molecules
ArticleSynthesis of Mesoporous TiO2/Boron-Doped
Diamond Photocatalyst and Its Photocatalytic Activity
under Deep UV Light (λ = 222 nm) Irradiation
Norihiro Suzuki 1, *,† , Akihiro Okazaki 2,† , Haruo Kuriyama 2 , Izumi Serizawa 2 , Aiga Hara 3 ,Yuiri Hirano 3 , Yukihiro Nakabayashi 1,‡ , Nitish Roy 1,§ , Chiaki Terashima 1 , Kazuya Nakata 1,3 ,Ken-ichi Katsumata 1 , Takeshi Kondo 3 , Makoto Yuasa 3 and Akira Fujishima 1
1

2
3

*
†
‡

§

Photocatalysis International Research Center, Research Institute for Science and Technology,
Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan; asdfghjk@tmu.ac.jp (Y.N.);
nitu.iitg@gmail.com (N.R.); terashima@rs.tus.ac.jp (C.T.); nakata@rs.tus.ac.jp (K.N.);
k.katsumata@rs.tus.ac.jp (K.-i.K.); fujishima_akira@rs.tus.ac.jp (A.F.)
ORC Manufacturing Co., Ltd., 4896 Tamagawa, Chino, Nagano 391-0011, Japan;
a-okazaki@orc.co.jp (A.O.); h-kuriyama@orc.co.jp (H.K.); i-serizawa@orc.co.jp (I.S.)
Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda,
Chiba 278-8510, Japan; 7216658@alumni.tus.ac.jp (A.H.); 7216661@alumni.tus.ac.jp (Y.H.);
t-kondo@rs.noda.tus.ac.jp (T.K.); yuasa@rs.noda.tus.ac.jp (M.Y.)
Correspondence: suzuki.norihiro@rs.tus.ac.jp
These authors contributed equally to this work.
Present address: Faculty and Graduate School of Urban Environmental Sciences, Department of Applied
Chemistry for Environment, Tokyo Metropolitan University, 1-1 minami-Osawa, Hachioji,
Tokyo 192-0397, Japan.
Present address: Department of Chemistry, University of North Bengal, Raja Rammohunpur,
Darjeeling, West Bengal-734013, India.

Received: 16 October 2018; Accepted: 19 November 2018; Published: 27 November 2018


Abstract: There is a need for highly efficient photocatalysts, particularly for water purification.
In this study, we fabricated a mesoporous TiO2 thin film on a boron-doped diamond (BDD) layer by asurfactant-assisted sol-gel method, in which self-assembled amphiphilic surfactant micelles were used
as an organic template. Scanning electron microscopy revealed uniform mesopores, approximately
20 nm in diameter, that were hexagonally packed in the TiO2 thin film. Wide-angle X-ray
diffraction and Raman spectroscopy clarified that the framework crystallized in the anatase phase.
Current–voltage (I–V) measurements showed rectification features at the TiO2 /BDD heterojunction,
confirming that a p–n hetero-interface formed. The as-synthesized mesoporous TiO2 /BDD worked
well as a photocatalyst, even with a small volume of TiO2 (15 mm × 15 mm × c.a. 1.5 µm in thickness).
The use of deep UV light (λ = 222 nm) as a light source was necessary to enhance photocatalytic
activity, due to photo-excitation occurring in both BDD and TiO2 .
Keywords: surfactant-assisted sol-gel method; mesoporous metal oxide; thin film; photocatalyst;
p-n heterojunction; water purification; deep UV light

1. Introduction
Water shortages are becoming urgent issues, owing to population expansion and rapid economic
growth in rising/developing countries. In such countries, the infrastructure for water purification isoften inadequate and people may be forced to use polluted water. However, installing a conventional
water purification system requires a considerable amount of time and investment, which has

Molecules 2018, 23, 3095; doi:10.3390/molecules23123095

www.mdpi.com/journal/molecules

Molecules 2018, 23, 3095

2 of 9prevented uptake. Therefore, there is a need for easily installable, simple, and inexpensive water
purification systems.
Photocatalysts are a low cost environmental clean-up technology, which are already widely usedin air purification. However, photocatalytic water purification is more difficult (compared to air
purification), because of its low efficiency. One reason for this low efficiency is that the light intensity
at the photocatalyst surface is weakened by the initial absorption of light by water, lowering its
intensity. Another reason is that molecules diffuse slower in water, which decreases the effectivenessof adsorption of pollutants and desorption of decomposed molecules.
To overcome these drawbacks highly efficient photocatalysts are needed, and several approaches
to develop such photocatalysts have been tested. One method involves introducing porosity into
the photocatalyst [1,2], with the aim of increasing the reactive surface area. In mesoporous Nb2 O5
photocatalysts, crystallites are exposed to mesopores, which promotes access to the active sites.
Photocatalytic activity can be enhanced by optimizing the porous structure [1]. Hybridization of
nonporous photocatalyst nanoparticles with porous ceramic nanoparticles is another effective way of
introducing porosity. Nanocomposites with TiO2 nanoparticles and mesoporous SiO2 nanoparticles
can adsorb dyes more effectively, owing to their large surface areas. These structural features make theinitial reaction rates of nanocomposites faster than those of nonporous TiO2 [2].
An alternative approach is to form p–n hetero-interfaces. When a p–n heterojunction is createdwith appropriate energetics, band bending takes place at the p–n heterojunction (to maintain the
Fermi level position), and an energy gradient is formed. This gradient promotes photocarrier
separation (i.e., suppresses photocarrier recombination), increasing the photocatalytic activity. Notably,
hybridization of an n-type metal oxide semiconductor (ZnO [3], TiO2 [4,5]) with p-type boron doped
diamond (BDD) has produced catalysts with high photocatalytic activity [3–5]. Nanostructuring has
also been used to increase the reactive surface area and provide an electric path for photogenerated
carriers to reach reaction surfaces [3,5].
In this study, we fabricated mesoporous TiO2 thin film by a surfactant-assisted sol-gel method,in which we used self-assembly of amphiphilic surfactant micelles as an organic template [6,7] on the
BDD layer. The large surface area of the mesopores and promotion of photocarrier separation at p–nheterojunction improved the photocatalytic activity of our TiO2 catalysts. Furthermore, we used deep
UV light (λ = 222 nm) as a light source. The band gap of BDD is 5.5 eV, and so electron transitions fromthe valence band (VB) to the conduction band (CB) occur when irradiated by deep UV light with awavelength shorter than 225 nm. In previous studies, an Hg lamp with a peak excitation at 365 nm
was used as a light source [4]. This light source can promote electronic transitions in TiO2 , which hasband gaps of 3.0 and 3.2 eV for rutile and anatase, respectively, although it has less effect on BDD.
Therefore, previous studies have not used BDD to its full potential as a photocatalyst. In this study,
we aimed to increase the number of photocarriers through photo-excitation of both TiO2 and BDD.
2. Results and Discussions
Figure 1 shows a top-view scanning electron microscope (SEM) image of the synthesized TiO2
film. As seen in the previous studies [8,9], hexagonally packed mesopores approximately 20 nm
in diameter formed (Figure 1a). However, unlike previous studies, we observed several cracks
in the low magnification images (Figure 1b). There are several possible reasons for this cracking.
First, the roughness of the underlying BDD layer: As shown in Figure S1, the BDD layer was not
smooth. Thus, the precursor solution was unevenly coated onto the BDD layer in the synthesis process.
Even if the as-prepared film was coated evenly, the calcination process might also cause breakage of thefilm. Shrinkage in the vertical direction of films typically occurs during the calcination process [10,11].
Compared with our previous studies in which the film thickness was several hundred nm, the thicknessof these as-prepared films was much thicker (i.e., on the order of µm). Therefore, the strain applied
to the film during the calcination process was more pronounced. Furthermore, because diamond
has a higher thermal conductivity (1000–2200 Wm−1 K−1 ) and a lower thermal expansion coefficientMolecules 2018, 23, 3095
Molecules
Molecules 2018,2018, 23,
23, xx FOR
FOR PEER
PEER REVIEWREVIEW

3 of 9

33 of
of 99

−6 K−1) than Si (thermal conductivity and thermal expansion coefficient at room
coefficient−6 K−×−6 KSi−1)(thermal
coefficient(1.18×1 )1010
than Si conductivity(thermal conductivity
thermalcoefficient
expansioncoefficientat room(1.18 × 10(1.18
than
and thermaland
expansionat room
temperature−1K−1 and
−6 K−1, respectively) [12], a large thermal/mechanical stress−1−
1
temperature
of
153
Wm
−1K−1
−6 K−1, respectively)of 153 Wm ofK153 and× and10−63.8
K−×
respectively)[12], a large
thermal/mechanicalstress actsstresson
temperature
Wm3.8
3.8
×1 , 1010[12],
a large thermal/mechanical
acts
on
the
TiO
2
/BDD
interfaceduringthecalcination
process.
However,
exposure
of
the
BDD
layer
theon
TiO
interfaceduringduring
the calcination
process.process.
However,
exposure exposureof the BDDoflayer
throughacts
theTiO2/BDD
interface
the calcination
However,the BDDlayer
2 /BDDthroughthesecracksisimportantforphotocatalyst
applications.
Because
deep
UV
light
(λ
=
222
these
cracksisimportantforphotocatalyst
applications.
Because
deep
UV
light
(λ
=
222
nm)
can
reach
through these cracks is important for photocatalyst applications. Because deep UV light (λ = 222 nm)
nm)
can
reach
BDD
surface,BDD,asas2, becomes possible.
the
BDDthesurface,in BDD, asin
as TiO
2 , becomes
can
reach
the
BDD photo-excitation
surface, photo-excitation
photo-excitation
inwellBDD,
as wellwell
as TiOTiOpossible.
2, becomes possible.

Figure1.
(a)High-magnificationand (b)low-magnification top-view
SEM
imagesthesynthesized
Figure1. 1.
(a)(a)
High-magnification
top-viewSEM
SEMimages
imagesofof
ofthesynthesized
FigureHigh-magnificationandand(b)(b) low-magnification
low-magnification top-view
thesynthesized
2 thin film on the boron doped diamond (BDD) layer.
mesoporousTiO
2 thindiamond(BDD)(BDD)layer.
layer.
mesoporous
mesoporousTiO
TiO
thinfilm
filmononthe
theboronboron doped diamond2

Wewide-angle
X-ray
diffraction(WAXD)
totheof
Weusedused wide-angle
diffraction(WAXD)
to examine
the crystallinityof the TiO2 frameworkWeusedwide-angleX-ray
X-ray
diffraction(WAXD)
to examine
examinethe crystallinity
crystallinityof thethe TiO
TiO22
framework
spectra(Figure
2a)
and
Raman
spectra(Figure2b).
Severaldiffraction
peaks
derivedfromspectra(Figure
2a)
and
Raman
spectra(Figure2b).
Severaldiffraction
peaks
derivedfromtheanataseframework spectra (Figure 2a) and Raman spectra (Figure 2b). Several diffraction peaks derived fromof TiOBDD
layer
appearedin theappeared
prepared insample,revealing sample,
that the frameworkof thethetheanatase
phase
TiO
2 and
BDD layer
the preparedrevealing that
2 andofthephaseanatase
phase
of
TiO
2 and BDD layer appeared in the prepared sample, revealing that theporous
TiO
layer
was
crystalline.
We
examinedtheelectricalproperties
ofthemesoporous
TiO
/BDD,
frameworkoftheporousTiO
2 layerwascrystalline.
We
examinedtheelectricalproperties
2
2
framework of the porous TiO2 layer was crystalline. We examined the electrical properties ofof the
the
and
their
I–V
curvesareshownin
Figure3.
Rectifying
properties
were
clearlyobserved,
indicatingthemesoporous
TiO
2/BDD, and their I–V curves are shown in Figure 3. Rectifying properties were clearlymesoporous TiO2/BDD, and their I–V curves are shown in Figure 3. Rectifying properties were clearly
formation of a p–n heterojunction.
observed,
observed, indicating
indicating thethe formation
formation ofof aa p–n
p–n heterojunction.
heterojunction.

Figure2.
(a) Wide-angle
diffraction(WAXD)and (b)andRaman
mesoporous
FigureWide-angle
X-ray
diffraction(WAXD)(b)Ramanofmesoporous
2 /BDD.
Figure2.2. (a)(a)
Wide-angleX-ray
X-ray
diffraction(WAXD)
and(b)spectrum
Raman ofspectrum
spectrumof TiO
mesoporous
As2/BDD.
a reference,
spectra of TiO(anatase)and
BDD arealso
included.
Asareference,
spectraofTiO
2 (anatase)
and
BDD
are
also
included.
TiO
2
TiO2/BDD. As a reference, spectra of TiO2 (anatase) and BDD are also included.

We examined the photocatalytic activity for the decomposition of methylene blue (MB) by
evaluating changes of the absorption spectra over time. The time course of the absorption spectraof the MB aqueous solution containing mesoporous TiO2 /BDD is shown in Figure 4. In the dark
(Figure 4a), the peak intensity of the MB absorption spectra decreased slightly at first and then remained
constant, indicating that the system had reached adsorption equilibrium. After starting light irradiation(Figure 4b), the absorbance of MB constantly decreased with a blue shift of the adsorption maximum(inset of Figure 4b), owing to photocatalytic decomposition of MB molecules [13]. This clarified thatthe mesoporous TiO2 thin film was photocatalytically active, although the volume of TiO2 was small

Molecules 2018, 23, 3095

4 of 9(15 mm × 15 mm × c.a. 1.5 µm in thickness). The decrease of the MB concentration over the timecourse experiment, as determined from the peak intensity of the absorption spectrum, is shown in
Figure 5a. Compared with mesoporous TiO2 on a glass substrate, the sample on BDD showed superior
photocatalytic activity. Because the film thickness of the mesoporous TiO2 was approximately the same
Figure of
3. theI–V typecharacteristics
of mesoporous
TiO2/BDD heterojunction.
shows the schematic
regardlessof substrate,enhanced photocatalytic
activity inInset
the mesoporous
TiO2 /BDD
illustration of the experimental setup.
was mainly derived from BDD. The effect can be explained as follows: First, as shown in the energy
diagram [Figure 5b], the energy gradient was created at a heterojunction and this gradient promoted
We examined the photocatalytic activity for the decomposition of methylene blue (MB) by
carrier separation. Second, the irradiation with deep UV (222 nm) light photo-excited both TiO2 andevaluating changes of the absorption spectra over time. The time course of the absorption spectra of
BDD. Thus, electron injection from the conduction band of BDD to TiO2 was effective, and more active
the MB aqueous solution containing mesoporous TiO2/BDD is shown in Figure 4. In the dark (Figure
oxygen species (O2 − ) formed by the reduction of dissolved oxygen [5]. In addition, photocatalysis on4a), the peak intensity of the MB absorption spectra decreased slightly at first and then remained
the BDD surface also occurs. As shown in Figure 5a, BDD operates as a photocatalyst under deep UV
constant, indicating that the system had reached adsorption equilibrium. After starting light(222 nm) light. Because some BDD is exposed by cracks (Figure 1b), MB molecules und UV light canirradiation (Figure 4b), the absorbance of MB constantly decreased with a blue shift of the adsorption
reach the BDD surface. Therefore, two parallel processes (photocatalysis on the TiO2 surface, and on
maximum (inset of Figure 4b), owing to photocatalytic decomposition of MB molecules [13]. Thisthe
BDD2018,
surface)occur
simultaneously
in mesoporous TiO2 /BDD.
Molecules
23, x FORPEER
REVIEW
4 of 9clarified that the mesoporous TiO2 thin film was photocatalytically
active, although the volume of
TiO2 was small (15 mm × 15 mm × c.a. 1.5 µm in thickness). The decrease of the MB concentration
over the time course experiment, as determined from the peak intensity of the absorption spectrum,is shown in Figure 5a. Compared with mesoporous TiO2 on a glass substrate, the sample on BDD
showed superior photocatalytic activity. Because the film thickness of the mesoporous TiO2 wasapproximately the same regardless of the type of substrate, enhanced photocatalytic activity in themesoporous TiO2/BDD was mainly derived from BDD. The effect can be explained as follows: First,
as shown in the energy diagram [Figure 5b], the energy gradient was created at a heterojunction andthis gradient promoted carrier separation. Second, the irradiation with deep UV (222 nm) light photoexcited both TiO2 and BDD. Thus, electron injection from the conduction band of BDD to TiO2 waseffective, and more active oxygen species (O2−) formed by the reduction of dissolved oxygen [5]. In
addition, photocatalysis on the BDD surface also occurs. As shown in Figure 5a, BDD operates as aphotocatalyst under deep UV (222 nm) light. Because some BDD is exposed by cracks (Figure 1b),
MB molecules und UV light can reach the BDD surface. Therefore, two parallel processes(photocatalysis
oncharacteristics
the TiO2 surface,and on the
BDD
surface) occur simultaneously
mesoporous
Figure 3.
ofInset shows
shows theinschematic
3. I–V characteristics
of mesoporous
mesoporous TiO
TiO22/BDD/BDD heterojunction. Inset
schematic
TiO2/BDD.
illustration of the experimental setup.

We examined the photocatalytic activity for the decomposition of methylene blue (MB) by
evaluating changes of the absorption spectra over time. The time course of the absorption spectra of
the MB aqueous solution containing mesoporous TiO2/BDD is shown in Figure 4. In the dark (Figure4a), the peak intensity of the MB absorption spectra decreased slightly at first and then remained
constant, indicating that the system had reached adsorption equilibrium. After starting lightirradiation (Figure 4b), the absorbance of MB constantly decreased with a blue shift of the adsorption
maximum (inset of Figure 4b), owing to photocatalytic decomposition of MB molecules [13]. Thisclarified that the mesoporous TiO2 thin film was photocatalytically active, although the volume of
TiO2 was small (15 mm × 15 mm × c.a. 1.5 µm in thickness). The decrease of the MB concentration
over the time course experiment, as determined from the peak intensity of the absorption spectrum,is shown in Figure 5a. Compared with mesoporous TiO2 on a glass substrate, the sample on BDD
showed superior photocatalytic activity. Because the film thickness of the mesoporous TiO2 wasFigure 4. Visible absorption spectra of methylene blue (MB) aqueous solutions sampled fromapproximately the same regardless of the type of substrate, enhanced photocatalytic activity in thethe solution containing mesoporous TiO /BDD at various times (a) in the dark and (b) during
mesoporous TiO2/BDD was mainly derived2 from BDD. The effect can be explained as follows: First,
photocatalytic activity test under 222 nm UV light.
as shown in the energy diagram [Figure 5b], the energy gradient was created at a heterojunction andthis gradientpromotedcarrier separation.
Second,the irradiation
with deep(222 nm)light photoTo clarify
the effectivenessof the deep
UV light(222 nm) source,
we UV
studiedthe wavelength
excited
both
TiO
2
and
BDD.
Thus,
electroninjectionfromtheconduction
band
of
BDD
to
TiO2 wasdependence of photocatalytic activity (Figure 6). When 308 nm UV light was used, no photo-excitation−
effective, in
andthemore
activespecies
2 )eV)
formedby thereductionof dissolved
oxygen[5].eV).
In
occurredBDD
as itsoxygen
band gap(ca.,(O5.5
is muchlargerthan thephoton energy(4.0
addition,
onphotocatalytic
the BDD surface
also occurs.
As shown inTiO
Figure5a,
BDD
operatesasaAsshownphotocatalysis
in Figure 6c, theactivityof the mesoporous/BDD
decreased,because2
photocatalyst
deep
UVof(222nm)
Becausesometraps
BDDinisthe
exposedcracks(Figure1b),electronsin theundervalenceband
BDD
andlight.
electronsin deep
valencebybandof TiO
2 cannotMB molecules und UV light can reach the BDD surface. Therefore, two parallel processes(photocatalysis on the TiO2 surface, and on the BDD surface) occur simultaneously in mesoporous
TiO2/BDD.

photocatalytic activity test under 222 nm UV light.

To clarify the effectiveness of the deep UV light (222 nm) source, we studied the wavelength
dependence of photocatalytic activity (Figure 6). When 308 nm UV light was used, no photoexcitation
occurred
Molecules
2018,
23, 3095 in the BDD as its band gap (ca., 5.5 eV) is much larger than the photon energy5(4.0of 9eV). As shown in Figure 6c, the photocatalytic activity of the mesoporous TiO2/BDD decreased,because electrons in the valence band of BDD and electrons in deep traps in the valence band of TiO2
bephotoexcited(Figure (FigureS2). Importantly,
there waslittledifferencein the photocatalytic
activitycannotbe photoexcitedS2). Importantly,
there
was
little differencein the photocatalyticofmesoporous
TiO
/BDD
and
mesoporous
TiO/glass
under
308
nm
UV
irradiation(Figure6c,d).
2
2 TiO2/glass under 308 nm UV irradiation (Figure
activity of mesoporous
TiO2/BDD and mesoporous
Thus,
heterojunction
only improved
effectivenessof the catalyst
when photocarriers
were excited
6c,d). theThus,
the heterojunction
only improved
effectivenessof the catalyst
when photocarriers
were
in
BDD.
Thus,
photo-excitation
in
BDD
and
electroninjection
into
TiO
have
importantinfluenceson2
excited in BDD. Thus, photo-excitation in BDD and electron injection
into TiO2 have importantthephotocatalytic
activity.
influences on the photocatalytic activity.

Figure 5.
5. (a) Time dependence of the MB
MB concentration,
concentration, as estimatedestimated from thethe absorption
absorption spectra
spectraduring the photocatalytic
activity
testingofmesoporous
TiO
/BDD
under
222
nm.
For
comparison,
photocatalytic activity testing of mesoporous TiO22/BDD underresults of mesoporous
mesoporous TiO
TiO22/glass,/glass,BDD
BDDsubstrate,
substrate,and
andphotolysis
also
included.
C and
C0 arephotolysis
arearealso
included.
C and
C0 arethetheremaining
initialconcentrations
MB,
respectively.(b)(b)Energy
Energydiagram
diagram ofof the mesoporous
remaining
andandinitialconcentrations
of of
MB,
respectively.
mesoporous
Molecules
23,andxand
FOR
PEER REVIEW
6 of 9TiO
transfermechanism ofof photocarriers.
photocarriers.
/BDD
transfermechanism
TiO2018,
22/BDDTime dependence
dependence ofof MB
MB concentration,
concentration, estimatedestimated fromfrom thethe visible
visible spectra during the
Figure 6. Timephotocatalytic
photocatalytic activity
activity testing
testing ofof(a,c)(a,c)mesoporous
mesoporousTiO
TiO2 2/BDD/BDD and (b,d)(b,d) mesoporous
mesoporous TiO
TiO22/glass/glass under(a,b) 222 nm and (c,d) 308 nm UV light.

The kinetics of the photocatalytic reaction were evaluated from the natural logarithm of the(C/C0) as the function of photoirradiation time, assuming that the first-order kinetic expression wasapplicable (Figure 7). The present system well obeyed the first-order kinetic and, from the slope ofthe linear regression lines, the reaction rate was calculated and summarized in Table 1. The reaction

Figure 6. Time dependence of MB concentration, estimated from the visible spectra during thephotocatalytic activity testing of (a,c) mesoporous TiO2/BDD and (b,d) mesoporous TiO2/glass under(a,b) 222 nm and (c,d) 308 nm UV light.

Molecules 2018, 23, 3095

6 of 9The kinetics of the photocatalytic reaction were evaluated from the natural logarithm of theof the photocatalytic
reaction
wereassuming
evaluated that
fromthethe first-ordernatural logarithm
the (C/C0 ) was(C/C0The) askinetics
the functionof photoirradiationtime,kineticofexpressionasthefunctionofphotoirradiation
time,
assuming
that
thefirst-orderkinetic
expression
wasapplicableapplicable (Figure 7). The present system well obeyed the first-order kinetic and, fromthe slope of(Figure7).regression
The present
systemobeyedkinetic
and, from theinslope
linearthelinearlines,
the wellreaction
ratethewasfirst-order
calculated
and summarized
Tableof1.theThereaction
regression
lines,
thereactionrate
wascalculated
andsummarized
in
Table
1.
The
reactionrate
wasrate was improved by hybridizing mesoporous TiO2 layer and BDD substrate under deep UV (222
improved
nm)
light. by hybridizing mesoporous TiO2 layer and BDD substrate under deep UV (222 nm) light.

Figure7. TheFigure 7.
The kinetics
kinetics ofofthethephotocatalytic
photocatalytic reaction
reaction ofof MB
MB underunder (a)(a)222222nm
nmand
and(b)(b)308308nm
nmUV
UVlight.
The
logarithm
of C/C0 is0 plotted
time,where
where
C and
C0 theare thelight.natural
The natural
logarithm
of C/Cis plottedversus
versusphotoirradiation
photoirradiation time,C and
C0 areremaining andofofMB,
respectively.
Linearregression
lineslines
for calculationof theof theremaining
andinitial
initialconcentrations
concentrations
MB,
respectively.
Linearregression
for calculation
reaction rate
that
BDD
does
notnotactactas aasphotocatalyst
under
308 nmreactionratearearealsoalsoincluded.
included.Note
Note
that
BDD
does
a photocatalyst
under
308 UV
nmlight.
UV light.
Table 1. Estimated reaction rates (min−1 ) in various conditions.

Photolysis
TiO2 /GlassTiO2 /BDD
BDD

222 nm

308 nm1.37 × 10−3

6.71 × 10−4
2.70 × 10−3
1.70 × 10−3
Does not act as a photocatalyst

5.22 × 10−3
7.93 × 10−3
5.88 × 10−3

3. Materials and Methods
3.1. Materials
Boron trioxide was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). The Si(111) wafer
was obtained from Matsuzaki Seisakusho Co., Ltd. (Ohda, Shimane, Japan). Diblock copolymer
PS(18000)-b-PEO(7500) was acquired from Polymer Source Inc. (Dorval, QC, Canada). Titanium
chloride, concentrated hydrochloric acid (35–37 wt%), tetrahydrofuran (THF), ethanol, and methylene
blue (MB) were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). These chemicals
were used as obtained.
3.2. Synthesis of BDD Layer
Boron doped diamond (BDD) thin film (c.a., 7 µm) was synthesized by a chemical vapor deposition(CVD) method, in which boron trioxide was selected as a boron source [14]. A Si (111) wafer was
selected as a substrate, and ground with diamond powder for 20 min. The wafer was then cut to a sizeof 15 mm × 15 mm, and ultrasonically cleaned with acetone, methanol, and distilled water for 10 min
each. The concentration of doped boron was set to be 10,000 ppm. The conductivity of the obtained
BDD electrode was examined with a resistivity meter Loresta-GX MCP-T700 (Mitsubishi Chemical
Analytech (Yamato, Kanagawa, Japan)), and estimated to be approximately 5 × 10−4 Ω cm.

Molecules 2018, 23, 3095

7 of 93.3. Synthesis of Mesoporous TiO2 Thin Film on BDD Layer
A mesoporous TiO2 thin film on the BDD electrode was prepared according to previous
studies [8,9]. The precursor solution was prepared as follows: PS(18000)-b-PEO(7500) (50 mg) was
dissolved in THF (1.5 mL) at 40 ◦ C. After cooling the solution to room temperature, ethanol (500 µL)
was added and the resulting mixture was stirred for 20 min. Separately, titanium chloride (150 µL)
was quickly added into concentrated HCl (200 µL). The solution was mixed until the yellow solidintermediate completely dissolved, and then distilled water (300 µL) was added. After stirring for
10 min, this solution was added dropwise to the surfactant solution with stirring for another 30 min.
The prepared precursor solution was dropped onto the BDD electrode and spin-coated at 3000 rpm for
30 s. To increase the film thickness, we performed the spin-coating 5 times. Each spin-coated layer was
thermally treated at 100 ◦ C for 5 min, before casting the following layer. Finally, the as-prepared film
was calcined at 400 ◦ C for 1 h (ramp ratio: 1 ◦ C/min). We confirmed that the calcination process did
not affect the conductivity of the BDD electrode in advance. The mesoporous TiO2 /glass sample wasalso prepared by the same process, for reference.
3.4. Characterization of Mesoporous TiO2 Thin Film
We examined morphological features with a scanning electron microscope (SEM) JSM-7600F(JEOL). The crystal structure was examined with an X-ray diffractometer D8 Discover (Bruker) andlaser Raman spectrometer NRS-5100 (JASCO). In wide-angle X-ray diffraction (WXRD) measurements,
X-rays were irradiated at a very small grazing angle to the synthesized thin film surface, to suppress
the background signal from the substrate. Current–voltage (I–V) measurements were performed on a
VersaSTAT3 (AMETEK) with manual probes. In this measurement, the mesoporous TiO2 layer wasplaced on transparent conductive fluorine doped tin oxide (FTO) glass substrates (Solaronix (Aubonne,
Switzerland)). Manual probes for anode and cathode were placed on the Si (substrate of BDD layer)
and FTO, respectively.
3.5. Photocatalytic Activity Test
The photocatalytic activity of the obtained thin film was evaluated from decomposition of
methylene blue (MB). A 70 mL portion of MB aqueous solution (20 µM) was prepared in a 100 mL
glass cup with a Teflon cap. The sample thin film was horizontally placed, in the dark, until adsorption
equilibrium was reached. Then, 1.2 mW/cm2 of 222 nm UV light from an excimer lamp (ORC
Manufacturing Co., Ltd. (Machida, Tokyo, Japan)) was irradiated through the quartz sample tube.
The glass cup was capped with a Teflon cap to prevented evaporation during the test, while the MB
solution was taken several times by opening the cap. The absorption spectrum of the MB solution wasmeasured with a UV-VIS spectrometer V-670 (JASCO). During the test, the MB solution was constantly
stirred. The experimental setup is illustrated in Figure S3. To examine the wavelength dependence,
another excimer lamp (ORC Manufacturing Co., Ltd.) emitting 308 nm UV light at 1.2 mW/cm2 ,was also used.
4. Conclusions
TiO2 thin films with well-ordered mesopores and framework crystallized in the anatase phasewere synthesized on a p-type BDD layer. The synthesized mesoporous TiO2 /BDD had a well-formed
p–n heterojunction and operated as an efficient photocatalyst. The use of deep UV light (λ = 222 nm) as
a light source increased the photocatalytic efficiency, as photo-excitation in BDD increased the numberof photocarriers available to produce active species. As BDD operates as an electrode, we expectthat photoelectrochemical water purification may be achieved by application of an electric bias.
These studies are now underway, and will be reported in the near future.

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5. Patents
The contents of this study are the subject of a Japanese Patent application (Application
No. 2017-124255).
Supplementary Materials: The following are available online, Figure S1: (a) Top- and (b) cross-sectional views of
the BDD layer, Figure S2: Energy diagram and transfer mechanism of photocarriers in mesoporous TiO2 /BDD
and TiO2 /glass under 222-nm and 308-nm UV irradiation, Figure S3: Schematic illustration of the experimental
setup for photocatalytic activity testing.
Author Contributions: A.O., A.H., Y.H., Y.N. and N.R. synthesized BDD layer. N.S. and A.O. fabricatedmesoporous TiO2 thin film on BDD layer and conducted its characterizations. A.O. performed a photocatalytic
activity test. C.T. gave scientific suggestions on this work. H.K. and I.S. supported this work financially. K.N.,
K.-i.K., T.K. and M.Y. contributed to arrange the research environment. A.F. acted as a supervisor.
Funding: This study was partly supported by the Joint Usage/Research Program of the Photocatalysis
International Research Center, Research Institute for Science and Technology, Tokyo University of Science andTokyo Ohka Foundation for The Promotion of Science and Technology.
Acknowledgments: We thank Andrew Jackson, from Edanz Group (www.edanzediting.com/ac) for editing a
draft of this manuscript.
Conflicts of Interest: The authors declare no conflicts of interest.

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Sample Availability: Samples of the compounds are not available from the authors.
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