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Applied Surface Science 254 (2008) 3033–3038

Low-temperature preparation of F-doped TiO2 film and its

photocatalytic activity under solar light

Jingjing Xu a,b, Yanhui Ao a,b, Degang Fu a,b,*, Chunwei Yuan a

a State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, Chinab School of Chemistry and Chemical Engineering, Southeast University, Nanjing 210096, China

Received 20 September 2007; received in revised form 21 October 2007; accepted 21 October 2007

Available online 4 November 2007

Abstract

A novel and simple method for preparing F-doped anatase TiO2 (defined as FTO) film with high photocatalytic activity was developed using

titanium-n-butoxide and NH4F as TiO2 and fluorine precursors under mild condition, i.e. low temperature (lower than 373 K) and ambient pressure.

The prepared samples were characterized by XRD, SEM, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectrum (DRS),

photoluminescence spectrum (PL) and TG–DSC analysis. The photocatalytic activity was evaluated by decomposing X-3B under artificial

solar light. The results showed that the crystallinity of TiO2 was improved by F-doping. F� ions can prevent the grain growth, and the

transformation of anatase to rutile phase was also inhibited. The doped fluorine atoms existed in two chemical forms, and the ones incorporated into

TiO2 lattice might take a positive role in photocatalysis. Compared with surface fluorination samples, FTO film exhibited better photocatalytic

activity. The high photocatalytic activity of FTO may due to extrinsic absorption through the creation of oxygen vacancies rather than the excitation

of the intrinsic absorption band of bulk TiO2. Furthermore, the FTO can be recycled with little photocatalytic activity depression. Without any

further treatment besides rinsing, after 6 recycle utilization, the photocatalytic activity of FTO film was still higher than 79%.

# 2007 Elsevier B.V. All rights reserved.

PACS : 81.07.�b

Keywords: Fluorine-doped; Photocatalysis; Low-temperature; Recycle

1. Introduction

Recently, many efforts have been devoted to developing

TiO2 heterogeneous photocatalyst since it was approved to be a

high efficient catalyst for environmental remediation and

energy conversion purpose [1]. However, its technological

application seems limited by several factors, among which the

most restrictive one is the need of using an ultraviolet (UV),

wavelength (l) <387 nm, as excitation source due to its wide

band gap (3.2 eV for anatase) [2], and can only capture less than

5% of the solar irradiance at the Earth’s surface. For the sake of

efficient use of sunlight, or use of the visible region of the

spectrum, the technology of enlarging the absorption scope of

* Corresponding author at: School of Chemistry and Chemical Engineering,

Southeast University, Nanjing 210096, China. Tel.: +86 25 83794310;

fax: +86 25 83793091.

E-mail address: xujj@seu.edu.cn (D. Fu).

0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2007.10.065

TiO2 may then appear as an appealing challenge for developing

the future generation of photocatalysts.

Several works reported that doping TiO2 with nonmetallic

elements, such as nitrogen, sulphur, carbon, shift the optical

absorption edge of TiO2 toward lower energy, thereby increasing

the photocatalytic activity in visible region [3–5]. It was also

demonstrated that F-doped TiO2 is effective for enhancing

photocatalytic activity. [6–8] Yu et al. [6] proposed that F-doping

converts Ti4+ to Ti3+ by charge compensation, and the existence

of Ti3+ can reduce the electron–hole recombination rate, and

subsequently enhances the photocatalytic activity. Li et al. [7]

confirmed that F-doping in TiO2 can induce visible-light-driven

photocatalysis by the creation of oxygenvacancies. However, the

methods of preparing F-doped samples usually need special

equipments or high temperature treatment. A few researchers

have revealed that fluorination of TiO2 powders in room

temperature can improve photocatalytic activity [9–13]. Minero

et al. claimed that surface fluorination of TiO2 improves the

photocatalytic oxidation rate of phenol and tetramethylammo-

Fig. 1. XRD patterns of FTO and TIO nanoparticles, (a) FTO and (b) TIO.

J. Xu et al. / Applied Surface Science 254 (2008) 3033–30383034

nium [10–12]. And they ascribed the phenomena to the enhanced

generation of mobile free OH radicals. However, this surface

fluorination calls for special pH range of the contaminated

solution, and that F� ions are easy to run off in recycle reactions.

In this article, a simple method is carried out for producing F-

doped TiO2 under low temperature (about 348 K) and ambient

pressure. The photocatalytic activity and the recycle ability of F-

doped TiO2 (defined as FTO) film are compared with pure TiO2

(defined as TIO) film, P25 film and surface fluorination sample

(defined as Surf-FTO) film under artificial solar light.

2. Experimental

2.1. Photocatalysts preparation

Ti(OBu)4 was chosen as a Ti precursor, and NH4F was

employed as F source for FTO samples. Firstly, NH4F was added

in the distilled water under vigorous stirring, the mixture of

Ti(OBu)4 and i-PrOH was then dropwise gradually into above

water whose pH value was adjusted by nitric acid at 2.0, until

Ti(OBu)4 was hydrolyzed completely. The molar ratio of F and Ti

were 1:100. Secondly, the solution was keep refluxing at 348 K

for 24 h to obtain the FTO sol. The sol was further dried in the

oven at 333 K, and finally, the uniform FTO powders were

available for analysis. TIO sample was prepared by the same

method mentioned above without adding NH4F. The surface

fluorination sample, Surf-FTO, was prepared following literature

[12] by adding NaF into the mixture of P25 and water, with the pH

value of 3.0. Glass plates, 70 mm � 20 mm, used as substrates

were dipped in the obtained FTO sol, Surf-FTO solution, TIO sol

and P25 solution for 30 min, respectively, and then dried in the

vacuum oven at 333 K for 2 h, then cooled in the atmosphere.

2.2. Characterization

The crystalline structure of TiO2 samples was measured by

X-ray diffractometer (XRD, XD-3A, Shimadazu Corporation,

Japan) using graphite monochromatic copper radiation (Cu Ka)

at 40 kV, 30 mA over the 2u range of 20–808. The morphology

and size of FTO and Surf-FTO films and particles were

observed by scanning electron microscope (SEM, Sirion, FEI),

and TEM (JEOL-2000EX), respectively. The binding energy

was identified by X-ray photoelectron spectroscopy (XPS) with

Mg Ka radiation (ESCALB MK-II). To investigate the

recombination and lifespan of photogenerated electrons/holes

in the photocatalysts, the photoluminescence (PL) emission

spectra of the samples were measured at room temperature by

LS-55 (Perkin-Elmer) illuminated with a 325-nm He–Cd laser.

A UV–vis spectrophotometer (Shimadzu UV-2100) was used to

record the diffuse reflectance spectra of samples. The crystal-

lization behavior of FTO was also monitored using a TG–DSC

instrument (TG 2000/2960, TA Instruments).

2.3. Photocatalysts characterization

The photocatalytic activity of FTO film was studied in the

photocatalytic degradation of Reactive Brilliant Red, X-3B in

aqueous solution, while Surf-FTO, TIO and P25 films were

used for comparison. A batch photoreactor system was used in

experiments. It consists of a cylindrical silica reactor with glass

plate at the bottom, and an external light source with vertical

irradiation. A 250 W halogen lamp of artificial solar light was

used as light source with average radiation intensity of

11.3 mW cm�2. A set of photocatalytic degradation experi-

ments were performed with the following procedure: a piece of

glass plate with sample film was dipped into 10 ml of X-3B

solution with an initial concentration of 50 mg l�1. Prior to

photoreaction, air was pumped into reactor in the dark for

30 min to reach adsorption–desorption equilibrium. Then, with

continuous pump, the reaction was irradiated by artificial solar

light from the top vertically. During the photoreaction, samples

were collected at a time interval of every 20 min for analysis.

As for many TiO2 samples, the active sites may occupied by

absorbed contaminated molecules or their by-product after

some time usage, moreover, the modified element may run off

from sample film. Accordingly, the recycle experiments were

designed to examine photoactivity on both FTO and Surf-FTO

films. After finishing a cycle, the film was rinsed and dried in

the atmosphere, without any other treatments. The recycle

experiment was carried on for 6 cycles.

3. Results and discussion

3.1. Characterization of F-TiO2 particles

The XRD behaviors of FTO and TIO are shown in Fig. 1. It

can be noted that both FTO and TIO have significant diffraction

peaks (25.528, 37.98, 48.28, 54.48, 62.48) representing the

characteristic of anatase phase [14], and a small amount of

brookite is existing. Moreover, the doping F atoms do not cause

any shift in peak position of that of TiO2 phase. This could be

understood that the ion radius of fluorine atom (0.133 nm) is

virtually the same as the replaced oxygen atom (0.132 nm) [15].

The average crystallite sizes of FTO and TIO, calculated from

the (1 0 1) peak of the XRD patterns are 3.78 and 5.54 nm,

Fig. 2. SEM images of FTO and Surf-FTO films, (a) FTO and (b) Surf-FTO. Fig. 3. TEM images of FTO and Surf-FTO particles, (a) FTO and (b) Surf-FTO.

Fig. 4. XPS spectra of F 1s region in both FTO and Surf-FTO samples (a) Surf-

FTO, (b) FTO, and the inset is the survey XPS spectrum of FTO.

J. Xu et al. / Applied Surface Science 254 (2008) 3033–3038 3035

respectively. It can be concluded that F-doping inhibits the

grain growth. And the increasing intensity of FTO sample

indicates that the crystallinity is improved upon F-doping.

The SEM and TEM images of FTO and Surf-FTO films and

nanoparticles are shown in Figs. 2 and 3, respectively. In SEM

images of FTO film, the surface is smoother than that of Surf-

FTO film, a probable explanation is that the morphologies of

the particles depended on the TiO2 precursors as well as

fluorine incorporation in TiO2 matrix. As for FTO, the existing

F-ions are not the same as that of Surf-TIO. As shown in the

TEM images, FTO possesses spherical particles with an

average particle diameter of about 5 nm, which is in good

agreement with XRD analysis. There exists some agglomera-

tion in Surf-FTO particles, while such phenomenon cannot be

detected in FTO image. This is probably due to the fact that F-

ions suppress the agglomeration by electrostatic repulsion.

The XPS survey spectrum of FTO indicates that the peak

related to Ti, O, F, and C elements. The C element can be

ascribed to the residual carbon from precursor solution, as our

samples are all prepared at low temperature. Fig. 4 gives the

high-resolution XPS spectra of the F 1s region of FTO and Surf-

FTO. An unsymmetrical F 1s peak was observed, especially for

FTO sample, which means that there exist two chemical forms

of F-atoms in FTO. Using Gaussian distributions, the F 1s peak

of FTO was divided into two separated peaks as shown in Fig. 4.

The peak 1 located at 684.3 eV corresponding to that of F atoms

adsorbed on TiO2 [16]. While the peak 2 located at 688.4 eV is

the F atoms in solid solution TiO2�xFx, which is probably

formed by nucleophilic substitution reaction of F� ions and

titanium alkoxide during the hydrolysis process [6,11]. As for

Surf-FTO, the F 1s peak is originated from the surface fluoride

formed by ligand exchange between F� and surface hydroxyl

group on TiO2. The F 1s peak only located at 684.3 eV, and no

Fig. 5. PL spectra of P25, Surf-F, and FTO samples.Fig. 6. The diffuse reflectance UV–vis spectra of P25, TIO, Surf-FTO, and FTO

samples.

Fig. 7. TG–DSC curve of FTO particles.

J. Xu et al. / Applied Surface Science 254 (2008) 3033–30383036

sign of F� ions in the lattice was detected, which is in

agreement with that of literature [16]. This means, in our

preparation, the fluorine atoms can incorporate in the TiO2

lattice, even at very low synthesized temperature, which takes a

positive role in photocatalysis [6].

PL emission spectra have been widely used to investigate the

efficiency of charge carrier trapping, immigration, transfer, and

to understand the fate of electron–hole pairs in semiconductor

particles [17]. Fig. 5 shows the PL spectra of FTO, Surf-FTO,

and P25. The excitonic PL intensity of these samples decreases

obeyed the following order: FTO < Surf-FTO < P25. This

indicates that an appropriate amount of F� doping may slow the

radiative recombination process of photogenerated electrons

and hole in TiO2, which may confirm that the lower the

intensity of PL spectra, the higher the photocatalytic activity.

Diffuse reflectance spectroscopy gives information about the

electronic absorption of the photocatalysts. In Fig. 6, the UV–

vis reflectance of P25, TIO and Surf-FTO are compared with

FTO. It is indicates that F-doping dose not cause any significant

red-shift in the fundamental absorption edge of TiO2. This

conclusion is consistent with Asahi, Yamaki, Ho and Li et al.

[3,15,18,19]. When doping with fluorine atoms in TiO2,

localized levels with high density appear below the VB of TiO2.

These levels consist of the F 2p state without any mixing with

VB or CB of TiO2. Accordingly, it is not expected to show high

photocatalytic activity under visible-light-region.

The thermal behavior of FTO was investigated with a TG/

DSC technique at temperature rang from room temperature to

1200 8C, and the TG–DSC patterns of the typical FTO is shown

in Fig. 7. It is well known that the thermal behavior of TiO2

usually depends on the chemical composition, preparation

condition and existing phases [20–22]. The endothermic peak at

about 100 8C is due to the desorption of the physically adsorbed

water and alcohol [21]. A small exothermal peak appeared at

about 180 8C is assigned to the decomposition of organic

substances in the powder. The relative broad exothermal peak at

790 8C observed is owning to the phase transformation from

anatase to rutile, while the usually transformation temperature is

around 600 8C. The TG curve can be divided into three stages.

The first stage is range from room temperature to 200 8C with a

moss loss of 14%, which represents the dehydration and loss of

residual solvent. In the temperature range of 200–400 8C, the

weight loss is about 5%, which is attributed to the loss of organic

substances, corresponding to a sharp exothermal process from

DSC curve. The third mass loss of 1.0% in the range of 400–

1200 8C is owning to the gradual removal of the organic residues.

It can be concluded from TG–DSC results that the phase of

sample FTO is mainly anatase, and the doping of fluorine atoms

can inhibit the transformation from anatase to rutile.

3.2. Photocatalytic activity

In order to investigate the photocatalytic activity of FTO,

degradation experiments of X-3B dye were studied under

artificial solar light and results are shown in Fig. 8. The blank

experiment without catalysts indicated that the merely photolysis

can be ignored as it is about 0.9% after illuminated for 80 min.

Concerning the catalysts, the degradation percent of X-3B

obeyed the following order: P25 < TIO < Surf-FTO < FTO.

Fig. 8. Kinetic of X-3B degradation for different samples.

Fig. 9. Variations in ln(C0/C) as a function of irradiation time and linear fits of

different samples.

Table 1

kapp and R data for each sample

kappa (min�1) Rb

P25 9.63 � 10�4 0.993

TIO 0.0060 0.997

Surf-FTO 0.011 0.997

FTO 0.022 0.999

a kapp is the apparent rate constant.b R is regression relative coefficient.

Fig. 10. Recycle photocatalytic experiments on FTO film for 6 cycles.

J. Xu et al. / Applied Surface Science 254 (2008) 3033–3038 3037

Both Surf-FTO and FTO showed better photodegradation effects

than pure TiO2. The apparent rate constant (kapp) has been chosen

as the basic kinetic parameter for the different photocatalysts,

since it enables one to determine photocatalytic activity

independent of the previous adsorption period in the dark and

the concentration of X-3B remaining in the solution. The

apparent first order kinetic equation:

ln

�C0

C

�¼ kapp � t (1)

was used to fit experimental data in Fig. 8. Where C is the

concentration of X-3B remaining in the solution at t, and C0 is

the initial concentration at t = 0 [23]. The variations in ln(C0/C)

as a function of irradiation time are given in Fig. 9. And kapp

data for FTO, Surf-FTO, TIO and P25 are exhibited in Table 1.

It shows that the kapp of FTO is obviously increases, about 2

times higher than that of Surf-FTO. It has been discussed above

that the FTO does not absorb in the visible spectrum by DRS

analysis. But according to recent study, it can also induce

visible-light-driven photocatalysis by extrinsic absorption

through the creation of oxygen vacancies [7]. Thus, the high

photocatalytic activity of FTO may due to that reason rather

than the excitation of the intrinsic absorption band of bulk TiO2.

The result is also in agreement with that of PL spectra analysis.

The regeneration of TiO2 photocatalyst was one of key steps

to making heterogeneous photocatalysis technology for

practical applications. The results of recycling of FTO and

Surf-FTO films are shown in Fig. 10 and Table 2. Fig. 10

exhibits the variation of X-3B concentration on FTO film of

every cycle. The photocatalytic activity of FTO almost kept the

same after three times reusing, and a little decline after six

cycles, and the decomposing rate was still above 79% after

recycle for 6 times. While for surf-FTO, after 6 cycles, the

photocatalytic activity decreased by 34%, as most F� ions were

absorbed on the surface of TiO2 physically [6], they were easy

to run off with the solution in repeated using. These results

mean that FTO can act as a useful photocatalyst to drive

photodegradation reaction. More importantly, FTO, as a novel

Table 2

Data for recycle photocatalytic experiments

Sample Decomposed rate (%)

Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5 Cycle 6

FTO 82.7 81.7 82.0 80.6 79.2 80.8

Surf-FTO 56.6 38.4 31.5 26.8 22.6 22.4

J. Xu et al. / Applied Surface Science 254 (2008) 3033–30383038

photocatalyst, can be reused for several times without great

decrease in photocatalytic activity during the reactions, exhibits

potential application in continual and long-time reactions.

4. Conclusion

F-doped TiO2 was prepared under mild condition by

hydrolysis of titanium-n-butoxide in abundant NH4F-H2O

acidic solution. The doped fluorine atoms existed in two

chemical forms, and the ones incorporated into TiO2 lattice

might take a positive role in photocatalysis. The FTO film

exhibited the highest photocatalytic activity for degradation of

X-3B in aqueous solution under artificial solar light, and it was

higher than that of P25. The high photocatalytic activity of FTO

may due to extrinsic absorption through the creation of oxygen

vacancies rather than the excitation of the intrinsic absorption

band of bulk TiO2. Without any special treatments, FTO film

can be applied to recycle use in photocatalytic reactions while

keeping high activity. Thus, the as-prepared FTO exhibits

potential application on industrial area.

Acknowledgment

This work is financially supported by the National Natural

Science Foundation of China (No. 60121101).

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