Visible-light-responsive nano-TiO2 with mixed crystal ... - CiteSeerX

INSTITUTE OF PHYSICS PUBLISHING

NANOTECHNOLOGY

Nanotechnology 17 (2006) 2490–2497

doi:10.1088/0957-4484/17/10/009

Visible-light-responsive nano-TiO2 with mixed crystal lattice and its photocatalytic activity Yao-Hsuan Tseng1,5 , Chien-Sheng Kuo2 , Chia-Hung Huang1 , Yuan-Yao Li2 , Po-Wen Chou3 , Chia-Liang Cheng3 and Ming-Show Wong4 1

Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu, 310, Taiwan 2 Department of Chemical Engineering, National Chung-Cheng University, Chia-Yi, 621, Taiwan 3 Department of Physics, National Dong-Hwa University, Hua-Lien, 974, Taiwan 4 Department of Materials Science and Engineering, National Dong-Hwa University, Hua-Lien, 974, Taiwan E-mail: [email protected]

Received 18 January 2006, in final form 20 March 2006 Published 24 April 2006 Online at stacks.iop.org/Nano/17/2490 Abstract Ultraviolet- and visible-light-responsive titania-based photocatalysts were synthesized and employed in the photocatalytic oxidation of NOx . Sol–gel processes using tetrabutyl orthotitanate and ethanol under acid catalyzed condition and controlled calcination were performed to synthesize titanium dioxide with a mixed crystal lattice of anatase, brookite and rutile phases. The TiO2 prepared under calcination at 200 ◦ C exhibited high photocatalytic activity for degradation of NOx under both ultraviolet (UV) and visible-light illumination. The experimental results showed that up to 70% removal of NOx could be obtained in a continuous flow type reaction system under irradiation with visible light. The calcination temperature has an important influence on the particle size and lattice structure of TiO2 . It is also found that the peculiar mixed-phase structure of TiO2 , evidenced from Raman, x-ray diffractometry (XRD), and UV–vis spectroscopy, was inferred to be an important factor for visible-light absorption and NOx removal activity under a wide range of visible-light illumination. (Some figures in this article are in colour only in the electronic version)

1. Introduction Titanium dioxide has been widely studied and applied to various fields, such as air purification (DeNOx , DeVOCs , and deodor), water treatment (decolorization), self-cleaning, superhydrophilicity, antifogging, antibacterial, etc [1–4]. With a band gap of 3.2 eV, the nano-sized anatase-type TiO2 absorbs photons in the UV range with wavelengths less than 388 nm [4] and evolves active oxygen species, such as OH radicals and O·− 2 ions, by reacting with water and oxygen adsorbed in the 5 Author to whom any correspondence should be addressed.

0957-4484/06/102490+08$30.00

surface of TiO2 . Despite the promising properties, application is now limited, for the UV region occupies only near 4% of the entire solar spectrum, while 45% of the energy belongs to visible light; most importantly, only visible light is available in the indoor environment. More practical applications can be achieved if the photocatalytic active region can be expanded to the visible-light region (400–700 nm); the photoenergy can be used more efficiently. As reported in the literature, sputtering [5], plasma [6], and ion implantation [7, 8] are usually used to dope trace impurities (Cr, W, N, V) in TiO2 for visible-light activity. In addition, other materials, such as CdS, TaN5 , TaON,

© 2006 IOP Publishing Ltd Printed in the UK

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Visible-light-responsive nano-TiO2 with mixed crystal lattice and its photocatalytic activity

2. Experimental details 2.1. Materials All reagents used in this study are grade (GR) chemicals, including tetrabutyl orthotitanate (TBOT), ethanol, nitric acid, nitric monoxide, and other reagents for synthesis and air purification experiments. Three kinds of LED device (blue, green, and red) and one black lamp were used to provide visible light and UV light, respectively. The main LED and UV peaks are located at 465 nm, 515 nm, 640 nm and 365 nm, respectively. The LED and UV photon energy distribution profiles are illustrated in figure 1. 2.2. Procedures 2.2.1. Synthesis of the visible-light-responsive TiO2 . The measured quality of tetrabutyl orthotitanate (50 mmol) fell slowly in 90 ml of anhydrate ethanol and 20 ml of deionized (D.I.) water, contained in a 250 ml flask. After complete dissolution, 4 ml of nitric acid was added to catalyze the hydrolysis and condensation reactions. The mixed solution was uniformly agitated at 500 rpm for 3 h, and then the precipitate of titanium hydroxide was produced. After drying at 110 ◦ C, the dried powder was calcined in air at controlled temperatures for 5 h. In this work, seven kinds of photocatalyst were synthesized with different calcination temperatures via the sol–gel process. Sample nomenclature was defined as follows: TiO2 X ◦ C—photocatalyst prepared by the sol–gel process with calcination at X ◦ C for 5 h, with temperatures of 150, 200, 250, 300, 400, 500, and 600 ◦ C, respectively. All synthesized products were identified using xray diffractometry (XRD) with Cu Kα radiation (Scintag XRD3000), micro-Raman spectrometry (Renishaw 1000B), scanning electron microscopy (SEM, Hitachi 4800I), powder

RLED UV

BLED

GLED

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and InVO4 [9, 10], are also used as visible-light-responsive photocatalysts. In the field of photocatalytic water splitting, these methods and materials have usually been studied and developed. However, for environmental purification, the cost of a photocatalyst must be inexpensive for any practical application. As a result, the feasibility of these methods and materials is low for industrial-scale production due to high price, environment pollution, and instability. In this work, an easier and low-cost method for preparing high-efficiency visible-light-responsive TiO2 is developed using the sol–gel method. The sol–gel method is widely used to prepare nano-sized metal oxide particles and films. Normally, the operation temperature of calcination is higher than 400 ◦ C for the preparation of anatase-type TiO2 . Some chemists found that the photoactivity of TiO2 is decreased by heat treatment over 600 ◦ C [11]. However, few studies on the photocatalysis property have been reported for TiO2 prepared at low calcination temperature. In this paper, we have developed a new preparation method of mixed crystal lattice photocatalyst using tetrabutyl orthotitanate as a precursor. The activities of the synthesized photocatalysts were evaluated upon the oxidation of NOx under illumination by visible light. The characteristics of synthesized photocatalysts were also investigated for a satisfactory explanation of visible-light activity.

300

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Figure 1. Photon energy distribution profiles of UV lamp and LEDs.

UV–visible spectrophotometry (Shimadzu UV-2450), and xray spectroscopy (XPS, VG ESCA Scientific Theta Probe) to establish the particle size, crystallinity, UV–visible absorption spectrum, and surface composition, respectively. 2.2.2. Oxidation of NOx with TiO2 . The degradation of NOx was performed in a continuous flow system, as depicted in figure 2. A round-shaped pyrex glass vessel was used to conduct the degradation of NOx . A sample dish was located inside the vessel containing the TiO2 powder for the experiment. A mini fan was attached inside the glass vessel to improve the mixing effect of the reaction mixture. Prior to being placed in the vessel, the TiO2 sample was pretreated by irradiation with UV light (of 1 mW cm−2 intensity) for 10 h, followed by rinsing with 200 ml of D.I. water to remove the possible contaminants from the sample surface. The light source was provided using an LED or black lamp, with an intensity of 1 mW cm−2 . The spectrum of the light source was measured using a spectrophotometer (Ocean Optics USB2000), on the basis of which the intensity could be calculated. The NOx degradation was carried out at room temperature using an air stream containing 1.0 ppm NO as feedstock. The NO gas was provided from a cylinder containing 100 ppm NO (N2 balance, from San Fu Chemical Co.) and diluted by a separate air stream. Two mass flow controllers (MFCs, Brooks 5850E) were used to simulate the relative humidity of atmosphere in the feeding stream. An air stream controlled by MFC #2 brought in the saturated water vapour from a homemade humidifier and mixed it with a separate air stream via MFC #1 to adjust the relative humidity. The humidified air stream was then mixed with the stream containing NO (via MFC #3) to form the feeding stream. The reaction gas in the feeding stream passed through the vessel containing TiO2 powder (0.5 g) at a flow rate of 1 l min−1 . For gas phase analysis, the NO and NO2 concentrations were continuously monitored by an on-line chemiluminescent NOx analyzer (Eco Physics CLD 700 AL). Furthermore, the regeneration of TiO2 photocatalyst was performed after degradation of NOx for 5 h under illumination by visible light. The used powder was moved out and rinsed 2491

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#1 MFC Ventt

Air cylinder

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Pyrex glass vessel ( Φ :14cm, H:3.6cm )

Humidifier NOx analyser analyzer

Mixer NO/N2 cylinder

#3 MFC

Mixing fan n

Sample dish ( Φ :9cm, H:0.5cm )

Figure 2. Schematic of continuous flow system for photocalaytic removal of NOx .

3. Results and discussion 3.1. Photocatalytic activity of prepared TiO2

O2 + e− → O− 2

(1)

OH− + h+ → OH·

(2)

H +

O− 2

→

HO·2 .

(3)

The nitric monoxide is oxidized to nitric acid or nitrous acid by active oxygen species. Based on the gas-phase chemistry concerning NOx [18], it can be expected that NO is converted to HNO3 as a consecutive photooxidation via an intermediate of NO2 : (4) NO + HO·2 → NO2 + OH· NO2 + OH· → HNO3 ·

NO + OH → HNO2 .

(5) (6)

Finally, the nitric acid forms at the catalyst surface. The activity of photocatalyst diminishes with the accumulation of 6 JIS R 1701-1 2003 air purification test procedure for photocatalytic materials, Japan Standards Association.

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In the photocatalytic application of TiO2 , the oxidation of NO is widely used for the examination of photocatalytic reactivity [2]6 and [12–14]. The electron–hole pair (e− – h+ ) generated upon light excitation is trapped at the surface of TiO2 as spatially separated redox active sites. On the surface of TiO2 irradiated with UV light, the formation of some kinds of reactive oxygen species, such as superoxide − ions (O·− 2 ), atomic oxygen (O), O , OH and HO2 radicals, has been reported [15–17]. The general mechanism of NOx oxidation by photocatalyst is shown in equations (1)–(6) as follows [2, 12–14]. The hydrogen ions (H+ ) and hydroxide ions (OH− ) are dissociated from water. The active oxygen species are produced on the TiO2 surface:

+

1.2

[ NOx ] ( ppm )

with 100 ml of D.I. water for 1 h to remove the adsorbed anions from the surface of TiO2 . The specimen was then dried at 70 ◦ C for 1 h to restore the active sites. The procedure was repeated several times. Comparing the results obtained during the operation cycle of degradation and regeneration, the longterm stability of TiO2 photocatalyst could be examined.

NO

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Figure 3. Reaction profile of NO over the TiO2 200 ◦ C photocatalyst during 2 h on stream: catalyst loading, 0.5 g; intensity of irradiation, 1 mW cm−2 ; relative humidity, 55%; inlet concentration of NO, 1 ppm; inlet flow rate, 1 l min−1 ; reaction temperature, 27 ◦ C.

acid. To illustrate the reaction behaviour of photocatalytic oxidation of NO, 0.5 g of TiO2 200 ◦ C and a light intensity of 1 mW cm−2 were used to conduct the experiment. The oxidation of NOx under UV and visible light was performed in a continuous flow system as shown in figure 3. At the beginning, 1 ppm of NO gas stream was introduced into the reactor under dark condition. The adsorption of NO on TiO2 was completed in a few minutes, and then the NO concentration was recovered to 1 ppm. The steady state of this photocatalyzed reaction was achieved as soon as the photocatalyst was illuminated by a red LED. The NO concentration decreased from 1 ppm to 0.21 ppm, and the NO2 concentration increased to 0.21 ppm. The NOx concentration, which is the sum of the NO and NO2 concentrations, was maintained at 0.42 ppm during half an hour of operation under red LED illumination. This indicated that the activity of TiO2 200 ◦ C was not decreased with time on stream. Theoretically, the NO oxidation activity of TiO2 will gradually decrease with time due to the surface of TiO2 being covered by nitric acid [2, 12, 13]. The explanation for the long-term activity of TiO2 200 ◦ C is attributed to the large surface area of 145 m2 g−1 . When the illumination was changed to a green LED, the concentrations of NO, NO2 , and NOx decreased

NO & NOx removal and NO2 generation ( µ mol/h )


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Figure 4. The dependence of NOx removal percentage on calcination temperature under LED illumination: catalyst loading, 0.5 g; intensity of irradiation, 1 mW cm−2 ; relative humidity, 55%; inlet concentration of NO, 1 ppm; inlet flow rate, 1 l min−1 ; reaction temperature, 27 ◦ C; operation time, 0.5 h.

Figure 5. The influence of regeneration cycle time on the removal rate of NOx under red LED irradiation: catalyst loading, 0.5 g; intensity of irradiation, 1 mW cm−2 ; relative humidity, 55%; inlet concentration of NO, 1 ppm; inlet flow rate, 1 l min−1 ; reaction temperature, 27 ◦ C; operation time, 5 h.

Table 1. The conversion and selectivity of NO and NO2 with TiO2 200 ◦ C under different light sources: catalyst loading, 0.5 g; intensity of irradiation, 1 mW cm−2 ; relative humidity, 55%; inlet concentration of NO, 1 ppm; inlet flow rate, 1 l min−1 ; reaction temperature, 27 ◦ C.

in the order of: TiO2 200 ◦ C > TiO2 250 ◦ C > TiO2 150 ◦ C > TiO2 300 ◦ C > TiO2 400 ◦ C > TiO2 500 ◦ C > TiO2 600 ◦ C > UV100. The order of activity is the same under different visible-light sources. The visible-light activity of prepared TiO2 sharply decreased under calcination temperatures higher than 300 ◦ C. The optimal temperature for visible-light activity is about 200 ◦ C in this experiment. The prepared photocatalysts of high visible-light response, TiO2 200 ◦ C, TiO2 250 ◦ C, and TiO2 150 ◦ C, are all calcinated at low temperature, and the photoactivity under UV, blue, green LEDs are not very different. This synthesis method is a quite energy-saving process for producing visible-light-responsive TiO2 , especially compared with ion impregnation and plasma treatment. According to the reaction scheme of the photo-oxidation of NO, the photocatalyst would be deactivated after longterm operation due to the coverage of active sites by adsorbed NO− 3 ions at the catalyst surface. However, the adsorbed NO− 3 ions could be easily rinsed out with D.I. water, enabling regeneration of the catalyst. Figure 5 indicates the removal rates of NOx during the degradation and regeneration operation cycle. The experiments were performed with TiO2 200 ◦ C under red LED irradiation. The experimental results showed that over 95% of the photocatalytic activity could be recovered after three degradation and regeneration cycles. This means that the prepared TiO2 could provide very good stability for NOx degradation. As a result, good stability, easy regeneration and high reactivity will provide a great opportunity for air purification by using titania-based photocatalysts in operation in both the UV and visible-light region.

Light source Conversion and selectivity

UV (%)

BLED (%)

GLED (%)

RLED (%)

A = Conversion of NO B = Selectivity of NO2 B/A = Ratio of the oxidized NO to NO2

88.3 11.4 12.9

88.2 16.2 18.4

86.2 18.1 21.0

78.3 22.0 28.1

to 0.13 ppm, 0.17 ppm, and 0.30 ppm, respectively. The conversion and selectivity of NO and NO2 with TiO2 200 ◦ C under different irradiations are listed in table 1. The amount of NOx removed decreased with the increase in the wavelength of illumination, and, on the contrary, the amount of NO2 generated was increased by increasing the wavelength of illumination. When comparing the reaction behaviour under UV and blue LED illumination, no difference in NO conversion was observed, and the NO2 concentration under the blue LED was higher than under UV. There are two reasons for this phenomenon. First, the number of induced active sites on the TiO2 surface decreases with the increase in the wavelength of illumination. Second, the NO2 is more difficult to oxidize than NO, so the decrease in reaction rate under visible light is mainly caused by the decrease in the oxidation rate of NO2 . The relation between NOx removal percentage and calcination temperature is displayed in figure 4. NOx removal is calculated by dividing the total concentration of effluent NOx (unchanged and formed NO2 ) to the concentration of influent NO. The commercial TiO2 powder UV100 (Sachtleben Co., Germany) possessed very little visible-light reactivity. The TiO2 200 ◦ C showed higher NOx removal activity than the other photocatalyts under visible-light irradiation. The NOx removal reaction rates of TiO2 200 ◦ C under UV and a red LED are 3.55 and 1.44 µmol h−1 g−1 -TiO2 , respectively. The activity of these photocatalysts under visible-light irradiation is

3.2. Characterization of prepared TiO2 3.2.1. SEM. Theoretically speaking, the photocatalytic activity is affected by the particle size of TiO2 , and 7 nm is the most reactive particle size, as reported in both Anpo’s and Yue’s papers [19, 20]. The observation of TiO2 morphology using SEM is shown in figure 6. The result reveals that the particle size was increased by increasing the 2493

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Figure 7. XRD patterns of various photocatalysts.

(g)

Table 2. Crystallite size of prepared samples. Sample

(d) 200nm

TiO2 TiO2 TiO2 TiO2 TiO2 TiO2

150 ◦ C 200 ◦ C 250 ◦ C 300 ◦ C 400 ◦ C 500 ◦ C

TiO2 600 ◦ C

Figure 6. SEM photograph of various photocatalysts: (a) TiO2 150 ◦ C, (b) TiO2 200 ◦ C, (c) TiO2 250 ◦ C, (d) TiO2 300 ◦ C, (e) TiO2 400 ◦ C, (f) TiO2 500 ◦ C, and (g) TiO2 600 ◦ C.

calcination temperature. The particle sizes for TiO2 150 ◦ C to TiO2 500 ◦ C are all smaller than 30 nm, and the size for TiO2 600 ◦ C is sharply increased to several hundreds of nanometres. The aggregation of particles, the transformation of crystallinity, and the crystal growth are all enhanced by severe heat treatment. The activity of TiO2 600 ◦ C was quite low as a result of small surface area and the existence of a single rutile phase. 3.2.2. XRD. Figure 7 illustrates the XRD patterns of the samples with different calcination temperatures. The crystal structures of TiO2 samples calcined under 500 ◦ C are mainly anatase (2θ = 25.4◦ , {101}), and small peaks of brookite (2θ = 30.8◦ , {121}) were also found. The existence of the brookite phase is more obvious in the micro-Raman analysis. The rutile phase began to appear at 500 ◦ C, and the crystal structure was completely transformed to rutile (2θ = 27.4◦ , {110}) at 600 ◦ C. The XRD results exhibit two peculiar phenomena. First, the coexistence of anatase and brookite phases was obtained with calcination at a low temperature. The crystallite defect of the mixed structure should be more obvious than pure anatase or rutile structure. Visible-light absorption is increased with by defects and impurities (e.g., nitrogen, sulfur, and carbon) in the TiO2 lattice [5, 13, 21, 22]. Hashimoto 2494

Crystallite sizea (nm) 4.1 5.5 6.3 6.3 9.2 16.5 32.8b 50.1b

a Determined using Scherrer’s equation(applicable from 3–200 nm). b Rutile lattice.

and coworkers also reported that the TiO2 film with brookite and anatase shows better photocatalytic hydrophilicity than the pure antase TiO2 film phase [23]. Second, the rutile phase appears at 500 ◦ C. Porter et al reported that the rutile phase of TiO2 was formed over 600 ◦ C, and completely transformed to rutile phase at 800 ◦ C [11]. The result showed that the synthesized TiO2 in this process is unstable due to the existence of carbon in the crystal. The carbon source might be introduced from the alkoxide group and improves the transformation of crystallinity [22]. Furthermore, the crystallite size of the samples was determined from the half-width of peaks by using Scherrer’s formula (d = 0.9λ/β cos θ ), as shown in table 2. The crystallite sizes of anatase and rutile phases were increased by the increase in calcination temperature, and the result is consistent with the SEM observations. 3.2.3. Raman. Raman spectroscopy can be used properly to examine the surface structure of TiO2 samples. Iida and Li both claim that this method is more sensitive for nanometresized crystals than x-rays [24, 25]. In this work, a lower laser power (30 mW) was used when taking the spectra to avoid laser heating or laser damage effects. The Raman spectra of TiO2 samples are displayed in figure 8. The mixed structure of brookite (Raman shift = 322, 360, and 480 cm−1 [26]) and anatase (Raman shift = 390, 518, and 638 cm−1 ) were detected for TiO2 150 ◦ C to TiO2 500 ◦ C, and the brookite phase was observed more obviously in the


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the surface of TiO2 particles. The peculiar entangled structure of the TiO2 surface probably results in a potential gradient at the interface, which facilitates the separation of holes and electrons. It may enhance the absorption of visible light and reduce the energy gap between the conduction band and the valence band. Hsu and Wu reported the similar phenomenon that the ZnO/ZnO2 composite photocatalyst shows superior photoactivity compared with either of its constituents [27]. However, the distribution of the rutile phase of TiO2 200 ◦ C and TiO2 250 ◦ C samples was only on the surface, so the same observation was not found with XRD. A reasonable explanation for this phenomenon is local oxidation by the residual nitric acid on the surface. In the sol–gel process, the nitric acid was added for hydrolysis and condensation reactions. The adsorption of nitric acid on the dried powder (heated at 110 ◦ C) and the TiO2 150 ◦ C samples was measured with ionic chromatography, so the samples for photoreaction must be pretreated. The boiling point of nitric acid is about 120 ◦ C, so the nitric acid evaporated from the TiO2 surface within calcination. The structure of prepared TiO2 is not very stable, so the partial surface of TiO2 is probably transformed to the rutile phase by the vaporization of nitric acid, which was highly oxidative. However, the same entangled structure was not found in Raman analysis for the other samples (TiO2 150 ◦ C and TiO2 300 ◦ C–600 ◦ C). The nitric acid is strongly adsorbed on the TiO2 surface at 150 ◦ C, so local server oxidation might not be carried out. At high temperature, the nitric acid would be evaporated too quickly from TiO2 to form the rutile phase. In addition, UV100 powder was immersed in nitric acid and heated at 200 ◦ C. However, the same entangled structure was not detected on the surface of the modified UV100. The crystallinity of UV100 is stable, so the surface structure is hard to change by nitric acid. Therefore, the mixed structure of photocatalyst is obtained via calcination at a temperature of 200–250 ◦ C. 3.3. UV–visible diffuse reflectance spectra

o

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TiO2 _600 C

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Figure 8. Raman spectra of photocatalysts prepared at various temperatures.

Raman spectra than in the XRD patterns. A small peak of the rutile phase (Raman shift = 440, and 608 cm−1 ) was also found in the Raman analysis of TiO2 500 ◦ C, and only the rutile structure was observed in the TiO2 600 ◦ C. As shown in figure 8(a), the Raman results for TiO2 150 ◦ C, 300 ◦ C, 400 ◦ C, 500 ◦ C, and 600 ◦ C are, in general, consistent with XRD analysis. However, an interesting result is for TiO2 200 ◦ C and TiO2 250 ◦ C samples, which may be responsible for most visible-light activity. In figures 8(b) and (c), two crystallite structures, rutile and mixed crystal (anatase and brookite), coexist on different surface areas of these two samples when Raman spectra were taken at two different points of the samples. It reveals that the rutile crystal and mixed crystal are probably entangled tightly on

The UV–visible diffuse reflectance spectra of the prepared samples and UV100 are shown in figure 9. The Kubelka– Munk function, F(R), is used as the equivalent of absorption. The TiO2 150 ◦ C, 200 ◦ C, 250 ◦ C, and 600 ◦ C exhibited new absorption bands in the visible-light region (λ > 400 nm), and the spectra of TiO2 300 ◦ C, 400 ◦ C, and 500 ◦ C were similar. No obvious absorption edge was observed for TiO2 200 ◦ C, and protracted absorption continued above 800 nm. The spectrum shows satisfactory evidence for good activity under visible-light irradiation. The absorption tails of TiO2 150 ◦ C In and 250 ◦ C were both extended close to 800 nm. figures 8(b) and (c), the mixed phases are observed in the Raman investigation of the TiO2 200 ◦ C and 250 ◦ C samples. It is possible that there exists an interface state due to the mixedphase surface structure which effectively reduces the band gap of the prepared samples that leads to absorption in the visible range. The formation mechanism of the mixed phases is not clear. Perhaps it is due to the impurities (carbonaceous species) existent on the prepared TiO2 surface, or perhaps it is caused by the calcination of the samples that transformed the less stable anatase phase to the more stable rutile phase. Furthermore, the carbonaceous species on the TiO2 surface also seem to 2495

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290 285 binding energy (eV)

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Figure 9. UV–visible absorption spectra of various photocatalysts.

Figure 10. C 1s XPS spectra of TiO2 200 ◦ C sample.

enhance visible-light absorption, and the carbonaceous species of TiO2 200 ◦ C was detected with XPS, as shown in figure 10. The peaks of C–C, C–O, and O=C–O components obviously appeared at 282–292 eV [13, 28], where the surface content of the C–C structure was more than that for C–O and O=C–O. The results of XPS analysis for TiO2 150 ◦ C, 200 ◦ C, 250 ◦ C, and 300 ◦ C are similar. The C–O and O=C–O were not chromosphors, so the C–C structure on the surface of TiO2 should be responsible for the visible-light absorption. The incorporation of carbonaceous species (C–C) is existent in the highly condensed and coke-like structure, so it could play the role of sensitizer to induce the visible-light absorption and response [13, 21, 22]. The carbonaceous species is either not existent or very scarce on the TiO2 surface over 400 ◦ C of calcination. The phenomena are also observed from the appearance of the TiO2 powder. The samples appeared to have a color from yellow to brown during calcination below 300 ◦ C, and changed to white at over 300 ◦ C. The visible-light absorption of TiO2 150 ◦ C was higher than that of TiO2 250 ◦ C, but the visible-light activity of TiO2 250 ◦ C was better than that of TiO2 150 ◦ C. The smaller visible-light activity of TiO2 150 ◦ C was due to the fact that the crystallinity was not well formed and the entangled structure on the surface was not formed either. The larger amount of carbonaceous species on the TiO2 150 ◦ C exhibited better visible-light absorption than TiO2 250 ◦ C. All of the prepared samples exhibited stronger visible-light absorption than UV100. The absorption edge of UV100 with complete content of anatase phase is close to 390 nm. The photocatalytic activity of TiO2 samples under visible light is mostly proportional to their visible-light absorption, except for TiO2 500 ◦ C and 600 ◦ C due to larger particle size and the existence of the rutile phase. The optimal calcination temperature for most visiblelight activity is 200 ◦ C, and the visible-light absorption of TiO2 samples would be sharply decreased over 300 ◦ C.

calcination at low temperature. Clear evidence for the mixed lattice structure of TiO2 was obtained from XRD and Raman analysis. UV–vis spectra depicted the strong visible-light adsorption of TiO2 200 ◦ C, which continued above 800 nm. The particle size of prepared TiO2 , which was evaluated and observed by XRD and SEM, respectively, was increased by increasing the calcination temperature. During the synthesis procedure, the calcination temperature should be controlled within 200–250 ◦ C for the existence of a mixed lattice structure and carbonaceous species on the surface, which exhibited the good visible-light activity of NOx removal. The visiblelight activity and absorption of prepared TiO2 is sharply decreased at calcination temperatures higher than 300 ◦ C due to the transformation of the lattice structure. The existence of carbonaceous species and mixed phases on the TiO2 lattice, observed from XPS and Raman spectroscopy, enhanced the absorption of visible light. Furthermore, the oxidation of NO2 under illumination by visible light was obviously inhibited in the photocatalytic oxidation of NOx . The NOx removal activity of prepared TiO2 was similar with a light source of wavelength less than 600 nm (UV to green LED). This simple and energysaving process for the production of visible-light-responsive photocatalysts will be applied practically to purify water and air, both in indoor and outdoor environments.

4. Conclusions A titania-based photocatalyst with a anatase, brookite and rutile mixed crystal lattice was synthesized by using tetrabutyl orthotitanate as the precursor via the sol–gel procedure and 2496

Acknowledgments The authors want to express their thanks to the Ministry of Economic Affairs for financial support of this work. C L Cheng would like to thank the National Science Council of Taiwan, Republic of China for financial support of this research under contracts NSC-94-2120-M-259-002 and NSC-94-2120M-259-001.

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