Research Article
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Cite This: ACS Sustainable Chem. Eng. 2018, 6, 11892−11903

Solar Cell with Photocatalyst Layers on Both the Anode andCathode Providing an Electromotive Force of Two Volts per Cell
Kazuki Urushidate,† Shigemitsu Matsuzawa,‡ Keisuke Nakatani,† Jifu Li,§ Takashi Kojima,§
and Yasuo Izumi*,††Department of Chemistry, Graduate School of Science, ‡The Institute for Global Prominent Research, and §Department of Applied
Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan

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S Supporting Information*

ABSTRACT: Solar cells and fuel cells are essential devices that convert
sustainable energy into electricity. However, the electromotive forces they
provide are in principle limited to less than 1 V, and consequently, they need
to be connected in series for practical use. In this study, photocatalyst layerswith thicknesses of 0.8−2.1 μm were applied to both electrodes. The primaryparticle size of TiO2 was optimized (15−21 nm), and its performance wasfurther improved by doping with organic dyes, e.g., anthocyanins. The
electron conductivity of TiO2 was found to be a primary factor in theperformance of the cells, but the ﬁlm ﬂatness also reduced resistance andimproved cell performance. Interestingly, the eﬃciency of TiO2 could be
evaluated based on the exchangeable surface O atoms via its 18O2 exchange
tests to suppress the reverse reaction step in water photo-oxidation, which occurs on the anode. UV and visible light wereabsorbed by TiO2 and the organic dyes, respectively, creating an electron ﬂow path from the valence band to the conduction
band of TiO2, then, the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of
the organic dyes, then the anode, and ﬁnally the cathode via the external circuit. The ﬂow of electrons and holes (separated fromelectrons in BiOCl on the cathode by UV−visible light) enabled a VOC value of 2.11 V and maximum power of 73.1 μW cm−2.
KEYWORDS: Solar cell, Photocatalyst, Open-circuit voltage, Organic dye, Anthocyanidin, Titanium oxide, Bismuth oxychloride,
Isotope tracing■

INTRODUCTION
The process of replacing fossil fuel energy sources withrenewable energy sources such as wind, waves, hydro,
geothermal, solar, and biomass is proceeding and cannot be
halted or reversed if a sustainable global environment is to berealized.1 Solar cells (SCs) are a key technology for the eﬀective
conversion of renewable energy into electricity. Accordingly,
silicon (crystal-type and amorphous-type), dye-sensitized, andperovskite SCs have been developed.2 In principle, theelectromotive force of a solar cell is determined by the
conduction band minimum (CBM), valence band maximum(VBM), and/or reaction potential of the constituents, e.g., theredox mediator.2,3 Accordingly, the values are mostly less than 1
V and do not vary signiﬁcantly for diﬀerent cell designs, andthus, multiple cells must be connected in series for practical use.
Several researchers have reported the possibility of photoanodes4−13 or photocathodes14 incorporated into fuel cells(FCs). The photoanodes consume fuels such as methanol,4,5
ethanol,5,6 hydrogen peroxide,7 urea,8 phenol,9 glucose,10−12glycerol,5 or starch13 and constitute “photofuel” cells when they
are combined with conventional cathodes, e.g., platinum−
carbon. The electromotive force they provide is determined by
the diﬀerence between the CBM and the reduction potential of
O2 in the cathode and, thus, is typically less than 1 V.2,3,15 If
photocatalysts are used on both electrodes and excited by light,© 2018 American Chemical Society

the electromotive force is primarily determined by the CBM and
VBM of the two photocatalysts. Zhang et al.10 rationalized thesuperiority of cells that use photocatalysts on both electrodes3,15,16 in terms of the higher open-circuit voltages (VOC,
1.59−1.69 V) they provide compared to those provided by FCs
that comprise only a photoanode or a photocathode. SCs do not
require fuel, and acidic water in the cell behaves as a redox
mediator (eqs 1 and 2).
hv

2H 2O → 4H+ + 4e− + O2
hv

4H+ + 4e− + O2 → 2H 2O(anode)(1)(cathode)(2)

The energy levels in such SCs are tunable by the choice of thetwo photocatalysts, with the only prerequisite for the choice isthat the common reaction potential for eqs 1 and 2 should bepositioned between the VBM and CBM on both the anode andcathode.3,15,16 However, Zhang et al. reported FCs withmaximum power densities (Wmax) of 36−82 μW cm−2 preparedusing glucose or phenol as the fuel, far superior to the Wmax of
11−16 μW cm−2 achieved using acidic water as the fuel.10 This
Received: May 11, 2018
Revised: June 29, 2018
Published: July 16, 2018
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paper limits the comparison to SCs comprising two photocatalysts on both electrodes. If platinum is used at the cathode, a
VOC of 0.80−1.73 V and Wmax of 75−750 μW cm−2 are reported
using FCs with a TiO2-based17 or BiVO4-based photoanode.18
Furthermore, based on photosynthesis, FCs using a tylakoid-C
nanotube anode and Pt cathode generated a VOC of 0.4 V and
Wmax of 5.3 μW cm−2.19
In this study, to exceed a VOC of 2.0 V and achieve a Wmax
higher than 70 μW cm−2, higher-voltage-type SCs withphotocatalysts on both electrodes were investigated using
various nanocrystalline forms of TiO2. Thus far, a VOC of 1.91
V and Wmax of 42.2 μW cm−2 were the best values reported forthis type of SCs.3 A breakthrough is needed to ﬁll the gap ascompared to the theoretical VOC value of 2.75 V.3,15 To
overcome the breakthrough, very thin (0.8−2.1 μm) TiO2 andBiOCl nanocrystalline photocatalyst ﬁlms were formed on theanode and cathode to minimize the diﬀusion overvoltage.
Furthermore, to achieve the target performance values (VOC >2.0 V and Wmax > 70 μW cm−2), the new method employedorganic dyes in the SC as a novel highlight of this study. The
TiO2 layers on the anodes were sensitized using anthocyanidintype organic dyes (Scheme 1a), such as pelargonidin,

Figure 1. UV−visible absorption spectra of (a) a 0.10 mM ethanolicpelargonidin chloride solution, (b) a 0.030 mM ethanolic PEC-TOMP04 solution, and an ethanol extract (120 mg L−1) of (c) rosehip/hibiscus ﬂowers (1/10 diluted), (d) rosehip ﬂowers (1/25 diluted), and(e) hibiscus ﬂowers (1/25 diluted).

anode and simultaneously suppressing the diﬀusion overvoltage
within the photocatalyst layer. Consequently, the VOC and Wmax
values were increased to 2.11 V and 73.1 μW cm−2 (per cell),respectively.

■

Scheme 1. Structural Formula of Anthocyanidin: (a) General
Formula and Three Typical Types of Anthocyanidin (b)Pelargonidin, (c) Delphinidin, and (d) Cyanidina

EXPERIMENTAL SECTION

Synthesis of TiO2 and BiOCl and Preparation of Photoelectrodes. TiO2 (JRC-TIO-14; purity >93.4%)23 was supplied by
Ishihara Sangyo Co. TiO2 (JRC-TIO-15; purity >99.5%)23 and TiO2
(Aeroxide P25; purity >99.5%)24 were supplied by the Japan Aerosil
Co. TiO2 (PEC-TOM-P03) was purchased from Peccell Technologies
as an organic/aqueous suspension. TiO2 (JRC-TIO-6; purity >99%)
was supplied by the Sakai Chemical Industry Co.23 TiO2 (particle size <5 μm, purity 99.9%) was purchased from Wako Pure Chemical anddenoted as “rutile-W”. These samples were used without further
puriﬁcation. The mean primary particle sizes and speciﬁc surface areas(SAs) for these TiO2 samples are summarized in Table 1. The
Brunauer−Emmett−Teller SAs were measured based on N2 adsorption
at several pressure points between P/P0 = 0.05 and 0.35 at 77.4 K (P0101.3 kPa).
The TiO2 powder (0.10 g) was well mixed with 1.0 mL of t-butanol,and 0.05 mL of ethylene glycol was added as a thickener. The
suspension was agitated using ultrasoniﬁcation (430 W, 38 kHz) for 10
min, and pasted onto an indium tin oxide (ITO) coated (thickness
0.12−0.16 μm, sheet resistivity 8−12 Ω/square) Pyrex glass substrate(area 2.5 cm × 2.5 cm, thickness 0.11 cm; Aldrich). In the ﬁlm
preparation of JRC-TIO-6 and P25, a slide method was also employed,i.e., 10 g of ﬂat Pyrex glass plate (area 10 cm2) was slowly slid on thepasted suspension. Each TiO2/ITO/Pyrex plate was dried in air at 373
K for 18 h and then the temperature was elevated to 573 K at 4 K min−1
where it was maintained for 30 min. The amount of TiO2 deposited on
the plate was 0.4 mg, and it covered an area of 1.0 cm × 1.3 cm. The
organic/aqueous suspension of PEC-TOM-P03 was directly pasted
onto an ITO-coated glass and treated following a similar procedure tothat used for the other TiO2 samples.
Pelargonidin chloride (compound 1; molecular weight 306.70;
ChromaDex, Inc., USA) ethanol solution (0.10 mM) was prepared as astandard for the anthocyanidins. The solution was red. The organic
dye/ethanol solutions were used as received (1.0 mM; PEC-TOM-P04,
Peccell Technologies). The molar absorption coeﬃcient was similar tothat for compound 1 within a variation of 13%. The color was orange.
Three drops (0.036 mL, 36 nmol) were cast onto TiO2/ITO/Pyrex
plates. The dye-adsorbed TiO2 ﬁlm was yellow. Anthocyanins wereextracted from 1.0 g of rosehip and hibiscus ﬂower herbal tea(Pompadour, Germany, No. 1519-88) using ethanol (10 mL) at 298 K
for 24 h. The hydroxy groups of the anthocyanin, e.g., at the 3- and/or 5position of the benzopyrylium ring (Scheme 1a), are dehydrated withsugar. Rosehip and hibiscus ﬂowers contain delphinidin and cyanidin,
respectively. The extract solution was transparent red, and theaThe pH dependence of the structure of pelargonidin is also
illustrated.

delphinidin, and cyanidin (Scheme 1b−d).20,21 These dyes
present colors between purple and orange and absorption peaksof the complementary color range between 532 and 553 nm(Scheme 1b−d and Figure 1). These absorptions are due toHOMO−LUMO electronic transitions.22 As a result, organicdyes work as an enhancer, increasing the electron ﬂow from the11893

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Table 1. Fundamental Physicochemical Properties of TiO2 and BiOCl Used on the Anode and Cathode, Respectively
entsample

a
b
c
d

TiO2
(0.4 mg)efg

JRC-TIO-6
rutile-We
BiOCl (0.6 mg)a

JRC-TIO-14
JRC-TIO-15
P25
PEC-TOM-P03bmean primaryparticle size (nm)speciﬁc surface
area (m2 g−1)

7.4a,c21a, 26b,c
21a,c
16a,c

308
50
60
82.7g

15a
61a,d

96.4g
8.0g
10.5g

mean ﬁlm
thickness
(μm)f

2.0
2.1
1.8
0.83

crystalline phaseanataseanatase:rutile = 8:2
anatase:rutile = 8:2
anatase dominant, rutile
and brookite minor
rutilerutilephotocurrent @ 1.0
V, SHE (μA)h

impedance
(MΩ)

60
79
79
68

247
137
137
85

23
23
24

101
77

91
120
108

23

ref

c

Fresh sample. Heated at 523 K. Based on XRD peak width (101 diﬀraction of anatase, d110 diﬀraction of rutile). eThe particle size was less than
5 μm. fBased on cross-sectional SEM. gBased on N2 adsorption at 77.4 K. hStandard hydrogen electrode (SHE).

concentration was 120 mg L−1. If the molar absorption coeﬃcient ofvisible light absorption for these anthocyanidins (Figure 1) is equivalent
to that for compound 1, the concentration was 0.13 mM and theaverage molecular weight was 920. Ten drops (0.12 mL, 16 nmol of theanthocyanidin) were cast onto a TiO2/ITO/Pyrex plate. The dyeadsorbed TiO2 ﬁlm was light red (Figure S1b and the Supporting
Information). For comparison purposes, anthocyanins were also
extracted from rosehip herbal tea (Tree of Life Co., Ltd., Japan, No.
02-520-7340) and hibiscus herbal tea (Tree of Life Co., Ltd., Japan, No.
02-620-6650) using ethanol (10 mL) at 298 K for 24 h. The extracts
were cast onto a TiO2/ITO/Pyrex plate in a manner similar to that used
for the Pompadour herbal tea.
BiOCl was synthesized from an aqueous solution using bismuth
trichloride.25 The obtained BiOCl powder (6.0 mg) was suspended indeionized water (60 μL; < 0.055 μS cm−1) supplied by a RFU424TA
system (Advantec) and agitated using ultrasoniﬁcation (430 W, 38
kHz) for 10 min. An aliquot of the suspension was pasted onto the ITOcoated Pyrex glass plate in a manner similar to that used to prepare theTiO2/ITO/Pyrex anodes. The amount of BiOCl deposited on each
ITO-coated glass plate was 0.6 mg, and it covered an area of 1.0 cm ×1.3 cm.
Characterization. Cross-sectional scanning electron microscopy(SEM) images were obtained using a JSM-6510 scanning electron
microscope (JEOL) 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.26,27 The incident angle of electrons with reference tothe sample surface was between 5° and 15°. The magniﬁcation was
between ×500 and ×20 000.
Ultraviolet (UV)−visible absorption spectra were obtained intransmission mode using a spectrophotometer (Model V-650,
JASCO) in the wavelength range 200−800 nm with D2 and halogen
lamps for wavelengths below and above 340 nm, respectively. The
ethanol solutions of pelargonidin chloride, organic dye (PEC-TOMP04), and herbal tea extract were set in quartz glass cells with 1.0 cm
sample thickness.
X-ray diﬀraction (XRD) patterns were obtained for the photocatalyst
ﬁlms. The XRD patterns were measured at a Bragg angle of 2θB = 20°−80° with a scan step of 0.02° and scan rate of 4 s per step using a D8
ADVANCE (Bruker) operated at 40 kV and 40 mA with Cu Kαemission (wavelength λ = 0.15419 nm) and a nickel ﬁlter. The mean
particle size (t) was determined using the Scherrer equation

t=

0.9λ
full width at half maximum × cos θBused to allow UV−visible light irradiation. The voltage of the WE wasswept between −1.00 and 1.50 V versus Ag/AgCl at a rate of 50 mV s−1
using a potentio/galvanostat (VersaSTAT 3-100, Princeton Applied
Research) in a N2 atmosphere at a ﬂow rate of 30 mL min−1, either
under irradiation with UV−visible light or in the dark.
Impedance measurements were performed for several ﬁlm-form
samples of TiO2 (0.4 mg) and BiOCl (0.6 mg, mean thickness 0.8 μm)
on ITO/Pyrex plates using a potentio/galvanostat (VSP, Bio-Logic). A
square Cu plate (6 mm × 6 mm × 30 μm) was attached to thesephotocatalyst ﬁlms, and the impedance between the Cu plate and ITO
ﬁlm was monitored using Au-coated Cu probes (KPRO-RY, Toyo
Corporation) in low-current mode. The applied alternative voltage was
100 mV, and the frequency was scanned between 10−1 Hz and 106 Hz.
18
O2 Exchange Photoreaction Tests. Isotope-labeled photoreaction tests were monitored using gas chromatography−massspectrometer (Model JMS-Q1050GC, JEOL).28 Helium (purity>99.9999%) was used as the carrier gas. The sampling loop compriseda Pyrex glass system vacuumed using rotary and diﬀusion pumps (10−6Pa) and connected to the JMS-Q1050GC using 1.5 m of deactivatedfused silica tube (No. 160-2845-10, Agilent; internal diameter 250 μm)
maintained at 393 K.
A photocatalyst sample (0.135 g, Table 1a−f) was placed in a quartz
photoreactor and evaluated at 295 K for 2 h at 10−6 Pa.1,28−32 Then,18
O2 photoexchange (0.55 kPa; 18O 97.8%, chemical purity > 99.9%,
Cambridge Isotope Laboratories, Inc.) with the photocatalyst wasmonitored at 295 K under irradiation by UV−visible light from a 500 W
xenon arc lamp (Model SX-UID502XAM, Ushio). The distance
between the exit of the lamp window and the photocatalyst was 160
mm. The exchange with 16O atoms at the surface was monitored using apacked column of 13X-S molecular sieves (GL Sciences, Inc.) set in the
JSM-Q1050GC.
Photodecomposition of H218O (0.25−1.8 kPa; 18O 97%, chemical
purity > 99.9%, Cambridge Isotope Laboratories, Inc.) with TiO2
samples (0.135 g) and those impregnated with silver nitrate (Wako,
purity 99.9%) was also monitored at 295 K using the JSM-Q1050GC
under irradiation from a Model SX-UID502XAM similar to the 18O2
photoexchange experiments.
SC Tests. The TiO2/ITO/Pyrex and BiOCl/ITO/Pyrex electrodes
were immersed in HCl solutions (40 mL in each compartment, initial
pH 2.0). The two compartments were separated by a 50 μm-thick H+conducting polymer ﬁlm (Naﬁon, DuPont; acid capacity > 9.2 × 10−4
equiv g−1).16,33,34 N2 (purity >99.999%) and O2 (purity >99.6%) were
bubbled 30 mm from each photoelectrode at a ﬂow rate of 100 mL
min−1. The SC was equipped with quartz windows (Φ = 80 mm) on
both sides. Both the TiO2 and BiOCl photocatalysts were irradiatedwith UV−visible light through the quartz windows using a two-way
branched quartz ﬁber light guide (Model 5Φ-2B-1000L, San-ei ElectricCo.) from the model SX-UID502XAM. The distance between the light
exit (Φ = 5 mm) and the surface of the TiO2 and BiOCl ﬁlm was 46 mm.
The light intensity was 91.3 mW cm−2 at the center of the photocatalyst
ﬁlm on each electrode.
The dependence of the current (i)−voltage (V) curves on the
conﬁguration of the anode and cathode was assessed in front−rear(3)

Photoelectrochemical Measurements. Cyclic voltammetry(CV) measurements were performed for several samples of TiO2
(Table 1) as the working electrode (WE; TiO2/ITO/Pyrex and an
Au/Ni-coated stainless steel clip; Toyo Corporation), glassy carbon as
the counter electrode (CE; Model 002012, Φpolyetheretherketone 6.0 mm,
ΦC 3.0 mm; BAS Incorporation), and Ag/AgCl as the referenceelectrode (RE; Model RE-1B, BAS Incorporation), immersed in ahydrochloric acid solution (80 mL, pH 4.0). A quartz glass window was
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Figure 2. Cross-sectional SEM images of TiO2 ﬁlms prepared from (a) PEC-TOM-P03, (b, b′) JRC-TIO-6, and (c) a BiOCl ﬁlm. The ﬁlm for b′ wasprepared via the slide method (see the Experimental Section).
conﬁguration with N2 and O2 ﬂows, respectively, by varying theresistance between 500 kΩ and 0.3 Ω.3,15,16 The front and rear
conﬁgurations allow perpendicular UV−visible light irradiation fromthe photocatalyst and Pyrex/ITO glass sides, respectively.3 For
comparison, a Xe arc lamp (300−500 W) and LED lights (BR244HK, SO-2123, Gentos Co.) at 3 W were also used for irradiationfrom both sides. Some of the i−V tests were conducted by irradiating
the anode with visible light ﬁltered with a L39 sharp cut ﬁlter (thickness
2.5 mm, Kenko) and the cathode being irradiated with UV−visible lightas above.

samples. It should be noted that the dyes were used in SC in
acidic pH solutions (2.0). Under the same pH conditions, theabsorption peak for the cyanidin in rosehip should be between
532 nm (pelargonidin) and 553 nm (delphinidin) based on thenumber of hydroxy groups (Scheme 1b−d).
The major peak at 532 nm for pelargonidin chloride
corresponds to the HOMO−LUMO gap of 2.33 eV. Thisvalue is consistent with the previously reported energy levels of
the HOMO (+0.13 V) and LUMO (−2.11 V) relative tostandard hydrogen electrode (SHE) for pelargonidin extracted
from Ixara Flora.36 The absorptions for cyanidin (≈543 nm) anddelphinidin (553 nm) correspond to the very similar HOMO−
LUMO gap values of 2.28 and 2.24 eV, respectively. Conversely,
the major peak at 415 nm for the organic dye in PEC-TOM-P04
corresponds to the relatively larger HOMO−LUMO gap of 2.99
eV.
Cross-Sectional SEM Measurements for the Electrodewith TiO2. In a previous study,3 the bulk density for the ﬂatTiO2 ﬁlm (2.0 mg) prepared from a PEC-TOM-P03 suspension
was calculated to be 3.3 ± 0.3 g cm−3 based on cross-sectional
SEM images (mean thickness 4.7 ± 0.5 μm). This value is 84%
of the density of anatase-type TiO2 crystals (3.89 g cm−3),37
demonstrating the formation of a dense, closely packed TiO2
ﬁlm on the ITO glass. In contrast, the mean thickness of the
TiO2 ﬁlm (0.4 mg) prepared from PEC-TOM-P03 in this studyis 2.1 ± 0.1 μm (Figure 2a and Table 1d), and the bulk density is
calculated as 1.47 ± 0.07 g cm−3 (Figure S2). This value is 38 ±2% of the density of anatase-type TiO2 crystals,37 indicating theformation of a relatively porous but suﬃciently ﬂat TiO2 ﬁlm on
the ITO glass.
The TiO2 ﬁlm (0.4 mg) prepared from JRC-TIO-06 is
inhomogeneous and comprises dispersed nanoparticles withdiameters of 2−20 μm (Figure 2b1), unlike the ﬂat ﬁlm preparedfrom PEC-TOM-P03 (Figure 2a). The mean thickness is 1.8 ±0.5 μm taking the uncovered ITO part of the ﬁlm area into
account (Figure 2b1−3 and Table 1e). Therefore, the bulk
density of these ﬁlms is 1.7 ± 0.5 g cm−3 (Figure S2). This value
is 40 ± 12% of the density of rutile-type TiO2 crystals (4.25 gcm−3),37 indicating an extensively loose packing arrangement. In
comparison, a ﬁlm formed via the slide method starting from
JRC-TIO-6 became more ﬂat (mean thickness 2.4 ± 0.3 μm,
Figure 2b′) and the bulk density of the ﬁlm is 1.3 ± 0.2 g cm−3
(Figure S2). The slide method successfully controlled the ﬁlm so■

RESULTS AND DISCUSSIONUV−Visible Spectra of Organic Dyes. UV−visible
absorption spectra were obtained for ethanol solutions of theorganic dyes to evaluate absorption wavelength for PEC-TOMP04 and the herbal tea extracts (Figure 1). Standard
pelargonidin chloride solution shows a major peak at 532 nm
and a shoulder peak at 430 nm in the visible-light range. The
color of the solution was complementary red and the molecule isprotonated at the 4′-position of the phenyl ring (Scheme 1b).
The 0.030 mM ethanol solution of PEC-TOM-P04 shows aresolved peak at 415 nm in the visible-light region (Figure 1).
The ethanolic rosehip/hibiscus herbal tea extract shows a broad
peak at 553 nm accompanied by a shoulder feature at 452 nm in
the visible-light range.
The absorption peak position depends primarily on the
structure of the aglycone (Scheme 1b−d) and the pH of thesolution (b and b′). To check absorption peak position for eachanthocyanidin, absorption spectra of the ethanol extract of therosehip ﬂower and that of the hibiscus ﬂower were measured
independently (Figure 1d and e). A peak at 456 nm appeared forthe cyanidin in rosehip ﬂowers corresponding to a shoulder
feature at 452 nm in rosehip/hibiscus tea (Figure 1c). A peak at
547 nm appeared for the delphinidin in hibiscus ﬂowers
corresponded to the peak at 553 nm in rosehip/hibiscus ﬂowers(Figure 1c).
The structural diﬀerences between pelargonidin, cyanidin,
and delphinidin is the number of hydroxy groups on the phenyl
ring, i.e., one, two, and three, respectively, and these lead todiﬀerences in absorption peak position, i.e., 500 nm forpelargonidin 3-O-glucoside, 510 nm for cyanidin 3-O-glucoside,and 516 nm for delphinidin 3-O-glucoside.35 Thus, theabsorption peak shift of ∼100 nm in ethanol shown in Figure
1 could be attributed to the eﬀects of organic acid contained in
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as to be more ﬂat and eﬀectively removed powders greater than
10 μm (Figure 2b and b′).
The bulk density of TiO2 could be controlled to ∼3.5 g cm−3
for the TiO2 ﬁlms of mean thickness greater than 4 μm, i.e. theweight greater than 1.8 mg (Figure S2). However, if the TiO2
weight is less than 1.8 mg (Figure 2a and b), the bulk density of
TiO2 varied between 1.3 and 3.5 g cm−3 depending on the
degree of coagulation of primary and/or secondary nanoparticles and mixed polyethylene glycol (Scheme 2).
Scheme 2. Conceptual Explanations for the Eﬀects of PrimaryParticle Size of TiO2 and Mixed PEG in the Photocatalyst
Film Used on the Anode of SC on the Photocurrent

Figure 3. CV results for several diﬀerent WEs in aqueous solutions of
pH 4.0 under N2 ﬂow with either irradiation by UV−visible light (red/thin) or in the dark (blue/thick). The source of TiO2 in thephotocatalyst was (a) JRC-TIO-14, (b) JRC-TIO-15, (c) P25, (d)PEC-TOM-P03, (e) JRC-TIO-6, and (f) rutile-W. The light wasapplied perpendicularly from the photocatalyst side.

JRC‐TIO‐6 (101 μ A) > P25 (79 μ A)
∼ JRC‐TIO‐15 (79 μ A) ∼ rutile‐W (77 μ A)
> PEC‐TOM‐P03 (68 μ A) > JRC‐TIO‐14 (60 μ A)
(4)

The photocurrents shown above are not simply correlated tothe speciﬁc SA, mean primary particle size, or crystalline phase(Table 1). The optimum mean particle size in terms of current is15−21 nm due to the balance of two competing factors for theelectron conductivity in TiO2 ﬁlm, i.e., (a) fast electron
conductance in the primary particles of TiO2, for which larger
particles are favorable (Scheme 2a) and (b) eﬀective packing ofprimary particles as a ﬁlm to minimize the resistance at theirinterfaces, for which smaller particles are favorable (Scheme 2b).
Nonconducting polyethylene glycol (PEG) was added to the
PEC-TOM-P03 (Table 1d) and would be disadvantageous interms of electron conductivity (Scheme 2c) compared to thepolymer-free sources JRC-TIO-6, P25, JRC-TIO-15, and rutileW (Table 1b, c, e, and f).
An electrochemical increment in the oxidative wave in therange >1.6 V for water oxidation is clearly observed using P25(Figure 3c) and JRC-TIO-6 (Figure 3e) in the dark. However,the peaks recorded in light become weaker and less clear thanthose recorded in the dark because water oxidation proceeds
both electrochemically and photocatalytically in the range lower
than 1.6 V.
Impedance Measurements of Photocatalyst Films. The
impedance of the TiO2 ﬁlms was evaluated based on the Cole−
Cole plots (Figure 4a). The resistance values were evaluatedbased on eq 5:

In a previous study,3 the bulk density of a relatively porous
BiOCl ﬁlm (2.5 mg) was 3.6 g cm−3 based on the cross-sectional
SEM images (mean thickness 5.4 μm). This value is 46% that oftetragonal BiOCl crystals (7.784 g cm−3),38 demonstrating theformation of a loose BiOCl ﬁlm on the ITO glass. In contrast, thethinner BiOCl ﬁlm in this study (0.6 mg) is relatively dense andﬂat with a mean thickness of 0.83 ± 0.24 μm (Figure 2c andTable 1g). The bulk density is thus 5.6 ± 1.8 g cm−3, which is 72± 23% that of tetragonal BiOCl crystals,38 and 1.57-times thatreported from the previous study (2.5 mg).3
CV Measurements of Several Types of TiO2 Film. CV
measurements using several types of TiO2 ﬁlm under N2
atmosphere in the presence or absence of UV−visible lightirradiation were performed (Table 1a−f). In this study, in the SC
comprising photocatalysts on both the anode and cathode, theelectrons photoexcited to the CB in TiO2 at the anode ﬂow into
the VB in BiOCl at the cathode via the external circuit. Based on
the diﬀerence of the two levels, the electromotive force is always
greater than 1 V.3,15,16 Therefore, the diﬀerence in currents
recorded in the presence and absence of UV−visible light for theCV region >1 V generally indicates the eﬃciency of charge
separation and the conductivity of photogenerated charges from
TiO2 to the cathode. An increment in current due toelectrochemical/photocatalytic oxidation of water is also
observed in the oxidation wave in the region above 1.6 V(Figure 3).
The diﬀerence in currents recorded in the presence andabsence of UV−visible light at 1 V versus SHE is in the order:(impedance real − a)2 + (impedanceimaginary − b)2i resistance yz
zz
= jjj
2
k
{

2

(5)

If the sample behaves as an ideal equivalent circuit comprising aparallel resistor and condenser, a perfect semicircle is drawn inthe ﬁrst quadrant. However, the impedance of TiO2 ﬁlms in thisstudy is aﬀected by the particle size distribution of TiO2 (Table
1) and the eﬀectiveness of the electric contact between TiO2
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reported for a TiO2 layer with no bias potential, as in thisstudy.39 The only exception is PEC-TOM-P03. In the previous
section on CV, the addition of PEG negatively reduced theelectron conductivity by more than 33 μA (order 4, Figure 3d,
and Scheme 2c). The ﬂatness of the photocatalyst ﬁlm would be
the other factor to reduce the resistance because the impedance
measurements were performed by attaching a ﬂat Cu plate tophotocatalyst ﬁlm. As is evident from the cross-sectional SEM
images, the PEC-TOM-P03 ﬁlm is signiﬁcantly more ﬂat andsmooth in comparison to the JRC-TIO-6 ﬁlm (Figure 2a and b).
Thus, while the CV performance of the TiO2 ﬁlm wasdetermined by the primary particle size (15−21 nm) in theprevious section (Scheme 2a and b), the eﬀects of the added
PEG and potential eﬀects of the ﬂatness of the TiO2 ﬁlm shouldbe essential for the impedance values for PEC-TOM-P03(Scheme 2c and d).
Similarly, the resistances of the BiOCl ﬁlms were also
evaluated (Figure 4b). The value of 206 MΩ for the 2.5 mg
ﬁlm in a previous study3 was decreased to 108 MΩ for the more
ﬂat 0.6-mg ﬁlm prepared in this study (Table 1g).
18
O2 Exchange Reaction Rates. The time courses for theexchange of 18O2 with O (oxo, hydroxy) and/or O of water on
diﬀerent TiO2 surfaces are summarized in Figure 5a−f. The
decrease in 18O2 balances with the sum of the increases in
18 16
O O, and 16O2, except in the case of PEC-TOM-P03, forwhich the decrease in 18O2 is signiﬁcantly faster, suggesting

Figure 4. Cole−Cole plots for TiO2 (a) and BiOCl ﬁlms (b) and thetheoretical data ﬁtted to eq 5.

nanocrystals. Thus, the value b needs to be included in eq 5, andwe tried to ﬁt the plot data to a complete circle. The resulting
order of resistance was
PEC‐TOM‐P03 (85 MΩ) ∼ JRC‐TIO‐6 (91 MΩ)< rutile‐W (120 MΩ) ∼ P25 (137 MΩ)∼ JRC‐TIO‐15 (137 MΩ) < JRC‐TIO‐14 (247 MΩ)(6)This order generally corresponds to that of the CV measurements divided into three groups, (i) JRC-TIO-6, (ii) rutile-W,
P25, and JRC-TIO-15, and (iii) JRC-TIO-14 (see order 4). A
similar charge transfer resistance value (7.1 MΩ) has been

Figure 5. Results of 18O2 exchange experiments using several TiO2 and BiOCl photocatalysts (0.135 g) under irradiation by UV−visible light. Initial
18
O2 pressure: 0.55 kPa. (a) JRC-TIO-14, (b) JRC-TIO-15, (c) P25, (d) PEC-TOM-P03, (e) JRC-TIO-6, (f) rutile-W, and (g) BiOCl.
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Table 2. Results of 18O2 Exchange Experiments
Initial decrement/formation rate
(μmol h−1 gcat−1)
entry
a
b
c
d
e
f
g

18

sample
TiO2

O2

−445
−135
−156
−78
−6.0
−32
−3.2

JRC-TIO-14
JRC-TIO-15
P25
PEC-TOM-P03
JRC-TIO-6
rutile-W

BiOCl

surface O

O16O

16

O2

calculated O atom ratio in 18O2 introduced vs surface O

nm−2μmol 0.135 g−1

315
103
106
17
6.4
34
1.9

154
20
39
3.0
0.98
1.4
0.13

1: 6.0 ± 0.2
1: 1.34 ± 0.02
1: 1.47 ± 0.02
1: 1.26 ± 0.05
1: >0.34 ± 0.02
1: 0.17 ± 0.01
1: >0.12 ± 0.01

8.3
11.4
9.9
6.5
>1.5
8.7
>4.7

560
130
140
120
>32
16
>11

18

excessive adsorption of 18O2 rather than a balanced exchange.
The formation rates of 18O16O are in the following order:

limit. Thus, the reverse reaction of the O2 formed on the TiO2
with protons and electrons, reforming water molecules, issigniﬁcant. In this study, the SC utilizing photocatalysts on bothelectrodes forms electrons as the result of water oxidation thatquickly transfer to the cathode via the external circuit, thus
eﬀectively suppressing the reverse reaction on the anode(Scheme 3).

JRC‐TIO‐14 (315 μmol h−1 gcat−1) > P25 (106 μmol h−1 gcat−1)∼ JRC‐TIO‐15 (103 μmol h−1 gcat−1)> rutile‐W (34 μmol h−1 gcat−1)> PEC‐TOM‐P03 (17 μmol h−1 gcat−1)> JRC‐TIO‐6 (6.4 μmol h−1 gcat−1)

Scheme 3. Energy Diagram for a Solar Cell Comprising TiO2
Enhanced with Pelargonidin on the Anode and BiOCl on the
Cathode(7)

This order is not directly correlated to that of speciﬁc SA.
However, O2 exchange is faster in general on anatase-type TiO2
than that on rutile-type TiO2 (Table 1).
When the ratio of the number of O atoms in the gas-phase18
O2 (18O 97.8%) to the 16O species on TiO2 (naturalabundance 16 O 99.8%) is 1:n, the molar ratio of18
O2:18O16O:16O2 reaches 1:2n:n2 at the equilibrium of theexchange reaction. Thus, the n values were determined for six
types of TiO2 (Table 2). Furthermore, the number of O atoms in
the unit area (nm2) on the TiO2 samples was calculated to be
6.5−11.4 based on the n and speciﬁc SA values (Table 2a−d, f),but this number is quite small (1.5) for JRC-TIO-6 because theO2 exchange reaction in this case is extremely slow and does notreach equilibrium within 28 h (Figure 5e). A simpliﬁed kinetic
equation for the 18O2 exchange rates is
r = kP(18 O2 )Nθ(16 O)

(8)
18

18

where k is the rate constant, P( O2) is the pressure of O2, N is
the total number of surface sites, and θ(16O) is the ratio of 16O
surface sites to N. P(18O2) and catalyst amount are constant in
the exchange tests in Figure 5, and the order of the formation
rates of 18O16O needs to be reinterpreted by dividing the rates(μmol h−1 gcat−1) with the Nθ(16O) value (mol) based on the nvalue given in Table 2.

i−V Dependence of SC Performance for Diﬀerent TiO2
Photocatalysts. First, i−V dependence was investigated forseveral kinds of TiO2 photocatalysts (Table 1a−f). The
photocatalyst on the cathode was always BiOCl. In ref 3,
PEC-TOM-P03 (2.0 mg) and BiOCl (2.5 mg) formed on ITO
electrodes using water as the solvent (Figure 6a) provided a VOCof 1.85 V. However, two problems remained in the previous
study:3 (i) non-negligible current leakage in the region below 0.7
V owing to electron ﬂow from the CB of TiO2 directly to oxygen
vacancy sites on/in BiOCl via the external circuit and (ii) linear
i−V dependence in the region 1.0−1.85 V (Wmax 42.2 μW cm−2,
Table 3i) in comparison to the typical convex curve for Si SCs,
increasing Wmax as the product area of the i and V values.
These two problems are well resolved using thinnerphotocatalyst ﬁlms in this study. For the thinner TiO2 ﬁlms,
the electron diﬀusion overvoltage in the TiO2 and the electrons
ﬂowing from the anode eﬀectively coupled with holes remaining
in the VB of BiOCl for the rear conﬁguration of the cathode(Scheme 3, right). Using PEC-TOM-P03 (0.4 mg) and BiOCl(0.6 mg) formed on ITO electrodes, the current leakage isnegligible in the lower voltage region and a convex i−V

rutile‐W (0.30 h−1) > JRC‐TIO‐15 (0.11 h−1) ∼ P25 (0.10 h−1)
> JRC‐TIO‐14 (0.076 h−1) > PEC‐TOM‐P03 (0.020 h−1)
∼ JRC‐TIO‐6 (< 0.027 h−1)(9)The exchange reaction was even slower using BiOCl
compared to those with TiO2. The reaction did not reach theequilibrium within 38 h elapsed, suggesting that the number of
O atoms per unit area (nm2) of BiOCl is more than 4.7 (Table
2g).
The photooxidation of H218O at 0.25−1.8 kPa using the TiO2
samples listed in Table 1 was assessed. The conversion is below
the detection limits for 18O2 and 18O16O using JSM-Q1050GC(data not shown). Furthermore, when the TiO2 is impregnatedwith 3.0 wt % Ag+, oxidation of H218O is still below the detection
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In other words, the Vmax values do not vary much, all falling in
the range 1.23−1.36 eV (Table 3a−f). The order of Wmax values(order 8) is consistent with the order of photocurrents obtained
from CV measurements (order 4) except for that of PEC-TOMP03. The electron conductivity in the TiO2 ﬁlm, rather than thenumber of active sites on the TiO2, is a critical factor for SC
performance in this study (Scheme 2a and b). However, thehomogeneity of TiO2 ﬁlms prepared from PEC-TOM-P03(Figure 2A) is also critical (Scheme 2c and d) because it leads tolower resistance (order 6 and Figure 4).
The VOC and Wmax values obtained using JRC-TIO-6 on thephotoanode and BiOCl on the photocathode (1.93 V and 56.1
μW cm−2) were moderately improved in comparison toreported values of VOC of 1.91 V and Wmax of 42.2 μW cm−2.3
The Wmax value by the SC free from fuel (56.1 μW cm−2) iscomparable to that previously reported for a phenol-FC using
CdS on the photoanode and Cu2O on the photocathode (60 μW
cm−2) and slightly smaller than those reported for a phenol/
glucose-FCs using TiO2 on the photoanode and C/Cu2O/Cu
on the photocathode (73 μW cm−2) or W:BiVO4 on thephotoanode and polyterthiophene on the photocathode (82 μW
cm−2).10
The exchange of 18O2 with 16O2 or surface O species has beencorrelated with catalytic activity in the total oxidation of
methane,40 N2O decomposition,41 NO x reduction withalkanes,42 and the CO−NO reaction.43 Furthermore, fundamental studies of this exchange in terms of basic sites44 or
Raman shifts45 have also been reported. Interestingly, in thecurrent study, the Wmax values (order 8 and Table 3) arenegatively correlated to 18O16O formation rates in the 18O2
exchange tests (order 7 and Figure 7a). However, the Wmax
values did not correlate to the formation rates if the numbers of
surface O atom in photocatalyst (Table 2) are considered (eq 9)in the kinetics (eq 8). Instead, the numbers of surface O atom
was plotted versus Wmax values and negative correlation was
clearly demonstrated (Figure 7b). Thus, water photooxidation

Figure 6. Current−voltage dependence using (a) PEC-TOM-P03 (2.0mg, 0.4 mg ■) and (b) JRC-TIO-14 (◇), JRC-TIO-15 (□), P25 (○),
PEC-TOM-P03 (■), JRC-TIO-6 (△), and rutile-W (▲) TiO2
photocatalysts on the anode. BiOCl was used on the cathode.

dependence curve is formed, providing a Wmax of 53.7 μW cm−2
while keeping the VOC value the same, i.e., 1.85 V (Table 3d andFigure 6a). Furthermore, using the same amounts of photocatalyst on the anode and cathode, the i−V curves are convex
and current leakage is negligible for all the types of TiO2
employed (Table 1a−f and Figure 6b). Thus, in the current
study, t-butanol as solvent and ethylene glycol as a thickener areeﬀective for the formation of thin TiO2 ﬁlms. The VOC values are
divided into three groups: (i) JRC-TIO-6 (1.93 V), (ii) JRCTIO-15, JRC-TIO-14, P25, and PEC-TOM-P03 (1.88−1.85 V),
and (iii) rutile-W (1.76 V). However, the maximal diﬀerence
between them is only 8.8%. Conversely, the imax values are in thesame order as that for Wmax and the diﬀerence is as large as 30%.
The order of Wmax values is
JRC‐TIO‐6 (56.1 μ W cm−2) > PEC‐TOM‐P03 (53.7 μ W cm−2)> rutile‐W (49.5 μ W cm−2) ∼ P25 (47.4 μ W cm−2)∼ JRC‐TIO‐15 (45.6 μ W cm−2)> JRC‐TIO‐14 (39.9 μ W cm−2)(10)

Table 3. Summary of i−V Dependence Tests
Wmax

photocatalyst
entry
a
b
c
c’
c”
c”’
c””
d
e
f
g
g’
g”
g’’’
h
h’
h”
i

anode
JRC-TIO-14 (0.4 mg)
JRC-TIO-15 (0.4 mg)
P25 (0.4 mg)
P25 (0.4 mg) + PEC-TOM-P04
P25 (0.4 mg) + rosehip−hibiscus
ﬂowersP25 (0.4 mg) + rosehip ﬂowersb
P25 (0.4 mg) + hibiscus ﬂowersb
PEC-TOM-P03 (0.4 mg)
JRC-TIO-6 (0.4 mg)
rutile-W (0.4 mg)
JRC-TIO-15 (0.4 mg)

P25 (0.4 mg)
P25 (0.4 mg) + PEC-TOM-P04
P25 (0.4 mg) + rosehip−hibiscus
ﬂowersPEC-TOM-P03 (2.0 mg)

cathodeBiOCl (0.6 mg)

light source power (W)Xe arc (500)

(450)
(400)
(300)
LED (3)Xe arc (500), L39 ﬁlter

BiOCl (2.5 mg)

Xe arc (500)iSC(μA cm−2)

VOC
(V)

Wmax
(μW cm−2)

imax(μA cm−2)

Vmax
(V)

41.5
65.7
54.0
78.0
69.2

1.88
1.88
1.87
2.00
2.04

39.9
45.6
47.4
73.1
64.7

31.9
34.5
34.9
53.5
45.5

1.25
1.32
1.36
1.37
1.42

82.7
85.2
49.9
57.9
55.5
47.3
43.3
26.5
<0.01
21.0
21.6
24.4

2.04
2.11
1.85
1.93
1.76
1.86
1.86
1.82
0.23
1.77
1.82
2.05

64.5
72.9
53.7
56.1
49.5
36.8
33.2
18.6
<0.002
10.6
16.5
29.4

50.5
53.5
41.2
45.6
40.2
29.0
24.6
11.3
<0.01
8.65
12.7
20.1

1.28
1.36
1.30
1.23
1.23
1.27
1.35
1.65
0.20
1.23
1.31
1.47

65.4a

1.85a

42.2a

41.9a

1.01a

a

Reported in ref 3. bA slide method was employed for pasted TiO2 suspension on ITO.
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increases from 47.4 to 73.1 μW cm−2, i.e., by a factor of 1.54,
mainly due to the increase in imax (53.5 μA cm−2).
The addition of 16 nmol of anthocyanin (delphinidin andcyanidin) extracted from rosehip and hibiscus ﬂowers to P25
was also evaluated in the same way (Figure 9a2). The VOC value
further increases to 2.04 V (Table 3c”). Accordingly, the cell
power also increases from 47.4 to 64.7 μW cm−2, i.e., by a factorof 1.36, mainly due to the increase in imax (45.5 μA cm−2).
The eﬀects of organic dye addition were also evaluated for an
anode under irradiation by light with a wavelength greater than
390 nm (Figure 9b). Most of the UV light is eliminated underthese conditions. The cell power falls to 22.4% that under fullUV−visible light (Table 3c and 3h) for the SC containing thedye-free TiO2 (P25) and BiOCl. When PEC-TOM-P04 is added
to the TiO2 ﬁlm, the cell power falls to 22.6% that under full light(Figure 9b1 and Table 3c’ and 3h’). Thus, the photoexcitation of
TiO2 is the major factor controlling the performance of the SCs,
and the PEC-TOM-P04 dye clearly promotes electron ﬂow
from/to TiO2.
In contrast, when the rosehip/hibiscus extract (16 nmol) is
added to P25, the cell power under visible-only irradiation falls
to 45.4% that under full light (29.4 μW cm−2, Figure 9b2 andTable 3c” and h”), which is signiﬁcantly more than that observed
for the dye-free test (22.4%) or that using PEC-TOM-P04(22.6%). Furthermore, the VOC value under UV−visible light ismaintained at 2.05−2.04 V under full visible light (Figure 9a2
and b2, blue). Thus, the dyes from the tea extract not only
promote electron ﬂow from/to TiO2, but enhance charge
separation in TiO2 and boost the Wmax value.
On the anode, the electrons in the VB in TiO2 are mostly
excited by UV light, whereas organic dyes, e.g., pelargonidin,
adsorbed on the TiO2 nanoparticles are excited by visible light(Scheme 3). The theoretical electromotive force of the SC in
this study is the diﬀerence (2.75 V) between the CBM of TiO2
and the VBM of BiOCl.3,15,16,46 The theoretical electromotive
force is decreased by the overvoltage (0.23 V) due to electron
diﬀusion in the TiO2 layer (Scheme 3, left).3 The organic dyes
are well adsorbed in the TiO 2 and should distributehomogeneously within a variation of 30% in the TiO2 layers(Figure S1). If the excited electrons in the CB of TiO2 ﬂow into
the unﬁlled HOMO of organic dye molecules in which an
electron has already been excited to the LUMO and then theﬂow into the CB of TiO2 again (Scheme 3, left), the eﬀective
diﬀusion path is shortened from water photooxidation sites on
the TiO2−ITO layer to the dye-ITO layer. Thus, the VOC values
increase from 1.87 V to 2.00−2.04 V (Figure 9) upon the
addition of organic dyes.
If we compare the eﬀects of PEC-TOM-P04, rosehip ﬂowers(cyanidin), and hibiscus ﬂowers (delphinidin), the Wmax values
were similar (73.1, 64.5, and 72.9 μW cm−2); however, aprogressive increase in Voc values was observed (2.00, 2.04, and2.11 V, respectively; Table 3c’, c”’, and c””). Thus, the structural
diﬀerences between pelargonidin, cyanidin, and delphinidin in
the number of hydroxy groups on the phenyl ring (one, two, andthree, respectively; Scheme 1) appear to be related to theelectromotive force of SC in this study, e.g., to elevate thepotential of CBM due to the increase of hydroxy groups(Scheme 3, left).
The increase of cell current by organic dyes (Figure 9) is
primarily due to the visible light, which is unutilized by TiO2
ﬁlm, contributing to HOMO−LUMO electron excitation in theorganic dyes, resulting in an increase in total electron ﬂow to theanode (Scheme 3, left). Such eﬀects explain the increase in Wmax

Figure 7. Negative correlation between (a) 18O16O formation rate and(b) surface O amount in photocatalyst in the 18O2 exchange reaction
and Wmax values generated using TiO2 on the anode.

proceeded faster and/or the reverse reaction from O2 to water
was eﬀectively suppressed over TiO2 comprising less exchangeable surface O atoms.
i−V Dependence of SC Output as a Function of
Incident-Light Power. The origin of cell power and thedependence of cell power on incident-light power werereconﬁrmed. Using JRC-TIO-15 and BiOCl photocatalysts, i−V dependence tests were performed by changing the power (x)of the incident Xe arc lamp (300−500 W) and also using LED
light (3 W) (Figure 8a). Based on the dependence observed, it is

Figure 8. (a) i−V dependence as a function of the power of UV−visible
light source using JRC-TIO-15 and BiOCl as the photocatalysts on
anode and cathode, respectively. (b) Current at 0.50 V as a function of
the power of UV−visible light source. Xe arc lamp with a branched ﬁberlight guide was operated between 500 and 300 W and LED lights wereoperated at 3 W.

apparent that light energy is transformed into electricity, and thecurrent at 0.5 V was plotted because of a sudden current rise
below 0.3−0.4 V owing to current leakage and the fact that thecurrent at 0.5 V is a good representation of current due toelectron ﬂow from the CB of TiO2 on the anode to the VB of
BiOCl on the cathode (Scheme 3). The dependence was ﬁt to i =0.000883x1.751 (Figure 8b). The power is nearly 2, suggestingthat the product of the concentration of electrons in the CB ofTiO2 and that of holes in the VB of BiOCl determines thecurrent based on formulation by reaction kinetics.16
Enhanced i−V Dependence of SC Performance by the
Addition of Organic Dyes. The eﬀect of the addition oforganic dyes to TiO2 (P25) upon cell voltage and current wasassessed. The i−V dependence for the SC using a P25 ﬁlm
supplemented with 36 nmol of organic dye (PEC-TOM-P04)
on the anode was compared to that for the SC using dye-free P25
ﬁlm on the anode under irradiation by UV−visible light (Figure9a1). The VOC and iSC values increase from 1.87 to 2.00 V and
54.0 to 78.0 μA cm−2, respectively, upon the addition of theorganic dye (Table 3c and 3c’). Accordingly, the cell power also
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Figure 9. Current−voltage dependence obtained on the anode: (1) using P25 (○) and P25 mixed with PEC-TOM-P04 (◇), (2) using P25 (○) andP25 mixed with ethanol rosehip/hibiscus extract (□), (3) using P25 (○) and P25 mixed with ethanol rosehip extract (□), and (4) using P25 (○) andP25 mixed with ethanol hibiscus extract (□). BiOCl was used on the cathode. Irradiated under (a) UV−visible light and (b) visible light only.

per cell. As UV light excites electrons in the VB to the CB ofTiO2 while visible light excites electrons in the HOMO to the
LUMO of organic dyes, new electron paths from VB to CB to
HOMO to LUMO and then to electrode are created, improving
the performance of the SC. With organic dyes, the charge
separation in the TiO2 is improved and the diﬀusion overvoltage
in the TiO2 layer is suppressed. Finally, electrons transfer via theexternal circuit to the cathode and eﬀectively combine withholes gathered at the interface between the ITO electrode andthe BiOCl ﬁlm layer excited by UV−visible light.

by the addition of PEC-TOM-P04 to TiO2 irradiated by UV−
visible light (by 1.54 times) and by visible light only (λ > 390 nm,
by 1.56 times; Figure 9a1 and b1).
However, this interpretation does not totally explain theincrease in Wmax upon the addition of the rosehip−hibiscus tea
extract to TiO2 irradiated under UV−visible light (by 1.36
times) and under visible light only (λ > 390 nm, by 2.77 times;
Figure 9a2 and b2). The improved charge separation in TiO2 byelectron ﬂow from the CB of TiO2 to the HOMO then LUMO
of the organic dyes is one contributing factor.

■■

CONCLUSIONS
Thin photoelectrodes were formed on ITO glass layers bydeposition of TiO2 to thicknesses of 1.8−2.1 μm for and BiOCl
to a thickness of 0.8 μm. The power eﬃciency of SCs containing
these photoelectrodes critically depends on the electron
conductivity of the TiO2 layer on the anode. TiO2 nanoparticles
of 15−21 nm were found to be favorable based on the balance
between the primary particle size, where larger is better, and theparticle−particle interface area, where smaller particles are
preferred. The electron conductance in commercial TiO2 fordye-sensitized SCs was worse than those of TiO2 types withcomparable primary particle sizes, but its impedance is the best
among all the TiO2 types tested in this study because
nonconducting polyethylene glycol is added and thus theresulting ﬁlm layer is quite ﬂat (thickness 2.1 ± 0.1 μm). The
eﬃciency of the TiO2 ﬁlm layer in SCs can be critically evaluatedbased on the amounts of 18O2 exchange with surface O species
over the TiO2 because the number of exchangeable O sites on
TiO2 negatively correlates with the rate of water photo-oxidation
at the anode owing to the contribution to the reverse reaction
from O2 to water. Thinner TiO2 ﬁlm layers are favorable for SCs
with higher Wmax and VOC values, and the addition of organicdyes, e.g., anthocyanins, on/in the TiO2 layer signiﬁcantly
increases the Wmax and VOC values, i.e., 73.1 μW cm−2 and 2.11 V

ASSOCIATED CONTENT

S Supporting Information*

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acssuschemeng.8b02166.
Color change of dropped organic dye extracted fromrosehip/hibiscus ﬂowers on TiO2 (P25) for 1 h and thecorrelation between ﬁlm thickness and the bulk density of
TiO2 ﬁlms (PDF)■

AUTHOR INFORMATION

Corresponding Author*Phone +81-43-290-3696, fax +81-43-290-2783, e-mailyizumi@faculty.chiba-u.jp.
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
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ACS Sustainable Chemistry & Engineering

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Research Promotion Program (2015−) from the Institute for
Global Prominent Research, Chiba University.

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