Carbon-Doped Mesoporous Anatase TiO2 Multi-Tubes Nanostructures for Highly Improved Visible Light Photocatalytic Activity

Article abstract of DOI:10.1021/acs.inorgchem.7b01864

Development of a high surface area and efficient visible light induced photocatalyst on a large scale is a promising task from the practical perspective. In this study, visible light active C-doped anatase TiO₂ multi-tubes were synthesized using banana (Musa acuminata) stem fiber as a sacrificial template, removed by calcination at 450 °C. During the calcination process, the lattice of anatase TiO₂ phase was doped with C, and obtained multi-tubes showed high surface area (～99 m²/g) with a mesoporous structure made of ～15 ± 3 nm nanoparticles. The synthesized TiO₂ multi-tubes showed an enhanced light absorption property in the whole visible light region and good thermal stability of the anatase phase up to 750 °C. The synthesized C-doped TiO₂ multi-tubes manifest an excellent photocatalytic activity for the reduction of Cr (VI) to Cr(III) under the visible light exposure. This process may have lots of practical importance as the method of synthesis of the catalyst is novel and the multi-tubes structure can be synthesized on a large scale through a quick and economical way with excellent photocatalytic activity. This novel multi-tubes structure may also be useful for photovoltaics, antimicrobial, and Li-batteries applications in the future.

Full text of DOI:10.1021/acs.inorgchem.7b01864

Article
pubs.acs.org/IC
Carbon-Doped Mesoporous Anatase TiO2 Multi-Tubes
Nanostructures for Highly Improved Visible Light Photocatalytic
Activity
Rahul Purbia, Rituraj Borah, and Santanu Paria*
Interfaces and Nanomaterials Laboratory, Department of Chemical Engineering, National Institute of Technology, Rourkela-769008,
India
S
* Supporting Information
ABSTRACT: Development of a high surface area and efficient visible light
induced photocatalyst on a large scale is a promising task from the practical
perspective. In this study, visible light active C-doped anatase TiO2 multi-
tubes were synthesized using banana (Musa acuminata) stem fiber as a
sacrificial template, removed by calcination at 450 °C. During the
calcination process, the lattice of anatase TiO2 phase was doped with C,
and obtained multi-tubes showed high surface area (∼99 m2/g) with a
mesoporous structure made of ∼15 3 nm nanoparticles. The synthesized
TiO2 multi-tubes showed an enhanced light absorption property in the
whole visible light region and good thermal stability of the anatase phase up
to 750 °C. The synthesized C-doped TiO2 multi-tubes manifest an excellent
photocatalytic activity for the reduction of Cr (VI) to Cr(III) under the
visible light exposure. This process may have lots of practical importance as
the method of synthesis of the catalyst is novel and the multi-tubes
structure can be synthesized on a large scale through a quick and economical way with excellent photocatalytic activity. This
novel multi-tubes structure may also be useful for photovoltaics, antimicrobial, and Li-batteries applications in the future.
nanostructure helps in narrowing the band gap which in turn
to improve the visible light absorption property to achieve
better photocatalytic performance. Therefore, significant efforts
have been made to develop a doped hollow morphology with
different metallic and nonmetallic elements. Generally, the
metallic doping such as Cr, V, Fe, Ag, etc., exhibited a lack of
thermal stability, atomic diffusion, and increased exciton
recombination in defect sites, which in turn shows low
photocatalytic activity. On the other hand, the doping of
nonmetallic elements such as B, S, F, C, and N shows
comparatively better performance compared to that of metallic.
Among nonmetallic elements, the carbon doping has received
more attraction owing to its excellent properties such as
narrowing of the band gap through the introduction of localized
electronic states of C into the TiO2 lattice, visible light
photosensitization, the large electron storage capacity to avoid
charge recombination, and thermal stability.1,10
For the synthesis of hollow nanostructures, template-assisted
and template-free approaches have been majorly used, where
the disadvantages of template-free approaches are controlling of
size/shape, low yield, reproducibility, use of additional
surfactant/stabilizer, expensive,9,11,12 etc. On the other hand,
the template-assisted methods are the most useful techniques
for the synthesis of hollow nanostructured materials with
1. INTRODUCTION
The emergence of different nanostructured TiO2 is becoming a
most popular photocatalyst in recent years for the application
of solar energy conversion and environmental remediation.
Among several semiconductor photocatalysts, TiO2 is the most
widely used photocatalyst in various applications because of its
high photocatalytic activity, low cost, nontoxic nature, chemical
inertness, and biocompatibility. However, development of a
highly efficient and low-cost TiO2 nanocatalyst is still a
challenging task because of the problems of fast charge-carrier
recombination, less surface area, large band gap, etc., while
synthesized by conventional methods. For the enhancement of
photocatalytic activity by means of physicochemical changes,
significant efforts have been made by developing different
morphologies for better structural properties with high surface
area and doping with different metallic or nonmetallic elements
for better performance in visible light.1−3 Regarding the
morphological aspect, it has been reported that the hollow
hierarchical TiO2 nanostructures morphologies are important
for photocatalytic applications because of improved mass
transfer at the interface,4,5 reflection6 and scattering6 of
incident light in interior cavities, structural stability,7 less
aggregation, easy to separation,8 and faster charge transfer9
when compared with other morphologies. In general, uses of
hollow nanostructures are a very common practice in catalysis
because of the higher specific surface area. It has also been
proven that the doping of different elements to a TiO2
Received: July 21, 2017
© XXXX American Chemical Society
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unique morphologies and properties because of better control
over uniform shell thickness, dimensions, and morphology of
the desired structure compared to the template-free methods,
in spite of the requirement of an additional sacrificial template
removal step in the process.13 As the green route synthesis of
nanomaterials is drawing significant attention in recent years
over the chemical routes, so, the uses of naturally occurring
biotemplates for the synthesis of hollow structures are also not
an exception. These natural templates are generally preferred
over the chemical templates because of their advantages such as
cheap, easily available, abundant, green and renewable, precise
dimensions, elimination of core synthesis step, etc.14−22 Several
naturally available biotemplates such as cotton,17,23−26 silk,27,28
filter paper, egg shell,21,29,30 cellulose,18,19,31,32 keratin,33 pollen
grain,34−36 DNA,16 virus,15 proteins,14 insects,20,21,29 etc., have
been reported for the synthesis of several micro/nanostructures
which are eventually useful for different applications. Recently,
the tubular nanostructure morphology of TiO2 has promising
scope in catalysis because of the high specific surface area as
well as directed electron transport.37,38 The tubular nanostruc-
ture morphology has been utilized with improved performance
in photocatalysis, solar cells, electrochromic devices, gas
sensors, and biomedical coatings applications.39 As per as
reported studies, tubular TiO2 morphologies are mostly
fabricated by time-consuming and expensive chemical methods
such as electrochemical oxidation,39 hydro/solvothermal
process,40 atomic layer deposition,41 sol−gel,42 and template-
assisted approach.43 In spite of several literatures on the
development of doped hollow TiO2 nanospheres after
removing different sacrificial cores (C,44 AgBr,45 S,46 etc.),
the development of a high surface area doped tubular structure
with pure anatase phase stability at high temperature is still a
challenging task to utilize the broad spectrum of solar light for
the photocatalysis. On the basis of the available literature, it is
very clear that the development of a doped tubular TiO2
nanostructure through an easy, scalable, sustainable, and cheap
route for the catalytic or other applications has great scientific
as well as practical importance.
To the best of our knowledge, this is the first time we report
a facile, low-cost, novel, green approach for the synthesis of
visible light active and high surface area C-doped TiO2 multi-
tubes nanostructures where the sacrificial template removal,
crystallinity, and doping take place during a single calcination
step. Herein, banana stem fiber, an agricultural waste product
that comprises the unique multi-tubular structure, has been
used as a template and dopant to fabricate a visible light
responsive pure anatase C-doped TiO2 nanostructure for
photocatalytic application. The as-synthesized TiO2 multi-
tubes reported here possess high-surface-area, high crystallinity,
low density, mesoporous nature, absorbing visible light, and
high thermal stability of anatase phase. The main advantages of
this process are green and sustainable, economical, short
processing time, scalable, and anatase phase stability. Since the
presence of the anatase phase is very important for the
photocatalysis applications, the developed material may be
useful for this application. Further, the photocatalytic reduction
of Cr(VI) was carried out under irradiation of different light
sources (LED and high-pressure Hg lamps), and the results
indicate that the C-doped TiO2 multi-tubes have better
photocatalytic activity compared to that of commercial P25
TiO2 powder. The better photocatalytic activity of the
developed catalyst for Cr(VI) reduction can be attributed to
the synergistic effects of large surface area, the presence of the
anatase phase, multiple reflections in hollow spaces, and higher
visible light absorption. This easy synthesis process may also be
applicable to synthesize other metal oxide nanostructures.
In general, hollow structures are made to enhance the
catalytic activity by enhancing active sites; additionally, in
photocatalytic applications, the light harvesting property in
terms of absorbance of hollow structures increases because of
the multiple reflection of light inside the hollow structure.
However, synthesis of a hollow nanostructure by a chemical
sacrificial template method is expensive and large scale
production for practical application is difficult. In chemical or
green sacrificial template methods, hollow spheres or tubes are
reported, but the synthesis of multi-tubes has not been reported
before. Because of the simplicity of the method with higher
production ability as well other above-mentioned advantages,
this study is novel.
2. EXPERIMENTAL SECTION
2.1. Materials. The required chemicals were purchased from the
following companies: Tetrabutyl orthotitanate (TBOT, Sigma-
Aldrich), anhydrous ethanol (Merck), and TiO2 (Aeroxide P25,
Sigma-Aldrich). All the reagents were analytical grade and used
without any further purification. All the experiments were conducted
using ultrapure water of 18.2 MΩ·cm resistivity (Millipore, Elix). A
banana stem was picked from the freshly cut matured banana plant.
The stem was then cut into thin slices using a knife, and the fibers
were collected from the stem during slicing. The fibers were
immediately dipped into deionized water and then in alcohol to
remove the sap.
2.2. Multi-tubes Synthesis. The fiber was washed with ethanol
and dried in an oven at 50 °C. The dried fibers were then immersed
into an alcoholic solution of titanium precursor (100 mM TBOT in
anhydrous ethanol solution) and sonicated for 10 min. After
sonication, the treated fibers were dried in an oven (50 °C for 3 h)
and then calcined in a muffle furnace at 450, 550, 650, and 750 °C for
2 h to form TiO2 multi-tubes.
2.3. Characterization. The phase, structure, and average
crystalline size of nanostructures were analyzed using a powder X-
ray diffractometer (Rigaku Japan/Ultima-IV) with Cu Kα radiation
(1.5406 Å) operating at a 40 kV and 30 mA. The morphology,
structure, and dimensions of the nanostructures were analyzed by
using a field-emission scanning electron microscope (FEI, NOVA) and
transmission electron microscope (FEI Tecnai, G20 F30). The optical
properties of the material were evaluated using a UV−vis−NIR
spectrophotometer (Shimadzu-3600) in the UV−visible range. The
Raman spectroscopy was performed using a Triple Raman
spectrometer (Horiba, T64000) equipped with 1800 grooves/mm
gratings, and Ar+ ion. Thermal gravimetric analysis (TGA) was
conducted (Shimadzu, DTG − 60H) at a heating rate of 20 °C min−1.
Nitrogen adsorption measurements were performed using an
automatic adsorption unit, Autosorb-1 (Quantachrome), at 77 K,
after degassing of samples for 3 h. The X-ray photoelectron
spectroscopy (XPS) was carried out (K-Alpha instrument, Thermo
Scientific) to know the chemical bonding states. The zeta (ζ)
potentials were measured using a Malvern zeta size analyzer (Nano ZS,
U.K.). Fluorescence measurement was performed using a spectro-
fluorometer (Hitachi, F-7000) with a slit width of 5 nm for the
excitation and emission. Fourier transform infrared spectroscopy (FT-
IR) spectra were carried out using an FT-IR spectrometer (Thermo
Fisher, Nicolet, iS-10).
2.4. Analysis of Hydroxyl Radicals (·OH). Hydroxyl radicals
generated by the as-synthesized photocatalyst under visible light
irradiation were measured by the fluorescence method. The
terephthalic acid (TA) was used as a probe molecule to detect ·OH
radicals. The 5 mg TiO2 multi-tubes were dispersed in the mixture of
TA (2 mM) and NaOH (0.5 mM) aqueous solution to make up 30
mL. The resulting solutions were separately irradiated by white LED
(2 × 12 W, 900 lm, COB, Phillips,) and UV−visible (125 W high-
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Figure 1. FE-SEM images of pure banana fiber: (a) the morphology of banana stem fiber and (b) the closer look at the fiber and in the inset is an
enlarged image of the fiber end. (c) TGA graph of banana stem fiber. (d) XRD spectra of banana stem fiber and TBOT coated banana stem fiber.
pressure mercury lamp) lights. Then, 1 mL of the suspension was
collected in a regular interval of 15 min and centrifuged for further
fluorescence measurements with an excitation wavelength of 315 nm.
The fluorescence emission intensity was measured at 425 nm
wavelength.
presented in (a) and (b) parts of Figure 1 show the uniform
diameter of the fibers (∼2−3 μm) throughout the lengths and
multiple fibers are attached together like a flat ribbon. An
enlarged image of a single fiber end presented in the inset of
Figure 1b shows that it has a hollow structure with a very thin
wall (∼0.5 μm). As these fibers were used a sacrificial template
to get the TiO2 multi-tubes, the complete burning temperature
was found from the TGA. The TGA plot in Figure 1c depicts
that the weight loss within 100 °C is because of unbound
moisture, then up to 200 °C because of bound moisture and
volatile matter, and finally the cellulosic material burns between
200 and 350 °C. Further, the surface characteristics of the fibers
were characterized using FT-IR spectroscopy. The FT-IR
spectral analysis of the banana fibers exhibits the vibrational
stretching peak of O−H (3310 cm−1), −CH2 (2898 cm−1), and
CC (1634 cm−1) bonds. Additionally, vibrational stretching
peaks of ester and carboxyl groups are also found at 1433−1115
and 1038 cm−1, respectively, as shown in Figure S1 (Supporting
Information). The FT-IR spectra of banana fibers confirm the
presence of cellulosic oxygen rich groups on the surface, which
makes the surface hydrophilic. The measurement of zeta
potential (ζ) was also done and found to be −33 0.6 mV; the
negative charge of the fiber is because of the presence of
carboxyl and other hydrophilic functional groups.
2.5. Measurement of Photocatalytic Activity. The photo-
catalytic activity of the synthesized TiO2 multi-tubes was indiviually
evaluated for the reduction of K2Cr2O7 in aqueous solution under
visible (2 × 12 W LED lamps, energy flux 160 W/m2) and UV−visible
light (125 W high-pressure mercury lamp, energy flux 210 W/m2)
source. The Hg lamp was used to see the effect of the combination UV
and visible light. During photocatalytic reaction, a UV filter was used
for LED light to eliminate the UV wavelengths, if any. The 20 mg of
TiO2 powder was added into 50 mL of K2Cr2O7 (50 ppm) solution
containing 10 mM of oxalic acid as a hole scavenger. In order to
exclude the adsorption−desorption equilibrium, the solution was kept
under the dark condition for 1 h before irradiation of light. After, the
solution was exposed to the light which was located vertically at 15 cm
from the sample. At every 15 min interval, aliquots were taken and
centrifuged to separate the photocatalyst. The concentration of
K2Cr2O7 was monitored by the absorbance at the maximum peak (365
nm) using UV−vis spectroscopy. Photocurrent measurements were
performed using a Metrohm Autolab, PGSTAT204 electrochemical
workstation with a Pt counter electrode and Ag/AgCl (3 M KCl)
reference electrode.
3. RESULTS AND DISCUSSION
3.2. Synthesis of TiO2 Multi-tubes. The banana fiber’s
multi-tube morphology was replicated by TiO2 through a two-
step synthesis methodology. First, in the coating step, fibers
were immersed in the alcoholic TBOT solution for adsorption
or thin layer coating of TBOT on the fiber surface. The
negative surface charge and the presence of oxygen rich polar
groups on the fiber surface facilitate the favorable adsorption of
3.1. Morphology of Template Fiber. When a banana
plant is mature, the corm stops producing new leaves and
begins to form a flower spike or inflorescence. The banana stem
is the thick flower stalk of the banana plant, grows up from the
ground, and forms the backbone of the herbaceous plant. The
fibers are available abundantly in a bunch inside the stem when
cut into two pieces. The FE-SEM images of the banana fiber
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Figure 2. (a−d) FE-SEM images of banana fiber templated TiO2 multi-tubes at different magnifications.
TBOT. In the second step, the multi-tubes were produced after
calcination of the TBOT adsorbed fiber at 450 °C in the
presence of air. In this step, TBOT was directly converted to
TiO2 because of pyrolytic transformation according to the
following reaction.
TEM images of multi-tubes calcined at 450 °C temperature.
From the TEM image (Figure 3a,b), the contrast difference
between the edges (dark) and central region (light) clearly
indicates the presence of a hollow structure of multi-tubes. The
high-resolution TEM image (Figure 3c) reveals that the tubes
consist of ∼15−18 nm size particles, which is consistent with
the FE-SEM image (Figure 2d). Lattice fringes with an
interplanar spacing (d) of 0.35 nm (d101) are clearly visible in
Figure 3d, which corresponds to the (101) planes of tetragonal
anatase phase of TiO2. The small pores of 5 0.7 nm in size
between interconnected nanoparticles along the tube are also
visible from the TEM image (Figure 3c). The high-angle
annular dark-field scanning transmission electron microscopic
(HAADF-STEM) images also confirm the tube structure and
interconnected NPs with small pores as shown in Figure 3e,f.
The ring patterns of the selected area electron diffraction
(SAED) pattern as shown in the inset of Figure 3f are indexed
to (101), (004), (200), (105), (204), (220), and (215) planes
of anatase phase. Further, the HAADF-STEM elemental
analysis and mapping of the TiO2 multi-tubes indicate the
existence of Ti, O, and C as shown in Figure 3g−j. The point
EDX mapping on TiO2 multi-tubes shows that the doped
carbon content is ∼6.12 (weight)% and 11.15 (atomic)%, as
shown in Figure S3 (Supporting Information); these results are
close to the FE-SEM analysis.
450 °C
Ti(OC4H9)4 ⎯⎯⎯⎯⎯→⎯ TiO2 + 2(C4H9OC4H9)
(1)
During the calcination process, the TiO2 nanostructure was
doped by the C atoms that originate from the cellulosic fiber by
replacing the O atom and formed Ti−O−C bonds in the
nanostructures. Therefore, the C-doped multi-tube structured
TiO2 was obtained.
3.3. Morphology and Structure of TiO2 Multi-tubes.
The FE-SEM images of TiO2 multi-tubes after calcination at
450 °C are shown in Figure 2. The low-magnification FE-SEM
image shows that the multi-tubular morphology of prepared
TiO2 is retained similar to that of raw banana fiber (Figure 2b−
d). The resulting TiO2 multi-tubes have a diameter of 2 0.5
μm and thickness of 170 10 nm, respectively. From Figure 2,
it can be seen that there is a slight shrinkage of the tubes after
burning the template at high temperature. The high-
magnification FE-SEM image shows that the tubes are
composed of interconnected nanoparticles with a size of
∼15−21 nm as shown in Figure 2d. In addition, the FE-SEM
elemental mapping of the TiO2 multi-tubes indicates the
existence of Ti, O, and C as shown in Figure S2 (Supporting
Information).
3.4. Crystallographic and Raman Spectroscopic
Analysis. The XRD characterization was performed to analyze
the crystallographic properties as illustrated in Figure 4a. The
XRD analysis of the banana fiber and precursor coated banana
fiber revealed both are amorphous in nature and show only a
Further, the morphology of the multi-tubes was also analyzed
by the transmission electron microscope. Figure 3 shows the
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Figure 3. HR-TEM images of synthesized TiO2 multi-tubes: (a) multi-tube structure, (b) single tube structure, (c) high magnification of single tubes
surface (yellow circles indicate pores), and (d) lattice fringes of TiO2 NPs. (e) HAADF-STEM image of tube. (f) High-magnification HAADF-
STEM image of tube surface. Inset of (f) shows SAED ring pattern of TiO2 multi-tubes. HAADF-STEM elemental mapping of TiO2 multi-tubes: (g)
selected area of elemental mapping with overlay of all elements, (h) mapping of Ti element (yellow), (i) mapping of O element (green), (j) mapping
of C element (red).
Figure 4. (a) XRD spectra of TiO2 multi-tubes calcined at different temperatures for 2 h (450, 550, 650, 750 °C). (b) Raman spectra of as-prepared
TiO2 annealed at 750 °C. (c) Nitrogen adsorption/desorption isotherm of as-prepared TiO2 annealed at 450 °C (inset pore size distribution graph).
cellulose diffraction peak at 2θ of 22.47° (200) as shown in
Figure 1d. However, the TiO2 multi-tubes are crystalline when
calcined between 450 and 750 °C for 2 h. Figure 4a shows the
XRD patterns of TiO2 multi-tubes after calcination, where the
diffraction peaks (2θ) of 25.4, 37.8, 48.1, 54.1, 55.2, and 62.8°
are indexed to only the tetragonal anatase phase of TiO2
(JCPDS card no. 83-2243). The crystallite sizes measured using
the Scherrer equation from the full-width half-maximum of the
(101) peak are 15.57, 18.98, 21.32, and 24.31 nm at 450, 550,
650, and 750 °C calcination temperatures, respectively. It can
be concluded that the crystallite size of anatase phase increases
with the increase in calcination temperature without changing
the phase. The anatase TiO2 is known to exhibit six Raman
active peaks (three Eg, two B1g, and one A1g).47 From the
Raman spectra of the TiO2 sample calcined at 750 °C, the
peaks of Eg (144 cm−1), Eg (198 cm−1), B1g (399 cm−1),
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Figure 5. Deconvolution XPS spectra of (a) carbon, (b) oxygen, (c) titanium element.
overlapped A1g-B1g (516 cm−1), and an overtone of Eg (639
cm−1) were observed under the active vibration mode as
displayed in Figure 4b. Thus, Raman analysis also confirms the
presence of pure anatase phase of TiO2 at high temperature,
which is consistent with the results of XRD. As the pure anatase
phase is important for catalytic applications, the developed
material may be useful for the better photocatalytic application.
3.5. Specific Surface Area and Porosity. The specific
surface area and porosity of as-prepared multi-tubes were also
determined by nitrogen adsorption/desorption isotherms
(Brunauer−Emmett−Teller or BET). The experimental results
exhibit a type IV isotherm with H3 hysteresis loop (IUPAC
classification) which is characteristic for mesoporous material
with slit-shaped pores as shown in Figure 4c. The pore size
distribution of the material was calculated by the Barrett−
Joyner−Halenda (BJH) method and showed a relatively narrow
distribution ranging from ∼2 to 8 nm (inset of Figure 4c). The
presence of mesopores is attributed to the voids between the
deposited TiO2 nanoparticles of the multi-tubes surface as
shown in Figure 3c. The BET surface area of mesoporous TiO2
multi-tubes calcined at 450 °C is 99 m2/g, and the value
decreases gradually when calcined at higher temperatures
because of increasing crystallite size (91, 80, 77 m2/g for 550,
650, 750 °C, respectively). This value is high enough and
comparable to those reported mesoporous carbon-doped TiO2
as shown in Table S1 (Supporting Information).
deconvolution, the carbon 1s spectra show three peaks at
binding energies of 284.4, 286.4, and 288.1 eV. The first peak at
284.4 eV is attributed to the presence of adventitious elemental
carbon on the surface of NPs, whereas the other two peaks are
from C−O (C−O−C) and CO bonds of carbonate-like
species because of oxidized carbon species.10,44,48−55 These
carbonate-like species may substitute in the lattice of the Ti site
or interstitial site. In addition, the absence of a peak around
282.0 eV indicates that the substitution of the oxygen in the
lattice of anatase TiO2 as Ti-C was not formed by the anionic
carbon species.44,48 On the basis of the above observations, it is
confirmed that carbon doping of carbonate-like species occurs
at the interstitial position of the TiO2 lattice during the
calcination of organic material of banana fibers. As depicted in
Figure 5b, the large peak at 529.1 eV is assigned to O2− ions in
the Ti−O bonds, and another peak at 531.8 eV is because of
chemically adsorbed surface oxygen species, such as OH− and
carbonates. The high-resolution Ti 2p spectrum of TiO2 shows
two principle peaks at binding energies 457.78 and 463.35 eV
corresponding to Ti4+ 2p1/2 and 2p3/2 oxidation states in Figure
5c.
To get the carbon content, we investigated the thermal
behavior of as-synthesized TiO2 multi-tubes at 450 °C by
thermogravimetric (TG) analysis as shown in Figure S4
(Supporting Information). The first weight loss in the range
29−200 °C was because of the removal of physically adsorbed
water molecules. The second weight loss observed between
200−800 °C can be attributed to the removal of carbon content
on TiO2 multi-tubes (94.7 to 89.6%), which was ∼5.17%. The
obtained value is also close to that with the HAADF-STEM
EDS mapping analysis (∼6.12%).
3.6. Surface Elemental Composition and Thermogra-
vimetric analysis. The surface elemental composition and
chemical states of as-synthesized TiO2 samples were examined
by XPS. The presence of Ti 2p, O 1s, and C 1s elements can be
clearly seen in the XPS spectra of the TiO2 sample calcined at
450 °C. It can be seen from the Figure 5a that, after
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Figure 6. (a) UV−vis spectra of as-synthesized carbon-doped TiO2 and commercial P25. (b) The photoluminescence spectra of P25 and carbon-
doped TiO2 tubes excited at 426 nm. (c) Fluorescence spectra of TiO2 multi-tube suspension containing 2 mM terephthalic acid at various
irradiation periods under Hg lamp irradiation. (d) The time dependence graph of the fluorescence intensity at 425 nm under under LED and Hg
lamps irradiation; the value of regression coefficient (R2) is 0.99 for both conditions.
3.7. Optical Property. To see the optical property of
carbon-doped TiO2, the UV−vis absorption of the as-
synthesized TiO2 multi-tubes and P25 was carried out and is
shown in Figure 6a. It is observed that the P25 suspension
shows an absorbance peak at a wavelength of 320 nm, which is
very close to that of pure TiO2 nanoparticles. In contrast, the
UV−vis absorption spectrum of the TiO2 multi-tubes shows
shifting of the peak toward the visible region with a peak at 350
nm wavelength because of carbon doping in TiO2 and more
interaction of incident light by multiple reflections in hollow
tubes. The change in light absorption characteristics of TiO2
after the doping should also show the change in the band gap of
TiO2. The Tauc plot was used to estimate the band gap energy
of the as-synthesized TiO2 and P25. The results indicate that
the band gap energies of TiO2 multi-tubes and P25 are 2.83 and
3.3 eV, respectively, which suggests that C-doped TiO2 should
exhibit better visible light photocatalytic activity compared to
P25 because of the lower band gap.
3.8. Photoluminescence Study. The PL spectrum is
commonly used to evaluate the separation and recombination
behavior of photoinduced charge carriers as well as the trapping
of charge carriers. Thus, the PL spectra were measured to
investigate the recombination process of photoexcited electrons
and holes using an excitation wavelength of 426 nm at room
temperature as depicted in Figure 6b. It can be seen that the
P25 TiO2 NPs have high electron−hole recombination
intensity, while the recombination decreases with carbon-
doped TiO2. This suppression in the recombination of
photoinduced electron−hole pairs probably suggests that the
carbon doping can improve the surface state of TiO2, which
could result in improving photocatalytic capability due to the
generation of more active species for photocatalytic reaction.
3.9. Hydroxyl Radical Analysis. The generation of ·OH
on the surface of TiO2 photocatalyst was analyzed by the
fluorescence method using TA as a probe molecule, which
reacts with ·OH radicals and forms highly fluorescence active 2-
hydroxyterephthalic acid (TAOH). The fluorescence intensity
of TAOH is proportional to the amount of ·OH produced on
the surface of photocatalysts. The maximum emission intensity
in fluorescence spectra was recorded at 425 nm when excited at
315 nm wavelength. The fluorescence intensities of the
suspension in the presence of TA under the LED and Hg
lamps irradiations were measured, and the results are shown in
Figure 6c,d. It can be seen that the fluorescence intensity
increases gradually with increasing irradiation time because of
formation of ·OH during the photocatalytic process under
visible and UV−visible light irradiation. Figure 6d shows a
linear increase in fluorescence intensity of TiO2 multi-tubes at
425 nm under LED and Hg lamps irradiation against the
illumination time. Consequently, we can conclude that ·OH
radicals formed at the TiO2 interface are proportional to the
light illumination time and obeying first-order reaction rate
kinetics. The formation rate of the OH· radicals could be
expressed by the slope of the plot shown in Figure 6b.
However, no fluorescence intensity increase was observed in
the absence of any light. This suggests that the fluorescence
signal is only because of the reaction of TA with ·OH formed
on the interface of the TiO2 during the light irradiation.
Generally, the greater the formation rate of OH· radicals, the
greater is the separation efficiency of electron−hole pairs. A
little lower efficiency for formation of ·OH in multi-tubes was
observed under LED light compared to the Hg lamp because of
low intensity of light and elimination of UV components. As
the photocatalytic activity has a positive correlation with the
G
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Figure 7. (a) The change in concentration of Cr(VI) with time during photocatalytic reduction by TiO2 multi-tubes and P25 nanoparticles under
LED and Hg lamps. (b) Fitting of experimental data with first-order rate equation; the regression coefficient (R2) values are 0.98, 0.99 and 0.99, 0.98
for TiO2 multi-tubes and P25 NPs under LED and Hg lamps, respectively. (c) Schematic illustration of the photocatalytic reaction mechanism of C-
doped TiO2 multi-tubes.
formation rate of ·OH radicals, it can be expected that this
material may be useful as a good photocatalyst.
at irradiation time (t). The rate constant values were found to
be 0.0038 and 0.0007 min−1 for TiO2 multi-tubes and P25 NPs
under the LED light irradiation, respectively, while the rate
constant values were found to be 0.0052 and 0.0023 min−1 for
TiO2 multi-tubes and P25 NPs under the Hg lamp light
irradiation, respectively. These results indicate that the
enhancement of reduction efficiencies of C-doped multi-tubes
compared to P25 is ∼69% and 247% under the exposure of
high pressure Hg and LED lights, respectively. A little lower
efficiency of multi-tubes under LED light is because of low
intensity of light and lower content of UV. Finally, these results
indicate the developed catalyst is promising for photocatalytic
activity under visible light as well as UV−visible mixed light
sources.
During the photocatalytic process, the light absorption by the
catalyst and charge separation properties play a vital role for
higher activity. The increase in visible light absorption property
obtained from the UV−vis spectrum (Figure 6b) of TiO2 multi-
tubes can also be attributed because of band gap reduction after
carbon doping, which eventually enhances the photocatalytic
activity. Figure 7c schematically shows the photocatalytic
mechanism of C-doped TiO2 multi-tubes under visible light.
The improved photocatalytic activity can be attributed to, first,
the carbonate-like species on TiO2 act as a photosensitizer,
3.10. Study of Photocatalytic Activity for Cr(VI)
Reduction. Photocatalytic activities of TiO2 multi-tubes were
investigated for the reduction of aqueous Cr(VI) solution with
oxalic acid as a hole scavanger. The white LED and Hg lamps
were used as a source of visible and UV−visible lights,
respectively, to see the efficiency of the developed catalyst.
Figure 7a,b illustrates the photoreduction of Cr(VI) in the
presence of TiO2 multi-tubes and P25 nanoparticles under LED
and Hg lamps light exposure. The Figure 7a shows that the
TiO2 multi-tubes exhibit enhanced photocatalytic activity
compared to that with pure P25 TiO2 nanoparticles under
both light sources. Under the Hg lamp, the TiO2 multi-tubes
are able to reduce 78% of Cr(VI) within 120 min exposure,
whereas that of P25 is only 46%. On the other hand, the TiO2
multi-tubes and P25 are able to reduce 66% and 19% of Cr(VI)
within 120 min exposure to LED light, respectively. Further,
the kinetics of the photocatalytic reduction of Cr(VI) was
studied and is depicted in Figure 7b. A linear relationship
between Cr(VI) concentration and the irradiation time was
obtained in both cases, when fitted with the first-order rate
equation ln(C0/Ct) = −kt, where C0 represents the initial
Cr(VI) concentration and Ct refers to the Cr(VI) concentration
H
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Article
Notes
which absorbs visible light and transfers photogenerated
electrons into the conduction band of TiO2. Second, the
doping of C in the TiO2 lattice leads to the formation of new
mid-gap energy states of 2p orbitals of carbon above the
valence band of O 2p orbitals of TiO2. Therefore, the band gap
of TiO2 decreases and optical absorptions shift toward the
visible region. Thus, the photogenerated electrons of C-TiO2
can easily migrate to the conduction band under the visible
light irradiation to improve the photocatalytic efficiency.
Finally, these photogenerated electrons reduce the Cr(VI) to
Cr(III). In addition, the photogenerated holes produced
hydroxyl radicals, which was suppressed by oxalic acid as a
hole scavenger. To support the higher photogenerated
electrons for C-doped multi-tubes, the photocurrent measure-
ments of the samples were analyzed and found to be much
higher efficiency than that of P25 (Supporting Information,
Figure S6).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support from the DST-SERB (ref. No. EMR/
2016/000810) for this project is gratefully acknowledged. The
authors are grateful to the CeNTAB, SASTRA University,
Thanjavur, Tamilnadu, India, for giving the opportunity to
access their XPS facility. The authors also acknowledge Mr.
Subhabrata Chakraborty, NIT Rourkela, for helping in FE-SEM
and HR-TEM characterizations.
REFERENCES
■
(1) Pelaez, M.; Nolan, N. T.; Pillai, S. C.; Seery, M. K.; Falaras, P.;
Kontos, A. G.; Dunlop, P. S. M.; Hamilton, J. W. J.; Byrne, J. A.;
O’Shea, K.; Entezari, M. H.; Dionysiou, D. D. A Review on the Visible
Light Active Titanium Dioxide Photocatalysts for Environmental
Applications. Appl. Catal., B 2012, 125, 331−349.
4. CONCLUSIONS
(2) Dhakshinamoorthy, A.; Navalon, S.; Corma, A.; Garcia, H.
Photocatalytic CO2 Reduction by TiO2 and Related Titanium
Containing Solids. Energy Environ. Sci. 2012, 5 (11), 9217−9233.
(3) Wang, Y.; Chen, Y.; Zuo, Y.; Wang, F.; Yao, J.; Li, B.; Kang, S.; Li,
X.; Cui, L. Hierarchically Mesostructured TiO2/graphitic Carbon
Composite as a New Efficient Photocatalyst for the Reduction of CO2
under Simulated Solar Irradiation. Catal. Sci. Technol. 2013, 3 (12),
3286−3291.
(4) Fang, B.; Bonakdarpour, A.; Reilly, K.; Xing, Y.; Taghipour, F.;
Wilkinson, D. P. Large-Scale Synthesis of TiO2Microspheres with
Hierarchical Nanostructure for Highly Efficient Photodriven Reduc-
tion of CO2 to CH4. ACS Appl. Mater. Interfaces 2014, 6 (17), 15488−
15498.
(5) Li, G.; Singh, R.; Li, D.; Zhao, C.; Liu, L.; Webley, P. a. Synthesis
of Biomorphic Zeolite Honeycomb Monoliths with 16 000 Cells per
Square Inch. J. Mater. Chem. 2009, 19 (44), 8372−8377.
(6) Gao, Z.; Wu, Z.; Li, X.; Chang, J.; Wu, D.; Ma, P.; Xu, F.; Gao, S.;
Jiang, K. Application of Hierarchical TiO2 Spheres as Scattering Layer
for Enhanced Photovoltaic Performance in Dye Sensitized Solar Cell.
CrystEngComm 2013, 15 (17), 3351−3358.
(7) Li, Y.-N.; Su, J.; Lv, X.-Y.; Long, Y.-F.; Wen, Y.-X. Yeast Bio-
Template Synthesis of Porous Anatase TiO2 and Potential Application
as an Anode for Sodium-Ion Batteries. Electrochim. Acta 2015, 182,
596−603.
In summary, a new and facile biomimetic template approach is
reported for the synthesis of visible light active, high surface
area (99 m2/g), and mesoporous (pore sizes ∼ 2−8 nm) C-
doped TiO2 multi-tubes. The significant advantages of this
biomimetic approach are a single step process, low-cost,
scalability, thermal stability of the anatase phase up to 750
°C, and multi-tubular morphology. The diameter of a single
tube is 2 0.5 μm with a wall thickness of 170 10 nm which
is made of ∼15 3 nm nanoparticles. The C doping was taking
place during the calcination step at the interstitial and surface
positions of the anatase TiO2 lattice. The TG and TEM-EDX
analyses show ∼6 wt % C doping in TiO2 multi-tubes. The C
doping in TiO2 induces visible light absorption through the
introduction of a localized electronic state, lowering the band
gap, and improves visible photosensitization. The generation of
·OH radicals on the surface of TiO2 multi-tubes was found to
be proportional to the visible light illumination time and
obeying first-order reaction rate kinetics. The maximum
photoreduction efficiencies of Cr(VI) were 66% and 78% for
TiO2 multi-tubes within 120 min exposure under visible and
UV−visible light sources, respectively, whereas those of P25
were only 19% and 46%. The enhancement of photocatalytic
activity of TiO2 multi-tubes under visible light can be explained
by two important factors: (i) visible light absorption and better
charge separation due to carbon doping, and (ii) the excellent
structural properties including anatase phase, hollow structure,
high surface area, and mesoporous structure.
(8) Xin, L.; Liu, Y.; Li, B.; Zhou, X.; Shen, H.; Zhao, W.; Liang, C.
Constructing Hierarchical Submicrotubes from Interconnected TiO2
Nanocrystals for High Reversible Capacity and Long-Life Lithium-Ion
Batteries. Sci. Rep. 2015, 4, 4479.
(9) Fattakhova-Rohlfing, D.; Zaleska, A.; Bein, T. Three-Dimensional
Titanium Dioxide Nanomaterials. Chem. Rev. 2014, 114 (19), 9487−
9558.
(10) Sakthivel, S.; Kisch, H. Daylight Photocatalysis by Carbon-
Modified Titanium Dioxide. Angew. Chem., Int. Ed. 2003, 42 (40),
4908−4911.
(11) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A.
High-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor
to Hydrocarbon Fuels. Nano Lett. 2009, 9 (2), 731−737.
(12) Roy, S. C.; Varghese, O. K.; Paulose, M.; Grimes, C. A. Toward
Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to
Hydrocarbons. ACS Nano 2010, 4 (3), 1259−1278.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01864.
■
S
Sample photographs, FT-IR spectra, TG graph, EDX
analysis, photocurrent measurement, and comparison
table (PDF)
(13) Liu, Y.; Goebl, J.; Yin, Y. Templated Synthesis of Nano-
structured Materials. Chem. Soc. Rev. 2013, 42 (7), 2610−2653.
(14) Prozorov, T.; Mallapragada, S. K.; Narasimhan, B.; Wang, L.;
Palo, P.; Nilsen-Hamilton, M.; Williams, T. J.; Bazylinski, D. A.;
Prozorov, R.; Canfield, P. C. Protein-Mediated Synthesis of Uniform
Superparamagnetic Magnetite Nanocrystals. Adv. Funct. Mater. 2007,
17 (6), 951−957.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sparia@nitrkl.ac.in or santanuparia@yahoo.com. Fax:
+91 661 246 2999.
ORCID
■
(15) Nam, K. T.; Kim, D.-W.; Yoo, P. J.; Chiang, C.-Y.; Meethong,
N.; Hammond, P. T.; Chiang, Y.-M.; Belcher, A. M. Virus-Enabled
Santanu Paria: 0000-0002-3053-7277
I
DOI: 10.1021/acs.inorgchem.7b01864
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Inorganic Chemistry
Article
Synthesis and Assembly of Nanowires for Lithium Ion Battery
Electrodes. Science 2006, 312 (5775), 885−888.
(16) Berti, L.; Burley, G. A. Nucleic Acid and Nucleotide-Mediated
Synthesis of Inorganic Nanoparticles. Nat. Nanotechnol. 2008, 3 (2),
81−87.
(17) Zheng, T.; Tian, Z.; Su, B.; Lei, Z. Facile Method To Prepare
TiO2 Hollow Fiber Materials via Replication of Cotton Fiber. Ind. Eng.
Chem. Res. 2012, 51, 1391−1395.
(18) Huang, J.; Matsunaga, N.; Shimanoe, K.; Yamazoe, N.;
Kunitake, T. Nanotubular SnO2 Templated by Cellulose Fibers:
Synthesis and Gas Sensing. Chem. Mater. 2005, 17 (13), 3513−3518.
(19) Li, Z.; Yao, C.; Wang, F.; Cai, Z.; Wang, X. Cellulose Nanofiber-
Templated Three-Dimension TiO2 Hierarchical Nanowire Network
for Photoelectrochemical Photoanode. Nanotechnology 2014, 25 (50),
504005.
(20) Tian, J.; Pan, F.; Xue, R.; Zhang, W.; Fang, X.; Liu, Q.; Wang,
Y.; Zhang, Z.; Zhang, D. A Highly Sensitive Room Temperature H2S
Gas Sensor Based on SnO2 Multi-Tube Arrays Bio-Templated from
Insect Bristles. Dalt. Trans. 2015, 44, 7911−7916.
(21) Zhu, S.; Zhang, D.; Li, Z.; Furukawa, H.; Chen, Z. Precision
Replication of Hierarchical Biological Structures by Metal Oxides
Using a Sonochemical Method. Langmuir 2008, 24 (12), 6292−6299.
(22) Paris, O.; Fritz-Popovski, G.; Van Opdenbosch, D.; Zollfrank, C.
Recent Progress in the Replication of Hierarchical Biological Tissues.
Adv. Funct. Mater. 2013, 23, 4408−4422.
(23) Fan, T.; Sun, B.; Gu, J.; Zhang, D.; Lau, L. W. M. Biomorphic
Al2O3 Fibers Synthesized Using Cotton as Bio-Templates. Scr. Mater.
2005, 53 (8), 893−897.
(24) Guo, H.; Mao, R.; Yang, X.; Wang, S.; Chen, J. Hollow
Nanotubular SnO2 with Improved Lithium Storage. J. Power Sources
2012, 219, 280−284.
(25) Song, P.; Wang, Q.; Yang, Z. Sensors and Actuators B: Chemical
Biomorphic Synthesis and Gas Response of In2O3 Microtubules Using
Cotton Fibers as Templates. Sens. Actuators, B 2012, 168, 421−428.
(26) Song, P.; Wang, Q.; Yang, Z. Biomorphic Synthesis of ZnSnO3
Hollow Fibers for Gas Sensing Application. Sens. Actuators, B 2011,
156 (2), 983−989.
(27) Zhao, M.; Li, Z.; Han, Z.; Wang, K.; Zhou, Y.; Huang, J.; Ye, Z.
Synthesis of Mesoporous Multiwall ZnO Nanotubes by Replicating
Silk and Application for Enzymatic Biosensor. Biosens. Bioelectron.
2013, 49, 318−322.
(28) Han, J.; Su, H.; Xu, J.; Song, W.; Gu, Y.; Chen, Y.; Moon, W.-J.;
Zhang, D. Silk-Mediated Synthesis and Modification of Photo-
luminescent ZnO Nanoparticles. J. Nanopart. Res. 2012, 14 (2), 726.
(29) Maddocks, A. R.; Harris, A. T. Biotemplated Synthesis of Novel
Porous SiC. Mater. Lett. 2009, 63 (9−10), 748−750.
(30) Dong, Q.; Su, H.; Zhang, D.; Zhu, N.; Guo, X. Biotemplate-
Directed Assembly of Porous SnO2 Nanoparticles into Tubular
Hierarchical Structures. Scr. Mater. 2006, 55, 799−802.
(31) Huang, J.; Ichinose, I.; Kunitake, T. Biomolecular Modification
of Hierarchical Cellulose Fibers through Titania Nanocoating. Angew.
Chem., Int. Ed. 2006, 45 (18), 2883−2886.
(32) Hu, W.; Chen, S.; Zhou, B.; Wang, H. Facile Synthesis of ZnO
Nanoparticles Based on Bacterial Cellulose. Mater. Sci. Eng., B 2010,
170 (1−3), 88−92.
(33) He, J.; Liu, Z.-W.; Fan, W.-B.; Liu, Z.-T.; Lu, J.; Wang, J. A
General Method for Faithful Replication of Keratin Fibers with Metal
Oxides. J. Mater. Chem. 2010, 20 (45), 10107.
(34) Wang, X.-X.; Tian, K.; Li, H.-Y.; Cai, Z.-X.; Guo, X. Bio-
Templated Fabrication of Hierarchically Porous WO3 Microspheres
from Lotus Pollens for NO Gas Sensing at Low Temperatures. RSC
Adv. 2015, 5 (37), 29428−29432.
(35) Li, H.; Wang, B.; He, X.; Xiao, J.; Zhang, H.; Liu, Q.; Liu, J.;
Wang, J.; Liu, L.; Wang, P. Composite of Hierarchical Interpenetrating
3D Hollow Carbon Skeleton from Lotus Pollen and Hexagonal MnO2
Nanosheets for High-Performance Supercapacitors. J. Mater. Chem. A
2015, 3 (18), 9754−9762.
(36) Bu, D.; Zhuang, H. Synthesis, Characterization, and Photo-
catalytic Studies of Copper-Doped TiO2 Hollow Spheres Using Rape
Pollen as a Novel Biotemplate. Catal. Commun. 2012, 29, 24−28.
(37) Varghese, O. K.; Paulose, M.; Grimes, C. A. Long Vertically
Aligned Titania Nanotubes on Transparent Conducting Oxide for
Highly Efficient Solar Cells. Nat. Nanotechnol. 2009, 4 (9), 592−597.
(38) Kuang, D.; Brillet, J.; Chen, P.; Takata, M.; Uchida, S.; Miura,
H.; Sumioka, K.; Zakeeruddin, S. M.; Gratzel, M. Application of Highly
̈
Ordered TiO2 Nanotube Arrays in Flexible Dye-Sensitized Solar Cells.
ACS Nano 2008, 2 (6), 1113−1116.
(39) Roy, P.; Berger, S.; Schmuki, P. TiO2 Nanotubes: Synthesis and
Applications. Angew. Chem., Int. Ed. 2011, 50 (13), 2904−2939.
(40) Liu, N.; Chen, X.; Zhang, J.; Schwank, J. W. A Review on TiO2-
Based Nanotubes Synthesized via Hydrothermal Method: Formation
Mechanism, Structure Modification, and Photocatalytic Applications.
Catal. Today 2014, 225, 34−51.
(41) Shin, H.; Jeong, D.-K.; Lee, J.; Sung, M. M.; Kim, J. Formation
of TiO2 and ZrO2 Nanotubes Using Atomic Layer Deposition with
Ultraprecise Control of the Wall Thickness. Adv. Mater. 2004, 16 (14),
1197−1200.
(42) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K.
Formation of Titanium Oxide Nanotube. Langmuir 1998, 14 (12),
3160−3163.
(43) Lv, Y.; Xu, Z. L.; Asai, H.; Shimada, N.; Nakane, K. Thoroughly
Mesoporous TiO2 Nanotubes Prepared by a Foaming Agent-Assisted
Electrospun Template for Photocatalytic Applications. RSC Adv. 2016,
6 (25), 21043−21047.
(44) Wang, H.; Wu, Z.; Liu, Y. A Simple Two-Step Template
Approach for Preparing Carbon-Doped Mesoporous TiO2 Hollow
Microspheres. J. Phys. Chem. C 2009, 113 (30), 13317−13324.
(45) Boxi, S. S.; Paria, S. Visible Light Induced Enhanced
Photocatalytic Degradation of Organic Pollutants in Aqueous Media
Using Ag Doped Hollow TiO2 Nanospheres. RSC Adv. 2015, 5 (47),
37657−37668.
(46) Ghosh Chaudhuri, R.; Paria, S. Visible Light Induced
Photocatalytic Activity of Sulfur Doped Hollow TiO2 Nanoparticles,
Synthesized via a Novel Route. Dalt. Trans. 2014, 43 (14), 5526−
5534.
(47) Ohsaka, T.; Izumi, F.; Fujiki, Y. Raman Spectrum of Anatase,
TiO2. J. Raman Spectrosc. 1978, 7 (6), 321−324.
(48) Zhang, L.; Koka, R. V. A Study on the Oxidation and Carbon
Diffusion of TiC in Alumina−titanium Carbide Ceramics Using XPS
and Raman Spectroscopy. Mater. Chem. Phys. 1998, 57 (1), 23−32.
(49) Liu, J.; Zhang, Q.; Yang, J.; Ma, H.; Tade, M. O.; Wang, S.; Liu,
J. Facile Synthesis of Carbon-Doped Mesoporous Anatase TiO2 for the
Enhanced Visible-Light Driven Photocatalysis. Chem. Commun. 2014,
50 (90), 13971−13974.
(50) Sun, H.; Bai, Y.; Cheng, Y.; Jin, W.; Xu, N. Preparation and
Characterization of Visible-Light-Driven Carbon−Sulfur-Codoped
TiO2 Photocatalysts. Ind. Eng. Chem. Res. 2006, 45 (14), 4971−4976.
(51) Li, Y.; Hwang, D.-S.; Lee, N. H.; Kim, S.-J. Synthesis and
Characterization of Carbon-Doped Titania as an Artificial Solar Light
Sensitive Photocatalyst. Chem. Phys. Lett. 2005, 404 (1−3), 25−29.
(52) Liu, B.; Liu, L.-M.; Lang, X.-F.; Wang, H.-Y.; Lou, X. W. D.;
Aydil, E. S. Doping High-Surface-Area Mesoporous TiO2 Micro-
spheres with Carbonate for Visible Light Hydrogen Production. Energy
Environ. Sci. 2014, 7 (8), 2592−2597.
(53) Ren, W.; Ai, Z.; Jia, F.; Zhang, L.; Fan, X.; Zou, Z. Low
Temperature Preparation and Visible Light Photocatalytic Activity of
Mesoporous Carbon-Doped Crystalline TiO2. Appl. Catal., B 2007, 69
(3−4), 138−144.
(54) Zhang, L.; Tse, M. S.; Tan, O. K.; Wang, Y. X.; Han, M. Facile
Fabrication and Characterization of Multi-Type Carbon-Doped TiO2
for Visible Light-Activated Photocatalytic Mineralization of Gaseous
Toluene. J. Mater. Chem. A 2013, 1 (14), 4497−4507.
(55) Bai, H.; Liu, Z.; Sun, D. D. Facile Preparation of Monodisperse,
Carbon Doped Single Crystal Rutile TiO2 Nanorod Spheres with a
Large Percentage of Reactive (110) Facet Exposure for Highly
Efficient H2 Generation. J. Mater. Chem. 2012, 22 (36), 18801−18807.
J
DOI: 10.1021/acs.inorgchem.7b01864
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