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Cite this: J. Mater. Chem. A, 2015, 3,
8389

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Photofuel cell comprising titanium oxide andbismuth oxychloride (BiO1xCl1y) photocatalysts
that uses acidic water as a fuel†Yukiko Fujishima,‡ab Seiji Okamoto,‡a Mao Yoshiba,‡a Takaomi Itoi,cShogo Kawamura,a Yusuke Yoshida,a Yuta Oguraa and Yasuo Izumi*a
Solar cells and fuel cells have been extensively investigated. However, the need for sustainability, durability,
and an electromotive force greater than 1.5 V per cell has not yet been fully realized. Herein, a photofuel cell
(PFC) comprising two photocatalysts TiO2 and BiOCl that uses acidic water as a fuel is described. The PFC is
designed such that water can be regenerated from the photogenerated O2 in the cell, and the electromotive
force is as high as the diﬀerence between the conduction band minimum of TiO2 and the valence band
maximum of BiOCl (theoretical: 2.75 V, experimental: 1.76 V). The photocurrents were in accordancewith a kinetic model based on the combination of the excited electron concentration at TiO2 and thehole concentration at BiOCl. The reaction between the surface sites of O and/or Cl-deﬁcient BiO1xCl1yspecies formed by ultraviolet-visible light irradiation with O2 was in situ monitored using Bi L3-edge
extended X-ray absorption ﬁne structure and X-ray photoelectron spectroscopy supported by Raman

Received 12th December 2014
Accepted 23rd February 2015

and UV-visible spectroscopy and could be involved in the cathodic O2 photoreduction in the PFC.
Furthermore, amorphous-like BiO1xCl species were observed on the surface of the BiOCl crystallites inhigh-resolution transmission electron microscopy. Importantly, this PFC that comprises TiO2 and BiOCl
is sustainable, recyclable, and has a series resistance that can be controlled by adjusting the BiOCl

DOI: 10.1039/c4ta06824f

particle size, and exhibits a more eﬃcient charge ﬂow because of band bending than that of a previously

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reported PFC based on the rectiﬁcation of the Schottky barrier.

1

Introduction

Fossil fuels have been utilized as the essential energy source forindustrialization. However, industrial CO2 emissions have led
to an increase in the atmospheric CO2 concentration (400 ppm),
and thus the eﬀects of this increase in terms of global warming
cannot be underestimated. The development of renewableenergy as a replacement for fossil fuels has been slow.1,2 Amongthese renewable energies, solar energy has the greatest potential; the solar energy that reaches the earth in 1 h is equivalent
to the energy consumed on the earth in 1 year.1,2 AlthoughaDepartment of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33,
Inage-ku, Chiba 263-8522, Japan. E-mail: yizumi@faculty.chiba-u.jp; Fax: +81-43290-2783; Tel: +81-43-290-3696

b

Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka
1-3, Shinjuku-ku, Tokyo 163-8601, Japanc
Department of Mechanical Engineering, Graduate School of Engineering, Chiba
University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan† Electronic supplementary information (ESI) available: The derivation of eqn (6)for PFC photocurrents. An HR-TEM video observation of BiOCl in Ar atmosphereand irradiated under UV-visible light for 1 h is available, an HR-TEM video
observation of BiOCl in Ar atmosphere and irradiated under UV-visible light for
1 h is available. See DOI: 10.1039/c4ta06824f‡ These authors contributed equally to this paper.

This journal is © The Royal Society of Chemistry 2015

silicon solar cells (Si SCs) have been commercialized, achieving
the widespread adoption of technologies that convert solar
energy into electricity oen requires government subsidies.3
Other types of SCs, including compound semiconductors anddye-sensitized SCs,4 and fuel cells (FCs) that use hydrogen as afuel,5 which potentially can be generated using solar energy,6have also been extensively investigated. However, all of therequisites (sustainability, durability, and an electromotive force
greater than 1.5 V per cell) have not yet been fully realized.
Recently, a new device, a photofuel cell (PFC) consisting of
the two photocatalysts TiO2 and silver-doped TiO2 on an electrode lm both immersed in acidic water and separated by aproton-conducting polymer (PCP), was demonstrated.7 The
Schottky barrier between TiO2 and Ag nanoparticles at thecathode rectied the cell currents. In the present study, n-TiO2
and p-BiOCl8 were used as the anode and cathode, respectively,
to enhance the photocurrent generation in the PFC by facilitating charge transfer at the interface between the photocatalyst
lm and the substrate in the acidic solution. The energy
diagram and electron ow in the PFC comprising TiO2 andBiOCl photocatalysts at pH 2 (typical conditions for PFC tests)
are shown in Scheme 1. The energy of the bottom of the
conduction band (CB) is 0.41 V @ pH 7 for TiO2 (ref. 9) and1.1 V @ pH 6.8 for BiOCl,10 whereas the band gap values for

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Scheme 1 Energy diagram and electron ﬂow for a PFC comprising
TiO2 and BiOCl.

anatase-type TiO2, rutile-type TiO2 and BiOCl are 3.2, 3.0,11,12
and 3.46 eV (ref. 13) (Fig. 1A-a), respectively.
The charges in the semiconductors are separated because ofultraviolet (UV) and a small amount of visible light irradiation.
The holes in n-TiO2 are likely to diﬀuse to the surface ofphotooxidize water, whereas the electrons in p-BiOCl are likely
to diﬀuse to the surface to photoreduce O2 molecules because ofband bending at the interface (Scheme 1).14,15 Consequently,
electron ow from the CB of TiO2 to the holes in the valenceband (VB) of BiOCl should become predominant with an
external circuit. The theoretical electromotive force for thesystem is 2.75 V (Scheme 1) when any overvoltage due to thePapercarrier level of the electrodes, charge recombination in thephotocatalysts, and reverse electron ow from the cathode toanode are not considered. In addition, the use of acidic water as“a fuel” is inexpensive and sustainable. It should be mentionedthat acidic water was not exactly a “fuel” consumed in FCs, butrather a regenerative medium to transfer the charge.7
The PFC in this study was also compared to photoelectrochemical electrolysis (or photoelectrocatalytic) cells (PECs)
already known in the literature (Scheme 2).16–33 Type 1 PECs
consist of a photoanode, e.g., TiO2 and WO3, a [photo]cathode,and a redox mediator, e.g., S2/Sn2 (ref. 16) and Fe2+/3+,17 to
diﬀuse the electrons from the cathode to the anode. Type 2
PECs are FCs that consist of a photoanode for fuel oxidation anda conventional cathode, e.g., Pt–carbon, for O2 reduction.18–23
The photoanode and cathode are separated by a PCP lm. Type
3 PECs (or tandem photolysis cells) consist of a photoanode and a[photo]cathode to form O2 and H2, respectively, from water.23–27
In the cells, a redox mediator is used to diﬀuse electrons;28,29
alternatively, protons diﬀuse from the anode to the cathode if aredox mediator is not employed.30,31 Type 4 PECs consist of aphotoanode to oxidize a fuel (not water) and a photocathode to
reduce protons.32,33
In summary, PFCs photooxidize water over a photoanodesimilar to Type 3 PECs and reduce O2 over a photocathodesimilar to Type 2 PECs and other FCs. Thus, the specicadvantage of a PFC is the photocatalytic reduction of O2 at thecathode, which is in contrast to PEC Types 1 to 4 (Scheme 2).

(A) UV-visible absorption spectra for solvo-BiOCl (a) as-synthesized, irradiated under UV-visible light in Ar for (b) 1 min and (c) 1.0 h, (d)sample c in the dark for 24 h, and (e) sample d exposed to air for 5 h. (B) Fitting to the Davis–Mott equation for the data in (A-a). The values for (a hn)2, (a  hn)2/3, (a  hn)1/2, or (a  hn)1/3 versus hn were plotted, and the intercept with the x-axis estimated the Eg value. (C) UV-visible
absorption spectra for solvo-BiOCl (a) as-synthesized, (b) irradiated under UV-visible light in air for 1.0 h, and sample b in the dark (c) for 24 h and(d) 92 h. XRD patterns for solvo-BiOCl (D) as-synthesized, (E) irradiated with UV-visible light under Ar for 9 h, and aqua-BiOCl (F) as-synthesized.
(G and G0 ) Raman spectra for BiOCl (a) as-synthesized and (b) irradiated using UV-visible light for 12 h under vacuum.
Fig. 18390 | J. Mater. Chem. A, 2015, 3, 8389–8404

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Scheme 2 Components and chemical reactions in a PFC and Type 1–4 PECs. The dotted arrow indicates the possible use of a photoelectrode ora general electrode, and the dotted line at the middle of the cell indicates the possible use of a PCP for separation.

Importantly, the combination of a photoanode and aphotocathode in a PFC enables a theoretical electromotive force
as high as 2.75 V (Scheme 1). In Type 1 PECs, the diﬀerence(1 V) between the CB minimum of the photoanode and thepotential of the redox mediator is the electromotive force.30,32 In
Type 2 PECs, the diﬀerence (1 V) between the CB minimum of
the photoanode and the electric potential of O2 reduction is theelectromotive force. In Type 3 PECs, the diﬀerence between the
CB minimum of the photoanode and the VB maximum of thephotocathode is the electromotive force if a redox mediator(IO3/I, Fe3+/2+, Br2/Br)28,29 is not used.30 Finally, in Type 4
PECs, the diﬀerence between the CB minimum of the photoanode and the VB maximum of the photocathode is the electromotive force. However, the electromotive force is less than 1
V when deep (positive) potential anodic WO3 and high (negative) potential cathodic Cu2O are used as photocatalysts for theselective oxidation of the fuel and selective reduction of
protons, respectively.33
A PFC is also diﬀerent from a dye-sensitized SC, which
consists of a dye on a semiconductor and a conventional
cathode.4 The diﬀerence (1 V) between the CB minimum of thephotoanode and the potential of the redox mediator is theelectromotive force in these systems.
Therefore, the electromotive force between the CB minimumof the photoanode and the VB maximum of the photocathode isadvantageous for PFCs and Type 3 and 4 PECs (Scheme 2), but
the potential for the reaction 2H+ + 2e / H2 (0 V @ 298 K and
pH 0) in PECs requires a more negative voltage for the CB of thephotocathode compared to the voltage (1.229 V @ 298 K and pH0) required for the reaction O2 + 4H+ + 4e / 2H2O in a PFC.
Consequently, for Type 3 and 4 PECs, the voltage of the VB
maximum of the photocathode is more negative, and thediﬀerence (electromotive force) with respect to the CB
minimum for the photoanode decreases (Scheme 2). Moreover,
the reactions in a PFC are completely reversible, allowing theregeneration of water; thus, PFCs are recyclable (Scheme 1).

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2 Experimental
2.1 Sample syntheses and preparation of thephotoelectrodes
TiO2 powder [1.00 g; P25, Degussa; anatase/rutile phases ¼ 7/3;
Brunauer–Emmett–Teller (BET) surface area (SBET) ¼ 60 m2 g1]
was suspended in 3.0 mL of deionized water (<0.06 mS cm1)and then stirred well. The obtained suspension was dried at 373
K for 24 h and heated in air at 673 K for 2 h. The resultant
powder was suspended in 75 mL of deionized water and placedon an indium tin oxide (ITO, thickness 1.2 to 1.6 mm)-coatedPyrex glass plate. The TiO2/ITO/Pyrex was dried at 373 K for 18 h
and heated in air at 573 K for 30 min. The amount of TiO2
deposited on the ITO-coated glass plate was 5.0 mg and covered
an area of 1.3 cm2.
BiOCl was synthesized using a solvothermal procedure.34
Bismuth nitrate pentahydrate (2.00 g; >99.5%, Wako Pure
Chemical) was dissolved in 80 mL of ethylene glycol (>99.5%,
Wako Pure Chemical), and then cetyltrimethylammonium
chloride (1.30 g; >95%, Wako Pure Chemical) was added to the
mixture. The solution was magnetically stirred at a rate of 900
rotations per minute (rpm) at 290 K for 1 h. Next, ethylene glycol
solution (4.5 mL) of potassium hydroxide (1.0 mol L1; 85%,
Wako Pure Chemical) was added. The solution was then introduced into a Teon-lined autoclave (Model TVS-N2-100, Taiatsu
Techno) and maintained at 433 K for 12 h. This solution wasltered using a polytetrauoroethene membrane lter (Model
Omnipore JGWP04700, Millipore) with a pore size of 0.2 mm,
and the collected white powder was subsequently washed with
deionized water (total 450 mL) and ethanol (total 450 mL) anddried at 323 K for 12 h. The obtained white BiOCl powder isdenoted as solvo-BiOCl (or simply BiOCl).
For comparison, some of the experiments using BiOCl
synthesized from an aqueous solution were also performed.35
For this process, 3.15 g of Bi trichloride (>97%; Wako Pure
Chemical) was dissolved in 100 mL of deionized water. The

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solution was agitated by ultrasonication (430 W, 38 kHz) for 10
min, and then magnetically stirred at 290 K in the dark for 2 h.
The precipitate was ltered using a JGWP04700 lter, washedwith ethanol (total 200 mL), and dried at 353 K for 12 h. The
obtained BiOCl is denoted as aqua-BiOCl.
The obtained BiOCl powders (solvo-BiOCl and aqua-BiOCl)
were each suspended in 38 mL of deionized water and placed
onto ITO-coated glass plates in a manner similar to that used for
the TiO2/ITO/Pyrex. The amount of BiOCl placed on each ITOcoated glass plate was 2.5 mg and covered an area of 1.3 cm2.
2.2

Characterization

Optical spectroscopic measurements were performed using a
UV-visible spectrophotometer (Model V-650, JASCO) with D2and halogen lamps for wavelengths below and above 340 nm,
respectively. An integrating sphere (Model ISV-469, JASCO) wasused for the diﬀuse reectance (DR) measurements. The
samples were placed in contact with the quartz window glass ina gas-tight DR cell (photographs in Scheme 3), and the interior
atmosphere was either argon or air. Measurements were performed at 290 K within the wavelength range of 200–800 nm
using 200 mg of the sample. The DR spectra were converted toabsorption spectra using the Kubelka–Munk equation.36,37 Theband gap value was estimated via simple extrapolation of theabsorption edge or by tting to the Davis–Mott equation:a  hn f (hn  Eg)n,(1)

where a, h, and n are the absorption coeﬃcient, Planck's
constant, and frequency of light, respectively, and n is 1/2,
3/2, 2, and 3 for the allowed direct, forbidden direct, allowed
indirect, and forbidden indirect electronic transitions,
respectively.38
High-resolution transmission electron microscopy (HRTEM) images were obtained using a Titan G2 60-300 (FEI) eld
emission-type transmission electron microscope operating at
300 kV. HR-TEM videos were observed using a JEM-2100F(JEOL) eld emission-type transmission electron microscope
operating at 200 kV. The samples were mounted on a conductive carbon-with-Cu grid mesh (100 mesh per inch). The electron currents at the sample position were estimated to be
between 500 and 100 pA cm2 on the basis of the counts on auorescence plate that was placed at a distance of 76 cm fromthe sample on the opposite side of the electron source. We
observed BiOCl samples aer suﬃcient astigmatism correction

Scheme 3

Proposed structural transformation for BiOCl.

8392 | J. Mater. Chem. A, 2015, 3, 8389–8404

using an amorphous carbon lm in the HR-TEM observation.
The intervals of the lattice planes were calculated for tetragonal
BiOCl with a ¼ 0.3891 nm and c ¼ 0.7369 nm,39,40 and thesewere compared to the observed lattice intervals.
X-ray diﬀraction (XRD) patterns were observed at a Bragg
angle of 2qB ¼ 10 to 70 with a scan step of 0.01 and a scan rateof 4 s per step using a D8 ADVANCE diﬀractometer (Bruker) at
the Center for Analytical Instrumentation, Chiba University.
The measurements were performed at 40 kV and 40 mA using
Cu Ka radiation and a nickel lter. The sample irradiated with
UV-visible light for 9 h under Ar atmosphere was also analyzed
using a gas-tight XRD holder with a semispherical cap (Model
A100-B37, Bruker). The mean particle size (t) of the BiOCl was
determined using the Scherrer equation
t¼

0:9l
:
Full width at half maximum  cos qB(2)

Raman spectra were obtained at 290 K using a model NRS5100 (JASCO) with the excitation wavelength of 532 nm for thesolvo-BiOCl samples as-synthesized and irradiated by UV-visible
light for 12 h under vacuum. The intensity of the laser was1 mW and a CCD detector was used at 203 K.
X-ray photoelectron spectra (XPS) were obtained using a
Model AXIS-NOVA, Shimadzu/Kratos equipped with an X-ray
source Al Ka (1486.6 eV). O 1s, Bi 4f5/2, and Bi 4f7/2 regions were
narrowly scanned. The C 1s (284.8 eV) was used as a reference
for energy calibration for adventitious hydrocarbon on thespecimen surface.41 BiOCl sample irradiated by UV-visible lightfor 2.5 h under Ar, and another BiOCl sample immersed in HClaqueous solution (pH 2.0) irradiated by UV-visible light for 2.5 hunder O2 were mounted on a sample holder and the sample
holder was introduced to a transfer vessel under Ar atmosphere.
The transfer vessel was connected to AXIS-NOVA, the gate valve
was opened, and then the sample was transferred inside AXISNOVA without any contact with air.
Bismuth L3-edge extended X-ray absorption ne structure(EXAFS) spectra were obtained at 290 K in the transmission
mode in the Photon Factory Advanced Ring and Photon Factoryat the High Energy Accelerator Research Organization (Tsukuba) on beam lines NW10A and 9C.42–45 The storage ring energy
was 6.5 and 2.5 GeV, and the ring current was 33.0–55.8 and 450
mA. A Si (311) and (111) double-crystal monochromator and aplatinum and rhodium-coated focusing cylindrical mirror wereinserted into the X-ray beam path. X-ray intensity was maintained at 65% of the maximum ux using a piezo translator
attached to the crystal. The slit opening size was 1 mm (vertical)
 2 mm (horizontal) in front of the ionization chamber. The
samples were thoroughly mixed with boron nitride and pressed
to form disks. Some samples were in Pyrex glass cells equippedwith polyethene naphthalate lm (PEN; Q51-25, Teijin-Dupont;
25 mm thick) windows on both sides, purged with Ar or lledwith HCl aqueous solution (pH 2.0) with O2 gas. The PEN lm
was transparent for X-rays, visible light, and a portion of the UV
light region (>370 nm) and enabled in situ monitoring of thephotocatalysts.36,46–49 The Bi L3-edge absorption energy wascalibrated to 13 426 eV for the spectrum of Bi metal.50

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The EXAFS data were analyzed using an XDAP package.51
The procedure has been described previously.36,47,52,53
Multiple-shell curve-t analyses were performed for the
Fourier-ltered k3-weighted EXAFS data in both k- and Rspaces using empirical amplitude and phase-shi parameters
theoretically generated using FEFF version 8.4 (ref. 54) for the
crystal structure of Bi metal powder or extracted from the
EXAFS data for Bi2O3 (>98%, Wako Pure Chemical) and BiCl3
powders. The interatomic distances (R) and associated coordination numbers (N) for Bi–Bi, Bi–O, and Bi–Cl pairs were set
to 0.3071 nm with an N value of 3, 0.3529 nm with an N valueof 3,55 0.2330 nm with an N value of 5,56 and 0.2500 nm with an
N value of 3 (Table 1d, e and g).57
Cyclic voltammetry (CV) measurements were performed for
BiOCl as the working electrode (WE) (BiOCl/ITO/Pyrex and a Au/
Ni-coated stainless clip; Toyo Corporation), glassy carbon as thecounter electrode (CE) (Model 002012, fpolyetheretherketone: 6.0mm, fC: 3.0 mm; BAS Incorporation), and Ag/AgCl as thereference electrode (Model RE-1B, BAS Incorporation),immersed in a hydrochloric acid solution of pH 4.0. The voltageof the WE was swept between 1.22 and 1.50 V versus Ag/AgCl at
a rate of 50 mV s1 using a potentio/galvanostat (Model

Table 1

VersaSTAT 3-100, Princeton Applied Research) in either O2 or N2
atmosphere at a ow rate of 100 mL min1 irradiated using UVvisible light (8 mW cm2 at the WE sample position) or in the
dark.

2.3

PFC tests

TiO2/ITO/Pyrex and BiOCl/ITO/Pyrex electrodes were immersed
in HCl solutions (40 mL in each compartment; initial pH was
between 2.0 and 4.0). The two compartments were separated by
a 50 mm-thick PCP lm (Naon, DuPont; acid capacity >9.2 
104 equivalent g1). N2 and O2 gases were bubbled 30 mm awayfrom each photoelectrode at a ow rate of 100 mL min1(Fig. S1†).7 The PFC was equipped with quartz windows (f ¼ 80mm) on both sides. Both the TiO2 and BiOCl photocatalysts
were irradiated with UV-visible light through the quartz
windows using a two-way branched quartz ber light guide(Model 5f-2B-1000L, San-ei Electric Co.) from a 500 W xenon
arc lamp (Model SX-UID502XAM, Ushio). The distance between
the light exit (f ¼ 5 mm) and the TiO2 or BiOCl lm was 46 mm.
The light intensity was 8 mW cm2 at the center of the photocatalyst lm on each electrode.

Curve-ﬁtting analysis of the Bi L3-edge EXAFS results for BiOCla
Bi–O

Bi–Cl

Bi–Cl

Bi–Bi

Bi–Bi

R (nm)
N
SampleDs2 (105 nm2)(a) As-synthesized

0.244(0.002)
4.0(0.7)
2.7(5.5)0.241(0.015)
1.9(0.9)
3.3(2.5)0.250(0.001)
2.2(0.2)
1.4(0.5)
0.247(0.001)
4.2(0.6)
6.0(1.4)
0.247(0.012)
5.1(1.6)
3.6(1.9)
0.23305—(b) In Ar & UV-visible, 19 h(c) In HCl (pH 2.0), O2 gas, & UV-visible,
35 min(d) Sample c, 12 min aer UV-visible wasoﬀ(e) Sample c, directly exposed to air(f) Bi2O3 (model)56(g) BiCl3 (model)57(h) BiOCl39(i) Bi metal (model)55

a0.2314
4—Goodness of t
3.5  1040.307(0.011)
4.0(1.7)
1.2(3.7)
0.276(0.021)
3.7(0.2)
5.5(1.6)
0.329(0.012)
3.1(0.7)
2.2(1.7)
0.331(0.012)
4.6(1.9)
5.7(1.8)0.301(0.009)
3.1(1.7)
2.2(3.6)

0.25003—0.30714—

1.6  105
1.4  105
3.2  1050.35001—0.3071
3—0.35293—

The values in parentheses are evaluated t errors.

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Static photocurrent generation tests were performed byconnecting an external parallel circuit for one route via a voltmeter and another route via an ammeter with a resistance of 0.5
U (Fig. S1†). Both the catalysts were exposed for 30 min to UVvisible light and then placed in the dark for 30 min, and thisprocess was typically repeated ve times. As a reference, a static
photocurrent test was performed using TiO2/ITO/Pyrex and
BiOCl/ITO/Pyrex electrodes immersed in H2SO4 and HCl solutions, respectively, both at pH 2.0. A static photocurrent test
comprising continuous UV-visible light exposure for 3 h was
also performed. The current (i) and voltage (V) between the twoelectrodes were monitored. The i–V characteristics were also
obtained using a similar parallel circuit; however, the resistancewas gradually varied from 500 kU to 0.3 U over 20 min.
In comparison, the BiOCl (solvo-BiOCl, aqua-BiOCl) nepowder (0.135 g) was irradiated using UV-visible light for 12 h ina quartz glass reactor2,36,48,53 under vacuum connected to rotaryand diﬀusion pumps (106 Pa), and then the reaction with 2.91
kPa of O2 was monitored by the pressure change using acapacitance manometer (Models CCMT-100A and GM-2001,
ULVAC). The rate constant k was determined by tting to theequation for the rst order reaction rateln

PO2  PO2 ;final¼ kt
PO2 ;initial  PO2 ;final(3)

where PO2,initial and PO2,nal are the initial and nal pressures of
O2, respectively.47

3 Results
3.1

Characterization of BiOCl

3.1.1 As-synthesized BiOCl
Solvo-BiOCl. The UV absorption edge for the as-synthesized
BiOCl was simply extrapolated to 358 nm (Fig. 1A-a). The Eg
value was evaluated on the basis of the t to eqn (1). As shown inthe ttings for (a  hn)1/n versus hn (n ¼ 1/2, 3/2, 2, and 3;
Fig. 1B), the Eg values (3.57–3.40 eV) when n was 1/2 or 3/2 wereconsistent with that given by simple extrapolation of theabsorption edge (3.46 eV) and results reported in the literature(3.46 eV).13 In fact, when the ln(a  hn) value was plotted versusln(hn  3.46), the n value was determined to be 1.030 from theslope. Thus, a direct transition from Cl 3p (and/or O 2p) to Bi 6p
was suggested.58
The XRD peak position and intensity ratio for the assynthesized BiOCl (Fig. 1D) were in accordance with thosereported in literature.13,59 The mean particle size of the BiOCl
was determined to be 24(12) nm using eqn (2). The mean
particle size in the (001) direction was smaller (13(2) nm)
compared to the mean particle size 40(13) nm in theperpendicular plane to the [001] direction, indicating a disk-likecrystalline growth.
Raman spectrum for as-synthesized BiOCl was obtained(Fig. 1G-a and G0 -a). For the tetragonal symmetry of BiOCl,13 A1g,
B1g, and Eg Raman vibrational modes were possible and if we
assume the ionic crystal of BiOCl as a “molecular” crystal,internal and external vibration modes were considered within
and beyond the “molecule”, respectively. The peaks at 200, 144,

8394 | J. Mater. Chem. A, 2015, 3, 8389–8404

and 62 cm1 were assigned to internal Eg, internal A1g (andminor external Eg), and external A1g for Bi–Cl stretching modes,
respectively.10 The weak peak at 396 cm1 was assigned to Eg
and/or B1g modes for the motion of oxygen.
The XP spectrum in the Bi 4f5/2 and Bi 4f7/2 regions was
obtained for as-synthesized BiOCl (Fig. 2A-a). The peaks
centered at 164.7 and 159.4 eV, correspond to the Bi3+ state of
BiOCl.41 These values are consistent with the reported values at
164.4–164.5 and 159.1–159.9 eV,58,60 respectively, if the C 1s peakenergy is assumed to be 284.8 eV.41
The Bi L3-edge EXAFS curve-tting analysis for the assynthesized BiOCl indicated Bi–O and Bi–Cl coordination at
0.244 and 0.307 nm, respectively (Table 1a and Fig. 3a), conrming the synthesis of BiO+ layers and intercalated Clanions.13
From the HR-TEM images of the as-synthesized BiOCl(Fig. 4a), the particle size was ranged between 10 and 20 nm,
which is nearly in accordance with the mean particle size (24nm) estimated on the basis of the XRD analysis. Lattice patterns
with intervals of 0.33–0.34 nm were assigned to the 011 planes(calculated interval 0.34 nm based on the crystal structure).39
Lattice patterns with intervals of 0.40 nm were also observed
and assigned to the 002 planes (calculated interval 0.37 nm).
Aqua-BiOCl. UV-visible spectrum and XRD peak position
obtained for the aqua-BiOCl were similar to those of the BiOCl(Fig. 1D and F); however, the mean particle size was signicantly
larger at 115(49) nm, as determined from the XRD peak width.
This trend was in accordance with the literature in which water
was the hydrolyzing agent in contrast to ethylene glycol, which
was the crystal growth inhibitor during the synthesis of BiOCl.61
The mean particle size in the [001] direction was larger(168(28) nm) compared to mean particle size (121(72) nm) in
the perpendicular plane to the (001) direction, indicating preferable crystalline growth in the c-axis.
3.1.2 BiOCl irradiated under Ar/vacuum. When the BiOCl
powder under Ar atmosphere was irradiated with UV-visible
light, the white color changed to brown and the light absorption
increased over the entire visible light range in 1 min (Fig. 1A-b).
When the irradiation was continued, the color further changed
to black (Scheme 3b, photograph) and light absorption

Fig. 2 XPS spectra in the Bi 4f5/2 and Bi 4f7/2 regions for as-synthesized BiOCl (a), BiOCl in Ar irradiated by UV-visible light for 2.5 h (b),and BiOCl in HCl aqueous solution with O2 irradiated by UV-visible
light for 2.5 h (c).

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Journal of Materials Chemistry A

Fig. 3 (a) Bi L3-edge EXAFS spectra for as-synthesized BiOCl, (b) sample “a” under Ar and UV-visible light for 19 h, and (c) sample “a” immersed ina pH 2.0 HCl solution under O2 and UV-visible light for 35 min, (d) 12 min after UV-visible light was oﬀ for sample “c”, and (e) sample “c” wasdirectly exposed to ambient air. (1) k3-weighted EXAFS c-function, (2) its associated Fourier transform, and best-ﬁt results in (3) k-space and (4) Rspace to the Fourier-ﬁltered transformed data. The red and blue lines represent the experimental and calculated values, respectively. The solidline represents the magnitude, and the dotted line indicates the imaginary part in panels 2 and 4.

Fig. 4 HR-TEM images observed for the (a) as-synthesized BiOCl and(b) a BiOCl irradiated with UV-visible light for 9 h in Ar atmosphere.

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progressively enhanced over the entire visible light region for
1 h (Fig. 1A-c).
The black color slowly diminished to gray aer 24 h under Ar
in the dark (Fig. 1A-d), and the gray color changed negligiblyaer 5 days under Ar and in the dark, but returned to white
within 5 h when the sample was exposed to air. In addition, theUV-visible spectrum of the recovered white sample was essentially identical to that for the as-synthesized BiOCl (Fig. 1A-e).
Thus, the species/phase responsible for the black color under Ar
and light exposure was reversibly transformed into the original
BiOCl crystallite in air.
Raman spectrum for BiOCl irradiated using UV-visible lightfor 12 h under vacuum was obtained (Fig. 1G-b and G0 -b). The
peaks at 200 (internal Eg), 144 [internal A1g (and minor externalEg)], and 62 cm1 (external A1g) for Bi–Cl stretching modes of
the as-synthesized BiOCl remained almost constant at 202, 147,
and 63 cm1, respectively.10 Furthermore, a new peak appeared
at 93 cm1 (Fig. 1G0 -b). A pair of peaks at 95 and 69 cm1 wasreported for metallic Bi;10 however, no peak at 69 cm1

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appeared in Fig. 1G-b. Therefore, we assume the new peak at
93 cm1 was due to the Bi–Cl stretching mode enabled for theblack, O-decient BiOCl.
XP spectrum in the Bi 4f5/2 and Bi 4f7/2 regions was obtainedfor BiOCl irradiated by UV-visible light for 2.5 h in Ar atmosphere (Fig. 2A-b). The peaks remained at the same position(164.7 and 159.4 eV) as that for the as-synthesized sample(Fig. 2A-a). In addition to the peaks due to the Bi3+ state, weakerpeaks appeared centered at 162.2 and 157.0 eV (arrows in
Fig. 2A-b and B-b). The two peaks shied by 2.5 and 2.4 eVcompared to the corresponding 4f5/2 and 4f7/2 peaks due to Bi3+,
respectively. Compared to chemical shi for the Bi 4f7/2 peak on
going from BiIIIOCl to Bi0 (3.1 eV),41 the weaker peaks at 162.2
and 157.0 eV could be obtained from reduced Bi sites at thevalence between 3 and 0.
Based on the EXAFS analyses, the N values for the interatomic Bi–O pair decreased from 4.0 to 1.9 when the assynthesized BiOCl powder was irradiated under Ar using UVvisible light for 19 h (Table 1a and b and Fig. 3a and b).
Conversely, aer irradiation, the N(Bi–Cl) value only changedslightly from 4.0 to 3.7. Bi atoms that lost neighboring lattice O
atoms may have formed shorter bonds with Cl: 0.276 nm
compared to 0.307 nm for the original crystalline BiOCl (Table
1a and b). Thus, these results suggest that the generation of O
deciencies in the crystalline BiOCl due to UV-visible irradiation in Ar atmosphere occurred. The curve tting to the model
function for a metallic Bi–Bi pair was unsuccessful (Fig. 3b).
Even aer UV-visible light irradiation in Ar atmosphere for 9h, the entire XRD pattern negligibly changed compared to thatfor the as-synthesized BiOCl (Fig. 1D and E). The mean particle
size did not change at 24(12) nm for the as-synthesized BiOCland one irradiated with UV-visible light under Ar. Thus,
following UV-visible light irradiation under Ar atmosphere, themajor phase was crystalline BiOCl, and a minor O-decient
amorphous-like phase resulted in the black color.
HR-TEM image for BiOCl irradiated with UV-visible light in
Ar atmosphere was obtained (Fig. 4b). Similar to the image forthe as-synthesized BiOCl (Fig. 4a), particles with sizes ranging
from 10 to 20 nm were observed, and lattice patterns withintervals of 0.33–0.35, 0.20, 0.22–0.24, and 0.27–0.30 nm wereassigned to the 011, 004, 112, and 110 planes (calculated latticeintervals of 0.34, 0.19, 0.22, and 0.28 nm), respectively.
During the HR-TEM observation, a portion of the external
nanocrystals of BiOCl transformed into spherical nanoparticles
without lattices (Fig. 4b3, right, bottom and 4b6, center, right,
and bottom; Fig. 4b7 is a zoomed image of 4b6). When observation was continued, the amorphous-like spherical nanoparticles transformed back to nanocrystals of BiOCl with a clear
lattice pattern (Fig. 4b4, right, bottom and 4b5, right andbottom). This phase transformation was reversible (a video isavailable as an ESI material†). Thus, the amorphous-likespherical nanoparticles were oxygen-decient BiO1xCl thatformed under the electron ux of the HR-TEM. The 110 and 112
lattice patterns in Fig. 4b5 (le and right) in the reversiblytransformed BiOCl crystals were continuous with 110 and 112
lattice patterns, respectively, present in the lower, unchanging
BiOCl particles. This fact suggests that O atoms were supplied

8396 | J. Mater. Chem. A, 2015, 3, 8389–8404

Paperfrom the lower BiOCl particles to the amorphous-like, oxygendecient BiO1xCl species to enable crystallization as BiOCl
from the boundary toward the BiO1xCl nanoparticles.
The observed reversible phase transformation should not be
an artifact because the amorphous-like spherical nanoparticles(Fig. 4b3, right, bottom and 4b6, center, right, and bottom, andFig. 4b7) were rarely observed for the as-synthesized BiOCl(Fig. 4a1–3). The adjustment of astigmatism was suﬃcient as isshown in the video (ESI material†).
3.1.3 BiOCl irradiated under O2/air and/or HCl aqueous
solution. Corresponding to the conditions of PFC tests, the on/oﬀ exposure of BiOCl to UV-visible light in air was performed,
and the UV-visible absorption spectra were monitored (Fig. 1C).
When fresh BiOCl was irradiated with UV-visible light for 1 h,
the color changed to black, and its light absorption increased
over the entire visible light region (Fig. 1C-b). The maximum
absorbance at 540 nm was 80% of that following irradiation
with light for 1 h under Ar (Fig. 1A-c). When the black sample
was stored in air in the dark for 24 h, the color faded to gray, andthe absorbance was 65% of that of the corresponding sample
stored under Ar in the dark for 24 h (Fig. 1C-c and A-d).
In addition, the BiOCl irradiated in air remained gray in thedark, even aer 92 h (Fig. 1C-d), which is in clear contrast to the
behavior of the BiOCl irradiated under Ar, which completely
turned white within 5 h upon exposure to air (Fig. 1A-e). Thus, it
can be concluded that the BiO1xCl formed when the BiOCl was
irradiated in air (or O2) with UV-visible light was stabilized by
the outer oxidized layers (Scheme 3e).
XP spectrum in Bi 4f5/2 and Bi 4f7/2 regions was also obtainedfor BiOCl irradiated by UV-visible light for 2.5 h immersed in an
HCl aqueous solution (pH 2.0) saturated with O2 (Fig. 2A-c).
Although the Bi3+ peaks were again predominant at 164.8 and
159.5 eV, weaker peaks appeared centered at 162.2 and 157.0 eV(arrows in Fig. 2A-c and B-c) similar to reduced Bi sites at thevalence between 3 and 0 for irradiated BiOCl in Ar atmosphere(Fig. 2B-b). The peak intensity of 4f5/2 and 4f7/2 peaks for
reduced Bi sites was diﬃcult to compare between Fig. 2B-b andB-c because the intensities of these peaks were quite weak.
The BiOCl lm immersed in HCl aqueous solution (pH 2.0)
irradiated by UV-visible light at beamline was analyzed using Bi
L3-edge EXAFS (Fig. 3c). The color of the lm was light gray. The
coordination number of both Bi–O and Bi–Cl decreased (from4.0 to 2.2 and from 4.0 to 3.1, respectively; Table 1c and Fig. 3c).
The oxygen vacancy could be formed similar to the conditions ofunder Ar and with UV-visible irradiation.
Then, UV-visible light was turned oﬀ aer 60 min of irradiation. A Bi L3-edge EXAFS spectrum was obtained at 12 min in
the dark (Fig. 3d). The shape of EXAFS oscillation was similar tothat for the sample irradiated by UV-visible light (Fig. 3c1), but
the reduced amplitude in the whole spectrum region irradiatedby light (Fig. 3c1) seemed to recover to the level for the assynthesized sample (Fig. 3a1). In fact, the N(Bi–O) and N(Bi–Cl)
values recovered to 4.2 and 4.6 (Table 1d), respectively, similar
to that for as-synthesized BiOCl (4.0; Table 1a). Thus, the O
and Cl vacancy formed in HCl aqueous solution irradiated by
UV-visible light is suggested to be transformed to the original
BiOCl crystalline state soon aer the light was turned oﬀ.

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Furthermore, a few percent of chlorine loss/recovery wasreported for BiOCl immersed in water in response to irradiation/stop of UV light and the transformation was reversible
based on the analysis using ion chromatography.34 The chlorine
vacancy based on the decrease of N(Bi–Cl) in EXAFS (Table 1c)
was qualitatively in accordance with the report.
Finally, the BiOCl lm that was subjected to ve 1 hour cyclesof the PFC test (2.5 h of irradiation) and directly exposed to air
was analyzed using Bi L3-edge EXAFS (Fig. 3e). The N(Bi–O)
values for the as-synthesized BiOCl (4.0) increased to 5.1,
whereas the N(Bi–Cl) values decreased from 4.0 to 3.1 (Table 1aand e). The R values, on the other hand, were essentially constant
at 0.244–0.247 and 0.301–0.307 nm. Notably, the N(Bi–O) value(5.1) was similar to that for Bi2O3 (5; Table 1f); however, theinteratomic distance was longer by 0.014 nm. In contrast to
reversible transformation between BiO1xCl1y and BiOCl by
on/oﬀ exposure to UV-visible light in HCl solution saturated
with O2, the BiO1xCl1y species was not totally transformed
back to the original BiOCl by direct exposure to air.
3.2 Photocurrent changes and the i–V dependence of thePFC
3.2.1 Static PFC tests. In static PFC cycle tests at pH 3.0,
following UV-visible irradiation, the photocurrents of the PFC
increased and stabilized within 0.84–3.4 min (Fig. 5A). In velight on/oﬀ cycles, the photocurrents converged to 32.7–34.0 mA.
Similarly, the photocurrents increased and converged to 4.29–
4.40 mA within 1.7–4.1 min at pH 4.0. In contrast, at pH 2.0, thecurrent gradually increased and stabilized within 20 min in the

Journal of Materials Chemistry Arst cycle, but did not fully stabilize in the following cycles. The
current nearly converged from 102 to 118 mA in ve cycles.
Separately, photocurrents converged to 106 mA within 60 min at
pH 2.0 when the irradiation was continued for 3 h (Fig. 5A0 ),suggesting reversible structure/state changes for BiOCl photocatalyst when the light was oﬀ. Throughout this study, the pH
values of the electrolyte changed negligibly (within a variationof 0.02) over 5 h.
When the UV-visible light was oﬀ, the photocurrent values
quickly decreased to zero within 0.84–4.1 min at pH 2.0, but
decreased more slowly (14–20 min) at pH 3.0, and most slowly(25–26 min) at pH 4.0. This trend suggests limited diﬀusion atlower proton concentrations. Thus, the nearly converged PFC
photocurrent values were plotted as a function of the electrolyte
solution proton concentrations (Fig. 5B), and it was found thatthe values increased proportionally to the square root of the
proton concentration.
When sulfuric acid solution (pH 2.0) was used as the electrolyte for the photoanode (TiO2) instead of HCl solution, theconverged photocurrent was 76 mA in the rst cycle, but gradually decreased to 77, 63, 64, and 64 mA with ve cycles(Fig. 5A00 ). When the UV-visible light was oﬀ, the photocurrents
decreased to zero within 3.0–6.5 min under this condition. This
response was signicantly slower than the relaxation of currents
in HCl solution for the photoanode (0.84–4.1 min; Fig. 5A, pH
2.0).
3.2.2 Reactivity of BiOCl with O2 gas. For comparison, assynthesized solvo-BiOCl ne powder was irradiated with UVvisible light for 12 h in vacuum and then placed into contact
with O2. The quantity of O2 (2.91 kPa) in a closed glass system

Fig. 5 (A) Time course of the photocurrents in the PFC comprised of the TiO2 and solvo-BiOCl photocatalyst electrodes, which were immersed
in HCl solutions at a pH of 2.0 (circle, ), 3.0 (square, ), and 4.0 (diamond, ). (A0 ) Continuous test at a pH of 2.0 (circle, ) was also plotted. (A00 )Time course of the photocurrents in the PFC comprised of the TiO2 and solvo-BiOCl photocatalyst electrodes, which were immersed in H2SO4and HCl solutions, respectively, both at a pH of 2.0 (circle, ) (B) correlation between the converged photocurrents within 5 h and the H+concentrations of the acidic water in the PFC or the square root of the H+ concentrations. The ﬁttings were to the linear function (lower x-axis) orto the kinetic model equation i ¼ A[H+]c1/2  B[H+]c1/2 (eqn (7), see text; upper x-axis). (C and C0 ) Time course of the O2 pressure in contact withsolvo-BiOCl (C) and aqua-BiOCl (C0 ) previously irradiated with UV-visible light for 12 h. (D) i–V dependence of the PFC comprising TiO2 andsolvo-BiOCl immersed in a pH 2.0 HCl solution. (E) i–V dependence of the PFC comprising TiO2 and aqua-BiOCl immersed in a pH 2.0 HCl
solution.

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Paper

corresponded to the dissolved amount of O2 in acidic water (40mL) at 290 K. The time course of the reaction for 26 h is shownin Fig. 5C. The rate constant k was 0.119 h1 by tting to eqn (3).
Because the initial reaction rate was signicantly faster than therate at 26 h, an alternative tting of the data only for the initial
20 min gave a k0 value of 1.37 h1 (Fig. 5C, inset).
In the corresponding reactivity test for aqua-BiOCl, the rate
constant k was 0.0939 h1 by tting to eqn (3) (Fig. 5C0 ), which
was 21% smaller compared to that for solvo-BiOCl (Fig. 5C). An
alternative tting of the data for the initial 20 min was diﬃcult
for aqua-BiOCl, and the t of the data for the initial 170 min
gave a k0 value of 0.261 h1 (Fig. 5C0 , inset).
Furthermore, based on the total O2 reacted with solvo-BiOCl
and aqua-BiOCl irradiated with UV-visible light for 12 h invacuum (6.1 and 3.6 mmol; Fig. 5C and C0 ), the stoichiometry of
the BiOCl subjected to the kinetic tests of Fig. 5C and C0 was
estimated to be BiO10.023Cl and BiO10.014Cl, respectively.
3.2.3 i–V characteristics. The i–V characteristics were thenstudied for a PFC comprising TiO2 and BiOCl at pH 2.0 (Fig. 5D).
The i value gradually increased as the cell voltage decreased
beginning at the open circuit voltage (VOC, 1.76 V), which issimilar to the i–V dependence for SCs.62 When the voltage wasless than 0.39 V, the current increased linearly from 24.7 to 77.7
mA (short circuit current, iSC), and the maximum electric power(Pmax) was 10.5 mW (1.01 V  10.5 mA) for a photocatalyst lm
area of 1.3 cm2 (Table 2A).
For comparison, the i–V characteristics of a PFC comprising
TiO2 and aqua-BiOCl at pH 2.0 were also investigated (Fig. 5E).
The major diﬀerence between the solvo-BiOCl and aqua-BiOClis the mean particle size: 24 versus 115 nm, respectively, basedon the XRD analysis (Fig. 1D and F). Although the VOC value(1.69 V) for the PFC based on the aqua-BiOCl was 0.07 V lessthan that for the PFC containing the solvo-BiOCl, the former iSC
value increased to 92.4 mA, which is 14.7 mA greater than thelatter. Consequently, the maximum electric power was 21.1 mW(1.15 V  18.3 mA) for a photocatalyst lm area of 1.3 cm2(Table 2B).
3.3

Fig. 6 CV for a BiOCl WE and glassy carbon CE in a pH 4.0 aqueous
HCl solution under a (a) N2 ﬂow and (b) O2 ﬂow during UV-visible lightirradiation (solid lines) or in the dark (dotted lines).

(0.09 V) coincides with the equilibrium voltage at the pH (Table
3B) for the reaction:
BiOCl + 2H+ + 3e / Bi + Cl + H2O(4)

Under light irradiation, another pair of oxidation andreduction peaks appeared at 0.75 and 0.19 V, respectively, as
shoulder peaks on the redox peaks due to eqn (4) (Fig. 6a). The
peak at 0.19 V was very weak. The center of the new peaks (0.28 V)
corresponds to the equilibrium voltage (0.32 V @ pH 0; Table
3A) for the reaction of the anodic dissolution of Bi and reduction back to Bi:
Bi3+ + 3e / Bi

(5)

CV for BiOCl

3.3.1 Under N2 atmosphere. The CV results for BiOCl in a
pH 4.0 aqueous HCl solution under N2 atmosphere are depicted
in Fig. 6A. In the dark, a pair of oxidation and reduction peaks
appeared centered at 0.57 and 0.39 V, respectively, versus astandard hydrogen electrode (SHE). The center of the two peaks

Table 2

3.3.2 Under O2 atmosphere. In the dark, only a weak
reduction peak for reaction 4 (0.39 V) appeared under O2
atmosphere (Fig. 6b). Furthermore, no distinct peaks wereobserved with irradiation of UV-visible light. This contrast
suggests a stabilized BiOCl-based phase under O2 atmosphere.

Electrochemical data for PFCs obtained from the i–V analysisa
Photocatalyst

Entry

Anode

CathodeVOC (V)iSC (mA)

Pmax (mW)

SourceA
B
C

TiO2b (25 nm)
TiO2b (25 nm)
TiO2b (25 nm)

Solvo-BiOCl (24 nm)
Aqua-BiOCl (115 nm)
Ag (3.0 wt%)–TiO2b (25 nm)

1.76
1.69
1.59

77.7
92.4
73.7

10.5
21.1
14.0

This work
This work
Ref. 7a
The pH of each HCl solution was 2.0. The area of each photocatalyst was 1.3 cm2. b Mean particle size estimated from the XRD pattern; anatase: 21
nm, rutile: 35 nm, anatase/rutile ratio ¼ 7 : 3.

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Table 3

Journal of Materials Chemistry A
Electrochemical data for BiOCl
CV peak (V) versus SHE

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Redox reaction conditions(A) Bi3+ + 3e ¼ Bi
O2, light
O2, dark
N2, light
N2, dark(B) BiOCl + 2H+ + 3e ¼ Bi + Cl + H2O
O2, light
O2, dark
N2, light
N2, dark(C) Bi2O3 + 6H+ + 6e ¼ 2Bi + 3H2O
(D) 4H+ + O2 + 4e ¼ 2H2O

Reduction wave

Oxidation wave

CenterEquilibrium voltage @ 298 K

—
—
0.19
—

—
—
0.75
—

—
—
0.28
—

0.3172 @ pH 0

—
0.39
0.39
0.39

—
—
0.57
0.57

—
0.09
0.09

4 Discussion
4.1

Phase transformation of BiOCl

The band gap for as-synthesized BiOCl powder was 3.46 eV(Fig. 1A-a), and the mean disk-like particle size was 24 nm(Fig. 1D). When the BiOCl powder was irradiated with UV-visible
light under Ar atmosphere, the color changed to black in 1 h(Fig. 1A-c and Scheme 3b). The black color turned to gray aer
24 h placed under Ar in the dark (Fig. 1A-d), and then back towhite when the powder was exposed to air for 5 h (Fig. 1A-e).
Thus, (a part of) the crystalline BiOCl changed to something else
following UV-visible light irradiation under Ar, but reversibly
converted back to BiOCl in air. Furthermore, a new Raman peak
appeared at 93 cm1 for BiOCl irradiated by UV-visible lightunder vacuum due to the Bi–Cl stretching vibration (Fig. 1G-b
and G0 -b). Because the peak was not detected for the assynthesized BiOCl, the new Bi–Cl stretching vibration mode wasenabled by the modulation of symmetry for O-decient BiOCl.
The generation of O defects in the BiOCl irradiated with UVlight was suggested on the basis of additional Bi 4f5/2 and 4f7/2
peaks at lower binding energy than the main peaks due to Bi3+
(Fig. 2b). The additional peaks were also reported in the literature.35,60,63 This interpretation was supported by the N(Bi–O)
decrease (4.0 to 1.9) based on Bi L3-edge EXAFS analysis for
BiOCl irradiated with UV-visible light under Ar (Table 1b andFig. 3b).
The origin of the black color and O deciency of the BiOCl
irradiated with UV-visible light under Ar atmosphere (Scheme
3b, photograph) was claried via HR-TEM analysis. A reversible
phase transformation from crystalline BiOCl to amorphous-likespherical nanoparticles was observed during the HR-TEM
analysis (Fig. 4b3–b6 and the video available in the ESI material†). In contrast, the amorphous-like spherical nanoparticles
were rarely observed for the as-synthesized BiOCl (Fig. 4a1–3).
Thus, it is concluded that the origin of the black color of theirradiated BiOCl is O-decient BiO1xCl. An amorphous-like
BiO1xCl phase present as spherical nanoparticles in the electron ow of the HR-TEM was observed to reversibly transform tocrystalline BiOCl with a clear lattice pattern once O atoms wereThis journal is © The Royal Society of Chemistry 2015

0.1697–0.0197 pH (0.0909 @ pH 4.0)

0.376–0.0591 pH (0.1396 @ pH 4.0)
1.229–0.0591 pH (0.993 @ pH 4.0)

supplied from inside the crystalline BiOCl grains to achievephase equilibrium (Fig. 4b3–6 and Scheme 3a and b).
In the HR-TEM images of BiOCl irradiated by UV light underAr, an outer, 1 nm-thick layer of the crystalline BiOCl was
disordered and considered to be amorphous-like BiO1xCl.35,58
Density functional theory (DFT) calculations reported for O
displacement in BiOCl from an original position in the latticeand generation of vacancies in the BiO+ layers revealed theformation of an additional energy state slightly below the CB of
BiOCl. Such vacancies would function as electron trapping sites;
however, they could also recombine with the displaced O underequilibrium conditions.64
The maximum electron current at the position of the sample
in the JEM-2100F was 500 pA cm2. Thus, considering theelementary charge, 4  109 electrons per second reached the 1.3
cm2 of sample. In comparison, the photon ux from the SXUID502XAM via the ber light guide was 2  1016 photons s1,
and generated photocurrents for the PFC were 7  1014 electrons per s (118 mA at the most; Fig. 5A) to the photocatalyst lm
area of 1.3 cm2 (Scheme 1). Although the physical situation for
the electron ow to BiOCl in the HR-TEM versus the charge
separation for TiO2 and BiOCl following irradiation and theresulting PFC current is diﬀerent, the eﬀective electron owduring the PFC tests was approximately 105 times greater thanthat in the HR-TEM apparatus.
In XPS spectra in Bi 4f5/2 and Bi 4f7/2 regions, weaker peaks
were detected at 162.2 and 157.0 eV on the lower energy side ofmajor peaks at 164.8(0.1) and 159.5(0.1) eV, respectively, for
BiOCl irradiated by UV-visible light in Ar or in HCl aqueous
solution saturated with O2 (Fig. 2b and c). The weaker peaks
suggest a reduced Bi state at the valence between 3 and 0,
shied by 2.5–2.6 eV toward the lower energy side from major
peaks due to the Bi3+ state. Furthermore, both an O and Cldecient structure was directly conrmed by the decrease inN(Bi–O) and N(Bi–Cl) values from 4 to 2.2 and 4 to 3.1, respectively (Fig. 3c and Table 1c), based on Bi L3-edge EXAFS in HClaqueous solution (pH 2.0) saturated with O2 irradiated by UVvisible light. Thus, BiO1xCl formed in Ar (Scheme 3b) further
lost a small amount of chlorine (BiO1xCl1y), but it

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transformed back to BiOCl when the light was oﬀ (Fig. 3d, Table
1d, and Scheme 3a), which is in accordance with literature.34 In
relation to the EXAFS results, a reduction peak due to Cl loss via
eqn (4) was observed at 0.39 V in HCl aqueous solution with N2
or O2 (Fig. 6a and b).
In summary, amorphous-like BiO1xCl spherical nanoparticles (Scheme 3b) would be present in the BiOCl samples
irradiated with UV-visible light under Ar and for which the UVvisible spectra (Fig. 1A-b–d), XRD patterns (Fig. 1E), Bi 4f5/2 and
4f7/2 XPS peaks (Fig. 2b), and Bi L3-edge EXAFS measurements(Fig. 3b) were obtained. Both O and Cl-decient BiO1xCl1yspecies could be present for the BiOCl lm on the photocathodeirradiated by UV-visible light immersed in HCl solution underO2 gas for XPS (Fig. 2c), Bi L3-edge EXAFS (Fig. 3c), and during
the PFC tests (Fig. 5A).
It is thought that the O and/or Cl-decient BiO1xCl1y or
BiO1xCl nanoparticles were covered with thin Bi2O3 layers(Scheme 3e) in samples subjected to UV-visible (Fig. 1C) and
EXAFS (Fig. 3e) analyses and used in the PFC tests in HCl withlight irradiation and direct exposure to air.
4.2

Factors for controlling PFC performance

The VOC, iSC, and Pmax values are based on i–V characteristics
and are summarized in Table 2. Although the theoreticalmaximum electromotive force for the PFC comprising TiO2 andBiOCl is 2.75 V in the energy diagram (Scheme 1), the best VOCvalue obtained in this study was 1.69–1.76 V (Fig. 5D and E). It isalso possible that the carrier level of the ITO [work function(WF) 4.6–4.8 eV] determined the voltage of the photoexcitedelectrons to the CB of TiO2 (WF 4.13–4.3 eV)65,66 if the ITO lmbehaved similar to a semiconductor rather than a conductor. If
this logic applies, the voltage loss due to the diﬀerence in the
WF is 0.3–0.67 V, and the theoretical maximum voltage of 2.75 V
would be reduced to 2.08–2.45 V.
Even though, the VOC values (1.69–1.76 V) were still higher
for the PFC comprising TiO2 and solvo/aqua-BiOCl than the
1.59 V reported for the PFC comprised of TiO2 and Ag–TiO2(Table 2).7 Depending on the energy diagram (Scheme 1), themaximum electromotive force is theoretically smaller for the
PFC comprised of TiO2 and BiOCl (2.75 V) than that for the PFC
comprised of TiO2 and Ag–TiO2 (3.0 V).7 In this context, thecombination of n- and p-semiconductors was advantageous forminimizing the overpotential compared to rectication by a
Schottky barrier.
4.3

Kinetics of the PFC

4.3.1 Derivation of kinetics. The photocurrents in thepresent PFCs were generated by the balance between thephotoexcitation and charge recombination both in the TiO2 andBiOCl photocatalysts, and the reaction rates of water photooxidation and O2 photoreduction (Scheme 1). The allowed indirect53,67 and direct58 (Fig. 1B) band gap electronic transitions
lead to the separation of electrons and holes in TiO2 and BiOCl,respectively, and equilibrium constants for charge separation(Ka and Kc) are assumed in the anodic TiO2 and cathodic BiOCl,respectively.

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The photooxidation over TiO2 was essentially irreversible
under N2 ow, which purged the generated O2. The superiorityof an HCl aqueous solution (pH 2.0) to H2SO4 aqueous solution(pH 2.0) as an electrolyte for the photoanode was demonstrated(Fig. 5A and A00 ). One of the major reasons would be thediﬀerence in molar conductivity of the solutions (411.1 versus
336.4 S cm2 mol1).68 The converged photocurrents gradually
increased in Fig. 5A when HCl solution was used, which is incontrast to the decreases in Fig. 5A00 when H2SO4 solution wasused. This diﬀerence suggests the inhibition of hole transferfrom the surface to a water molecule due to the blocking of Ti
sites by sulfate ions. Larger blocking eﬀects of Ti sites by theanions of electrolyte were reported for the photodecompositionof 2-chloro-20 ,60 -diethyl-N-(methoxymethyl)acetanilide and theorder of the eﬀects was in the order69
PO43 > NO3 > SO42 > Cl.

(6)

The rate and rate constant for this reaction are denoted as rox
and kox, respectively. The O2 photoreduction at BiOCl was inequilibrium with the product (water), and the constant isdenoted as Kred. Based on the principles of PFCs, the forward
electron ow rate from TiO2 to BiOCl via the external circuit
could be proportional to both the excited electron concentration in TiO2 and the hole concentration in BiOCl (Scheme 1). In
addition, the reverse electron ow rate could be proportional toboth the unreacted excited electron concentration at BiOCl andthe unreacted hole concentration at TiO2. Thus, the eﬀective
electron ow rate (photocurrent i) is formulated in eqn (7). The
derivation is shown in eqn (S1)–(S4) (ESI†).
i¼k

1 0 1 0 1
0
4 K 6 K8
Ka0 koxcred1ðrox Þ41   1½O2 c 8 Hþ c 21
1ðrox Þ4 Kc0 6
1
k1 0 1
1
1 ;
0
4 K
8 ½O  8 ½Hþ  2
kox
2 c
red
c
0

(7)

where [O2] and [H+] with a subscript “c” denote the concentrations of O2 and H+ in the acidic solution around the cathode,and the rate and equilibrium constants with the prime symbol
denote the constants multiplied with essentially constantconcentrations for the predominant species in the acidic solution/photocatalysts (see ESI† for a detailed denition).
4.3.2 Comparison of kinetics to experimental data. Fittingof the experimental data for the pH dependence in eqn (7) ispresented in the upper x-axis of Fig. 5B. The second term was
negligible in the tting to eqn (7) (i ¼ 1182[H+]1/2  0.07458[H+]1/2), demonstrating that the reverse electron ow from the
CB of BiOCl to the VB of anodic TiO2 via the external circuit was
minimal. Moreover, the net photocurrents were in fact
proportional to [H+]1/2 (Fig. 5B, lower x-axis), which is inaccordance with eqn (7).
A kinetic equation similar to eqn (6) was reported for the PFC
comprising TiO2 and Ag–TiO2.7 The tted equation for the pH
dependence was i ¼ 626.7[H+]1/2  0.009977[H+]1/2. The coefcient of the second term was even smaller than that for the p–n
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Schottky barrier between the TiO2 and Ag nanoparticles.
Notably, the combination of TiO2 and BiOCl was nearly a factor
less eﬃcient with respect to rectication.
Conversely, the coeﬃcient of the rst term was greater forthe p–n type PFC compared to that for the Schottky-type PFC(1182 versus 626.7, respectively). Faster charge transfer rates(holes to H2O at the anode and electrons to O2 at the cathode) at
the interfaces between the photocatalysts and the acidic solution due to band bending14,15 could enhance the rst term: theiSC values of 77.7–92.4 mA (Table 2A and B) versus 73.7 mA for the
Schottky-type PFC7 (Table 2C).
Another reason for this diﬀerence is the layered structure of
BiOCl. If a Cl 3p electron is excited to Bi 6p (Fig. 1A and C) upon
irradiation with UV-visible light, the holes and electrons could
preferably diﬀuse in the Cl and BiO+ layers, respectively, bothof which are in the 001 plane.13 The crystallites of solvo-BiOCltended to grow in the perpendicular direction to [001], whereas
the crystallites of aqua-BiOCl tended to grow in the direction of[001] (Fig. 1D and F). Therefore, the charges separated in bulk
would eﬀectively diﬀuse in the 001 plane and favorably reachthe photocatalyst surface, e.g. {100} and {110}, for aqua-BiOCl,13
which is in accordance with the PFC performances (Table 1A
and B). Alternatively, aqua-BiOCl was advantageous to solvoBiOCl because the larger number of crystallites led to a smaller
serial resistance70 of the cathode photocatalyst lm.
4.3.3 Redox of Bi sites. The peaks for the reduction of
BiOCl to Bi0 and the reverse oxidation reaction (eqn (4)) wereobserved in a pH 4.0 acidic solution under N2 (Fig. 6a and Table
3B). When the electrode was irradiated with UV-visible light,
weaker peaks for the reduction of Bi3+ to Bi0 and the reverse
oxidation reaction (eqn (5)) were also detected (Table 3A). In
contrast, in a pH 4.0 acidic solution under O2 in the dark, only avery weak peak for the reduction of BiOCl to Bi0 was observed,
and during irradiation, no CV peaks appeared (Fig. 6b).
The Bi0 species formed at voltages lower than 0.19 V during
the CV experiments was not detected via XRD, XPS, EXAFS, or
HR-TEM analysis of BiOCl irradiated under Ar or O2. The color
change from white to black (under Ar) or gray (under air/O2) was
due to O-decient BiO1xCl species formed because of UVvisible light irradiation. Under vacuum and irradiation of UVvisible light for 12 h, the stoichiometry of the sample was estimated to be BiO10.023Cl for solvo-BiOCl and BiO10.014Cl foraqua-BiOCl based on a kinetic study with O2 gas (Fig. 5C and
C0 ). Amorphous-like BiO1xCl particles (5 nm) were observedin the HR-TEM image (Fig. 4b3, 5–7), and these particles
reversibly transformed to crystalline BiOCl (Fig. 4b4 and 5)
because of the balance of electron ows in the HR-TEM apparatus and the O supply from the supporting BiOCl grains.
The presence of BiO1xCl1y species in HCl aqueous solution(pH 2.0) and O2 gas irradiated by UV-visible light was proposedby weaker intensity peaks at 162.2 and 157.0 eV at the lower
binding energy side for 4f5/2 and 4f7/2 for Bi3+ states of BiOCl
crystallites in XPS (Fig. 2c) and reduced N(Bi–O) and N(Bi–Cl)
values (Table 1c) given by Bi L3-edge EXAFS (Fig. 3c). No peaks
corresponding to Bi0 and no metallic Bi–Bi bonds were detectedin XPS and EXAFS, respectively.

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Journal of Materials Chemistry A

4.3.4 Reaction steps. The converged photocurrent (118 mA,
Fig. 5A) of the PFC at pH 2.0 corresponds to 1.8 mmol e h1
gBiOCl1, which is ten times faster than the reaction rate of theblack solvo-BiO1xCl with O2: k  0.206 mmol (initial O2amount)/0.135 gBiOCl ¼ 0.18 mmol O2 h1 gBiOCl1 based on thetting for 26 h, but is of the same order as the initial reaction
rate k0 during the rst 20 min (2.1 mmol O2 h1 gBiOCl1). The
initial higher rate suggests that the reaction with O2 rst occurs
at surface BiO1xCl sites (or BiO1xCl1y in HCl aqueous solution), rather than at bulk sites. The k value for 26 h was almost
reproduced for solvo- and aqua-BiOCl (1.37 versus 0.0939 h1;
Fig. 5C and C0 ), but the initial fast reaction at surface BiO1xCl
sites was diﬃcult to follow for aqua-BiOCl due to the lower
dispersion (larger mean particle size, see section 3.1.1, Fig. 1D
and F).
On the basis of these results, a mechanism for photocatalysis
by BiOCl has been proposed. In the PFC, a part of BiOCl on thephotocathode immersed in HCl at a pH of 2.0–4.0 is thought tobe photoreduced to BiO1xCl1y (Scheme 3d) in a manner
similar to that described in eqn (4):BiOCl (surface) + 2xH+ + (2x + y)e /BiO1xCl1y (surface) + yCl + xH2O

(8)

Reaction (4) was observed as a CV peak at 0.39 V in an O2
atmosphere, but only in the dark (Fig. 6b). The reaction with O2
at the BiO1xCl1y surface sites irradiated by UV-visible light(2.1 mmol O2 h1 gcat1, Fig. 5C) follows eqn (9):2BiO1xCl1y (surface) + xO2 / 2BiOCl1y (surface)(9)

Summing up eqn (8) and (9), eqn (10) is obtained:
4H+ + O2 + 4e / 2H2O

(10)

in which the partial dissolution of Cl was neglected because it
was not related to the photoconversion of H2O and O2. The total
current generated in Fig. 5A (pH 2.0) corresponds to 9.7 mmole, whereas the quantity of the BiOCl mounted on the cathodewas 9.6 mmol. Because the oxygen vacancy (x) in BiO1xCl irradiated by UV-visible light was estimated to be 2.3% (Fig. 5C), thecathode reaction could be catalytic.
Suggested active species for the catalysis using BiOCl-based
photocatalysts are summarized in Table 4. Due to the exposure
to UV and/or visible light, black/gray BiO1xCl was suggested tobe the active species for dyes and 2-naphthol degradation.35,58,60
The Cl dissolution via the reaction of cCl and electron was also
reported to form BiOCl1x(OH)x.34 On the other hand, metallic
Bi nanoparticles were suggested on the BiOCl or BiO1xCl
surface using a reducing agent, e.g. oxalate and ethanol eﬀective
for dyes, acetophenone, and p-chlorobenzaldehyde degradation.10,63 Furthermore, the reactivity of O-defects to form cO2and O22 was proposed over (001) and (010) surfaces of
BiO1xCl, respectively, for the O2 reduction based on DFT
calculations.71
In this study, in situ spectroscopic monitoring of BiOCl
during photocatalysis was performed for the rst time by Bi L3-

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Table 4 Suggested active species using BiOCl and the derivatives as photocatalysts

Substrate

MediumType of light

Catalyst

Suggested active species

Ref.

Rhodamine B
Rhodamine B
Methyl orange
Methyl orange
PhenolRhodamine B
2-Naphthol degrad.
Methylene blue
Methyl orange
Rhodamine B
Methyl blue
Eosin B
Coomassie brilliant blue
Acetophenone
p-Chlorobenzaldehyde
Rhodamine B
Rhodamine B
O2 reduction
O2 reduction

Water & air
Water & airDil. HCl aq & N2
Dil. HCl aq & air
Water & air
Water & air
Water & air
Water & air
Water & air
Water & air
Water & air
Water & air
Water & air
Water & air
Water & air
Water & air
Water & air——

UV (254 nm)
Vis (>420 nm)
High P Hg
High P Hg
High P Hg
High P Xe (>400 nm)
Vis
W-halogen (>420 nm)
Wideband
Vis
Vis
Vis
Vis
Vis
Vis
UV (365 nm)
Xe (>420 nm)
(DFT)
(DFT)

(001)-preferred BiOCl(001)-preferred BiOCl
BiOCl
BiOCl
BiOCl
BiOCl under UV in Ar(010)-preferred BiOCl
UV-reduced BiOCl
C2O42-reduced BiOCl
EtOH/HCl-reduced BiOCl
EtOH/HCl-reduced BiOCl
EtOH/HCl-reduced BiOCl
EtOH/HCl-reduced BiOCl
EtOH/HCl-reduced BiOCl
EtOH/HCl-reduced BiOCl
Ultrathin BiOCl
Ultrathin BiOCl(001) face(010) face

BiOCl
BiOCl
BiOCl1x(OH)x
BiOCl1x(OH)x
BiOCl1x(OH)x
BiO1xCl
BiO1xCl
BiO1xCl
Bi0–BiOCl
Bi0–BiO1xCl
Bi0–BiO1xCl
Bi0–BiO1xCl
Bi0–BiO1xCl
Bi0–BiO1xCl
Bi0–BiO1xCl
Bi-defect
Bi-defectO-defect to form cO2O-defect to form O22

60
60
34
34
34
35
60
58
10
63
63
63
63
63
63
72
72
71
71

edge EXAFS (Fig. 3c). No special reducing agent10,63 was used inthis study. Thus, active species BiO1xCl1y (Scheme 3d) is
proposed based on the XPS (Fig. 2c), EXAFS (Fig. 3c), andreactivity test with O2 (Fig. 5C) being consistent with the suggested active structure in ref. 34, 35, 60, and 58 and the reaction
route via eqn (9) is in accordance with ref. 71.
In addition to plausible incorporation as active sites in eqn(9), the O-decient state of BiO1xCl(1y) is slightly lower thanthat of the CB of BiOCl. Consequently, it can eﬀectively trap
photoexcited electrons59,73 and contribute to the photocatalysis
via transfer of the trapped electrons to O2 molecules in a similarmanner to that observed for dye photodecomposition in thepresence of O2 (ref. 35, 58 and 73) and as predicted by DFT
calculations for the (001) surface.71

4.4

Energetic eﬃciency of the PFC

The quantum eﬃciency of the PFC was evaluated on the basis of
the absorption (6.9%) by the photocatalyst of the incident light(1.7  1017 photons per s) that the photocatalyst layers were
exposed to and the short circuit current (92.4 mA to 5.7  1014
electrons per s; Fig. 5E). The overall eﬃciency of the cell current
per absorbed light was 0.024 and is considered as the product of
the eﬃciencies of the anode (TiO2) and the cathode (BiOCl). If asimilar eﬃciency is assumed for electron generation at the CBof the anode and hole generation at the VB of the cathode, thenthe eﬃciencies of the anode and cathode were each approximately 0.16 (0.16  0.16 ¼ 0.024). Practically, as the absorbance at the photocatalyst lm was low (6.9%), the true overall
eﬃciency was lower (0.0017), that may be divided into thecontribution of anode and cathode: 0.041  0.041 ¼ 0.0017.
Next, the contribution of the entropy of mixing (DSmix) was
considered for an equivalent amount of N2 and O2 supplied tothe photoanode and photocathode, respectively, in comparison
to the concentration cells.74 When the maximum power was

8402 | J. Mater. Chem. A, 2015, 3, 8389–8404

produced by the PFCs comprised of TiO2 and solvo-BiOCl(Fig. 5D) or aqua-BiOCl at pH 2.0 (Fig. 5E), the power generationeﬃciencies were 10.5 mW (1.01 V  10.5 mA) and 21.1 mW (1.15 V
 18.3 mA), respectively, versus the energy generated because of
mixing of N2 and O2 (TDSmix) per second:
TDSmix ¼  RTðnN2 ln xN2 þ nO2 ln xO2 Þ¼  8:31  290  2 

10:5  1061ln ¼ 0:091 mW;
4  9:65  104 2
(11)

and
TDSmix ¼  8:31  290  2 

18:3  1061ln ¼ 0:16 mW;
4  9:65  104 2(12)respectively. Thus, the contribution of the entropy of mixing of
the feed gases was negligible, and the light energy for band gap
excitation at both the anode and cathode exclusively generated
power up to 21.1 mW (Table 2B). The electromotive force of the
PFC can thus be explained on the basis of the energy diagram in
Scheme 1, including the equilibrium charge separation at theanode and cathode following irradiation, rather than the simple
diﬀerence in the concentrations of O2 and the Gibbs freeenergies for the O2 fractions at the anode and cathode for theconcentration of the cells.74

5 Conclusions
A PFC comprising two photocatalysts (TiO2 and BiOCl) was
demonstrated to achieve VOC values of 1.69–1.76 V on the basisof the diﬀerence in the CB minimum of TiO2 and the VB
maximum of BiOCl, which are larger than those for PECs, dyesensitized SCs, and a PFC comprising TiO2 and Ag–TiO2.7 Thelarger particle size (mean 115 nm) of aqua-BiOCl was advantageous by facilitating electron transfer from particle to particle,

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whereas the smaller particle size (mean 24 nm) of solvo-BiOCl
was advantageous for suppressing charge recombination within
the nanoparticles. The maximum power was 21.1 mW for the
PFC comprised of TiO2 and aqua-BiOCl, and the energetic eﬃciency per absorbed light at the photocatalysts was estimated as
the product (0.024) of electron generation at TiO2 (0.16) andhole generation at BiOCl (0.16) for recombination via theexternal circuit.
A portion of the BiOCl nanoparticles was transformed into
amorphous-like BiO1xCl nanoparticles because of irradiation
with UV-visible light, and then reversibly transformed back in a
vacuum (HR-TEM and Raman) or under Ar (UV-visible spectroscopy and XPS) when the light was turned oﬀ. These Odecient nanoparticles were the origin of the black color
appearance of the irradiated BiOCl. In addition, a BiOCl sample
suﬃciently irradiated with UV-visible light reacted with thequantity of O2 gas corresponding to the dissolved amount in the
HCl solution in the PFC at a rate (2.1 mmol O2 h1 gBiOCl1)during the initial 20 min that was on the same order as thephotocurrent of the PFC (1.8 mmol e h1 gBiOCl1), suggestingthat surface O-decient BiO1xCl sites may contribute to thephotocatalytic O2 reduction in the PFC. The reduced Bi sites
observed for BiOCl in Ar irradiated by UV-visible light was also
detected but further lost a part of Cl as BiO1xCl1y in HClaqueous solution (pH 2.0) in O2 irradiated by UV-visible light by
Bi 4f XPS and Bi L3-edge EXAFS. Again, the reaction of O-decient BiO1xCl1y sites with O2 and subsequent reaction with
protons and electrons suggested a catalytic cycle for O2 reduction in the PFC.

Acknowledgements
The authors are grateful for nancial support from the Feasibility Study Stage of A-STEP (AS251Z00906L, AS231Z01459C)
from the Japan Science and Technology Agency, the Iwatani
Naoji Foundation (2011–2012), and a Grant-in-Aid for ScienticResearch C (26410204, 22550117) from the Japan Society for the
Promotion of Science. X-ray absorption experiments were conducted under the approval of the Photon Factory Proposal
Review Committee (2013G159). We appreciate Dr Takeo Soejima at JASCO for Raman measurements and Yoshihide Yoshida and Tohru Maki at the Analytical & Measuring Instruments
Division, Shimadzu Corporation for XPS measurements.

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