Merging Cation Exchange and Photocatalytic Charge Separation Efficiency in an Anatase/K2Ti4O9 Nanobelt Heterostructure for Metal Ions Fixation

Article abstract of DOI:10.1021/acs.inorgchem.8b00538

Efficient collection and safe disposal of toxic metals ions from aqueous solutions is critical for applications in environmental remediation. Although extensive efforts have been devoted to the synthesis of functional TiO₂ materials, photocatalytic reduction (photoreduction) of aqueous metal ions into solid metals remains a challenge. We designed a TiO₂ nanoparticle-decorated layered titanate (K₂Ti₄O₉) material that retained the cation exchange ability of K₂Ti₄O₉ but also possessed the enhanced charge separation efficiency of K₂Ti₄O₉. Combining cation exchange with enhanced charge separation efficiency results in a heterostructured material with remarkably high activity for the photoreduction of metal ions. Initially we demonstrated how the photocatalyst can efficiently reduce aqueous Ni²⁺ cations, whereas the benchmark TiO₂-based P25 catalyst showed little to no activity. The resulting Ni-deposited heterostructure can then be used as a catalyst for visible light-induced photocatalytic H₂ evolution in water.

Full text of DOI:10.1021/acs.inorgchem.8b00538

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Merging Cation Exchange and Photocatalytic Charge Separation
Efficiency in an Anatase/K2Ti4O9 Nanobelt Heterostructure for Metal
Ions Fixation
Yusuke Ide,*,† Wataru Shirae,‡ Toshiaki Takei,† Durai Mani,†,§ and Joel Henzie†
†International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki
Tsukuba, Ibaraki 305-0044, Japan
‡Graduate School of Creative Science and Engineering, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan
§Center for Nanoscience and Technology, Anna University, Chennai 600025, India
S
* Supporting Information
ABSTRACT: Efficient collection and safe disposal of toxic metals ions
from aqueous solutions is critical for applications in environmental
remediation. Although extensive efforts have been devoted to the
synthesis of functional TiO2 materials, photocatalytic reduction
(photoreduction) of aqueous metal ions into solid metals remains a
challenge. We designed a TiO2 nanoparticle-decorated layered titanate
(K2Ti4O9) material that retained the cation exchange ability of K2Ti4O9
but also possessed the enhanced charge separation efficiency of K2Ti4O9.
Combining cation exchange with enhanced charge separation efficiency
results in a heterostructured material with remarkably high activity for
the photoreduction of metal ions. Initially we demonstrated how the
photocatalyst can efficiently reduce aqueous Ni2+ cations, whereas the
benchmark TiO2-based P25 catalyst showed little to no activity. The
resulting Ni-deposited heterostructure can then be used as a catalyst for
visible light-induced photocatalytic H2 evolution in water.
catalysts,13−19 they exhibit poor photocatalytic activity without
INTRODUCTION
■
any modification.20,21 Typically, titanate nanosheets are
Using light to reduce metal ions in water and deposit them on
modified by exfoliating them in the presence of other functional
solid surfaces is a topic covering a wide range of scientific and
materials.14−19 This process hybridizes the two materials, but
practical purposes, with applications spanning the removal of
frequently causes the titanate to lose its original ion exchange
ability. In the pursuit of higher photocatalytic activity, most
existing work in the literature does not address the loss of ion
exchange ability, or attempt to discover ways to mitigate loss.
We hypothesized that layered titanate-based hybrid photo-
catalysts that maintained their original cation exchange
properties will possess extraordinary performance for the
photocatalytic reduction of metal ions if they were properly
synthesized.
Herein, we report on design of a TiO2 (anatase)-modified
layered titanate (K2Ti4O9) photocatalyst via a simple one-pot
hydrothermal reaction. This photocatalyst has remarkably high
performance for the photoreduction of Ni2+ cations from water,
whereas P25-type TiO2 is inactive. Ni was chosen as the target
aqueous cation because it is a common water contaminant that
occurs in high concentrations in the environment, and is also a
valuable metal in various technologies.22,23
toxic ions to the collection of useful metals. Photocatalytic
collection of ions is a safer disposal process than most
adsorption and ion exchange processes because the adsorbed
metal ions can be spontaneously reduced to fix metals on
surfaces.1 There are many kinds of photocatalysts, but TiO2 is
one of the most stable and cost-effective materials. However,
despite great progresses on advanced TiO2 synthesis methods,
the photocatalytic reduction of metal ions (e.g., Ni2+ and Cd2+)
is still challenging, in part because TiO2 has poor adsorption
affinity for metal ions, which is necessary for photoreduction to
occur.2−6
Layered inorganic solids including layered clay minerals have
historically been used as ion exchange materials owing to their
large surface areas and large adsorption capacities. Their high
capacities stem from the structure of the material; layered
materials composed of nanometer-thick nanosheets have better
chemical and thermal stabilities than their organic counter-
parts.7,8 For example, layered titanates are composed of titania
nanosheets separated by interlayers of alkali metal cations and
are known to be excellent ion exchange materials.9−12 Although
layered titanates have been widely investigated as photo-
Received: March 1, 2018
© XXXX American Chemical Society
A
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The present hydrothermal reaction for TiO2-based materials
builds on a method previously developed by our group.24−26
For example, amorphous TiO2 is chemically less resistant to
etching agents than crystalline phases. P25-type TiO2 is mainly
composed of anatase and rutile TiO2 and contains some
fraction of amorphous TiO2, which we exploit by dissolving it
and then recrystallizing on the main crystals under hydro-
thermal conditions. The new crystallites are anatase and rutile
phases, and the new heterostructure system behaves as a high-
performance photocatalyst.24 In this study, K2Ti4O9 nanobelts
were exposed to similar hydrothermal conditions. To our
surprise, the edges of the nanobelts selectively dissolved and
recrystallized into anatase nanoparticles, forming heterostruc-
ture photocatalysts that retain the cation exchange ability of the
original titanate (Figure 1).
peaks assigned to K2Ti4O9 to weaken, while new XRD peaks
matching anatase appeared. The basal spacing of Hyd-K2Ti4O9,
0.88 nm, was identical to K2Ti4O9, suggesting that no
intercalation reaction occurred during the hydrothermal
reaction. UV−vis spectroscopy of the Hyd-K2Ti4O9 sample
also confirmed the presence of anatase, which has a narrower
band gap (i.e., longer-wavelength absorption edge) than
K2Ti4O9 (Figure 2, right).17−19
Figure 3a,b shows SEM images of K2Ti4O9 and Hyd-
K2Ti4O9. The K2Ti4O9 nanobelt-shaped particles had thick-
nesses of up to a few hundreds of nanometers and lengths of up
to several microns. The Hyd-K2Ti4O9 sample was also shaped
like a nanobelt. However, high-magnification SEM micrographs
for Hyd-K2Ti4O9 (Figure 3c) revealed the presence of tiny
particles covering the surface of each nanobelt. HRTEM
(Figure 3d) showed that these smaller particles (ca. 10−30 nm)
were anatase, consistent with the XRD and UV data (Figure 2).
Both phases are bonded together by a particle interface
according to HRTEM, selected area electron diffraction
(SAED), high-angle annular dark field scanning TEM
(HAADF-STEM), and energy-dispersive X-ray spectroscopy
(EDX) measurements shown in Figure S1. In sum, these results
prove that Hyd-K2Ti4O9 is a heterostructure material composed
of K2Ti4O9 nanobelts decorated with anatase nanoparticles.
Interestingly, SEM (Figure 3b) and TEM and HRTEM
(Figure 3e,f) observations showed that many Hyd-K2Ti4O9
nanobelts had uneven edges that extended into nanowires,
whereas the original K2Ti4O9 nanobelts had relatively smooth
edges. Since the thickness and length of K2Ti4O9 nanobelts
scarcely changed after the hydrothermal reaction, the edges of
K2Ti4O9 nanobelts were selectively dissolved and recrystallized
into anatase to form Hyd-K2Ti4O9 during the hydrothermal
reaction. It is well-known that K2Ti4O9 can be exfoliated into
nanosheets without significant change in the lateral size of the
layers in alkali solutions containing tetrabutylammonium
hydroxide.17−19 In contrast, P25-type TiO2 (composed of
anatase, rutile, and amorphous phases) treated under the
similar hydrothermal conditions causes the amorphous phase to
selectively dissolve and convert into new anatase and rutile
crystals in an uncontrolled manner.24 Dissolution in layered
K2Ti4O9 titanate nanobelts happens primarily at the edges,
which can be dissolved more easily. This is the first report on
the intraparticle regioselective chemical weakness of layered
titanates.
Figure 1. Scheme for design of anatase-modified layered titanate
(K2Ti4O9) having the original cation exchange ability.
RESULTS AND DISCUSSION
■
K2Ti4O9 nanobelts were prepared by a solid state reaction
between K2CO3 and TiO2 (anatase) according to the
literature.27 We hydrothermally treated K2Ti4O9 under alkali
conditions in the presence of tetrapropylammonium hydroxide
(TPA) and ammonium fluoride (NH4F). According to our
previous experiments, both reagents should promote the
dissolution of K2Ti4O9, while NH4F acted as mineralizer to
recrystallize the dissolved species.24−26 We named the hydro-
thermal product Hyd-K2Ti4O9. XRD analysis (Figure 2, left a
and b) revealed that the hydrothermal reaction caused the
More importantly, after the hydrothermal reaction, the cation
exchange ability of K2Ti4O9 was retained. We examined the
adsorption of Ni2+ on different materials from water containing
NiCl2 and methanol (pH = 4.7), which was the same solution
used in photocatalytic Ni2+ deposition experiments described in
detail below. As shown in Figure 4a, K2Ti4O9 effectively
adsorbed Ni2+ with a maximum adsorption amount of >1 mmol
g−1, which was almost identical to the cation exchange capacity
(1.2 mmol g−1 for divalent cations at this pH region).10 This
result shows that cation exchange of interlayer K+ and
replacement with Ni2+ is almost complete. We compared the
Ni2+ adsorption of TiO2 (P25) and found it was negligible,
consistent with previous reports.2,4 This is understandable
because the surface of P25 is positively charged below the
isoelectric point (ca. 6.4)28 and repels cations. On the other
hand, Hyd-K2Ti4O9 had a cation exchange ability comparable
to that of K2Ti4O9. The slight decrease in the maximum
amount of the adsorbed cation can be explained by the fact that
a part of the K2Ti4O9 surface was converted into anatase.
Figure 2. (left) XRD patterns and (right) UV−vis spectra of (a)
K2Ti4O9, (b) Hyd-K2Ti4O9, and (c) Hyd-K2Ti4O9 after the photo-
catalytic Ni2+ reduction. Insets show the expanded figure at low 2θ
region and the photographs of each sample.
B
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Figure 3. SEM images of (a) K2Ti4O9 and (b) Hyd-K2Ti4O9. (c) High-magnification SEM and (d) HRTEM images of Hyd-K2Ti4O9. (e) TEM and
(f) HRTEM images of the edge of Hyd-K2Ti4O9, showing the presence of K2Ti4O9 nanowires.
TiO2-based photocatalyst. This reaction causes the NBT to
convert into insoluble Formazan that deposited on the surface
of TiO2.20,29 K2Ti4O9 did not generate any O2 according to
−
the NBT assay (Figure 4b), whereas Hyd-K2Ti4O9 generated a
−
considerably higher amount of O2 than our JRC TIO-1
anatase benchmark sample, even though the anatase
component in Hyd-K2Ti4O9 and the JRC TIO-1 were similar
in size (the JRC TIO-1 was supplied by Catalysis Society of
Japan). Considering the fact that the conduction band potential
(ECB) of K2Ti4O9 is almost identical to that of anatase,17−19
electron transfer from photoexcited K2Ti4O9 to anatase and
vice versa is thermodynamically favorable, and both compo-
nents can synergistically enhance the charge separation of the
combined system (Figure 4b, inset).
As expected, Hyd-K2Ti4O9 showed a remarkably high activity
for the photoreduction of Ni2+ in water to Ni under irradiation
with a solar simulator. The photoreduction tests were
conducted using methanol as a sacrificial reagent. TiO2 (and
K2Ti4O9) cannot directly reduce Ni2+ because its ECB is lower
(more positive) than the reduction potential of the Ni2+/Ni
couple at ambient pH (pH < 7). However, TiO2-based
materials can indirectly reduce Ni2+ cations with radical
reductant species formed by the oxidization of an organic
additive (e.g., methanol) via the TiO2-photogenerated holes.2−5
As shown in Figure 4c, P25 reduced/deposited only a tiny
amount of Ni2+ into Ni metal, consistent with previous
reports.2−6 Pure K2Ti4O9 showed no proficiency for this
reaction. In contrast, Hyd-K2Ti4O9 efficiently reduced Ni2+ to
Ni under the identical irradiation conditions. The Ni deposition
was also confirmed by XRD and UV−vis analysis (Figure 2c):
the gray color of the product indicates the deposited Ni is
partially oxidized. We called the present reduction reaction
“photoinduced reduction” hereafter because the structure of
Hyd-K2Ti4O9 changed after the reaction.
Figure 4. (a) Ni2+ adsorption rates from water containing methanol on
P25, K2Ti4O9, and Hyd-K2Ti4O9. (b) O2− evolution rates on K2Ti4O9,
anatase, Hyd-K2Ti4O9. Inset shows energy diagram and scheme for
electron transfer between anatase and K2Ti4O9 components in Hyd-
K2Ti4O9 under light irradiation. (c) Ni2+ deposition rates from water
containing methanol on P25, K2Ti4O9, and Hyd-K2Ti4O9 under light
irradiation.
In addition to the cation exchange ability, the enhanced
charge separation efficiency of Hyd-K2Ti4O9 was examined to
determine if it could catalyze the photoreduction of metals. The
−
O2 yield from TiO2 was quantitatively monitored using
−
The location of the Ni metal in the recovered product was
nitroblue tetrazolium (NBT). NBT reacts with O2 generated
by a reaction between O2 and photoexcited electrons of the
used to clarify part of the mechanism for photoinduced Ni2+
C
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deposition by Hyd-K2Ti4O9. The cross-sectional HRTEM and
HADDF-STEM images of the Ni-deposited Hyd-K2Ti4O9
(Figure 5) reveal that Ni (or NiO) was deposited inside the
adsorption was examined in detail. Figure 6a shows the
photocatalytic activity of different materials for the complete
Figure 6. (a) CO2 evolution rates from methanol in water on P25,
K2Ti4O9, and Hyd-K2Ti4O9 under light irradiation. (b) Adsorption
isotherms of methanol on P25 and Hyd-K2Ti4O9.
oxidation of methanol into CO2 in water. Hyd-K2Ti4O9 had
considerably enhanced activity compared with the starting
material K2Ti4O9. This confirms the O2− evolution data (Figure
4b) and strongly suggests the formation of methanol-derived
radical species, capable of reducing Ni2+ to Ni during the
photoirradiation. The CO2 evolution rate on Hyd-K2Ti4O9 was
moderate compared to P25, a benchmark TiO2 for oxidation of
organic compounds.34,35 It is likely that the moderate methanol
oxidation activity of Hyd-K2Ti4O9 is afforded by the longer
lifetime of the radical species. On the other hand, Hyd-K2Ti4O9
adsorbed methanol more strongly and had a higher capacity
than P25 according to the adsorption isotherm (Figure 6b).
These results confirmed the above scenario.
The results presented above indicate that Hyd-K2Ti4O9 can
be used for the efficient removal and safe disposal of metal ions,
which cannot be achieved by conventional TiO2 photocatalytic
systems. Since Ni2+ ions and NiO clusters are known to
sensitize TiO2 photocatalysts,36−38 we tested the resulting Ni-
deposited Hyd-K2Ti4O9 materials for visible light-induced
photocatalytic H2 evolution via water splitting. As shown in
Figure 7, the Ni@Hyd-K2Ti4O9 material exhibited enhanced
Figure 5. Cross-sectional (a) HRTEM image and (b) HADDF-STEM
image and the corresponding EDX elemental map of Hyd-K2Ti4O9
after photocatalytic Ni deposition.
titanate particles, rather than on the outer surface of the particle
(additional STEM images are shown in Figure S2, together
with their corresponding EDX elemental maps and EDX
spectra). Additionally, many of the Ni particles had a plate-like
shape, strongly suggesting that the two-dimensional interlayer
nanospace served as a template during the growth of the Ni
nanoparticle.30,31 Considering that the XRD peak due to the
basal spacing was shifted to the lower 2θ region and
substantially broadened after the Ni deposition (Figure 2c),
we concluded that Hyd-K2Ti4O9 effectively concentrates Ni2+
from water into the interlayer space and reduces it there in the
presence of methanol and the assistance of photoirradiation.
The HADDF-STEM of Ni-deposited Hyd-K2Ti4O9 also
showed that a part of Ni (or NiO) particles was deposited on
the external surface (Figure S3). In the XPS spectrum (Figure
S4), Ni-deposited Hyd-K2Ti4O9 had a peak at 856 eV, which is
assigned to NiO interacting with supports including TiO2.32,33
From XRD, HRTEM/HAADF-STEM, and XPS data described
above, we can conclude that Ni deposited on the surface of the
particle was oxidized to NiO, while the Ni deposited inside the
particle remained metallic. We cannot, however, rule out the
possibility that the presence of both Ni and NiO results from
difference in reactivity toward O2 between anatase and
K2Ti4O9.
Figure 7. Rate of H2 evolution from water containing methanol from
P25, Hyd-K2Ti4O9, Ni@Hyd-K2Ti4O9 (3 h and 24 h irradiation
products in Figure 4c) and NiO@P25 (Ni loading of 0.5 wt %) under
photoirradiation.
activity at visible wavelengths (λ > 450 nm) because it is a
strong light absorber in this region (Figure 2, right). The
apparent quantum yield at 450 nm was 0.0002%. In contrast,
P25 and Hyd-K2Ti4O9 did not show activity under identical
irradiation conditions because their absorption cross section is
almost zero at λ > 450 nm.
To gain a deeper insight into the mechanism, the ability of
the photocatalyst to drive methanol oxidation and methanol
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Formazan deposits on the photocatalyst. Therefore, by quantifying the
amount of NBT removed from aqueous media, the amount of
generated O2 can be quantified.
Methanol Oxidation. The sample (15 mg) was dispersed in water
(5 mL) containing 5 vol % of methanol in a Pyrex glass tube (34 mL),
and then aerated by O2 bubbling. The glass tube was sealed with a
rubber septum and irradiated by a solar simulator under stirring. The
headspace CO2 was quantified by a Shimadzu GC-2010 plus gas
chromatograph equipped with a BID detector.
As a reference, we prepared NiO cluster-grafted TiO2
(NiO@P25) showing λ > 450 nm light absorption (Figure
S5) via chemisorption of nickel(II) acetylacetonate, followed by
calcination.36,37 The NiO@P25 was not photocatalytically
active under identical conditions. This result suggests the
important role of the Ni species present in the Ni@Hyd-
K2Ti4O9 material, including plate-like Ni nanoparticles which
may enhance activity.
−
H2 Evolution. The sample (15 mg) was dispersed in water (5 mL)
containing 5 vol % of methanol in a Pyrex glass tube (34 mL), and
then deaerated by Ar bubbling. The glass tube was sealed with a rubber
septum and irradiated by a 150 W Xe lamp with a long-pass filter
(>450 nm). The headspace H2 was quantified by a Shimadzu GC-8A
gas chromatograph equipped with a TCD detector. For the apparent
quantum yield (AQY) calculation, the glass tube was irradiated with a
monochromated light for 3 h using a 500 W Xe lamp (Ushio) and an
SM-25 monochromator (Bunkoukeiki). The number of incident
photons was determined using an S1337-1010BQ silicon photodiode
(Bunkoukeiki). AQY was defined by the following equation: AQY (%)
= [number of evolved H2 molecules × 2]/[number of incident
photons] × 100.
CONCLUSION
■
We reported a simple hydrothermal method to selectively
dissolve the edges of layered titanate (K2Ti4O9) nanobelts.
Anatase nanocrystals recrystallize on the K2Ti4O9 surfaces,
forming heterostructures that exhibit (i) superb cation
exchange ability comparable to the titanate starting material
and (ii) enhanced charge separation efficiency. The hetero-
structures possess enhanced activity for the photoreduction of
Ni2+ in water into Ni, while a benchmark TiO2 photocatalyst
was inactive. Numerous 1D and 2D titanate compounds can be
easily synthesized like K2Ti4O9. Therefore, this chemically
exploitable weakness in the titanate edges can be used to design
a variety of different heterostructured photocatalysts for the safe
capture and disposal of toxic metal cations and recovery of
valuable metals from the environment.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00538.
EXPERIMENTAL SECTION
■
Synthesis of Hyd-K2Ti4O9. A 40 wt % aqueous solution of TPA
was purchased from Tokyo Chemical Industry and used as received.
TPA (0.78 g), NH4F (0.014 g), and K2Ti4O9 (0.2 g) were added in a
Teflon-lined stainless steel autoclave, and the mixture was heated at
170 °C for 1 week. After the hydrothermal reaction, the product was
washed with ethanol and dried at 60 °C.
HRTEM and HADDF-STEM images, EDX elemental
maps of Hyd-K2Ti4O9 and Ni-deposited Hyd-K2Ti4O9,
XPS spectra of Ni-deposited Hyd-K2Ti4O9 and P25,
UV−vis spectra of P25 and NiO-grafted P25 (PDF)
Materials Characterization. XRD patterns of all samples were
examined using a Smart Lab, RIGAKU. The UV−vis spectra of the
samples were measured using a JASCO V-570UV-vis spectropho-
tometer. The morphology and structure of all samples were observed
via FE-SEM (JEOL JEM-6500F) and (high-resolution) TEM (JEOL
JEM 2100F). XPS was performed with a JEOL JPS-9010 instrument.
All binding energies were calibrated in relation to the C 1s line from
adventitious carbon (285 eV). Nitrogen and methanol vapor
adsorption/desorption isotherms were recorded using a MicrotrakBel
Belmax. The sample powders were evacuated at 120 °C for 3 h before
the measurements.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: IDE.Yusuke@nims.go.jp.
ORCID
Yusuke Ide: 0000-0002-6901-6954
Joel Henzie: 0000-0002-9190-2645
Author Contributions
The manuscript was written through contributions of all
authors.
Ni2+ Adsorption and Reduction. The sample (15 mg) was
dispersed in a mixed solution of an aqueous NiCl2·6H2O (213 mg/L, 5
mL) solution and methanol (5 mL) in a Pyrex glass tube (34 mL), and
then deaerated by Ar bubbling. The glass tube was sealed with a rubber
septum and irradiated by a solar simulator (San-Ei Electric, λ > 300
nm, 1000 Wm−2) under stirring. After separation of the mixture, the
amount of Ni2+ in the supernatant was measured with inductively
coupled plasma atomic emission spectroscopy ICP-AES (Agilent 710-
ES). The solid was analyzed by XRD and UV−vis spectroscopy to
confirm the deposition of Ni semiquantitatively. Ni2+ adsorption was
carried out without the photoirradiation. The amount of Ni2+ in the
supernatant decreased while no Ni deposition was detected (and the
color of the solid was light green, not but gray as seen in Figure 2c).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly supported by JSPS KAKENHI Grant
Number 26708027. We thank Shinpei Enomoto (Waseda
University) for TEM observation.
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DOI: 10.1021/acs.inorgchem.8b00538
Inorg. Chem. XXXX, XXX, XXX−XXX