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Low temperature synthesis of TiO2−xNy powders and films with visible light responsive photocatalytic activity
Solid State Communications 137 (2006) 132–137 www.elsevier.com/locate/ssc
Low temperature synthesis of TiO2KxNy powders and films with visible light responsive photocatalytic activity Shu Yin *, Ken Ihara, Masakazu Komatsu, Qiwu Zhang, Fumio Saito, Takashi Kyotani, Tsugio Sato Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 9808577, Japan. Received 19 April 2005; received in revised form 1 November 2005; accepted 3 November 2005 by T.T.M. Palstra Available online 1 December 2005
Abstract Nitrogen-doped titania powders and films with excellent visible light photocatalytic activity were successfully prepared by an unadulterated ‘low-temperature process’ below 60 8C using mechanochemical doping and oxygen plasma treatment. q 2006 Elsevier Ltd. All rights reserved. PACS: 61.72.Ww Keywords: A. Nitrogen doped titania; B. Mechanochemical synthesis; C. Atomic force microscopy; D. Photocatalytic activity; E. X-ray photoelectron spectroscopy
1. Introduction Titania is the most effective photocatalyst and is widely applied in the purification of air and water, deodorization, antibacterial and self-cleaning coating and other environment applications [1–4]. However, titania can only be induced by UV light because of its large band gap value of ca. 3 eV. Asahi et al. [5,6] reported that nitrogen doped titania with very highvisible light photocatalytic activity could be prepared by heat treatment of titania in NH3 (67%)–Ar atmosphere at 600 8C or sputtering TiO2 target in an N2 (40%)–Ar gas mixture followed by annealing in N2 gas at 550 8C for 4 h. Recently, we initially found that nitrogen-doped titania TiO2KxNy with different phase compositions could be synthesized through a soft solution process around 190 8C [7,8]. It was also found that nitrogen-doped rutile titania powders could be prepared by a low-temperature mechanochemical treatment of titania and hexamethylenetetramine (C6H12N4, HMT) mixed powders [9,10]. However, during the mechanochemical treatment using HMT as a nitrogen reagent, carbon was formed according to the following reaction (Eq. (1)): C6 H12 N4 / 6C C 4NH3 * Corresponding author. Tel./fax: C81 22 217 5598. E-mail address: [email protected] (S. Yin).
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.11.010
(1)
Residual by-products, such as C and adsorbed organic molecules on the surface of the titania, suppress the photocatalytic activity, making the following calcination at 400 8C necessary to realize the visible light photocatalytic activity [9]. In the present research, in order to realize a ‘complete lowtemperature operation process’ for the preparation of nitrogendoped titania powders, a planetary milling of titania with urea ((NH2)2CO) and ammonium carbonate (NH4)2CO3 instead of hexamethylenetetramine was carried out. In addition, in order to utilize the photocatalyst effectively, it is important to fix the powder as a film on various substrates. Usually, organic binders are used for the formation of titania films, combined with high temperature treatment to remove the binders [11,12]. In the present research, a low-temperature oxygen plasma reaction treatment below 60 8C was suggested for the formation of TiO2KxNy film. Consequently, not only TiO2KxNy powders but also TiO2KxNy films could be prepared by way of the complete low-temperature synthesis process. By this low-temperature process, it is possible to fix the photocatalyst onto various kinds of substrate materials, including thermally unstable materials such as wood and plastic basal plate. It is a novel fabrication process in order to produce functional materials. No similar results were reported by other researchers. 2. Experimental Commercial titania powder P25 (Degussa), ammonium carbonate and urea were used as raw materials. P25 titania
S. Yin et al. / Solid State Communications 137 (2006) 132–137
powder was mixed with 10 wt% ammonium carbonate or urea before introduction into a reaction vessel. A planetary ball mill (Fritsch, P-7) was used for grinding of the samples. Seven zirconia balls of 15 mm diameter and 4 g of mixed powder were introduced to a zirconia vessel of 45 cm3 inner volume. The grinding was operated at 700 rpm of rotation speed for 1 h. For comparative insight, some data of the TiO2KxNy powders prepared by mechanochemical treatment of P25 with HMT were discussed. The phase constitution of the product was determined by X-ray diffraction analysis (XRD) (Shimadzu, XD-D1). A commercial borosilicate glass slide (Matsunami glass Ind. Ltd, Japan) 0.25 mm in thickness and 22 mm in diameter was used as a substrate with TiO2KxNy film by spin coating at 3000 rpm for 15 s (Mikasa Spincoater 1H-D7), using the TiO2KxNy powders obtained from mechanochemical treatment of P25 TiO2-urea mixture followed by washing with only distilled water. The coating liquid was prepared by uniformly mixing 1 g TiO2KxNy powder with 2 g of industrial grade nitrocellulose, 5 g of ethyl acetate and 5 g of butyl acetate using a paint shaker and a 50 g ball of zirconia 2.7 mm in diameter for 40 h. The binder in the films was removed using a treatment of 100 W oxygen plasma at 0.6 Torr for 5 min with an O2 flow rate of 30 cm3 minK1 (Yamato Plasma Reactor PR301S). For comparison, as-prepared TiO2KxNy powders and films were calcined at 400 8C for 1 h. The surface structure of the films was characterized by AFM (Seiko, Instruments Inc. SII Nanopics 1000). The absorption edge and band gap energy of the products were determined from the onset of the diffuse reflectance spectrum of the sample, measured using an UV-VIS spectrophotometer (Shimadzu, UV-2000). The binding energy of N1s was measured using an X-ray photoelectron spectrometer (Perkin–Elmer PHI5600). The photocatalytic activity for the oxidative destruction of nitrogen monoxide under irradiation of various light wavelength was carried out by the same method reported in previous papers [8,10,13]. The film sample with an area of 380 mm2 on a glass substrate was used for the photocatalytic characterization. 3. Results and discussion Fig. 1 shows the XRD patterns of the starting P25 powder and the samples prepared by the mechanochemical treatment. The P25 titania powder consists of 77 wt% anatase and 23 wt% rutile. By planetary ball milling with 10 wt% HMT, urea or ammonia carbonate powder, anatase gradually transformed to rutile with a small quantity of brookite. All mechanical treatments were carried out at room temperature. The temperature of mechanochemical treatment vessel increased a little but was less than 50 8C even after extended milling. Generally, the transformation temperature of anatase to rutile can reach temperatures higher than 700 8C. These results strongly suggested that high mechanical energy accelerated the phase transformation of anatase to brookite and thermodynamically stable rutile. Single phase rutile could be obtained by decreasing the amount of the nitrogen source from 10 to 5 wt%, although the data was not shown here. The coexisting nitrogen source powder seemed to relieve the mechanical stress and
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Fig. 1. XRD patterns of (a) P25 titania and of the TiO2KxNy samples prepared by ball milling P25 with (b) 10 wt% HMT, (c) 10 wt% urea and (d)10 wt% ammonium carbonate at 700 rpm for 60 min. All samples were prepared using zirconia vessel and balls then washed with distilled water. ( ) anatase, (C) brookite, (;) rutile.
delay the phase transformation of titania [10]. The XRD patterns showed no broadening upon ball milling, indicating that no obvious particle size decrement occurred during the mechanochemical treatment, although part of phase transformation from anatase to rutile was observed. The BET specific surface area measurement also agreed with the above result, because no obvious variation of BET specific surface area was observed after mechanochemical treatment. In photocatalyst research, anatase titania is usually considered to be more active than rutile crystalline [14–16]. The enhancement is ascribable to the differences of the Fermi level and surface area of the solid. In addition, rutile usually showed harder agglomeration and larger particle size than anatase, since rutile is normally prepared by calcination of anatase at high temperatures. In the present research, it might be suggested that the rutile phase crystalline possessed similar particle size, BET specific surface area and crystallinity to that found in the initial consistency of anatase. In our previous research, it was also found that photocatalytic oxidation activity strongly related to the crystallinity and the BET specific surface areas. Conversely, the amorphous titania with high specific surface area showed no photocatalytic activity because of the existence of substantial crystal defects, which act as the recombination center of holes and electrons [17]. Even if on X-ray invisible amorphous phase was formed during the mechanochemical treatment, it might be accepted that there was no positive effect on the increment of photocatalytic activity. Thus, it is difficult to decide that the activity of the photocatalyst is by XRD
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analysis only. It is considered that the chemical reaction between titania and its nitrogen source proceeds at the same time with phase transformation. In many cases, mechanical stress induces the formation of a fresh, oxygen-rich surface of oxide, resulting in the destruction of weak chemical bonding and the formation of new binding on the surface. It might be suggested that the NH3 released from HMT, urea or ammonia carbonate diffused and adsorbed on the surface of titania first, then reacted with the activated titania surface, finally leading to the formation of nitrogen-doping in the titania powders. In order to remove the residual unreacted nitrogen source in the final powders, and to realize the complete low-temperature process, the ball-milled samples were washed with distilled water three times and dried at 50 8C for 1 day instead of calcination in air at 400 8C. Ammonium carbonate could be decomposed to ammonia and carbon dioxide gas directly during mechanochemical treatment (Eq. (2)), and the urea decomposed to biuret (Eq. (3)), which could then be removed by way of normal washing process. ðNH4 Þ2 CO3 / 2NH3 C CO2 C H2 O
(2)
2ðNH2 Þ2 CO/ NH3 C NH2 CONHCONH2 ðbiuretÞ
(3)
Fig. 2 shows the diffusion reflectance spectra of P25 titania and the samples prepared by ball milling P25 with HMT, urea and ammonium carbonate at 700 rpm for 1 h, followed by calcination or a washing process. The absorption edge of the sample was determined by Eq. (4) Eg Z
1239:8 l
(4)
where Eg is the band gap (eV) of the sample, l (nm) is the wavelength of the onset of the spectrum. Although the powder prepared from the TiO2–HMT mixture followed by calcination showed a clear two-step absorption in visible-light region (non-marked lines), that which followed by a washing process was black and showed less reflectance in the visible light region (marked lines), indicating that the residual carbon could not be removed by the washing process alone. On the other hand, the samples prepared by a washing process using urea and
Fig. 2. Diffuse reflectance spectra of P25 titania and of the TiO2KxNy samples prepared by ball milling P25 with (a) 10 wt% HMT (b) 10 wt% urea and (c) 10 wt% ammonium carbonate ((NH4)2CO3) followed by calcination at 400 8C (non-marked lines) or washing with distilled water(marked lines).
Fig. 3. DeNOx ability of the TiO2KxNy powders prepared by mechanochemical doping process using HMT, urea and ammonium carbonate ((NH4)2CO3) followed by calcination or washing process, together with those of P25 before and after ball-milling without any additives. Open bars: as prepared by mechnochemical doping; Oblique lined bars: calcined at 400 8C for 1 h; solid bars: washed with distilled water instead of calcination.
ammonium carbonate as nitrogen sources were yellowish and showed similar DRS spectra with those after calcination at 400 8C, indicating the formation of nitrogen doping during the planetary milling. P25 titania showed an absorption edge at 408 nm corresponding to the band gap of 3.04 eV. The powders prepared by ball milling P25 titania with urea and ammonium carbonate showed two absorption edges at 400–408 nm (3.04–3.10 eV) and 540–565 nm (2.19–2.30 eV). It is thought that the first and second edges are related to the band structure of original titania and a newly formed N2p band which is located above the O2p valence band [5,6], respectively. It is seen that the band gap could be greatly narrowed by the mechanochemical doping of nitrogen in titania at temperatures as low as 50 8C. Fig. 3 shows the visible light photocatalytic activity of TiO2KxNy prepared by mechanochemical reaction followed by a washing processes, together with that of P25 before and after ball milling without any additives. For comparative analysis, the data of the powders prepared by calcination in air at 400 8C were also shown in Fig. 3 (Oblique lined bars). After ball milling without any additive, the photocatalytic activity of P25 decreased by ca. 10–20%, probably due to the formation of lattice defects during high-energy ball milling. Similarly, as-prepared TiO2KxNy powders (open bars) showed low activity, probably caused by the existence of the residual nitrogen reagents and by-products. After a washing process (solid bars), the powders obtained from the titania–HMT mixture still showed very low activity, indicating that washing processes cannot remove the carbonaceous by-products. In this case, the calcination process is necessary to remove the carbonaceous by-products and to realize the high visible light responsive activity. Conversely, the powders prepared by washing process (solid bars) from titania–urea and titania– ammonium carbonate mixtures showed similar high-level activity as those prepared by calcination. The as-prepared powders from titania–ammonium carbonate also showed highphotocatalytic activity, indicating that the ammonium carbonate could be decomposed directly by the mechanochemical treatment. Based on the above results, it is obvious that an
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unadulterated ‘low-temperature process’ and the process simplification of the TiO2KxNy preparation could be realized by selecting an appropriate nitrogen source compound. Under the irradiation of visible light at a wavelength O510 nm, the photocatalytic activity of P25 was quite low because of its large band gap energy of 3.04 eV, however, nearly 17–22% of nitrogen monoxide could be continuously removed by the TiO2KxNy prepared by ball milling P25 titania-10 wt% urea or ammonium carbonate at 700 rpm for 1 h. The photocatalytic activity was several times higher than that of P25 titania, taking blank results into consideration. In the case of irradiation of the light at a wavelength O400 and O290 nm, similar results were observed. In other words, the powder prepared by ball milling with urea and ammonium carbonate possessed excellent photocatalytic activity not only in the visible-light region, but also in near-UV and UV light regions. It was also found that when the light was turned off, NO concentration returned to its initial level of 1 ppm within 10 min, indicating that light energy is necessary for the removal of nitrogen monoxide. Fig. 4 shows the N1s spectra of P25, and the TiO2KxNy powders prepared by mechanochemical doping. No peak was observed in the commercial Degussa P25 powders, indicating that no nitrogen was contained in the P25 powders. The binding energy around 396 eV, which related to the existence of Ti–N binding, was confirmed in the TiO2KxNy samples prepared by using various nitrogen reagents, indicating that Ti–N binding was actually formed in the lattice of the titania crystal during the mechanochemical treatment. It is accepted that in the nitrogen-doped TiO2KxNy photocatalyst, small amounts of nitrogen are located at the position of oxygen site of the coordinated TiO6 octahedron [5,6]. It is known that the XPS
Fig. 4. N1s XPS spectra of the commercial powder P25, and the powders prepared by the planetary ball milling of P25 with 10 wt% HMT, urea and (NH4)2CO3, The measurement was carried out after ArC ions sputtered for 3 min. For comparison, the spectrum of TiN reagent was also recorded.
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peak around 396 eV is caused by the doping between titanium and nitrogen. On the other hand, the peak around 400 eV is related to the N–N, N–O or N–C binding energy, and that around 402 eV is related to the N–H binding energy, which is caused by the adsorption of nitrogen containing molecules on the photocatalyst surface [5,6]. According to the integral intensity of the peaks, around 396 and 400 eV, it could be calculated that about 63, 41 and 58% of the nitrogen consistent in the samples were really incorporated into the TiO2 lattices
Fig. 5. AFM images of the TiO2KxNy films (a) as-prepared by spin coating at 3000 rpm; followed by (b) calcination at 400 8C for 1 h and (c) 100 W oxygen plasma treatment below 60 8C for 5 min.
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Table 1 Surface roughness and photocatalytic activities of the TiO2KxNy films Sample As prepared After calcination at 400 8C for 1 h After 100 W O2 plasma treatment below 60 8C for 5 min. Degussa P25 (commercial) film Blank (no sample) a
Surface roughness, Ra (nm)
DeNOx photocatalytic activitya (%) O510 nm
O400 nm
O290 nm
34.8 42.6 72.8
– 8.25 7.21
– 16.5 13.3
– 44.3 40.8
– –
4.0 3.4
10 6.4
41 20
Continuously flowed 1 ppm NO-50 vol% air mixed (balance N2) gas (200 cm3 minK1); sample area: 380 mm2; retention reaction time: 112 s.
(peak at 396 eV) by using HMT, urea and (NH4)2CO3, respectively. It was obvious that the intensity of the Ti–N peak around 396 eV of the samples in the present research was in the sequence of: HMTO(NH4)2CO3Ourea. According to the integral intensity ratio of N1s around 396 eV to those of the total peaks of N1s, O1s and Ti2p, it might be calculated that about 0.36, 0.25 and 0.26% of the nitrogen was actually doped at the oxygen site of the TiO6 octahedrons by mechanochemical doping using HMT, urea and (NH4)2CO3, respectively. Fig. 5 shows the AFM images of the TiO2KxNy films prepared by oxygen plasma treatment together with those prepared by calcination at 400 8C. In the present research, in order to get a photocatalyst film with high surface area, a high concentration of binder was added in the coating solution. In most cases, the thickness of the film is about 500 nm. The asprepared film without any further treatment (Fig. 5a) possessed a comparatively smooth surface. After calcination at 400 8C, as shown in Table 1, the surface roughness (Ra) increased from 34 to 43 nm, where Ra is the average arithmetic roughness, calculated by the data analysis program Nanopics 1000, based on the Japanese Industry Standard ‘JIS B0601-1994’. The film obtained by O2 plasma treatment for 5 min temperatures as low as 60 8C showed much higher surface roughness than that by calcination at 400 8C for 1 h. The oxygen plasma treatment not only removed the organic binder at low temperature, but also suppressed crystalline growth during the removal of the binder. It has been reported that organic species in films can be removed by low-temperature oxygen plasma treatment [18–20]. By applying oxygen plasma at 5 W, the oxygen plasma treatment is generally effective to a depth of at least 10–12 nm. In the present research, it was found that oxygen plasma treatment at 100 W was effective up to several hundred nanometers. It might be related to the high concentration of organic binder in the film. A higher degree of roughness in the photocatalyst film might result to increase the adsorptive ability of the reactive species, and lead to higher photocatalytic activity. As shown in Table 1, the visible light responsive photocatalytic activity of the TiO2K xNy film prepared by oxygen plasma treatment showed the same level as that obtained by calcination. About 7.21 and 13.3% of 1 ppm NO could be continuously removed under the irradiation of visible light at wavelengths of lO510 nm and lO400 nm, respectively. In addition, the films also showed excellent photocatalytic activity under UV light irradiation. It is expected that TiO2KxNy films on some thermal-unstable substrates such
as wood, paper and plastic can be realized using the present method. 4. Conclusion In summary, the following conclusions may be drawn: TiO2KxNy powders are successfully prepared by way of a complete low-temperature process, i.e. a novel mechanochemical reaction of titania with urea and/or ammonium carbonate below 50 8C followed by washing with water. TiO2KxNy film with high surface area could be prepared by way of a low temperature process, i.e. a spin-coating of TiO2KxNy powder using an organic binder followed by oxygen plasma treatment below 60 8C within a short time period. The nitrogen-doped yellowish titania powders and films possessed excellent light absorption until 565 nm, and showed excellent visible light induced photocatalytic ability under irradiation of visible light at a wavelength of O510 nm. Acknowledgements This research was partially supported by the Ministry of Education, Culture, Sports, Science and Technology, a Grantin-Aid for the COE project (Giant Molecules and Complex Systems) and a Grant-in-Aid for Science Research (No. 14750660) and by the Steel Industrial Foundation for the Advancement of the Environmental Protection. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [2] B.O. Regan, M. Gratzel, Nature 353 (1991) 737–739. [3] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima, J. Photochem. Photobiol. A: Chem. 98 (1996) 79–86. [4] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [5] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [6] T. Morikawa, R. Asahi, T. Ohwaki, K. Aoki, Y. Taga, Jpn. J. Appl. Phys. 40 (2001) L561–L563. [7] Y. Aita, M. Komasu, S. Yin, T. Sato, J. Solid State Chem. 177 (2004) 3235–3238. [8] S. Yin, Y. Aita, M. Komasu, J. Wang, Q. Tang, T. Sato, J. Mater. Chem. 15 (2005) 674–682. [9] S. Yin, Q. Zhang, F. Saito, T. Sato, Chem. Lett. 32 (2003) 358–359. [10] S. Yin, H. Yamaki, M. Komatsu, J. Wang, Q. Zhang, Q. Tang, F. Saito, T. Sato, J. Mater. Chem. 13 (2003) 2996–3001.
S. Yin et al. / Solid State Communications 137 (2006) 132–137 [11] W. Chen, J. Zhang, Q. Fang, S. Li, J. Wu, F. Li, K. Jiang, Sens. Actuators B 100 (2004) 195–199. [12] S. Portal, R.M. Almeida, Phys. Status Solidi A (2004) 2012941– 2012947. [13] S. Yin, H. Hasegawa, M. Daisaku, M. Ishitsuka, T. Sato, J. Photochem. Photobiol. A: Chem. 163 (2004) 1–8. [14] R.I. Bickley, T. Gonzalez-Carreno, J.S. Lees, L. Palmisano, R.J. Tilley, J. Solid State Chem. 92 (1991) 178–190. [15] J. Augustynski, Electrochem. Acta 38 (1993) 43–46.
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[16] Z. Ding, G. Lu, P. Greenfield, J. Phys. Chem. B 104 (2000) 4815– 4820. [17] S. Yin, T. Sato, Ind. Eng. Chem. Res. 39 (2000) 4526–4530. [18] J. Huang, I. Ichinise, T. Kunitake, A. Nakao, Langmuir 18 (2002) 9048– 9053. [19] T. Asanuma, T. Matsutani, C. Liu, T. Mihara, M. Kiuchi, J. Appl. Phys. 95 (2004) 6011–6016. [20] Y. Wang, R. Kumar, X. Zhou, J. Pan, J. Chai, Thin Solid Film 473 (2005) 132–136.
Low temperature synthesis of TiO2KxNy powders and films with visible light responsive photocatalytic activity Shu Yin *, Ken Ihara, Masakazu Komatsu, Qiwu Zhang, Fumio Saito, Takashi Kyotani, Tsugio Sato Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 9808577, Japan. Received 19 April 2005; received in revised form 1 November 2005; accepted 3 November 2005 by T.T.M. Palstra Available online 1 December 2005
Abstract Nitrogen-doped titania powders and films with excellent visible light photocatalytic activity were successfully prepared by an unadulterated ‘low-temperature process’ below 60 8C using mechanochemical doping and oxygen plasma treatment. q 2006 Elsevier Ltd. All rights reserved. PACS: 61.72.Ww Keywords: A. Nitrogen doped titania; B. Mechanochemical synthesis; C. Atomic force microscopy; D. Photocatalytic activity; E. X-ray photoelectron spectroscopy
1. Introduction Titania is the most effective photocatalyst and is widely applied in the purification of air and water, deodorization, antibacterial and self-cleaning coating and other environment applications [1–4]. However, titania can only be induced by UV light because of its large band gap value of ca. 3 eV. Asahi et al. [5,6] reported that nitrogen doped titania with very highvisible light photocatalytic activity could be prepared by heat treatment of titania in NH3 (67%)–Ar atmosphere at 600 8C or sputtering TiO2 target in an N2 (40%)–Ar gas mixture followed by annealing in N2 gas at 550 8C for 4 h. Recently, we initially found that nitrogen-doped titania TiO2KxNy with different phase compositions could be synthesized through a soft solution process around 190 8C [7,8]. It was also found that nitrogen-doped rutile titania powders could be prepared by a low-temperature mechanochemical treatment of titania and hexamethylenetetramine (C6H12N4, HMT) mixed powders [9,10]. However, during the mechanochemical treatment using HMT as a nitrogen reagent, carbon was formed according to the following reaction (Eq. (1)): C6 H12 N4 / 6C C 4NH3 * Corresponding author. Tel./fax: C81 22 217 5598. E-mail address: [email protected] (S. Yin).
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.11.010
(1)
Residual by-products, such as C and adsorbed organic molecules on the surface of the titania, suppress the photocatalytic activity, making the following calcination at 400 8C necessary to realize the visible light photocatalytic activity [9]. In the present research, in order to realize a ‘complete lowtemperature operation process’ for the preparation of nitrogendoped titania powders, a planetary milling of titania with urea ((NH2)2CO) and ammonium carbonate (NH4)2CO3 instead of hexamethylenetetramine was carried out. In addition, in order to utilize the photocatalyst effectively, it is important to fix the powder as a film on various substrates. Usually, organic binders are used for the formation of titania films, combined with high temperature treatment to remove the binders [11,12]. In the present research, a low-temperature oxygen plasma reaction treatment below 60 8C was suggested for the formation of TiO2KxNy film. Consequently, not only TiO2KxNy powders but also TiO2KxNy films could be prepared by way of the complete low-temperature synthesis process. By this low-temperature process, it is possible to fix the photocatalyst onto various kinds of substrate materials, including thermally unstable materials such as wood and plastic basal plate. It is a novel fabrication process in order to produce functional materials. No similar results were reported by other researchers. 2. Experimental Commercial titania powder P25 (Degussa), ammonium carbonate and urea were used as raw materials. P25 titania
S. Yin et al. / Solid State Communications 137 (2006) 132–137
powder was mixed with 10 wt% ammonium carbonate or urea before introduction into a reaction vessel. A planetary ball mill (Fritsch, P-7) was used for grinding of the samples. Seven zirconia balls of 15 mm diameter and 4 g of mixed powder were introduced to a zirconia vessel of 45 cm3 inner volume. The grinding was operated at 700 rpm of rotation speed for 1 h. For comparative insight, some data of the TiO2KxNy powders prepared by mechanochemical treatment of P25 with HMT were discussed. The phase constitution of the product was determined by X-ray diffraction analysis (XRD) (Shimadzu, XD-D1). A commercial borosilicate glass slide (Matsunami glass Ind. Ltd, Japan) 0.25 mm in thickness and 22 mm in diameter was used as a substrate with TiO2KxNy film by spin coating at 3000 rpm for 15 s (Mikasa Spincoater 1H-D7), using the TiO2KxNy powders obtained from mechanochemical treatment of P25 TiO2-urea mixture followed by washing with only distilled water. The coating liquid was prepared by uniformly mixing 1 g TiO2KxNy powder with 2 g of industrial grade nitrocellulose, 5 g of ethyl acetate and 5 g of butyl acetate using a paint shaker and a 50 g ball of zirconia 2.7 mm in diameter for 40 h. The binder in the films was removed using a treatment of 100 W oxygen plasma at 0.6 Torr for 5 min with an O2 flow rate of 30 cm3 minK1 (Yamato Plasma Reactor PR301S). For comparison, as-prepared TiO2KxNy powders and films were calcined at 400 8C for 1 h. The surface structure of the films was characterized by AFM (Seiko, Instruments Inc. SII Nanopics 1000). The absorption edge and band gap energy of the products were determined from the onset of the diffuse reflectance spectrum of the sample, measured using an UV-VIS spectrophotometer (Shimadzu, UV-2000). The binding energy of N1s was measured using an X-ray photoelectron spectrometer (Perkin–Elmer PHI5600). The photocatalytic activity for the oxidative destruction of nitrogen monoxide under irradiation of various light wavelength was carried out by the same method reported in previous papers [8,10,13]. The film sample with an area of 380 mm2 on a glass substrate was used for the photocatalytic characterization. 3. Results and discussion Fig. 1 shows the XRD patterns of the starting P25 powder and the samples prepared by the mechanochemical treatment. The P25 titania powder consists of 77 wt% anatase and 23 wt% rutile. By planetary ball milling with 10 wt% HMT, urea or ammonia carbonate powder, anatase gradually transformed to rutile with a small quantity of brookite. All mechanical treatments were carried out at room temperature. The temperature of mechanochemical treatment vessel increased a little but was less than 50 8C even after extended milling. Generally, the transformation temperature of anatase to rutile can reach temperatures higher than 700 8C. These results strongly suggested that high mechanical energy accelerated the phase transformation of anatase to brookite and thermodynamically stable rutile. Single phase rutile could be obtained by decreasing the amount of the nitrogen source from 10 to 5 wt%, although the data was not shown here. The coexisting nitrogen source powder seemed to relieve the mechanical stress and
133
Fig. 1. XRD patterns of (a) P25 titania and of the TiO2KxNy samples prepared by ball milling P25 with (b) 10 wt% HMT, (c) 10 wt% urea and (d)10 wt% ammonium carbonate at 700 rpm for 60 min. All samples were prepared using zirconia vessel and balls then washed with distilled water. ( ) anatase, (C) brookite, (;) rutile.
delay the phase transformation of titania [10]. The XRD patterns showed no broadening upon ball milling, indicating that no obvious particle size decrement occurred during the mechanochemical treatment, although part of phase transformation from anatase to rutile was observed. The BET specific surface area measurement also agreed with the above result, because no obvious variation of BET specific surface area was observed after mechanochemical treatment. In photocatalyst research, anatase titania is usually considered to be more active than rutile crystalline [14–16]. The enhancement is ascribable to the differences of the Fermi level and surface area of the solid. In addition, rutile usually showed harder agglomeration and larger particle size than anatase, since rutile is normally prepared by calcination of anatase at high temperatures. In the present research, it might be suggested that the rutile phase crystalline possessed similar particle size, BET specific surface area and crystallinity to that found in the initial consistency of anatase. In our previous research, it was also found that photocatalytic oxidation activity strongly related to the crystallinity and the BET specific surface areas. Conversely, the amorphous titania with high specific surface area showed no photocatalytic activity because of the existence of substantial crystal defects, which act as the recombination center of holes and electrons [17]. Even if on X-ray invisible amorphous phase was formed during the mechanochemical treatment, it might be accepted that there was no positive effect on the increment of photocatalytic activity. Thus, it is difficult to decide that the activity of the photocatalyst is by XRD
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analysis only. It is considered that the chemical reaction between titania and its nitrogen source proceeds at the same time with phase transformation. In many cases, mechanical stress induces the formation of a fresh, oxygen-rich surface of oxide, resulting in the destruction of weak chemical bonding and the formation of new binding on the surface. It might be suggested that the NH3 released from HMT, urea or ammonia carbonate diffused and adsorbed on the surface of titania first, then reacted with the activated titania surface, finally leading to the formation of nitrogen-doping in the titania powders. In order to remove the residual unreacted nitrogen source in the final powders, and to realize the complete low-temperature process, the ball-milled samples were washed with distilled water three times and dried at 50 8C for 1 day instead of calcination in air at 400 8C. Ammonium carbonate could be decomposed to ammonia and carbon dioxide gas directly during mechanochemical treatment (Eq. (2)), and the urea decomposed to biuret (Eq. (3)), which could then be removed by way of normal washing process. ðNH4 Þ2 CO3 / 2NH3 C CO2 C H2 O
(2)
2ðNH2 Þ2 CO/ NH3 C NH2 CONHCONH2 ðbiuretÞ
(3)
Fig. 2 shows the diffusion reflectance spectra of P25 titania and the samples prepared by ball milling P25 with HMT, urea and ammonium carbonate at 700 rpm for 1 h, followed by calcination or a washing process. The absorption edge of the sample was determined by Eq. (4) Eg Z
1239:8 l
(4)
where Eg is the band gap (eV) of the sample, l (nm) is the wavelength of the onset of the spectrum. Although the powder prepared from the TiO2–HMT mixture followed by calcination showed a clear two-step absorption in visible-light region (non-marked lines), that which followed by a washing process was black and showed less reflectance in the visible light region (marked lines), indicating that the residual carbon could not be removed by the washing process alone. On the other hand, the samples prepared by a washing process using urea and
Fig. 2. Diffuse reflectance spectra of P25 titania and of the TiO2KxNy samples prepared by ball milling P25 with (a) 10 wt% HMT (b) 10 wt% urea and (c) 10 wt% ammonium carbonate ((NH4)2CO3) followed by calcination at 400 8C (non-marked lines) or washing with distilled water(marked lines).
Fig. 3. DeNOx ability of the TiO2KxNy powders prepared by mechanochemical doping process using HMT, urea and ammonium carbonate ((NH4)2CO3) followed by calcination or washing process, together with those of P25 before and after ball-milling without any additives. Open bars: as prepared by mechnochemical doping; Oblique lined bars: calcined at 400 8C for 1 h; solid bars: washed with distilled water instead of calcination.
ammonium carbonate as nitrogen sources were yellowish and showed similar DRS spectra with those after calcination at 400 8C, indicating the formation of nitrogen doping during the planetary milling. P25 titania showed an absorption edge at 408 nm corresponding to the band gap of 3.04 eV. The powders prepared by ball milling P25 titania with urea and ammonium carbonate showed two absorption edges at 400–408 nm (3.04–3.10 eV) and 540–565 nm (2.19–2.30 eV). It is thought that the first and second edges are related to the band structure of original titania and a newly formed N2p band which is located above the O2p valence band [5,6], respectively. It is seen that the band gap could be greatly narrowed by the mechanochemical doping of nitrogen in titania at temperatures as low as 50 8C. Fig. 3 shows the visible light photocatalytic activity of TiO2KxNy prepared by mechanochemical reaction followed by a washing processes, together with that of P25 before and after ball milling without any additives. For comparative analysis, the data of the powders prepared by calcination in air at 400 8C were also shown in Fig. 3 (Oblique lined bars). After ball milling without any additive, the photocatalytic activity of P25 decreased by ca. 10–20%, probably due to the formation of lattice defects during high-energy ball milling. Similarly, as-prepared TiO2KxNy powders (open bars) showed low activity, probably caused by the existence of the residual nitrogen reagents and by-products. After a washing process (solid bars), the powders obtained from the titania–HMT mixture still showed very low activity, indicating that washing processes cannot remove the carbonaceous by-products. In this case, the calcination process is necessary to remove the carbonaceous by-products and to realize the high visible light responsive activity. Conversely, the powders prepared by washing process (solid bars) from titania–urea and titania– ammonium carbonate mixtures showed similar high-level activity as those prepared by calcination. The as-prepared powders from titania–ammonium carbonate also showed highphotocatalytic activity, indicating that the ammonium carbonate could be decomposed directly by the mechanochemical treatment. Based on the above results, it is obvious that an
S. Yin et al. / Solid State Communications 137 (2006) 132–137
unadulterated ‘low-temperature process’ and the process simplification of the TiO2KxNy preparation could be realized by selecting an appropriate nitrogen source compound. Under the irradiation of visible light at a wavelength O510 nm, the photocatalytic activity of P25 was quite low because of its large band gap energy of 3.04 eV, however, nearly 17–22% of nitrogen monoxide could be continuously removed by the TiO2KxNy prepared by ball milling P25 titania-10 wt% urea or ammonium carbonate at 700 rpm for 1 h. The photocatalytic activity was several times higher than that of P25 titania, taking blank results into consideration. In the case of irradiation of the light at a wavelength O400 and O290 nm, similar results were observed. In other words, the powder prepared by ball milling with urea and ammonium carbonate possessed excellent photocatalytic activity not only in the visible-light region, but also in near-UV and UV light regions. It was also found that when the light was turned off, NO concentration returned to its initial level of 1 ppm within 10 min, indicating that light energy is necessary for the removal of nitrogen monoxide. Fig. 4 shows the N1s spectra of P25, and the TiO2KxNy powders prepared by mechanochemical doping. No peak was observed in the commercial Degussa P25 powders, indicating that no nitrogen was contained in the P25 powders. The binding energy around 396 eV, which related to the existence of Ti–N binding, was confirmed in the TiO2KxNy samples prepared by using various nitrogen reagents, indicating that Ti–N binding was actually formed in the lattice of the titania crystal during the mechanochemical treatment. It is accepted that in the nitrogen-doped TiO2KxNy photocatalyst, small amounts of nitrogen are located at the position of oxygen site of the coordinated TiO6 octahedron [5,6]. It is known that the XPS
Fig. 4. N1s XPS spectra of the commercial powder P25, and the powders prepared by the planetary ball milling of P25 with 10 wt% HMT, urea and (NH4)2CO3, The measurement was carried out after ArC ions sputtered for 3 min. For comparison, the spectrum of TiN reagent was also recorded.
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peak around 396 eV is caused by the doping between titanium and nitrogen. On the other hand, the peak around 400 eV is related to the N–N, N–O or N–C binding energy, and that around 402 eV is related to the N–H binding energy, which is caused by the adsorption of nitrogen containing molecules on the photocatalyst surface [5,6]. According to the integral intensity of the peaks, around 396 and 400 eV, it could be calculated that about 63, 41 and 58% of the nitrogen consistent in the samples were really incorporated into the TiO2 lattices
Fig. 5. AFM images of the TiO2KxNy films (a) as-prepared by spin coating at 3000 rpm; followed by (b) calcination at 400 8C for 1 h and (c) 100 W oxygen plasma treatment below 60 8C for 5 min.
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Table 1 Surface roughness and photocatalytic activities of the TiO2KxNy films Sample As prepared After calcination at 400 8C for 1 h After 100 W O2 plasma treatment below 60 8C for 5 min. Degussa P25 (commercial) film Blank (no sample) a
Surface roughness, Ra (nm)
DeNOx photocatalytic activitya (%) O510 nm
O400 nm
O290 nm
34.8 42.6 72.8
– 8.25 7.21
– 16.5 13.3
– 44.3 40.8
– –
4.0 3.4
10 6.4
41 20
Continuously flowed 1 ppm NO-50 vol% air mixed (balance N2) gas (200 cm3 minK1); sample area: 380 mm2; retention reaction time: 112 s.
(peak at 396 eV) by using HMT, urea and (NH4)2CO3, respectively. It was obvious that the intensity of the Ti–N peak around 396 eV of the samples in the present research was in the sequence of: HMTO(NH4)2CO3Ourea. According to the integral intensity ratio of N1s around 396 eV to those of the total peaks of N1s, O1s and Ti2p, it might be calculated that about 0.36, 0.25 and 0.26% of the nitrogen was actually doped at the oxygen site of the TiO6 octahedrons by mechanochemical doping using HMT, urea and (NH4)2CO3, respectively. Fig. 5 shows the AFM images of the TiO2KxNy films prepared by oxygen plasma treatment together with those prepared by calcination at 400 8C. In the present research, in order to get a photocatalyst film with high surface area, a high concentration of binder was added in the coating solution. In most cases, the thickness of the film is about 500 nm. The asprepared film without any further treatment (Fig. 5a) possessed a comparatively smooth surface. After calcination at 400 8C, as shown in Table 1, the surface roughness (Ra) increased from 34 to 43 nm, where Ra is the average arithmetic roughness, calculated by the data analysis program Nanopics 1000, based on the Japanese Industry Standard ‘JIS B0601-1994’. The film obtained by O2 plasma treatment for 5 min temperatures as low as 60 8C showed much higher surface roughness than that by calcination at 400 8C for 1 h. The oxygen plasma treatment not only removed the organic binder at low temperature, but also suppressed crystalline growth during the removal of the binder. It has been reported that organic species in films can be removed by low-temperature oxygen plasma treatment [18–20]. By applying oxygen plasma at 5 W, the oxygen plasma treatment is generally effective to a depth of at least 10–12 nm. In the present research, it was found that oxygen plasma treatment at 100 W was effective up to several hundred nanometers. It might be related to the high concentration of organic binder in the film. A higher degree of roughness in the photocatalyst film might result to increase the adsorptive ability of the reactive species, and lead to higher photocatalytic activity. As shown in Table 1, the visible light responsive photocatalytic activity of the TiO2K xNy film prepared by oxygen plasma treatment showed the same level as that obtained by calcination. About 7.21 and 13.3% of 1 ppm NO could be continuously removed under the irradiation of visible light at wavelengths of lO510 nm and lO400 nm, respectively. In addition, the films also showed excellent photocatalytic activity under UV light irradiation. It is expected that TiO2KxNy films on some thermal-unstable substrates such
as wood, paper and plastic can be realized using the present method. 4. Conclusion In summary, the following conclusions may be drawn: TiO2KxNy powders are successfully prepared by way of a complete low-temperature process, i.e. a novel mechanochemical reaction of titania with urea and/or ammonium carbonate below 50 8C followed by washing with water. TiO2KxNy film with high surface area could be prepared by way of a low temperature process, i.e. a spin-coating of TiO2KxNy powder using an organic binder followed by oxygen plasma treatment below 60 8C within a short time period. The nitrogen-doped yellowish titania powders and films possessed excellent light absorption until 565 nm, and showed excellent visible light induced photocatalytic ability under irradiation of visible light at a wavelength of O510 nm. Acknowledgements This research was partially supported by the Ministry of Education, Culture, Sports, Science and Technology, a Grantin-Aid for the COE project (Giant Molecules and Complex Systems) and a Grant-in-Aid for Science Research (No. 14750660) and by the Steel Industrial Foundation for the Advancement of the Environmental Protection. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37–38. [2] B.O. Regan, M. Gratzel, Nature 353 (1991) 737–739. [3] I. Sopyan, M. Watanabe, S. Murasawa, K. Hashimoto, A. Fujishima, J. Photochem. Photobiol. A: Chem. 98 (1996) 79–86. [4] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [5] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269–271. [6] T. Morikawa, R. Asahi, T. Ohwaki, K. Aoki, Y. Taga, Jpn. J. Appl. Phys. 40 (2001) L561–L563. [7] Y. Aita, M. Komasu, S. Yin, T. Sato, J. Solid State Chem. 177 (2004) 3235–3238. [8] S. Yin, Y. Aita, M. Komasu, J. Wang, Q. Tang, T. Sato, J. Mater. Chem. 15 (2005) 674–682. [9] S. Yin, Q. Zhang, F. Saito, T. Sato, Chem. Lett. 32 (2003) 358–359. [10] S. Yin, H. Yamaki, M. Komatsu, J. Wang, Q. Zhang, Q. Tang, F. Saito, T. Sato, J. Mater. Chem. 13 (2003) 2996–3001.
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