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pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2020, 8, 1406−1416

Polarizability and Catalytic Activity Determine Good Titanium Oxide
Crystals but Not Homogeneity in Solar Cells Using Photocatalysts on
Both Electrodes
Kazuki Urushidate,† Jifu Li,‡ Keisuke Hara,†,§ Takashi Kojima,‡ and Yasuo Izumi*,††Department of Chemistry, Graduate School of Science and ‡Department of Applied Chemistry and Biotechnology, Graduate
School of Engineering, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan§
Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku, Tokyo 163-8601, JapanS Supporting Information*

ABSTRACT: TiO2 is advantageous when used on the anode of a solar cell (SC) andachieves an electromotive force of 2 V when using another photocatalyst on the cathode(dual-photoelectrode fuel cell). However, both suﬃcient photocatalytic activity and electronic
conductivity are required for the ﬁlm, and these two requirements compete with each other.
Herein, uniformly sized and shaped spheroidal, cubic, and “rhombic” TiO2 nanoparticles weresynthesized, forming ﬁlms using casting, slide, ball milling-slide (BS), and ball millingmechanical (BD) methods. The ac/dc impedances of the ﬁlms followed the order “rhombic”-ball milling-slide > “rhombic”-slide> cubic-slide > spheroidal-ball milling-mechanical > P25-casting ≈ P25-slide ≈ spheroidal-BS ≈ spheroidal-slide ≈ P25-ballmilling-mechanical, demonstrating that “rhombic” and cubic TiO2 ﬁlms were unsuitable for use on the SC photoanode. The
reverse reaction rates of cathode reaction (water photooxidation), that is, the 18O2 exchange reaction, were evaluated, which
followed the order Cubic-TiO2 > Spher-TiO2 > Rhomb-TiO2 ≈ P25-TiO2, demonstrating that the synthesized TiO2 ﬁlms were
unsuitable for water oxidation. Polarizability and catalytic activity of TiO2 crystals were critical to maximize the SC performance,and ﬂuorescence peak intensity at 372−366 nm was well correlated with SC performance. P25-slide on a photoanode at pH
1.78 and BiOCl-Cast on a photocathode at pH 2.00 enabled a maximum power of 85.2 μW cm−2 and an open-circuit voltage of
1.94 V.
KEYWORDS: solar cell, dual-photoelectrode fuel cell, photocatalyst, water oxidation, TiO2, polarizability, impedance, isotope tracing■

INTRODUCTION
Photoanodes have been investigated for various applications,for example, perovskite solar cells (SCs),1 dye-sensitized
SCs,2,3 photofuel cells,4,5 solar water splitting,5,6 and sensors.7,8
Optimization of photoelectrodes is complex because theirperformance depends on the balance between the electronic
conductivity9,10 and chemical performance; for example, for a
photoanode, the electron injection eﬃciency from the highest
occupied molecular orbital of the dye to the conduction band(CB) of the semiconductor and then to the anode11 must bebalanced against the catalytic reaction eﬃciency of photocatalytic oxidation on the anode. Dense and porous ﬁlms,
respectively, are required to achieve these attributes.12
The development of SCs with dual-photocatalyst systems on
both the anode and cathode, called dual-photoelectrode fuelcells elsewhere,13−15 is highly desirable to enable a high opencircuit voltage (VOC) exceeding 2 V.12,16−19 Speciﬁcally, ourpower system utilizes the following reactions at the anode andcathode, respectively.
2H 2O + 4h+ → O2 + 4H+(1)

O2 + 4H+ + 4e− → 2H 2O(2)kinetics on the cathode (eq 2), an acidic electrolyte isadvantageous rather than the neutral one, for example,
KCl.17,19 Furthermore, TiO2 is advantageous as a photocatalyst
on the anode, and various commercially available TiO2
particles were employed for preparing photoelectrode thinﬁlms. However, their size and shape were nonuniform,
hindering the determination of the characteristics and eﬀects
on the photoelectrode performance of individual particles.
In this study, we synthesized TiO2 particles with a uniform
size and shape. Three types of model particles, spheroidal,
cubic, and “rhombic” TiO2, were synthesized to evaluate theeﬀects of uniform size and shape. Spheroidal and cubic
particles were prepared by hydrothermal treatment of aprecursor gel with an added structure-directing agent.20−22
Uniformly sized “rhombic” particles were synthesized by asolvothermal method23 and are called “rhombic” because oneof the facets appeared rhombic based on the anatase unit cellof the tetragonal crystal system.
The performance of a SC with photocatalyst layers on boththe anode and cathode12,16−19 was improved by optimizing theﬁlm application method, that is, casting, slide, mechanical, andReceived: September 18, 2019
Revised: November 11, 2019
Published: December 5, 2019

As light energy is the power source in this system, it acts as a
SC and water acts as the redox mediator. Based on the reaction© 2019 American Chemical Society1406

DOI: 10.1021/acssuschemeng.9b05576
ACS Sustainable Chem. Eng. 2020, 8, 1406−1416

ACS Sustainable Chemistry & Engineering

Research Article

Table 1. Basic Properties of TiO2 Samples Used in This Study
entry

type

mean primary particle size (nm)a

speciﬁc SA (m2 g−1)b

crystal phase

a
b
c
d
e

P25
JRC-TIO-6
spheroidal
cubic“rhombic”

21 ± 515 ± 5(31 ± 11) × (13 ± 2)
28 ± 5(22 ± 2) × (11 ± 4)

60
96.4
115
161
211

anatase/rutile = 8:2
rutileanataseanataseanatasepreferred face

ref.
12
12

except for (0 0 1)
{1 0 0}, (0 0 1)
{1 0 1}, (0 1 0)a

Based on TEM. bBrunauer−Emmett−Teller SA based on N2 adsorption at 77.3 K.

Scheme 1. Synthetic Steps for Spher-TiO2, Cubic-TiO2, and Rhomb-TiO2

2.0 mL of Suspension 1 were added to 9.0 mL of Stock solution 2.
The mixed solution was diluted with an aqueous solution of NaOH
from 1.0 to 0.1 mol L−1 and then with pure water (total 20 mL). The
ﬁnal concentrations were 0.25 mol L−1 Ti and 0.25 mol L−1
ethylenediamine, and the solution pH was adjusted to 11.5 by adding
NaOH aqueous solution. The solution was heated at 373 K for 24 h.
The resulting gel was transferred to a polytetraﬂuoroethylene(PTFE)-lined autoclave and hydrothermally treated at 413 K for 72
h to form white crystals. This sample was denoted as Spher-TiO2(Scheme 1, middle and Figure 1a).

ball milling, for TiO2 and BiOCl ﬁlms on the anode andcathode, respectively, and the pH of the HCl electrolyte. A SC
utilizing photocatalysts on both electrodes achieved anelectromotive force of 0.90−2.11 V using TiO 2 andpolyterthiophene,13,24 W-doped BiVO4/Bi4V2O11 and undoped
BiVO4/Bi4V2O11,25 and TiO2/anthocyanidin and BiOCl.12
Dual photoelectrodes were also simultaneously utilized toremove waste materials.14,15,26,27 In this study, the key
parameters to improve the SC performance were investigatedby scanning electron microscopy (SEM), ﬂuorescence spectroscopy, ﬁlm impedance measurements, exchange reactions in
18
O2 gas, and Bi L3-edge extended X-ray absorption ﬁne
structure (EXAFS).

■

EXPERIMENTAL SECTION

Synthesis of TiO2 and BiOCl. TiO2 (Aeroxide P25; purity >99.5%) was supplied by Japan Aerosil Co. TiO2 (JRC-TiO-6, purity >99%) was supplied by Sakai Chemical Industry Co., Japan (Table 1 a
and b).
The spheroidal TiO2 nanoparticles were prepared as follows.20,21
First, a suspension of seed particles was prepared to control the size ofthe spheroidal TiO2 particles (Scheme 1, left). Titanium isopropoxide(TIPO, 7.3 mL, Ti(OCH(CH3)2)4, >95.0%, Wako Pure Chemical,
Japan) and perchloric acid (HClO4, 1.1 mL, 60%, Wako Pure
Chemical) were mixed and diluted to 99 mL with pure water(conductivity < 0.2 μS cm−1) to concentrations of 0.25 mol TIPO L−1
and 0.1 mol HClO4 L−1. The solution was heated at 373 K for 24 h,
resulting in a suspension comprising nanocrystal TiO2. Then, 1.1 mL
of HClO4 was added, resulting in a stable suspension. This was named
Suspension 1, and the ﬁnal concentrations were 0.25 mol TIPO L−1
and 0.2 mol HClO4 L−1.
Separately, Stock solution 2 was prepared by mixing TIPO andtriethanolamine (N(CH2CH2OH)3, >99.0%, Wako Pure Chemical)
to a concentration of 0.50 mol L−1 Ti and a molar ratio of 1:2 in purewater (Scheme 1, middle). Ethylenediamine (H2N(CH2)2NH2, 0.34
mL, >99.0%, Wako Pure Chemical) as a structure-directing agent andFigure 1. TEM images of as-prepared Spher- (a), Cubic- (b), andRhomb-TiO2 nanoparticles (c).
The cubic TiO2 particles were prepared as follows (Scheme 1,
middle).22 Sodium oleate (C17H33CO2Na, >99%, Wako Pure
Chemical) as a structure-directing agent was added to 10 mL of
Stock solution 2. The concentrations of Ti4+ and sodium oleate andthe solution pH were then adjusted to 0.25 mol L−1, 0.05 mol L−1,
and 10.5, respectively, by adding progressively diluted NaOH aqueous
solution followed by pure water. The prepared solution was heated at
373 K for 24 h. The resulting gel was transferred to a PTFE-lined
autoclave and hydrothermally treated at 413 K for a week to form
white crystals. The obtained sample was denoted as Cubic-TiO2(Figure 1b).
The “rhombic” TiO2 particles were prepared via a method
developed by Wu et al. (Scheme 1, right).23 TIPO (0.25 mL),
benzyl alcohol (C6H5CH2OH, 5.0 mL, >99.0%, Wako Pure1407

DOI: 10.1021/acssuschemeng.9b05576
ACS Sustainable Chem. Eng. 2020, 8, 1406−1416

P25-BD
Spher-Slide
Spher-BS
Spher-BD
JRC6-Cast
Cubic-Slide
Cubic-BS
Rhomb-Slide
Rhomb-BS

a″
b
b′
b″
c
d
d’
e
e’

0.49
0.38
0.33
0.50
0.45
0.39
0.40
0.48
0.28
0.31
0.39
0.38
0.40
0.35

wt
(mg)

BiOCl-Slide
BiOCl-BS
BiOCl-Cast

BiOCl-Cast

type

0.64
0.51
0.43
0.46
0.55
0.65
0.65
0.75
0.70
0.60
0.68
0.55
0.70
0.45

wt
(mg)

catalyst (cathode)

1.78
2.00

2.00

pH

Wmax
(μW cm−2)
47.4b61.1 ± 4.0
85.2
33.0
23.7
43.7
55.0
55.0
42.1
56.1b
43.1
21.4
32.9 ± 15.0
11.6

VOC (V)1.87b
1.96 ± 0.05
1.94
1.85
1.80
1.91
1.90
1.78
1.86
1.93b
1.92
1.62
1.92 ± 0.21
1.71
2.0 ± 0.1
1.0 ± 0.2
1.32 ± 0.05
2.48 ± 0.05
1.3 ± 0.2b
1.1 ± 0.6
2.0 ± 0.1
2.2 ± 0.4
1.6 ± 0.1

0.8 ± 0.1
0.9 ± 0.1

bulk density(g cm−3)

d

23
61

d

14.4

0.47
0.48
0.63
4.8

0.76
0.65

500 mV ac,
2 V dc

172
150

127
83
112
126
91b,c
97

135
69

500 mVac

impedance (MΩ)

TiO2

0.23

<0.027
0.23

6.4b
214
106

0.078

0.10

(h−1)

162

106b

(μmol h−1 gcat−1)

O16O formation rate in
18
O2

18

1:0.61 ± 0.03

1:>0.34 ± 0.02
1:1.39 ± 0.02

1:2.8 ± 0.2

1:1.47 ± 0.02

O atom ratio in 18O2 introduced
vs surface O

HCl aqueous solution at pH 2.0 was used in both the anode and cathode unless otherwise noted. bIn reference 12. cac 100 mV applied. dUnable to obtain quantitative value.

P25-Cast
P25-Slide

a
a′

a

type

ent

catalyst (anode) at
pH 2.00

1.4

>1.5
3.6

10.1

9.9

surface O(nm−2)

Table 2. Physicochemical Parameters for TiO2 Photocatalyst on the Anode and the SC Power Using the TiO2 Photocatalyst on the Anode and BiOCl-Cast of the SCa

ACS Sustainable Chemistry & Engineering
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an ITO/Pyrex plate at 3 cm s−1. The BiOCl/ITO/Pyrex plate wasdried in air at 373 K for 18 h. The temperature was then increased to573 K at 4 K min−1 and maintained at 573 K for 30 min. The
prepared ﬁlm was denoted as BiOCl-slide.
BiOCl was also treated by ball milling. BiOCl powder (1.5 g) was
mixed and dispersed with deionized water (15.0 mL). YTZ-5 balls (40g) were added to the suspension, and the mixture was rotated at 100
rpm for 24 h. The suspension was slid to form a BiOCl thin ﬁlm on an
ITO/Pyrex plate as described in the previous paragraph. The
temperature was then increased to 573 K at 4 K min−1 andmaintained at 573 K for 30 min. The prepared ﬁlm was denoted as
BiOCl-BS.
Characterization. The speciﬁc surface area (SA) of the samples
was measured using an automatic speciﬁc SA/pore size distribution
analyzer (BELSORP-max, Microtrac BEL, Japan). Transmission
electron microscopy (TEM) images were obtained using a H7650A transmission electron microscope (Hitachi, Japan) at anaccelerating voltage of 100 kV. Cross-sectional SEM images wereobtained using a JSM-6510 scanning electron microscope (JEOL,Tokyo, Japan) at an accelerating voltage of 15 kV. A tungsten ﬁlament
was used in the electron gun. The photocatalyst ﬁlm on the ITO/
Pyrex plate was cut and mounted on an aluminum sample holderusing an adhesive. The incident angle of electrons with reference tothe sample surface was between 5 and 15°. The magniﬁcation was
between ×500 and ×20 000.
Absorption−ﬂuorescence spectra were recorded on model FP-8600(JASCO; Chiba Iodine Resource Innovation Center, Chiba
University) using a 150 W Xe arc lamp equipped with aphotomultiplier tube within the excitation range from 250 to 300
nm and ﬂuorescence range from 300 (or 340) to 800 nm. TiO2
powder (2.0 mg) was mixed with water (4.0 mL) and ultrasonicated(430 W, 38 kHz) for 2 h. All spectra were recorded for thesuspensions in a quartz cell at 295 K. Bismuth L3-edge EXAFS spectra
were obtained at 290 K in transmission mode using a Si (3 1 1)
double-crystal monochromator29−34 calibrated to 13 426 eV35 andwere analyzed using the Athena and Artemis package36 using refs
37−42. The details are given in the Supporting Information.
Impedance Measurements. Impedance measurements were
performed for ﬁlm samples of TiO2 (0.4 mg) and BiOCl (0.6 mg)
on ITO/Pyrex plates using a potentio/galvanostat (VersaSTAT 3100, Princeton Applied Research, Oak Ridge, TN, USA) equippedwith a low current option (VersaSTAT-LC). An iron nail (front
curvature radius 0.5 mm) was perpendicularly in contact with the
TiO2/BiOCl ﬁlm surface utilizing a micrometer, and the ITO/Pyrex
plate was electrically in contact via a Au/Ni-coated stainless steel clip
(Toyo Corporation, Japan).12 The applied alternating voltage was 500
mV, and the frequency was scanned between 102 and 106 Hz. Anotherseries of impedance measurements were performed by applying bothan alternating voltage of 500 mV and a direct voltage of 2 V.
18
O2 Exchange Photoreaction Tests. Isotope-labeled photoreaction tests were performed using gas chromatography−massspectrometry (GC-MS, JMS-Q1050GC, JEOL).12,29 Helium (purity> 99.9999%) was used as the carrier gas. The sampling loop
comprised a Pyrex glass system placed under vacuum using rotary anddiﬀusion pumps (10−6 Pa) and connected to the gas chromatograph−mass spectrometer using a 1.5 m deactivated fused silica tube (no.
160-2845-10, Agilent, Santa Clara, CA, USA; internal diameter 250μm) maintained at 393 K.
A photocatalyst sample (0.135 g) was placed in a quartz
photoreactor and evacuated at 295 K for 2 h at 10−6 Pa. Then,18
O2 photoexchange (0.55 kPa; 18O 97.8%, chemical purity >99.9%,
Cambridge Isotope Laboratories, Inc., Tewksbury, MA, USA) withthe photocatalyst was monitored in a Pyrex closed circulation system(265.0 mL) at 295 K under irradiation with UV−visible light from a
500 W xenon arc lamp (SX-UID502XAM, Ushio, Japan). The
distance between the exit of the lamp window and the photocatalyst
was 160 mm. The exchange with 16O atoms (natural 16O abundance
99.76%) at the photocatalyst surface was monitored using a column
packed with 13X-S molecular sieves (GL Sciences, Inc., Japan) set in
the GC-MS.

Chemical), and oleylamine (CH3(CH2)7CHCH(CH2)8NH2, 2.0 mL,>50%, Tokyo Chemical Industry, Tokyo, Japan) as a structuredirecting agent were mixed and stirred at 293 K for 30 min. Then, 0.1
mL of pure water was added. Solvothermal treatment was conductedin a PTFE-lined autoclave at 453 K for 24 h, resulting in white
crystals. All the TiO2 synthesized in this study was in anatase phase,
which has a tetragonal crystal system. In comparison to Spher-TiO2,
which grew in the c-direction (c = 0.95143 nm) and cubic-TiO2,
which exposed fundamental {1 0 0} and (0 0 1) facets, this TiO2
exposed a rhombic (0 1 0) facet neighboring relatively unstable
rectangular (1 0 1) and (1̅ 0 1) facets protected by oleylamine during
the synthesis. Therefore, the crystals appeared to have a monoclinic
crystal system, but based on their preferential exposure of the rhombic(0 1 0) facet (Figure 1c), we denoted them as “rhombic TiO2”,23
abbreviated as Rhomb-TiO2.
BiOCl was synthesized from an aqueous solution using bismuth
trichloride (>97%, Wako Pure Chemical).28
Preparation of Photoelectrodes. The TiO2 powder (0.10 g)
was well mixed with 1.0 mL of t-butanol, and 50 μL of ethylene glycol
was added as a thickener. The suspension was ultrasonicated (430 W,
38 kHz) for 10 min. The paste (36 μL) was cast on an indium tinoxide (ITO)-coated (thickness 0.12−0.16 μm, sheet resistivity 8−12
Ω/square) Pyrex glass substrate (area 2.5 × 2.5 cm, thickness 0.11
cm; Sigma-Aldrich, St. Louis, MO, USA).12,16 The prepared ﬁlm wasdenoted as “TiO2 type”-Cast.
A slide method was also employed for the preparation of TiO2 ﬁlm;that is, a 10 g ﬂat Pyrex glass dish (bottom area 10 cm2) under theforce of gravity was laterally slid on the pasted suspension (36 μL) onan ITO/Pyrex plate at 3 cm s−1 to achieve a homogeneous ﬁlm
thickness. The TiO2/ITO/Pyrex plate was dried in air at 373 K for 18
h. The temperature was then increased to 573 K at 4 K min−1 andmaintained at 573 K for 30 min. The amount of TiO2 deposited on
the plate was 0.39 mg (±0.11 mg) throughout this study (Table 2),
covering an ITO area of 1.0 × 1.3 cm. The prepared ﬁlm was denoted
as “TiO2-type” slide.
For comparison, ball milling was also employed to prepare TiO2
ﬁlms. TiO2 powder (0.10 g) was dispersed with t-butanol (1.0 mL)
and ethylene glycol (36 μL). Zirconia balls comprising 5% Y2O3 (25g, φ = 5 mm; Nikkato Co., Japan, Type YTZ-5) were added to thesuspension, and this was rotated at 50 rotations per minute (rpm) for
24 h. Thereafter, the mixture was agitated using dual-frequency
ultrasound (23 and 43 kHz, 130 W). TiO2 paste (1.3 g) was separated
from the YTZ-5 balls. The paste (36 μL) was dropped on an ITO/
Pyrex glass plate, and a ﬂat Pyrex glass dish (10 g) under the force ofgravity was laterally slid on the pasted suspension on the ITO/Pyrex
plate at 3 cm s−1. The TiO2/ITO/Pyrex plate was dried in air at 373
K for 18 h, and then the temperature was increased to 573 K at 4 K
min−1 and maintained at 573 K for 30 min. The prepared ﬁlm wasdenoted as “TiO2-type” BS.
As with the TiO2-BS ﬁlm, the mixed TiO2 paste (36 μL) was
rotated at 50 rpm for 24 h while dropping the YTZ-5 balls, pasted onan ITO/Pyrex glass plate, and the mixture was ﬂattened using a ﬁlm
applicator (Paul N. Gardner Company Inc., Pompano Beach, FL,
USA, Digital Microm II), maintaining an interval of 10 μm between
the blade and the substrate and laterally moving at 0.6 cm s−1.
Thereafter, the TiO2/ITO/Pyrex plate was dried in air at 373 K for 18h, and then the temperature was increased to 573 K at 4 K min−1 andmaintained at 573 K for 30 min. The prepared ﬁlm was denoted as“TiO2-type” BD.
The obtained BiOCl powder (6.0 mg) was suspended in deionizedwater (60 μL, <0.055 μS cm−1) and ultrasonicated (430 W, 38 kHz)for 10 min. An aliquot of the suspension was pasted on the ITOcoated Pyrex glass plate. The amount of BiOCl deposited on each
ITO-coated glass plate was 0.60 mg (±0.17 mg), covering an area of
1.0 × 1.3 cm. The prepared ﬁlm was denoted as BiOCl-Cast (Table
2).
A slide method was also employed for BiOCl, that is, a 10 g ﬂat
Pyrex glass dish (ﬂat area 10 cm2) under the force of gravity waslaterally slid on a pasted suspension (36 μL) prepared by mixing
BiOCl (0.10 g), t-butanol (1.0 mL), and ethylene glycol (0.05 mL) on
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DOI: 10.1021/acssuschemeng.9b05576
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SC Tests. The TiO2/ITO/Pyrex and BiOCl/ITO/Pyrex electrodes
were immersed in hydrochloric acid solution (40 mL in eachcompartment, initial pH 2.00). The two compartments wereseparated by a 50 μm-thick H+-conducting polymer ﬁlm (Naﬁon,
Dupont, Wilmington, DE, USA; acid capacity >9.2 × 10−4 equiv.
g−1).12,16−19 N2 (purity > 99.999%) and O2 (purity > 99.6%) (50mm) were bubbled from each photoelectrode to the anode andcathode compartments, respectively, at a ﬂow rate of 100 mL min−1.
The SC was equipped with quartz windows (φ = 80 mm) on bothsides. Both the TiO2 and BiOCl photocatalysts were irradiated with
UV−visible light from a 500 W xenon arc lamp (SX-UID502XAM)
through quartz windows using a two-way branched quartz ﬁber lightguide (5φ-2B-1000L, San-ei Electric Co., Japan). The distance
between the light exit (φ = 5 mm) and the surface of the TiO2 andBiOCl ﬁlm was 46 mm. The light intensity was 91.3 mW cm−2 at thecenter of the photocatalyst ﬁlm on each electrode.
The dependence of the current (i)−voltage (V) curves on theanode and cathode conﬁgurations was assessed in front−rear
conﬁguration12,16 by varying the resistance between 500 kΩ and 0.3
Ω over 20 min. The front and rear conﬁgurations allowed
perpendicular UV−visible light irradiation from the photocatalyst
and Pyrex/ITO glass plate, respectively.
Furthermore, the pH dependence of the i−V characteristics wasinvestigated by varying the pH of the HCl electrolyte in the cathodecompartment between 2.00 and 1.78. The pH of the HCl electrolyte
in the anode was ﬁxed at 2.00, and TiO2-Slide and BiOCl-Cast wereused on the photoanode and photocathode, respectively.

Figure 2. Side and top view SEM images of (a) P25-Cast, (a′) P25Slide, (a″) P25-BD, (b) Spher-Slide, (b′) Spher-BS, (b″) Spher-BD,(c) Cubic-Slide, (c′) Cubic-BS, (d) Rhomb-Slide, and (d′) RhombBS ﬁlms.

■

RESULTS AND DISCUSSIONTEM Observation of Synthesized TiO2 Nanoparticles.
Figure 1 shows the TiO2 particles synthesized in this study.
Regarding the homogeneity of these crystals, on progressively
increasing the Ti molar ratio in seed particles (Suspension 1)
to Stock solution 2, the length of the major c-axis of needlelike
TiO220 became shorter and the crystals became spheroidal inshape (Spher-TiO2), and the particle size distribution became
wider: (31 ± 11 nm, major) × (13 ± 2 nm, minor) when the
Ti molar ratio was 1:9 for Suspension 1 with Stock solution 2(Figure 1a and Table 1c). The mean oblateness ((major axis −minor axis)/major axis) of Spher-TiO2 was 0.6.
Cubic-TiO2 (Figure 1b) and Rhomb-TiO2 (Figure 1c)
nanoparticles with narrow particle size distributions andunique shapes were successfully synthesized; the size
distributions were 28 ± 5 nm and (22 ± 2 nm, major) ×(11 ± 4 nm, minor), respectively (Table 1d,e). Using the routeinvolving stock solution and sodium oleate (Scheme 1,
middle), most of the particles had a cubic shape that exposedthe {1 0 0} and (0 0 1) facets (Figure 1b) because of thecontrol by the strong adsorption of oleate.22 The crystal shapeof the Rhomb-TiO2 particles was similar to that reported in ref
23, exposing the {1 0 1} and (0 1 0) facets with a dihedral
angle of 43.4° between the (1 0 1) and (1 0 1) facets (Figure1c). Furthermore, the particles tended to be tightly packed andconnected at the {1 0 1} facets because they had a uniform
particle size and shape because of capillary forces during drying
on the TEM grid.
SEM Observation of TiO2 Films on Electrode. The
SEM images conﬁrmed the ﬁlm quality of Spher-TiO2, CubicTiO2, Rhomb-TiO2, and various other types of TiO2 ﬁlms
formed on ITO/Pyrex plates (Table 2). The ﬁlm preparation
methods, cast, sliding, BS, and BD, were also compared. Basedon the side view as shown in Figure 2, upper rows, the meanﬁlm thickness (t μm) was evaluated. The bulk density (ρ gcm−3) of the TiO2 ﬁlm was then calculated based on eq 3
(Table 2).

ρ=(0.39 ± 0.11) × 10−3
3.00 ± 0.85=
−4t(1.0 × 1.3)t × 10

(3)

Using conventional P25-TiO2, the ﬁlm thickness decreased
from 4.8 μm (Cast, Figure 2a) to 3.1 μm (Slide, Figure 2a′)and then to 1.5 μm (BD, Figure 2a″), corresponding toprogressive increases in the bulk density of 0.8, 0.9, and 2.0 g
cm−3, respectively (Table 2a,a′,a″). The ﬁlm ﬂatness was
satisfactory for these ﬁlms, and the side views demonstrate thatthe porous ﬁlms became more stacked in the above orderbecause of the eﬀect of the applicator (weight of the glass dishor mechanical applicator).
Similarly, using Spher-TiO2, the ﬁlm thickness decreased
from 3.1 μm (Slide, Figure 2b) to 2.8 μm (BS, Figure 2b′) andthen to 0.86 μm (BD, Figure 2b″), corresponding toprogressive increases in the bulk density of 1.0, 1.32, and2.48 g cm−3, respectively (Table 2b,b′,b″), compared to P25TiO2. Several coagulated parts on the micrometer scale wereseen for the Spher-Slide ﬁlms (Figure 2b, top view) that were
suﬃciently ﬂattened by ball milling to form Spher-BS andSpher-BD (Figure 2b′,b″). These changes were due to a
progressive decrease in the pores in the TiO2 ﬁlm and/or
liberation of coagulated crystals because of the diﬀerences in
the TiO2 ﬁlm formation method.
In a previous study,12 JRC6-Cast was the thinnest
photocatalyst ﬁlm (t = 1.8 μm, Figure 2c). The mean primaryparticle size and crystalline phase were 15 nm and rutile incontrast to those for P25 (21 nm and an anatase and rutile
mixture with a ratio of 8:2) (Table 1a,b). Because of thesmaller primary particle size, the JRC6-Cast ﬁlm was thinner
and thus stacked more eﬀectively than P25-Cast with athickness of 4.8 μm (Figure 2a). However, as opposed to theTiO2 type (Table 1), the ﬁlm preparation method was morecritical, and a much denser ﬁlm was formed via the BD method(2.48−2.0 g cm−3 for Spher-BD and P25-BD) in comparison
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to JRC6-Cast (1.3 g cm−3)12 (Table 2b″,a″,c and Figure
2b″,a″).
The TiO2 types were then compared for ﬁlms formed by theslide method. As the mean ﬁlm thicknesses were 3.1, 3.1, 2.8,
and 1.2 μm for P25-Slide, Spher-Slide, Cubic-Slide, andRhomb-Slide, respectively (Figure 2a′,b,c,d), the TiO2 bulk
density followed the order (Table 2a’,b,d,e)

Weak peaks at 451, 468−469, and 482 nm were also
observed with the excitation at 250−300 nm, associated withthe surface trap sites, for example, Ti or O defect to VB.43 The
correlation between the trap site peak intensity and the Wmax
values is not clear at the moment.
Impedance of TiO2 Films. The impedance of various
TiO2 ﬁlms formed in this study was evaluated while applying
500 mV ac. The Cole−Cole plot showed part of a semicircle
beginning at the origin in the ﬁrst quadrant for each TiO2 ﬁlm
sample (Supporting Information, Figure S1a). The diametercorrelated with the ﬁlm impedance. Based on the ﬁt with acircular function, the order of impedances (Table 2) was

Rhomb‐Slide > Cubic‐Slide ≈ Spher‐Slide ≈ P25‐Slide

The density was more than doubled by using Rhomb-Slide(2.2 g cm−3) with a homogeneous size and shape, and this
could be explained based on the smaller mean primary particle
size (22 × 11 nm) in comparison to Cubic-TiO2 (28 nm) or
Spher-TiO2 (31 × 13 nm; Table 1c−e and Figure 1) and theclose packing of the “rhombic” crystals with a homogeneous
size and shape (Figure 1c). Coagulated parts were observed for
Cubic-Slide and Spher-Slide ﬁlms in contrast to ﬂat RhombSlide or P25-Slide ﬁlms (Figure 2a′,b,c,d). As for Spher-BS,
Cubic-BS, and Rhomb-BS, the ball milling and slide method
ultimately ﬂattened the ﬁlms (Figure 2b′,c′,d′).
Fluorescence Spectra of TiO2 Suspensions. The
ﬂuorescence spectra were compared among the TiO2 samples
with excitation at 250 and 300 nm (Figure 3a,b). Because of

Rhomb‐Slide > Rhomb‐BS > P25‐Cast ≈ P25‐BD ≈ Spher‐BD ≈ Spher‐BS > Cubic‐Slide > Spher‐Slide > P25‐Slide

When 2 V dc was also applied to the TiO2 ﬁlm
simultaneously with 500 mV ac (Figure S1b), the order of
impedance changed to
Rhomb‐BS > Rhomb‐Slide > Cubic‐Slide > Spher‐BD > P25‐Cast ≈ P25‐Slide ≈ Spher‐BS ≈ Spher‐Slide ≈ P25‐BD

Overall, the impedance of the ﬁlms prepared via the slide
method, especially Rhomb-Slide and Cubic-Slide, relatively
increased, suggesting that they were less polarizable byapplication of dc, while ﬁlms comprising anisotropic nanoparticles were eﬀectively polarized by 2 V dc (Scheme 2, top).
Scheme 2. Reasons for the Dependence of the Impedancewith Applied ac/dc, 18O2 Photoexchange Rates, and Wmax on
the Particle Size and Shape of TiO2 Used on the Anode andthe Energetics of O2 Exchange and Water Oxidation
ReactionsFigure 3. Fluorescence spectra of aqueous TiO2 suspensions withexcitation at (a) 250 and (b) 300 nm. TiO2 sample powder (2.0 mg)(Spher-, Cubic-, Rhomb-, and P25-TiO2) was suspended in 4.0 mL ofwater.

the excitation both at 250 and 300 nm, a predominant
ﬂuorescence peak appeared at 372−366 nm, for all the TiO2
samples. This occurred because of the band edge emission of
TiO2.43 Therefore, an increase in the excitation wavelength for
ﬂuorescence at 372−366 nm leads to a corresponding
monotonous decrease in the excitation spectra for all the
TiO2 samples. Therefore, we compared the peak intensity withthe excitation at 250 nm (Figure 3a)
Rhomb‐TiO2 > Cubic‐TiO2 > Spher‐TiO2 > P25‐TiO2

The blue shift of the band edge emission peak from 372 nm(Spher, Cubic, Rhomb) to 408 nm (P25) which appeared in
Figure 3a should be the de-excitation at the band gap (BG) for
anatase phase (Spher, Cubic, Rhomb). Also, the shift of excitedelectrons from the CB of anatase phase (80% of P25, Table 1a)to the CB of rutile phase (20% of P25) followed by band edge
emission at BG of rutile phase.43 For the same reason, we havea band edge emission shift from 366 to 420 nm shown in
Figure 3b. The order above is contrary to the Wmax values
derived by the TiO2 photocatalyst ﬁlm prepared slide methodon the anode of SC (see the section on SC Tests). This isbecause the electronic excitation at BG contributes to thephotooxidation as charge separation at the anode (eq 1). In
fact, ﬂuorescence at 372−420 nm is an alternative process as aresult of electronic excitation at BG.

Yun et al. reported that the capacitance of TiO2 nanoparticles followed the order
Spherical (mean 25 nm)< ellipsoidal (39 × 132 nm)< ellipsoidal (27 × 156 nm)
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samples were 161, 115, 211, 60, and 96.4 m2 g−1 (Table 1) inthe order, and again, no correlation with the 18O2 exchange
rate was found. Conversely, JRC6-TiO2 is only one pure rutilephase sample in this study and the 18O16O formation rate was
the lowest: 6.4 μmol h−1 gcat−1. A diﬀerent type of rutile TiO2
with a surface O population of 8.7 nm−2 and a speciﬁc SA of
8.0 m2 g−1 was tested in another study,12 and the 18O16O
formation rate was also suﬃciently slow (34 μmol h−1 gcat−1)
compared to the rates for samples in this study. Thus, anatase
TiO2 was favorable in the 18O2 exchange reaction with surface
O. Furthermore, nonpolar {1 0 0} facets of anatase comprising
50% of surface Ti and 50% of surface O (e.g., on Cubic-TiO2)
would be advantageous as oxo Lewis base sites rather thanhydroxy sites at, for example, polar (0 0 1) or {1 0 1} (e.g., on
Rhomb-TiO2) facets. A weaker interaction between O2 and theanatase TiO2 (1 0 1) face was also reported.45
SC Tests: Dependence on TiO2. The i−V dependence
was tested using BiOCl-Cast on the photocathode and various
types of TiO2 ﬁlms prepared via various methods on thephotoanode. First, the eﬀects of the ﬁlm preparation method
were compared for P25-TiO2 (Figure S2a and Table 2a,a′,a″)and Spher-TiO2 samples (Figure S2b and Table 2b,b′,b″). In
both series, TiO2 ﬁlms prepared by the slide method enabledhigher VOC values and generation of the maximum cell power(Wmax)

which matched well the aspect ratios of the ﬁlms formed on
ITO/Pyrex.44 As the capacitance is deﬁned for two ﬂat plates
as eq 4,
Capacitance = dielectric constantplate area
×
distance between plates(4)

the dielectric constant increased with the aspect ratio of
nanoparticles. In this context, in comparison to Rhomb-TiO2
and Cubic-TiO2 crystals, P25-TiO2 and Spher-TiO2 crystals
were anisotropic, had greater dielectric constants, tended to
become more polarized, and were expected to show smaller
impedance during application of both ac and dc.
18
O2 Exchange Photoreaction with TiO2 Films. The
photoexchange reaction was conducted using Spher-TiO2,
Cubic-TiO2, and Rhomb-TiO2 powders. Gas-phase 18O2
reacted with surface 16O to form 18O16O, and the formed
18 16
O O reacted with surface 16O to form secondary 16O2
(Figure 4). When using Cubic-TiO2, the amount of 18O2 gas

P25‐Slide (1.96 V)> P25‐BD (1.91 V)> P25‐Cast (1.87 V)

Figure 4. Time course of 18O2 (0.55 kPa) photoexchange reaction
using (a) Spher-TiO2, (b) Cubic-TiO2, and (c) Rhomb-TiO2
photocatalysts (each 0.135 g).

P25‐Slide (61.1 μW cm−2)> P25‐Cast (47.4 μW cm−2)12

decreased rapidly within 0.5−1 h of the reaction and reachedisotope-exchange equilibrium after 8 h of reaction (Figure 4b).
Corresponding to the 18O2 decrease, 18O16O was formed at an
initial rate of 214 μmol h−1 gcat−1 (Table 2d), followed by thesecondary formation of 16O2. Among TiO2 samples, the orderof the rate of 18O2 decrease was≈ P25‐BD (43.7 μW cm−2)
Spher‐Slide (1.90 V)> Spher‐BD (1.86 V)

Cubic‐TiO2 > Spher‐TiO2 > Rhomb‐TiO2> Spher‐BS (1.78 V)
18

16

which was consistent with the initial formation rate of O O(Table 2b−e)Spher‐Slide (55.0 μW cm−2)≈ Spher‐BS (55.0 μW cm−2)

Cubic‐TiO2 > Spher‐TiO2 > Rhomb‐TiO2 ≈ P25> Spher‐BD (42.1 μW cm−2)‐TiO2 ≫ JRC6‐TiO2

The SC test results (VOC, Wmax) were well reproduced using
diﬀerent P25-Slide ﬁlms on the anode with a deviation of 2.6−
6.5% (Table 2a′), owing to the ﬂatness of the ﬁlm (Figure 2a′).
The ﬁlm preparation method was then ﬁxed via slide, and theperformances of various TiO2 types were systematicallycompared (Figure S2c). The orders of VOC and Wmax values
were

Clearly, the exchange reaction in 18O2 was faster with TiO2
crystals having a uniform size and shape than with conventional P25-TiO2 and JRC6-TiO2.12 If we assume complete
Cubic-TiO2 (anatase phase), 67% of {1 0 0} and 33% of (0 01) faces were exposed.22 For Rhomb-TiO2 (anatase phase),∼two-thirds of thermodynamically unstable {1 0 1} facets and∼one-third of stable (0 1 0) facets were exposed.23 Meanwhile,
the major axis of Spher-TiO2 coincided with the c-axis ofanatase, and all facets other than (0 0 1) appeared to be
exposed.20,22
Based on the 18O2 exchange tests, the surface (exchangeable)
O atom populations were evaluated as 3.6, 10.1, 1.4, 9.9, and>1.5 nm−2 (Table 2) for ﬁlm samples in the order of 18O16O
formation above (Cubic-TiO2, Spher-TiO2, Rhomb-TiO2, P25TiO2, and JRC6-TiO2); however, no correlation with the 18O2
exchange rate was found. The speciﬁc SAs of these TiO2

P25‐Slide (1.96 V) > Cubic‐Slide (1.92 V) ≈ Rhomb‐Slide (1.92 V) > Spher‐Slide (1.90 V)

P25‐Slide (61.1 μ W cm−2) > Spher‐Slide (55.0 μ W cm−2)> Cubic‐Slide (43.1 μ W cm−2) > Rhomb‐Slide (32.9 μ W cm−2)
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Critical Factors in Controlling the Wmax Value of the
SC. To determine the critical factors in determining the SC
performance, the correlations between six parameters, VOC,
Wmax for the SC (Table 2 and Figures S2 and 4), bulk density,
ac impedance (Figure S1a), ac/dc impedance (Figure S1b),
and the 18O16O formation rate in 18O2 (Figure 4) for 11 TiO2
ﬁlms used on the anode (Table 2), were examined. If weassumed a linear relationship and accepted a correlation only
for cases with a correlation coeﬃcient exceeding 0.5 (eq 5),
four correlations with Wmax were veriﬁed (Figure 6), whereas
no correlations were found with VOC.

These orders clearly demonstrated that TiO2 crystals with auniform size and shape were unfavorable with respect to boththe electromotive force and cell power when used on thephotoanode in the SC. The test results (VOC and Wmax) varied
greatly using diﬀerent Rhomb-Slide ﬁlms on the anode with adeviation of 11−63% (Table 2e), mostly because of the less ﬂat
ﬁlm compared to P25-Slide (Figure 2a′,d). Cubic-BS andRhomb-BS were also tested; however, their performance was
the poorest (Table 2d′,e′).
SC Tests: Dependence on pH. The P25-Slide ﬁlm was
the best for the SC performance on the photoanode (Table
2a′, Figure S2a,c). The VOC of 1.96 V was based on the energylevel diﬀerence between the CB minimum (CBM) of TiO2 andthe valence band maximum (VBM) of BiOCl,12 which
exceeded the electromotive force reported for similar SCs:
TiO2 and polyterthiophene (0.90 V),13,24 and W-doped
BiVO4/Bi4V2O11 and undoped BiVO4/Bi4V2O11 (1.54 V).25
Furthermore, the eﬀects of the electrolyte pH on the SC
performance were assessed using the optimized combination ofphotocatalysts: P25-Slide and BiOCl-Cast (Table 2a′). Bychanging the pH of the HCl electrolyte in the cathode from2.00 to 1.78 while maintaining the pH at 2.00 in the anode, iSC
was increased by 66% from 61.7 to 102.4 μA cm−2, and Wmax
was also increased by 39% from 61.1 to 85.2 μW cm−2 (Figure
5 and Table 2a′). Conversely, when we set the pH of thecathode to 1.78 from the beginning, iSC and Wmax decreased to55.9 and 38.9 μW cm−2 (data not shown).

Correlation coefficient Rn∑i = 1 (xi − x ̅ )(yi − y ̅ )=nn{∑i = 1 (xi − x ̅ )2 }1/2 {∑i = 1 (yi − y ̅ )2 }1/2
> 0.5

(5)

Figure 6. Correlations between Wmax and the impedance of TiO2
ﬁlms on applying 500 mV ac (a), between Wmax and the impedance ofTiO2 ﬁlms when both 500 mV ac and 2 V dc were applied (b),between Wmax and the 18O16O formation rate in the 18O2 exchange
reaction (c), and between Wmax and the bulk density of TiO2 ﬁlms
(d).

Figure 5. Dependence of the i−V test results on the pH of the HClsolution for the SC using P25-Slide on the photoanode and BiOClCast on the photocathode. Anode compartment: initial pH 2.00
(ﬁxed) and cathode compartment: initial pH 1.78 (○) and 2.00 (□).

A negative correlation was veriﬁed between the ac
impedance and Wmax (R = 0.769, Figure 6a), demonstratingthat electronic conductivity both in the TiO2 ﬁlm and at theinterface from the TiO2 ﬁlm to the ITO electrode was essential
to maximize the SC power. The correlation coeﬃcient between
the ac/dc impedance and Wmax increased (R = 0.924, Figure6b), but the smaller impedances (0.47−0.76 MΩ) for ﬁve ﬁlms
(P25-TiO2 and Spher-TiO2) did not faithfully correlate with
Wmax. Polarization caused by applying 2 V dc at the anode wasplausible (Scheme 2, top) during the SC test because of theenergy bias between the CBM of TiO2 and VBM of BiOCl due
to irradiation with UV−visible light.12,16,18 The ac/dc
impedance inversely correlated with the electronic conductivity
in the TiO2 ﬁlm and also at the interface between the TiO2
ﬁlm and ITO. Cubic-TiO2 and Rhomb-TiO2 with a uniform
size and shape were unsuitable based on the impedances(Figure 6b), probably because of packing problems in the ﬁlm(Scheme 2, top, right). Conversely, Wmax could be controlled
through the cathode using the BiOCl photocatalyst when the
ac/dc impedance was below 0.76 MΩ (Figure 6b).

The proton concentration in the cathode increased by 1.66
times at pH 1.78 compared to that at pH 2.00. The increase in
the current density in the voltage range 0−1.4 V (Figure 5)
was due to the increase in the kinetic rate of the cathodereaction (eq 2) consistent with the diﬀerence in proton
concentration if we assume a ﬁrst-order reaction of [H+].
Conversely, the voltage range 1.4−2.0 V was controlled not bykinetics but by the equilibrium based on the applied external
resistance because the electron energy diﬀerence in bothelectrodes on light excitation12,16−19 and/or suﬃcient electrons
for the reduction reaction of O2 were not supplied through theresistance of external circuit. The local site structure of Bi3+ did
not change between pH 1.78 and 2.00 based on Bi L3-edge
EXAFS; the interatomic distance and its associated coordination number for Bi−O and Bi−Cl interatomic pairs changed
negligibly at pH values between 2.00 and 1.78 (Table S1a,b),
excluding the possibility of pH-dependent changes in the Bi3+
site structure and reactivity (Figure S3).
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Wmax was also negatively correlated with the 18O16O
formation rates in 18O2 with various TiO2 samples (R =0.793; Figures 4 and 6c). The 18O16O formation rates weredivided by the number of surface (exchangeable) 16O atoms toevaluate the rate constant k of the 18O16O formation reaction(Table 2). The 18O16O formation proceeds through an18
O−18O bond dissociation step (Scheme 2, bottom, reverse
reaction). This step is a reverse reaction of O−O formation,
which is the key step in the water photooxidation to O2
occurring on the anode (Scheme 2, bottom, forward reaction).
The correlation between ﬂuorescence peak intensity (Figure 3)
and SC power (Table 2) existed among TiO2 ﬁlms,
demonstrating the eﬃciency of O2 photoreduction reaction
over these ﬁlms.
Assuming the Arrhenius-type rate dependence and (nearly)
equivalent pre-exponential factors for the 18O2 exchange
reaction, the diﬀerence in the apparent activation energies,
ΔE1 for Rhomb-Slide (0.23 h−1) and ΔE2 for JRC6-Cast(<0.027 h−1), is 5.2 kJ mol−1. Conversely, water photooxidation is the forward reaction in Scheme 2, bottom, andproceeds faster for JRC6-Cast (lower activation barrier fromleft to right) than for Rhomb-Slide. In this context, Cubic-TiO2
and Rhomb-TiO2 with a uniform size and shape were the mostunfavorable for the forward reaction in Scheme 2, bottom, andthus gave the lowest Wmax (Figure 6c).
We have ensured that the 18O2 exchange tests were carefully
performed using TiO2 powder, not ﬁlm while Wmax varied
according to the ﬁlm preparation method (Slide, Cast, BS, andBD) for P25 and Spher-TiO2 (Table 2). Currently, it isuncertain whether the exchange rate is dependent on the ﬁlm
preparation method.
A linear correlation between the bulk density of the TiO2
ﬁlm and Wmax existed, but this was weak (R = 0.517; Figure6d) and only discussed in the Supporting Information.
In summary, the crystals of Cubic-TiO2 and Rhomb-TiO2
were homogeneous in size and shape. However, thehomogeneity led to smaller polarizability, resulting in greater
impedance on application of 2 V dc to the anode. A further
disadvantage was conﬁrmed by the 18O2 exchange reaction on
TiO2, which showed that the reverse reaction of wateroxidation was catalyzed. To solve this situation, anisotropic
TiO2 crystals proved favorable for polarizability, and a wider
particle size distribution was favorable for forming ﬁlms with alower bulk density to maximize the surface exposure ofphotooxidation sites. Spher-Slide and P25-Slide were the mostfavorable, enabling 85.2 μW cm−2 when combined with BiOClCast.

Both ﬁlm impedance and the 18O2 exchange rate were highfor Cubic-TiO2 and Rhomb-TiO2 with a uniform size andshape, demonstrating their poor performance on the photoanode, while nonuniform anisotropic Spher-TiO2 andcommercial P25 achieved satisfactory values of theseparameters and proved advantageous as photocatalysts on
the anode of the SC in this study. With regard to the methodof preparing the ﬁlms on each electrode, the slide method wasadvantageous in maximizing VOC and Wmax for TiO2 (anode),while the casting method was superior to the slide method forforming a BiOCl ﬁlm on the cathode with a perpendicular
crystal orientation.
The polarizability and photocatalytic activity of TiO2 crystals
were the most eﬀective control factors. The impedance of TiO2
on application of both 500 mV ac and 2 V dc was closely
correlated with the polarizability of TiO2; anisotropic TiO2
showed higher polarizability and lower ac/dc impedance,
achieving the best SC performance.

■

ASSOCIATED CONTENT* Supporting InformationS

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acssuschemeng.9b05576.
Cole−Cole plot for various TiO2 ﬁlms on ITO/Pyrex,
i−V test graphs and the dependence on BiOCl,
experimental results of Bi L3 -edge EXAFS, andcorrelation between bulk density of TiO2 ﬁlm and SC
power (PDF)■

AUTHOR INFORMATION

Corresponding Author*E-mail: yizumi@faculty.chiba-u.jp. Phone: +81-43-290-3696.
Fax: +81-43-290-2783.
ORCID

Yasuo Izumi: 0000-0001-8366-1864
Notes

The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
The authors are grateful for the ﬁnancial support from theGrant-in-Aid for Scientiﬁc Research C (17K05961) from theJapan Society for the Promotion of Science and Leading
Research Promotion Program (2015−2019) from the Institutefor Global Prominent Research, Chiba University. X-ray
absorption experiments were performed with the approval of
the Photon Factory Proposal Review Committee (2018G649).
The authors thank Dr. S. Matsuzawa, the Institute for Global
Prominent Research, Chiba University, for fundamental
electrochemical tests and R. Akaogi and N. Ageji, Department
of Applied Chemistry and Biotechnology, Chiba University, forthe synthesis of “rhombic” TiO2.

■

CONCLUSIONSThe crystal size, shape, homogeneity, and method of preparing
a ﬁlm of TiO2 on the photoanode of a SC were critical for the
SC performance. A SC using P25-Slide and BiOCl-Cast on its
electrodes was optimized to achieve a VOC of 1.94 V and a
Wmax of 85.2 μW cm−2. An appropriate particle size range (15−30 nm) was obtained, and needlelike TiO2 was controlled toform spheroids with a mean size of 31 × 13 nm. The
performance of the SC (Wmax) critically depended on ﬁlm
impedance measurements using ac and both ac and dc and the18
O2 exchange rate. The order of SC performance was
supported by the major peak intensity of ﬂuorescence spectra
at 372−366 nm.

■

REFERENCES(1) Jung, E. H.; Jeon, N. J.; Park, E. Y.; Moon, C. S.; Shin, T. J.;
Yang, T.-Y.; Noh, J. H.; Seo, J. Efficient, Stable and Scalable
Perovskite Solar Cells Using Poly(3-Hexylthiophene). Nature 2019,
567, 511−515.
(2) McGehee, M. D. Paradigm Shifts in Dye-Sensitized Solar Cells.
Science 2011, 334, 607−608.
(3) Xu, P.; Huang, T.; Huang, J.; Yan, Y.; Mallouk, T. E. DyeSensitized Photoelectrochemical Water Oxidation Through a Buried
Junction. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 6946−6951.
1414

DOI: 10.1021/acssuschemeng.9b05576
ACS Sustainable Chem. Eng. 2020, 8, 1406−1416

ACS Sustainable Chemistry & Engineering

Research Article(4) Horiuchi, Y.; Toyao, T.; Takeuchi, M.; Matsuoka, M.; Anpo, M.
Recent Advances in Visible-Light-Responsive Photocatalysts for
Hydrogen Production and Solar Energy Conversion − From
Semiconducting TiO2 to MOF/PCP Photocatalysts. Phys. Chem.
Chem. Phys. 2013, 15, 13243−13253.
(5) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical
Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A.
2006, 103, 15729−15735.
(6) Lee, D. K.; Lee, D.; Lumley, M. A.; Choi, K.-S. Progress on
Ternary Oxide-Based Photoanodes for Use in Photoelectrochemical
Cells for Solar Water Splitting. Chem. Soc. Rev. 2019, 48, 2126−2157.
(7) Du, X.; Jiang, D.; Liu, Q.; Hao, N.; Wang, K. Ingenious DualPhotoelectrode Internal-Driven Self-Powered Sensing Platform for thePower Generation and Simultaneous Mirocystin Monitoring Based on
the Membrane/Mediator-Free Photofuel Cell. Anal. Chem. 2019, 91,
1728−1732.
(8) Ji, W.; Yan, K.; Chen, Y.; Gao, J.; Zhang, J. A Visible LightInduced Self-Powered Aptasensing Platform for Kanamycin Detection
Based on Mediator-Free Photofuel Cell. Sens. Actuators, B 2019, 292,
129−135.
(9) Zhao, Y.; Zeng, Q.; Liu, X.; Jiao, S.; Pang, G.; Du, X.; Zhang, K.;
Yang, B. Highly Efficient Aqueous-Processed Polymer/Nanocrystal
Hybrid Solar Cells with an Aqueous-Processed TiO2 Electron
Extraction Layer. J. Mater. Chem. A 2016, 4, 11738−11746.
(10) Kim, H.; Ou, K.-L.; Wu, X.; Ndione, P. F.; Berry, J.; Lambert,
Y.; Mélin, T.; Armstrong, N. R.; Graham, S. Investigation of UltraThin Titania Films as Hole-Blocking Contacts for Organic Photovoltaics. J. Mater. Chem. A 2015, 3, 17332−17343.
(11) Xu, P.; Mallouk, T. E. Charge Transfer Dynamics in Aqueous
Dye-Sensitized Photoelectrochemical Cells: Implications for Water
Splitting Efficiency. J. Phys. Chem. C 2019, 123, 299−305.
(12) Urushidate, K.; Matsuzawa, S.; Nakatani, K.; Li, J.; Kojima, T.;
Izumi, Y. Solar Cell with Photocatalyst Layers on Both the Anode andCathode Providing an Electromotive Force of Two Volts per Cell.
ACS Sustainable Chem. Eng. 2018, 6, 11892−11903.
(13) Zhang, B.; Fan, W.; Yao, T.; Liao, S.; Li, A.; Li, D.; Liu, M.; Shi,
J.; Liao, S.; Li, C. Design and Fabrication of a Dual-Photoelectrode
Fuel Cell toward Cost-Effective Electricity Production from Biomass.
ChemSusChem 2017, 10, 99−105.
(14) Lu, Y.; Chu, Y.; Zheng, W.; Huo, M.; Huo, H.; Qu, J.; Yu, H.;
Zhao, Y. Significant Tetracycline Hydrochloride Degradation andElectricity Generation in a Visible-Light-Driven Dual Photoelectrode
Photocatalytic Fuel Cell Using BiVO4/TiO2 NT Photoanode andCu2O/TiO2 NT Photocathode. Electrochim. Acta 2019, 320, 134617.
(15) He, Y.; Zhang, C.; Hu, J.; Leung, M. K. H. NiFe Layered
Double Hydroxide/BiVO4 Photoanode Based Dual-Photoelectrode
Photoatalytic Fuel Cell for Enhancing Degradation of Azo Dye
Electricity Generation. Energy Procedia 2019, 158, 2188−2195.
(16) Yoshiba, M.; Ogura, Y.; Tamba, M.; Kojima, T.; Izumi, Y. A
Solar Cell for Maximizing Voltage Up To the Level Difference of Two
Photocatalysts: Optimization and Clarification of the Electron
Pathway. RSC Adv. 2017, 7, 19996−20006.
(17) Ogura, Y.; Yoshiba, M.; Izumi, Y. Recyclable Photofuel Cell for
Use of Acidic Water as a Medium. Oil Gas Sci. Technol. 2015, 70,
853−862.
(18) Fujishima, Y.; Okamoto, S.; Yoshiba, M.; Itoi, T.; Kawamura,S.; Yoshida, Y.; Ogura, Y.; Izumi, Y. Photofuel Cell Comprising
Titanium Oxide and Bismuth Oxychloride (BiO1−xCl1−y) Photocatalysts that Uses Acidic Water as a Fuel. J. Mater. Chem. A 2015, 3,
8389−8404.
(19) Ogura, Y.; Okamoto, S.; Itoi, T.; Fujishima, Y.; Yoshida, Y.;
Izumi, Y. A Photofuel Cell Comprising Titanium Oxide and Silver(I/0) Photocatalysts for Use of Acidic Water as a Fuel. Chem. Commun.
2014, 50, 3067−3070.
(20) Kojima, T.; Suzuki, R.; Uekawa, N.; Kakegawa, K. Fabricationof Porous Titania Using Anisotropic Particles. J. Ceram. Soc. Jpn.
2007, 115, 160−164.

(21) Sugimoto, T.; Zhou, X.; Muramatsu, A. Synthesis of Uniform
Anatase TiO2 Nanoparticles by Gel-sol method: 3. Formation Process
and Size Control. J. Colloid Interface Sci. 2003, 259, 43−52.
(22) Sugimoto, T.; Zhou, X.; Muramatsu, A. Synthesis of Uniform
Anatase TiO2 Nanoparticles by Gel-sol Method: 4. Shape Control. J.
Colloid Interface Sci. 2003, 259, 53−61.
(23) Wu, B.; Guo, C.; Zheng, N.; Xie, Z.; Stucky, G. D. Nonaqueous
Production of Nanostructured Anatase with High-Energy Facets. J.
Am. Chem. Soc. 2008, 130, 17563−17567.
(24) Zhang, B.; He, L.; Yao, T.; Fan, W.; Zhang, X.; Wen, S.; Shi, J.;
Li, C. Simultaneous Photoelectrocatalytic Water Oxidation andOxygen Reduction for Solar Electricity Production in Alkalie Solution.
ChemSusChem 2019, 12, 1026−1032.
(25) dos Santos, W. S.; Rodriguez, M.; Hhoury, J. M. O.;
Nascimento, L. A.; Ribeiro, R. J. P.; Mesquita, J. P.; Silva, A. C.;
Nogueira, F. G. E.; Pereira, M. C. Bismuth Vanadate Photoelectrodes
with High Photovoltage as Photoanode and Photocathode in
Photoelectrochemical Cells for Water Splitting. ChemSusChem
2018, 11, 589−597.
(26) Liu, J.; Xia, M.; Chen, R.; Zhu, X.; Liao, Q.; Ye, D.; Zhang, B.;
Zhang, W.; Yu, Y. A Membrane-Less Visible-Light Responsive Micro
Photocatalytic Fuel Cell with the Laterally-Arranged CdS/ZnS-TiO2
Photoanode and Air-Breathing CuO Photocathode for Simultaneous
Wastewater Treatment and Electricity Generation. Sep. Purif. Technol.
2019, 229, 115821.
(27) He, Y.; Yuan, R.; Leung, M. K. H. Highly Efficient AgBr/BiVO4Photoanode for Photocatalytic Fuel Cell. Mater. Lett. 2019, 236, 394−
397.
(28) Ye, L.; Deng, K.; Xu, F.; Tian, L.; Peng, T.; Zan, L. Increasing
Visible-Light Absorption for Photocatalysis with Black BiOCl. Phys.
Chem. Chem. Phys. 2012, 14, 82−85.
(29) Zhang, H.; Itoi, T.; Konishi, T.; Izumi, Y. Dual Photocatalytic
Roles of Light: Charge Separation at the Band Gap and Heat via
Localized Surface Plasmon Resonance To Convert CO2 into CO over
Silver−Zirconium Oxide. J. Am. Chem. Soc. 2019, 141, 6292−6301.
(30) Zhang, H.; Kawamura, S.; Tamba, M.; Kojima, T.; Yoshiba, M.;
Izumi, Y. Is Water More Reactive Than H2 in Photocatalytic CO2
Conversion into Fuels Using Semiconductor Catalysts Under High
Reaction Pressures? J. Catal. 2017, 352, 452−465.
(31) Morikawa, M.; Ahmed, N.; Yoshida, Y.; Izumi, Y. Photoconversion of Carbon Dioxide in Zinc−Copper−Gallium Layered
Double Hydroxides: The Kinetics to Hydrogen Carbonate andFurther to CO/Methanol. Appl. Catal., B 2014, 144, 561−569.
(32) Yoshida, Y.; Mitani, Y.; Itoi, T.; Izumi, Y. Preferential Oxidationof Carbon Monoxide in Hydrogen Using Zinc Oxide Photocatalysts
Promoted and Tuned by Adsorbed Copper Ions. J. Catal. 2012, 287,
190−202.
(33) Izumi, Y.; Konishi, K.; Obaid, D. M.; Miyajima, T.; Yoshitake,H. X-ray Absorption Fine Structure Combined with X-ray
Fluorescence Spectroscopy. Monitoring of Vanadium Sites in
Mesoporous Titania, Excited under Visible Light by Selective
Detection of Vanadium Kβ5,2 Fluorescence. Anal. Chem. 2007, 79,6933−6940.
(34) Izumi, Y.; Kiyotaki, F.; Yagi, N.; Vlaicu, A.-M.; Nisawa, A.;
Fukushima, S.; Yoshitake, H.; Iwasawa, Y. X-ray Absorption Fine
Structure Combined with X-ray Fluorescence Spectroscopy. Part 15.
Monitoring of Vanadium Site Transformations on Titania and in
Mesoporous Titania by Selective Detection of the Vanadium Kα1
Fluorescence. J. Phys. Chem. B 2005, 109, 14884−14891.
(35) Bearden, J. A. X-Ray Wavelengths. Rev. Mod. Phys. 1967, 39,
78−124.
(36) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS:
Data Analysis for X-ray Absorption Spectroscopy Using IFEFFIT. J.
Synchrotron Radiat. 2005, 12, 537−541.
(37) Wyckoﬀ, R. W. G. Crystal Structures 1, 2nd ed.; Interscience
Publishers: New York, 1963; pp 294−296.
(38) Keramidas, Κ. G.; Voutsas, G. P.; Rentzeperis, P. I. The Crystal
Structure of BiOCl. Z. Kristallogr. 1993, 205, 35−40.
1415

DOI: 10.1021/acssuschemeng.9b05576
ACS Sustainable Chem. Eng. 2020, 8, 1406−1416

ACS Sustainable Chemistry & Engineering

Research Article(39) Huang, W. L.; Zhu, Q. Electronic Structures of Relaxed BiOX
(X = F, Cl, Br, I) Photocatalysts. Comput. Mater. Sci. 2008, 43, 1101−
1108.
(40) Cucka, P.; Barrett, C. S. The Crystal Structure of Bi and of
Solid Solutions of Pb, Sn, Sb, and Te in Bi. Acta Crystallogr. 1962, 15,865−872.
(41) Jiang, N.; Spence, J. C. H. Can Near-Edge Structure of the Bi L3
Edge Determine the Formal Valence States of Bi? J. Phys.: Condens.
Matter 2006, 18, 8029−8036.
(42) Nyburg, S. C.; Ozin, G. A.; Szymański, S. T. The Crystal andMolecular Structure of Bismuth Trichloride. Acta Crystallogr., Sect. B:
Struct. Crystallogr. Cryst. Chem. 1971, 27, 2298−2304.
(43) Pan, D.; Zhao, N.; Wang, Q.; Jiang, S.; Ji, X.; An, L. Facile
Synthesis and Characterization of Luminescent TiO2 Nanocrystals.
Adv. Mater. 2005, 17, 1991−1995.
(44) Yun, H. J.; Lee, H.; Joo, J. B.; Kim, W.; Yi, J. Influence ofAspect Ratio of TiO2 Nanorods on the Photocatalytic Decompositionof Formic Acid. J. Phys. Chem. C 2009, 113, 3050−3055.
(45) Li, Y.-F.; Selloni, A. Theoretical Study of Interfacial ElectronTransfer from Reduced Anatase TiO2(1 0 1) to Adsorbed O2. J. Am.
Chem. Soc. 2013, 135, 9195−9199.

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DOI: 10.1021/acssuschemeng.9b05576
ACS Sustainable Chem. Eng. 2020, 8, 1406−1416