material with a typical type IV
isotherm and a H3 hysteresis loop at a relative pressure of
0.8–0.98,41 and its BET specific surface area is 5.8 m2 g−1 (Fig.
S2a†). It can be seen in Fig. S2b† that BiOI begins to
decompose in the airflow at an elevated temperature of 280
°C. From 280 to 450 °C, the total weight decreases by
approximately 25%. By increasing the temperature from 450
to 650 °C, the weight loss could be ascribed to the phase
transformation for producing a metastable Bi5O7I. Finally, it
is in good agreement with the previous report that the sample
transforms from Bi5O7I to Bi2O3 with ca. 2–3% of weight loss
beyond 700 °C.42 We also determined the chemical functional
groups of BiOI using the FTIR technique (Fig. S2c†),
displaying several peaks located at approximately 1000–2000
cm−1 and broadband at 3000–3600 cm−1. These peaks at 1070,
1300, and 1600 cm−1 are identical to those characteristic
peaks of BiOI in the previous report,43 and the band at 3000–
3600 cm−1 can be associated with the hydroxyl groups of the
surface-adsorbed H2O on the BiOI surface.
in Fig. 3b–d. The peaks at the binding energies of ca. 159 eV
and 164 eV could be attributed to Bi 4f7/2 and Bi 4f5/2
(Fig. 3b), respectively, which are the characteristics of Bi3+ in
BiOI.44 In addition, two peaks at approximately 620 eV and
631.5 eV, which can be assigned to the I 3d5/2 and I 3d3/2
states, were observed. These peaks can be ascribed to the −1
valence state of I in the BiOI sample.45 Moreover, the
spectrum for the O 1s core level of BiOI can be deconvoluted
into two peaks located at 530.7 eV and 532.8 eV,
corresponding to Bi–O bonding in the [Bi2O2]2+ layer of BiOI
and the OH group. This OH group mainly results from
surface-adsorbed H2O molecules.46 Therefore, it is confirmed
to be the pure BiOI phase and its I-intercalated [Bi2O2]2+
layered nature.
Fig. 4 shows the time courses of O2 evolution from a water
solution containing different electron mediators and pH
adjusters, such as NaIO3, FeCl3, AgNO3, and HNO3, using
BiOI as the photocatalyst under metal halide lamp
irradiation. In principle, O2 evolved steadily at a rate of 1540
μmol h−1 for the BiOI sample in the presence of 1 × 10−2
M
NaIO3 as an electron mediator. Different concentrations of
NaIO3 were applied to investigate the O2 evolution using BiOI
(Fig. S3a†), revealing that the O2 evolution is in proportion to
the NaIO3 concentration. However, no O2 gases could be
detected without the addition of a catalyst even in the
presence of NaIO3 as an electron mediator. For the other
electron mediator and pH adjuster, the O2 evolution amounts
reached a steady value of less than 1800 μmol and 3000
μmol, respectively. Fig. S3b and c† showed the concentration
and pH-dependent time courses of O2 evolution for AgNO3
and HNO3, respectively, showing that AgNO3 and FeCl3 are
not good candidates for promoting O2 evolution for BiOI. In
the cases of AgNO3, FeCl3, and HNO3, the low O2 evolutions
indicated the deactivation of the sample. Fig. S4† shows the
XRD and XPS plots of BiOI after the photocatalytic reaction
using FeCl3 and AgNO3, revealing that the crystalline
structure of BiOI was destroyed. We can observe the presence
of AgI- and Fe-based iodide compounds after the reaction.
This agrees with the deactivation of the sample in the
photocatalytic reaction with prolonged time. Most
The chemical states of BiOI were determined by XPS
(Fig. 3), showing a typical survey spectrum composed of Bi
4d, Bi 4p, Bi 4f, I 3d, and O 1s states for BiOI (Fig. 3a). The
XPS spectra of the Bi 4f, I 3d, and O 1s core states are shown
Fig. 4 Photocatalytic O2 evolution using BiOI in the presence of
different electron mediators (1 × 10−2 M) and pH adjuster under metal
halide lamp irradiation.
Fig. 3 XPS spectra for (a) survey scan, (b) Bi 4f, (c) I 3d, and (d) O 1s
level of BiOI.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
importantly, the high activity of BiOI in the presence of the
−
NaIO3 solution could be attributed to the IO3 /I− redox
potential of 0.67 V (vs. ENHE), which is suitable for receiving
photoexcited electrons from BiOI for reduction (see eqn (1)),
thus, enhancing the charge separation to trigger the
photoinduced hole at the BiOI side for oxidizing H2O to
produce O2 (see eqn (3)). The proposed reaction mechanism
is shown as follows:
−
+
BiOI + light → eCB + hVB
−
IO3 + 3H2O + 6eCB− → I− + 6OH−
(1)
(2)
(3)
(4)
I− + 6OH− + 6h+ → IO3 + 3H2O
−
H2O + hVB+ → O2 + 4H+
2H+ + 2eCB → H2
−
Fig.
6
(a) XRD patterns, (b) SEM image, (c) Bi 4f level for XPS
spectrum, and (d) chemical composition obtained from XPS for BiOI
after the photocatalytic O2 production reaction. The inset of (b) shows
the EDS spectrum of BiOI.
It is important to determine the photocatalytic cycle test
for a sustainable photocatalyst. The long-term photocatalytic
stability to produce O2 using BiOI in the presence of NaIO3
as an electron mediator using xenon lamp to simulate solar
irradiation is shown in Fig. 5. Evidently, BiOI has high
photocatalytic stability to produce around 7500–9000 μmol in
5 h for each cycle, and the production rate is almost
BiOI, revealing that the samples exhibit pure BiOI tetragonal
phase without any impurity, which is mainly because the Ru
content is relatively low. In addition, the main diffraction
peaks of 1Ruim/BiOI and 1Rupd/BiOI at approximately 29.7°
did not shift toward higher or lower degrees (Fig. S5†),
indicating the lattice of BiOI did not expand or shrink after
Ru loading. The XPS survey spectra of these samples also
suggest that there are Bi 4d3, 4d5, and 4f and I 3d, 3p3, and
3p1 levels for the BiOI sample without any impurity.
Interestingly, the Bi 4f, I 3d, and O 1s states in Fig. 7c–e
shifted toward relatively high binding energies compared to
those peaks in Fig. 3. This can be attributed to the higher
electronegativity of Ru (2.2) compared to that of Bi (2.02),
which may lead to electrons near the Bi, I, and O states
moving toward Ru sites, thereby increasing the binding
energy of the Bi 4f and I 3d levels. And the electron
movement and the increasing binding energy of the Bi 4f
level in 1Rupd/BiOI suggest the photoexcited electrons could
probably flow toward Ru sites more easily, thereby improving
its photocatalytic activity. The Bi 4f7/2, 4f5/2, I3d5/2, and 3d3/2
levels and the Bi–O bond can still be observed in Fig. 7c–e,
suggesting the well-preserved structure of BiOI after Ru
modification. A small peak at around 625.6 eV was observed
to 1Ruim/BiOI in Fig. 7d. The peak could be attributed to the
oxidized specie, i.e., I2O5,47 on the sample surface after heat
treatment of 300 °C of the impregnation method. The Ru
3d5/2 and 3d3/2 states can be seen in Fig. 7f, and these states
indicate the coexistence of the metal Ru phase and the RuO2
phase on BiOI. The C–C bond at approximately 284.7 eV is
set to fill the region envelope. Most importantly, the presence
of Ru and RuO2 on BiOI could serve as accommodating sinks
for photoexcited holes to further improve the charge
separation and photocatalytic O2 evolution.36,37
unchanged, indicating that BiOI could be
a potential
photocatalyst. Moreover, the XRD, SEM, and XPS
measurements of BiOI after the photocatalytic reaction are
shown in Fig. 6, indicating that the crystalline structure,
appearance and morphology, and the binding energy for the
Bi 4f level of BiOI all remain unchanged. The chemical
composition obtained from the elemental analysis (Fig. 6d)
also suggested that the stoichiometric ratio of BiOI after the
reaction is consistent with that before the reaction.
To further improve the photocatalytic activity of BiOI, Ru
was loaded onto the BiOI surface to increase the active sites
for water splitting. Two methods, impregnation and
photodeposition, were employed for loading Ru on BiOI.
Fig. 7a shows the XRD patterns of 1Ruim/BiOI and 1Rupd
/
Fig. 5 Cycle test of photocatalytic O2 evolution using BiOI in the
presence of NaIO3 as an electron mediator with intermediate
evacuation using xenon lamp to simulate solar irradiation.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Fig. 7 (a) XRD and XPS spectra for (b) survey scan, (c) Bi 4f, (d) I 3d, (e) O 1s, and (f) Ru 3d levels of the 1Ruim/BiOI and 1Rupd/BiOI samples.
The 1Ruim/BiOI and 1Rupd/BiOI samples were investigated
by HRTEM images (Fig. 8a and b). The lattice fringes of
1Ruim/BiOI and 1Rupd/BiOI samples are visible, showing the
of Ru on the BiOI surface. The white-line peaks for the
1Ruim/BiOI and 1Rupd/BiOI samples were observed around
2842 eV, as well as that of RuO2. The result indicates that the
oxidation state of Ru cocatalyst is mainly composed of RuO2
(Ru4+), which is generally considered to be an active oxidative
cocatalyst.48
Different weight contents of Ru were employed to be
individually impregnated or photodeposited onto the surface
of BiOI, followed by the photocatalytic O2 production. Fig. 9
shows the time courses of O2 evolution from a water solution
containing NaIO3 as an electron mediator using Ruim/- or
good crystallinity. The d-spacing of 1Rupd/BiOI and 1Ruim
/
BiOI samples were estimated to be around 0.88 nm and 0.3
nm, which referred to the (001) and (012) of BiOI,
respectively. It can be seen that numerous small circular
particles, which are attributed to Ru, were homogeneously
distributed on the surface of the BiOI sample with a size of
approximately 1–3 nm. This suggested that these Ru particles
at the BiOI surface could act as a cocatalyst to receive
photoexcited charges for the photocatalytic reaction. In the
inset of Fig. 8a and b, these samples consist of thin and
transparent sheet-like structure, which is similar to the
HRTEM result in Fig. 2c. Moreover, as shown in Fig. 8c, we
employed Ru LIII-edge XAFS to determine the oxidative states
Rupd/BiOI as
a photocatalyst under metal halide lamp
irradiation. Interestingly, all BiOI samples after Ru-
impregnation showed significantly decreased activities. This
is mainly because the appearance of BiOI had been destroyed
from the flower-like structure to an irregular sheet-like
stacking configuration (Fig. S6a†), resulting from heat
treatment, which is certainly required in impregnation to
influence the morphology of BiOI and hence its
photocatalytic activity. This is consistent with the above TGA
that BiOI is metastable between heating temperatures of 250–
450 °C (Fig. S2b†). Conversely, the photodeposited Ru on the
BiOI surface could extensively improve the photocatalytic O2
production. Particularly, 1Rupd/BiOI and 0.5Rupd/BiOI had O2
production rates of approximately 2730 μmol h−1 and 2360
μmol h−1, respectively, which are much higher than that of
pristine BiOI (1540 μmol h−1) under metal halide lamp
irradiation. The high activity to produce O2 for Rupd/BiOI
samples could be attributed to that the RuO2 cocatalyst can
effectively receive photoexcited holes from BiOI to enhance
the charge separation owing to its partially filled π* band at
metal–oxygen (d-p) level, which is similar to that in the
reported studies.37,49 In addition, we also evaluated the
photocatalytic stability of 1Rupd/BiOI in the presence of
Fig. 8 (a and b) HRTEM images and (c) XAFS spectrum of 1Rupd/BiOI
and 1Ruim/BiOI samples. Inset of (a) and (b) shows the TEM images of
the corresponding samples. Metallic Ru and RuO2 were used as
references in the XAFS spectrum.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
Fig. 9 Photocatalytic O2 production using different weight contents of Ru on BiOI through the (a) impregnation and (b) photodeposition method
in the presence of NaIO3 as an electron mediator under metal halide lamp irradiation.
NaIO3 under xenon lamp irradiation (Fig. 10), revealing that
O2 evolved steadily with time and that the sample can be
reused for at least eight cycles. The O2 evolution amount of
Rupd/BiOI in this study is higher than the reported values
using Pt-doped α-Fe2O3,50 doped TiO2,51 decorated
g-C3N4,52,53 and conjugated polymers.54 Fig. S6b† shows the
XRD patterns of 1Rupd/BiOI before and after the
photocatalytic O2 production, revealing the structural stability
and that no impurities can be found. In addition, Fig. S7†
displays the SEM images and the corresponding EDS
mapping of Bi, O, I, and Ru elements for 1Rupd/BiOI before
and after the reaction. Evidently, the appearance and
morphology of 1Rupd/BiOI remained unchanged as a sheet-
stacked flower-like configuration (Fig. S7a and f†). In the
elemental mapping, Bi, O, I, and Ru elements were
homogeneously distributed before and after the reaction. But
in the high-resolution XPS spectra for the used sample (Fig.
S8†), the peak of Bi 3d level moved toward lower binding
energy compared to that in Fig. 7, indicating its reduced
state.55 Besides, the O 1s level shifted to high binding energy
(Fig. S8b†), showing the formation of O–I bond
(electronegativity of I is larger than that of Bi). Moreover, the
I 3d level in Fig. S8c† also shifted to larger binding energy
than that in the fresh sample, which means the presence of
anionic vacancies.56,57 On the other hand, the additional two
peaks at around 625 and 637 eV were assigned to iodate or
iodide,58 indicating the surface adsorbed redox-couples.
These results suggest that Bi could be reduced by
photoexcited electrons, and the I atom of BiOI could leach
during the photocatalytic reaction to react with the surface
adsorbed O2− to form a weak O–I bond to proceed with an
interfacial reduction, which may contribute to improving the
Z-scheme water splitting for O2 generation in our case. At the
same time, generated I− could compensate for the vacancies
of BiOI, which means this could only occur in the BiOI case
in the presence of NaIO3 as a mediator. But the interfacial
reduction–oxidation still needs further determination.
Nevertheless, the I-contained RuO2/BiOI still could be an
effective photocatalyst for O2 production, especially in the
presence of NaIO3 as an electron mediator.
This study presents the UV-vis spectroscopic measurement
for the RuO2-loaded BiOI samples in Fig. 11a, showing that
the absorption edges of these samples were similar, with
values of approximately 660 nm. The calculated band gap
values of BiOI and 1Rupd/BiOI were obtained to be 2.02 eV
and 1.92 eV, respectively, from the Tauc plot in the inset of
(a). Interestingly, Ru modification did not change the
absorption shoulder and its band gap energy. Instead, Ru
modification on BiOI only resulted in a significant increase
in absorption in the range of 680–800 nm, and the intensity
became relatively strong when the Ru content increased. This
interesting feature can be ascribed to the surface plasmonic
effect associated with the excitation of collective electron
oscillation on the metallic Ru/RuO2 surface.49 Moreover, this
surface plasmonic effect could not only attract the
photoexcited hole but also produce an extra trap state to
improve the visible light-harvesting ability of the Rupd/BiOI
sample. The UPS was employed to elucidate the band edge of
BiOI (Fig. 11b), revealing that the linear extrapolation of the
tangent line was located at approximately 6.2 eV (inset of
Fig. 11b). This value is consistent with the reported values of
5.9–6.1 eV.59 In combination with the band gap energy
obtained from the UV-vis spectra, an energy band structure
Fig. 10 Cycle test of photocatalytic O2 evolution using 1Rupd/BiOI in
the presence of NaIO3 as an electron mediator with intermediate
evacuation using xenon lamp to simulate solar irradiation.
of BiOI along with the redox potentials of H+/H2, IO3 /I−, and
−
O2/H2O was proposed in Fig. 11c. It can be noted that the CB
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Fig. 11 (a) UV-vis, (b) UPS spectra, and (c) proposed energy band position of RuO2-loaded BiOI.
of BiOI is very close to the redox potential of H2 production References
−
but much higher than that of IO3 /I−, indicating that the
1 C. W. Chang and C. Hu, Chem. Eng. J., 2019, 383, 12116,
DOI: 10.1016/j.cej.2019.123116.
2 C. Hu, Y. R. Lin and H. C. Yang, ChemSusChem, 2019, 12,
1794–1806, DOI: 10.1002/cssc.201802257.
3 Z. Ni, Y. Sun, Y. Zhang and F. Dong, Appl. Surf. Sci.,
2016, 365, 314–335, DOI: 10.1016/j.apsusc.2015.12.231.
4 M. Arumugam and M. Y. Choi, J. Ind. Eng. Chem., 2020, 81,
237–268, DOI: 10.1016/j.jiec.2019.09.013.
5 C. Karthikeyan, P. Arunachalam, K. Ramachandran, A. M. Al-
Mayouf and S. Karuppuchamy, J. Alloys Compd., 2020, 828,
154281, DOI: 10.1016/j.jallcom.2020.154281.
photoexcited electrons have sufficient over-potential to react
with IO3 to produce I−. Conversely, with Ru modification,
−
the photoexcited holes could move toward the RuO2 site to
oxidize H2O to produce O2 because the reactive potential
position of BiOI is much lower than the oxidative potential of
O2/H2O. As a result, RuO2-loaded BiOI can be an effective
photocatalyst for photocatalytic O2 evolution but is not
suitable for producing H2.
Conclusions
6 A. Malathi, J. Madhavan, M. Ashokkumar and P.
Arunachalam, Appl. Catal., A, 2018, 555, 47–74, DOI:
10.1016/j.apcata.2018.02.010.
7 G. Zhang, Z. A. Lan, L. Lin, S. Lin and X. Wang, Chem. Sci.,
2016, 7, 3062–3066, DOI: 10.1039/C5SC04572J.
8 S. Chen, T. Takata and K. Domen, Nat. Rev. Mater., 2017, 2,
17050, DOI: 10.1038/natrevmats.2017.50.
9 H. F. Liang, A. N. Gandi, D. H. Anjum, X. B. Wang, U.
Schwingenschlögl and H. N. Alshareef, Nano Lett., 2016, 16,
7718–7725, DOI: 10.1021/acs.nanolett.6b03803.
In the present study, RuO2-loaded BiOI was first used as an
effective photocatalyst for O2 evolution in the presence of
NaIO3 as the electron mediator under simulated solar
irradiation. The XAFS and HR-TEM results revealed that
RuO2 was homogeneously distributed on the BiOI surface as
a co-catalyst; the UPS and UV-vis spectra showed that RuO2-
loaded BiOI exhibited an appropriate valence band potential
for generating O2. Highly dispersed RuO2 on the BiOI surface
could serve as an electron sink for enhancing light-harvesting
ability while improving the photocatalytic activity for O2
evolution at a rate of approximately 2730 μmol h−1 for eight
cycle tests. In summary, this study prepared an effective
RuO2-loaded BiOI as an OEP to promote light-harvesting and
charge separation to boost the photocatalytic O2 evolution
under simulated solar irradiation.
10 K. Maeda, ACS Catal., 2013, 3, 1486–1503, DOI: 10.1021/
cs4002089.
11 Y. O. Wang, H. Suzuki, J. J. Xie, O. Tomita, D. J. Martin, M.
Higashi, D. Kong, R. Abe and J. W. Tang, Chem. Rev.,
2018, 118, 5201–5241, DOI: 10.1021/acs.chemrev.7b00286.
12 Q. L. Xu, L. Y. Zhang, J. G. Yu, S. Wageh, A. A. Al-Ghamdi
and M. Jaroniec, Mater. Today, 2018, 21, 1042–1063, DOI:
10.1016/j.mattod.2018.04.008.
Conflicts of interest
13 M. S. Zhu, Z. C. Sun, M. Fujitsuka and T. Majima, Angew.
Chem., Int. Ed., 2018, 57, 2160–2164, DOI: 10.1002/
anie.201711357.
14 R. B. Chandran, S. Breen, Y. X. Shao, S. Ardo and A. Z.
Weber, Energy Environ. Sci., 2018, 11, 115–135, DOI: 10.1039/
C7EE01360D.
15 H. J. Li, W. G. Tu, Y. Zhou and Z. G. Zou, Adv. Sci., 2016, 3,
1500389, DOI: 10.1002/advs.201500389.
16 Y. R. Lin, G. V. C. Dizon, K. Yamada, C. Y. Liu, A. Venault,
H. Y. Lin, M. Yoshida and C. Hu, J. Colloid Interface Sci.,
2020, 567, 202–212, DOI: 10.1016/j.jcis.2020.02.017.
There are no conflicts to declare.
Acknowledgements
This research was supported by the Ministry of Science and
Technology, Taiwan (MOST 107-2221-E-033-032-MY3, 108-
2221-E-033-034-MY3). The Ru LIII-edge XAFS measurements
were carried out at BL-9A in KEK-PF (2018G589), and Prof.
Masaaki Yoshida was financially supported by Kiban C
(17K05843) and the Electric Technology Research Foundation
of Chugoku.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Catalysis Science & Technology
Paper
17 A. Iwase, Y. H. Ng, Y. Ishiguro, A. Kudo and R. Amal, J. Am.
Chem. Soc., 2011, 133, 11054–11057, DOI: 10.1021/ja203296z.
18 H. Kato, M. Hori, R. Konta, Y. Shimodaira and A. Kudo,
Chem. Lett., 2004, 33, 1348, DOI: 10.1246/cl.2004.1348.
19 Z. H. Chen, W. L. Wang, Z. G. Zhang and X. M. Fang, J. Phys.
Chem. C, 2013, 117, 19346–19352, DOI: 10.1021/jp406508y.
20 J. X. Low, B. Z. Dai, T. Tong, C. J. Jiang and J. G. Yu, Adv.
Mater., 2019, 31, 1802981, DOI: 10.1002/adma.201802981.
21 S. Wang, B. C. Zhu, M. J. Liu, L. Y. Zhang, J. G. Yu and M. H.
Zhou, Appl. Catal., B, 2019, 243, 19–26, DOI: 10.1016/j.
apcatb.2018.10.019.
22 K. Sayama, K. Mukasa, R. Abe, Y. Abe and H. Arakawa,
J. Photochem. Photobiol., A, 2002, 148, 71–77, DOI: 10.1016/
S1010-6030(02)00070-9.
23 H. M. Zhu, B. F. Yang, J. Xu, Z. P. Fu, M. W. Wen, T. Guo,
S. Q. Fu, J. Zuo and S. Y. Zhang, Appl. Catal., B, 2009, 90,
463–469, DOI: 10.1016/j.apcatb.2009.04.006.
24 Y. Yu, C. Y. Cao, H. Liu, P. Li, F. F. Wei, Y. Jiang and W. G.
Song, ACS Catal., 2012, 28, 1677–1683, DOI: 10.1039/
C3TA14494A.
25 J. C. Wang, L. Zhang, W. X. Fang, J. Ren, Y. Y. Li, H. C. Yao,
J. S. Wang and Z. J. Li, ACS Appl. Mater. Interfaces, 2015, 71,
8631–8639, DOI: 10.1021/acsami.5b00822.
26 A. Malathi, P. Arunachalam, A. N. Grace, J. Madhavan and
A. M. Al-Mayouf, Appl. Surf. Sci., 2017, 412, 85–95, DOI:
10.1016/j.apsusc.2017.03.199.
27 A. Malathi, P. Arunachalam, J. Madhavan, A. M. Al-Mayouf
and M. A. Ghanemb, Colloids Surf., A, 2018, 537, 435–445,
DOI: 10.1016/j.colsurfa.2017.10.036.
39 I. C. Stefan, Y. B. Mo, M. R. Antonio and D. A. Scherson,
J. Phys. Chem. B, 2002, 106, 12373–12375, DOI: 10.1021/
jp026300f.
40 Y. N. Wang, K. J. Deng and L. Z. Zhang, J. Phys. Chem. C,
2011, 115, 14300–14308, DOI: 10.1021/jp2042069.
41 O. Mehraj, B. M. Pirzada, N. A. Mir, M. Z. Khan and S. Sabir,
Appl. Surf. Sci., 2016, 387, 642–651, DOI: 10.1016/j.
apsusc.2016.05.166.
42 L. Q. Ye, L. H. Tian, T. Y. Peng and L. Zan, J. Mater. Chem.,
2011, 21, 12479–12484, DOI: 10.1039/C1JM11005E.
43 L. H. Ai, Y. Zeng and J. Jiang, Chem. Eng. J., 2014, 235,
331–339, DOI: 10.1016/j.cej.2013.09.046.
44 H. Liu, W. R. Cao, Y. Su, Y. Wang and X. H. Wang, Appl. Catal.,
B, 2012, 111, 271–279, DOI: 10.1016/j.apcatb.2011.10.008.
45 W. E. Morgan, J. R. Vanwazer and W. J. Stec, J. Am. Chem.
Soc., 1973, 95, 751–755, DOI: 10.1021/ja00784a018.
46 L. A. Brook, P. Evans, H. A. Foster, M. E. Pemble, A. Steele,
D. W. Sheel and H. M. Yates, J. Photochem. Photobiol., A,
2007, 187, 53–63, DOI: 10.1016/j.jphotochem.2006.09.014.
47 D. K. Smith, J. McCollum and M. L. Pantoya, Phys. Chem. Chem.
Phys., 2016, 18, 11243–11250, DOI: 10.1039/C5CP06998J.
48 Z. U. Rahman, N. Wei, M. Feng and D. A. Wang, Int. J.
Hydrogen Energy, 2019, 44, 13221–13231, DOI: 10.1016/j.
ijhydene.2019.03.176.
49 M. T. Uddin, Y. Nicolas, C. Olivier, T. Toupance, M. M.
Muller, H. J. Kleebe, K. Rachut, J. Ziegler, A. Klein and W.
Jaegermann, J. Phys. Chem. C, 2013, 117, 22098–22110, DOI:
10.1021/jp407539c.
50 H. Liu, K. Tian, J. Ning, Y. Zhong, Z. Zhang and Y. Hu, ACS
Catal., 2019, 9, 1211–1219, DOI: 10.1021/acscatal.8b03819.
51 F. Amano, R. Tosaki, Y. Sato and Y. Higuchi, J. Solid State
Chem., 2018, 258, 79–85, DOI: 10.1016/j.jssc.2017.09.030.
52 C. Ye, J. X. Li, Z. J. Li, X. B. Li, X. B. Fan, L. P. Zhang, B.
Chen, C. H. Tung and L. Z. Wu, ACS Catal., 2015, 5,
6973–6979, DOI: 10.1021/acscatal.5b02185.
28 Y. Bu, J. Xu, Y. Li, Q. Liu and X. Zhang, RSC Adv., 2017, 7,
42398–42406, DOI: 10.1039/C7RA06462D.
29 A. Malathi, P. Arunachalam, V. S. Kirankumar, J. Madhavan
and A. M. Al-Mayouf, Opt. Mater., 2018, 84, 227–235, DOI:
10.1016/j.optmat.2018.06.067.
30 Y. C. Huang, H. B. Li, M. S. Balogun, W. Y. Liu, Y. X. Tong,
X. H. Lu and H. B. Ji, ACS Appl. Mater. Interfaces, 2014, 6,
22920–22927, DOI: 10.1021/am507641k.
31 Y. C. Huang, H. B. Li, W. J. Fan, F. Y. Zhao, W. T. Qiu, H. B.
Ji and Y. X. Tong, ACS Appl. Mater. Interfaces, 2016, 8,
27859–27867, DOI: 10.1021/acsami.6b10653.
53 C. Ye, X. Z. Wang, J. X. Li, Z. J. Li, X. B. Li, L. P. Zhang, B.
Chen, C. H. Tung and L. Z. Wu, ACS Catal., 2016, 6,
8336–8341, DOI: 10.1021/acscatal.6b02664.
54 L. Wang, Y. Wan, Y. Ding, Y. Niu, Y. Xiong, X. Wu and H. Xu,
Nanoscale, 2017, 9, 4090–4096, DOI: 10.1039/C7NR00534B.
55 Y. Li, Y. Hu, C. A. Morrison, W. Wu, H. Han and N.
Robertson, Sustainable Energy Fuels, 2017, 1, 308–316, DOI:
10.1039/C6SE00061D.
32 Y. C. Huang, H. X. Hu, S. X. Wang, M. S. Balogun, H. B. Ji
and Y. X. Tong, Appl. Catal., B, 2017, 218, 700–708, DOI:
10.1016/j.apcatb.2017.07.028.
33 W. W. Dai and Z. Y. Zhao, J. Am. Ceram. Soc., 2016, 99,
3015–3024, DOI: 10.1111/jace.14311.
34 J. An, Y. Du, T. Wang, C. Wang, W. Hao and J. Zhang, Rare
Met., 2008, 27, 243–250, DOI: 10.1016/S1001-0521(08)60123-
0.
35 G. J. Lee, Y. C. Zheng and J. J. Wu, Catal. Today, 2018, 307,
197–204, DOI: 10.1016/j.cattod.2017.04.044.
36 S. Y. Mi, Y. X. Liu and W. D. Wang, Chin. J. Chem. Phys.,
2016, 29, 585–590, DOI: 10.1063/1674-0068/29/cjcp1603057.
37 Q. Gu, Z. Gao, S. Yu and C. Xue, Adv. Mater. Interfaces,
2016, 3, 1500631, DOI: 10.1002/admi.201500631.
56 M. Zi, J. Li, Z. Zhang, X. Wang, J. Han, X. Yang, Z. Qiu, H.
Gong, Z. Ji and B. Cao, Phys. Status Solidi A, 2015, 212,
1466–1470, DOI: 10.1002/pssa.201532015.
57 A. Liu, H. Zhu, W. T. Park, S. J. Kang, Y. Xu, M. G. Kim and
Y. Y. Noh, Adv. Mater., 2018, 30, 1802379, DOI: 10.1002/
adma.201802379.
58 K. Li, Y. Zhao, P. Zhang, C. He, J. Deng, S. Ding and W. Shi, Appl.
Surf. Sci., 2016, 390, 412–421, DOI: 10.1016/j.apsusc.2016.08.095.
59 R. L. Z. Hoye, L. C. Lee, R. C. Kurchin, T. N. Huq, K. H. L.
Zhang, M. Sponseller, L. Nienhaus, R. E. Brandt, J. Jean, J. A.
Polizzotti, A. Kursumović, M. G. Bawendi, V. Bulović, V.
Stevanović, T. Buonassisi and J. L. MacManus-Driscoll, Adv.
Mater., 2017, 29, 1702176, DOI: 10.1002/adma.201702176.
38 K. Wenderich and G. Mul, Chem. Rev., 2016, 116,
14587–14619, DOI: 10.1021/acs.chemrev.6b00327.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.