Visible light photoelectrochemical activity of K4Nb 6O17 intercalated with photoactive complexes by electrostatic self-assembly deposition

Article abstract of DOI:10.1016/j.jssc.2005.09.038

Electrostatic self-assembly deposition (ESD) results in the intercalation of Ru(bpy)₃²⁺ or methylene blue (MB) into the niobate layered oxide right after the cations come into contact with [Nb ₆O₁₇]^4- nanosheets. Monolayers can be obtained by the exfoliation of proton exchanged K₄Nb₆O₁₇ (KNbO) in an aqueous tetrabutylammonium (TBA) solution as revealed by the atomic force microscopy micrographs. UV-vis spectra show that intercalated films are able to absorb in the visible light range. Heat-treatment of Ru(bpy) ₃²⁺ resulted in the red-shift in the absorption spectra, which was assigned to the enhancement in the interaction between the complex molecules and [Nb₆O₁₇]^4- host layer. Intercalated niobate layered oxides are able to produce photocurrent as a result of the electron transfer from the excited guest molecule to the host layer under visible light illumination. Ru(bpy)₃²⁺ intercalated niobate layered oxide shows photocatalytic activity under visible light illumination to produce H₂ from water-methanol solution.

Full text of DOI:10.1016/j.jssc.2005.09.038

ARTICLE IN PRESS
Journal of Solid State Chemistry 179 (2006) 33–40
www.elsevier.com/locate/jssc
Visible light photoelectrochemical activity of K4Nb6O17 intercalated
with photoactive complexes by electrostatic self-assembly deposition
Ã
Ugur Unal , Yasumichi Matsumoto, Naoko Tamoto, Michio Koinuma, Masato Machida,
Kazuyoshi Izawa
Faculty of Engineering, Department of Applied Chemistry, Kumamoto University, Kurokami 2-39-1, Kumamoto 860-8555, Japan
Received 13 July 2005; received in revised form 23 September 2005; accepted 24 September 2005
Available online 25 October 2005
Abstract
Electrostatic self-assembly deposition (ESD) results in the intercalation of Ru(bpy)23+ or methylene blue (MB) into the niobate layered
oxide right after the cations come into contact with [Nb6O17]4ꢀ nanosheets. Monolayers can be obtained by the exfoliation of proton
exchanged K4Nb6O17 (KNbO) in an aqueous tetrabutylammonium (TBA) solution as revealed by the atomic force microscopy
micrographs. UV-vis spectra show that intercalated films are able to absorb in the visible light range. Heat-treatment of Ru(bpy)32+
resulted in the red-shift in the absorption spectra, which was assigned to the enhancement in the interaction between the complex
molecules and [Nb6O17]4ꢀ host layer. Intercalated niobate layered oxides are able to produce photocurrent as a result of the electron
transfer from the excited guest molecule to the host layer under visible light illumination. Ru(bpy)23+ intercalated niobate layered oxide
shows photocatalytic activity under visible light illumination to produce H2 from water–methanol solution.
r 2005 Elsevier Inc. All rights reserved.
Keywords: Layered oxides; Niobate; Exfoliation; Intercalation; Methylene blue; Photochemistry; Water splitting
1. Introduction
space with alkylammoniums to make the intercalation
possible [6–13]. Intercalation or ion-exchange process,
however, might take days or weeks, and only partial
exchange is obtained. Recently, we have reported electro-
static self-assembly deposition (ESD) technique for the
intercalation of several complexes into the layered titanate
oxide [14]. The technique provides a quick and reliable way
to introduce any complex molecule or a cation into the
interlayer of these layered oxides. Intercalation takes place
immediately after mixing the solutions of negatively
charged titanate nanosheets and complex cations under
controlled pH [14].
Photocatalytic and photoluminescent properties of
KNbO have been widely studied by many researchers.
Domen et al. showed that KNbO, hosting various cations
in the interlayer is able to catalyze the photolysis of water
efficiently [15–22]. The process, however, takes place under
UV light because of the large band-gap of the host oxide.
Since the use of visible light in photocatalytic reactions in
order to utilize readily available sunlight is important,
several modifications have been carried out in the inter-
Layered oxide materials have been widely studied in the
past couple of decades, because of their unique, highly
applicable chemical and physical properties. Preparation,
characterization and application of several series of
inorganic layered oxides in different fields are of common
interest. Among them is K4Nb6O17 (KNbO) with its
unique layered structure. KNbO possesses two alternative
interlayer spaces formed by repeating [Nb6O17]4ꢀ layers
between which K+ exist to hold the layers together and to
maintain the charge balance. Hydration and ion exchange
behavior of these alternative interlayers are different from
each other [1]. Introduction of wide range of cations and
molecules into the interlayer of KNbO has been carried out
on the basis of the ion-exchange principles [2–5] and
intercalation of large molecules, such as dyes or Ru(bpy)32+
complex were accomplished after opening up the interlayer
Ã
Corresponding author. Fax: +81 96 342 3679.
E-mail address: ugurunal@chem.kumamoto-u.ac.jp (U. Unal).
0022-4596/$ - see front matter r 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2005.09.038

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34
layer or on the surface of the photocatalyst to do so. For
instance, narrow band-gap semiconductors were intro-
duced into the interlayer and these systems were reported
to be able to utilize visible light in photocatalytic reactions
[23–25]. In addition, Kim et al. and several other
researchers have reported photocatalytic KNbO systems
working under visible light in the presence of photoactive
dyes or complexes [26–29]. In these systems, an electron is
injected into the host layer from the excited photoactive
molecules on the surface under visible light. The activity of
similar molecules in the interlayer also indicated that
electron transfer from the interlayer molecule to the host
layer is possible under illumination with visible light. These
results were concluded after detailed studies of host–guest
interaction with spectral analyses [30–33].
In this work, we have investigated the exfoliation of
[Nb4O17]4ꢀ sheets and the intercalation of Ru(bpy)23+
complex and MB into the interlayer of KNbO by ESD
technique. Photoelectrochemical properties of KNbO films
deposited by electrophoretic deposition method were
previously reported by our laboratory [34]. In current
research, photoelectrochemical properties of intercalated
compounds were discussed in terms of the interaction
between the interlayer molecules and the [Nb4O17]4ꢀ host
layer. In addition, photocatalytic activity of Ru(bpy)32+
intercalated KNbO to release H2 in water/methanol
solution under visible light illumination was reported.
The concentrations of dye solutions were 1 mM and
0.5 mM, respectively. Prior to the addition, the pH of the
exfoliation solution was adjusted to 8 with 0.1 M HCl
carefully. The addition resulted in an immediate precipita-
tion consisting of a single phase niobate layered oxides
intercalated with the Ru(bpy)23+ or methylene blue
molecules. Precipitation occurs as a result of electrostatic
principles governing the deposition with ESD mechanism,
where negatively charged [Nb6O17]4ꢀ nanosheets combine
with the dye cations. pH adjustment is inevitable in this
method, because the initial pH values of each solution is
quite far away from each other. The exfoliation solution
has a pH of around 12, where pH of the Ru(bpy)23+ or MB
solution is around 6. [Nb6O17]4ꢀ sheets precipitate as
HNbO at the pH values lower than 5 as a result of H+
intercalation into the interlayer instead of a desired cation.
In addition, Ru(bpy)+3 or MB might produce hydroxides at
pH value of the exfoliation solution. Thus, ESD reaction
was carried out in a reasonable pH range, which was
chosen as 8 and estimated from the titration curves of both
solutions.
The topography of the deposited films was observed by
Nanoscope E system Atomic Force Microscopy (AFM).
The crystal structure and the orientation were analyzed
from XRD patterns (using CuKa radiation, Rigaku RINT-
2500VHF) of the thin films deposited simply by applying a
number of drops of ESD precipitate on a Pt substrate and
drying naturally. The compositions of the deposited films
were analyzed with an ICP spectrophotometer (Nippon
Jarrel Ash IRIS Advantage) and an X-ray photoelectron
spectrometer (XPS, VG Scientific S-probe). The ICP
analyses were made after dissolving the films in HCl
solution. Fourier-transformed infrared spectra (FT-IR
Perkin Elmer) of the films were obtained with a KBr
technique. An appropriate amount of each sample was
mixed with 0.3–0.5 g KBr and pressed into a pellet. The
samples were analyzed right after preparing the pellets.
UV-vis absorption spectra of the deposited oxides were
measured using an UV-vis spectrometer (Jasco V-550).
TG–DTA curves were obtained by thermal analysis (Seiko
TG/DTA).
2. Experimental section
Layered potassium niobate (K4Nb6O17) was prepared by
a conventional solid-state method. The mixture of Nb2O5
(Wako Pure Chemical, 99.9%) and K2CO3 (Wako Pure
Chemical, 99.9%) was calcined at 1473 K for 15 min in a Pt
crucible [16]. The final product of K4Nb6O17 powder was
pulverized in a mortar to decrease particle size to less than
about 10 mm. The K4Nb6O17 sample was hydrated at this
stage (K4Nb6O17 ꢁ 3H2O) according to its XRD pattern.
The powder was treated with 0.5 M H2SO4 for 24 h in order
to protonate the interlayers. The proton exchanged powder
was exfoliated by introducing tetrabutylammonium (TBA)
or ethylamine (EA) molecules into the interlayer. The
reaction was carried out in an aqueous tetra(n-butyl)
ammonium hydroxide or EA solution for 24 h. Exfoliation
occurs as a result of the penetration of large TBA or EA
molecules into the interlayer, opening up the interlayer to
delaminate the powder into single nanosheets. The
subsequent centrifugation of the solution under 2000 rpm
for 30 min yielded colloidal suspension of individual
[Nb6O17]4ꢀ sheets.
All electrochemical experiments were carried out in a
conventional three-electrode electrochemical cell with a Pt
counter electrode and a saturated Ag/AgCl reference
electrode. The working electrode potentials referred to this
reference electrode unless otherwise stated in this paper. A
500 W ultra-high pressure Hg lamp was used as the light
source to measure the photoelectrochemical properties.
Cyclic voltammograms (CV) were measured under a
potential sweep rate of 20 mV sꢀ1. 0.1 M K2SO4 solution
was used as a supporting electrolyte solution with a pH of
6.5. N2 saturation was made in the electrolytes before the
electrochemical measurements.
The ESD method was used in the intercalation step [14].
The intercalation of Ru(bpy)23+ complex ions (Aldrich,
Tris (2,20-bipyridil) dichlororuthenium (II) hexahydrate,
99.95%) or methylene blue (Chroma, Methylene Blue med.
Puriss.) into the interlayer was performed by the addition
of 20 mL of colloidal TBA exfoliated [Nb6O17]4ꢀ sheets
into the aqueous solution of Ru(bpy)23+ or methylene blue.
A quartz cell is used as a reaction chamber to conduct
photocatalytic experiments. 0.06 g of Ru(bpy)23+ interca-
lated sample, which was heat-treated at 300 1C for 2 h was
dispersed in 200 mL of 4 M CH3OH aqueous solution. The

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35
reaction was carried out under stirring and the system was
fed with N2 prior to the measurement. Xe lamp (300 W)
was utilized to irradiate the system, during which the light
with a wavelength shorter than 420 nm was cut by a cutoff
filter. The rate of H2 evolution was measured by an online
gas chromatograph system (Shimadzu, GC-8A), which was
connected to the reaction cell.
in Fig. 1b, the right image. The exfoliation in this case is
most likely to occur throughout the interlayer I only.
Typical XRD pattern of the KNbO was observed in the
hydrated form at room temperature as given in Fig. 2. The
˚
layer distance of 9.5 A corresponds to its hydrated form
with three water molecules, K4Nb6O17 ꢁ 3H2O. The proton
exchange brought about only a very slight change in the
˚
layer distance from 9.5 to 9.6 A according to the shift in
3. Results and discussion
3.1. Synthesis
The topographic images of the surface of the exfoliated
nanosheets were taken by AFM and are given in Fig. 1a.
AFM images are consistent with the preservation of the
original crystal structure after the exfoliation. The drawn
rectangle shows the surface of a unit cell through b-axis
and connects the uppermost oxygen atoms, which appear
as white dots on the AFM pattern. The a ꢂ c dimensions
are in accordance with the unit cell parameters of KNbO.
In Fig. 1b, the height profile through the drawn axis on the
left image indicates the thickness of an individual
nanosheet, which is determined to be around 1 nm for
nanosheets exfoliated with TBA. The value is very close to
the thickness of a monolayer and almost half of the
thickness of a bilayer. The lateral sizes of the exfoliated
particles are around 200 nm. Thus, AFM images are clear
evidences that exfoliation of the layers was accomplished
and TBA can exfoliate proton exchanged niobate to
individual [Nb6O17]4ꢀ sheets. Exfoliation occurs when
bulky ammine molecules penetrate into the acidic inter-
layer of proton exchanged form of a layered oxide and
extend the interlayer gallery to break up the forces holding
the layers together to form single nanosheets [35–37].
Saupe et al. reported that the exfoliation yields bilayers
initially, but monolayers can be formed subsequently [35].
On the other hand, ethylamine exfoliates niobate layered
oxide to bilayers, probably that of layer II, as it can be seen
Fig. 2. XRD patterns of K4Nb6O17 ꢁ 3H2O, its proton exchanged form,
exfoliated nanosheets, Ru(bpy)23+ intercalated sample, its heat-treated
form at 300 1C and MB intercalated sample, respectively, from bottom to
top.
Fig. 1. AFM images of exfoliated [Nb6O17
(left image) and ethylamine (right image).
] ]
4ꢀ nanosheets. (a) Atomic resolution image of a [Nb6O17 4ꢀ nanosheet, and (b) nanosheets exfoliated with TBA

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(040) peak. Protonation is only partial for KNbO and
K/Nb ratio is 0.09 after the proton exchange according to
the XPS data. The composition of the proton exchanged
form is estimated as K0.54H3.46Nb6O17. The film deposited
from its exfoliated form by TBA yielded a substantial
sideration, the intercalation of Ru(bpy)23+ complex occurs
in both layers as a result of ESD method. Otherwise, the
˚
layer distance of 18.5 A would be short for niobate layered
oxide, hosting Ru(bpy)23+ only in one layer. The intercala-
tion model is given in Fig. 3a. We can conclude that this
intercalation could be B-type [8], thus the corresponding
peak is assigned to the diffraction from (040) plane.
Similarly, MB is placed in both interlayers with its planar
structure and the two possible intercalation models are
illustrated in Fig. 3b. If MB molecules lie in the interlayer
parallel to [Nb6O17]4ꢀ layers with their long axis, it forms a
double layer arrangement when taking the size of the
molecule [13] and the interlayer space into consideration.
Assuming that it forms a single layer arrangement, the
molecule should align in the interlayer slightly inclined to
the [Nb6O17]4ꢀ layers with their long axis. Consequently,
the corresponding peak can be attributed to (040)
diffraction, and the intercalation is B-type, because MB
molecules were intercalated into both interlayers. The co-
intercalation of proton and/or hydronium ions also occurs
to surround the interlayer complex molecules and has an
effect on the photoelectrochemical properties, which was
also the case for Ru(bpy)23+ intercalated titanium layered
oxides [14]. Heat-treatment resulted in the slight shift of the
corresponding peak to higher degrees as a result of the
removal of the surrounding water or hydronium ions and
enhancement in the photoelectrochemical properties [14].
Similar behavior was observed for Ru(bpy)23+ intercalated
niobate layered oxides as seen in Fig. 2. (040) peak gives
˚
increase in the layer distance to 22.6 A, which is in
agreement with the result reported by Domen et al. [36].
The corresponding peak in the XRD pattern of the film in
Fig. 2 was attributed to the diffraction from the (020)
plane, which shows that the TBA exist in the interlayer I
only. Both FTIR and XPS data of the film show the
presence of TBA molecules in the interlayer. Intercalation
of Ru(bpy)23+ cation and MB into the interlayer by ESD
˚
method carried the layer distance to 18.5 A (2y ¼ 4:781)
˚
and 21.4 A (2y ¼ 4:121), respectively, as tabulated in Table 1.
Layer distances were calculated from the corresponding
peaks in the XRD pattern of the films deposited from ESD
precipitations.
ESD method is governed by electrostatic principles and
interaction between [Nb6O17]4ꢀ layers and Ru(bpy)32+ or
MB molecules brings about a combination between
nanosheets and Ru(bpy)23+ or MB to form intercalated
niobate layered oxide after mixing both solutions. Com-
pared to the conventional ion-exchange methods, the
advantage of ESD method comes from its rather con-
trollable and quick nature. The intercalation of the
molecules takes place immediately as a result of electro-
static interaction forces between oppositely charged species
[14]. Taking the size of Ru(bpy)23+ molecule into con-
˚
the layer distance of 16.6 A, which is a reasonable value to
accommodate Ru(bpy)23+ in the interlayer without any
hydrate.
Table 1
Chemical compositions and layer distances of the starting and intercalated
niobate layered oxides
The chemical compositions of the Ru(bpy)23+ and MB
intercalated niobate layered oxides are given in Table 1.
Atomic ratios were calculated on the basis of XPS data
and the amount of water was calculated from the TG/DTA
data. The chemical composition of the Ru(bpy)23+ inter-
calated sample was determined by taking Ru/Nb ratio,
which was estimated from the XPS data as 0.05.
˚
Layer distance (A)
Chemical composition
K4Nb6O17 ꢁ 3H2O
9.5
9.6
K0.54H3.46Nb6O17
K0.54[Ru(bpy)3]0.3TBA0.5H2.66Nb6O17
18.5
21.4
K0.54MB0.66TBA0.5H2.3Nb6O17
Fig. 3. Intercalation models of: (a) Ru(bpy)23+ and (b) MB intercalated niobate layered oxides.

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Ru(bpy)23+ molecule is known to occupy an area of
0.84 nm2 in the interlayer [30] and the charge density of
the [Nb6O17]4ꢀ layer is known as 0.126 nm2 per charge. The
theoretical Ru/Nb ratio can be estimated by taking the
ratio of a complex cation to the number of Nb atoms in an
area that a Ru(bpy)23+ molecule occupies. Assuming that
Ru(bpy)23+ complex molecules are aligned in the interlayer
in a single-layer arrangement and are intercalated into
both layers, the maximum Ru(bpy)23+/[Nb6O17]4ꢀ ratio
would be 0.4 according to the estimation. Consequently,
the experimental value indicates that ESD resulted in the
occupation of almost all possible sites with Ru(bpy)32+
molecules and the intercalated molecules were packed
densely in the interlayer. On the other hand, MB/[Nb6O17]4ꢀ
was estimated as 0.66, which is much larger than the ratio
reported by Kaito et al. [13]. These results show that ESD
method produces Ru(bpy)23+ or MB intercalated niobate
layered oxides with higher atomic ratios than intercalation
compounds obtained by conventional ion-exchange meth-
ods [30,32]. XPS and FTIR analyses also revealed that
Ru(bpy)23+ and MB intercalated compounds contains K+
cations and TBA in the interlayer together with the
complexes. Thus, the remaining sites are occupied with
TBA, K+ and protons to maintain the charge balance.
Fig. 4 shows the FTIR spectra of exfoliated and
Ru(bpy)23+ intercalated niobate sheets and its heat-treated
form. The FTIR spectrum of exfoliated nanosheets in Fig.
4a shows similar peaks to that of the FTIR spectrum of
TBA itself, because TBA molecules coexist in the
interlayer. Ru(bpy)23+ intercalated compound shows a
broad OH stretching vibration peak centered at
3400 cmꢀ1, which can be attributed to the hydrogen
bonded water. Typical bipyridine bands on the spectrum
are also observable in the region of 1400–1600 cmꢀ1. The
slight shift of these peaks to lower wavenumbers after the
heat-treatment may be attributed to the strain applied on
the molecules as a result of the shrinkage in the interlayer
space. The peaks due to the C–H stretching bands of TBA
at 2870 and 2955 cmꢀ1 observed In Fig. 4a and b
disappeared after the heat-treatment at 300 1C because of
the decomposition and removal of TBA molecules by this
temperature as seen in Fig. 4c. The heat-treatment of the
Ru(bpy)23+ intercalated sample does not bring about any
other significant change in the FTIR spectra (Figure b and
c), because Ru(bpy)23+ molecules remain intact at 300 1C.
As seen in Fig. 5, Ru(bpy)32+ intercalated niobate layered
oxide has three weight loss region on its TGA curve in the
range of room T–600 1C. Initially, free or loosely bound
water is removed in the room temperature–100 1C region,
which is followed by the subsequent removal of interlayer
water molecules by 210 1C. Interlayer TBA molecules are
also removed in this region. The initiation of the downward
curve on the TGA diagram after 300 1C is due to the
decomposition of Ru(bpy)23+ complex molecule in the
interlayer, which starts at 320 1C by oxidation. The
decomposition and removal of the molecule in the
interlayer give rise to an exothermic peak at 400 1C on
the DTA curve. Ru(bpy)23+ intercalated titanate layered
oxide also exhibited the same weight loss trend on its
TG/DTA diagram [14].
The UV absorption spectra of the exfoliated [Nb6O17]4ꢀ
layers and Ru(bpy)23+ intercalated niobate before and after
heat-treatment is given in Fig. 6a–c, respectively. The
absorption edge of the niobate around 330 nm is red-
shifted to 600 nm and the band is broadened after the
intercalation of Ru(bpy)23+ into the interlayer. Absorption
starting at this wavelength is assigned to the metal-to-
ligand charge transfer (MLCT) of the Ru(bpy)23+ mole-
cules in the interlayer. The shift for the intercalated
molecules with respect to the peak for its aqueous solution
was also reported in other studies related to zeolites [38] or
layered materials [8,31,32] and it was assigned to the
strained complex molecules [8] and to the chelate-oxide
layer and chelate–chelate interaction in the interlayer [32].
Fig. 4. FTIR spectra of (a) exfoliated nanosheets, and Ru(bpy)32+
intercalated niobate layered oxide (b) before and (c) after heat-treatment
at 300 1C.
Fig. 5. TG/DTA curves of Ru(bpy)32+ intercalated niobate layered oxide.

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Furthermore, the heat-treatment brought about further
red-shift of the absorption edge. Ru(bpy)23+ intercalated
titanate layered oxide also experienced the same behavior
of red-shift by heat-treatment [14]. We have assigned the
red-shift to the enhancement in the host–guest interaction
as a result of the removal of water or hydronium ions
surrounding the interlayer complex molecules by heat-
treatment at 300 1C. The UV absorption spectrum of MB
intercalated sample is also given in Fig. 6d. The very broad
absorption band in the visible light region is a result of the
presence of MB in the interlayer. The spectrum shows that
MB exists as a dimmer, higher aggregate and monomer
according to bands around 600 and 500 nm [13,39].
had a better response in the visible light range as seen on its
UV-vis spectra in Fig. 6. In addition, the heat-treated
sample was more stable in the interlayer after the heat-
treatment [14]. MB intercalated film was measured without
any treatment because of the decomposition of the
molecule even at low temperatures. Thus, MB molecules
in the interlayer are more likely to deintercalate into the
electrolyte during photoelectrochemical measurements as a
result of weaker interaction with the host layer and less
stability in the interlayer. The cyclic voltammetry measure-
ments of the intercalated niobate layered oxide films were
started immediately after the sample was immersed into the
N2 pre-saturated electrolyte to ensure that the interlayer
complex molecules do not deintercalate into the electrolyte
during the initial stages of the measurement. The cyclic
voltammograms of Ru(bpy)23+ and MB intercalated
niobate layered oxide films are given in Fig. 7a and b,
respectively. Cyclic voltammograms were measured in
0.1 M K2SO4 solution. Illumination of the system was
carried out by blocking the path of the light periodically to
observe photocurrent. Films produced both anodic and
cathodic photocurrents under visible light illumination in
which the light with a wavelength shorter than 420 nm was
cut by a cutoff filter. The visible photoresponse is in
harmony with the result obtained by the UV-vis spectra in
Fig. 6. The maximum current density was estimated as 4
and 8 mA/cm2 for Ru(bpy)23+ and MB intercalated films,
respectively. Higher current density for MB intercalated
film might be attributed to a large amount of MB in the
interlayer of niobate layered oxide. Under visible light
illumination, Ru(bpy)32+ molecule in the interlayer is
excited to its MLCT state to form Ru(bpy)23+* by
absorbing the visible light, which leads to the photocurrent.
The possible electron transfer mechanism is illustrated in
3.2. Photoelectrochemical properties
Ru(bpy)23+ intercalated film was heat-treated at 300 1C
prior to the measurement, because the heat-treated sample
Fig. 6. UV-vis absorbance spectra of the films deposited from
(a) exfoliated niobate nanosheets, and Ru(bpy)23+ intercalated niobate
layered oxide (b) before and (c) after heat-treatment at 300 1C, and (d) MB
intercalated niobate layered oxide.
Fig. 7. Cyclic voltammograms of (a) Ru(bpy)23+ and (b) MB intercalated niobate layered oxide films.

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Fig. 8. Electrons produced by the excitation from the
HOMO level to the LUMO level of the Ru(bpy)23+ under
the illumination in the visible light range will jump into the
conduction band (CB) level of the [Nb6O17]4ꢀ host layer
and will move through the band. On the other hand, the
produced hole will also move by the tunneling mechanism
[14]. Similarly, electron transfer occurs from excited MB*
molecules to the [Nb6O17]4ꢀ host layer under visible light
illumination. The photoelectrochemical reactions will
evaluate H2 and O2 gases according to the energy positions.
Ru(bpy)23+ intercalated niobate layered oxide produced
H2 in the methanol solutions under illumination in the
visible light range. The amount of hydrogen evolution is
given in Fig. 9. The maximum amount of evolution was
reached at 0.28 mmol after 1.5 h. The cessation of the
evolution rate might be attributed to the deintercalation of
Ru(bpy)23+ molecules into the methanol solution, because a
gradual change in the color of the solution was observed
during the experiment. The reaction takes place in the
interlayer of the niobate layered oxide. [Nb6O17]4ꢀ host
layer gains electrons from the excited Ru(bpy)23+ molecules
in the interlayer and these electrons react with the
interlayer H+ to produce H2. The result is in agreement
with the cyclic voltammogram data in terms of electron
transfer mechanism as given above and the mechanism
suggested by Furube et al. [33]. No H2 evolution was
observed when the reaction was carried out in pure water.
In addition, MB intercalated niobate layered oxide did not
produce any H2 even in methanol–water solution. The
inactivity of MB intercalated niobate layered oxide is
probably a result of the deintercalation of interlayer MB
into the solution soon after the contact. Thus, charge
transfer does not occur from deintercalated MB molecules
to the host layer. As mentioned previously, interlayer MB
molecules are not stable and might deintercalate into the
solution easily.
4. Conclusion
The intercalation of Ru(bpy)23+ complex and MB into
the interlayer of niobate layered oxides was achieved by
ESD method. Individual [Nb6O17]4ꢀ nanosheets exfoliated
in TBA solution combine with cations to yield the
intercalation compounds. XRD patterns revealed that
Ru(bpy)23+ complex and MB were intercalated into both
layers of niobate layered oxide. Co-intercalation of TBA
molecules was also observed. Intercalated compound
absorbs in the visible light range due to the presence of
photoactive molecules that are responsive to visible light in
the interlayer. Heat-treatment at 300 1C enhances the
interaction between the interlayer Ru(bpy)23+ complex
and host layer, so that electrons formed by excitation
from the HOMO level to LUMO level of the complex
yields visible light photocurrent. Electron transfer occurs
from the excited interlayer molecules to the [Nb6O17]4ꢀ
host layer to take part in reactions in the interlayer.
Ru(bpy)23+ intercalated sample was able to produce H2 in
methanol/water system under visible light illumination, but
not in pure water.
Fig. 8. Possible electron transfer mechanisms for Ru(bpy)23+ intercalated
niobate layered oxides under illumination in the visible light range.
Acknowledgments
This work was supported by the Core Research for
Evolutional Science and Technology (CREST) program of
the Japan Science and Technology Co.(JST), and a Grant-
in-Aid for Scientific Research (No 15350123) from the
Ministry of Education, Culture , Sports, Science, and
Technology.
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