RSC Advances
PAPER

Cite this: RSC Adv., 2017, 7, 19996

A solar cell for maximizing voltage up to the level
diﬀerence of two photocatalysts: optimization andclariﬁcation of the electron pathway
Mao Yoshiba,a Yuta Ogura,a Masayuki Tamba,b Takashi Kojimab and Yasuo Izumi*aFuel cells (FCs) and solar cells (SCs) are indispensable devices for a hydrogen energy society. The voltages
obtained are less than 1 V per cell for most FCs and SCs. Herein, we use a recently developed SC comprising
two photocatalysts. In principle, both TiO2 and BiOCl are photo-excited, and the energy diﬀerence between
the conduction band (CB) minimum of TiO2 for excited electrons (0.11 V) and the valence band
(VB) maximum of BiOCl for holes (2.64 V) can provide a theoretical electromotive force of 2.75 V. ThisSC converts light energy into an electromotive force corresponding to the level diﬀerence of the two
photocatalysts permanently mediated by the redox of water/O2. The diﬀusion overpotential of electrons
in the photocatalysts (0.23–0.41 V) and the leakage current (0.38 V) are experimentally evaluated. The
contact between the TiO2 ﬁlm and ITO layer is improved by the better dispersion of the TiO2 suspension
at a lower pH than that of the isoelectric point. Cyclic voltammetry data suggest the formation of O/Cl
vacancy sites during the SC tests and the superiority of the rear orientation of the BiOCl photocatalyst
Received 2nd March 2017
Accepted 21st March 2017

on the cathode and eﬀective photo-oxidation of water over TiO2, whereas impedance measurementssuggest a smaller impedance for the tight and uniform TiO2 ﬁlm in comparison to the porous BiOCl ﬁlm.
Thus, in the optimized conﬁguration of the electrodes (irradiation from the other side of the

DOI: 10.1039/c7ra02600e

photocatalyst), the leakage current and the diﬀusion overpotential in the catalyst layers are eﬀectively

rsc.li/rsc-advances

suppressed to realize an open-circuit voltage of 1.91 V and cell output of 55.8 mW per 1.3 cm2.

1

Introduction

Fuel cells (FCs) and solar cells (SCs) are indispensable energy
devices for a sustainable society, which do not emit CO2 and do
not require fossil fuels, respectively.1 In principle, their opencircuit voltages (VOC) are determined on the basis of thediﬀerence in the reaction potentials at the electrodes for polymer electrolyte FCs2 and microbial FCs,3 the diﬀerence in the
Fermi levels (EF) for Si SCs, the diﬀerence between the
conduction band (CB) minimum and the reaction potential fordye-sensitized (DS) SCs,4,5 and the diﬀerence between the EF of
perovskite and the highest occupied molecular orbital of holetransport materials for perovskite SCs.6,7 The maximum VOC for
most FCs and SCs is 1 V per cell; however the series connectionof these cells to obtain higher voltage values is complex anddisadvantageous for portable and/or outdoor use.
Thus, herein, an SC with an optimized VOC comprising two
photocatalysts is designed. TiO2 and BiOCl are chosen as thephotocatalysts for anode and cathode, and the energy diﬀerence

between the CB minimum of TiO2 for excited electrons (0.11
V) and the valence band (VB) maximum of BiOCl for holes (2.64
V) can provide a theoretical electromotive force of 2.75 V. TiO2 isan n-type semiconductor and advantageous to accept electrons
from water during oxidation.16,17 In contrast, a p-type semiconductor is advantageous to transfer the photo-excited electrons to O2 molecules. The feasibility of these advantages of then–p combination in the SC is demonstrated.16 In this study, theoptimization of cell conguration, reason for the complex
current–voltage dependence, fundamental understandings forthe electron (e) pathway in the SC, and reasons for the overpotential are claried. In contrast to various photoFCs, the SC
for the use of glucose,8,9 the SCs comprising a photoanode anda Pt-cathode,10,11 and photoelectrocatalytic cells,12–14 the developed SC directly extracts the V corresponding to the band-gap
values (3 V) per cell without using fuel.

2 Experimental
2.1

aDepartment 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 Applied Chemistry and Biotechnology, Graduate School of Engineering,
Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan

19996 | RSC Adv., 2017, 7, 19996–20006

Sample syntheses and preparation of photoelectrodes

A TiO2 organic/aqueous suspension (mean TiO2 particle size
15.6  3.3 nm; PEC-TOM-T03, Peccell Technologies, Inc.) was
placed on an indium tin oxide (ITO; thickness 0.12–0.15 mm)coated Pyrex glass plate (area: 2.5 cm  2.5 cm and thickness:
0.11 cm; Aldrich). The suspension was controlled to be acidic,

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RSC Advances

below the isoelectric point of TiO2 (4.8–6.7), and well-dispersedby the repulsion of the positive charges of the TiO2 nanoparticles.15 The TiO2/ITO/Pyrex plate 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 2.0 mg, and it
covered an area of 1.0 cm  1.3 cm.
BiOCl was synthesized from an aqueous solution.16,18 3.15 g
bismuth trichloride (>97%; Wako Pure Chemical) was dissolvedin 100 mL deionized water (<0.055 mS cm1) supplied by
a RFU424TA system (Advantec). The solution was agitated using
ultrasound (430 W, 38 kHz) for 10 min and then magneticallystirred at 290 K in the dark for 2 h. The precipitate was lteredusing a polytetrauoroethylene-based membrane lter(Omnipore JVWP04700, Millipore; pore size 0.1 mm), washedwith ethanol (total 200 mL), and dried at 353 K for 12 h.
The obtained BiOCl powder was suspended in 38 mL deionized water and placed on an ITO/Pyrex plate in a manner similar
to that used for the TiO2/ITO/Pyrex. The amount of BiOCl placedon each ITO-coated glass plate was 2.5 mg, and it covered anarea of 1.0 cm  1.3 cm.
2.2

Characterization

Cross-sectional scanning electron microscopy (SEM) images
were observed using a JSM-6510 scanning electron microscope(JEOL) at an accelerating voltage of 20 kV. A tungsten lament
was used in the electron gun. The photocatalyst lm on the
ITO/Pyrex plate was cut and mounted on an aluminum
sample holder using an adhesive. The incident angle of e's
with reference to the normal of the sample surface was between
75 and 85 . The magnication was between 250 and 50 000
times.
X-ray diﬀraction (XRD) patterns were obtained for the photocatalyst powder and the lm. In comparison, XRD was also
performed on the ITO/Pyrex plate. The XRD patterns of thepowder were measured at a Bragg angle of 2qB ¼ 10–60 witha scan step of 0.02 and scan rate of 4 s per step using D8
ADVANCE (Bruker) operated at 40 kV and 40 mA with Cu Ka
emission (wavelength l ¼ 0.15419 nm)19 and a nickel lter. The
XRD patterns of the lm were observed under similar conditions but with a scan rate of 0.1 s per step using a Mini Flex
(Rigaku) operated at 30 kV and 15 mA. The mean particle size (t)was determined using the Scherrer equation
t¼
2.3

0:9l
:
full width at half maximum  cos qB(1)

SC tests

The TiO2/ITO/Pyrex and BiOCl/ITO/Pyrex electrodes were
immersed in hydrochloric acid solutions (40 mL in eachcompartment, initial pH 2.0; Fig. 1). The two compartments
were separated by a 50 mm-thick H+-conducting polymer lm(Naon, DuPont; acid capacity > 9.2  104 equivalent per g1).
N2 and O2 gases were bubbled 30 mm away from each photoelectrode at a ow rate of 100 mL min1. The distance was xed
both in the front and rear congurations for the photocatalysts(Fig. 1). The SC was equipped with quartz windows (F ¼ 80 mm)
on both sides. Both the TiO2 and BiOCl photocatalysts wereThis journal is © The Royal Society of Chemistry 2017

Fig. 1 i–V dependence of the SC comprising TiO2 and BiOCl both on
ITO/Pyrex electrodes, when they are immersed in HCl solutions at
a pH of 2.0. (a) Both electrodes irradiated from the photocatalyst side(front–front), (b) the anode and the cathode irradiated from theelectrode and the photocatalyst side (rear–front), (c) the anode andthe cathode irradiated from the photocatalyst and the electrode side(front–rear), respectively, and (d) both electrodes irradiated from theelectrode side (rear–rear).

irradiated with UV-visible light through the quartz windows
using a two-way branched quartz ber light guide (Model 5F2B-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 surface of the TiO2 or BiOCl lm was 46
mm. The light intensity was 91.3 mW cm2 at the center of thephotocatalyst lm on each electrode (Fig. 2).
The dependence of the i–V curves on the conguration of theanode and cathode was tested in four combinations, as depicted
in Fig. 1a–d, with an N2 and O2 ow. As a control, 40 mL ofhexane layer was added on the top of the aqueous HCl layer (40mL) in the anode and the two compartments of anode andcathode were owed with N2 and O2, respectively. Furthermore,
the gas supply was stopped, and the compartments were isolated before the start of the i–V test.
The i–V dependence by the irradiation of one electrode only
was monitored in the front–front conguration (Fig. 1a), in
which the rear sides of the electrodes were covered by a 3 mm-

RSC Adv., 2017, 7, 19996–20006 | 19997

RSC AdvancesFig. 2 The light intensity at the photocatalyst (left: 2.0 mg of TiO2 andright: 2.5 mg of BiOCl), on the other side of the ITO/Pyrex electrode,and at the photocatalyst on the other side of the SC.

thick polytetrauoroethylene (PTFE) plate (area ¼ 2.0 cm 
2.0 cm). As a control, PTFE plates were not used and only thecathode was irradiated in the i–V test for the practical feasibilityof irradiation from one side only.
Static photocurrent generation tests were performed byconnecting the external parallel circuit for one route via a voltmeter and for another route via an ammeter with a resistance of
0.5 U (Fig. 1). Both catalysts were exposed to UV-visible light for
30 min and then kept in the dark for 30 min, and this process
was repeated ve times. The i–V characteristics were obtainedby changing the resistance between the two electrodes from 500
kU to 0.3 U over 30 min for the four anode–cathode congurations (Fig. 1a–d).
2.4[Photo]electrochemical measurements

Cyclic voltammetry (CV) measurements were performed for
TiO2 and BiOCl as the working electrode (WE; TiO2/ITO/Pyrex or
BiOCl/ITO/Pyrex and an Au/Ni-coated stainless clip; Toyo
Corporation), glassy carbon as the counter electrode (CE; Model
002012, Fpolyetheretherketone: 6.0 mm, FC: 3.0 mm; BAS Incorporation), and Ag/AgCl as the reference electrode (RE; Model RE1B, BAS Incorporation), immersed in an HCl solution of pH
4.0 (Fig. 3). The voltage of the WE was swept between 1.00 and
1.50 V for TiO2, 1.22 and 1.50 V for BiOCl versus Ag/AgCl at
a rate of 50 mV s1 using a potentio/galvanostat (VersaSTAT 3100, Princeton Applied Research) in either an O2 or a N2
atmosphere at a ow rate of 500 mL min1 irradiated with UV-

Fig. 3 Design and electrode conﬁguration of the electrochemical cellfor CV measurements.

19998 | RSC Adv., 2017, 7, 19996–20006

Paper

visible light (91.3 mW cm2 at the WE sample position) or in thedark.
Impedance measurements were performed for TiO2 (meanthickness 4.7  0.3 mm) and BiOCl (mean thickness 5.4  1.9mm) on ITO/Pyrex plates using a potentio/galvanostat (VSP, BioLogic). A Cu plate (area 0.28 cm2) was attached on these photocatalyst lms, and the impedance between the Cu plate andITO lm was monitored using Au-coated Cu probes (KPRO-RY,
Toyo Corporation). The applied alternative voltage was 500 mV,
and the frequency was scanned between 101 Hz and 106 Hz.
The light transmission was checked using a photosensor anda counter (PCM-01, Prede and KADEC-UP, North One, respectively). The quartz windows of the SC absorbed/reected 8.7% of
the light, and 2.0 mg of the TiO lm and 2.5 mg of the BiOCllm both on ITO/Pyrex absorbed/reected/scattered 58% and
63% of the light, respectively. Moreover, 3.7–4.2% of the lightreached the electrode on the other side (Fig. 2).

3 Results and discussion
3.1

Characterization

TiO2 and BiOCl photocatalyst lms were formed over indium tinoxide (ITO)-coated Pyrex glass plates, as observed by crosssectional SEM (Fig. 4). The lm area was 1.0 cm  1.3 cm,
and the mean thickness of the photocatalysts was 4.7  0.3 mm
for TiO2 (Fig. 4A2) and 5.4  1.9 mm for BiOCl (Fig. 4B2). The
TiO2 nanoparticles (mean particle size ¼ 15.6 nm  3.3 nm)
were closely packed via an acidic well-dispersed suspension to
constitute uniform layers (Fig. 4A), whereas the BiOCl particles(mean size ¼ 127  43 nm) were less packed via a neutral
aqueous suspension and showed porosity (Fig. 4B).
The XRD patterns of the powder and the TiO2 and BiOCl
lms were obtained (Fig. 5). Four peaks appear at 2qB ¼ 21.4,
30.3, 35.4, and 50.5 for ITO/Pyrex (Fig. 5A), which are assigned
to the 2 1 1, 2 2 2, 4 0 0, and 4 4 0 diﬀractions of the ITO lm,
respectively. The broad peak at 20–30 is due to the amorphous
phase of the Pyrex glass.
Fig. 5B depicts the XRD pattern of the TiO2 powder. Peaks
appear at 2qB ¼ 25.4, 37.1, 38.7, 48.0, 53.9, and 55.1 , which are

Fig. 4 Cross-sectional SEM images of TiO2 (A) and BiOCl (B) particles
in wide (1) and close (2) views.

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RSC Advances
Table 1 The peak intensity ratio in the XRD patterns of the TiO2 (A) andBiOCl (B) ﬁlms and powders, and the intensity ratio of the corresponding peaks of the ﬁlms versus the powders(A) TiO2
TiO2

Peak intensity ratio

Crystal phase
Anatase

RutileDiﬀraction
plane

Film

PowderFilm/powder

101
200
105
211
110
101
004

1.00
0.18
0.10
0.11
0.20
0.10
0.16

1.00
0.26
0.15
0.17
0.10
0.05
0.17

1.00  0.03
0.71  0.10
0.66  0.17
0.65  0.15
1.99  0.26
1.95  0.48
0.92  0.15

(B) BiOCl

Fig. 5 XRD pattern of (A) ITO/Pyrex, (B) TiO2 powder, (C) TiO2 ﬁlm on
ITO/Pyrex, (D) BiOCl powder, and (E) BiOCl ﬁlm on ITO/Pyrex.

assigned to the 1 0 1, 1 0 3, 1 1 2, 2 0 0, 1 0 5, and 2 1 1
diﬀractions of the anatase phase, respectively. In addition,
peaks appear at 2qB ¼ 27.5, 36.1, 38.0, and 56.8 , which areassigned to the 1 1 0, 1 0 1, 0 0 4, and 2 2 0 diﬀraction of therutile phase, respectively, and a peak appears at 2qB ¼ 31.0 andis assigned to the 1 2 1 diﬀraction of the brookite phase.
In contrast, for the TiO2 lm on ITO/Pyrex (Fig. 5C), the 1 0 1,
2 0 0, 1 0 5, and 2 1 1 diﬀraction peaks at 2qB ¼ 25.4, 48.1, 54.0,
and 55.1 due to the anatase phase, the 1 1 0, 1 0 1, and 0 0 4
diﬀraction peaks at 2qB ¼ 27.5, 36.2, and 37.9 due to the rutile
phase, and the 2 2 2, 4 0 0, and 4 4 0 diﬀraction peaks at 2qB ¼
30.3, 35.3, and 50.6 due to the ITO lm appear. The peakintensity ratios in the XRD patterns of the TiO2 powder and theTiO2 lm are listed with reference to the 1 0 1 peak intensity(Table 1A). Furthermore, the intensity ratio of the corresponding peaks for lm versus powder was also calculated, as listed inTable 1A. Clearly, based on the values, the thermodynamicallyunstable 2 0 0, 1 0 5, and 2 1 1 planes of the anatase phase areunfavorable in the TiO2 lm, contrary to the thermodynamicallystable 1 1 0 and 1 0 1 planes of the rutile phase that are
signicantly favoured because the TiO2/ITO/Pyrex electrode washeated in air at 573 K and stabilized prior to the characterization and SC tests. Thus, the thermodynamically stable planes ofTiO2 were prioritized during the preparation of the TiO2 lm on
ITO/Pyrex (see Section 2.1).
In the XRD patterns of both the powder and the lm of BiOCl(Fig. 5D and E), peaks at 2qB ¼ 12.1, 24.3, 26.0, 32.7, 33.6, 36.7,
41.0, 46.8, 49.8, 55.3, and 58.8 appeared and were assigned to
the 0 0 1, 0 0 2, 1 0 1, 1 1 0, 1 0 2, 0 0 3, 1 1 2, 2 0 0, 1 1 3, 2 0 2, and2 1 2 diﬀraction of the tetragonal structure of BiOCl, respectively.20 The peak intensity ratio in each XRD pattern for theThis journal is © The Royal Society of Chemistry 2017

BiOCl

Peak intensity ratio

Diﬀraction
plane

Film

PowderFilm/powder

001
002
101
110
102
003

3.37
1.00
0.47
0.25
0.42
0.61

2.84
1.00
0.40
0.25
0.59
0.64

1.18  0.01
1.00  0.01
1.17  0.02
1.01  0.04
0.71  0.02
0.95  0.01

BiOCl powder and lm is listed with reference to the 0 0 2 peakintensity (Table 1B). Furthermore, the intensity ratio of thecorresponding peaks of the lm versus the powder was also
calculated (Table 1B). Based on the values, the thermodynamically stable 0 0 1 and 1 0 1 faces are favored, whereas the thermodynamically unstable 102 face is unfavourable due to theheating at 573 K in air prior to the characterization and SC tests.
3.2

i–V dependence of SC

3.2.1 Dependence on electrode conguration. First, thecurrent i–V characteristics were obtained for the two photocatalysts positioned at the UV-visible light irradiated front(Fig. 1a). The convex i–V characteristics result in a VOC value of
1.43 V (Fig. 6A-a and Table 2A-a), and the current–voltage curve
shape is similar to that for Si SCs and DSSCs.4,5 However, thecurrent did not converge following the convex curve, butsuddenly rose linearly up to short-cut i (iSC) of 186 mA in therange below 0.5 V. This curve looks like a series combination of
diﬀerent SCs, where their impedance does not match very wellor where a part of the SC is shaded.21,22
Next, the TiO2 lm was positioned on the rear side toward
the irradiation of light, while the BiOCl lm remained on thefront side toward the light (Fig. 1b). Although the VOC value
increased to 1.66 V, both the convex and linear characteristics inthe entire V range and the range below 0.3 V, respectively, were
suppressed and resulted in an iSC of 66.7 mA and a maximum
power (PMAX) of 23.8 mW corresponding to 36% and 44% of the
RSC Adv., 2017, 7, 19996–20006 | 19999

RSC AdvancesPaperoriginal values (Fig. 1a), respectively (Table 2A-a and b). Thus,
the number of eﬀectively excited e's at the anode was smaller
than the number of holes at the cathode, and the i value of SC
was determined by the former number. The decrease in thenumber of excited e's at the anode in Fig. 1b is attributed to
the following: (1) the ltering of the light by the ITO/Pyrex
electrode in front of TiO2 in the rear conguration and (2) tothe fact that only the TiO2 catalyst layer near the ITO electrodecontributed to the supply of excited e's, and the holes photooxidized water (Fig. 1b), whereas most of the TiO2 catalyst was
utilized in the frontal conguration (Fig. 1a). Compared to thetotal absorption, scattering, and/or reection of TiO2/ITO/Pyrex(63%, Fig. 2), the absorption, scattering, and/or reection of the
ITO lm and Pyrex plate were 8.0% and 0.7%, respectively.
Thus, the contribution of the ITO lm in the rear conguration
is not negligible but that of TiO2 (thickness 4.7  0.3 mm) is
predominant under the conditions of this study. Conversely, theoverpotential due to the diﬀusion of e's in the catalyst layer
was minimal in the rear conguration of the anode (Scheme 1,le), leading to an increase in the VOC value by 0.23 V (Table 2Aa and b).
Then, the BiOCl photocatalyst lm was positioned on the rearside toward the irradiation of light, while the TiO2 lm wasirradiated from the front side (Fig. 1c). Considering the i–V
dependence at the front–front conguration (Fig. 1a), the convex
curve expanded to a higher V range (Fig. 6A-a and c), resulting ina maximal VOC of 1.84 V (Table 2A-c). The linear i increase in therange below 0.4 V was mostly suppressed below 20 mA, and the i–
V dependence became similar to that for Si SCs and DSSCs. The
improvement in the complex i–V curve, as in Fig. 6A-a, to a simple
convex curve, as in Fig. 6A-c and d is similar to the improvement
in a series combination of diﬀerent SCs, in which their impedance was not matched very well or in which a part of the SC wasshaded.21,22 Further improvement would be possible, particularly
by the control of uniformity of the BiOCl lm and the secureinterface between BiOCl and the electrode.
The total e ow from the anode to cathode during an i–V
test wasð0
ATotal ¼iðV ÞdV :(2)
VOC

Fig. 6 i–V dependence curves of SC comprising TiO 2 and BiOCl
photocatalysts in four conﬁgurations of anode and cathode, as
illustrated in Fig. 1a–d(A), in the front–rear conﬁguration anda hexane layer (40 mL) added on the aqueous layer in the anode(B), and (a) when both electrodes are irradiated, (a0 ) when only
the anode is irradiated, and (a00 ) when only the cathode is irradiatedin the front–front conﬁguration (Fig. 1a) (C). i–V dependence
curve when only the anode (a0 ) or cathode (a00 ) is irradiated in thefront–front conﬁguration (D). N2 and O2 gases were supplied to theanode and cathode in tests A, C, and D while the compartments of
the anode and cathode were purged by N2 and O2, respectively,
and isolated in test B. The rear sides of the electrodes were coveredby PTFE plates in tests C whereas PTFE plates were not used in tests
A, B, and D.

20000 | RSC Adv., 2017, 7, 19996–20006

The PMAX and ATotal values shown in Fig. 1a were essentiallypreserved in the results shown in Fig. 1c (changing by 1.02 and0.93 times, respectively; Table 1A-a and c) and Fig. 6A-c,
substituting the increase in the VOC value for the linear increase
to iSC in Fig. 6A-a in the V range below 0.4 V.
In the rear conguration of the cathode, the photoexcitede's were transferred from TiO2 to the cathode and werequickly combined with the remaining holes at the VB of the
BiOCl lm near the cathode (Scheme 1, right). The holes in the
VB of BiOCl tended to concentrate near the interface with the
ITO due to the band-bending of the p-type semiconductor(Scheme 1, right),16 and the shorter diﬀusion length of e's
at the interface in the cathode enabled greater VOC values(Table 2A-c).

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PaperTable 2

RSC Advances
Electrochemical data for the SC comprising both TiO2 and BiOCla obtained from the i–V analyses(A) Dependence on the anode–cathode congurationConguration ofphotocatalysts to UVvisible light
Entry

Anode

Cathode

a
b
c
c0 b
d

Front
Rear
Front

Front
Front
Rear

Rear

Rear

N2,
O2 supply
FlowedClosedFlowedVOC (V)iSC (mA)

PMAX (mW)

ATotal (mW)

1.43
1.66
1.84
1.84
1.88

186
66.7
85.0
84.4
84.4

53.9
23.8
55.0
43.7
55.8

99.3
43.6
92.1
84.4
93.0

VOC (V)iSC (mA)

PMAX (mW)

ATotal (mW)

1.56
0.38
0.28
<0.01

163
79.6
1.5
<0.1

53.4
2.80
0.29
<0.01

103
7.61
0.31
<0.01

(B) Dependence on the UV-visible irradiation to anode and/or cathodec
Presence/absence of lightirradiation
Entry

Anode

Cathode

a
a0
a00
a000

Light
Light
Dark
Dark

Light
Dark
Light
Dark

N2,
O2 supply
Floweda
The area of the photocatalysts: 1.3 cm2. b Hexane (40 mL) was on the top of aqueous HCl (40 mL) in the anode. c Both electrodes were irradiatedfrom the photocatalyst side, but the rear side was covered by 3 mm-thick plastic plates.

Scheme 1 Energy diagram and e ﬂow paths for the SC comprising
TiO2 and BiOCl. Path A provides the optimized electromotive force inthe rear–rear conﬁguration of the anode and the cathode toward theUV-visible light irradiation (Fig. 1d) owing to an eﬀective charge
combination at the interface between the electrodes and the photocatalysts, whereas path B provides the same in the front–front
conﬁguration (Fig. 1a). The leakage current from the CB of TiO2 to theOV sites of BiOCl is also indicated.

Finally, the rear–rear conguration for the TiO2 and BiOCl
photocatalysts (Fig. 1d) was tested. The dependence curve was
essentially identical to that of the front–rear conguration(Fig. 6A-c and d). Thus, the number of eﬀective holes at theThis journal is © The Royal Society of Chemistry 2017

cathode in Fig. 1c and d (rear conguration) would be smaller
than the number of excited e's at the anode, and the i value ofSC was determined by the former number.
Associated with this, the thickness of the TiO2 layer (4.7  0.3mm) on the anode varied between 0.8 and 14.1 mm in the front–
rear conguration (Fig. 7). The VOC (1.81–1.91 V) and PMAX
values (36.7–55.0 mW) were almost independent of the thickness
and in agreement with the hypothesis in the previous paragraphthat the i value of the SC is determined by the number of holes
at the cathode in Fig. 1c and d.
As a future option of this SC, which does not need a gas
supply, N2 and O2 gases were rst owed to the anode andcathode in the front–rear conguration (Fig. 1c) at a rate of 100
mL min1 and then stopped prior to the i–V test (Fig. 6B). 40 mLof hexane on the top of HCl aqueous solution in the anodecompartment was also used to circulate the photocatalyticallyformed O2.23 The obtained electrochemical data (VOC 1.84 V, iSC
84.4 mA, ATotal 84.4 mW; Table 2A-c0 ) is quite similar to that for
the test under gas ow (Table 2A-c) within the variance of 0.916–
1.00 times; however, the PMAX value in the closed SC was 79.5%of that in the SC with a gas ow. Since the number of holes atthe cathode determine the i value of the SC in the front–rearconguration, the ATotal value is essentially preserved in the
results shown in Fig. 6A-c and B and Table 2A-c and c0 independent of the changes in the anode conditions (N2 owstopped and upper hexane layer in the anode versus essentiallyno eﬀect of abundant O2 ow stopped).
3.2.2 Dependence on the irradiation on the photoelectrodes. To identify each e pathway in the SC, in order torealize the convex i–V dependence expanding toward VOC values

RSC Adv., 2017, 7, 19996–20006 | 20001

RSC AdvancesFig. 7 Dependence of the i–V characteristics on the thickness of the
TiO2 layer in the SC comprising TiO2 and BiOCl in the front–rear
conﬁguration. The thickness of the BiOCl ﬁlm was ﬁxed at 5.4  1.9mm.

of 1.84–1.88 V (Fig. 6A-c and d) and to suppress linear i increase
to iSC in the V range less than 0.4 V (Fig. 6A-a), the two photocatalysts were irradiated one by one during the i–V tests(Fig. 6C). To eliminate the eﬀects of transmitted light throughthe other side of the electrode (3.7–4.2% of the direct light to theelectrode; Fig. 2), UV-visible light was irradiated from the photocatalyst side, and the rear side of the electrode was covered by
a 3 mm-thick PTFE plate (area ¼ 2.0 cm  2.0 cm).
The i–V dependence in the front–front conguration (Fig. 6Aa) is reproduced in Fig. 6C-a, in which the VOC, PMAX, and ATotal
values are essentially the same within the variance of 0.99–1.09
times, whereas the iSC value decreases by a factor of 0.88 times
due to the light being blocked from the rear side by the PTFE
plate (Table 2A-a and B-a).
When only the anode was irradiated with UV-visible light,
a current as high as 79.6 mA was observed only in the V range
below 0.4 V (Fig. 6C-a0 ), and the ATotal value decreased by a factorof 0.074 times compared to that obtained under light irradiation from both sides (Table 2B-a and a0 ). Testing the front–front
conguration, we observed a sharp linear i increase at the Vrange below 0.4 V (Fig. 6A-a and C-a). Thus, the linear i increaseat the low-V region is a leakage current from the CB of TiO2 on
the anode to the O vacancy (OV) sites in BiOCl on the cathode(Scheme 1). Conversely, when only the cathode was irradiated(Fig. 6C-a00 ), the VOC, iSC, PMAX, and ATotal values were only 0.3–
18% of the corresponding values for Fig. 6C-a due to thepolarization of TiO2.
The future feasibility of irradiation from one side only was
also tested when the PTFE plates on the rear side of the electrodes

20002 | RSC Adv., 2017, 7, 19996–20006

Paper

were not used under the conditions of irradiation of anode only
(Fig. 6D-a0 ) and cathode only (Fig. 6D-a00 ). The iSC values weresatisfactory in the former (134 mA) compared to values for thetests where irradiation was from both sides (186–66.7 mA; Table2A); however, the convex section was suppressed in these tests
because the UV-visible light spot from the bers in this study(Fig. 2) tended to extend (the spot radius was two times at 9.2 mmfrom the light exit), and only 3.7–4.2% of light reached theelectrode on the other side (Fig. 2). This problem would bepartially mitigated by use under sunlight or translatory, parallel
light.
Scheme 1 illustrates the band-gaps of 3.00 V and 3.46 V (ref.
16 and 20) for the TiO2 and BiOCl photocatalysts, respectively.
The potential for surface reactions between water and O2(1.11 V, pH 2.0) occurring on both electrodes is also presented.
The diﬀerence in the VOC values obtained for front irradiation(Fig. 1a) and rear irradiation (Fig. 1b) toward TiO2 on the anodeallows us to evaluate the eﬀective diﬀusion overpotential fore's in the TiO2 layer to the electrode in the front conguration(0.23 V) compared to direct e injection into electrode in therear conguration (Table 2A-a and b and Scheme 1, le).
Similarly, the diﬀerence in the VOC values obtained for front
irradiation (Fig. 1a) and rear irradiation (Fig. 1c) toward BiOClon the cathode can be used to evaluate the eﬀective diﬀusion
overpotential in BiOCl from the electrode to the reduction sitesof O2 in the front conguration (0.41 V), compared to the direct
charge recombination of the e's and the holes at the interface
between the electrode and the BiOCl layer (Table 2A-a and c, andScheme 1, right). Associated with this, greater impedance isobserved for the BiOCl lm compared to that for the TiO2 lm(see Section 3.4.3).
Moreover, the EF for ITO (4.12–4.78 eV with reference tovacuum) is also considered.24 Compared to the CB minimum for
TiO2 (4.33 eV versus vacuum), a voltage loss of 0–0.45 V during
photoexcited e diﬀusion from TiO2 to ITO and then to thecircuit is plausible. Thus, the theoretical electromotive force inthe front–front conguration (Fig. 1a) is 2.11–1.66 V (Scheme 1,
path B), whereas in the rear–rear conguration (Fig. 1d) it isgreater: 2.75–2.30 V (Scheme 1, path A). Compared to theexperimental VOC values (1.43 and 1.88 V; Table 2A-a and d),
unknown overvoltage values of 0.68–0.23 V and 0.87–0.42 V,
respectively, are observed, presumably due to the resistance
between the photocatalysts and electrodes. Particularly, theporosity, size and conguration of the BiOCl lm's crystals(Fig. 2B) need to be optimized for better e transfer at theinterface.
The VOC value for the leakage currents from the anode to the OV
sites was also evaluated (0.38 V, Fig. 6C-a0 and Table 2B-a0 ) by
irradiating only the anode. This value accordingly gave thepotential of the OV sites [0.27 V versus standard hydrogen electrode(SHE); Scheme 1] which was almost consistent with the values
based on density functional calculations (0.25 to 0 V).16,18,25

3.3

Photocurrent changes of SC

In the front–front anode–cathode conguration (Fig. 1a), thephotocurrent quickly increased in response to UV-visible light

This journal is © The Royal Society of Chemistry 2017

Paperirradiation, reaching 176, 179, 180, 183, and 190 mA in the rst,
second, third, fourth, and h cycles of irradiation, respectively(Fig. 8a). The photocurrent quickly decreased to zero when theirradiation was stopped. The gradual increase in the converged
photocurrents in ve cycles could be related to the OV sites'
increase in BiOCl when the irradiation under UV-visible light wascontinued, as evidenced by the Bi L3-edge extended X-ray absorption ne structure (EXAFS; the decrease in Bi–O atomic pairs
starting from 4 to 2.2).16 OV sites are considered active sites for O2
reduction (Scheme 1, right), but also eﬀective e receptors fromthe anode resulting in a leakage current. Namely, the photocurrents in Fig. 8a included a signicant portion of leakage currents(Scheme 1) because the resistance was 0.5 U (Fig. 1a), which is near
the lower limit in the i–V tests (500 kU–0.3 U; Fig. 6A-a).
The static photocurrent test in the rear–front conguration(Fig. 1b) is plotted in Fig. 8b, which corresponds to the i–V test
shown in Fig. 6A-b. The converged currents were 91.3, 112, 114,
114, and 109 mA in the ve irradiation cycles, and they wereapparently suppressed in comparison to the ones shown in
Fig. 6A-a in the front–front conguration because of the lightltering by the electrode in front of TiO2. Thus, only the TiO2
photocatalyst layer in the proximity of the ITO electrodecontributed to the photocurrents.
The photocurrents (109–120 mA) in the front–rear conguration (Fig. 8c) were more stabilized and slightly improved incomparison to those in the rear–front conguration (Fig. 8b).
Finally, the photocurrents in the rear–rear conguration
gradually increased from 125, 123, 133, 134, and 140 mA in theve irradiation cycles (Fig. 8d) mostly due to the increase in theactive OV sites in BiOCl similar to the test in the front–front
conguration (Fig. 8a). The order of converged i values in Fig. 8
is consistent with that of iSC (Table 2A):

RSC Advances

photocurrent tests (Fig. 8) versus 30 min during the i–V tests(Fig. 6A).
3.4 CV and impedance measurements for thephotoelectrodes
3.4.1 CV for BiOCl. The CV curves for the electrodecomprising BiOCl in the dark and an N2 atmosphere show a pairof peaks at 0.49 V and 0.42 V versus SHE both in the front andrear congurations (Fig. 3 and 9D, blue and red, respectively).
The center (0.04 V) is essentially identical with the equilibrium
voltage given by eqn (4) at 298 K and pH 4.0 (0.0909 V; Table 3A).
BiOCl + 2H+ + 3e % Bi + Cl + H2O(4)

Under these conditions, a part of the Cl of BiOCl dissolves
in aqueous HCl,24 and the oxidation peak for BiOCl at 0.49 V
becomes more pronounced. The minor diﬀerence of the peakintensity in the two congurations may be because the supplied
N2 gas diﬀuses and purges O2 more eﬀectively in the rearconguration rather than in the front conguration (Fig. 3) andfacilitates the dissolution of Cl.
Under UV-visible light irradiation and an N2 atmosphere, thepair of peaks at 0.49 V and 0.42 V becomes more pronounced(Fig. 9C and Table 3A) due to the photodissolution of Cl, whichis in accordance with previous observations26 and EXAFS
measurements under light, demonstrating the decrease in the
Bi–Cl interatomic pair.16 A new pair of peaks appears at 0.93 and0.25 V, particularly in the rear conguration. The center(0.34 V; Table 3B) is essentially identical with the equilibrium
voltage at 298 K, pH 0 (0.3172 V; Table 3B) for eqn (5).
Bi3+ + 3e % Bi

a (front–front) > d (rear–rear)  c (front–rear) > b (rear–front)
(3)

(5)

The converged currents in Fig. 8b–d were 1.66–1.41 times
greater than the corresponding iSC values (Table 2A) because of thediﬀerence in the total irradiation time resulting in a higher
concentration of OV sites, which was 150 min during the static

CV for the BiOCl ﬁlm on ITO/Pyrex (WE) and the glassy C CE in
an aqueous solution of pH 4.0 under an O2 ﬂow (A and B) or an N2 ﬂow(C and D) under irradiation with UV-visible light (A and C) or in the dark
(B and D). Blue: irradiated from the photocatalyst side, and red: fromthe rear side of the electrode.
Fig. 9Fig. 8 Time course of the SC comprising both TiO2 and BiOCl photocatalysts. (a) Front–front, (b) rear–front, (c) front–rear, and (d) rear–
rear conﬁguration (see Fig. 1a–d).

This journal is © The Royal Society of Chemistry 2017

RSC Adv., 2017, 7, 19996–20006 | 20003

RSC Advances
Table 3

Paper

Electrochemical data for BiOCl (A and B) and TiO2 (C and D)
CV peak (V) versus SHE

Redox reaction conditions

Reduction
wave

Oxidation
wave

Center(A) BiOCl + 2H+ + 3e ¼ Bi + Cl + H2O
O2, light
O2, dark
N2, light
N2, dark

—
0.42
0.42
0.42

—
—
0.49
0.49

—
—
0.04
0.04

0.0909(B) Bi3+ + 3e ¼ Bi
O2, light
O2, dark
N2, light
N2, dark

—
—
0.25
—

—
—
0.93
—

—
—
0.34
—

0.3172(C) 2H+ + 2e % H2
O2, light
O2, dark
N2, light
N2, dark

0.27
0.27

—
—

—
—

(D) O2 + 2H+ + 2e % H2O
O2, light
O2, dark
N2, light
N2, dark

<0.1
<0.1
—
—

>1.6
>1.6
—
>1.6

—
—
—
—

0.24

The CV curve in Fig. 9C suggests the presence of reducedBi(32xy)+O1xCl1y, particularly in the rear conguration
because of the faster e transfer between the electrode and
BiO1xCl1y. The faster e transfer should be related to thehigher VOC values obtained in the rear conguration of BiOCl in
Fig. 1c and d (1.84–1.88 V; Table 2A-c and d).
In the dark and an O2 atmosphere (Fig. 9B), only a very weak
reduction peak appears at 0.42 V (Table 3A) via eqn (4), whichsuggests that BiOCl is stabilized under O2. Under light and an
O2 atmosphere (Fig. 9A), the redox of the Bi0/3+ species via eqn(4) and (5) disappears completely. Nevertheless, minor O and Cl
vacancy sites are observed under these conditions by in situ
EXAFS and can form active sites for O2 reduction.16,18,27,28 The O2
reduction reaction is indirectly monitored as the more negative
slope of the reduction wave in Fig. 9A and B, in comparison tothat in Fig. 9C and D, in the V range below 0.4 V. However, theslope is exclusively dependent on the potential and independent of light irradiation (Fig. 9A and B). The electrode conguration exhibits only a minor eﬀect on the slope. The advantageof the rear conguration for BiOCl (Fig. 6A-c and d) may be thestabilization of BiOCl by O2 in the rear conguration (Fig. 1cand d) rather than in the front conguration (Fig. 1a and b)
because the photocatalyst faces the O2 supply source andsuppresses the number of OV sites, resulting in less leakage
current to the OV sites (Scheme 1) in the rear conguration(Fig. 1c and d, and 3).
3.4.2 CV for TiO2. In the CV curve for the electrodecomprising TiO2 in the dark and a N2 atmosphere, a weak peakfor H+ oxidation to form H2 appears at the potential of 0.27 V
versus SHE for TiO2/ITO/Pyrex in the front conguration (Fig. 320004 | RSC Adv., 2017, 7, 19996–20006

Equilibrium
voltage@298 K, pH 4.0

0.993

and 10D, blue). This value is essentially identical to theequilibrium voltage of eqn (6) at 298 K and pH 4.0 (0.24 V;
Table 3C).
2H+ + 2e % H2

(6)

Under UV-visible light irradiation and a N2 atmosphere(Fig. 10C), peaks at 0.27 V also appear owing to H+ oxidation(Table 3C).
In the dark and an O2 atmosphere (Fig. 10B), no distinct
peaks appear; however, the reduction wave is signicantly
enhanced to 150 mA, demonstrating the electro-reduction of
O2 to H2O.
O2 + 2H+ + 2e % H2O(7)The equilibrium voltage of eqn (7) was 0.993 V at 298 K, pH
4.0 (Table 3D),16 but it was diﬃcult to compare it to the negative
slope in Fig. 10B. Under light and an O2 atmosphere (Fig. 10A),
the O2 reduction was again monitored as the more negative
slope of the reduction wave; however, the slope was exclusively
dependent on the potential. Conversely, water oxidation(reverse reaction of eqn (7)) was clearly accelerated in theoxidative wave in the V range above 1.5 V (Fig. 10A) by UV-visible
light irradiation in comparison to the CV curve under O2 anddark conditions (Fig. 10B), which demonstrates the eﬀective
photo-oxidation by TiO2 on the anode (Scheme 1, le).
The CV for the electrode comprising the TiO2 lm in the rearconguration (Fig. 10, red) was basically similar to that for theelectrode in the front conguration (Fig. 10, blue). The onlyThis journal is © The Royal Society of Chemistry 2017

PaperFig. 10 CV for the TiO2 ﬁlm on ITO/Pyrex (WE) and the glassy C CE in
an aqueous solution of pH 4.0 under an O2 ﬂow (A and B) or N2 ﬂow (Cand D) under irradiation with UV-visible light (A and C) or in the dark (Band D). Blue: irradiated from the photocatalyst side, red: from the rearside of the electrode.

clear diﬀerence was the steeper slope in the oxidation wave forthe front conguration under N2 and UV-visible light (Fig. 10C)
because most of the TiO2 was utilized in the front conguration(Fig. 3A), that is under the condition of the anode of the SC,
versus some of the UV-visible light was absorbed by the electrode and only the TiO2 in the proximity of the electrodeparticipated in the H2O oxidation in the rear conguration(Fig. 3B). This diﬀerence supports the diﬀerent eﬃciencies of
the i–V dependence in Fig. 6A-a (front–front conguration) and6A-b (rear–front conguration).
3.4.3 Impedance measurements. The Bode plot and Cole–
Cole plot were obtained based on the impedance measurements
for the BiOCl lm compared to the TiO2 lm (Fig. 11). The
greater arc in the Cole–Cole plot for BiOCl (Fig. 11B, square,blue) than that for TiO2 (circle, red) demonstrates the greater
impedance for BiOCl mostly owing to the connection between
the nanoparticles (mean size ¼ 127  43 nm), which isconsistent with the greater overpotential evaluated for BiOCl(0.41 V) than that for TiO2 (0.23 V) based on the VOC values inTable 2A.
3.5

Eﬃciency of the SC comprising two photocatalysts

The incident light intensity was 91.3 mW (see Section 2.3) at thecenter of the photocatalyst lm on the electrodes. The quantum
eﬃciency of this SC was evaluated based on the amount ofincident light (1.7  1017 photons per s1)29,30 absorbed (6.9%)
by the photocatalyst to photocatalyst layers and the iSC value(186 mA  1.2  1015 e' per s s1). The overall eﬃciency thatwas evaluated as the cell current per absorbed photons was
0.096, and this could be considered as the result of the anode(TiO2) and cathode (BiOCl) eﬃciencies.16,17 The cell currents
were created by the combination of excited e's formed at theThis journal is © The Royal Society of Chemistry 2017

RSC AdvancesFig. 11 (A) Bode plots and (B) Cole–Cole plots based on impedance

measurements at AC 500 mV and the frequency range of 101 and
106 Hz for TiO2 (circle, red; mean thickness 4.7 mm) and BiOCl (square,
blue; mean thickness 4.4 mm) on ITO/Pyrex plates.

CB of the anode with holes formed at the VB of the cathode. If
a similar eﬃciency was assumed for the e's at the anode andthe holes at the cathode, the product of 0.31  0.31 would be
0.096. By the optimization of the SC in the rear–rear conguration as well as the intimate connection between TiO2 and the
ITO layer (Scheme 1), the eﬃciency of the preliminary SC
comprising TiO2 and Ag–TiO2 (0.04)17 and that comprising TiO2
and BiOCl (0.024)16 was improved to 0.096 in this study.
The intimate contact between TiO2 and the ITO/Pyrex anode
was clearly strengthened by the positive charge repulsion in theacidic suspension of TiO2 and dispersion to constitutea uniform lm (Fig. 4A) in contrast to the neutral aqueous
suspension of TiO2 and Ag–TiO2 in preliminary SCs.16,17 Thus,
the contact between BiOCl and ITO should be improved tosuppress the overvoltage of 0.87–0.42 V for the SC in this study
in the rear–rear conguration (Scheme 1) other than the voltage
loss due to e diﬀusion in the photocatalyst layers and the EF for
the ITO layer to realize the VOC of 2.75–2.30 V per cell.

4 Conclusions
An SC comprising two photocatalysts, TiO2 and BiOCl, enables
the VOC of 1.91 V and PMAX of 55.8 mW per cell (1.3 cm2) in the
optimized rear conguration of the anode and the cathode,eliminating the diﬀusion overvoltage of 0.23 V and 0.41 V in thephotocatalyst layer and suppressing the leakage current (0.38 V)
directly from the CB of TiO2 to the OV sites of BiOCl. XRD
suggests the preferable exposure of the stable crystal faces of the
TiO2 and BiOCl lms used in the SC. The balance of the numberof excited electrons at TiO2 and that of remaining holes at BiOCl

RSC Adv., 2017, 7, 19996–20006 | 20005

RSC Advances

determine the iSC values in each conguration of the photoelectrodes. The CV data support the formation of OV sites in
BiOCl during the SC tests and the superiority of the rearconguration, whereas the impedance data suggest a smaller
impedance for a tight and uniform TiO2 lm in comparison to
the porous BiOCl lm. The eﬃciency of this optimized SC was
improved to 0.096.

Acknowledgements
For this study, the authors received nancial support fromGrant-in-Aid for Scientic Research C (26410204) from theJapan Society for the Promotion of Science, Feasibility Study
Stage of A-STEP (AS262Z00159L, AS251Z00906L) from the Japan
Science and Technology Agency, and the Leading Research
Promotion Program (2015–) from the Institute for Global
Prominent Research, Chiba University.

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This journal is © The Royal Society of Chemistry 2017