Synthesis of titanium dioxide with oxygen vacancy and its visible-light sensitive photocatalytic activity

Materials Research Bulletin 46 (2011) 531–537

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Synthesis of titanium dioxide with oxygen vacancy and its visible-light sensitive photocatalytic activity Sitao Yang a, Weiping Tang b, Yoshie Ishikawa a, Qi Feng a,* a b

Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, 2217-20 Hayashi, Takamatsu 761-0396, Japan Research Institute for Solvothermal Technology, 2217-43 Hayashi, Takamatsu 761-0301, Japan

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 June 2010 Received in revised form 2 December 2010 Accepted 3 January 2011 Available online 12 January 2011

A novel active carbon reducing process was developed for the synthesis of titanium dioxide with oxygen vacancy. In this process a nanocomposite of hydrolyzed titanium(IV) tetra-isopropoxide and the active carbon was annealed in air. The formation reaction, visible-light absorption, and visible-light sensitive photocatalytic activity of the titanium dioxide with oxygen vacancy samples were investigated using XRD, TG-DTA analyses, FE-SEM, EDS, and measurements of electric conductivity, BET specific surface area and photocatalytic activity. The nonstoichiometric titanium dioxide with oxygen vacancy sample has a rutile structure and its chemical formula can be written as Ti(IV)1 xTi(III)xO(2 x/2)&x/2, where & is oxygen vacancy. The oxygen vacancy was introduced into the rutile structure by reducing reaction of the active carbon in a phase transformation process from anatase to rutile. The samples showed visible-light absorption with an absorption edge around 570 nm and high surface visible-light sensitive photocatalytic activity. ß 2011 Elsevier Ltd. All rights reserved.

Keywords: A. oxides B. chemical synthesis C. X-ray diffraction D. catalytic properties D. defects

1. Introduction TiO2 materials have potential applications as the efficient photocatalysts for the water splitting to produce hydrogen, degradation of organic pollution, and dye-sensitized solar cells [1–4]. Since TiO2 is a wide band gap semiconductor with band gap of about 3.0 eV, it can only use ultraviolet light in a wavelength range below 390 nm for the photocatalytic reaction. In sunlight, however, 90% energy is in wavelength range of visible light, and the energy of ultraviolet light in the wavelength range below 390 nm is only about 5%. Therefore, development of visible-light sensitive photocatalyst is important from the viewpoint of efficient utility of sunlight to the photocatalytic reaction. Up to now, many attempts have been done to develop visiblelight sensitive photocatalyst by narrowing down the band gap of TiO2. Doping TiO2 with other elements has been reported as an effective method. The visible-light sensitive TiO2 photocatalyst has been realized by doping with transition metals, such as Fe and Cr [5,6]. However, the transition metal doped TiO2 is unstable and easily loses the visible-light sensitivity. Another problem is that the doped transition metal ions in TiO2 can act as the recombination center of the photo-excited electrons and holes, which reduces the photocatalytic activity [7,8]. To solve these problems, doping TiO2 with non-metal elements, such as N, F, S and C, have been

* Corresponding author. Tel.: +81 87 864 2402; fax: +81 87 864 2438. E-mail address: [email protected] (Q. Feng). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.01.004

carried out [9–13]. Asahi et al. have reported an N-doped TiO2 and its photocatalytic activity. The N-doped TiO2 has an absorption edge around 500 nm, and shows high visible-light sensitive photocatalytic activity [9]. The Ti(IV) ions in TiO2 can be reduced to Ti(III) ions by reacting with reducing agent at high temperature. The reduction reaction accompanies introducing oxygen vacancy into the structure and formation of nonstoichiometric TiO2 with oxygen vacancy (TiO2 x). It has been reported that after introducing oxygen vacancy, the white color of TiO2 changes to yellow or blue color, and new energy levels locating in a range of 0.75–1.18 eV below the conduction band of TiO2 are formed, which can be assigned to the energy state of the oxygen vacancy [14,15]. Although the yellow or blue color of TiO2 x suggests that the oxygen vacancy introduced TiO2 x could be used as a visible-light sensitive photocatalyst, but only a few studies have been carried out on the photocatalytic reaction of TiO2 x. Nakamura et al. have reported the first research work on the relationship between the oxygen vacancy and the photocatalytic activity of TiO2 x [15]. They used ESR to identify the electron trapped on oxygen vacancy and investigated the relationship between intensity of the ESR signal and the visiblelight sensitive photocatalytic activity for the decomposition of NOx. The results revealed that the visible-light activity of the TiO2 x samples can be ascribed to the energy state of oxygen vacancy locating between the valence band and the conduction band of TiO2. Since TiO2 is an important material for utility of solar energy, a large number of studies have been reported on the synthesis of this

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material [16–19]. However, only a few of studies have been reported on the preparation of TiO2 x. Cronemeyer has reported that a single crystal TiO2 x of rutile phase with blue color can be prepared by annealing rutile single crystal TiO2 in hydrogen gas in a temperature range of 600–800 8C [14]. An anatase phase of TiO2 x powders with light yellow color has been prepared by a plasma treatment of TiO2 fine powders (ST-01, anatase phase) in an quartz glass chamber with H2 atmosphere of 2 Torr at 673 K [15]. Up to now, the most synthesis methods for the preparation of TiO2 x were carried out by reducing TiO2 with H2 gas in a closed chamber. These methods are expensive, and the reactions are difficult to be controlled, which may be a reason why only a few studies were carried out on TiO2 x. In the present paper, we describe a novel synthesis process for TiO2 x powders, formation reaction of TiO2 x in the process, and their visible-light sensitive photocatalytic activity. In the synthesis process, active carbon is used as the reducer and reacted with titanium tetra-isopropoxide in air. In comparison with the traditional H2 reducing process, the active carbon reducing process has some advantages, such as low cost and simple operation.

pellet as electrodes, and the impedance was measured using Solartron SI 1287 electrochemical interface. The resistance was evaluated from the impedance using Solartron 1255B frequency response analyzer with Zplot electrochemical impedance software. High resolution X-ray fluoresce (HRXRF) of Ti Ka spectrum was carried out on Technos XFRA 190 at exited voltage of 35 kV and current of 80 mA. Energy-dispersive spectroscopy (EDS) was measured on transmission electron microscope (JEOL JED2300T). Before the EDS measurement, the samples were cleaned by an ion cleaner for 10 min to remove carbon contamination on the sample surface. 2.3. Photocatalytic characterization

In a typical procedure for synthesis of TiO2 x samples, active carbon (3 g) was mixed with titanium(IV) tetra-isopropoxide (71 g) in a glove box filled with Ar gas, followed by adding H2O (18 g) into the mixture to hydrolyze titanium(IV) tetra-isopropoxide in the mixture, and then a grey-colored precursor was obtained. In this case, the molar ratio of C/Ti in the precursor was 1:1. The precursor was dried at 100 8C for 24 h. To prepare TiO2 x, the precursor was annealed at a desired temperature for 8 h in air (one-step process). Other series of samples were prepared also under the similar synthesis conditions, but changed the molar ratio of C/Ti in the precursor. The samples were designated as S-Y-X, where Y and X represent C/Ti molar ratio and annealing temperature, respectively. In order to study the mechanism of oxygen vacancy formation, another series of samples were prepared by further annealing the S-1-500 sample (two-step process), and the samples were named as S-1-500-X, where X represent annealing temperature at second step. Reference TiO2 samples (S-0-X) were prepared also under the similar conditions but without active carbon addition (C/Ti molar ratio = 0).

Photocatalytic activity was evaluated by photoelectrochemical method and methylene blue (MB) degradation method. In the photoelectrochemical measurement, the titanium oxide film was used as working electrode, and irradiated in a quartz cell containing 0.1 mol/L of sodium sulfate supporting electrolyte solution using a Xe lamp (Asahi Spectra USA LAX-Cute, VIS 400– 700 nm) with a light intensity of 3000 W/m2. The masked-off irradiated area was 4.8 cm2. A Pt plate and an Hg/Hg2Cl2/KClsat electrode were used as counter and reference electrodes. An external bias was applied, and photocurrent was measured using the electrochemical analyzer. The titanium oxide film on conducting glass (FTO coated glass) was prepared by coating the conducting glass surface with titanium oxide paste, and then calcined at 450 8C for 60 min. In the methylene blue (MB) degradation method, an incandescent lamp of 300 W was used as the irradiation visible-light source. The TiO2 x sample (0.1 g) was added in a 5 ppm MB aqueous solution (100 mL), and the solution was irradiated with 300 W incandescent lamp located at 1 m from the MB solution. The MB degradation amount was evaluated from the MB concentrations before and after the irradiation determined using a Shimadzu UV2450 spectrophotometer. A blank experiment was carried simultaneously using 5 ppm MB solution without addition of TiO2 x sample. The adsorption amount of MB on TiO2 x sample surface was evaluated by measuring the MB concentration before and after added the TiO2 x sample into the MB solution for 60 min in dark room. The decrease of MB by the adsorption was removed from the degradation by the photocatalytic reaction. For comparison, the Ishihara ST-01 sample (anatase phase, BET surface area 349 m2/g) was used as the standard sample for the photocatalytic activity.

2.2. Physical analysis

3. Results and discussion

TG-DTA analysis was carried out in air with a heating rate of 10 K/min and Al2O3 as a reference sample on a Shimadzu DTG-50 thermal analyzer. X-ray diffraction (XRD) analysis was carried out on a Rigaku RINT 1200 X-ray diffractometer with a graphitemonochromator using Cu Ka radiation (l = 0.15405 nm). A Shimadzu UV-390 spectrophotometer equipped with an integrating sphere was used for measuring UV–Vis reflection spectra of the powder samples, and BaSO4 powder sample was used as the white standard. Absorption spectra were obtained by transformation of the reflection spectra to the absorption spectra. Morphology of the samples was observed using FE-SEM (JEOL, JSM-6700FZ). AQUANTAC-HROMEAUTO-SORB1-C apparatus was employed for measuring the nitrogen adsorption–desorption isotherm at 77 K with a sample out-gassed at 298 K for 3 h below 10 3 mmHg. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. In the conductivity measurement, TiO2 powder sample was pressed into a disk-shaped pellet of 1 cm in radius and about 1.7 mm in thickness. Silver paste was coated on both sides of the

3.1. Synthesis of TiO2

2. Experimental 2.1. Material preparation

x

In the synthesis process, when water was added into the mixture of titanium(IV) tetra-isopropoxide and active carbon, hydrolysis reaction of titanium(IV) tetra-isopropoxide occurred, and then a grey-colored precursor was obtained. An XRD analysis indicated that the precursor was an amorphous phase (Fig. 1(a)). When the precursor was annealed, the amorphous phase transformed to anatase phase from 300 8C, and then the anatase phase transformed to rutile phase from 500 8C with increasing the annealing temperature (Fig. 1). A mixture of anatase and rutile phases was formed in a temperature range of 500–600 8C. The yellow-colored samples can be obtained above 600 8C. For the reference samples (S-0-X samples) without active carbon addition, the precursor was also an amorphous phase after the hydrolysis reaction of titanium(IV) tetra-isopropoxide at room temperature, and the amorphous phase transformed to anatase phase, and then to rutile phase after annealing in air, similar to the sample with active carbon addition, but the phase transformation

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(g)

(e)

(d)

30

35

40

45

50

55

60

65

1

Percentage of rutile / %

0.9 0.8 S-0-X S-1-X

0.5 0.4 0.3 0.1 0 300

400

500

600

700

800

900

Temp / ºC Fig. 2. Dependences of proportion of rutile crystal phase in the S-1-X samples and S0-X samples on the annealing temperature.

Endotherm

100

260ºC DTA

10

90

425ºC

0 70ºC TGA

80

-20 -30

Mass loss / %

Exotherm

30

70

-40 0

(c)

-30

80

TGA

-50

(a)

(b)

70 60

200

400

600

800

water in the sample (Fig. 3). The exothermal peaks around 260 8C and 425 8C, each with a weight loss, correspond to the burning of organic compounds which are remained in the sample after the hydrolysis reaction of titanium tetra-isopropoxide. The XRD result suggests that the decomposition of the organic compound accompanies transformation of the amorphous phase to anatase phase. In the TG-DTA curves of the precursor sample (S-1-25) with active carbon (Fig. 4), the endothermic peak around 70 8C and the exothermal peak around 250 8C, each with a weight loss, were observed similar to the reference sample. These peaks can be attributed to the evaporation of the water and the burning of organic compounds similar to the case of the reference sample. The exothermal peak around 465 8C with a weight loss corresponds to the burning of the active carbon in the sample. The XRD results revealed that the phase transformation from anatase to rutile also occurred around this temperature (Fig. 1). It is interesting that the pure active carbon shows the exothermal peak of its burning around 560 8C, which is higher than the peak to the active carbon burning in the precursor (Fig. 4), suggesting that the TiO2 in the precursor accelerates the oxidation reaction of the active carbon. The TG-DTA results suggest that the phase transformation from anatase to rutile occurs mainly after burning the active carbon. 3.2. Visible-light absorption properties

0.2

-10

70ºC

70

temperature from anatase to rutile was different and the color of the products were white. The phase transformation temperature from anatase to rutile for the precursor with active carbon addition is higher than that without active carbon, as shown in Fig. 2. The fraction of rutile phase in the samples was evaluated from the XRD data using an empirical equation reported in literature [20]. The phase transformation reactions were investigated by TGDTA analysis. In the TG-DTA curves of the precursor of reference sample (S-0-25) without active carbon, the endothermic peak with a weight loss around 70 8C can be ascribed to the evaporation of the

20

90

Fig. 4. DTA-TG curves of (a, b) S-1-25 sample with active carbon and DTA curve of (c) active carbon.

Fig. 1. XRD patterns of S-1-X samples prepared at (a) 25, (b) 300, (c) 400, (d) 500, (e) 600, (f) 700, and (g) 900 8C, respectively. *: anatase phase, &: rutile phase.

0.6

465oC

Temp / ºC

2θ /º

0.7

10

0

(c)

25

100

-70

(b) (a) 20

DTA

DTA

-10

Exotherm

Intensity (a.u)

(f)

560ºC

250ºC

30

Mass loss / %

Endotherm

50

200

400

600

800

Temp / ºC Fig. 3. DTA-TG curves of the S-0-25 reference sample without active carbon.

After the burning of the active carbon and the phase transformation from anatase to rutile above 500 8C, the sample changed it color from grey to yellow. Since it has reported that Cdoped TiO2 shows visible light absorbance around 500 nm [13], analyses of carbon content in the samples were carried using EDS and HRXRF. The EDS analysis indicated that the sample prepared at 300 8C contains carbon component, and no carbon component is observed in the samples prepared above 500 8C (see Fig. S1). This result is agreement with the result that the C-doped TiO2 prepared by annealing amorphous TiO2 obtained by sol–gel process loses carbon when it is heated up to 400 8C in air [21,22]. The HRXRF analysis also suggested no carbon component in the sample prepared at 500 8C (see Fig. S2). On the basis of these results, the yellow color of the samples prepared above 500 8C can be ascribed to the formation of Ti(III) from Ti(IV) by reducing reaction of the active carbon. The visible-light absorption characters of the sample prepared by the active carbon reducing process described above were investigated by UV–Vis spectroscopy. The UV–Vis absorption spectra of S-1-X samples prepared above 500 8C are shown in Fig. 5, where all active carbon was burnt out in this temperature range. For comparison, the spectra of commercial anatase and rutile TiO2 samples are shown also in the figure. The commercial anatase and

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1.6

2

1.4

Absorption intensity

Absorption Intensity

1.8 Anatase

1.6 1.4 1.2

S-1-900

1 0.8

S-1-600

0.6

S-1-500

0.4

Rutile

1

Ti/C 1:10

0.8 0.6

1:4

0.4

1:1

0.2

0.2 0 200

1.2

2:1 300

400

500

600

700

800

Wave length / nm

0 200

400

600

800

Wave length / nm

Fig. 5. Absorption spectra of the S-1-X samples and commercial anatase and rutile samples.

Fig. 7. Absorption spectra of the samples synthesized at different Ti/C mole ratios.

rutile samples show absorption edges at 390 and 410 nm, respectively. The samples prepared by the active carbon reducing process show visible-light absorption with an absorption edge around 570 nm that is different to 535 nm for the C-doped TiO2 [13]. The visible-light absorption intensity increases with increasing reaction temperature, as shown in Fig. 6. Especially, the absorption intensity increases dramatically when the annealing temperature increased from 500 to 600 8C. The visible-light absorption intensity increases also with increasing active carbon content in the precursor (Fig. 7). The spectra of the samples prepared above 600 8C are similar to that prepared by the H2 reducing process [15]. The results described above reveal that the appearance of visible-light absorption can be attributed to the reducing reaction of Ti(IV) to Ti(III). The reducing reaction causes the formation a nonstoichiometric TiO2 x with oxygen vacancy in the crystal structure. The chemical formula of the nonstoichiometric TiO2 x can be written as Ti(IV)1 xTi(III)xO(2 x/2)&x/2, where & is oxygen vacancy. The formation of oxygen vacancy is due to the upkeep of charge balance in the crystal after the reduction of Ti(IV) to Ti(III).

tivity is due to high concentration of the conducting electron in S1-600 sample. The Ti(III) ion formed by the reducing reaction corresponds to Ti(IV) ion plus free electron, and the free electron can act as charge carrier in the crystal. Therefore, the increase of Ti(III) ion concentration causes the increase of the charge carrier concentration. The particle morphology of the products was investigated by FE-SEM. In the S-0-25 sample without active carbon, aggregate of amorphous TiO2 nanoparticles is observed in Fig. 8(a). The active carbon has a large particle size with a smooth surface (Fig. 8(b)). In the S-1-25 sample, the surface of active carbon particles was coated by the nanoparticles of amorphous TiO2, forming a nanocomposite of the active carbon and TiO2. After annealing at 400 8C, the amorphous nanoparticles of TiO2 were transformed to anatase nanoparticles with size of about 20 nm (Fig. 8(d) and (e)). A porous structure was formed by connecting the anatase nanoparticles together. With increasing the annealing temperature, the particle size of TiO2 x grew up to about 40 nm at 500 8C (Fig. 8(f)) and to 500 nm at 800 8C. The S-1-400 and S-0-400 samples presented very high BET special surface area, and the surface area decreased down dramatically in a temperature range of 400–500 8C (Table 1), which corresponds to the results of SEM study, meaning that the crystal size increases quickly and the porous structure disappears in the temperature range. The decrease rate of surface area with annealing temperature increase for S-1-X samples was slower than that for S0-X samples, suggesting the active carbon in the precursor inhibits the crystal growth, which will be discussed in the next section.

3.3. Physical properties of TiO2

x

The reduction of Ti(IV) to Ti(III) in the synthesis process can be confirmed by electrical conductivity measurement [14]. The impedance measurement results (see Fig. S3) indicated that the S-1-600 sample has a much larger conductivity of 7.2  10 5 (V m) 1 than a commercial rutile TiO2 powder sample (1.7  10 6 (V m) 1) without oxygen vacancy. The large conduc-

3.4. Formation mechanism of TiO2 process

0.6

x

in the active carbon reducing

Absorption intensity

S-1-X 0.5

S-1-500-X

0.4 0.3 0.2 0.1 0 400

500

600

700

800

900

1000

Annealing temperature / ºC Fig. 6. Change of absorption intensity of visible-light at 450 nm with change of annealing temperature and C/Ti mole ratio. ~: S-1-X samples, &: S-1-500-X samples prepared by annealing S-1-500 (two-step process).

In the formation process of the TiO2 x with oxygen vacancy, there are two phase transformations. One is transformation from amorphous phase to anatase phase around 300 8C. Since the active carbon is stable around this temperature, it will not take part in the reaction (Fig. 4). Therefore, the transformation reaction is similar to the normal transformation reaction from the amorphous TiO2 phase prepared by sol–gel process to the anatase phase. It has reported that the C-doped anatase phase can be formed by annealing the amorphous TiO2 prepared by sol–gel process [21,22], suggesting that the C-doping reaction occurs in the transformation process. The C-doped anatase phase, however, loses the carbon component after heating-up to 400 8C in air, indicating that the Cdoped anatase phase is unstable at high temperature in air. Another phase transformation reaction is from anatase phase to rutile phase around 500 8C, and the active carbon affects the transformation reaction. The results of Fig. 2 reveal that the active

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Fig. 8. FE-SEM images of (a) S-0-25 precursor, (b) active carbon, (c) S-1-25 precursor, (d, e) S-1-400, and (f) S-1-500.

carbon in the precursor hindered the transformation from anatase phase to rutile phase. The retard of anatase-to-rutile transformation has been observed also in a mixture of anatase and SiO2 or Al2O3 [20,23–25]. This fact can be explained by the phase transformation mechanism of anatase-to-rutile. The transformation rate is determined by the rate of nucleation of rutile, because the crystal growth rate of rutile is very quickly [26,27]. There are three types of nucleation of rutile on anatase particles, interface nucleation, surface nucleation, and bulk nucleation, which occurs at the contact part of anatase particles, surface of anatase particles, and bulk of anatase crystal, respectively, as shown in Fig. 9(a) [25]. The activation energy of the nucleation increases in the order of Table 1 The BET specific surface area (m2/g) for S-1-X and S-0-X samples. Annealing temperature (8C)

S-1-X S-0-X

400

500

600

700

800

900

80 85

16 12

9 6

5 4

5 –

3 –

interface nucleation < surface nucleation < bulk nucleation, meaning that the transformation is dominated by interface nucleation at low temperature such as 500 8C, by surface nucleation at intermediate temperature such as 700 8C, and by bulk nucleation at very high temperature such as 1000 8C. The active carbon addition would decrease the interface of contacting anatase particles and free surface of anatase particles, which decreases the interface nucleation and surface nucleation. We think that the anatase-to-rutile transformation in the present study is dominated by the interface nucleation, because the transformation reaction occurs at relatively low temperature. On the basis of the results above, we give a reaction mechanism for the formation of TiO2 x with oxygen vacancy in the active carbon reducing process, as shown in Fig. 9(b). The precursor transforms from amorphous structure to anatase TiO2 after annealing up to 300 8C. In this temperature range, the active carbon is stable and coated by TiO2 nanoparticles (Fig. 8(c)). In the temperature range of 400–500 8C, the anatase TiO2 is reduced to TiO2 x by the active carbon, accompanying consumption of the active carbon and the interface nucleation of rutile (Fig. 8(e) and

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5 S-1-600

Current per m2 / mA

4 3 Light on

2

Light off

1 0 ST-01

1 0 --300 - 200 -100 0

100 200 300 400 500 600 700 800 900 1000

Potential vs SCE / V Fig. 10. Current–voltage curves of S-1-600 film sample with oxygen vacancy and ST01 film sample without oxygen vacancy in aqueous solution of 0.1 mol/L of sodium sulfate under intermittent irradiation with Xe lamp (400–700 nm).

(f)). The reducing reaction causes formation Ti(III) and oxygen vacancy on the particle surface of anatase phase. At above 500 8C, the rutile nuclei grow and the anatase particles are transformed to rutile phase, which accompany diffusion of the oxygen vacancy from the particle surface to the bulk. The results of Fig. 6 revealed that the visible-light absorption intensity of S-1-X samples increased dramatically in temperature range of 500–600 8C. Since all active carbon was consumed in the temperature range of 400–500 8C (Fig. 4), we think that the increase of the visible-light absorption intensity in the temperature range above 500 8C is due to the diffusion of oxygen vacancy into the crystal bulk, but not due to the reducing reaction. To confirm this fact, we prepared TiO2 x by two-step process. In the first step, the S1-500 sample was prepared by same method described above. In the second step, the S-1-500 sample was annealed again at higher temperature than 500 8C. The absorption spectrum results reveal that the visible-light absorption intensity increases with increasing the annealing temperature in the second step similar to the case of one-step process as described above (Fig. 6), although no residual active carbon in the S-1-500 sample. We think that the increase of the visible-light absorption intensity can be attributed to the formation of TiO2 x rutile phase with uniform distribution of the oxygen vacancy in the crystal bulk by diffusing the oxygen vacancy from the crystal surface to it bulk (Fig. 9(b)), but not to the concentration increase of the oxygen vacancy, because the temperature increase will cause the concentration decrease of oxygen vacancy. The uniform distribution of the oxygen vacancy can enhance the efficiency of visible light adsorption. For the formation of TiO2 x with the uniform distribution of oxygen vacancy, the high temperature annealing is necessary, because activation energy of the diffusion of oxygen vacancy is large. 3.5. Photocatalytic activity of TiO2

101% Blank

100% 99%

S-1-500

98% 97%

C/C0

Fig. 9. Schematic illustrations of (a) different types of nucleation in phase transformation from anatase to rutile, and (b) formation mechanism of TiO2 x with oxygen vacancy in carbon reducing process.

samples show visible-light absorption. The visible-light sensitive photocatalytic activity of the TiO2 x samples can be demonstrated by the results of photoelectrochemical study. Fig. 10 shows typical current–voltage curves obtained by irradiating intermittently a TiO2 x film electrode with oxygen vacancy and a TiO2 film electrode without oxygen vacancy as a reference, respectively, with a Xe lamp (l = 400–700 nm). The TiO2 x film electrode and the TiO2 film electrode were prepared using S-1-600 sample and Ishihara ST-01 sample (anatase phase), respectively. When the potential was scanned from 0.3 to 1.0 V (vs. SCE), which was below the water decomposition potential (1.0 V O2/H2O vs. SCE), a large photocurrent was observed for the TiO2 x film electrode by the visible-light irradiation, and no photocurrent was observed without visible-light irradiation. For the TiO2 film electrode, however, only a small photocurrent was observed. This means the TiO2 x sample can be well sensitized by visible-light. The hydrogen gas evolution at the Pt counter electrode and oxygen gas evolution at the TiO2 x film working electrode were observed, accompanying the photocurrent, suggesting the decomposition of water in the photoelectrochemical reaction. Fig. 11 shows the results of the photocatalytic degradation of methylene blue (MB) under visible-light irradiating conditions. Ishihara ST-01 was used as the standard sample for the comparison. The photocatalytic activity increases in the order of

96%

S-1-900 S-1-800 S-1-700

95% 94% 93%

ST-01

92% 91%

S-1-600

90% 89% 0

x

We investigated visible-light sensitive photocatalytic activity of the TiO2 x samples prepared in the present study, because these

100

200

300

400

Time / minutes Fig. 11. Photocatalytic degradation of methylene blue by S-1-X samples and ST-01 standard sample under fluorescent light irradiating conditions.

S. Yang et al. / Materials Research Bulletin 46 (2011) 531–537

Degradation amount μmol/m2

6

the crystal bulk causes the increase of the visible-light absorption intensity. The TiO2 x samples with oxygen vacancy show high surface visible-light sensitive photocatalytic activity, and can be utilized as a new type of visible-light sensitive photocatalyst.

5 S-1-900

4

S-1-700 S-1-600

3

S-1-800

2

5. Acknowledgment

1 ST-01

0 -1

537

0

This work was supported in part by Grants-in-Aid for Scientific Research (B) (No. 17310074) from Japan Society for the Promotion of Science.

S-1-500

0.1

0.2

0.3

0.4

0.5

0.6

Absorption intensity at 450nm Fig. 12. Dependence of MB degradation amount of per surface area after fluorescent light irradiating for 5 h on the visible-light absorption intensity at 450 nm of the S1-X and ST-01 samples.

S-1-500 < S-1-900 < S-1-800 < S-1-700 < S-1-600 for the S-1-X samples, which corresponds to the increasing order of their BET surface area (Table 1), except S-1-500 sample that presents the lowest visible-light sensitive photocatalytic activity. The S-1-600 presents the higher photocatalytic activity than that of ST-01, and other samples show the lower photocatalytic activity than that of ST-01, because ST-01 has very large BET surface area (349 m2/g). Since the photocatalytic reaction occurs on the photocatalyst surface, the photocatalytic activity on the crystal surface is very important to understand the properties of the material. To characterize the photocatalytic activity on the crystal surface, we estimated the MB degradation amount by per BET surface area of the powder sample, and plot it as a function of the visible-light absorption intensity at 450 nm, as shown in Fig. 12. The visiblelight sensitive photocatalytic activity of per surface area increased almost linearly with increasing the visible-light absorption intensity. This result reveals that the visible-light sensitive photocatalytic activity can be attributed to the visible-light absorption that results from formation of the oxygen vacancy in the crystal. 4. Conclusion The active carbon can be used as the reducer for the synthesis of TiO2 x with oxygen vacancy. The TiO2 x samples prepared by the active carbon reducing process show the visible-light absorption with an absorption edge around 570 nm. The oxygen vacancy is introduced into the rutile structure by the reducing reaction accompanying the phase transformation from anatase to rutile. The diffusion of the oxygen vacancy from the crystal surface into

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