# Preparation of Fluorine-DopedTiO2 Photocatalysts with Controlled Crystalline Structure

Authors: N. Todorova; T. Giannakopoulou; G. Romanos; T. Vaimakis; Jiaguo Yu; C. Trapalis
Journal: International Journal of Photoenergy
 (2008-02-05)

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## Abstract

Nanocrystalline F-dopedTiO2 powders were prepared by sol-gel route. The thermal behavior of the powders was recorded by DTA/TG technique. The crystalline phase of the fluorinated TiO2 powders was determined by X-ray diffraction technique. It was demonstrated that F-doping using CF3COOH favors the formation of rutile along with anatase phase even at low temperature. Moreover, the rutile's phase content increases with the increase of the quantity of the fluorine precursor in the starting solution. The surface area of the powders and the pore size distribution were studied by N2 adsorption-desorption using BET and BJH methods. X-ray photoelectron spectroscopy (XPS) revealed that the fluorine is presented in the TiO2 powders mainly as metal fluoride in quantities ∼16 at %. The F-doped TiO2 showed a red-shift absorption in UV-vis region which was attributed to the increased content of rutile phase in the powders. The powders exhibited enhanced photocatalytic activity in decomposition of acetone.


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## Body

## 1. Introduction

In the last decades,  the TiO2 is regarded as a promising material for organic pollutants remediation due to its properties [1]. Its wide application though is still retarded by the low efficiency of the photo-oxidative process. The
development of titania-based photocatalysts activated by visible light (λ≥385 nm) remains a great challenge. Anion doping of TiO2 films and powders with elements like nitrogen [2–5], sulfur [6, 7], carbon [8, 9], and fluorine [10–18] has been investigated in order to red-shift the absorbance to be achieved. Narrowing of the band gap of TiO2 upon doping was often considered
responsible for the enhanced visible light activity of these materials.
Theoretical DOS calculations revealed intermediate energy levels available in
the band gap of TiO2 for most of the elements except fluorine [1].
Park and Choi [10] reported that surface
fluorination (F-TiO2) enhances the photocatalytic degradation of
certain substrates due to the electron-trapping sites formation. The enhanced
photocatalytic activity of F-doped powders has been attributed by Li et al. [11–13] to several beneficial effects produced by F-doping
and mainly to the creation of surface oxygen vacancies. Yu et al. [14]
reported fluorine ions incorporation in the TiO2 lattice using an NH4F source. They attributed the stronger absorption in the UV-visible range to the
presence of the F− ions that favor anatase phase formation and
improve the crystallinity of TiO2. However, Yamaki et al. [15]
claimed that F-doping causes a modification of the conduction band edge and
introduction of impurity states near its bottom. Our recent study on
fluorine-modified titania films [18] revealed improved anatase crystallinity
upon fluorine doping without the effective band gap of TiO2 to be
affected.Anatase is usually produced when TiO2 is synthesized by
sol-gel method [19].  Calcination of
anatase at high temperatures is applied to obtain rutile phase. Mixtures of
anatase and rutile (e.g., Degussa P25 consists of ~75% anatase and ~25% rutile)
are reported to exhibit enhanced photocatalytic activity in degradation of
organic pollutants as the electron-hole recombination rate is decreased in the
composite system [20]. Cheng et al. [21] reported
that rutile phase is developed preferably from a solution with low pH. Klein et al. [22] produced rutile (100%) titania
powders at low temperature using nitric and hydrochloric acids as catalysts.In this work, we study the influence of CF3COOH fluorine
precursor on the structure and the properties of TiO2 powders.


## 2. Experimental

Fluorine- (F-)
doped TiO2 powders have been prepared by sol-gel route.  Tetraethyl orthotitanate (TEOTi) Merck was used as titanium source. Different amounts of fluoroacetic
acid (Aldrich Chemical Co., Milwaukee, Wis, USA) (atomic
ratios F/Ti=0,10/90,20/80) were
dissolved in deionized water (molar ratio H2O/TEOTi =18) under
stirring at room temperature and the TEOTi was added to the solution dropwise.  The
sols were stirred in closed beakers at room temperature for 24 hours for
completion of the hydrolysis process. Consequently, they were dried at 90°C
in air for 12 hours for water and alcohol evaporation. The xerogels obtained
were denoted as TF0, TF10, and TF20 for the undoped TiO2 and the
samples with F/Ti=10/90 and 20/80 atomic ratios,
respectively. The as-prepared xerogels were calcinated at 400, 500, and 600°C
for 1 hour in a Carbolite muffle furnace with heating rate of 5°C/min.Differential
thermal analysis (DTA) and thermogravimetry (TG) measurements were performed
using NETZSCH equipment model STA 449. The samples with a mass ~14 mg
were placed in alumina crucible and heated in air flow (30 mL/min) from room
temperature up to 1000°C. The heating rate applied was 10°C/min.The crystalline structure of the powders was investigated
using X-ray diffraction (XRD) technique. SIEMENS D500 diffractometer with
secondary graphite monochromator and CuKα radiation was used to obtain the X-ray
diffraction patterns of the samples. The measurements were performed using the
following combination of slits: 1.0°/1.0°/1.0° as aperture diaphragms, 0.15° as detector diaphragm, and 0.25° as diffracted beam monochromator diaphragm. The measured 2θ range between 20° and 80° was scanned in steps of 0.03°/2s. The size of the crystallites
responsible for the Bragg reflection was determined using the Scherrer equation
(1)d=kλb⁢cosθ,
where d is the crystallite diameter in Å, k the shape constant (0.9), λ the wavelength in Å, θ the Bragg
angle in degrees, and b the observed
peak width at half-maximum peak height in rad. The phase content (%) of the
samples was calculated using the formula [14]
(2)WR=ARKAAA+AR,
where WR is the content of the rutile phase (WA=100−WR), AA and AR are the integrated intensities of anatase
(101) and rutile (110) peaks, respectively, and KA is a coefficient equal to 0.886.The surface areas of the powders were determined by the Branauer-Emmet-Teller
(BET) method. The equipment COULTER 5200 USA
was used for the nitrogen
adsorption/desorption measurements at 77 K. All the samples were degassed at
573 K for 24 hours.The presence of the fluorine in the TiO2, the valence state
of the dopant, as well as the bonding element was analyzed by
X-ray photoelectron spectroscopy (XPS). Non-monochromatized MgKα X-rays from
a dual-anode VSW TA-10 source were used. The ultra-high vacuum (UHV) chamber
was baked at 150°C for 12 hours to reach the 10−10 mbar
pressure range.  Measurements were
performed for the samples without Ar ion sputtering (as received), as well as
after a 45-minute light Ar+ ion sputtering in order to remove most
of the superficial carbon contamination.UV-vis
diffuse reflectance spectra of the samples in the wavelength range 200–900 nm were obtained using a UV-2100 Shimadzu
instrument. The widths of the effective band gap of the samples were determined
using Kubelka-Munk phenomenological theory [23].The photocatalytic activity of the powders was evaluated by
photocatalytic oxidation of acetone (CH3COCH3) using a 7L
reactor (INNOVA). An amount of 0.3 g
of each powder was used for the preparation of aqueous suspensions which were
placed in petri dishes and dried at 100°C for 2 hours. The samples
were placed into the reactor where acetone was injected. The initial humidity
in the reactor was controlled by dryer containing CaCl2. The acetone
vapour was allowed to reach adsorption equilibrium with the photocatalyst prior
measurements. Its initial concentration was about 400 ppm. A 15 W UV lamp (Cole
Parmer Instr. Co., Chicago, Ill,
USA) was used for illumination. The illumination time applied was 60 minutes.
The amounts of acetone, carbon dioxide, and water vapour in the reactor were
monitored. During the photocatalytic reaction, the acetone concentration
decreased steadily. The photocatalytic activity of the powders was
quantitatively evaluated by comparing the apparent reaction rate constants ka of the decomposition reactions given that the degradation of acetone is known
to be a reaction of pseudo first order [19].


## 3. Results and Discussion

The TG, DTG, and DTA curves of undoped and F-doped TiO2 powders are presented in Figure 1. For the undoped TiO2 (Figure 1(a)),
it can be observed that the total loss of weight (~25%) took place in three not
well-defined steps. The first is the main loss of weight (~22.3%) that took
place between ~30 and ~180°C and it was related to the evaporation
of water and alcohol included in the xerogel. The second (~2.5%) and the third
(~0.4%) weak losses took place between ~180–430°C
and ~430–675°C,
respectively, during which the remained water and organics (~2.9%) were
gradually removed from the powders. The DTA curve reveals a weak exothermic
phenomenon from 400°C to 672°C and it was connected to
the process of crystallization of anatase. The
endothermic effect after 670°C and the shoulder at 732°C were attributed to the process of anatase to rutile transformation. The DTA, TG, and DTG curves of the TF10 fluorinated xerogel are presented
in Figure 1(b). It can be observed that the addition of fluorine
containing substance results in complication of
TG and DTA curves in comparison with the undoped sample. The
mass decrease of the samples TCF10 took place in three steps. The first step of
about 7.3% took place in the range of 30 to 150°C and the second
step of about 17% took place from 150 to 350°C. The latter was
connected to the decomposition and removal of organic radicals (CF3COO−).
In addition, in the range from about 390 to 700°C, mass loss of
about 3% took place that can be ascribed to the release of less volatile
components and reaction products. Beyond ~700°C the DTA curve exhibits
endothermic tendency which might be attributed to the anatase to rutile
transformation. The thermal behavior of TF20
xerogel (not given) was similar to the TF10 sample.DTA, TG, DTG patterns of undoped (a) and fluorinated (b) titania powders.
(a)(b)The X-ray diffraction patterns of the calcinated at 400, 500, and 600°C
samples are presented in Figure 2. In our study, pure anatase was recorded for
the undoped titania (Figure 2(a)) after thermal treatment at every temperature
mentioned above. The XRD patterns of F-doped
TiO2 powders (Figures 2(b) and 2(c)) revealed that a second
crystalline phase, that of rutile, was developed along with the anatase phase
at temperatures as low as 400°C. The average
sizes of the crystallites (d, nm), as well as the anatase/rutile phase
composition (W, %) of the powders are presented in Table 1. From the
results displayed in Figure 2 and Table 1, it is evident that the
crystallization process of TiO2 depends on the presence and the
quantity of the fluorine precursor, as well as the calcination temperature. The
increase in the size of the crystallites with the increase of the
calcination temperatures can be observed for all the samples. It is also
evident that the polymorph phase composition is dependent on the quantity of the CF3COOH
in the starting solution. It can be deduced that fluorine-doped
titania powders with controlled anatase/rutile phase composition can be
synthesized by altering the molar ratio of CF3COOH/TEOTi in the
starting solution and the calcination temperature.Table 1
Phase content W (%) and average diameterd (nm) of the crystallites in the TiO2 powders calcinated at different temperatures  (A: Anatase, R: Rutile).
Nominal nameCalcination temperatures400°C500°C600°CW (%)d (nm)W (%)d (nm)W (%)d (nm)TF0A:1008.5A:10012.0A:10018.0F/Ti = 0TF10A:72.510.5A:45.721.5A:9.315.5F/Ti = 10/90R:27.410.6R:54.320.8R:90.720.8TF20A:9.110.2R:10014.1R:10036.1F/Ti = 20/80R:90.9XRD patterns of undoped (a) and fluorinated TiO2 powders with starting ratios F/Ti = 10/90 (b) and F/Ti = 20/80 (c).
(a)(b)(c)The
influence of the F− ions on the crystallization process of TiO2 has been studied and improved anatase crystallinity upon F-doping has been reported
[14, 18]. Up to our knowledge, there is no report on direct
synthesis and fluorination of TiO2 powder with controlled
anatase/rutile content. Moreover, the increase of the F/Ti molar ratio in the
starting solution (sample TF20) leads to formation of pure rutile even at 400°C.
The crystallization of rutile alone or along with anatase phase at low
temperatures is attributed to the acidic character of the fluorine precursor
which provides low pH conditions during peptization. At these acidic
conditions, the edge-shared bonding among the [TiO6] octahedrons which
is responsible for anatase formation during calcination is suppressed, while
the corner-shared bonding that is responsible for the rutile formation is favored
[21].Nitrogen adsorption-desorption isotherm
measurements were performed to determine the specific surface area, the
porosity, and the pore size distribution in the samples. The N2 adsorption-desorption isotherms for the undoped TiO2 and the fluorinated samples calcinated at 400°C clearly show a hysteresis in the desorption branch (Figure 3). These
samples exhibited the highest BET surface area. Their pore size distribution
curves obtained from the desorption branch are depicted in Figure 4. From the
figures and the results presented in Table 2, it can be observed that the BET
surface areas decrease significantly while the pore size distribution becomes
wider with the increase of the calcination temperature for all samples. A
decrease in the porosity with the increase of the calcination temperature is
also evident. The BET values for the fluorinated powders are much lower than
those for the undoped TiO2 powders when calcinated at the same
temperatures. These findings are connected to the presence of fluorine
containing organic precursor in the xerogels and the exothermic phenomena of
its burning occurring during calcination. The heat release leads to the
agglomeration of TiO2 nanoparticles and to the decrease of the BET
surface area. The process contributes to the improvement of the TiO2 crystallinity as it was revealed by XRD results.Table 2
BET surface area and porosity of TiO2 powders calcinated at different temperatures.
Nominal nameCalcination temperatures400°C500°C600°CBETSSA (m2/g)Porosity (%)BETSSA (m2/g)Porosity (%)BETSSA (m2/g)Porosity (%)TF0156.0655.3492.1947.7731.6732.34F/Ti = 0TF1048.4832.1532.8922.4118.7910.23F/Ti = 10/90TF2014.1322.5720.9113.0630.297.32F/Ti = 20/80Nitrogen adsorption-desorption isotherms of undoped (a) and F-doped (b) and (c) powders  calcinated at different temperatures.
(a)(b)(c)Pore size distribution in the undoped TiO2 calcinated at different temperatures (a) and in the TF10 and TF20 powders calcinated at 400°C (b).
(a)(b)The chemical composition of the sample TF20 calcinated
at 400°C was studied by XPS analysis. After the removal of the
superficial carbon contaminant (3-4 monolayers), the elements detected were
titanium, oxygen, and fluorine. The spectra obtained for Ti 2p, O 1s,  and F 1s regions (Figure 5) revealed that the
powder consists of TiO2, where titanium and oxygen are present as Ti4+ and O2−. The valence states of the titanium and the oxygen were
determined from the Ti 2p3/2–Ti 2p1/2 splitting (~5.7 eV) and the difference
of ~71.5 eV between the binding energies of the Ti 2p3/2 peak (458.5±0.1 eV) and the
O 1s peak (530.0±0.1 eV). The F 1s spectrum (Figure 5(c)) is nonsymmetric and
the contribution near 684.3 eV indicates that the fluorine is presented in the
sample mainly as TiF4 [24] and/or to physically adsorbed F− on TiO2 [19]. The contribution
near 685.4 eV could be related to TiOF2 structures [12] and the
tail near 688 eV to nonstoichiometric solid solution of F in TiO2 of
the TiO2−xFx type [12, 19]. It can be also associated with
C–F bonding in CF3 residual groups [25].
The fluorine concentration was estimated as ~14 at % before and ~16 at % after the Ar+ ions sputtering. This level of doping can be connected to the high
temperature of decomposition of the fluorine precursor which is evident from
the thermal analysis.XPS spectra for Ti 2p (a), O 1s (b), and F 1s (c) regions of TF20 sample calcinated at 400°C.
(a)(b)(c)The diffuse reflectance spectra (R) of the samples were used to assess
the absorption of the photocatalysts in the UV-vis region. The undoped TF0
sample and the TF20 samples revealed typical anatase and rutile spectra,
respectively, and for this reason they are not shown. The absorption curves of
the fluorinated TF10 samples with various anatase/rutile ratios are presented
in Figure6(a). The P25 absorption curve is also presented for comparison
because of its mixed phase composition (anatase ~75% and rutile ~25%). To
determine the band gap of the powders, the Kubelka-Munk method based
on the diffuse reflectance spectra was employed. The (F(R)hv)1/2 versus
hv plots of the
P25 and the TF10 samples are presented in Figure 6(b), where F(R) = (1−R)/2R. It can be observed that the band gap values obtained for the Degussa
P-25 and the TF10 sample calcinated at 400°C are close
(3.08 eV and
3.07 eV, resp.). This fact can be connected to the similar anatase/rutile phase
content of the samples. This is also consistent with the literature [11, 12, 19]
and support the conception that the fluorine doping does not affect the band
gap of TiO2. With the increase of the calcination temperature, the widths
of the band gap for the fluorinated samples decrease from 3.07 to 3.03 eV. This
“narrowing” of the effective band gap is attributed to the increase of the rutile
content in the powders which is in accordance with our XRD results.Absorbance spectra (a) and Kubelka-Munk plot for band gap evaluation (b) of fluorinated (TF10) TiO2 and P25 powders.
(a)(b)The
increase in the CO2 concentration due to the decomposition of
acetone and the calculated apparent rate constant of the photocatalytic
reaction are presented in Figures 7 and 8, respectively. From our
results, it can be concluded that the fluorinated TiO2 powders
exhibit enhanced activity in comparison to the reference photocatalyst Degussa
P25. The best results are recorded for the powders calcinated at 400°C
which can be connected to the mixed anatase/rutile phase composition and the
higher BET surface area in comparison to the samples calcinated at 500 and 600°C.
In addition, the activity of TF20 samples treated at 400 and 500°C
exceeds significantly the activity of the corresponding TF10 powders. This fact
was attributed to the rutile rich composition of the powders and the higher
level of fluorine doping. The drastic decrease in the activity of the treated
at 600°C samples is attributed to the decrease of the BET surface
area, as well as to the absence of anatase phase in the powders.Increase of the CO2 concentration due to decomposition of acetone for fluorinated TF10 (a) and TF20 (b) titania powders.
(a)(b)Photocatalytic activity of fluorinated TF10 (a) and TF20 (b) titania powders evaluated by decomposition of acetone.
(a)(b)


## 4. Conclusions

Simultaneous fluorination and controlled low-temperature anatase/rutile phase crystallization of nanosized TiO2 powders were achieved using sol-gel route. It was found that the anatase/rutile crystalline ratio depends on the amount of fluorine precursor used for fluorination as well as on the temperature of thermal treatment.The crystallization of pure rutile or anatase/rutile mixture at low temperatures is attributed to the acidic character of the fluorine precursor. Improved crystallinity and enhanced absorbance of the powders in the UV-vis region are recorded upon F-doping. The fluorine is presented in the powders mainly as metal (Ti) fluoride.The width of the TiO2 band gap was not affected by the presence of fluorine. The red shift of the absorption edge is attributed to the increased rutile content in the fluorine doped TiO2 powders. The prepared fluorine doped TiO2 powders calcinated at 400°C exhibited higher photocatalytic activity than that of Degussa P-25 for decomposition of acetone under UV-vis illumination.



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*Source: 534038-2008-03-11.xml*