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Chemical Engineering Science 63 (2008) 5066 5070www.elsevier.com/locate/ces

Hydrothermal synthesis of TiO2 photocatalysts in the presence of NH4F andtheir application for degradation of organic compounds

Kohsuke Mori, Keiichi Maki, Shinichi Kawasaki, Shuai Yuan, Hiromi Yamashita

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

Received 13 April 2007; received in revised form 22 June 2007; accepted 26 June 2007

Available online 30 June 2007

AbstractTiO2 photocatalysts were synthesized by hydrothermal method from tetraisopropyl orthotitanate (TPOT) in the presence of NH4F with

different NH4F/Ti molar ratios (HT-TiO2 (F0, 0.01, 0.05, 0.25, and 1)). The formation of well-crystallized anatase phase of TiO2 and suppression

of phase transition to rutile were observed even at high calcination temperature. The amount of employed NH 4F influenced on the crystal size

and surface area. XPS analysis showed that F ions coordinated with Ti in the lattice. In comparison to the commercial TiO 2 powder (P-25),

the HT-TiO2 samples with high F ion contents exhibited high absorption in the UVvisible range with a shift to the longer wavelength in the

band gap transition. The measurement of the water adsorption ability suggested that the F ion doping did not significantly influence on their

hydrophobicity. The HT-TiO2 samples exhibited high photocatalytic activity for the degradation of i-BuOH diluted in water. The photocatalytic

activities were apparently affected by surface area and crystallinity of TiO2 phase depending on the amount of employed NH4F.

2007 Elsevier Ltd. All rights reserved.

Keywords: Photocatalyst; TiO2; NH4F; Hydrothermal synthesis; Photocatalytic degradation

1. Introduction

Photocatalytic decomposition of organic pollutants on semi-

conductor surfaces is of vital interest with respect to both

fundamental understanding and potential practical utilization

(Yamashita and Anpo, 2003; Horikoshi et al., 2002, Minero

et al., 2000a,b). Titanium dioxide TiO2 is a promising photo-

catalyst exploiting low cost, nontoxity as well as strong ox-

idation ability under UV light irradiation (Mills and Hunte,

1997; Fujishima et al., 2002). Especially, TiO2 thin film can

be used in a wide variety of practical applications such as self-

cleaning materials in automobiles, buildings, and householdglazing (Wang et al., 1997). In spite of the above characteris-

tics, most of applications of TiO2 photocatalyst so far are lim-

ited to UV-light irradiation because the light absorption edge

of pure TiO2 is less than 380 nm. Moreover, hydrophilic nature

of TiO2 surface prevents the efficient transfer of hydrophobic

Corresponding author. Tel./fax: +81 6 6879 7457.

E-mail address: yamashita@mat.eng.osaka-u.ac.jp (H. Yamashita).

0009-2509/$- see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ces.2007.06.030

organic compounds from solution or atmosphere to TiO2 par-

ticle surface, which resulted in the decrease of photocatalytic

activity. Therefore, the great demand for the development of

TiO2 photocatalysts with enhanced activity is still high.

During the past decade, TiO2 photocatalysts operating ef-

fectively under visible-light irradiation have been presented by

the implantation of transition metal cations, e.g. Fe, Cr, and V

(Yamashita and Anpo, 2004). Another noble metals loading,

such as Pt and Ag, can decrease the recombination of photogen-

erated electrons and holes, which promote interfacial electron

transfer (Zou et al., 2004). Asahi et al. (2001) also demonstrated

that the doping of nitrogen into TiO2 provided the visible-lightsensitivity. The induced visible-light sensitivity of the N-doped

TiO2 was due to the narrowing of the band gap by hybridiza-

tion of the N 2p and O 2p orbitals, or formation of an isolated

narrow N 2p band above the valence band. As an alternative

method, the doping of F ion within the TiO2 lattice has re-

ceived much attention. TiO2 doped with F in the lattice using

NaF as a fluorine source showed enhanced photocatalytic ac-

tivity (Park and Choi, 2004; Minero et al., 2000a,b). F-doped

TiO2 were also prepared by spray pyrolysis method (Li et al.,

2005). It has been reported that the improved photocatalytic

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K. Mori et al. / Chemical Engineering Science 63 (2008) 50665070 5067

activity could be ascribed to the increase in the anatase crys-

tallinity induced by F ion. The introduction of F ion is also

promising to reduce the hydrophilic nature of TiO2 particle sur-

face, which could enhance the affinity of TiO2 surface toward

organic compounds by the replacement of O.H group by

F (Yuan et al., 2001). The modification of hydrophilic TiO2

surface also could be achieved by HF or F 2 treatment (Ponget al., 1995).

In this study, we designed TiO2 photocatalyst in the pres-

ence of NH4F under hydrothermal conditions (HT-TiO2). The

effects of initial NH4F contents on the characteristics of TiO2were investigated by several physicochemical characterizations.

Moreover, its successful utilization was investigated for the

photocatalytic degradation of isobutanol (i-BuOH) diluted in

water.

2. Experimental

Tetraisopropyl orthotitanate (TPOT: Ti(OC3H7)4

), ethanol,

NH4F, and i-BuOH are commercial products. All chemicals

were used as received. The synthesis of HT-TiO2 were per-

formed by hydrothermal method using TPOT as the starting

material in a mixed NH4F.H2Oethanol solution. The molar

ratio of the mixture was as follows: Ti : H2O : EtOH=1 : 5 : 5.

The above mixture was transferred into an autoclave and kept

at 433 K for 48 h. After centrifugation of the resultant mixture,

followed by washing, drying, and calcination at 823 K for 5 h,

HT-TiO2 was obtained. NH4F/Ti molar ratios were set at 0,

0.01, 0.05, 0.25, and 1, respectively. The products were de-

noted as HT-TiO2 (F0), HT-TiO2 (F0.01), HT-TiO2 (F0.05),

HT-TiO2 (F0.25), and HT-TiO2 (F1), respectively. As a ref-

erence sample, TiO2 was also prepared by the conventionalsolgel method. The mixture of TPOT, H2O, and EtOH (1 : 5 :5) was stirred at room temperature for 24 h, and then heated

at 358 K for 24 h. After centrifugation of the resultant mixture,

followed by drying, and calcination at 823 K for 5 h, solgel

TiO2 was obtained.

X-ray diffraction patterns of all samples were measured by

Rigaku RINT2500 diffractometer with Cu K radiation ( =

1.5406 A). By using an ASAP 2000 system (Shimadzu), the

BET method was applied for the determination of the specific

surface area. The diffuse reflectance absorption spectra were

recorded with Shimadzu UV-2200A photospectrometer. X-ray

photoelectron spectroscopy was recorded with JEOL micro-probe system (JPS-9010) using the Mg K line. SEM micro-

graph was obtained with Hitachi S-5200.

The photocatalytic activity of samples was evaluated by the

photodegradation ofi-BuOH. A 0.05 g of catalyst was dispersed

in 25 ml of aqueous i-BuOH solution (2.61mmoll1). After

stirring under dark conditions for 30min, the solution was bub-

bled by oxygen for another 30 min. Then the solution was irra-

diated using UV light (> 280nm) from a 100W high-pressure

Hg lamp. The progress of the reactions was monitored by

gas chromatography (GC-14B, Shimadzu). The photocatalytic

activity was estimated from the initial decrease in the concen-

tration of i-BuOH after preadsorption of i-BuOH on the cata-

lyst under dark conditions. Water adsorption isotherms of the

catalysts were measured at 293 K using a conventional vacuum

system.

3. Results and discussion

Fig. 1 shows the XRD patterns of hydrothermally synthe-

sized and calcined at 823 K HT-TiO2 samples with differentNH4F loading levels, together with the reference TiO2 pre-

pared by the conventional solgel method. In all the HT-TiO2samples, very sharp peaks corresponding to the anatase phase

of TiO2 could be observed. No other phases such as rutile

and brookite could be detected. The crystallinity of the anatase

phase (I/Isol.gel) in HT-TiO2 was estimated by the intensity

of (1 0 1) reflection relative to that of the solgel TiO2 and the

results are given in Table 1. The HT-TiO2 (0.25) showed the

highest I /Isol.gel value. By applying the Scherrer equation for

(1 0 1) reflection, the average crystallite size of samples can

be determined to be 20 nm (HT-TiO2 (F0)), 18 nm (HT-TiO2

(F0.01)), 20 nm (HT-TiO2 (F0.05)), 24 nm (HT-TiO2 (F0.25)),and 21 nm (HT-TiO2 (F1)), respectively. The crystallite sizes

were not dependent on the amount of NH4F employed, but the

HT-TiO2 (0.01) has the smallest anatase crystallite size among

the samples prepared. The results of the BET surface areas re-

flected the crystallite sizes determined by XRD, as summarized

in Table 1. The surface area decreased with increasing the crys-

tal size. On the contrary, additional peaks corresponding to the

rutile phase at 27.5 and 36.0 were observed in the solgel

TiO2 (Fig. 1a). It should be noted that the present hydrothermal

synthesis using TPOT as a Ti source in the presence of NH4F

inhibits the phase transformation from anatase to rutile.

In order to further verify the thermal stability of the HT-TiO2,

the samples were calcined at a higher temperature of 923 K andthe XRD patterns of them are given in Fig. 2. The result of

the TiO2 prepared by the solgel method was also depicted as

20 30 40 50 60

Intensity/a.u.

(c) HT-TiO2(0.01)

2 / degree

(e) HT-TiO2(0.25)

(f) HT-TiO2(1)

(b) HT-TiO2(0)

(a) sol-gel TiO2

(d) HT-TiO2(0.05)

Fig. 1. XRD patterns of TiO2 prepared by the solgel method and hydrother-

mally synthesized HT-TiO2 samples with different amount of NH4F after

calcination at 823 K.

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Table 1

Crystal size, surface area, and amount of H2O adsorption of the HT-TiO2samples calcined at 823K

Sample I /Isol.gela Surface area H2O adsorption

b

(m2 g1) (molm2)

HT-TiO2 (0) 1.03 55 18

HT-TiO2 (0.01) 1.05 59 17HT-TiO2 (0.05) 1.20 48 20

HT-TiO2 (0.25) 1.44 33 16

HT-TiO2 (1) 1.17 47 15

aThe intensity of (1 0 1) reflection relative to that of the solgel TiO2.bThe values at P0/P1 = 1.

20 30 40 50 60

Intensity/a.u.

(a) sol-gel TiO2

2 / degree

(c) HT-TiO2(0.01)

(e) HT-TiO2(0.25)

(b) HT-TiO2(0)

(f) HT-TiO2(1)

(d) HT-TiO2(0.05)

Fig. 2. XRD patterns of TiO2 prepared by the solgel method and hydrother-

mally synthesized HT-TiO2 samples with different amount of NH4F after

calcination at 923 K.

reference sample. The HT-TiO2 (F0) and HT-TiO2 (F0.01)

showed the formation of the mixture of anatase and rutile

phases. With increasing the NH4F contents, the phase transfer

to rutile was completely suppressed and all peaks could be in-

dexed as the anatase phase of crystalline TiO2. On the other

hand, the rutile phase was dominant in the solgel TiO2 with asmall amount of anatase phase. Similar phenomenon was also

reported in the F-doped TiO2 powder prepared by hydrolysis

of titanium tetraisopropoxide in a mixed NH4F.H2O solution

(Li et al., 2005). These results clearly indicate that F ions

enhance the crystallization of anatase phase, and meanwhile

suppress the crystallization into rutile phase.

Fig. 3 shows the F1S XPS spectrum of the HT-TiO2 (F1).

Generally, the F1S binding energy at around 684eV corresponds

to the adsorbed F ions on TiO2 surface, while that at 688 eV is

ascribed to the F ions in the TiO2 lattice (Wang and Sherwood,

2004). Although the intensity of the peak of F1S was weak in

the HT-TiO2 (F1), one major peak ascribed to the F ions in

the TiO2 lattice exhibited strong evidence of F ions was in-

680685690695

Binding Energy / eV

Absorbance/a.u.

Fig. 3. XPS spectrum of F1S in HT-TiO2 (1).

corporated into TiO2 crystals. On the other hand, no noticeable

signals assignable to N1S binding energy could be observed in

the HT-TiO2 (F1), suggesting that N atoms originating from

the NH4F scarcely present.

The differences in the H2O adsorption capability were com-

pared and the results are listed in Table 1. The F ion dop-

ing did not significantly affect the amount of adsorbed H2O

molecules, which is ascribed to that most of the F ions exist

in the TiO2 lattice rather than simply being adsorbed on the

surface, as confirmed by XPS analysis. However, the amount

of adsorbed H2O molecules slightly decreased on the HT-TiO2(F0.25) and HT-TiO2 (F1), which may indicate that the hy-

drophobic properties were enhanced by the replacement of O.H group by F.

Fig. 4 shows the SEM image of the HT-TiO2 (F0) andHT-TiO2 (F0.25) samples calcined at 823 K. The gross mor-

phology of HT-TiO2 remained unchanged by the addition of

the NH4F. In spite of the smaller surface area of the HT-TiO2(F0.25) sample compared to the HT-TiO2 (F0), the noticeable

agglomeration of TiO2 particles could not be observed.

F ions doping clearly influenced on the light absorption

characteristics. UVvis spectra of the HT-TiO2 samples are

shown in Fig. 5. Compared with the pure TiO2 (P-25), the

HT-TiO2 (F0.25) and HT-TiO2 (F1) samples showed a stronger

absorption in the UV range and a slight shift to visible-light

range (> 430 nm). HT-TiO2 (F0) and HT-TiO2 (F0.01) samples

exhibited similar absorption edge to that of the pure TiO2. Ithas been reported that the F ion doping does not cause a shift

in the fundamental absorption edge of TiO2 (Ho et al., 2006).

This results is consistent with the theoretical band calculations

for F ions doped TiO2 (Morikawa et al., 2001; Yamaki et al.,

2003). Although we could not detect the nitrogen atoms by the

XPS analysis, an extremely small amount of N contamination

from NH4F in the HT-TiO2 (F0.25) and HT-TiO2 (F1) samples

might have shifted the band edges to the longer wavelength.

Increase of the absorption intensity as well as response range

would increase the number of photogenerated electrons and

holes to participate in the photocatalytic reaction.

The reaction rates for the photocatalytic degradation of

i-BuOH diluted in water were shown in Fig. 6. The reaction

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Fig. 4. SEM image of (a) HT-TiO2 (0), and (b) HT-TiO2 (0.25) calcined at

823K.

rate of the commercially available P25 was determined to

be 15molh1. The HT-TiO2 (F0.05) was the most active

photocatalyst among the catalyst tested, and the HT-TiO2(F0.25) showed similar activity to that of the HT-TiO2 (F0.01).

HT-TiO2 (F0) and HT-TiO2 (F1) apparently exhibited de-

creased photocatalytic activities. The observed activities of the

photocatalysts are combined effects of many factors including

phase structure, surface area, crystallinity. The relatively highphotocatalytic activity of the HT-TiO2 (F0.01) is partially

attributed to its large surface area and small crystallite size.

In the case of the HT-TiO2 (F0.25), high crystallinity of the

anatase phase is a main factor for the high activity (Yu et al.,

2002; Murakami et al., 2007). A photoabsorption ability in the

wider range observed in the UVvis spectrum may also plays

an important role. The highest activity of the HT-TiO2 (F0.05)

is attributed to both relatively high crystallinity as well as high

surface area. It has been reported that F doping in the lattice

could convert some Ti4+ to Ti3+ originated from the charge

compensation caused by the replacement of F ions with O2

ions in the lattice (Yu et al., 2002). The Ti3+ could trap pho-

togenerated holes, which resulted in charge recombination in

400 450 500 550

Wavelength / nm

0

1

2 10

-3

ec

ab

f

d

4000

0.02

0.04

0.06

0.08

Wavelength / nm

Kubelka-Munk

K

ubelka-Munk

a

b

e

df

c

500

Fig. 5. UVvis spectra of (a) TiO2 (P-25), (b) HT-TiO2 (0), (c) HT-TiO2(0.01), (d) HT-TiO2 (0.05), (e) HT-TiO2 (0.25), and (f) HT-TiO2 (1).

0 5 10 15 20

HT-TiO2(0)

HT-TiO2(0.25)

HT-TiO2(1)

HT-TiO2(0.01)

HT-TiO2(0.05)

Conversion ofi-BuOH /molh1

P25

Fig. 6. Photocatalytic degradation of i-BuOH diluted in water on P25 and

various HT-TiO2 synthesized with different amount of NH4F.

bulk and lower photocatalytic activity. It can be said that an

excessive F doping in the HT-TiO2 (F1) plays a negative role

in photocatalysis.

4. Conclusions

F-doped TiO2 photocatalysts were prepared by a hydrother-

mal method in the presence of NH4

F. This is a promising

technique to synthesize the well-crystallized anatase TiO2, sup-

pressing the transition to rutile phase. The amount of employed

NH4F influenced on the crystal size and surface area as well as

photocatalytic activity. The HT-TiO2 (F0.05) was demonstrated

to be most active for the degradation ofi-BuOH diluted in wa-

ter. The photocatalytic activity of the HT-TiO2samples is dom-

inated by the crystallinity of anatase phase, and surface area.

Acknowledgements

The present work is supported by the Grant-in-Aid for

Scientific Research (KAKENHI) from Ministry of Educa-

tion, Culture, Sports, Science and Technology (No. 1734036),

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5070 K. Mori et al. / Chemical Engineering Science 63 (2008) 50665070

(No. 17360388) and (No. 18656238). This work is partly per-

formed under the project of collaborative research at the Joining

and Welding Research Institute (JWRI) of Osaka University.

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