Langmuir 2004, 20, 9821-9827
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Ta3N5 Nanoparticles with Enhanced Photocatalytic Efficiency under Visible Light Irradiation Qinghong Zhang and Lian Gao* State Key Lab of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China Received May 13, 2004. In Final Form: July 23, 2004 Nanocrystalline Ta3N5 particles with a surface area of more than 33 m2/g were synthesized by nitridation of nanosized Ta2O5 particles using NH3 as the reactant gas. It was found that nanocrystalline Ta2O5 was converted into Ta3N5 completely (by X-ray diffraction, XRD) at 700 °C within 5.0 h, which was much lower than the temperature 900 °C for the complete nitridation of micrometer-sized Ta2O5 powder. The oxide precursor and the resulting nitride were characterized by XRD analysis, transmission electron microscopy, UV-vis diffuse reflectance spectra, and BET surface area techniques. The nitrogen contents in the prepared Ta3N5 powders were quantitatively determined with a CHN elemental analyzer. Nanocrystalline Ta3N5 showed an absorption edge of around 600 nm, and Ta3N5 in the size of about 26 nm exhibited a blue shift of 15 nm in the adsorption edge. The photocatalytic activity of the prepared Ta3N5 under UV-vis and visible light irradiation was compared to that of nanocrystalline TiO2-xNx using the photocatalytic degradation of methylene blue (MB) as a model reaction. The Ta3N5 nanoparticles showed the significantly enhanced photocatalytic activity for the degradation of MB in comparison with the larger-sized Ta3N5. Moreover, the nanocrystalline Ta3N5 showed much higher photocatalytic activity under visible light irradiation compared with TiO2-xNx in the same size.
1. Introduction In the past decade, a new field of materials chemistry and physics has emerged that emphasizes the rational synthesis and the enhanced and improved properties of nanocrystalline materials. Many synthetic routes based on the wet chemical method have been developed and focused on semiconductor nanoparticles, which display a variety of fundamentally interesting photophysical properties that are a direct result of their size and dimensionality.1,2 Because many semiconductor compounds have potential or demonstrated technological importance in photoluminesence, solar energy conversion, and photocatalysis, the tuning of these properties by adjusting crystallite size and nanoarchitecture is an attractive prospect.2 Unlike well-investigated nanocrystalline oxides, the preparation of nanocrystalline nitrides is more difficult because it generally proceeds at high temperature. A drastic growth and exaggregation of the resulting nitride particles occur inevitably during the high-temperature processing. Metal nitrides are conventionally synthesized by the high-temperature reaction of nitrogen, ammonia, or the hydrogen/nitrogen mixture with the metal or its oxide or chloride, typically at the temperature of around 1000 °C over long periods. Problems associated with the preparation include the huge energy costs, incorporation of unreacted metal in the product, and lack of control over crystallinity and particle sizes. These difficulties have inspired a great deal of work on alternative synthetic routes.3-12 Much later, nitride powders with high specific * Corresponding author. E-mail:
[email protected]. (1) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (c) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (d) Hu, Z. S.; Oskam, G.; Penn, R. L.; Pesika, N.; Searson, P. C. J. Phys. Chem. B 2003, 107, 3124. (2) (a) Zhang, Z.; Wang, C.; Zakaria, R.; Ying, J. Y. J. Phys. Chem. B 1998, 102, 10871. (b) Zhang, J. Z. Acc. Chem. Res. 1997, 30, 423. (3) Hector, A. L.; Parkin, I. P. Chem. Mater. 1995, 7, 1728. (4) Parkin, I. P.; Rowley, A. T. J. Mater. Chem. 1995, 5, 909.
surface area have been prepared under moderate-temperature conditions to meet the demands of catalysis applications.13-16 Bulk synthesis of nitride includes precursor decomposition and various self-propagating reactions.17,18 However, some carbon and other impurities generally remained in the nitride product,8 which might prevent many applications. Since the discovery of the photoinduced decomposition of water on TiO2,19 there has been extensive research activity focused on the synthesis and photophysical property characterization of nanocrystalline TiO2. The stimulus behind this research is the potential for converting light to electrical energy or chemical energy by solardriven band gap excitation of TiO2. Unfortunately, TiO2 has a wide band gap (3.2 eV for anatase and 3.0 eV for rutile), which limits its practical solar energy applications (the most intense region of the solar spectrum centered (5) Hansen, N. A. K.; Hermann, W. A. Chem. Mater. 1998, 10, 1677. (6) Holl, M. M. B.; Wolczanski, P. T.; Proserpio, D.; Bielecki, A.; Zax, D. B. Chem. Mater. 1996, 8, 2468. (7) Ritala, M.; Kalsi, P.; Riihela¨, D.; Kukli, K.; Leskela¨, M.; Jokinen, J. Chem. Mater. 1999, 11, 1712. (8) Marchand, R.; Tessier, F.; Disalvo, F. J. J. Mater. Chem. 1999, 9, 297. (9) O’Loughlin, J. L.; Wallace, C. H.; Knox, M. S.; Kaner, R. B. Inorg. Chem. 2001, 40, 2240. (10) Holl, M. M. B.; Kersting, M.; Pendley, B. D.; Wolczanski, P. T. Inorg. Chem. 1990, 29, 1518. (11) Zhu, L.; Ohashi, M.; Yamanaka, S. Chem. Mater. 2002, 14, 4517. (12) Sharma, R.; Naedele, D.; Schweda, E. Chem. Mater. 2001, 13, 4014. (13) (a) Kwon, H.; Choi, S.; Thompson, L. T. J. Catal. 1999, 184, 236. (b) Claridge, J. B.; York, A. P. E.; Brungs, A. J.; Green, M. L. H. Chem. Mater. 2000, 12, 132. (14) Kim, J. H.; Kim, K. L. Appl. Catal., A 1999, 181, 103. (15) Gouin, X.; Marchand, R.; L’Haridon, P.; Laurent, Y. J. Solid State Chem. 1994, 109, 175. (16) Hada, K.; Tanabe, J.; Omi, S.; Nagai, M. J. Catal. 2002, 201, 10. (17) Shin, J.; Ahn, D.-H.; Shin, M.-S.; Kim, Y.-S. J. Am. Ceram. Soc. 2000, 83, 1021. (18) Bradshaw, S. M.; Spicer, J. L. J. Am. Ceram. Soc. 1999, 82, 2293. (19) Fujishima, A.; Honda, K. Nature 1972, 238, 37.
10.1021/la048807i CCC: $27.50 © 2004 American Chemical Society Published on Web 09/21/2004
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Langmuir, Vol. 20, No. 22, 2004
Zhang and Gao
Table 1. Specific Surface Area, Crystallite Size, Crystalline Phase, and Nitrogen Contents of the Resulting Tantalum Nitride Powders sample
precursor
temp, °C
time, h
surface area, m2 g-1
phase
crystallite size, nm
A B C D E F G
amor Ta2O5 amor Ta2O5 NC Ta2O5a amor Ta2O5 Ta2O5 (Merck)b Ta2O5 (Merck) Ta2O5 (Merck)
600 700 700 800 700 800 900
8 5 5 5 5 5 5
35.2 33.7 23.0 18.9 2.5 9.2 8.7
amor Ta3N5 + TaON Ta3N5 Ta3N5 Ta3N5 + Ta2O5 Ta3N5 + TaON Ta3N5
nitrogen content, wt %
18 26 32 69 75
n.d. 9.45 10.77 10.96 2.92 10.17 10.75
a The nanocrystalline (NC) Ta O powder was obtained by the calcination of amorphous Ta O powder at 700 °C for 5 h, and the specific 2 5 2 5 surface areas of NC Ta2O5 and amor Ta2O5 are 29.0 and 110.1 m2/g, respectively. b The specific surface area of Merck Ta2O5 is 0.46 m2/g (crystallite size ca. 1.4 µm). It is seen that the surface area of the resulting Ta3N5 powder is much higher than its original oxide powder. The abnormal increase of surface area of Ta3N5 can be explained by considering the density difference between Ta2O5 and Ta3N5. “n.d." denotes that the nitrogen content in these samples was not determined by CHN analysis.
at ∼2.6 eV).20 Several attempts have been made to improve the performance of TiO2 as a photocatalyst under UV illumination and extend its absorption and conversion capacity into the visible portion of the solar spectrum.21,22 The band gaps of Ta3N5 and TaON are 2.08 and 2.4 eV,23,24 respectively, which make them suitable as visible-lightdriven photocatalyst25-27 and pigment.28 Some routes have been reported to prepare Ta3N5 including the thermal decomposition of organic tantalum compounds29 and the ammolysis of TaCl57 and TaS2.8 The reaction of micrometer-sized Ta2O5 with NH3 at 850 °C for a time as long as 15 h was also reported to synthesize phase-pure Ta3N5.25 With respect to other visible-light-driven photocatalysts, such as metal ions doped TiO2,21 nitrogen-22a-e and fluorine-doped TiO2,22f In1-xNixTaO4,30 AgInZn7S9,31 LaTiO2N,32 and Sm2Ti2S2O5,33 both Ta3N5 and TaON have relatively simple compositions and structures. Thus, they may be easily synthesized and used as promising photocatalysts. However, the preparation of nanocrystalline Ta3N5 has not been reported, and the possibly sizedependent optical properties and photocatalytic activity of Ta3N5 nanocrystals are also unclear up to now.
Wet chemical methods are intensively used to synthesize semiconductor nanocrystals with tunable morphologies.1,2 We previously reported that nanocrystalline oxides were used as precursors to prepare nitrides at relatively lower temperatures for a shorter time compared to the condition that oxide powders in the larger size were used as precursors.34-36 In this paper, it is demonstrated that semiconductor Ta3N5 nanocrystals with an average crystallite size of about 18 nm can be prepared at 700 °C for 5 h while a much higher temperature (up to 900 °C) is necessary for the complete nitridation of micrometer-sized Ta2O5 powder. UV-vis diffuse reflectance spectroscopy (DRS) spectra of Ta3N5 nanocrystals show a weak size dependence of optical properties, and a blue shift of 15 nm in the absorption band edge has been observed for the Ta3N5 in the average crystallite size of about 26 nm. Compared to the larger-sized Ta3N5 (75 nm), both the nanocrystalline Ta3N5 in the size of about 18 nm and about 26 nm show the significantly enhanced photocatalytic activity for the degradation of methylene blue (MB).
(20) Granqvist, C. G. Adv. Mater. 2003, 15, 1789. (21) (a) Choi, W.; Termin, A.; Hoffmann, M. R. J. Phys. Chem. 1994, 98, 13669. (b) Paola, A. D.; Marci, G.; Palmisano, L.; Schiavello, M.; Uosaki, K.; Ikeda, S.; Ohtani, B. J. Phys. Chem. B 2002, 106, 637. (c) Klosek, S.; Raftery, D. J. Phys. Chem. B 2001, 105, 2815. (d) Anpo, M.; Takeuchi, M. J. Catal. 2003, 216, 505. (22) (a) Sato, S. Chem. Phys. Lett. 1986, 123, 126. (b) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (c) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (d) Gole, J. L.; Stout, J. D.; Burda, C.; Lou, Y. B.; Chen, X. B. J. Phys. Chem. B 2004, 108, 1230. (e) Diwald, O.; Thompson, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 6004. (f) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. Z. Chem. Mater. 2002, 14, 3808. (23) Fang, C. M.; Orhan, E.; de Wijs, G. A.; Hintzen, H. T.; de Groot, R. A.; Marchand, R.; Saillard, J.-Y.; de With, G. J. Mater. Chem. 2001, 11, 1248. (24) Chun, W.-J.; Ishikawa, A.; Fujisawa, H.; Takata, T.; Kondo, J. N.; Hara, M.; Kawai, M.; Matsumoto, Y.; Domen, K. J. Phys. Chem. B 2003, 107, 1798. (25) Hitoki, G.; Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Domen, K. Chem. Lett. 2002, 31, 736. (26) Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Chem. Commun. 2002, 1698. (27) Hara, M.; Hitoki, G.; Tanaka, T.; Hondo, J. N.; Kobayashi, H.; Domen, K. Catal. Today 2003, 78, 555. (28) Jansen, M.; Letschert, B. E. F. E. P. Patent. No. 592867 A1, 1993. (29) Winter, C. H.; Jayaratne, K. C.; Proscia, J. W. Mater. Res. Soc. Symp. Proc. 1994, 327, 103. (30) Zou, Z.; Ye, J.; Sayama, K.; Arakawa, H. Nature 2001, 414, 625. (31) Kudo, A.; Tsuji, I.; Kato, H. Chem. Commun. 2002, 1958. (32) (a) Kasahara, A.; Nukumizu, K.; Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Phys. Chem. A 2002, 106, 6750. (b) Kasahara, A.; Nukumizu, K.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Phys. Chem. B 2003, 107, 791. (33) Ishikawa, A.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. J. Am. Chem. Soc. 2002, 124, 13547.
2.1. Synthesis of Ta2O5 and Ta3N5 Nanoparticles. Tantalum pentachloride (99.9% TaCl5, Acros Organics) was used as a starting material for the preparation of Ta2O5 nanoparticles. An appropriate amount of TaCl5 was dissolved in absolute alcohol to obtain a 0.3 M TaCl5 solution. The clear solution was stirred for 15 min, and then the dilute ammonia ethanolic solution was added into the TaCl5 ethanolic solution. The used ammonia (NH3‚ H2O) in the molar amount was equivalent to the HCl formed by the hydrolysis of TaCl5. The precipitate was collected by the filtration, washed with distilled water and ethanol, and dried at 110 °C for 24 h. Then, the dried precipitate was calcined at 500 °C for 2 h to remove the adsorbed and structural water in amorphous tantalum oxide. For the preparation of crystalline Ta2O5 nanoparticles, the above amorphous tantalum(V) oxide was calcined at 700 °C for 5 h. For the preparation of Ta3N5 nanoparticles, 2.0 g of the oxide powder was put into a tube furnace and subjected to nitridation in the flow of ammonia gas. The flow rate of NH3 gas was 0.5 L min-1, and the total pressure of the tube reactor was slightly higher than 1 atm. The nitridation temperature ranged from 600 to 1000 °C, while the nitridation time was 5-8 h. The sample was taken from the furnace after it was cooled to room temperature in the flow of N2 gas. By comparison, a commercially available crystalline Ta2O5 (Merck, S ) 0.46 m2 g-1) was also used as precursor oxide for nitridation reaction. The detailed conditions for the preparation of each sample are summarized in Table 1.
2. Experimental Section
(34) Li, J. G.; Gao, L.; Sun, J.; Zhang, Q. H.; Guo, J. K.; Yan, D. S. J. Am. Ceram. Soc. 2001, 84, 3045. (35) Li, Y. G.; Gao, L.; Li, J. G.; Yan, D. S. J. Am. Ceram. Soc. 2002, 85, 1294. (36) Gao, L.; Zhang, Q. H.; Li, J. G. J. Mater. Chem. 2003, 13, 154.
Photocatalytic Efficiency of Ta3N5 Nanoparticles Irie et al. reported the optimized conditions for the preparation of visible-light-driven TiO2-xNx photocatalyst.22c According to their work, a commercial Degussa P-25 TiO2 photocatalyst (which is a mixture of 80% anatase and 20% rutile, and with a specific surface area of 50 m2/g) with high photocatalytic activity was nitrided in the flow of ammonia gas at 550 °C for 3 h to obtain nanocrystalline TiO2-xNx photocatalyst. The detailed information of Degussa P-25 can be found in the published work by Ying et al.2a 2.2. Sample Characterization. The powder phase composition was identified by X-ray diffraction (XRD) equipment (model D/max 2550V, Rigaku Co., Tokyo, Japan), using Cu KR (λ ) 1.5406 Å) radiation. The broadening of the XRD peak at 2θ ) 22.9° (d001) for orthorhombic Ta2O5 and 2θ ) 17.2° (d002) for orthorhombic Ta3N5 was used to calculate the crystallite size for both nanocrystals according to the well-known Scherrer equation. Elemental analysis for carbon (C), hydrogen (H), and nitrogen (N) contents in the nitride powder was carried out on a PerkinElmer CHN analyzer (model 2400-II, MA, U.S.A.), and O2 was introduced into the system for the complete combustion nitride powder. For CHN analysis, the parallel experiment has been carried out three times for each sample, and the average value of the CHN contents is given in Table 1. The morphology and size of the resultant nitrides were observed using transmission electron microscopy (TEM; model JEM-200CX, JEOL, Tokyo, Japan) and field emission scanning electron microscopy (FESEM; model JSM-6700F, JEOL, Tokyo, Japan). The BET specific surface area measurement was performed on a nitrogen adsorption apparatus (model ASAP 2010, Micromeritics Instruments, Norcross, GA). DRS spectra were recorded on a Shimadzu UV3101 PC instrument, using BaSO4 as the reference sample, in the range 200-800 nm. Thermogravimetry (TG) and differential scanning calorimetry (DSC) were performed on NETZSCH STA 449C at the rate of 5 °C min-1 in air flow. 2.3. Photocatalytic Degradation of MB over Ta3N5 Nanoparticles. The MB (C16H18N3SCl‚3H2O) was used as supplied, and its degradation was tested as a model reaction to evaluate the photocatalytic activity of the resulting Ta3N5 powders. The photocatalytic experiments were carried out by adding 0.8 g of Ta3N5 powder into a 450-mL Pyrex photoreactor containing 400 mL of 5.35 × 10-5 M (20 mg L-1) MB solution. The cooling water in a quartz cylindrical jacket around the lamp was used to keep the reaction temperature constant (22 ( 2 °C). The mixture was sonicated before illumination and stored in dark for a further 30 min to obtain the saturated absorption of MB. The stirring suspension was illuminated by means of 300-W medium-pressure Hg lamp with a filter (ZJB 340 filter glass, Shanghai Nonferrous Glass, Ltd.) to cut off the light with wavelength
