New insights into fluorinated TiO2 (brookite, anatase and rutile) nanoparticles as efficient photocatalytic redox catalysts

Article abstract of DOI:10.1039/c4ra17076h

The synthesis of anionic-modified nanostructures with specific properties is often hindered by difficulty in tuning the material compositions without sacrificing phase purity and sample uniformity. Here, we present a novel methodology using NH₄

Full text of DOI:10.1039/c4ra17076h

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New insights into fluorinated TiO2 (brookite,
anatase and rutile) nanoparticles as efficient
photocatalytic redox catalysts†
Cite this: RSC Adv., 2015, 5, 34302
a
a
a
a
Yafang Wang,ab Liping Li, Xinsong Huang, Qi Li and Guangshe Li
*
The synthesis of anionic-modified nanostructures with specific properties is often hindered by difficulty in
tuning the material compositions without sacrificing phase purity and sample uniformity. Here, we present
a novel methodology using NH4F as fluorine source and a high-temperature ionic diffusion that facilitate
surface fluorination of TiO2 without changes of phase structure. Using this method, we prepared
fluorinated TiO2 nanostructures of different phases (brookite, anatase and rutile) and investigated their
structural and photocatalytic redox properties. Photocatalytic selective oxidation of MO and selective
reduction of water in producing hydrogen under UV-light irradiation are employed as probe reactions to
test the redox properties of the fluorinated TiO2 of different phases. The results show that the
photocatalytic redox properties of TiO2 are highly dependent on phase structure and surface fluorination.
Among all the phases, the oxidation activity of fluorinated TiO2 can be ranked as brookite > anatase >
rutile. Strikingly, the photocatalytic reduction activity for these fluorinated TiO2 followed the same
sequence. The variations of photocatalytic redox activity are most likely due to the synergism of phase
composition control and surface fluorination. This could effectively delay electron–hole recombination,
and generate more free cOH radicals but less free electrons. The methodology reported here is simple
and general, which might be used to design and control the structures and properties of anionic-modified
nanostructures for a variety of applications in environmental cleaning and energy conversion.
Received 26th December 2014
Accepted 7th April 2015
DOI: 10.1039/c4ra17076h
www.rsc.org/advances
electron transfer rates for effective organic degradation.7
Recently, modication of TiO2 surface by F species has attracted
remarkable attention.10–12 It is important to gure out how and
1. Introduction
Among all advanced oxidation and reduction processes,
heterogeneous photocatalysis is popularly recognized as an
effective method for dye degradation,1 and photocatalytic water
splitting, which is based on semiconductor oxides (e.g., TiO2,
ZnO, WO3, etc.).2–4 Presently, the full exploitation of photo-
catalytic processes is largely hampered by the lack of efficient
photocatalysts.5–7 Surface modication is promising method in
preparing high-performance catalysts, since it could alter the
adsorption properties of inorganic8 and organic species9 on the
semiconductors' surface, and thus inuence the rate of photo-
induced reactions and their pathways. One may expect that
the optimized surface charge distributions and tuned
substrate–surface interactions can promote the interfacial
why the presence of F anions can affect the photocatalytic redox
processes, so as to lay a foundation for further development in
photocatalysis. However, despite of extensive investigation, for
TiO2 of different phases, the understanding on the operative
mechanism of uorination effect are still unsystematic and
ambiguous.13 Diverse interpretations have been proposed, but
no consensus is reached so far. For instance, adsorption of F
anions on the TiO2 surface decreases the amount of surface
hydroxyl groups, leading to the formation of hTi–F groups
which are able to serve as effective trapping sites by tightly
holding the electrons due to the strong electronegativity of
uorine, and consequently depressed the recombination of
electrons and holes.14 Xu et al. suggested that the enhanced
production of free cOH radicals is due to desorption of surface-
bound cOH radicals by F anions in the solution through the
formation of cOH-Fꢀ hydrogen bond.15 Minero et al. reported
the uorination effect and suggested that the displacement of
surface hydroxyl groups by uoride suppressed the formation of
surface-trapped radicals (TiOc) but enhanced the formation of
free cOH radicals.16
aKey Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute
of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002,
P. R. China. E-mail: guangshe@irsm.ac.cn; Fax: 86-591-63179426
bSchool of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot
010021, P. R. China
† Electronic supplementary information (ESI) available: Experimental details;
XRD, the calculated lattice parameters, unit cell volume, crystalline size and
specic surface area, TEM, HRTEM, particle length distributions, kinetics plots
for pseudo rst order reaction of MO degradation, TPR and so on. See DOI:
10.1039/c4ra17076h
Among the three main crystallographic forms of TiO2,
naturally metastable brookite is the least investigated in
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comparison with the comprehensively studied anatase and 700 ꢁC, respectively, for 4 h. These samples were named
rutile phases because of the difficulties encountered in B-FT400-700, respectively.
acquiring its pure form.17 In the recent years, in spite of the
To study the effect of surface uorination, a reference
study of brookite TiO2 is increasing,18,19 photocatalytic redox sample was also prepared using the above procedure (calcina-
performance of uorinated brookite is rare. One may wonder if tion in N2 atmosphere at given temperatures), while NH4F was
uorinated brookite TiO2 can show a photocatalytic activity not involved. Named as B-T400.
superior to other counterparts. Then, one may nd new routes
of improving photocatalytic redox performance for obtain effi- 2.3 Preparation of uorinated anatase TiO2
cient energy and environmental cleaning.
In this work, we conducted a comprehensive study on a
series of uorinated TiO2 of different phases (brookite, anatase
and rutile), in order to provide better understanding into pho-
tocatalytic processes. These uorinated TiO2 were prepared
with a novel methodology which enables us to realize surface
uorination while maintain their pristine phase structure
The sample was also prepared using the similar two-step
processes, but with some modications. Firstly, anatase TiO2
nanoparticles were prepared, based on an approach reported by
Xing et al.20 A mixture of 35 ml absolute ethyl alcohol (EtOH),
2.0 mL H2O, and 0.5 mL HNO3 was named as solution A, and a
mixture of 8.0 mL TBOT and 32.0 mL EtOH was named as
solution B. Solution B was added dropwise to solution A under
unchanged at the same time. Photocatalytic selective oxidation
vigorous stirring, and this mixture was stirred continuously for
methyl orange degradation and selective reduction hydrogen
30 min and immediately transferred to a 100 mL Teon-lined
production from water are employed as probe reaction to test
the redox properties of the uorinated TiO2 (brookite, anatase
and rutile). These two reactions can serve as the criterion of
oxidation and reduction abilities, respectively. Among all pha-
ses, uorinated brookite TiO2 photocatalyst is highly stable, and
both the photocatalytic oxidation and reduction activity of
uorinated TiO2 can be ranked as brookite > anatase > rutile.
The reasons underlying these experimental observations were
discussed in terms of the synergism of phase composition
control and surface uorination.
ꢁ
stainless steel autoclave, which was allowed to react at 180 C,
for 12 h. Aer solvothermal reactions, the precipitate was
washed with water and EtOH, dried, and ground to obtain the
nal anatase nanoparticles.
During the second surface uorination process, 0.6 g of the as-
prepared anatase TiO2 was mixed with 0.10932 g NH4F, and
sufficiently ground in agate mortar. Aer that, the mixture was
calcined in N2 atmosphere at a given temperature of 400 ꢁC for 4 h.
The sample was named A-FT400.
For comparison, anatase nanoparticles without surface
uorination were also prepared aer calcination in N2 atmo-
sphere at given temperatures but without NH4F involved.
Named as A-T400.
2. Experimental section
2.1 Materials
2.4 Preparation of uorinated rutile TiO2
All chemical reagents used were analytical grade, purchased
from Sinopharm Chemical Reagent Corp, P. R. China, and were
employed without further purication. Water puried by a
Milli-Q water system (Millipore) was used throughout.
The sample was also prepared using the similar two-step
processes, but the titanium source was replaced by potassium
titanyl oxalate dihydrate for the rst step of hydrothermal
reactions. 1 mmol potassium titanyl oxalate dihydrate was
added to 60 mL HCl solution (1 M) under stirring. Sometime
later, 6 mmol oxalic acid were added to this mixed solution, and
transferred to a 100 mL Teon-lined stainless steel autoclave.
Aer reaction at 180 ꢁC for 12 h, the precipitate was washed with
water, dried, and ground to obtain nanoparticles.
2.2 Preparation of uorinated brookite TiO2
The sample synthesis involves two-processes. The rst process
is the synthesis of pure-phase brookite using a solvothermal
method. In a typical synthesis, 5 mL of tetra-butyl titanate
(TBOT) was added in 25 mL absolute ethyl alcohol. Aer suffi-
cient stirring, this mixed solution was added dropwise into
30 mL deionized water under vigorous stirring. Subsequently, 5
g of urea were mixed and dissolved in this solution with agita-
tion. Finally, 5 mL of sodium lactate liquor (60%) were dropped
in the mixed solution while stirring for about 30 min. All these
mixed solutions were removed to autoclaves, which were sealed
and heated in an oven at 200 ꢁC for 20 h. Aer reactions, a white
product of brookite phase was separated, washed respectively
with ethanol and deionized water for 3 times, and dried in an
Similar to the procedures for uorinated brookite and
anatase, 0.6 g of rutile TiO2 was mixed with 0.10932 g NH4F.
Aer sufficient grinding in agate mortar and subsequent calci-
ꢁ
nations in N2 atmosphere at 400 C for 4 h, uorinated rutile
TiO2 was obtained. The sample was named R-FT400.
For comparison, rutile nanoparticles without surface uori-
nation were also prepared, according to the procedures
mentioned above for brookite and anatase. Named as R-T400.
2.5 Sample characterization
ꢁ
oven at 60 C.
XRD analysis was conducted on a Rigaku MiniFlex II diffrac-
The second process is the surface uorination. 0.6 g the as- tometer at a voltage of 30 kV and a current of 15 mA with Cu-Ka
˚
prepared brookite TiO2 was mixed with 0.10932 g NH4F. Aer radiation (l ¼ 1.5406 A), employing a scanning step width of
sufficient grinding in an agate mortar, the mixture was calcined 0.02ꢁ and 8 s measurement times per step in the range of
in N2 atmosphere at given temperatures of 400, 500, 600, and 10–80ꢁ. 10–15 wt% Ni powder was added in the samples to act
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ꢁ
as an internal standard for peak positions calibration. The lamp and keep the temperature at 5 C The light source used
lattice parameters for all samples were further calculated by was a 300 W xenon lamp equipped with a UV-cutoff lter
Rietveld renement method using GSAS program. The Raman (l > 300 nm). Before visible light irradiation, we must ensure
spectra were recorded at room temperature with a HORIBA that the photocatalytic reactor was sealed, and the residual air
LabRAM HR confocal microscope spectrograph with a spectral was deprived by vacuum pump. The experiment time was also
resolution of 0.6 cmꢀ1 and excitation lines at 532 nm. TEM set at 3 h. The photo-generated gas was detected in every 1 h by a
graphs of the samples were obtained on a Tecnai G2 F20 gas chromatograph (Fuli 9790II) with TCD detector, which
S-TWIN eld emission transmission electron microscope with a purchased from Fuli Analytical Instrument Corp, Zhejiang, PR
maximum acceleration voltage of 200 kV. Prior to TEM China.
measurements, samples were dispersed in absolute ethanol and
deposited on a carbon lm coated copper grid. UV-visible
2.7 Determination of reactive species (hydroxyl radical, cOH)
diffuse reectance spectra of the samples were collected using
In order to measure the relative concentration of hydroxyl
a Varian Cary 500 UV-vis-NIR spectrometer, and were converted
radicals (cOH), the terephthalic acid (TA) uorescence method
was employed in this study since TA can react with cOH to form
highly uorescent 2-hydroxyterephthalic acid (TA-OH). The
from reection to absorption according to the Kubelka–Munk
method. An X-ray photoelectron spectroscopy (XPS) measure-
ment was carried out on an ESCA-LAB MK II apparatus using a
mixture of TA solution was prepared from 5 ꢂ 10ꢀ4 molar of TA
monochromatic Al Ka X-ray source operating at 150 W. During
and 2 ꢂ 10ꢀ3 molar of NaOH in deionized water. Then, 0.01 g of
the XPS measurements, emission lines were calibrated using
the C 1s signal at 284.6 eV. Photoluminescence (PL) spectra of
the uorinated brookite TiO2 at 400 ꢁC was dispersed in 50 mL
of the TA aqueous solution. The solution was collected at every
the samples were carried out at room temperature by a Varian
5 min during the irradiation procedure in order to estimate the
Cary Eclipse Fluorescence Spectrometer. Temperature pro-
generated TA-OH, which can be detected by uorescence spec-
troscopy at 425 nm.
grammed reduction (TPR) tests were carried out in a conven-
tional lowꢀa1pparatus with a TCD detector at a heating rate of
5
ꢁC min using 10 vol% H2 in Ar to examine the redox
behaviour of the samples. The owing rate was set at 30 mL
3. Results and discussion
3.1 Crystallinity, specic surface area, and structure
minꢀ1. Before TPR tests, 50 mg catalyst was pretreated in Ar at
ꢁ
200 C for 2 h.
properties for uorinated TiO2 of different phases
Fig. 1 shows the XRD patterns of the uorinated samples
obtained aer calcinations under N2 ow. All diffraction lines
are relatively strong, indicating a high crystallinity for all
samples. Further, the peak positions and relative intensities of
the diffraction lines match well with standard diffraction data
for different TiO2 phases, i.e. brookite, anatase and rutile. For
instance, the peaks observed at 2q values of 25.33, 25.68, 30.80,
36.25, 40.15, 48.01 and 55.23 for the top diffraction pattern of
Fig. 1 could be indexed to the (120), (111), (121), (012), (022),
(231) and (241) planes of an orthorhombic phase brookite,
respectively, consistent with the standard XRD data for brookite
TiO2 (JCPDS, no. 39-1360). The other two XRD patterns are
2.6 Photocatalytic redox activity tests of the samples
Methyl orange degradation. Photocatalytic oxidation activi-
ties of the samples were assessed by methyl orange (MO)
degradation as a probe reaction under UV-light irradiation
(>300 nm). UV-light was obtained by employing a 300 W Xe
lamp equipped with a 300 nm cut-off lter to remove the
undesired radiations. The quantity ratio of catalyst to MO
solution (10 mg Lꢀ1) was set at 0.1 g : 100 mL, and MO solution
containing the catalyst was magnetically stirred for 3 h to
establish an adsorption and desorption equilibrium before
illumination. The lamp was located at a distance of 15 cm away
from the surface of the suspension placed in a beaker open to
air. Sampling was proceeded from the illuminated suspension
at a certain time interval, and the extracted suspension was
centrifugated at a rate of 8000 rpm to remove the solid powders.
The resulting supernatant was measured by a Perkin-Elmer UV
lambda 35 spectrophotometer to acquire its UV-vis adsorption
spectra.
Hydrogen generation. The photocatalytic reduction activities
of hydrogen production for the photocatalyst samples were
examined. In a typical photocatalytic experiment, 50 mg
samples were added into the reactor with 60 mL de-ionized
water in it, then 20 mL methanol were added to the solution
which acted as sacricial agent to capture photo-generated
holes. In order to improve the photocatalytic activity of the
samples, a certain amount of H2PtCl6 solution as co-catalyst was
added to the reactor. Thermostatic water owed through a
Fig. 1 XRD patterns of the fluorinated (at 400 ꢁC) brookite, anatase
jacket of the Pyrex reactor remove extra heat produced by the and rutile TiO2.
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˚
consistent with the standard XRD data for tetragonal phase a ¼ 9.1888(4), b ¼ 5.4591(2) and c ¼ 5.1466(2) A. This obser-
anatase TiO2 (JCPDS, no. 21-1272) and tetragonal rutile TiO2 vation is surprising, since high temperature calcinations
(JCPDS, no. 21-1276), respectively.
usually lead to lattice shrinkage due to the larger grain sizes.
It should be noted that the uorinated TiO2 (brookite, Surface uorination at 400 ꢁC led to a further increase in lattice
˚
anatase and rutile) samples are all present in pure phases, parameters at a ¼ 9.1982(2), b ¼ 5.4649(1) and c ¼ 5.1536(1) A.
exactly inherited from their respective pristine structure (see Interestingly, the axial ratio, c/a, representing the lattice
XRD data of un-uorinated TiO2 in Fig. S1†), without any symmetry, remains almost constant at 0.560. The lattice
detectable secondary phases. It is thus clear that surface uo- expansion of brookite TiO2 observed upon high-temperature
rination does not have signicant inuence on the phase calcinations and surface uorination should be closely related
structure of TiO2.
to the presence of defects.22
To study the impacts of calcinations and surface uorination
Similarly, for anatase and rutile TiO2, calcinations and
on the microstructures of the TiO2 with different phases, XRD surface uorination led to a gradual increase in lattice param-
patterns for all samples were carefully analyzed. The average eters and unit cell volume.
crystallite sizes of the samples were estimated from the full
Raman spectroscopy is sensitive enough to distinguish
width at half maxima of the respective peaks of XRD data at 2q brookite, anatase and rutile TiO2. The solid was then charac-
values of 29–60ꢁ, using Scherrer's equation in eqn (1),21
D ¼ kl/b cos q
terized with Raman spectroscopy, and the results are shown in
Fig. 2. For uorinated brookite TiO2 at 400 ꢁC, there were
14 Raman bands in the range from 100 to 700 cmꢀ1, which can
be assigned to A1g (133, 156, 195, 251, 414, 550, and 639 cmꢀ1),
B1g (217 and 289 cmꢀ1), B2g (369, 465 and 586 cmꢀ1), and B3g
(326 and 505 cmꢀ1) of brookite, respectively.23 It is worth noting
that characteristic Raman vibrations for anatase at 518 cmꢀ1
and rutile at 442 cmꢀ1 are absent in the spectrum. This
(1)
where k is a constant equal to 0.89, l is the X-ray wavelength
equal to 0.154 nm, b is the full width at half maximum, and q is
the half diffraction angle. The calculated crystallite size for TiO2
(brookite, anatase and rutile) were listed in Table 1. B-T, A-T and
R-T are meaning of brookite, anatase or rutile TiO2 that
prepared with solvothermal method; the B-T400, A-T400 and
R-T400 are meaning of calcinated brookite, anatase or rutile
ꢁ
TiO2 at 400 C; B-FT400, A-FT400 and R-FT400 are meaning of
uorinated brookite, anatase or rutile TiO2 at 400 ꢁC, respec-
tively. From Table 1, one can see that the crystallite sizes for
TiO2 (brookite, anatase and rutile) increased, while specic
surface area decreased with calcination and uorination.
Besides the impacts on the crystallite sizes and surface area,
calcinations and surface uorination also alter the lattice
parameters of TiO2 with different phases. The lattice parame-
ters for all samples were analyzed using GSAS structural
renement. For the as-prepared brookite TiO2, the rened
lattice parameters were a ¼ 9.1844(5), b ¼ 5.4542(3) and c ¼
˚
5.1458(3) A, respectively, which are all compatible with those of
˚
Fig. 2 Raman patterns of the fluorinated (at 400 ꢁC) brookite, anatase
and rutile TiO2.
the standard values (i.e., a ¼ 9.18, b ¼ 5.45, and c ¼ 5.16 A). Aer
400 ꢁC calcinations, the lattice parameters slightly increased to
Table 1 Lattice parameters, unit cell volume, crystalline size and specific surface area of the samples: as-prepared, after 400 ꢁC calcinations or
surface fluorination
Lattice parameters
3
Samples
Unit cell volume (A )
Crystalline size (abc)a (nm)
BET (m2 gꢀ1
)
˚
a (A)
˚
b (A)
˚
c (A)
˚
B-T
9.1844(5)
9.1888(4)
9.1982(2)
3.7870(2)
3.7943(2)
3.8100(2)
4.6036(6)
4.6062(6)
4.6062(2)
5.4542(3)
5.4591(2)
5.4649(1)
3.7870(2)
3.7943(2)
3.8100(2)
4.6036(6)
4.6062(6)
4.6062(2)
5.1458(3)
5.1466(2)
5.1536(1)
9.5168(8)
9.5296(7)
9.5634(7)
2.9586(4)
2.9650(4)
2.9640(1)
257.77(3)
258.17(2)
259.1(1)
136.49(2)
137.19(2)
138.83(2)
62.70(3)
62.91(3)
62.89(1)
21.0
21.0
27.5
9.4
31
28
17
121
93
25
23
13
13
B-T400
B-FT400
A-T
A-T400
A-FT400
R-T
9.6
12.4
24.1
28.2
26.3
R-T400
R-FT400
a
(abc) is (120), (101) and (110) for brookite, anatase and rutile, respectively.
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indicates that the titanium oxide presents in the form of pure
Similar variation trend of strain was also observed for
brookite phase. Other TiO2 polymorphs thus obtained are also anatase with the exception of the magnitude. For instance, the
phase pure TiO2, as evidenced by their respective Raman spec- strain was compressive at ꢀ0.36% for the as-prepared anatase
ꢁ
trum: for example, the major Raman bands at 147, 199, 399, TiO2, which increased to ꢀ0.09% aer calcinations at 400 C.
518 and 641 cmꢀ1 are associated with ve Raman-active modes Upon surface uorination at 400 ꢁC, compressive strain changes
of anatase phase TiO2 with the symmetries of Eg, Eg, B1g, B1g and to tensile again at +0.78%. These results indicate that the lattice
Eg, respectively;23,24 for “rutile”, the Raman peaks at 143 cmꢀ1 is strain could be tuned from the compressive to tensile by uo-
for the mode of B1g symmetry, 249 and 442 cmꢀ1 are for Eg rination for both brookite and anatase.
mode while 611 cmꢀ1 is for A1g mode.25 Except for these
Different from the cases for brookite and anatase, the strain
distinctive vibrations, no additional peaks can be found in each was tensile at 0.78% for the as-prepared rutile TiO2, which was
spectrum. These observations conrm that uorinated increased to +0.87% aer calcinations at 400 ꢁC or +0.83% upon
ꢁ
brookite, anatase and rutile synthesized in this work are in pure surface uorination at 400 C. No changes in the strain nature
phase.
were observed. Furthermore, by comparing the lattice parame-
Effective strains of the samples are calculated by the broad- ters listed in Table 1 and the strain variations in Fig. 3, one can
ening effects of the diffraction peaks using the Williams and clearly see that the increasing trend of lattice parameter is
Hall theorem in eqn (2),26,27
coincident with that of the strain. It is most likely that the
strains varied with calcinations or surface uorination are
responsible for the increases in lattice parameters.
b cos q/l ¼ 1/D + h sin q/l
(2)
The surface uorination of TiO2 nanoparticles was further
investigated by TEM and HR-TEM. For uorinated brookite
TiO2 at 400 ꢁC, the as-prepared particles are uniform, showing a
relatively narrow particle size distribution (Fig. S2 and S3†). The
particle shape is rod-like, as indicated from TEM image in
Fig. 4a. Onto the surfaces of brookite nanorods, there deposited
a thin layer with a thickness of about 0.5 nm, which might be
the uorinated layers (see HRTEM image for a single brookite
particle in Fig. 4b). The existence of surface uorinated layers
were also proved by a designed experiment that compared the
where b is the full width at half maximum (FWHM), q is the
diffraction angle, l is the X-ray wavelength, D is the effective
particle size, and h is the effective strain. The strain is calculated
from the slope, and the particle size (D) is calculated from the
intercept of a plot of b cos q/l vs. sin q/l.
For the as-prepared brookite TiO2, the strain was determined
to be compressive at ꢀ0.07%, which became ꢀ0.01% when the
brookite TiO2 was calcinated at 400 ꢁC. Strikingly, the nature of
the strain changed from the compressive to tensile at +0.15%
ꢁ
upon surface uorination at 400 C (Fig. 3a1–3 and d).
Fig. 3 Comparisons between Williamson–Hall plots of as-prepared (black), calcinated (red) and fluorinated (blue) samples for (a1–a3) brookite,
(b1–b3) anatase and (c1–c3) rutile TiO2, and (d) calculated lattice strain.
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chemical ingredients when treated under autogenous pressure
conditions inside the autoclave at 180 ꢁC. The material obtained
from the autoclave is subjected to calcinations at 400 ꢁC in the
presence of NH4F for 4 h in order to realize surface uorination
for brookite TiO2. In order to prove NH4F can fully participate in
the reaction, keep the brookite structure and appropriate
specic surface area, we select 400 ꢁC as the optimum synthesis
temperature. The present method is very attractive in surface
modication of TiO2 because it won't cause phase evolution
even when uorinated at 400 ꢁC. We have conducted a series of
comparative investigations to optimize the uorination method
and eventually obtained the above preparation route, changes
in any step may lead to a mixed phase of brookite and rutile
TiO2 (Fig. S4 and S6†).
Fig. 4 (a) TEM and (b) HRTEM images of brookite TiO2 after surface
fluorinations at 400 ꢁC. Insets show the corresponding SAED patterns
and a schematic illustration.
3.2 Optimum photocatalytic redox performance of
uorinated TiO2 with different phases
photocatalytic oxidation performance between NaOH-treated
and untreated samples (Fig. S10b†). The lattice fringes with
an interplanar spacing of 0.3528 nm correspond to the (120)
crystal plane of orthorhombic phase brookite TiO2. The SAED
pattern (inset of Fig. 3a) shows a set of rings consisted of spots,
which conrms the random orientations of nanoparticles.
The formation of the uniform uorinated TiO2 is benecial
from the two-step reaction processes adopted in this work: the
rst step of pure-phase TiO2 synthesis, and the second step of
uniform surface uorination using NH4F as uorine source and
a high-temperature ionic diffusion. Take brookite TiO2 for
example, the schematic reaction path of reactants during the
synthesis is illustrated in Scheme 1, which is based on literature
data analyses and our XRD, TEM and Raman observations. It is
well documented that brookite TiO2 nanoparticles can be
prepared through a low-basicity hydrothermal process utilizing
urea as an in situ OHꢀ source, and sodium lactate as the com-
plexant and surfactant.2,18 Surface uorination increases the
rate of photocatalytic oxidation process through a hydroxyl
radical-mediated path.7 With these information in mind, a
novel concept of establishing the uorinated brookite TiO2 in
the monomer level is proposed in the present study: tetra-butyl
titanate as a titanium source, urea as an in situ OHꢀ source,
sodium lactate as the complexant, and surfactant and NH4F as
uorine source are suitable starting species to facilitate the
formation of surface uorination of brookite TiO2. With these
starting species, brookite TiO2 was initially formed in terms of
the procedure in Scheme 1 through the reaction among various
Photocatalytic oxidation activities of TiO2 of different phases
with and without surface uorination were comparatively
studied by taking a model reaction of MO photodegradation
under UV-visible light irradiation (l > 300 nm) at room
temperature. Depending on the phases of TiO2, surface uori-
nation may show different impacts on the photocatalytic
oxidation activities. For brookite, surface uorination led to an
apparent enhancement in photocatalytic activity, as indicated
by the time-dependent degradation spectra of MO aqueous
solutions during UV-vis light irradiation (Fig. S5†). This activity
is signicantly improved, when comparing to those un-
ꢁ
uorinated samples (either 400 C calcinations or as-prepared
brookite). Different from the case of brookite, surface uori-
nation of anatase or rutile led to a decrease in photocatalytic
activity (Fig. S6†), therefore, the photocatalytic oxidation activity
is affected by the synergistic effect of phase structure control
and surface uorination. Namely, the increased photocatalytic
oxidation of surface uorination was effected by phase struc-
ture. Interestingly, when comparing all uorinated samples,
uorinated brookite TiO2 nanoparticles showed an optimum
photocatalytic oxidation activity towards MO removal: almost
100% of MO molecules were degraded in 30 min (Fig. 5a). For
the calcinated TiO2, whether brookite or anatase, rutile, only
partly MO was removed in the same time (Fig. 5c).
The kinetics of MO decolourisation of uorinated or calci-
nated TiO2 in different phases, brookite, anatase and rutile are
presented in Fig. 5b and d. It is seen that the photocatalytic
degradation process exactly follows a pseudo-rst order reac-
tion in eqn (3),28
ꢀln(C/C0) ¼ kt
(3)
where k is the apparent rate constant (minꢀ1), C0 is the initial
concentration of dye and C is the concentration of dye at time
(t). As the dye concentration in these experiments remains in
the regime where the Beer–Lambert law holds, the concentra-
tion can be substituted by the dye absorbance at a given wave-
length (typically the peak absorption).
Scheme 1 Schematic illustration for the formation of surface fluori-
nated TiO2 nanoparticles after calcination at 400 ꢁC, taking brookite as
an example.
From Fig. 5b, for brookite, surface uorination led to an
apparent rate constant of 0.1141 minꢀ1, which is about 6 times
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Fig. 5 (a) and (c) Photocatalytic degradation towards MO; (b) and (d) the corresponding kinetics plots for pseudo first order reaction of
decolourisation in the presence of fluorinated TiO2 (brookite, anatase and rutile) under UV-visible light irradiation.
larger than that of 0.0196 minꢀ1 for uorinated anatase, and even because uorine ion has strong electronegativity; consequently,
17 times larger than that of 0.0067 minꢀ1 for uorinated rutile. the hTi–F groups formed on the surface were able to serve as
Such a superior photocatalytic oxidation activity towards the effective trapping sites by tightly holding the electrons.
degradation of MO has not been ever reported before for uori-
In addition, it was observed that the calcinated brookite TiO2
nated TiO2. One cannot explain this superior activity in terms of showed the highest photocatalytic reduction activity among all
the surface area, since the uorinated brookite TiO2 didn't catalysts. Its H2 production rate is almost 5 times of that for
possess the highest specic surface area (Table 1). Therefore, it is calcinated rutile TiO2 (R-T400-Pt). Moreover, for either uori-
necessary to examine other factors, which will be discussed below. dated or calcinated TiO2, the photocatalytic reduction activity
In Fig. 5d, the apparent rate constants of calcinated brookite, follows same sequence: brookite > anatase > rutile.
anatase and rutile TiO2 are 0.0127, 0.0154 and 0.0067 minꢀ1
,
respectively. Seem obvious, the effect of surface uorination is
superior to the calcination on enhanced photocatalytic oxida-
tion activity.
3.3 Insights into the optimum photocatalytic redox
performance of uorinated brookite TiO2
Above analyses indicate that only the photocatalytic oxida-
tion performance of brookite TiO2 is enhanced aer uorida-
tion. Is the photocatalytic reduction ability inuenced
simultaneously? How uoridation and calcinations affect pho-
tocatalytic redox activity for different phase of TiO2? To answer
these questions, photocatalytic reduction activity on hydrogen
production of all the uoridated and calcinated TiO2 (brookite,
anatase and rutile) are characterized in the following.
The activities of hydrogen evolution observed are shown in
Fig. 6. The inset displays hydrogen evolution over time at the
presence of brookite TiO2. As seen from Fig. 6, when the UV-
ꢁ
As stated above, uorinated brookite TiO2 at 400 C displayed
an optimum photocatalytic redox performance among all other
TiO2 phases. In order to further understand how surface uo-
rination affects the catalytic activity of brookite TiO2, we per-
formed a detailed surface analysis.
ꢁ
visible light irradiates over the reaction system for 3 h at 5 C,
ca. 50 mmol gꢀ1 H2 was generated for the calcinated brookite
TiO2 (B-T400-Pt), while for the uorinated brookite TiO2 (B-FT400-
Pt), only 12.9 mmol gꢀ1 H2 are generated. Similarly, surface
uorination of anatase or rutile led to a decrease in photocatalytic
reduction activity on hydrogen production from water splitting.
Therefore, the surface uorination affected primarily the photo-
catalytic reduction activity rather than phase structure. Namely,
the decreased photocatalytic reduction of surface uorination
wasn't effected by phase structure. For the uorinated TiO2
(brookite, anatase and rutile), decreased photocatalytic reduction
activity is likely due to the decrease of free electrons. This is
Fig. 6 The photocatalytic activities of hydrogen production in the
presence of as-prepared and fluorinated TiO2 (brookite, anatase and
rutile) under UV-visible light irradiation. The inset displays hydrogen
evolution as a function of irradiation time for sample BT-400 and
B-FT400.
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Fig. 7 (a) Survey XPS spectrum and (b) Ti 2p, (c) F 1s, and (d) O 1s spectra for calcinated or fluorinated brookite at 400 ꢁC.
adsorbed oxygen for calcinated brookite TiO2. Based on the
larger electronegativity of F ions than O ions and positive BE
shis in binding energy for Ti 2p and O 1s levels, it is probable
that surface uorination of brookite may have dual effects on
the surface structures: (i) surface Ti–O(F) polyhedra were dis-
torted due to the sub-surface oxygen replacement of F replaced
the oxygen, as reported elsewhere;31 and (ii) surface Ti–O bonds
may become more co-valent in chemical nature with the
replacement of oxygen by F led to the reduction of bulk oxygen
was more difficult, which has been indicated by the oxygen
activation at lower temperatures (see H2-TPR results in Fig. S7†).
Surface structure relaxation. Associated with the surface
lattice distortion is the surface structure relaxation, which could
be indicated by the thermodynamic stability. Here, we used
XRD to examine the thermodynamic stabilities of uorinated
brookite at different calcination temperatures. As demonstrated
in Fig. 8, below the calcinations temperature of 600 ꢁC, brookite
structure was maintained, showing XRD patterns exactly the
same with an orthorhombic brookite structure (JCPDS, no.
29-1360), while no other peaks from nitrogen and uorine-
Surface compositions. The surface compositions (like F, Ti,
C, and O species) for the uorinated and calcinated brookite
TiO2 at 400 ꢁC was characterized by XPS. As indicated by the XPS
survey spectrum in Fig. 7a, there appear signals assigned to Ti,
O, F, and C elements. The presence of C element can be
ascribed to the residual carbon from precursor solution. Con-
cerning the F species, as demonstrated in Fig. 6c (red solid line),
one single F 1s signal peak was observed at 684.3 eV for the
uorinated brookite TiO2, as the characteristic of chemical
bonding of F species to Ti4+ in forming sub-surface Ti–F species,
as reported elsewhere for surface uorination.29 Further, all
these F species cannot diffuse to the interior lattice to substitute
O species in forming solid solution TiO2ꢀxFx, since the typical
signal for F atoms in solid solution TiO2ꢀxFx (Fig. 7c, dash
line)30 was absent at 689.3 eV. Apparently, the presence of sub-
surface Ti–F species for uorinated brookite TiO2 is consistent
with our HRTEM observation, which expects to eliminate the
possibility of generating more lattice defects (e.g., those arose
from F substitution for lattice oxygen) and moreover to reduce
the electron–hole recombination rate for improved catalytic
activity.
ꢁ
derived phases were found. When calcined beyond 600 C, the
brookite structure was destabilized to show some amounts of
rutile TiO2 (JCPDS, no. 21-1276). It is obvious from these
observations that surface uorination could greatly enhance the
thermal stability of brookite through surface structure relaxa-
tion by prohibiting brookite from phase transformation to
rutile.
Surface lattice distortion. Fig. 7c compares Ti 2p spectrum
for uorinated brookite TiO2 with that for 400 ꢁC calcinated
brookite. For uorinated brookite TiO2, Ti 2p spectrum consists
of two distinct photoelectron signals Ti 2p1/2 and Ti 2p3/2 at
463.9 and 458.2 eV, respectively. The spin-orbital splitting
between both signals is 5.70 eV, which is comparable with that
of 5.74 eV reported previously in literature.27 Both signals are
slightly positively shied in binding energy, when comparing to
Another important indication of surface structure relaxation
is represented by the crystal growth and lattice shrinkage with
calcination temperatures. The crystallite size for uorinated
ꢁ
those for 400 C calcinated brookite. Similar positive shi has
ꢁ
brookite TiO2 at calcination temperature of 400 C was about
also been observed in O 1s signal. For instance, the O 1s spec-
trum for uorinated brookite TiO2 consists of one strong bulk
oxygen (O2ꢀ) photoelectron signal at 529.7 eV, which positively
shied to 529.4 eV for calcinated brookite TiO2 (Fig. 7d).
Strikingly, a shoulder signal at 531.5 eV associated with surface
28 nm, which slightly increased to 32 nm aer calcinations at
600 ꢁC. When calcined beyond 600 ꢁC, the brookite structure
was destabilized to show a larger crystallite size of about 38 nm.
Further, with increasing the calcination temperature, the peak
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conrmed the position of the shallow minimum of the
conduction band is strongly dependent on the lattice contrac-
tion. They observed a reduction in the indirect band-gap with
lattice expansion and an increase in band-gap with lattice
contraction,34 which is consistent with our observations upon
surface uorination. Fundamentally, the varied lattice param-
eter may lead to changes in density of states (O-2p and Ti-3d)
and total energy, which affect the band gap. This seems not
ꢁ
applicable for uorinated brookite aer calcinations at 600 C
or above, since we observed no apparent changes in band gap in
spite of lattice contraction. It is quite likely that the increased
concentration of defects may help to balance the band gap.
Surface reaction kinetics. Surface uorination would give
rise to surfaces negatively charged, which are able to draw the
photogenerated holes to semiconductor surface by electrostatic
force. Thus, promoted surface reaction kinetics is highly
expected, since more holes are available at the interface, which
could remarkably accelerate the oxidation of surface-adsorbed
solvent water to free cOH radicals. The kinetics of MO decol-
ourisation for the present uorinated brookite aer calcina-
tions at different temperatures is presented in Fig. 10. It can be
seen that MO photodegradation follows pseudo-rst order
kinetics in terms of the eqn (2). With increasing the calcination
temperature, the apparent rate constant decreased froꢀm1
0.1141 minꢀ1 at 400 ꢁC, to 0.0793 minꢀ1ꢁat 500 ꢁC, 0.0081 min
Fig. 8 XRD patterns of the as-prepared brookite TiO2 and the fluori-
nated brookite TiO2 obtained after fluorination at 400, 500, 600, and
700 ꢁC. Symbol * represents Bragg peak of rutile TiO2.
positions of the diffraction peak (120) for brookite TiO2 showed
a systematic shi towards higher two-theta degrees, which
accompanies a decrease in lattice parameter (Table S1†).
Band gap energies. To obtain the band gap energies of the
uorinated brookite TiO2, Schuster–Kubelka–Munk absorption
function ((ahn)1/n) was plotted against the photon energy (hn),
according to the following eqn (4),32
(ahn)1/n ¼ A(hn ꢀ Eg)
(4)
ꢀ1
ꢁ
at 600 C, and to 0.0172 min at 700 C (Fig. 10). Clearly, the
where A is a proportionality constant, h is Planck's constant, n is
the frequency of vibration (hence hn is photon energy), and a is
an absorption coefficient. The value of n depends on the type of
optical transition of the semiconductor (n ¼ 2 for indirect
transition).33 The approximate band gap can then be deter-
mined from the straight line x-intercept as shown in Fig. 9.
Compared to the as-prepared brookite TiO2, the uorinated
maximum rate constant of uorinated brookite TiO2 was
reached at 400 C. Since the apparent rate constant of the as-
ꢁ
prepared brookite TiO2 was 0.0274 minꢀ1, surface uorination
effect at 400 ꢁC is represented by approximately 4 times
enhancement in photocatalytic activity when comparing to the
as-prepared brookite TiO2. Otherwise, higher uorination
ꢁ
temperatures like 600 or 700 C will show poor photocatalytic
ꢁ
brookite aer calcinations at 600 C or above did show signi-
activity as a result of the increased crystalline size and
decreased specic surface area. Concerning the uorinated
brookite TiO2 at 400 ꢁC, the specic surface area is the highest,
as indicated by BET analysis (Table S1†), and therefore more
active sites are available;35 Moreover, the band gap of uori-
nated brookite TiO2 at 400 ꢁC was smallest (Fig. 9).
As a result, reaction rate of photocatalytic oxidation MO
degradation is optimized at 400 ꢁC. Hence, combining the
above analysis, one can see that only if the phase is brookite and
the temperature is appropriate, can the optimum photocatalytic
cant changes in band-gap energies. Interestingly, the uori-
nated brookite at 400 ꢁC with lattice expansion (Shown in Table
S1†) showed a decreased band gap (0.03 eV) and the samples
uorinated at 500 ꢁC with lattice contraction showed an
increased band gap (0.07 eV). It is well known that the variation
of band gaps is closely related to the change in lattice param-
eters and the associated intrinsic defects in lattice.22 For the
present uorinated brookite, the defect states will be conrmed
by PL measurements as discussed below. Ekuma and Bagayoko
Fig. 9 (a) Schuster–Kubelka–Munk absorption function of fluorinated brookite TiO2 after calcinations at given temperatures and (b) calculated
band gap energies.
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efficiency of charge carrier trapping, monitoring the migration,
transfer, and to understand the fate of electron–hole pairs in
semiconductor particles.37 This is because PL emission spectra
are generated from the radiative recombination of excited
electrons and holes.38 A higher PL intensity indicates a higher
radiative recombination rate of electron–hole pairs under light
excitation. Herein, PL spectra were used to examine the impact
of surface uorination on the separation of photo-generated
electron–hole pairs in brookite TiO2. As indicated in Fig. 12,
un-uorinated brookite TiO2 showed a fast recombination rate
of electrons and holes, which is demonstrated by the strong and
wide PL spectrum for the as-prepared brookite TiO2 when
excited at 290 nm. However, aer surface uorination, PL peaks
of the uorinated brookite decreased greatly in intensity, which
indicates that the radiative recombination rate of electrons and
holes is hindered. The variation of PL intensity may result from
the change of defect state on the shallow level of TiO2 surface.39
This phenomenon demonstrates that the photogenerated elec-
tron–hole pairs can be effectively separated in the uorinated
brookite, which accounts for the enhanced photocatalytic
oxidation activity, as reported elsewhere for other TiO2 surface
anionization.36 However, the effectively separated electrons and
holes were not the only factor that contributes to the enhanced
photocatalytic oxidation activity. Instead, the photocatalytic
activity is inuenced by various factors such as band gap,
crystalline size, specic surface area and phase structure etc.
Fig. 10 The reaction kinetics for pseudo first order reaction of MO
decolourisation of (a) as-prepared brookite TiO2 and the fluorinated
brookite TiO2 after calcinations at temperatures of (b) 400, (c) 500, (d)
600, and (e) 700 ꢁC.
oxidation activity be obtained. Besides, the adsorption effi-
ciency of MO in dark at different pH values for the uorinated
ꢁ
brookite TiO2 aer calcinations at 400 C is given in Fig. S8.†
Strong binding of F species to surfaces. Since the binding of
F species to brookite surface directly determines the reproduc-
ibility and durability, which are two critical issues for the long-
term use of any catalysts in practical applications,36 herein we
tested the photocatalytic stability of uorinated brookite TiO2 at
400 ꢁC toward the photodegradation of MO. We reused the
uorinated brookite ve times, and each experiment was per-
formed under the identical conditions (Fig. 11). It was found
that aer ve cycles, the photocatalytic activity retained nearly
90% relative to its initial activity. In particular, aer 5th catalytic
cycle, uorinated brookite TiO2 remained its phase structure
under UV-visible light irradiation, indicating that the current
brookite upon surface uorination at 400 ꢁC is highly stable,
which can be seen from XRD patterns in Fig. S9.† In short,
surface uorination of brookite at 400 ꢁC led to a strong F
binding to the surface and furthermore electrostatic force,
which yields an efficient photocatalytic oxidation activity and
high stability for MO degradation under UV-light irradiation.
Separation of photogenerated electron–hole pairs. It is well
established that the photoluminescence (PL) is powerful in the
ꢁ
When calcinated at higher temperature (>400 C), for the uo-
rinated brookite TiO2, the band gap increased, specic surface
area gradually decreased and the phase structure may change to
rutile. Therefore, the higher calcinated temperature doesn't
necessarily contribute to the enhanced photocatalytic oxidation
activity.
Oxidizing species. For majority of photocatalytic oxidation
processes, hydroxyl radicals (cOH) are commonly recommended
as the primary oxidizing species. In this regard, terephthalic
acid photoluminescence probing technique (TA-PL) is
a
powerful testing method and has been widely used in detection
of hydroxyl radicals.40 2-Hydroxyl-terephthalic acid, which is
generated when terephthalic acid captures the hydroxyl radi-
cals, performs a strong uorescence characteristic, so that
hydroxyl radicals could be detected indirectly by monitoring the
uorescence intensity changes of TA solution. Herein, the
Fig. 11 Cycling runs of MO degradation in the presence of the fluo- Fig. 12 PL spectra of the as-prepared brookite TiO2 and the fluori-
rinated brookite TiO2 at 400 ꢁC.
nated brookite TiO2 after calcinations at 400, 500, 600, and 700 ꢁC.
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and a subsequent high-temperature ionic diffusion process in a
N2 atmosphere without sacricing the phase structures of TiO2.
Surface uorination not only alters the microstructures of TiO2
(like surface compositions, particle sizes, surface Ti–O(F) poly-
hedra symmetry, and lattice dimensions), but also leads to the
surface structure relaxation which enhance the thermal stability
of TiO2. Our investigation indicates that uorinated brookite
TiO2 has the highest photocatalytic redox activity, followed by
its anatase counterpart, while rutile TiO2 exhibits the most
sluggish behaviour. The photocatalytic redox activity can
ranked as brookite > anatase > rutile. Base on all the experi-
mental observations, it can be concluded that the photocatalytic
oxidation activity is affected by surface uorination, phase
structure and calcination temperature; on the other hand, the
photocatalytic reduction activity is mainly affected by surface
uorination and is independent of phase structure or calcina-
tions temperature. The improved oxidation performance but
decreased reduction performance could be ascribed to the
enhanced desorption of free cOH radicals of surface-bound cOH
radicals and decreased free electron, respectively. The syner-
gism between surface uorination and its electrostatic force,
effectively separates the photogenerated electron–hole pairs,
thereby increasing the lifetime of the electron–hole separation.
Besides, the holes, and cO2ꢀ also synactic play important roles
Fig. 13 Fluorescence spectra of a TA–OH solution generated by
fluorinated brookite TiO2 after calcinations at 400 ꢁC under UV-visible
light irradiation.
uorinated brookite TiO2 aer at 400 ꢁC was employed to detect
the generated cOH on the surface. As indicated in Fig. 13, there
appeared a cOH-trapping photoluminescence spectrum for uo-
rination brookite TiO2 at 400 ꢁC in a terephthalic acid solution at
room temperature under UV-light irradiation. That implies the
formation of uorescent products during the photocatalytic
process, which is likely due to the specic reaction between cOH in the photocatalytic oxidation reaction. The present synthesis
method and the comprehensive photocatalysis study on TiO2
may provide hints for preparing high-performance photo-
catalyst and shed lights on better understanding into the factors
that inuences photocatalysis processes.
and terephthalic acid. The uorescence intensity at 425 nm
increased when UV light was irradiated continuously, which
means that the photocatalytic performance became higher with
prolonging the time of light irradiation. In the same time, the
uorinated anatase and rutile TiO2 aer at 400 ꢁC were employed
to detect the generated cOH on the surface. As indicated in
Fig. S11.† It clearly indicates that cOH is not formed in the pho-
Acknowledgements
tocatalytic reaction. Therefore, the photocatalytic oxidation This work was partly supported by the NSFC (no. 91022018,
activity for uorinated anatase and rutile TiO2 were not enhanced. 21025104) and National Basic Research Program of China (no.
As the uorescence intensity increased with the number of 2011CBA00501, 2013CB933203) for nancial support.
minutes, it can be deduced that the amount of hydroxyl radicals
(cOH) is proportional to the irradiation time. It has been reported
that the formation rate of cOH radicals is related to the separation
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