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ARTICLE IN PRESS
M.V. Dozzi et al. / Applied Catalysis A: General xxx (2015) xxx–xxx
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fuels. Previous studies evidenced that among different TiO2 based
TiO2 powders with controlled physical properties (e.g., specific
surface area, crystallite size and composition) may be obtained,
which guarantee good performance in both organics mineraliza-
a recent investigation carried out employing a series of fluorinated
Pt/TiO2 photocatalysts evidenced that 5 at.% F for O substitution in
TiO2 was optimal for achieving high H2 production rate through
this reaction [36].
sampling depth is within 10 nm. During measurements the vac-
uum level was around 10−9 Torr. In order to neutralize the surface
electrostatic charge of nonconductive samples an electron gun was
used. Furthermore, the charging effect on the analysis was also
corrected considering the binding energy (BE) value of C(1s), due
to adventitious carbon, at 285.0 eV. The quantitative analysis data
were reported as atomic percentage of elements; the normalization
was performed without including hydrogen.
2.3. Preparation of titania films
In the present work, we try to elucidate the origin of the better
performance of fluorinated flame-made TiO2 materials containing
different amounts of fluorine, in a systematic investigation aimed
at correlating their photoactivity, in both powder and film form,
with their surface and bulk properties.
The home-made FSP x nanopowders were deposited on FTO
transparent electrodes by the following procedure. A TCO22-7 FTO
glass was first cut in the appropriate size (1.5 cm × 1.0 cm) and care-
fully cleaned by sonication for 30 min first in distilled water and
then in a 1:1 acetone-isopropyl alcohol solution. The electrodes
were then dried in oven at 70 ◦C for 2 h and calcined at 500 ◦C for
4 h.
2. Experimental
The conductive layer of the FTO glass was coated with a paste
containing the FSP x samples and spin coated in a Laurell WS-
650MZ-23NPPB spin coater at 600 rpm for 180 s. The paste was
obtained by mixing 60 mg of fluorine-doped TiO2 powder with
0.5 mL of ethanol and 0.02 mL of acetic acid, followed by sonication
for about 90 min. Then 0.4 mL of ethanol were further added to the
paste, which was stirred overnight before being deposited and spin
coated on FTO. The FTO-coated films were then calcined at 300 ◦C
for 3 h, with a heating ramp of 10 ◦C min−1. Their thickness was
720 nm 120 nm, as determined by a DeltaXT profilometer. The
area of each film was carefully fixed at 1.0 cm2 (1.0 cm × 1.0 cm).
2.1. Photocatalysts preparation
A series of fluorine-doped TiO2 samples was synthesized in
continuous and single step by flame spray pyrolysis [28,29,37].
These samples were labeled as FSP x with x corresponding to the
nominal F/O percent molar ratio, ranging from 1.0 to 10%. The
precursor organic solution was prepared by dissolving 18.3 mL of
titanium(IV)-isopropoxide in a 10 vol.% propionic acid in xylene
fluorine source.
2.4. Photoactivity tests
The so-obtained liquid organic solutions were fed to the burner
at 4 mL min−1 by the syringe pump through the capillary tube
placed at the center of the vertical nozzle [36] and dispersed by oxy-
gen (5 L min−1) at 2 bar constant pressure drop across the burner
nozzle. The produced powders (yield ≈50%) were collected on a
glass fiber filter (Whatman GF/A, 26 cm in diameter) placed on
top of a cylindrical steel vessel surmounting the flame reactor and
connected to a vacuum pump (Busch Seco SV 1040C).
2.4.1. Photocatalytic decomposition of formic acid
All photocatalytic formic acid (FA) degradation runs were per-
formed under atmospheric conditions in a magnetically stirred
60 mL cylindrical quartz reactor, inserted in a home made housing
consisting of a black box mounted on an optical bench [24]. The irra-
diation source was an Osram, model Powerstar HCI-T, 150 W/NDL
lamp, mounted on a Twin Beam T 150 R reflector, mainly emitting
at > 340 nm. The light intensity on the reactor in the 300–400 nm
range was 5 × 10−8 Einstein s−1, as calculated from the emission
spectrum of the lamp and its full emission intensity, which was
regularly checked with an optical power meter.
All chemicals employed in the synthesis of the materials and in
the preparation of all solutions were purchased from Aldrich. Water
purified by a Milli-Q water system (Millipore) was used throughout.
2.2. Photocatalysts characterization
The irradiated aqueous suspensions always contained 0.1 g L−1
of photocatalyst and
a FA initial concentration equal to
X-ray powder diffraction (XRPD) patterns were recorded on a
Philips PW3020 powder diffractometer, by using the Cu K␣ radia-
tion (ꢀ = 1.54056 Å). Quantitative phase analysis was made by the
Rietveld refinement method [38], using the “Quanto” software. The
average anatase crystallite size was calculated according to the
Scherrer equation, from the integral XRD peak width calculated
as the ratio between the peak area and peak intensity obtained
by fitting with a Gaussian function the profile of the most intense
reflection at 2ꢁ = 25.4◦.
Diffuse reflectance (R) spectra of the photocatalyst powders
were recorded on a Jasco V-670 spectrophotometer equipped with
a PIN-757 integrating sphere, using barium sulphate as a reference,
and then converted into absorption (A) spectra (A = 1–R). The BET
specific surface area (SSA) was measured by N2 adsorption at liq-
uid nitrogen temperature (77 K) on a ASAP 2020 apparatus, after
out-gassing in vacuo at 150 ◦C for at least 2 h.
1.0 × 10−3 mol L−1. After preliminary ultrasound treatment for
30 min, the suspension was magnetically stirred in the dark for
15 min to attain the adsorption equilibrium of the substrate on
the photocatalyst surface, before starting irradiation. Stirring was
continued during the photocatalytic runs. 2 mL-samples of the
suspension were withdrawn from the photoreactor at different
time intervals during the runs and centrifuged employing an EBA-
20 Hettich centrifuge. The supernatant was analysed for residual
FA content by ion chromatography with conductivity detection,
employing a Metrohm 761Compact IC instrument, after calibra-
tion for formate ion concentration in the 0–50 ppm range. During
FA degradation the pH of the suspensions increased, from initial
values around 3.7 to ca. 4.8, as a consequence of FA mineralization
to CO2 and H2O.
X-ray photoemission spectroscopy (XPS) data were collected
by a PHI-5500—Physical Electronics spectrometer, equipped with
an aluminium anode (K␣ = 1486.6 eV) as monochromatized source,
operating at a 200 W of applied power, 58.7 eV pass energy and
0.5 eV energy step. The analysis area is around 0.5 mm2 and the
The same photoreactor and setup were employed to test the
efficiency of the prepared photocatalysts in the oxidation of tereph-
thalic acid (benzene 1,4-dicarboxylic acid), following the procedure
described in Ref. [39]. 10 mg of each photocatalyst were first dis-
persed in 54 mL of ultrapure water by 30 min-long sonication.