catalysts
Article
Three-Dimensional TiO2 Structures Incorporated
with Tungsten Oxide for Treatment of Toxic Aromatic
Volatile Compounds
Joon Yeob Lee and Wan-Kuen Jo *
Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Korea;
[email protected]
* Correspondence: [email protected]; Tel.: +82-539-506-584; Fax: +82-539-506-579
Academic Editors: Shaobin Wang and Xiaoguang Duan
Received: 24 February 2017; Accepted: 20 March 2017; Published: 23 March 2017
Abstract: This study assessed 3D WO3 –TiO2 nanoflowers (WTNF) synthesized by a combined
hydrothermal–ultrasonication–impregnation method for their applicability to the treatment of
aromatic volatile compounds under visible-light illumination. The scanning electron microscopy
exhibited the formation of 3D structures in the prepared WTNF samples. The X-ray diffraction
patterns and energy dispersive X-ray results indicated a successful incorporation of WO3 into TNF
structures. The UV-visible spectroscopy showed that the prepared WTNF samples can be functioned
under visible light irradiation. The output-to-input concentration ratios of toluene and o-xylene
with WTNF samples were lower than those of TiO2 nanoflowers. These findings were illustrated
on the basis of charge separation ability, adsorption capability, and light absorption of the sample
photocatalysts. The input-to-output concentration ratios of the target chemicals were lowest for 10 M
NaOH and highest for 5 M NaOH. The photocatalytic degradation efficiencies of WTNF sample
photocatalysts increased with increasing WO3 content from 0.1% to 1.0%, and dropped gradually
with increasing WO3 content further to 4.0%. Light-emitting-diodes (LEDs) are a more highly
energy-efficient light source compared to a conventional lamp for the photocatalytic degradation of
toluene and o-xylene, although the photocatalytic activity is higher for the conventional lamp.
Keywords: nanoflower; NaOH concentration; WO3 content; light-emitting-diode; conventional lamp
1. Introduction
Exposure to aromatic volatile compounds indoors has become an important environmental
issue because it is closely linked to the adverse health risk of building occupants. There are a wide
range of indoor sources of aromatic pollutants, such as building finishing materials, furniture, and
household products, resulting in higher indoor pollution for these pollutants relative to outdoor
concentrations [1]. Most aromatic volatile compounds display high carcinogenic chronic effects
and non-carcinogenic chronic effects such as damage to liver, kidneys, the central nervous system,
and the respiratory system [2,3]. Moreover, many people spend most of their personal time in indoor
environments, thus justifying the application of mitigation strategies for indoor aromatic volatile
compound concentrations to reduce the health risks of building occupants.
The photocatalysis of titanium dioxide (TiO2 ) is an advanced oxidation method that can
efficiently be applied in the treatment of a variety of environmental pollutants [4–9]. Nevertheless,
the environmental application of TiO2 is obstructed by a wide band gap, which is restricted to the
ultraviolet (UV) region [6,8,10]. Photocatalysis of TiO2 is also hampered by high recombination rates
of charge carriers, thus resulting in a low quantum yield [6]. To solve these problems, a great deal
of techniques have been developed to modify the surface characteristics of TiO2 [11]. The surface
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modification of TiO2 with tungsten trioxide (WO3 ), which is a narrow-band gap semiconductor
(2.4–2.8 eV), is an interesting technique for expanding the light absorbance capability of TiO2 to
the visible range [12,13]. WO3 can act as an electron acceptor, thus lowering the recombination
rate of photoinduced electrons and holes on the surface and bulk spaces of TiO2 and elevating its
photocatalytic activity [14]. The electron acceptation by WO3 was ascribed to a fast reaction rate
of W6+ to W5+ . Additionally, in recent studies, WO3 -incorporated TiO2 powders revealed higher
photocatalytic activity compared to pure TiO2 for the destruction of certain water and air pollutants
with visible light or UV exposure [12,14–16].
Conversion of TiO2 structural dimension is also a potential technique for the enhancement of
photocatalytic activity of TiO2 since it affects the movement of electrons and holes, light applicability,
and adsorption capacity [17]. Previous researchers have prepared one-dimensional titanate
nanowires and two-dimensional TiO2 nanosheets to enhance the functional properties of TiO2 [18,19].
Horváth et al. [20] noted that some of titanate compounds are not active as photocatalysts under certain
conditions. In particular, a three-dimensional (3D) photocatalyst exhibits a high surface area-to-volume
proportion, accelerating the movement of charge carriers and the transport of pollutant molecules
to the surface of photocatalyst, and increasing light absorbance capacity [17,21]. Some researchers
have found that 3D TiO2 -incorporated architectures had a higher photocatalytic activity relative to
that of zero-dimensional (0D) TiO2 -incorporated architectures for the treatment of aqueous-phase
p-chlorophenol, methylene blue, methyl orange, and rhodamine B [22–25].
With the merits of WO3 incorporation into TiO2 (WO3 -TiO2 ) and 3D TiO2 for photocatalytic
activities, their combination is proposed to provide a synergistic effect for the treatment of
environmental pollutants. Unfortunately, the applicability of 3D WO3 -TiO2 hybrids to gas-phase
pollutant treatments is hardly reported in scientific literature. Accordingly, in this study, 3D WO3 -TiO2
nanoflowers (WTNF) were synthesized using a combined hydrothermal–ultrasonication–impregnation
method to examine their applicability to the treatment of aromatic volatile compounds under visible
light illumination. The photocatalytic treatment tests were performed under different conditions by
varying the amount of WO3 content, the concentration of NaOH, and the light source. The amount
of semiconductors incorporated into TiO2 is an important parameter for the photocatalytic activity
of semiconductor-embedded TiO2 hybrids [26]. Additionally, the concentration of NaOH, which is
used during the ultrasonication process, can also influence the formation of 3D architectures and thus
their photocatalytic activities for the degradation of aromatic volatile compounds [27]. Light source
type is another important parameter for the photocatalytic performance of many photocatalysts [28].
For comparison, a reference photocatalyst (3D TiO2 nanoflower sample, TNF) was additionally
prepared, and its characteristics and photocatalytic activity were investigated.
2. Results and Discussion
2.1. Fabricated Photocatalysts
The natures of the fabricated samples were inspected by XRD analysis, SEM/EDX, and UV-visible
spectroscopy. Figure 1 shows the powder XRD results of pure TiO2 nanoflowers (TNF) and
WO3 -TiO2 nanoflowers with different WO3 loadings (WTNF-0.1, WTNF-0.5, WTNF-1.0, WTNF-2.0,
and WTNF-4.0). All the samples revealed both anatase and rutile phase peaks with a primary peak at
2θ = 25.34◦ corresponding to the (101) plane and a large peak at 2θ = 27.29◦ corresponding to the (110)
plane, respectively. These patterns are similar to the XRD results obtained from commercial P25 TiO2 ,
which were reported in previous studies [29,30]. Notably, WTNF-4.0 exhibited additional two peaks
at 2θ = 24.19◦ corresponding to the (110) plane and at 2θ = 49.91◦ corresponding to the (220) plane.
These findings indicate that WO3 was successfully incorporated into TiO2 structures in the WTNF-4.0.
This assertion is supported by EDX images (Figure 2), which display the presence of WO3 in all WTNF
samples. Meanwhile, there were no WO3 -associated peaks for TNF. The EDX images shows that
the WTNF samples consist of Ti and WO3 elements, whereas the TNF samples consist only of Ti
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(Figure 2), which display the presence of WO3 in all WTNF samples. Meanwhile, there were no
(Figure
2), whichpeaks
display
the presence
of images
WO3 inshows
all WTNF
samples.
thereofwere
no
WO3-associated
for TNF.
The EDX
that the
WTNFMeanwhile,
samples consist
Ti and
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2017,
7,
97
3
of
11
WO
peaks for
EDXconsist
imagesonly
shows
that
the WTNF
samplesshown
consistinofthe
TiXRD
and
WO33-associated
elements, whereas
theTNF.
TNFThe
samples
of Ti
elements.
Pt elements
WO
3 elements, whereas the TNF samples consist only of Ti elements. Pt elements shown in the XRD
images
of all samples are ascribed to a Pt coating for a sample pretreatment process. However,
images
of
all samples
are
ascribed
to a3 Pt
coating for a of
sample
pretreatment
process.WTNF-0.5,
However,
other WTNF
samplesshown
with
WO
concentrations
2.0%
or less
elements.
Pt elements
insmall
the XRD
images
of all samples are
ascribed
to a(WTNF-0.1,
Pt coating for
a sample
other
WTNF
samples
withdid
small
WO3 concentrations
of 2.0%
or less (WTNF-0.1,
WTNF-0.5,
WTNF-1.0,
and
WTNF-2.0)
not
reveal
WO
3 peakswith
in their
XRD
These
results
are
pretreatment
process.
However,
other
WTNF
samples
small
WOimages.
concentrations
of
2.0%
ormost
less
3
WTNF-1.0,
and
WTNF-2.0)
did
not
reveal
WO3bepeaks
in their
XRD
images. instrument
These results
areinmost
likely
due
to
low
amounts
of
WO
3
that
cannot
detected
by
the
analytical
used
this
(WTNF-0.1, WTNF-0.5, WTNF-1.0, and WTNF-2.0) did not reveal WO3 peaks in their XRD images.
likely
to low amounts
of WO
3 that cannot be detected by the analytical instrument used in this
study.due
Additionally,
Figure
depicts
theamounts
SEM images
the prepared
photocatalysts.
All
These
results
are most
likely3due
to low
of WOof3 that
cannot besample
detected
by the analytical
study.
Additionally,
Figure
3
depicts
the SEM regardless
images of thethe
prepared
sample of
photocatalysts.
All
samples exhibited
3D nanoflower
structures,
incorporation
WO3 or not.
As
instrument
used in this
study. Additionally,
Figure 3 depictsofthe
SEM
images of the
prepared
sample
samples exhibited 3D nanoflower structures, regardless of the incorporation of WO3 or not. As
photocatalysts.
All samples
3D nanoflower
structures, regardless of the incorporationmethod
of WO3
such, these results
suggest exhibited
that the combined
hydrothermal–ultrasonication–impregnation
such,
these
results
suggest
that
the combined
hydrothermal–ultrasonication–impregnation
method
or
not.
As
such,
these
results
suggest
that
the
combined
hydrothermal–ultrasonication–impregnation
employed in this study can be applied for the synthesis 3D WTNF samples.
employed
in this study
can
be applied
the synthesis
3D WTNF
method employed
in this
study
can be for
applied
for the synthesis
3Dsamples.
WTNF samples.
Figure 1. X-ray diffraction patterns of pure TiO2 nanoflowers (TNF) and WO3‒TiO2 nanoflowers
1.
diffraction
patterns
pure
TiOTiO
(TNF)
and and
WO3WO
-TiO3‒TiO
with
Figure
1. X-ray
X-ray
diffraction
patternsofof
pure
2 nanoflowers
(TNF)
2 nanoflowers
2 nanoflowers
2 nanoflowers
with different
WO
3 loadings (WTNF-0.1, WTNF-0.5,
WTNF-1.0, WTNF-2.0,
and WTNF-4.0).
different
WO3 WO
loadings
(WTNF-0.1,
WTNF-0.5,
WTNF-1.0,
WTNF-2.0,
and WTNF-4.0).
with
different
3 loadings
(WTNF-0.1,
WTNF-0.5,
WTNF-1.0,
WTNF-2.0,
and WTNF-4.0).
Figure 2.
Energy-dispersive X-ray spectroscopy of pure TiO2 nanoflowers (TNF) and
Figure 2. Energy-dispersive X-ray spectroscopy of pure TiO2 nanoflowers (TNF) and WO3‒TiO2
WO
-TiO
nanoflowers
with different
WO3 loadings
(WTNF-0.1,
WTNF-0.5, WTNF-1.0, WTNF-2.0,
3
Figure
2. 2Energy-dispersive
of pure
TiO
2 nanoflowers (TNF) and WO3‒TiO2
nanoflowers
with different X-ray
WO3 spectroscopy
loadings (WTNF-0.1,
WTNF-0.5,
WTNF-1.0, WTNF-2.0, and
and
WTNF-4.0).
nanoflowers
WTNF-4.0). with different WO3 loadings (WTNF-0.1, WTNF-0.5, WTNF-1.0, WTNF-2.0, and
WTNF-4.0).
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Figure
3. 3.
Scanning
electron
microscopy
of
pure
TiO22 nanoflowers
nanoflowers(TNF)
(TNF)and
andWO
WO
3‒TiO2 nanoflowers
nanoflowers
Figure
3.
Scanning
nanoflowers
(TNF)
and
WO
3 -TiO
2
Figure
Scanningelectron
electronmicroscopy
microscopyof
of pure
pure TiO
TiO
3‒TiO
2 nanoflowers
with
different
WO
3
loadings
(WTNF-0.1,
WTNF-0.5,
WTNF-1.0,
WTNF-2.0,
and
WTNF-4.0).
with
different
WO
WTNF-1.0,WTNF-2.0,
WTNF-2.0,and
andWTNF-4.0).
WTNF-4.0).
with
different
WO
3 loadings(WTNF-0.1,
(WTNF-0.1, WTNF-0.5,
WTNF-0.5, WTNF-1.0,
3 loadings
The
UV-visible
absorption
spectra
the
sample photocatalysts
photocatalystswere
weredetermined
determined to
examine
The
UV-visible
absorption
spectraofof
of
the
sample
examine
The
UV-visible
absorption
spectra
the
sample
photocatalysts were
determined totoexamine
the
thethe
characteristics
ofoftheir
electronic
structures
(Figure 4).
4). Unmodified
Unmodified3D
3DTiO
TiO
2 structures
(TNF)
characteristics
their
electronic
structures
(Figure
2 structures
(TNF)
characteristics of their electronic structures (Figure 4). Unmodified 3D TiO2 structures (TNF) revealed
revealed
region with
with aaband
bandgap
gapedge
edgeatat3.30
3.30
revealeda ahigh
highabsorption
absorptionproperty
property in
in the
the UV region
eV,eV,
butbut
no no
a high absorption property in the UV region with a band gap edge at 3.30 eV, but no significant
significant
absorbance
1), which
whichwas
wasininaccordance
accordancewith
with
the
results
TiO
2
significant
absorbanceininthe
thevisible
visiblerange
range (Table
(Table 1),
the
results
of of
TiO
2
absorbance in the visible range (Table 1), which was in accordance with the results of TiO2 powder,
powder,
whichwere
werereported
reportedin
inprevious
previous studies [31,32].
spectra
of of
powder,
which
[31,32].On
Onthe
thecontrary,
contrary,the
theUV-visible
UV-visible
spectra
which were reported in previous studies [31,32]. On the contrary, the UV-visible spectra of the WTNF
WTNF
samplesrevealed
revealedbroad
broadlight
light absorbance
absorbance in
toto
2.97
eVeV
(Table
thethe
WTNF
samples
in aa band
bandgap
gaprange
rangefrom
from2.92
2.92
2.97
(Table
samples revealed broad light absorbance in a band gap range from 2.92 to 2.97 eV (Table 1). Moreover,
1).
Moreover,
the
absorption
intensities
in
the
visible
range
became
greater
for
WTNF
samples
with
1). Moreover, the absorption intensities in
visible range became greater for WTNF samples with
thehigh
absorption
intensities in
the visible
greater for3 WTNF
samples
with highand
WO3
WO
3 concentrations,which
which
likely is
isrange
due to
tobecame
the
the
electronic
high WO
3 concentrations,
likely
due
the embedded
embeddedWO
WO3that
thatchanges
changes
the
electronic and
concentrations,
which
likely
is due to the embedded WO that changes the electronic and surface
surface
naturesof
ofTiO
TiO
2 structures [14]. For WTNF samples,3 electrons excited from the valence band
surface
natures
2 structures [14]. For WTNF samples, electrons excited from the valence band
natures
of
TiO
structures
[14].
For
WTNF
samples,
excited
from the valence
of WO3
2 conduction band
WO
3 to its
under
visible
light electrons
illumination
are transferred
to the band
conduction
of of
WO
3 to its conduction band under visible light illumination are transferred to the conduction
to band
its conduction
band
under visible
lightlight
illumination
areAccordingly,
transferred the
to the
conduction
band of
TiO
2, thus
increasing
their visible
absorption.
UV-visible
absorption
band ofof
TiO
2, thus increasing their visible light absorption. Accordingly, the UV-visible absorption
TiO
visible
lightthat
absorption.
Accordingly,
the can
UV-visible
absorption
results
2 , thus
results
ofincreasing
the WTNF their
samples
indicate
the fabricated
architectures
be efficiently
functioned
results of the WTNF samples indicate that the fabricated architectures can be efficiently functioned
of for
thethe
WTNF
samples
indicate
that
the
fabricated
architectures
can
be
efficiently
functioned
for the
degradation of aromatic volatile compounds with exposure to visible light.
for the degradation of aromatic volatile compounds with exposure to visible light.
degradation of aromatic volatile compounds with exposure to visible light.
Figure 4. UV-visible spectra of pure TiO2 nanoflowers (TNF) and WO3‒TiO2 nanoflowers with
different
WO3 loadings (WTNF-0.1,
WTNF-0.5,
WTNF-1.0,
and2WTNF-4.0).
Figure
of pure
TiO
nanoflowers
(TNF)WTNF-2.0,
and WO3 -TiO
with different
2TiO
Figure 4.4.UV-visible
UV-visiblespectra
spectra
of pure
2 nanoflowers (TNF) and
WOnanoflowers
3‒TiO2 nanoflowers with
WO3 loadings (WTNF-0.1, WTNF-0.5, WTNF-1.0, WTNF-2.0, and WTNF-4.0).
different WO3 loadings (WTNF-0.1, WTNF-0.5, WTNF-1.0, WTNF-2.0, and WTNF-4.0).
Catalysts 2017, 7, 97
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Table 1. Properties of pure TiO2 nanoflowers (TNF) and WO3‒TiO2 nanoflowers with different WO3
Catalysts
2017, 7, 97
loadings
(WTNF-0.1, WTNF-0.5, WTNF-1.0, WTNF-2.0, and WTNF-4.0).
5 of 11
Total Pore Volume,
Band Gap,
3
−1
Table 1. Properties of pure TiO2 nanoflowers (TNF) and WO
with different WO3
cm 3g-TiO2 nanoflowerseV
Photocatalyst
SBET, m2 g−1
loadings (WTNF-0.1,
WTNF-0.5, WTNF-1.0,
WTNF-4.0).
TNF
102.1 WTNF-2.0, and0.30
WTNF-0.1
122.6
Photocatalyst
SBET , m2 g−1
WTNF-0.5
133.4
TNF
102.1
WTNF-1.0
135.4
WTNF-0.1
122.6
WTNF-2.0
WTNF-0.5
133.4107.4
WTNF-4.0
WTNF-1.0
135.4104.8
0.37
Total Pore Volume, cm3 g−1
0.31
0.30
0.36
0.37
0.25
0.31
0.22
0.36
WTNF-2.0
107.4
WTNF-4.0
104.8
2.2. Photocatalytic Activity of Sample
Photocatalysts
0.25
0.22
3.30
2.97
2.96
3.30
2.96
2.97
2.95 2.96
2.92 2.96
Band Gap, eV
2.95
2.92
The photocatalytic activity of the sample photocatalysts was surveyed for the degradation of
2.2. Photocatalytic Activity of Sample Photocatalysts
selected aromatic volatile compounds under different operating conditions. Using an uncoated
The
photocatalytic
activity ofno
thediscernible
sample photocatalysts
surveyed
forvolatile
the degradation
of
Pyrex
reactor
with illumination,
degradation was
of the
aromatic
compounds
selected
aromatic
volatile
compounds
under
different
operating
conditions.
Using
an
uncoated
were observed. Equilibrium in the concentrations of toluene and o-xylene on the catalyst surface was
Pyrex
reactor
with1 illumination,
no discernible
of the aromaticdegradation
volatile compounds
were
achieved
within
h after gas flows.
Figure 5 degradation
depicts the photocatalytic
efficiencies
of
observed.
Equilibrium
in
the
concentrations
of
toluene
and
o-xylene
on
the
catalyst
surface
was
TNF and WTNF samples with different WO3 loadings, which were obtained after observing
achieved
within
1 h after gas flows.
Figure
5 depicts
the photocatalytic
degradation
efficienciesratios
of TNF
equilibrium
in concentrations
of the
target
chemicals.
The output-to-input
concentration
of
and
WTNF
samples
with
different
were that
obtained
after observing
equilibrium
in
WTNF
samples
were
lower
than WO
those
of TNF, which
indicating
the degradation
efficiencies
of the
3 loadings,
concentrations
the target
ratios of WTNF
former samplesofwere
higherchemicals.
than thoseThe
of output-to-input
the latter. These concentration
findings are illustrated
on thesamples
basis of
were
lower
than those
of TNF,
indicating
that the degradation
efficiencies
thesample
formerphotocatalysts.
samples were
charge
separation
ability,
adsorption
capability,
and light absorption
ofof
the
higher
those of
the latter.
These findings
are6illustrated
theWTNF
basis ofsamples
charge separation
ability,
The PLthan
emission
intensity
displayed
in Figure
shows thatonthe
possessed greater
adsorption
capability,
and
light
absorption
of
the
sample
photocatalysts.
The
PL
emission
intensity
charge separation abilities relative to TNF, because high PL emission intensity typically represents a
displayed
in Figure 6rate
shows
that theand
WTNF
possessed
greatersurface
charge areas
separation
abilities
high recombination
of electron
holesamples
pairs [22,33].
The specific
and total
pore
relative
to
TNF,
because
high
PL
emission
intensity
typically
represents
a
high
recombination
rate
volumes of some WTNF samples (WTNF-0.1 and WTNF-0.5) were higher than the corresponding
of
electron
and
holesample,
pairs [22,33].
The specific
surface
areas and
total pore
volumes
of some
WTNF
values
of the
TNF
which leads
to higher
adsorption
capacities
for WTNF
samples
(Table
1).
samples
(WTNF-0.1
and WTNF-0.5)
were
higher
than the corresponding
the TNF
sample,
In addition,
WTNF samples
displayed
a higher
absorption
intensity in thevalues
visibleof
range
compared
to
which
leads
to
higher
adsorption
capacities
for
WTNF
samples
(Table
1).
In
addition,
WTNF
samples
TNF (Figure 4).
displayed a higher absorption intensity in the visible range compared to TNF (Figure 4).
Figure 5. Photocatalytic degradation efficiencies (PDE, %) of (a) toluene and (b) o-xylene as determined
Figure 5. Photocatalytic degradation efficiencies (PDE, %) of (a) toluene and (b) o-xylene as
using pure TiO2 nanoflowers (TNF) and WO3 -TiO2 nanoflowers with different WO3 loadings
(TNF)
and WO3‒TiO2 nanoflowers with different WO3
determined
using pureWTNF-1.0,
TiO2 nanoflowers
(WTNF-0.1, WTNF-0.5,
WTNF-2.0,
and WTNF-4.0).
loadings (WTNF-0.1, WTNF-0.5, WTNF-1.0, WTNF-2.0, and WTNF-4.0).
Figure
Figure 55 also
also shows
shows that
that the
the photocatalytic
photocatalytic degradation
degradation efficiencies
efficiencies of
of WTNF
WTNF sample
sample
photocatalysts
increased
with
increasing
WO
content
from
0.1%
to
1.0%,
and
dropped
gradually
with
3 3 content from 0.1% to 1.0%, and dropped
photocatalysts increased with increasing WO
gradually
increasing
WO3 content
furtherfurther
to 4.0%.
indicateindicate
that there
is an
optimal
3 content
with increasing
WO3 content
toThese
4.0%. results
These results
that
there
is an WO
optimal
WO3
for
the
preparation
of
WTNF
sample
photocatalysts.
The
order
of
WTNF
samples
in
photocatalytic
content for the preparation of WTNF sample photocatalysts. The order of WTNF samples in
degradation
efficiency
was efficiency
the same as
that
charge
separation
efficiency
(Figure
6). Specifically,
photocatalytic
degradation
was
theofsame
as that
of charge
separation
efficiency
(Figure 6).
WTNF-1.0 displayed the highest charge separation efficiency, while WTNF-4.0 showed the least
efficiency among the surveyed WTNF samples. Adsorption capacity and light absorption of WTNF
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6 of 11
Specifically, WTNF-1.0 displayed the highest charge separation efficiency, while WTNF-4.0 showed
the least efficiency among the surveyed WTNF samples. Adsorption capacity and light absorption of
WTNF sample
photocatalysts
in the
same
of their photocatalytic
degradation
efficiencies
sample
photocatalysts
were in were
the same
order
oforder
their photocatalytic
degradation
efficiencies
(Table 1
(Table
1 and4,Figure
4, respectively).
and
Figure
respectively).
Figure 6.
6. Photoluminescence
spectroscopy of
of pure
pure TiO
TiO22 nanoflowers
nanoflowers (TNF)
(TNF) and
and WO
WO33‒TiO
Figure
Photoluminescence emission
emission spectroscopy
-TiO22
3 loadings
(WTNF-0.1,
WTNF-0.5,
WTNF-1.0,
and
nanoflowerswith
with
different
nanoflowers
different
WO3WO
loadings
(WTNF-0.1,
WTNF-0.5,
WTNF-1.0,
WTNF-2.0,WTNF-2.0,
and WTNF-4.0).
WTNF-4.0).
The effects of photocatalytic operating conditions on the degradation efficiency were investigated
The effects of photocatalytic operating conditions on the degradation efficiency were
using WTNF-1.0, which exhibited the highest photocatalytic activity among the WTNF samples.
investigated using WTNF-1.0, which exhibited the highest photocatalytic activity among the WTNF
Figure 7 shows the photocatalytic degradation efficiencies of toluene and o-xylene with WTNF-1.0
samples. Figure 7 shows the photocatalytic degradation efficiencies of toluene and o-xylene with
according to NaOH concentrations used for an ultrasound process. Specifically, the input-to-output
WTNF-1.0 according to NaOH concentrations used for an ultrasound process. Specifically, the
concentration ratios of the target chemicals were lowest for 10 M NaOH and highest for 5 M NaOH,
input-to-output concentration ratios of the target chemicals were lowest for 10 M NaOH and highest
indicating that the photocatalytic degradation efficiency was highest for the former and lowest for
for 5 M NaOH, indicating that the photocatalytic degradation efficiency was highest for the former
the latter. These results indicate that there is an optimal WO3 content for the preparation of WTNF
and lowest for the latter. These results indicate that there is an optimal WO3 content for the
sample photocatalysts. The higher photocatalytic degradation efficiency for WTNF-1.0 prepared using
preparation of WTNF sample photocatalysts. The higher photocatalytic degradation efficiency for
a higher NaOH concentration (10 M) is attributed to better 3D nanoflower structures, which were
WTNF-1.0 prepared using a higher NaOH concentration (10 M) is attributed to better 3D nanoflower
shown in our preliminary study. In addition, Figure 8 shows the photocatalytic degradation efficiencies
structures, which were shown in our preliminary study. In addition, Figure 8 shows the
of toluene and o-xylene with WTNF-1.0 according to light source (conventional daylight lamp and
photocatalytic degradation efficiencies of toluene and o-xylene with WTNF-1.0 according to light
white and violet LEDs). The input-to-output concentration ratio of the target chemicals with the
source (conventional daylight lamp and white and violet LEDs). The input-to-output concentration
conventional daylight lamp was lower than those of the white and violet LEDs, indicating a higher
ratio of the target chemicals with the conventional daylight lamp was lower than those of the white
photocatalytic efficiency for the former light source. These findings are ascribed to the higher light
and violet LEDs, indicating a higher photocatalytic efficiency for the former light source. These
power of the conventional daylight lamp (8 W power and 400–720 nm wavelength) compared to the
findings are ascribed to the higher light power of the conventional daylight lamp (8 W power and
white (0.32 W and 450 nm wavelength) and violet LEDs (0.32 W and 400 nm wavelength). However,
400‒720 nm wavelength) compared to the white (0.32 W and 450 nm wavelength) and violet LEDs
the photocatalytic degradation efficiencies normalized to supplied electric powers were higher for the
(0.32 W and 400 nm wavelength). However, the photocatalytic degradation efficiencies normalized
LEDs than for the conventional daylight lamp: white LED, 0.16 and 0.34%/W for toluene and o-xylene,
to supplied electric powers were higher for the LEDs than for the conventional daylight lamp: white
respectively; violet LED, 0.28 and 1.25%/W for toluene and o-xylene, respectively; conventional
LED, 0.16 and 0.34%/W for toluene and o-xylene, respectively; violet LED, 0.28 and 1.25%/W for
daylight lamp, 0.10 and 0.12%/W for toluene and o-xylene, respectively. These results indicate that
toluene and o-xylene, respectively; conventional daylight lamp, 0.10 and 0.12%/W for toluene and
LEDs are a more highly energy-efficient light source for the photocatalytic degradation of toluene
o-xylene, respectively. These results indicate that LEDs are a more highly energy-efficient light
and o-xylene with WTNF-1.0, although the photocatalytic activity is higher for the conventional
source for the photocatalytic degradation of toluene and o-xylene with WTNF-1.0, although the
daylight lamp.
photocatalytic activity is higher for the conventional daylight lamp.
Catalysts 2017, 7, 97
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7, 97
97
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77 of
of 11
11
Figure 7.7.Photocatalytic
Photocatalytic
degradation
efficiencies
%) toluene
of (a) toluene
and (b)aso-xylene
as
Figure
degradation
efficiencies
(PDE,(PDE,
%) of (a)
(b) o-xylene
determined
Figure 7. Photocatalytic
degradation
efficiencies
(PDE,
%) of (a) and
toluene
and (b) o-xylene
as
determined
usingaccording
WTNF-1.0
to NaOH concentration.
using
WTNF-1.0
toaccording
NaOH concentration.
determined
using WTNF-1.0
according
to NaOH concentration.
Photocatalytic
degradation
efficiencies
%) toluene
of (a) and
toluene
and (b)aso-xylene
as
Figure 8.8.Photocatalytic
degradation
efficiencies
(PDE,(PDE,
%) of (a)
(b) o-xylene
determined
Figure 8. Photocatalytic degradation efficiencies (PDE, %) of (a) toluene and (b) o-xylene as
determined
usingaccording
WTNF-1.0
according
light source (conventional
lamp
white
and
using
WTNF-1.0
to light
sourceto(conventional
daylight lamp daylight
and white
and and
violet
LEDs).
determined using WTNF-1.0 according to light source (conventional daylight lamp and white and
violet LEDs).
violet LEDs).
A suggested photocatalytic degradation mechanism of toluene and o-xylene with WTNF sample
A suggested
photocatalytic
degradation
mechanism
of toluene
andgap
o-xylene
WTNF
photocatalysts
is further
described. Under
a light exposure
exceeding
the band
of WO3with
, the electron
A suggested
photocatalytic
degradation
mechanism
of toluene
and o-xylene
with
WTNF
sample
photocatalysts
is
further
described.
Under
a
light
exposure
exceeding
the
band
gap
of
WO3,
generated
in
the
valence
band
of
WO
is
transferred
to
the
conduction
band
of
WO
.
This
transferred
sample photocatalysts is further described.
Under a light exposure exceeding the 3band gap of WO3,
3
the electron
in the valence
band of WOband
3 is transferred to the conduction band of WO3. This
electron
cangenerated
again be moved
to the conduction
of TNF due
lower band
gapofposition
of
the electron
generated
in the valence
band of WO3 is transferred
toto
thethe
conduction
band
WO3. This
transferred
electron
can
again
be
moved
to
the
conduction
band
of
TNF
due
toproduce
the lower
band
gap
- and
TNF.
The
positive
holes
in
the
valence
band
of
WO
react
with
OH
H
O
to
·
OH,
and
the
transferred electron can again be moved to the conduction
band of TNF 2due to the lower
band gap
3
‒ and H2O to
position of TNF.
positive
holes in
thewith
valence
of WO
reactreactive
with OH
- then
photoinduced
and The
transferred
electrons
react
O2 toband
generate
·O2 -33. The
OH‒ and
position of TNF.
The positive
holes in
the valence
band
of WO
react with OH
and·OH
to
2 2O
produce·OH,
and
the
photoinduced
and
transferred
electrons
react
with
O
2 to generate·O2‒. The
react
with limonene
andphotoinduced
toluene to produce
CO2 , CO, and
other byproducts.
produce·OH,
and the
and
transferred
electrons
react
with
O
2 to generate·O2‒. The
reactive OH and·O2‒‒ then react with limonene and toluene to produce CO2, CO, and other
reactive OH and·O2 then react with limonene and toluene to produce CO2, CO, and other
byproducts.
3.
Materials and Methods
byproducts.
3.1.
Preparation
of Methods
Samples
3. Materials
and
3. Materials and Methods
The procedure for the synthesis of 3D WO3 -TiO2 hybrids with different WO3 concentrations is
3.1. Preparation of Samples
3.1. Preparation
of Samples
summarized
as follows:
the preparation of TiO2 powder, morphological transformation of TiO2 powder
The procedure
forphotocatalyst),
the synthesis ofand
3Dembedment
WO3‒TiO2 hybrids
3 concentrations
is
into TNF
(a reference
of WO with
intodifferent
3D TiO WO
nanoflowers.
For the
The procedure for the synthesis of 3D WO3‒TiO2 hybrids 3with different2 WO3 concentrations is
summarized
as
follows:
the
preparation
of
TiO
2
powder,
morphological
transformation
of
TiO
preparation of TiO2 powder, 2.0 g of Surfactant P-123 (Sigma-Aldrich, Milwaukee, WI, USA) was mixed2
summarized as follows: the preparation of TiO2 powder, morphological transformation of TiO2
powder
intoofTNF
(a reference
photocatalyst),
into
3D TiO
with
400 mL
deionized
water and
stirred for 30and
min,embedment
after which of
120WO
mL3 of
titanium
IV2 nanoflowers.
isopropoxide
powder into TNF (a reference photocatalyst), and embedment of WO3 into 3D TiO2 nanoflowers.
For theSigma-Aldrich)
preparation of was
TiO2added
powder,
2.0 mixture
g of Surfactant
P-123for
(Sigma-Aldrich,
WI, USA)
(TTIP,
to the
and stirred
another 30 min.Milwaukee,
The suspension
was
For the preparation of TiO2 powder, 2.0 g of Surfactant P-123 (Sigma-Aldrich, Milwaukee, WI, USA)
was mixed with
deionized water
and stirred
30 min, after
which
120 mLat
of2000
titanium
ultrasonicated
for400
90 mL
min,ofconditioned
overnight
at roomfor
temperature,
and
centrifuged
rpm
was mixed with 400 mL of deionized water and◦ stirred for 30 min, after which◦ 120 mL of titanium
IV isopropoxide
(TTIP,
Sigma-Aldrich)
was
added
to15
theh mixture
and at
stirred
another
30 min.
for
20 min. The solid
products
were dried
at 80
C for
and calcined
550 for
C for
1 h to provide
IV isopropoxide (TTIP, Sigma-Aldrich) was added to the mixture and stirred for another 30 min.
The2 suspension
TiO
powders. was ultrasonicated for 90 min, conditioned overnight at room temperature, and
The suspension was ultrasonicated for 90 min, conditioned overnight at room temperature, and
centrifuged
at 2000 TiO
rpm for
20 min.
Thehydrothermally
solid products were
dried at 80 °Ctreated
for 15 to
h and
calcined
The fabricated
powders
were
and ultrasonically
obtain
3D TiOat2
centrifuged at 2000 rpm2 for 20 min. The solid products were dried at 80 °C for 15 h and calcined at
550 °C for 1Titanium
h to provide
TiO2 powders.
structures.
tetrachloride
(TiCL4 , Sigma-Aldrich) (50 mL), 70 mL of NaOH (5, 10, 15 or 20 M)
550 °C for 1 h to provide TiO2 powders.
Catalysts 2017, 7, 97
8 of 11
The fabricated TiO2 powders were hydrothermally and ultrasonically treated to obtain 3D TiO2
of 20
11
tetrachloride (TiCL4, Sigma-Aldrich) (50 mL), 70 mL of NaOH (5, 10, 15 8or
M) (Sigma-Aldrich), and 25 mL of H2O2 (30%) (Sigma-Aldrich) was mixed with 300 mL of deionized
water. TiO2 powders (220 mg) were added to this solution, before it was hydrothermally treated in
(Sigma-Aldrich), and 25 mL of H2 O2 (30%) (Sigma-Aldrich) was mixed with 300 mL of deionized
an autoclave at 90 °C for 3 h. Then, this mixture was centrifuged at 10,000 rpm for 15 min, after
water. TiO2 powders (220 mg) were added to this solution, before it was hydrothermally treated in
which the products were mixed with the 400 mL of HCL (Sigma-Aldrich). This suspension was
an autoclave at 90 ◦ C for 3 h. Then, this mixture was centrifuged at 10,000 rpm for 15 min, after which
ultrasonicated for 40 min using an ultrasonicator (VCX750, Sonics & Materials, Newtown, CT,
the products were mixed with the 400 mL of HCL (Sigma-Aldrich). This suspension was ultrasonicated
USA). The precipitates were cleaned using distilled water and ethanol (Sigma-Aldrich), and the
for 40 min using an ultrasonicator (VCX750, Sonics & Materials, Newtown, CT, USA). The precipitates
cleaned output was then dried at 50 °C for 20 h and calcined at 550 °C for 1 h to obtain 3D TiO2
were cleaned using distilled water and ethanol (Sigma-Aldrich), and the cleaned output was then
structures.
dried at 50 ◦ C for 20 h and calcined at 550 ◦ C for 1 h to obtain 3D TiO2 structures.
WTNFs were fabricated by embedding WO3 into 3D TiO2 structures. A pre-scheduled amount
WTNFs were fabricated by embedding WO3 into 3D TiO2 structures. A pre-scheduled amount
of pentahydrated ammonium paratungstate ((NH
4)10W12O41·5H2O, Sigma-Aldrich) was added to a
of pentahydrated ammonium paratungstate ((NH4 )10 W12 O41 ·5H2 O, Sigma-Aldrich) was added to
solution of C2H5OH (10 mL) and distilled water (30 mL). Three-dimensional TiO2 powder (1 g) was
a solution of C2 H5 OH (10 mL) and distilled water (30 mL). Three-dimensional TiO2 powder (1 g) was
added to the solution and stirred for 15 h. Then, this mixture was conditioned in a dry oven (110 °C)
added to the solution and stirred for 15 h. Then, this mixture was conditioned in a dry oven (110 ◦ C)
for 2 h, dried at 380 °C for 2 h, and calcined at 550 °C for 1 h to produce WTNF samples. During the
for 2 h, dried at 380 ◦ C for 2 h, and calcined at 550 ◦ C for 1 h to produce WTNF samples. During
drying and calcination processes, WO3 was formed by the reaction of (NH4)10W12O41·5H2O and
the drying and calcination processes, WO3 was formed by the reaction of (NH4 )10 W12 O41 ·5H2 O
incorporated into TiO2 structures. The amounts of ((NH4)10W12O41·5H2O were 0.08, 0.40, 0.80, 0.16,
and incorporated into TiO2 structures. The amounts of ((NH4 )10 W12 O41 ·5H2 O were 0.08, 0.40, 0.80,
and 0.32 g, which were determined to obtain WTNFs with WO3-to-TiO2 percentages of 0.1%, 0.5%,
0.16, and 0.32 g, which were determined to obtain WTNFs with WO3 -to-TiO2 percentages of 0.1%,
1.0%, 2.0% and 4.0%, respectively (They are denoted here as WTNF-0.1, WTNF-0.5, WTNF-1.0,
0.5%, 1.0%, 2.0% and 4.0%, respectively (They are denoted here as WTNF-0.1, WTNF-0.5, WTNF-1.0,
WTNF-2.0, and WTNF-4.0, respectively).
WTNF-2.0, and WTNF-4.0, respectively).
The properties of the synthesized samples were examined using X-ray diffraction (XRD,
The properties of the synthesized samples were examined using X-ray diffraction (XRD, Rigaku
Rigaku D/max-2500 diffractometer, Rigaku Corp., Tokyo, Japan), scanning electron
D/max-2500 diffractometer, Rigaku Corp., Tokyo, Japan), scanning electron microscopy/energy
microscopy/energy dispersive X-ray (SEM, Hitachi S-4300 & EDX-350 FE, Hitachi, Tokyo, Japan),
dispersive X-ray (SEM, Hitachi S-4300 & EDX-350 FE, Hitachi, Tokyo, Japan), photoluminescence
photoluminescence emission spectroscopy (PL, SpectraPro 2150i, Acton Research, Princeton, NJ,
emission spectroscopy (PL, SpectraPro 2150i, Acton Research, Princeton, NJ, USA), UV-visible
USA), UV-visible spectroscopy (Varian CARY 5G, Varian, Cary, NC, USA), and N2 physisorption
spectroscopy (Varian CARY 5G, Varian, Cary, NC, USA), and N2 physisorption (ASAP 2020,
(ASAP 2020, Micromeritics, Norcross, GA, USA).
Micromeritics, Norcross, GA, USA).
Catalysts
2017, Titanium
7, 97
structures.
3.2. Photocatalysis of Aromatic Compounds
3.2. Photocatalysis of Aromatic Compounds
The experimental setup for the photocatalytic tests of the prepared photocatalysts is shown in
The experimental setup for the photocatalytic tests of the prepared photocatalysts is shown in
Figure 9. Major components of the experimental system include an air cylinder for clean air supply,
Figure 9. Major components of the experimental system include an air cylinder for clean air supply,
rotameters for flow measurements, a water bath for humidification, a heated chamber for mixing of
rotameters for flow measurements, a water bath for humidification, a heated chamber for mixing of
air and standard substrates, a syringe pump for automatic injection of target compounds, a 3-way
air and standard substrates, a syringe pump for automatic injection of target compounds, a 3-way
valve for gas sampling, and a photocatalytic reactor with a conventional lamp or a photocatalytic
valve for gas sampling, and a photocatalytic reactor with a conventional lamp or a photocatalytic
reactor with white or violet LEDs. A dipping method was applied for the coating of the reactor with
reactor with white or violet LEDs. A dipping method was applied for the coating of the reactor with
individual photocatalyst samples. Clean air supplied from the air cylinder was passed through the
individual photocatalyst samples. Clean air supplied from the air cylinder was passed through the
water bath unit for humidification at a specified relative humidity. The humidified air was delivered
water bath unit for humidification at a specified relative humidity. The humidified air was delivered
into the reactor with a direction perpendicular to the reactor in order to increase the mass transport
into the reactor with a direction perpendicular to the reactor in order to increase the mass transport of
of incoming gas to the catalyst surface. The photocatalytic operating conditions were adjusted as
incoming gas to the catalyst surface. The photocatalytic operating conditions were
adjusted as follows:
follows: sample amounts coated onto the inner wall of the reactor, 0.45
mg cm‒2; input concentration
−
2
sample amounts coated onto the inner wall of the reactor, 0.45 mg cm ; input−1concentration of target
of target chemicals, 0.1 ppm; relative humidity, 40%; gas flow rate, 1.0 L min .
chemicals, 0.1 ppm; relative humidity, 40%; gas flow rate, 1.0 L min−1 .
Figure 9. Schematic diagram of experimental setup; 1: air cylinder; 2: rotameter; 3: water bath;
4: heated chamber; 5: syringe pump; 6: 3-way valve; 7: photocatalytic reactor with a conventional lamp;
8: photocatalytic reactor with LEDs.
Catalysts 2017, 7, 97
9 of 11
Before conducting main photocatalytic experiments, the experimental system was cleaned using
clean air overnight while light illumination to degrade any compounds attached to the system.
An uncoated reactor was exposed to visible light in order investigate the effect of light on the
degradation of the target chemicals. Adsorption equilibrium of aromatic volatile compounds between
the photocatalyst and gas was checked by examining if the chemical concentrations were equal in
input and output air. Once the adsorption equilibrium was identified, the light source was turned on
to start the main photocatalytic experiments.
Air samples for the measurements of aromatic volatile compounds and other organic vaporous
compounds were obtained at upstream and downstream sampling ports of the photocatalytic reactor.
Air sampling was carried out by drawing air from the sampling ports to Tenax GC-contained tube to
concentrate target chemicals. A thermal desorption unit (ATD 350, Perkin Elmer, Waltham, MA, USA)
was employed to transfer sampled compounds to a gas chromatograph/mass spectrometer (GC/MS)
(Clarus 680, Perkin Elmer, Waltham; Clarus SQ8 T, Perkin Elmer, Waltham) for analysis. The adsorbent
tube was thermally treated at 260 ◦ C for 12 min, and the chemical species were concentrated at −30 ◦ C
on an internal trap. Subsequently, the internal trap was heated to 250 ◦ C to deliver the chemical species
to the analytical system. The initial temperature in the GC oven was adjusted at 40 ◦ C for 6 min and
ramped at 5 ◦ C/min to 220 ◦ C for 3 min. A contro analysis showed no co-eluting compounds during
entire analytical processes. The quantitative determination of chemical species was done based on the
calibration equations established using at least five different concentrations. For the quality control for
sample analyses, blank samples were examined to check sample contamination, and spiked samples
were investigated to check any analysis variation. The minimum detection thresholds varied between
0.5 and 1.3 ppb, depending on compound.
4. Conclusions
In the present study, WTNF composites with different WO3 concentrations were fabricated
and their photocatalytic activity for treatment of toluene and o-xylene at an indoor concentration
under visible light exposure was surveyed. The spectral results demonstrated the formation of 3D
structures in the prepared WTNF samples, the incorporation of WO3 into TNF structures, and the
potential activity of WTNF sample photocatalysts under visible light irradiation. WTNF samples
exhibited superior photocatalytic function to the reference TNF sample for degradation of indoor-level
toluene and o-xylene with visible light exposure. The photocatalytic degradation efficiencies
of WTNF composites depended upon the WO3 -to-TiO2 ratios. There was an optimal NaOH
concentration for the preparation of WTNF photocatalysts using an ultrasound treatment process.
LEDs are suggested as an energy-efficient light source for the photocatalytic degradation of toluene
and o-xylene with WTNF samples. Overall, WTNF photocatalysts prepared by the combined
hydrothermal–ultrasonication–impregnation method can be applied for the treatment of indoor level
aromatic volatile pollutants.
Acknowledgments: This work was supported by the National Research Foundation of Korea grant funded by
the Korea government (Ministry of Science and Future Planning) (No. 2016R1A2B4009122).
Author Contributions: Wan-Kuen Jo conceived and designed the experiments; Joon Yeob Lee performed the
experiments; Wan-Kuen Jo and Joon Yeob Lee analyzed the data.
Conflicts of Interest: The authors declare no conflict of interest.
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