二氧化钛光催化 英文参考文献

二氧化钛光催化 英文参考文献
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题目：
Visible light-activated N-F-codoped TiO2 nanoparticles for the photocatalytic degradation of microcystin-LR in water
正文：
1. Introduction
The development of nanotechnology for the synthesis of
nanomaterials is providing unprecedented opportunities to deal
with emerging environmental problems associated with water
contamination along with worldwide energy-related concerns [1].
Currently, advanced oxidation technologies (AOTs) and nanotechnologies
(AONs) have been extensively investigated for the
destruction of toxic and recalcitrant organic compounds and
inactivation of microorganisms in water and air [2–12]. Titanium
dioxide (TiO2), a well-known semiconductor with photocatalytic
properties, is a widely used AON for water and air remediation [6–
10]. It has proven to be highly effective in the nonselective
degradation of organic contaminants due to high decomposition
and mineralization rates. However, conventional TiO2 requires
ultraviolet (UV) radiation (l < 400 nm) to overcome its wide band
gap energy (\x023.2 eV for anatase phase) for photocatalytic
activation [4,11]. This is a technological limitation when aiming
at implementation of large scale sustainable technologies with
renewable energy sources such as solar light, since UV radiation
accounts only for 5% of the total solar spectrum compared to the
visible region (\x0245%) [12,13]. Several attempts have been directed
towards the development of modified TiO2 with visible light
response by dye sensitization, metal (Fe, Co, Ag) [14,15] and
nonmental (N, F, C, S) [4,16–23] doping of the catalyst to reduce
TiO2 band gab energy requirements for photocatalytic activation.
In some metal doping approaches, the resulting visible light
photocatalytic activity has some drawbacks including increase in
the carrier-recombination centers (electron–hole pair species
generated after photo-excitation of the catalyst) and low thermal
stability of the modified material [14]. Moreover, metal leaching
and possible toxicity diminish the potential of employing metaldoped
TiO2 for drinking and wastewater treatment applications. A
more successful approach involves nonmetal doping of TiO2.
Nitrogen doping of TiO2 for visible-light driven photocatalysis
revealed band gap narrowing from the mixing of nitrogen 2p
states with oxygen 2p states on the top of the valence band at
substitutional lattice sites in the form of nitride (Ti–N) or
oxynitride (Ti–O–N). A different arrangement is the formation of
oxyanion species at the interstitial lattice sites creating localized
intergap states [24]. Both configurations make it possible to shift
the optical absorption towards visible light, thus, allowing
photocatalytic activity in the visible region [11,22,23]. Fluorine
doping is also effective to induce modifications of the electronic
structure of TiO2 by the creation of surface oxygen vacancies due to
charge compensation between F\x04 and Ti4+ but without producing a
significant change in the optical absorption of TiO2 [21]. Moreover,
codoping of TiO2 with nitrogen and fluorine has demonstrated high
photocatalytic activity in the visible region with beneficial effects
induced by both dopants [25–27]. Huang et al. confirmed strong
visible-light absorption and high photocatalytic activity of N-FTiO2
for p-chlorophenol and Rhodamine B degradation under
visible light irradiation [26]. Xie et al. effectively decomposed
methyl orange with visible light-induced N-F-TiO2 photocatalyst
[27]. Both attributed their findings to the synergistic effect of
nitrogen and fluorine doping.
In addition to nonmetal doping, structural properties of TiO2 are
of significant importance to enhance its physicochemical properties
and photocatalytic response. For instance, the use of self-assembly
surfactant-based sol–gel methods has been reported as an effective
approach to tailor-design the structural properties of TiO2 nanoparticles
and films from molecular precursors [6,8–10]. The
hydrocarbon surfactant is used as pore directing agent and to
control the hydrolysis and condensation rates of the titanium
precursor in the sol formulation. This method has the capacity to
yieldtailor-designedTiO2withhighsurface area,highporosity, small
crystal size with narrow pore size distribution and high photocatalytic
activity under UV [8–10] and visible light irradiation [4].
One of the aims of this work is to develop highly efficient N-Fcodoped
TiO2 nanoparticles with enhanced structural properties
and high photocatalytic activity under visible light irradiation
using a novel sol–gel route employing a nonionic fluorosurfactant
as pore directing agent and fluorine dopant and ethylenediamine
as nitrogen source. Fluorosurfactants or fluorinated surfactants,
have been used mainly as antistatic, antifogging and wetting
agents, and paint coating additives [28]. Only recent studies have
focused on the use of fluorinated surfactants as pore template for
mesoporous silica materials [29–32], signifying a great potential
for novel ceramic materials.
The second aim of this work is to focus on the application of
such nanoparticles in engineered water treatment processes for
the destruction of environmental contaminants of worldwide
concern. Drinking water treatment plants are facing more
prevalent occurrence of cyanobacterial harmful algae blooms
(Cyano-HABs) and the release of their toxins in their water sources.
These toxins are considered a serious health risk due to their high
solubility in water, toxicity (i.e., hepatotoxicity, neurotoxicity, and
carcinogenicity) and chemical stability. Among them, microcystin-
LR (MC-LR) is one of the most commonly found cyanotoxins in
Cyano-HABs and the most toxic derivative of the group of
microcystins [33]. Conventional TiO2 has been proven to be
effective in the treatment of MC-LR under UV radiation [34,35].
Recent work demonstrated high degradation rates of MC-LR with
nitrogen-doped TiO2 nanoparticles [4]. In this study, we present
results on the destruction of MC-LR with N-F-TiO2 nanoparticles
under visible light irradiation.
2. Experimental
2.1. Synthesis of visible light-activated TiO2 nanoparticles
To prepare the modified sol–gel solution, a nonionic fluorosurfactant
(Zonyl FS-300 (FS), \x0250% solids in H2O, RfCH2CH2O(CH2
CH2O)xH; Rf = F(CF2CF2)y where x = 14 and y = 3, Fluka), acting as
both pore directing agent and fluorine source, dissolved in
isopropanol (i-PrOH), was used. Acetic acid (Fisher) was added
to maintain a low pH (\x026.4). Before adding the titania precursor,
anhydrous ethylenediamine (EDA, Fisher) was added in the
solution as nitrogen source. Then, titanium(IV) isopropoxide (TTIP,
97%, Aldrich) was added dropwise under vigorous stirring and
more acetic acid was added for peptidization. The final sol obtained
was transparent, homogeneous and stable after stirred overnight
at room temperature. Afterwards, the sol was dried at room
temperature for 24 h and then calcined in a multi-segment
programmable furnace (Paragon HT-22-D, Thermcraft) where
the temperature was increased at a ramp rate of 60 8C/h to 100 8C
and maintained for 1 h. Then it was increased up to 400 8C under
the same ramp rate, maintained for 2 h and cooled down naturally
to finally obtain a yellowish powder. The FS:i-PrOH:acetic
acid:EDA:TTIP molar ratio employed in the sol–gel for the
preparation of the denoted Particle 1 was 0.01:0.65:1.0:0.1:0.05.
Specifically, the i-PrOH/EDA molar ratio was 2.85 and 14 for
Particles 2, and 3, respectively. Nitrogen-doped TiO2 (Particle 4)
and fluorine-doped TiO2 (Particle 5) where synthesized without FS
and EDA, respectively, maintaining the same final volume by the
addition of more isopropanol. Reference TiO2 was synthesized
using the same procedure but without the addition of nitrogen and
fluorine sources. The synthesized nanoparticles were compared
with Kronos vlp 7000, a commercially available visible lightactivated
TiO2 photocatalyst (Kronos International Inc., D-51373).
2.2. Characterization of synthesized TiO2
An X-ray diffraction (XRD) analysis was performed with a
Kristalloflex D500 diffractometer (Siemens) using Cu Ka
(l = 1.5406A˚ ) radiation, to study the crystal structure and
crystallinity of the TiO2 nanoparticles. The Brunauer–Emmett–
Teller (BET) surface area, pore volume, porosity, Barret–Joyner–
Halenda (BJH) pore size and distribution (based on nitrogen
adsorption and desorption isotherms) were determined by Tristar
300 (Micromeritics) porosimeter analyzer. The samples were
purged with nitrogen gas for 2 h at 150 8C using Flow prep 060
(Micromeritics). A high resolution-transmission electron microscope
(HR-TEM) with field emission gun at 200 kV was employed
to obtain crystal size and crystal structure at the nanoscale. The
samples in ethanol were dispersed using an ultrasonicator (2510RDH,
Bransonic) for 15 min and fixed on a carbon-coated copper grid
(LC200-Cu, EMS). The particle morphology was characterized by an
environmental scanning electron microscope (ESEM, Philips XL 30
ESEM-FEG) at an accelerating voltage of 30 kV. The point of zero
charge (PZC) was measured using a Zetasizer (Malvern Instruments).
The fine elemental composition and electronic structure
was determined with an X-ray photoelectron spectroscope (XPS,
PerkinElmer Model 5300) with Mg Ka X-rays at a takeoff angle of
458 and vacuum pressure of 10\x048 to 10\x049 Torr. The binding
energies were calibrated with respect to C1s core level peak at
284.6 eV. To investigate the optical band gap of the synthesized
TiO2 nanoparticles, the UV–vis absorption spectra were obtained
with a UV–vis spectrophotometer (Shimadzu 2501 PC) mounted
with an integrating sphere accessory (ISR1200) using BaSO4 as
reference standard.
2.3. Photocatalytic evaluation with microcystin-LR under visible light
The photocatalytic activity of the synthesized TiO2 nanoparticles
was evaluated for the degradation of MC-LR. A borosilicate
vessel (i.d. 4.7 cm) was employed as photocatalytic reactor. An
aqueous solution, previously adjusted at the desired pH with
H2SO4 or NaOH without any buffer, was spiked with an aliquot of
MC-LR standard (Calbiochem Cat #. 475815) to achieve an initial
concentration of 1.0 \x03 0.05 mg/L. A solution with TiO2 nanoparticles
was dispersed using an ultrasonicator (2510R-DH, Bransonic) for 24 h
and transferred to the reactor containing MC-LR for a final volume
solution of 10 ml. The reactor was completely sealed and mixed to
minimize mass transfer limitations. Two 15W fluorescent lamps
(Cole-Parmer) mounted with UV block filter (UV420, Opticology) to
eliminate spectral range below 420 nm were employed to irradiate
the reactors. The intensity of the radiation was below the detection
limit when employing an IL 1700 radiometer (International Light)
with a 365 nm sensor. The light intensity was determined using a
broadband radiant power meter (Newport Corporation) for a total
visible light intensity of 7.81 \x05 10\x045Wcm\x042. During irradiation, a fan
was positioned near the reactor to cool it down. Sampling was done at
specific periods of time and the samples were quenched with
methanol to stop any further reaction, filtered (L815, Whatman) to
remove the suspended nanoparticles, transferred to 0.2 ml glass
inserts and placed in sample vials. MC-LR samples were analyzed by
liquid chromatography (LC, Agilent Series 1100) equipped with a
photodiode array detector set at 238 nm under isocratic conditions:
60% (v/v) of 0.05% trifluoroacetic acid (TFA) in MilliQ water and 40%
(v/v) of 0.05% TFA in acetonitrile with a flow rate of 1 ml/min.
The column employed was a C18 Discovery (Supelco) column
(4.6 mm \x05 150 mm, 3 mm particle size) kept at 40 8C with an
injection volume of 50 ml [7]. The handling of the toxin must be
done with extreme care since it is highly toxic and irritant if exposed.
Therefore, all the experiments were conducted in an Advance
Sterilchemgard III Class II biological safety cabinet (Baker Company,
Sanford, ME) with full exhaust.