A photocatalytically inactive microporous titanate nanofiber as an excellent and versatile additive to enhance the TiO2 photocatalytic activity

Article abstract of DOI:10.1039/c4ta06027j

The photocatalytic activity of TiO₂ under simulated solar light, evaluated by the oxidative decomposition of organic compounds in water, was substantially enhanced when a new and photocatalytically inactive TiO₂-based nanostructured material (a microporous titanate nanofiber) was just mixed with the starting suspension, as a result of the unique electron capture ability of the titanate to retard charge recombination.

Full text of DOI:10.1039/c4ta06027j

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Photocatalytically inactive microporous titanate
nanofiber as an excellent and versatile additive to
enhance TiO2 photocatalytic activity
Cite this: DOI: 10.1039/x0xx00000x
Y. Ida*ab and K. Komaguchi*c
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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and the unique properties such as wellꢀrestrained photocatalytic
activity and extremely low refractive index.12 Herein, we report an
extraordinary and important finding that the photocatalytic activities
of TiO2 under solar light can be substantially enhanced by just using
the photocatalytically inactive MPTNF as the additive of the
The photocatalytic activity of TiO2 under simulated solar
light, evaluated by the oxidative decomposition of organic
compounds in water, was substantially enhanced when a new
and photocatalytically inactive TiO2-based nanostructured
material (microporous titanate nanofiber) was just mixed
with the starting suspension, as a result of the unique electron
capture ability to retard charge recombination.
reactions. We highlight
a successful enhancement in the
photocatalytic activity of a commercial TiO2, P25 (mainly composed
of anatase and rutile), which has frequently been used as a
benchmark for photocatalysis because it is not easy to find and
synthesize TiO2 photocatalysts showing activities higher than that of
P25.6b We anticipate MPTNF to be a key material for ultraꢀhighly
active and lowꢀcost green photocatalysis.
Development of highly active solid photocatalysis systems is
among the most attractive topics not only for environment
purification but also for fuel production and fine chemical
synthesis.1ꢀ3 While acknowledging tremendous effort to design
advanced photocatalysts like those including noble and rare metals,
lowꢀprice photocatalysts are preferential for practical use. TiO2 is
one of the most promising photocatalysts because of its
inexpensiveness, combined with longꢀterm chemical stability and
negligible toxicity. One weakness of TiO2 is that it absorbs only
ultraviolet light, occupying 3ꢀ5% of the incident solar light, and then
shows negligible and low activity when used under visible light and
solar light irradiation, respectively. One approach for highly active
TiO2 photocatalysis under such irradiation conditions in an
economically benign fashion is to design new TiO2 having
noticeable structures or properties such as high surface area and high
crystallinity,4 anatase/rutile heterojunction5 and predominantly
exposed reactive facets.6 Another is to develop operating
environments (e.g. temperature,7 light irradiation direction,8
additives,9 solvent compositions,10 headꢀspace atmospheres[11]) to
enhance the activities of abundant TiO2, though it is not easy to find
such environments.
We have recently reported the synthesis of a new TiO2ꢀbased
nanostructured material, a microporous titanate nanofiber (MPTNF),
Fig. 1. (A) Xꢀray diffraction patterns of (a) K2Ti2O5, (b) MPTNF and (c)
P25 (mainly composed of anatase and rutile), (B) SEM image of
MPTNF, (C) Ar adsorption (open circles)/desorption (filled circles)
isotherms of MPTNF (a) before and (b) after crystal violet adsorption
and (D) UVꢀvis spectra of (a) MPTNF and (b) P25. Inset shows the pore
size distribution of MPTNF and crystal violetꢀadsorbed MPTNF by
HorwarthꢀKawazoe method applied to the adsorption isotherms.
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MPTNF was synthesized via the simple hydrothermal treatment of resulted from interꢀnanofiber mesopores, which were observed in
a layered titanate, K2Ti2O5.12 The chemical formula of MPTNF was Fig. 1B). An inorganic cation (Ca2+) effectively adsorbed inside the
H0.65K0.75Ti2.15O5 (K and partial H are exchangeable cations),12 particle, while a larger organic cation, crystal violet (CV, molecular
which was different from that of the starting layered titanate. size of 1.4×1.4×0.4 nm3), adsorbed much less effectively because
MPTNF showed the Xꢀray diffraction pattern different from that of CV was difficult to penetrate along a nanofiber axial direction due to
the starting layered titanate and those of conventional TiO2 and diffusion limitation.12 After CV adsorption (mainly in the vicinity of
titanates such as anatase, rutile, layered titanates and titanate the particle outer surface), Ar uptake at lower partial pressure
nanotube (Fig. 1A). MPTNF had a nanofibrous morphology with a dramatically decreased (Fig. 1C a vs b). Such significant decrease
diameter up to 10 nm and a length up to several hundreds of nm must not be observed if MPTNF has randomly oriented micropores
(Fig. 1B). From various analyses, it was suggested that MPTNF had or microchannels perpendicular to or across a nanofiber axial
several microchannels (tunnels) with a width of ca. 0.5 nm along a direction, and similar phenomenon has been observed for a clay
nanofiber axial direction in a nanofiber.12 An example analysis is mineral sepiolite having ordered microchannels (so called “tunnels”
presented in Fig. 1C. The Ar adsorption/desorption isotherms of with a crossꢀsection dimension of about 1.1×0.4 nm2) in a
MPTNF exhibited the abrupt uptake of Ar at lower partial pressure, microfibrous particle.13 The photocatalytic activity of MPTNF was
indicating the presence of micropores or microchannels (hysteresis evaluated by the oxidation of cyclohexane in acetonitrile and 2ꢀ
propanol in water under simulated solar light irradiation (λ >
320nm) to be negligible, which presumably resulted from the unique
structure (e.g. low crystallinity as shown in Fig. 1A).12
On the basis of our previous successes to control the
photocatalytic activities of semiconductor photocatalysts using
photocatalytically indifferent materials, such as layered silicates and
mesoporous silicas, as additives,9cꢀg we conceived to use MPTNF as
the additive of TiO2 photocatalysis. First, we tried to enhance the
photocatalytic activity of P25, because P25 is one of the best TiO2
photocatalysts for various reactions.14 Here we evaluated the
photocatalytic activity of P25 and P25/MPTNF mixture by the
decomposition of formic acid to evolve CO2 from an aerated
(molecular O2ꢀsaturated) aqueous solution, a representative reaction
to test the performance of photocatalysts for organic pollutant
removal.6b,15 Fig. 2A shows the time course of CO2 evolution from
an aerated aqueous solution of formic acid containing P25 or
P25/MPTNF mixture under simulated solar light irradiation (λ > 320
nm). With P25, the amount of CO2 for formic acid decomposition
linearly increased with irradiation time as reported previously.6b,15
To our surprise, P25/MPTNF mixture showed considerably higher
photocatalytic activity than did P25. MPTNF was inactive for the
reaction (Fig. 2B), which was consistent with our previous results.12
Therefore, a probable synergetic effect of the coꢀpresence of P25 and
MPTNF was revealed (MPTNF acted neither to concentrate organic
molecules near P25 surface nor to scatter light to lead to better light
adsorption of P25, enhancing the photocatalytic activity as described
below). The enhancement of the photocatalytic activity of P25 was
up to three times by changing amounts of the added MPTNF (Fig.
Fig. 2. (A) Time courses of CO2 evolution from aerated aqueous
solutions (5 mL) of formic acid containing (a) P25 (15 mg) and (b)
P25/MPTNF mixture (15 mg/3 mg) under simulated solar light (λ > 320
nm). (B) CO2 evolution rates at different P25/MPTNF, P25/amorphous
TiO2 and P25/rutile mixing ratios (mg/mg). Mesh bar indicates CO2
evolution rate for reused P25/MPTNF mixture after washing with water.
2B). On the other hand, other photocatalytically inactive or lowꢀ
16
active TiO2, such as amorphous TiO2 and rutile (JRCꢀTIOꢀ6)11e,14
,
did not synergetically enhance the activity of P25 (the activity of
P25/amorphous TiO2 (or P25/rutile) mixture was similar to the sum
of the activities of each TiO2, Figure 2B). Furthermore, P25/MPTNF
was successfully reused without losing the original activity (Fig. 2B,
mesh bar), as a result of the chemical stability of MPTNF.12 All
results described above indicates an importance of our MPTNF as
the additive of TiO2 photocatalytic reactions.
MPTNF was further used to enhance activities of other TiO2
photocatalytic reactions. Fig. 3A compares time courses of
photocatalytic decomposition of methanol in aerated aqueous
solutions containing P25 and P25/MPTNF mixture under simulated
solar light irradiation (λ > 320 nm). MPTNF could enhance the
photocatalytic activity of P25 also for this reaction. Moreover, as
shown in Fig. 3B, the photocatalytic activity for the oxidative
decomposition of formic acid on anatase, not but P25, was also
significantly enhanced when MPTNF was added into the starting
suspension. These results confirm the versatility of MPTNF as
additives to enhance photocatalytic reactions.
Fig. 3. Time courses of CO2 evolution under simulated solar light
irradiation (λ > 320 nm) from an aerated aqueous solutions (5 mL) of (A)
methanol containing (a) P25 (15 mg) and (b) P25/MPTNF mixture (15
mg/3 mg) and (B) formic acid containing (a) anatase (15 mg) and (b)
anatase/MPTNF mixture (15 mg/3 mg). (C) Rates for photocatalytic
degradation of phenol in aerated aqueous solutions (40 mL) containing
P25/MPTNF mixtures (mg/mg) at different mixing ratios under a full arc
of Xe lamp (λ > 320 nm).
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Fig. 3C shows the rates for the photocatalytic degradation of
phenol in an aerated aqueous solutions containing P25 and
P25/MPTNF mixtures at different mixing ratios under a full arc of
Xeꢀlamp irradiation (λ > 320 nm). MPTNF could enhance the
activity of P25 also for this reaction, suggesting again the versatility
of this material. It should be noted here that when the reaction was
conducted with MPTNF, only twoꢀfifteenth (ca. 13%) amounts of
the photocatalyst were required to obtain the comparable reaction
rate without MPTNF. This motivates us to use considerably reduced
amounts of highly active stateꢀofꢀtheꢀart photocatalysts, like those
including novel and rare metals, with MPTNF for much lowerꢀcost
photocatalysis.
Fig. 4. Photographs of aqueous suspensions containing (left) MPTNF,
(middle) P25 and (right) P25/MPTNF mixtures. 6, 15 and 15/6 mg of
powders, respectively, were suspended in 5 mL of water.
We investigated the possible mechanism for the enhanced
photocatlytic activities of TiO2 in the presence of MPTNF. It has
been reported that when photocataysts particles are mixed with
adsorbents ones like clay minerals, adsorbents can concentrate
substrates near photocatalysts to promote the photocatalysis. This
did not apply to the present case, because MPTNF hardly adsorbed
formic acid, methanol and phenol in water (probably due to the
narrow channels and the surface hydrophilicity12). Also, we
considered the possibility that MPTNF efficiently scattered light to
lead to better light absorption of P25. Fig. 4 shows the photographs
of aqueous suspensions containing P25, MPTNF and P25/MPTNF
mixture. MPTNF was considerably more transparent than P25 when
suspended in water, which was as a result of much lower refractive
index (ca. 1.7).12 Therefore, the positive light scattering effect of
MPTNF on P25 photocatalysis was not predominant.
While, the photocatlytic decomposition of orgnic compounds into
CO2 often proceeds effciently only under aerated conditions and the
plausible mechanism involves capture of photoexcited electrons with
molecular O2 to produce relatively stable superoxide radical anion
and retard electronsꢀholes recombination17 (although another
involves production of peroxy radicals, as a caririer for radical chain
reaction, through reaction with radical species evolved by the
reaction of holes with organic substrates18). Here, we investigated
electron transfer from P25 to MPTNF by monitoring Ti3+ formed in
P25 rutile, by trapping the photoexcited electrons,19 with electron
spin resonance (ESR) analysis. As shown in Fig. 5A, when irradiated
by UV light (λ > 330 nm) in the presence of molecular O2, P25
created a broad signal due to Ti3+ (g = 1.980), in addition to signals
Fig. 5. (A) ESR spectra of P25, MPTNF, P25/MPTNF mixtures with
different mixing ratios by weight, amorphous TiO2 and rutile. The
samples (10 mg) were treated with O2 under UV light irradiation (λ > 330
nm) at room temperature and measured at 77 K after evacuation. (B)
Variation of Ti3+ yields per P25 contents as a function of P25/MPTNF
mixing ratios (mg/mg).
−
due to superoxideꢀtype oxygen anion (O2 ; gxx = 2.003, gyy = 2.010,
gzz = 2.022) which was formed by the reduction of O2 with the
photoexcited electrons.20 On the other hand, when P25/MPTNF
mixtures were irradiated under the identical condition, Ti3+ yeild
(based on P25 contents) decreased depending on amounts of the
added MPTNF (Fig. 5A and B), suggesting efficient electron transfer
from photoexcited P25 to MPTNF to retard electronsꢀholes
recombination on P25 and enhance the photocatalytic activity. Such
electron transfer may occur, because the conduction band potential
of MPTNF is similar to that of P25 on the basis of speclation from
their band gaps (Fig. 1D). As shown in Fig. 5C, P25/MPTNF
showed much higher photocatalytic activity than P25 also under the
irradiation of monochromatic light, 390 nm, under which only P25
was excited (Fig. 5C). This confirms the above hypothesis. The
photocatalytic activities of P25/MPTNF mixtures did not depend on
amounts of the added MPTNF (Fig. 2B), which probably resulted
from that redudant MPTNF, which did not interact with P25, just
absorbed UV light to reduce UV light absorption by P25.
Fig. 6. (A) Particle size distribution determined by DLS of (black) P25
(15 mg), (orange) MPTNF (6 mg) and (green) P25/MPTNF mixture (15/6
mg) suspended in water (5 mL). (B) TEM image of P25/MPTNF mixture
(1/0.2 mg) suspended in water (15 mL) after evaporation of water.
Fig. 5C (gray bars) shows the photocatalytic formic acid
decompositon rates on P25 and P25/MPTNF mixture in deaerated
from formic acid mainly via radical chain reaction initiated by holes,
aqueous solutions (in the absence of O2). P25 hardly gave CO2, where electrons is not included18). This results again support our
while, P25/MPTNF mixture produced significant amounts of CO2 hypothesis that MPTNF can capture electrons photoexcited on
adjacent P25, like molecular O2, to retard electronsꢀholes
recombination on P25 (the degree (31%) in lowering of CO2
even under such deaerated condition (CO2 was thought to be evolved
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4TA06027J
(SanꢀEi Electric, λ > 320 nm, 1000 W mꢀ2) under stirring. The gas in
evolution rates without O2 for P25/MPTNF mixture was slightly
larger than that (20%) for P25, therefore, we could not rule out the
possibility that MPTNF captured electrons from P25, namely, acts as
a good additive for P25, only in the presence of O2 for some reason).
the container was withdrawn with a gasꢀtight syringe and quantified
using
a
BID gas chromatograph (Shimadzu BIDꢀ2010 plus)
Micropacked ST column. In some of the
equipped with
a
experiments, irradiation was performed after bubbling the starting
mixture with Ar to remove dissolved O2. Phenol degradation was
also performed in the stainlessꢀmade container (75 mL) equipped
with Pyrex window. In this case, the sample was suspended in an
O2ꢀsaturated aqueous solution (40 mL) containing 60 ppm of phenol,
the suspension was irradiated by a full arc of Xe lamp (Bunko Keiki,
150 W, λ > 320 nm) with magnetic stirring, and the amount of the
remained phenol in the supernatant was quantified by a HPLC
(Shimadzu LCꢀ20AD) with a Union UKꢀC18 column.
ESR analysis. The ESR spectra were recorded on a JEOL JES REꢀ
1X spectrometer (Xꢀband). The magnetic field was calibrated and
radical yields were determined using Mn2+/MgO as a standard
marker. The samples were mixed in a mortar and a portion (10 mg)
of the mixture was placed in a quartz ESR tube, which was
evacuated at 423 K for 1 h and cooled to room temperature. O2 (20
Torr) was introduced into the tube and kept for 10 min. The tube was
photoirradiated at room temperature using the 500 WꢀXe lamp at λ >
330 nm for 5 min. The tube was then evacuated for 10 min to
remove excess O2 and analysed at 77 K.
To further discuss the feasibility of electrons transfer between P25
and MPTNF during photocatalytic reactions, we investigated the
mixing state of P25 and MPTNF in the aqueous suspension. Fig. 6A
shows the particle size distribution of P25, MPTNF and
P25/MPTNF mixture suspended in water determined by dynamic
light scattering (DLS). P25 showed bimodal distribution with
particle sizes of ca. 300 and 7000 nm, which are assigned to small
and large aggregates, respectively, of the primary particles (ca. 20ꢀ50
nm). MPTNF also showed bimodal size distribution at ca. 240 and
1100 nm, which corresponds to aggregated MPTNF particles
observed in the SEM image (Fig. 1B). On the other hand,
P25/MPTNF mixture showed a peak at ca. 4000 nm which P25 and
MPTNF solely did not show, in addition to a peak at ca. 240 nm like
P25 and MPTNF solely. From these results, it is thought that when
just mixed in water, a part of P25 and MPTNF can form aggregates
in which P25 particles can directly contact to MPTNF particles. We
observed the TEM image of P25/MPTNF mixed suspension after the
evaporation of water, finding that P25 and MPTNF particles
contacted each other (Fig. 6B).
DLS analysis. DSL was performed on a Micromeritics
NanoPlus zeta potential and nano particle analyser. The
measurement was carried out with water (5 mL) containing P25
(15 mg), MPTNF (6 mg) or P25/MPTNF (15 mg/6 mg)
mixture, after ultrasonication for 3 min and the subsequent
stirring for 20 min.
Finally, we discussed the possible reason for the efficient electron
capture and O2 reduction abilities of MPTNF. As shown in Fig. 5A,
MPTNF exhibited a larger gzz value for O2 than conventional TiO2
−
such as P25, amorphous TiO2 and rutile. This indicates weaker
− 21
intractions between MPTNF and O2 . We then believe that
MPTNF can effectively adsorb and reduce O2 by promptly releasing
−
the onceꢀgenerated O2 from its surface and then effectively capture
Notes and references
International Center for Materials Nanoarchitectonics (MANA),
electrons newly photogenerated on neighboring TiO2, as a result of
the unique structure. Because a wide variety of TiO2ꢀbased materials
with different structures is available22 and electron migration
behaviors in TiO2ꢀbased materials are known to be affected by the
structures,23 further systematic studies are worth conducting to
clarify the mechanism of the unique electron capture and O2
reduction abilities of MPNTF.
a
National Institute for Materials Science (NIMS), 1ꢀ1 Namiki, Tsukuba,
Ibaraki 305ꢀ0044, Japan.
b
Graduate School of Creative Science and Engineering, Waseda
University, 1ꢀ6ꢀ1 Nishiwaseda, Shinjukuꢀku, Tokyo 169ꢀ8050, Japan.
c
Graduate School of Engineering, Department of Applied Chemistry,
In summary, we have reported that the photocatalytic activities of
TiO2 for the decomposition of organic compounds in water can be
substantially enhanced by just adding a new and photocatalytically
inactive TiO2ꢀbased nanostructured materials, microporous titanate
nanofiber, into the starting suspensions. This resulted from the
unique electron capture ability of the titanate from photoexcited
TiO2 to retard the charge recombination. We are currently
investigating the effect of microporous titanate nanofiber and other
nanostructured materials as additives on various photocatalytic
reactions.
Hiroshima University, 1ꢀ4ꢀ1 Kagamiyama, HigashiꢀHiroshima 739ꢀ8527,
Japan.
†
This work was partly supported by JSPS KAKENHI Grant Number
26708027.
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J. Name., 2012, 00, 1-3 | 5

Journal of Materials Chemistry A
Page 6 of 6
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DOI: 10.1039/C4TA06027J
Table of contents
When just mixed with a new and photocatalytically inactive material (microporous titanate
nanofiber), TiO2 showed largely enhanced photocatalytic activities.