Environ. Sci. Technol. 2008, 42, 3622–3626
Improvement in Photocatalytic Activity of TiO2 under Visible Irradiation through Addition of N-TiO2 IN-CHEOL KANG,* QIWU ZHANG, SHU YIN, TSUGIO SATO, AND FUMIO SAITO Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan
Received November 23, 2007. Revised manuscript received February 18, 2008. Accepted February 25, 2008.
This work is focused on improvement in photocatalytic activity of anatase-TiO2 (a-TiO2) photocatalyst under visiblelight irradiation by adding nitrogen-doped TiO2 (N-TiO2) for depression of recombination rate between photoexcited electron and hole. The composites (a-TiO2/N-TiO2) were prepared by grinding in ethanol solvent at 200 rpm for 15 min with change in weight ratio of N-TiO2. In addition to the characterizations by X-ray diffraction and X-ray photoelectron spectroscopy, measurements of existing singlet oxygen by chemiluminescens method and photocatalytic activity by using NOx decomposition were conducted. The increases in singlet oxygen and photocatalytic activity have been observed and the phenomena are discussed based on the efficient prevention of recombination between photoexcited electron and hole within the prepared composite.
Introduction Photocatalysts have been applied in various areas such as purification of air/water, deodorization, water splitting, and conversion of solar energy into electrical energy, etc. (1-5). Until now, many kinds of photocatalysts have been developed, and particularly TiO2 has been considered most promising for the strong oxidizing ability (3.0 eV) of the photogenerated hole in the TiO2, of which excellent oxidizing ability allows the removal of various toxic organic compounds and harmful materials. However, the excitation of pure TiO2 by just ultraviolet irradiation has limited realization of effective utilization of solar energy because of 3-4% UV light in solar irradiation. For further improvement of effective utilization of solar energy, nonmetal (6-13)/transition-metal (14) doping into photocatalysts has been carried out to prepare visible sensitive photocatalyst via band gap narrowing of photocatalyst, resulting in achievement of visible sensitive photocatalyst. In our previous works, nitrogen-doped TiO2 (6) and carbon-doped TiO2 (9) were prepared by using a mechanochemical method, which showed good photocatalytic activity under visible irradiation region. However, these need further development for improvement of visible photocatalytic activity. * Corresponding author tel.: +81-22-217-5136; fax: +81-22-2175136; e-mail:
[email protected]. 3622
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The commercial photocatalyst P-25 shows a visible photocatalytic activity even though it is just a mixture consisting of pure anatase and rutile TiO2 without doping, which is attributed to an effective separation of photoinduced electron-hole (15, 16). In P-25 photocatalyst, the rutile phase functions as hole acceptor and anatase functions as electron acceptor, resulting in long life-span of photoinduced electron and hole. In the case of P-25 photocatalyst, the rutile phase releases photoinduced electrons and holes, of which the electrons move to anatase TiO2 while the hole remains in rutile TiO2 so that rutile induces oxidation while anatase induces reduction. Rutile TiO2 has 3.0 ev band gap energy, so its photoactivity under visible wavelength is not so sensitive. Therefore, if a more visible sensitive photocatalyst replaces rutile, better visible active photocatalyst may be realized. In the present research, with nitrogen-doped TiO2 instead of the pure rutile phase, anatase-TiO2/N-doped TiO2 (a-TiO2/N-TiO2) composites were prepared with various weight ratios of N-TiO2, and were characterized by XRD, specific surface area (SSA), UV-vis spectroscopy, chemiluminescence, and NOx decomposition activity techniques. The fundamental information about such composite based on the above characterizations is reported.
Experimental Section Preparation of N-TiO2. Even though the doping amount of nitrogen into TiO2 greatly affects photocatalytic activity, there is no standard index with respect to optimum doping amount, because the photocatalytic activity of product doped by nitrogen is dependent on preparing method, starting materials, constitution of phase, and the kind of reagent as a nitrogen source (8, 17-19). In the present work, N-TiO2 was prepared with the procedure reported by Yin et al. (7, 8). In particular, ref 8 reported the influence of nitrogen doping amount on photocatalytic activity. On that basis, accordingly, N-TiO2 was prepared as follows. Anatase-TiO2 powder (3.6 g) (a-TiO2, purity min 98.5%, Wako Pure Chem. Inc., Japan) was ground with (NH4)2CO3 reagent (0.4 g) as a nitrogen source by using a planetary ball mill (Pulverisette-7, Fritsch, Germany) consisting of a PSZ (partial stabilized zirconia) pot having 45 cm2 inner space and seven zirconia balls of 15 mm in diameter, at 700 rpm for 2 h, followed by heating at 200 °C for 1 h with heating rate of 200 °C/hr in air. Preparation of a-TiO2/N-TiO2 Composites. The a-TiO2 and N-TiO2 were mixed in ethanol (C2H5OH(95); 95%, Wako
FIGURE 1. XPS profiles of starting materials (a) anatase-TiO2 and (b) N-TiO2. 10.1021/es702932m CCC: $40.75
2008 American Chemical Society
Published on Web 04/16/2008
FIGURE 2. XRD profiles depending on the weight ratio of N-TiO2: (a) 10:0, (b) 8:2, (c) 6:4, (d) 4:6, (e) 2:8, and (f) 0:10. (A and R indicate anatase and rutile TiO2, respectively).
TABLE 1. Specific Surface Area and Average Crystallite Size with Increase in Weight Ratio of N-TiO2: (a) 10:0, (b) 8:2, (c) 6:4, (d) 4:6, (e) 2:8, and (f) 0:10 phase ratio (mol%) average crystallite specific surface rutile size (nm) area (m2/g)
a-TiO2:N-TiO2 anatase (a) 10:0 (b) 8:2 (c) 6:4 (d) 4:6 (e) 2:8 (f) 0:10
1 0.99 0.97 0.96 0.9 0.63
0 0.01 0.03 0.04 0.1 0.37
47.84 44.33 44.48 42.78 39.70 13.65
9.78 19.46 29.59 40.37 50.37 61.68
Pure Chem. Indus., Japan) with weight ratio from 10:0 to 0:10 (a-TiO2/N-TiO2). The grinding operation was performed by relatively mild grinding conditions as follows. The total sample (2 g), solvent (30 mL), and balls (Φ ) 5 mm; 40 g) were put into the mill pot, and then the grinding was started to run at 200 rpm for 15 min. After grinding operation the residual solvent was removed by heating at around 80 °C with magnetic stirring, and finally, the sample was kept in an oven at 80 °C for 12 h. Characterizations. X-ray photoelectron spectroscopy (XPS) (PHI 5600 ESCA system, Ulvac-Phi Inc., Japan) was conducted to investigate the chemical binding states of the starting materials (a-TiO2 and N-TiO2). The products were sputtered with 3 kv Ar+ ion for 5 min, on a 4 × 4 mm2 area, and the XPS scanning was recorded with Mg KR X-ray by 20 times × 3cycles. The phase of products was determined by using X-ray diffraction analysis (XRD) (RAD-B, Rigaku Co. Ltd., Japan) using Cu KR radiation. The phase molar ratio of anatase and rutile was calculated by using peak intensity according to the Spurr and Myers method (20). Wr ) 1 ⁄ [1 + 0.8(Ia ⁄ Ir)]
(1)
Wa ) 1 - Wr
(2)
where Wa and Wr are, respectively, the mole ratio of anatase and rutile, Ia and Ir are the peak intensity of anatase d(101) and rutile d(110). The average crystallite size (D) was
FIGURE 3. UV-vis profiles with increase in weight ratio of N-TiO2: (a) 10:0, (b) 8:2, (c) 6:4, (d) 4:6, (e) 2:8, and (f) 0:10. calculated by following Scherrer’s formula (21), depending on the weight ratio of N-TiO2. D ) (0.9λ) ⁄ (β1⁄2cosθ)
(3)
where λ is the wavelength (0.15418 nm) of the X-ray, β1/2 is line-width at medium height of anatase d(101) and rutile d(110), and θ is the diffracting angle. The average crystallite size was calculated by the following equation: Dave ) (Da × (Ia ⁄ (Ia + Ir)) + Dr × (Ir ⁄ (Ia + Ir)))
(4)
where Dave is average crystallite size, and Da and Dr are crystallite size of anatase d(101) and rutile d(110), respectively. Ia and Ir are peak intensity of anatase d(101) and rutile d(110), respectively. The specific surface area (SSA) of products was measured by nitrogen adsorption-desorption isothermal measurements at 77 K (ASAP-2010, Micromeritics, Shimadzu, Japan) based on a BET method. The optical absorption edge of the products was recorded by UV-vis spectrophotometry (UV-2000, Shmadzu, Japan) from 700 to 200 nm wavelength region. VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Mmechanism of electron-hole separation in a-TiO2/ N-TiO2 composites during photocatalysis. activities. The measurement was conducted as follows (22). The sample was placed on a glass holder plate (hollow place 20 × 15 × 0.5 mm2) and set in the center of the reactor box. A 450-W high-pressure mercury lamp was used as the light source, in which the wavelength was controlled by various filters, i.e., pyrex glass for cutting off the light of wavelength >290 nm, Kenko L41 Super Pro (W) filter >400 nm and Fuji triacetyl cellulose filter >510 nm. One ppm NOx-50 vol.% air mixed gas was flowed into the reactor box at a rate of 200 cm3/min, and the concentration of NOx gas was measured at the outlet of the reactor box (373 cm3).
Results and Discussion
FIGURE 4. Chemiluminescence of singlet oxygen of products (a) a-TiO2, (b) N-TiO2, and (c) a-TiO2/N-TiO2 (4:6), respectively. The low-level luminous intensity of chemiluminescence of singlet oxygen (1O2) was measured by a multi luminescence spectrometer (MLA-GOLDS; Tohoku Electric Ind. Japan) at 25 °C in air. The sample was placed in the stainless steel chamber (20 mm diameter and 10 mm depth). The sample was irradiated for 5 s by four different types of LED light, i.e., UV light (375 nm wavelength), blue light (470 nm wavelength), and green light (530 nm wavelength), red light (630 nm wavelength). The luminous intensity of chemiluminescence of singlet oxygen (1O2) having 634 nm wavelength was calculated as follows: the luminous intensity of singlet oxygen was measured by using two kinds of filters, λ < 620 nm and λ 470 nm wavelength), but just against UV light (375 nm wavelength). On the other hand, the N-TiO2 shows relatively strong luminous intensity of singlet oxygen against irradiation of UV light, blue light, and even against green light. As expected, the luminous intensity of singlet oxygen became stronger against irradiation of shorter wavelength, due to the generation of much singlet oxygen. And, as for product (c) consisting of a-TiO2/N-TiO2 (4:6), which shows stronger luminous intensity and longer life-span than that of N-TiO2. In other words, that the product (c) (a-TiO2/N-TiO2) shows stronger luminous intensity and longer life-span of singlet oxygen even with less N-TiO2 compared with product (b), well identifies that effective separation of photoinduced electron and hole takes place in product (c) finally, leading to depression of recombination rate between photoinduced electron and hole.
In a-TiO2/N-TiO2 composites, N-TiO2 supplies electrons and holes against visible irradiation and acts as a hole acceptor, while a-TiO2 acts only as an electron acceptor, in which the accepted electrons in a-TiO2 move to the surface and then are trapped by O2 (electron acceptor) (25). Therefore, that the electron acceptor (a-TiO2) has a high specific surface is an important factor for enhancing contacting degree of O2 with electrons on the surface. In addition, for more effective transfer of electrons to electron acceptor (a-TiO2), it needs good interparticle contact which might enhance the electron-hole separation (26, 27). Subsequently, the electron acceptor (a-TiO2) might have to have high specific surface area for better photocatalytic activity via effective separation of between electron and hole. Figure 5 schematically illustrates the separating mechanism between electron and hole in a-TiO2/N-TiO2 composites. As shown above, the electrons are trapped on trapping state (Ti3+) of a-TiO2, at below 0.8 eV from bottom of conduction band (28), while the hole is trapped on N2p of N-TiO2 (29). In the case of >510 nm wavelength, the photoinduced electron and hole in a-TiO2 are absent, while there exist the photoinduced electron and hole in N-TiO2. Here, the photoinduced hole moves to the surface of N-TiO2 and then induces oxidation, while the photoinudced electrons move to trapping site of a-TiO2 and then move to the surface of a-TiO2, which induces reduction. On the other hand, the separating mechanism between electron-hole under >400 nm wavelength may be a little different from that of the case of >510 nm wavelength. In the case of irradiation of >400 nm wavelength, there exist photoinduced electron and hole in both a-TiO2 and N-TiO2, so the photoinduced hole of a-TiO2 is trapped on N2p of N-TiO2 while the photoinduced electron of N-TiO2 is trapped on trapping site of a-TiO2. Subsequently, higher photocatalytic activity under >400 nm wavelength might be achieved for generation of much photoinduced electron and hole, and efficient separation between electron and hole. Figure 6 shows NOx gas decomposition activity for examining photocatalytic ability under irradiation of >510 nm (A) and >400 nm (B) in wavelength depending on the weight ratio of N-TiO2. In Figure 6, the dotted line indicates the expected values, which were calculated as follows: NOx decomposition activity ) (Dea × Wa) + (DeN × WN) (8) where Dea and DeN are respectively assigned to NOx decomposition rate of a-TiO2 and N-TiO2, and Wa and WN are assigned to weight ratio of a-TiO2 and N-TiO2, respectively. The solid line indicates experimental results. Neither anatase VOL. 42, NO. 10, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TiO2 nor N-TiO2, we compare the activity of the composite (a-TiO2/N-TiO2) to that of expected results represented by the dotted line calculated by eq 8. The dotted line is used as the baseline to show the expected activity when there is no interaction between two samples (a-TiO2 and N-TiO2). In the case of irradiation under >510 nm wavelength all of products show higher NOx decomposition activity than that of the results expected, except two products (9:1 and 8:2). This trend is similar to that shown in Figure 6B. It may be due to decrease in specific surface area relatively for the increase in a-TiO2 ratio versus N-TiO2, resulting in low electron transferability and interparticle contactability for low specific surface area and large particle size of a-TiO2(shown in Table 1). Nevertheless, the product (a-TiO2/ N-TiO2 ) 6:4) shows significantly improved photocatalytic activity by about 1.4 times compared with that of an expected value, for efficient separation between electron and hole.
Acknowledgments I.C.K. is grateful to the Korean Government (MOST) for the financial support provided through the Korea Science and Engineering Foundation Grant (2005-215-D00146).
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