Visible light photodegradation of rhodamine B over VDF/CTFE copolymer-templated crystalline mesoporous titania

Article abstract of DOI:10.1007/s11164-012-0554-5

Mesoporous TiO₂ with anatase crystalline structure (MTiO ₂/F₂₃₁₉) has been synthesized by using vinylidene fluoride/chlorotrifluoroethylene copolymer (1:9 in mole, F₂₃₁₉) as template. The synthesized mesoporous titania samples were characterized by a combination of various physicochemical techniques, such as X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and N₂ adsorption/desorption. It was found that without any external doping, MTiO₂/F₂₃₁₉ exhibited significantly higher photocatalytic activities for the degradation of rhodamine B (RhB) dye than P25 TiO₂ under visible light irradiation. Furthermore, the UV-Vis absorption maximum of the dye solution exhibited a gradual hypsochromic shift due to de-ethylation and degradation of RhB dye. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11164-012-0554-5

Res Chem Intermed (2012) 38:2383–2391
DOI 10.1007/s11164-012-0554-5
Visible light photodegradation of rhodamine B
over VDF/CTFE copolymer-templated crystalline
mesoporous titania
Guoli Zi
Fuzhi Wang
Jiao He Jiaqiang Wang
•
Yi Wang
•
Kai Zheng
•
Huan Zhao
•
•
•
Wenjun Zhang
•
Zhiying Yan
•
Received: 7 January 2012 / Accepted: 2 April 2012 / Published online: 25 April 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Mesoporous TiO with anatase crystalline structure (MTiO /F2319) has
2
2
been synthesized by using vinylidene fluoride/chlorotrifluoroethylene copolymer
1:9 in mole, F2319) as template. The synthesized mesoporous titania samples were
(
characterized by a combination of various physicochemical techniques, such as
X-ray diffraction, scanning electron microscopy, high-resolution transmission
electron microscopy, and N adsorption/desorption. It was found that without any
2
external doping, MTiO /F
2
exhibited significantly higher photocatalytic activities
2319
for the degradation of rhodamine B (RhB) dye than P25 TiO under visible light
2
irradiation. Furthermore, the UV-Vis absorption maximum of the dye solution
exhibited a gradual hypsochromic shift due to de-ethylation and degradation of RhB
dye.
Keywords Crystalline mesoporous TiO ꢀ Visible light photocatalysis ꢀ
2
F2319 copolymer ꢀ Rhodamine B
Introduction
Photocatalytic processes involving TiO semiconductor particles under UV light
2
illuminations have been shown to be potentially advantageous and useful in the
treatment of wastewater pollutants [1]. However, TiO2 requires UV light for
photoreactions to proceed. This greatly limits the use of sunlight as an energy source
for the photoreactions, because sunlight is comprised of less than 5 % ultraviolet
light. To develop more light-efficient catalysts, there is an urgent need to develop
photocatalytic systems that are able to operate effectively under solar light irradiation
G. Zi ꢀ Y. Wang ꢀ K. Zheng ꢀ H. Zhao ꢀ F. Wang ꢀ W. Zhang ꢀ Z. Yan ꢀ J. He ꢀ J. Wang (&)
Department of Applied Chemistry, The Universities’ Center for Photocatalytic Treatment of
Pollutants in Yunnan Province, Yunnan University, Kunming 650091, People’s Republic of China
e-mail: jqwang@ynu.edu.cn
123

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G. Zi et al.
[
2, 3]. Due to the direct correlation between the photocatalytic performance and the
microstructure of a semiconductor photocatalyst, searching for new photocatalysts
with an appropriate crystal structure has been the intensively pursued topic in
heterogeneous photocatalysis [4]. It is well known that the effectiveness of TiO2
photocatalyst is very sensitive to its crystallinity, particle size, and crystal phase [5],
and mesoporous TiO has been shown to be potentially advantageous because it
2
shows higher surface areas, much more uniform and controllable pore size, and
pore morphologies compared to randomly organized nanocrystalline TiO [6]. For
2
example, poorly crystalline nanoparticles can result in limited photocatalytic effects
since the defects in TiO can act as recombination sites of generated electrons (e-)
2
and holes (h?) [7], while single-crystalline TiO provides rapid electron transfer
2
with high efficiency [8]. Therefore, one of the strategies of making highly active
TiO is to give particle high crystallinity, which may reduce the possibility of
2
electron–hole pairs recombination at a defective site [5]. At the same time, anatase
is the preferred phase of TiO with catalytic effects superior to other phases [9].
2
Nevertheless, the fabrication of crystalline mesoporous TiO is not easy to succeed
2
because of the sensitivity of the structure toward the heat-treatment procedure and
crystallization [10]. We successfully synthesized mesoporous TiO materials with an
2
anatase crystalline framework to retain their structural integrity and mesoporosity
when calcined at 773 K.
Up to now, the general synthetic strategies for mesoporous TiO have been
2
developed using a variety of templates, such as phosphates amine, ionic surfactants,
block polymer, nonionic surfactants, and nonsurfactant templates [5, 11–15].
However, both templates and dopants are essential for the synthesis of visible-light
photocatalysts based on mesoporous TiO . Recently, in our group, 14 commercial
2
synthetic dyes were directly used as structure-directing agents that convert to
carbonaceous materials after calcination in synthesis of thermally stable meso-
structured TiO [12]. Without any external doping, this in situ carbon enhanced the
2
visible light photocatalytic activities for the degradation of aqueous dyes. In this
paper, we have extended our previous work on dye templates to a new block
copolymer template (F2319). We also want to obtain visible-light photocatalytic
activity without any external doping by using F2319 as a template for the preparation
of mesoporous TiO2.
Vinylidene fluoride/chlorotrifluoroethylene (VDF/CTFE) random copolymers are
synthesized by copolymerizing VDF with CTFE in different molar ratios. F2319 has
much higher decomposition temperature than those fluoride-free copolymers such
as P123 (EO PO EO ). The molecular weight of the F polymer we used was
2319
2
0
70
20
5
3.0 9 10 g/mol. Recently, grafted VDF/CTFE polymers have been used as
*
templates to synthesize TiO thin films [16] and Ag-loaded TiO nanostructures [17]
2
2
through a sol–gel process. However, hydrothermal synthesis of mesoporous TiO2
using pure VDF/CTFE random polymers as template has not been investigated as
far as we know. In particular, previous studies have not yet addressed the direct use
of F2319 as a template in the synthesis of thermally stable crystalline mesoscopic
ordering and visible-light photocatalytic activity.
Herein we report on the synthesis and characterization of mesoporous TiO2
with an anatase crystalline structure (MTiO /F2319) templated by a VDF/CTFE
2
1
23

Visible light photodegradation of rhodamine B
2385
random copolymer, i.e., VDF/CTFE copolymer with a 1:9 molar ratio (F2319).
MTiO /F exhibited significant visible light photocatalytic activity without any
external doping.
2
2319
Experimental
Synthesis
In a typical preparation process, 1.0 g F2319 ([(–CF –CH –) –(–CF –CFCl–) ]
b m
2
2
a
2
a:b = 1:9) copolymer was put into 90 mL of ethyl acetate, and an ultrasonic
treatment for more than a week was carried out to help F2319 copolymer dissolve.
Afterwards, 10 mL of distilled water was added slowly into the supernatant, and
then 8 mL of tetrabutyl titanate was added drop-wise under continuous stirring.
Subsequently, the mixture was stirred for 24 h and then transferred into a Teflon
bottle and treated under autogenous pressure at 363 K for 7 days, filtered, washed,
and then dried. Finally, the sample was calcined in air at 773 K for 8 h and then a
powder was obtained.
Characterizations
X-ray powder diffraction (XRD) experiments were conducted on a D/max-3B
spectrometer with Cu Ka radiation. Scans were made in the 2h range 5–90° with a
scan rate of 10°/min (wide-angle diffraction). Pore size distributions, BET surface
areas, and pore volumes were measured by nitrogen adsorption/desorption using a
Micromeritics Tristar II Surface Area and Porosity Analyzer. Prior to the analysis,
the samples were degassed at 150 °C for 1 h. Scanning electron microscopy
(
SEM) images were taken on an FEI Quanta200 FEG microscope at an accelerating
voltage of 15 kV with the pressure in the sample chamber of 1 Torr. High-
resolution transmission electron microscopy (HRTEM) imagines were taken on a
2
TEM (TECNAI G S-TWIN F20).
Photocatalytic activity
The reaction of the photocatalytic degradation of rhodamine B (RhB, C H
8 31
2
ClN O ) was carried out in a glass beaker containing 50 mL of a model solution
2
3
(10 ppm) and 25 mg of photocatalyst. The suspensions were magnetically stirred in
dark for 24 h to attain adsorption–desorption equilibrium between dye and
mesoporous TiO . After the system reached the adsorption–desorption equilibrium
2
and dissolved oxygen, they were exposed for 5 h under visible light. The
photocatalytic experiments were carried out under visible light with an 800-W
Xe-illuminator in which a colored glass filter (k [ 420 nm) was utilized for the
purpose of allowing only visible radiation. Laboratorial film was used to seal
the beaker so that the volume of the solutions decreased little after the experiment.
The solutions were mixed with a magnetic stirrer during the reaction process. The
concentration of residual RhB was measured at an interval of 1 h by monitoring
123

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G. Zi et al.
UV-Vis spectrum over 200–800 nm after centrifugation at 2,500 rpm for 10 min
and subsequently filtered through a Millipore filter (pore size, 0.45 lm). All the
absorbance-concentration standard curves were obtained using this apparatus. The
determination wavelength is 553 nm, which is the maximum absorption wavelength
of RhB as detected by UV-Vis spectrometer. The decrease due to adsorption can be
deducted after the adsorption equilibrium was achieved. Therefore, photodegrada-
tion yield is defined as: [18]
Co ꢁ Ca ꢁ Cb
photodegradation yield ¼
ꢂ 100 %
ð1Þ
Co
C is the initial concentration of dye, C is the concentration after photodegradation
o
a
of dye, and C is the decrease concentration because of the direct photolysis.
b
Results and discussion
Synthesis of mesoporous TiO2
In order to use F2319 copolymer as template in a hydrothermal process, it had to be
dissolved in an appropriate solvent. F2319 copolymer is difficult to dissolve in
ordinary solvents such as absolute ethyl alcohol, frequently used in a hydrothermal
process. Therefore, ethyl acetate was used as solvent and a long-term ultrasonic
treatment was also employed. In addition, the F2319 copolymer can serve as a
scaffold for the mesostructure in titania crystallization before they decompose at
*
723 K. This temperature is much higher than the decomposition temperature of
traditional PEO-based block copolymer such as F127 and P123 templates (*523 K)
19]. Therefore, a high temperature of 773 K was employed in order to remove the
[
template completely, higher than ordinary calcination temperature (up to 673 K) in
a typical process of anatase TiO synthesis. The high calcinations temperature
2
implies that the synthesized mesoporous TiO has a high thermal stability.
2
The characterizations of photocatalyst
Figure 1 shows the N adsorption/desorption isotherm and BJH pore-size distribu-
2
tion plots (inset) of MTiO /F2319. Typical IV adsorption/desorption isotherm is
2
observed, which indicates the mesoporous structure of prepared material. This
phenomenon is in good agreement with those mesoporous TiO templated by other
2
traditional templates, such as block copolymers or surfactants. The specific surface
area, pore volume, and average pore diameter of MTiO /F
2
were 61.7 m /g,
2
2319
0
was similar to that of mesoporous TiO films templated by P123 block copolymer
.15 cc/g, and 9.0 nm, respectively. The surface area of the as-synthesized sample
2
2
58 m /g) [20], but lower than by F127 block copolymer (122 m /g) [21]. This
2
(
could be attributed to the use of different templates and the higher calcination
temperatures. The BJH pore-size distribution of MTiO /F
2
indicates that the
2319
catalysts have regular mesoporous channels.
1
23

Visible light photodegradation of rhodamine B
2387
100
0
.012
8
6
4
2
0
0
0
0
0
0.008
0
0
.004
.000
0
20
40
60
80
100
120
140
Pore width (nm)
0
.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P )
0
2 2
Fig. 1 N adsorption/desorption isotherm and BJH pore-size distribution (inset) of MTiO /F2319
2
0
40
60
80
100
2
Theta (degrees)
2
Fig. 2 X-ray diffraction pattern of MTiO /F2319
XRD pattern of MTiO /F
2
is shown in Fig. 2. Several diffraction peaks, which
2319
are well-matched with the diffraction patterns of anatase TiO , are observed in the
2
wide-angle diffraction region. It can be clearly seen that the diffraction peaks are
sharp and intensive, suggesting the formation of MTiO /F
crystals with only one
2
2319
pure crystalline phase present, namely anatase. This again suggests that F2319 polymer
template has a high-temperature decomposition-resistant character and inhibited
phase transition from anatase to other phase in the process of crystallization.
123

2388
G. Zi et al.
The average size of anatase nanocrystals calculated by the Scherrer equation is to be
*
20 nm.
SEM and high-resolution TEM images of MTiO /F
are shown in Fig. 3a, b,
2319
2
and c, respectively. SEM reveals that this sample has no regular morphology and
highly aggregated (Fig. 3a). The crystallinity of anatase phase in the mesoporous
wall is observed from the HRTEM images of MTiO /F2319. Figure 3b shows that the
2
highly aggregated nanocrystal composed of disordered primary nanoparticles
(
20–30 nm in size), which is in agreement with the domain size (ca. 20 nm)
obtained from XRD by applying the Scherrer equation. The synthesis of well-
crystallized particles with mesoporous wall in the literature was limited. Figure 3c
shows clear lattice fringes. It also clearly confirmed that the mesoporous TiO are
2
highly crystalline and almost no amorphous structures can be observed. The
crystalline framework enables the superior performance of photocatalytic activities
for the degradation of RhB dye.
Photocatalytic activity
In photocatalytic experimentation, after reaching the adsorption equilibrium, the
variations for the concentrations of the studied materials are summarized in Fig. 4.
The photodegradation yields using MTiO /F
2
and P25 TiO as photocatalysts are
2
2319
included. The photodegradation yield of RhB in the presence of the MTiO /F
2
2319
can be up to 71.4 % under visible light, while P25 TiO only had 55.4 % under
2
visible light (Table 1). The degradation of RhB on P25 TiO under visible light
2
irradiation can be attributed to the self-sensitization of RhB since RhB served
as sensitizer in the process of dye sensitized TiO2 photocatalysis [22]. More
importantly, it is obvious that MTiO /F
2
composite exhibited higher photocat-
2319
alytic activity under visible light (k [ 420 nm) than P25 TiO2 although the
photodegradation yield of MTiO /F
reaction time. Therefore, MTiO /F
and P25 TiO increased with increasing
had apparently better visible photocatalysis
2319
2
2319
2
2
than P25 TiO2.
The variation of UV-Vis spectra of RhB during photodegradation over MTiO2/
F2319 under visible light (k [ 420 nm) is shown in Fig. 5. It is seen that the
absorption peaks corresponding to RhB diminished under visible light irradiation
2
Fig. 3 SEM (a), HRTEM images (b–c) of MTiO /F2319
1
23

Visible light photodegradation of rhodamine B
2389
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
P25 TiO2
MTiO /F2319
2
0
1
2
3
4
5
Irradiation Time (hour)
Fig. 4 Comparison of photodegradation yields of rhodamine B over MTiO
under visible light irradiation (k [ 420 nm)
2 2
/F2319 and Degussa P25 TiO
2 2
Table 1 Photocatalytic activities of MTiO /F2319 and P25 TiO under visible light (k [ 420 nm) in
different reaction times (%)
Catalysts
Photodegradation yield of RhB (%) under visible light (k [ 420 nm)
1
h
2 h
3 h
4 h
5 h
MTiO
2
/F2319
42.7
15.0
54.0
23.6
56.7
39.6
62.1
49.9
71.4
55.4
P25 TiO
2
0-5h
200
300
400
500
600
700
800
Wavelength (nm.)
Fig. 5 UV-Vis spectra of RhB during photodegradation over MTiO
visible light irradiation (k [ 420 nm)
2
/F2319 at different times under
123

2390
G. Zi et al.
0
0
Scheme 1 De-ethylation and degradation processing of N,N,N ,N -tetraethylated rhodamine (RhB)
molecule
and indicate the degradation of RhB. No new absorption bands, especially, the
absorption bands of aromatic moieties and other similar intermediates, were
observed. Besides, visible light irradiation of the aqueous RhB-MTiO /F
2
2319
dispersion leads to a wavelength shift of the band to shorter wavelengths accompany
with decrease in absorption. In general, the spectral shift from 553 to 497 nm with a
concomitant diminishing of absorption peaks during 5 h. The wavelength shift
depicted in Fig. 5 is caused by de-ethylation of RhB because of attack by one of the
0
0
active oxygen species on the N-ethyl group [4]. De-ethylation of the fully N,N,N ,N -
tetraethylated rhodamine molecule (i.e., RhB) has the wavelength position of its
major absorption band moved toward the blue region (Scheme 1(1–4)) [4, 23].
When the de-ethylated process is fully completed, the absorption band shifts to
4
97 nm and RhB is turned to rhodamine. Rhodamine is then gradually decomposed
due to the further destruction of the conjugated structure (Scheme 1(5)) [23, 24].
The above results are analogous to those of the competitive photodegradation
reactions between the polyoxyethylene chain and the aromatic ring in polyoxyethy-
lated nonylphenol. During the initial period of photodegradation of RhB, competitive
reactions between de-ethylation and cleavage of the RhB chromophore ring structure
occur, with de-ethylation predominating. Irradiation by visible light for longer
periods of time leads to further decomposition of the de-ethylated rhodamine
intermediates as indicated by changes in peak intensity at 497 nm (Fig. 5) [4, 25].
The formation of large-pore mesoscopically ordered titania oxides via such a
simple procedure suggests that the F2319 moleculars are the ideal building blocks for
synthesis of visible light photocatalysts, even though F2319 moleculars are expected
to behave in a more complex manner than those conventional PEO-based block
copolymer templates. It is also interesting to evoke some reasons why the
mesoporous titanium dioxides prepared by using F2319 as template exhibited visible
light photocatalytic activities. Obviously, the crystalline framework, large pore size,
and high thermal stability of these new mesoporous titanias are expected to afford
better activity toward photocatalytic reactions. Indeed, mechanism details of the
formation of mesoporous materials are still far from understood.
Conclusions
In summary, F2319 were successfully used as template to synthesize mesoporous
anatase crystalline TiO . The synthesized MTiO /F sample without any external
2319
2
2
1
23

Visible light photodegradation of rhodamine B
2391
doping exhibited higher photocatalytic activity for the degradation of RhB under
visible light irradiation than P25 TiO . Both de-ethylation and degradation of RhB
2
take place under visible light irradiation in the presence of MTiO /F
2
particles.
2319
We believe that the synthetic strategy demonstrated here could also be extended to
other mesoporous materials with visible-light photocatalytic activity, such as, SiO2
and Al O and other photosensitizing agents studies of which are ongoing.
2
3
Acknowledgments The authors thank the Natural Science Foundation of China (Project NSFC-YN
U1033603, 20863009, 21063016,), Yunnan Province (Project 20008CD065), Specialized Research Fund
for the Doctoral Program of Higher Education (200806730005), Program for Innovative Research Team
(in Science and Technology) in University of Yunnan Province (IRTSTYN) the R&D Program of
Kunming (11H-011101, 2011-15) for financial support.
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