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J. Phys. Chem. C 2007, 111, 3624-3628
Fabrication and Photocatalytic Properties of HLaNb2O7/(Pt, Fe2O3) Pillared Nanomaterial Jihuai Wu,* Yinghan Cheng, Jianming Lin, Yunfang Huang, Miaoliang Huang, and Shancun Hao Institute of Materials Physical Chemistry, Huaqiao UniVersity, Quanzhou, Fujian, 362021, China ReceiVed: NoVember 13, 2006; In Final Form: January 6, 2007
A novel intercalated nanomaterial HLaNb2O7/(Pt, Fe2O3) was fabricated by successive intercalated reaction of HLaNb2O7 with [Pt(NH3)4]Cl2 aqueous solution, with n-C6H13NH2/C2H5OH organic solution, and with [Fe3(CH3CO2)7(OH)(H2O)2]NO3 aqueous solution, followed by UV light irradiation. The intercalation of n-C6H13NH2 resulted in the formation of supramolecule HLaNb2O7‚C6H13NH2 and the interlayer gallery expansion of HLaNb2O7. HLaNb2O7/(Pt, Fe2O3) possessed a gallery height of less than 0.5 nm and specific surface area of 24.46 m2/g, which indicated the formation of an intercalated nanomaterial and a porous material. HLaNb2O7/Fe2O3 and HLaNb2O7/(Pt, Fe2O3) showed a broad absorption over 450-600 nm, which indicated photocatalytic activity under visible light irradiation. The photocatalytic activity of HLaNb2O7/Fe2O3 intercalated material was superior to that of unsupported Fe2O3 and was enhanced by the co-incorporation of Pt. With use of HLaNb2O7/(Pt, Fe2O3) as a catalyst, the photocatalytic hydrogen evolution was more than 110 cm3‚h-1‚g-1 in the presence of methanol as a sacrificial agent under irradiation with λ > 290 nm from a 100-W mercury lamp. Furthermore, the catalyst showed photocatalytic activity even under visible light irradiation.
1. Introduction The photocatalytic hydrogen evolution using solar energy is a potentially clean and renewable source. Since the first photocatalyst titanium dioxide suitable for hydrogen evolution from water splitting was reported1 several decades ago, considerable efforts2-5 have been devoted to develop a semiconductor photocatalyst for practical application. But, the yield of hydrogen gas by these catalysts still settles in magnitude of µmol‚h-1‚g-1, which renders the overall process impractical. Incorporation of a semiconductor catalyst in the interlayer region of a lamellar compound via chemical reactions is a promising method for fabricating a nanocomposite consisting of host layers with ultrafine particles in the interlayer and enhancing photocatalytic activity of the semiconductor, since in this system, the photogenerated electrons and holes are effectively separated due to the charge transfer from the guest to the host semiconductor layer. On the other hand, the distance the photoinduced holes and electrons must diffuse before reaching the interface decreases, and the holes and electrons can be effectively captured by the electrolyte in the solution.6 Yamanaka et al.,7 Enea and Bard,8 Yoneyama and co-workers,9,10 and Sato and co-workers11,12 reported the incorporation of extremely small particles of Fe2O3, TiO2, CdS, and CdS-ZnS mixtures, 290 nm from a 100-W mercury arc for 3 h.
TABLE 1: Gallery Height, Band Gap Energy, Specific Surface Area, and Element Content of the Samples
sample HLaNb2O7 HLaNb2O7/Pt Fe2O3 HLaNb2O7/Fe2O3 HLaNb2O7/(Pt, Fe2O3)
gallery height/ nm
band energy/ eV
specific area/ m2‚g-1
0.19 0.36
3.10 3.10 2.3 2.2 2.8 2.2 2.8
4.18 7.67 10.25 30.31 24.46
0.38 0.45
content/% Pt
Fe
0.74 0 0.39
69.75 4.43 3.91
Pt are similar with an absorption edge at 400 nm, but due to the Pt intercalation, the absorption edge of the latter is broadened. On the other hand, the absorption edge of Fe2O3 is at 540 nm,23 and the Fe2O3 intercalation into the interlayer of HLaNb2O7, HLaNb2O7/Fe2O3, and HLaNb2O7/(Pt, Fe2O3) shows broad reflection spectra with two onsets at ca. 450 and 600 nm, the former contributed to the red shift of the host HLaNb2O7 layer and the latter due to the red shift of incorporated guest Fe2O3. The red shift of HLaNb2O7 and Fe2O3 phenomena of band gap energy may be due to the coupling effect of host and guest semiconductor and the quantum size effect of intercalated nanomaterial.6,24-27 Similar phenomena also were observed in HTaWO6, HNbWO6, H2Ti4O9, and H4Nb6O17 systems.11-16 According to the formula ∆E ) 1240/λ (nm), the band gap energy ∆E (eV) for each specimen can be calculated by its wavelength of adsorption edges λ (nm) on its inflection of adsorption spectrum, which is 3.10 eV for HLaNb2O7 and HLaNb2O7/Pt, both 2.2 and 2.8 eV for HLaNb2O7/Fe2O3 and HLaNb2O7/(Pt, Fe2O3), respectively. The gallery height, contents of Fe and Pt elements incorporated, band gap energies, and specific surface areas of prepared catalysts are summarized in Table 1. The amounts of Fe and Pt elements are 3.91-4.43 and 0.39-∼0.74 wt %, respectively. The specific surface areas of HLaNb2O7/Fe2O3 and HLaNb2O7/ (Pt, Fe2O3) are 5-6 times greater than that of HLaNb2O7, which further confirms the intercalation of Pt and Fe2O3 guests and the formation of the porous material (maybe some Fe2O3 did not intercalate into the interlayer of HLaNb2O7 and deposited on the surface of HLaNb2O7). 3.3. Photocatalytic Properties. The amount of gas produced from 500 mL of 10 vol % methanol solutions containing 1 g of dispersed unsupported Fe2O3, HLaNb2O7, HLaNb2O7/ Pt, HLaNb2O7/Fe2O3, and HLaNb2O7/(Pt, Fe2O3) at 60 °C under irradiation with λ > 290 nm from a 100-W mercury arc for 3 h is shown in Figure 7. The gas produced was ascertained as
Figure 8. Amount of H2 produced from 500 mL of 10 vol % methanol solution containing 1 g of dispersed unsupported Fe2O3, HLaNb2O7, HLaNb2O7/Pt, HLaNb2O7/Fe2O3, and HLaNb2O7/(Pt, Fe2O3) at 60 °C by irradiating with λ > 400 nm from a 100-W mercury arc for 3 h.
hydrogen gas by analysis with gas chromatography, and the amount of formaldehyde (measuring with a formaldehyde analyzer 4160 model, Interscan America) in the solution increased with the increase of the irradiation time. From the above facts, all samples show photocatalytic activity to evolve hydrogen gas. The amount of hydrogen gas produced increased in the following sequence, unsupported Fe2O3 < HLaNb2O7 < HLaNb2O7/Fe2O3 < HLaNb2O7/Pt < HLaNb2O7/(Pt, Fe2O3). These results suggest that the photocatalytic activity of Fe2O3 is enhanced when it is intercalated in the interlayer of HLaNb2O7, especially when Pt is intercalated together with them. Figure 8 expresses the amount of hydrogen gas produced from 500 mL of 10 vol % methanol solutions containing 1 g of dispersed unsupported Fe2O3, HLaNb2O7, HLaNb2O7/ Pt, HLaNb2O7/Fe2O3, and HLaNb2O7/(Pt, Fe2O3) at 60 °C under irradiation with λ > 400 nm from a 100-W mercury arc for 3 h. As expected, due to their wide band gap energies (>3 eV), no hydrogen gas evolution is observed in the presence of HLaNb2O7 and HLaNb2O7/Pt. It is notable that evolution of hydrogen gas in the presence of HLaNb2O7/Fe2O3 is three times greater than that of unsupported Fe2O3 and is greatly enhanced by co-intercalation of Pt with Fe2O3. Similar phenomena were reported in H2Ti4O9, H4Nb6O17, HNbWO6, and HTaWO6 systems,11-16 but the photocatalytic effect of the HLaNb2O7
3628 J. Phys. Chem. C, Vol. 111, No. 9, 2007 system is better than that of the other systems, which may be due to the better photochemical activity for La rare earth metal ion with f-f energy transition in the host layer. Other layered hosts containing rare earth metal ion with f-f energy transition may have similar photocatalytic activities, but further study is necessary for clarification. Additionally, comparing the results of Figures 7 and 8, the photocatalytic activity of all samples under irradiation with λ > 290 nm is more than that with λ > 400 nm, which is due to greater incidence light intensity and higher light energy for the former than the latter. According to the mechanism in the introduction, the holes in the valence band and the electrons in the conduction band of the particle can be used effectively for the above reaction with an increase in the specific surface area. And the photoinduced electrons and holes in the interlayer space could be effectively separated by the heterogeneous electron transfer from guest semiconductor or metal to host layer. Therefore, the recombination of the electrons and holes is suppressed.11,12 This is one of the reasons why HLaNb2O7/(Pt, Fe2O3) intercalated material (24.46 m2/g) possessed a higher photocatalytic activity than unsupported HLaNb2O7 (4.18 m2/g) and Fe2O3 (10.25 m2/g). The platinum can promote the charge separation on the ferric oxide particle surface and lead to a great increase in the photocatalytic activity. This is why the photocatalytic activity of HLaNb2O7/(Pt, Fe2O3) is higher than that of HLaNb2O7/ Fe2O3. In addition, as mentioned above, it is thought that the visible light photocatalytic activity is caused by an absorption of light energy smaller than 3.1 eV (λ > 400 nm), and since the band gap of HLaNb2O7 and HLaNb2O7/Pt is greater than 3.1 eV, no visible light photoactivities are observed for HLaNb2O7 and HLaNb2O7/Pt samples. 4. Conclusion In summary, Fe2O3 together with Pt was incorporated into the interlayer of HLaNb2O7 by successive reaction of HLaNb2O7 with [Pt(NH3)4]Cl2 aqueous solution, with n-C6H13NH2/C2H5OH organic solution, and with [Fe3(CH3CO2)7(OH)(H2O)2]NO3 aqueous solution, followed by UV light irradiation. The intercalation of C6H13NH2 results in the formation of supramolecule HLaNb2O7‚C6H13NH2 and the interlayer gallery expansion of HLaNb2O7. The height of the Fe2O3 pillar in HLaNb2O7/ Fe2O3 and HLaNb2O7/(Pt, Fe2O3) is less than 0.5 nm. HLaNb2O7/ Fe2O3 and HLaNb2O7/(Pt, Fe2O3) show a broad reflection over
Wu et al. 450-600 nm. The photocatalytic activity of HLaNb2O7/Fe2O3 intercalated material is superior to those of unsupported Fe2O3 and is enhanced by the co-incorporation of Pt. With use of HLaNb2O7/(Pt, Fe2O3) as a catalyst, the photocatalytic H2 evolution is more than 110 cm3‚h-1‚g-1 in the presence of methanol as a sacrificial agent under irradiation with λ > 290 nm from a 100-W mercury lamp. Furthermore, they show photocatalytic activity even under visible light irradiation. Acknowledgment. The authors thank the Natural Science Foundation of China (Nos. 50572030, 50372022, and 50082003) for financial support. References and Notes (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) Kato, H.; Kudo, A. J. Phys. Chem. B 2001, 105, 4285. (3) Zhuo, Z. G.; Yue, J. H.; Hironori, A. Nature 2001, 414, 625. (4) Fujiahima, A.; Rao, T. N.; Tryk, Q. A. J. Photochem. Photobiol. C 2000, 1, 1. (5) Kim, H. G.; Hwang, D. W.; Kim, J. J. Chem. Soc., Chem. Commun. 1999, 1077. (6) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (7) Yamanaka, S.; Doi, T.; Sako, S. Mater. Res. Bull. 1994, 19, 61. (8) Enena, O.; Bard, A. J. J. Phys. Chem. 1986, 90, 301. (9) Yoneyama, H.; Haga, S.; Yamanaka, S. J. Phys. Chem. 1989, 93, 4833. (10) Miyoshi, H.; Mori, H.; Yoneyama, H. Langmuir 1991, 7, 503. (11) Sato, T.; Yamamoto, Y.; Uchida, S. J. Chem. Soc., Faraday Trans. 1996, 92, 5089. (12) Sato, T.; Masaki, K.; Sato, K. J. Chem. Technol. Biotechnol. 1996, 67, 339. (13) Wu, J. H.; Uchida, S.; Sato, T. J. Photochem. Photobiol. A 1999, 128, 129. (14) Wu, J. H.; Lin, J. M.; Sato, T. J. Mater. Chem. 2001, 11, 3343. (15) Wang, L. L.; Wu, J. H.; Huang, M. L. Scr. Mater. 2004, 50, 465. (16) Wu, J. H.; Uchida, S.; Fujishiro, Y.; Sato, T. Int. J. Inorg. Mater. 1999, 1, 253. (17) Kawai, T.; Sakata, T. J. Chem. Soc., Chem. Commun. 1980, 694. (18) Gerischer, H.; Heller, A. J. Phys. Chem. 1991, 95, 5261. (19) Gopalakrishnan, J.; Bhat, V. Mater. Res. Bull. 1987, 22, 423. (20) Shannon, R. P.; Prewitt, C. T. Acta Crystallogr. B 1961, 25, 925. (21) Tsuneo, M.; Tetsuya, F.; Naosuke, M. Bull. Chem. Soc. Jpn. 1993, 66, 1548. (22) Izawa, H.; Kikkawa, S.; Koizumi, M. Polyhedron 1983, 2, 741. (23) Wang, L. L.; Wu., J. H.; Li, T. H.; Cheng, Y. H. J. Porous Mater. 2005, 12, 23. (24) Kamat, P. V.; Shanghavi, B. J. Phys. Chem. B 1997, 101, 7675. (25) Nasr, C.; Hotchandani, S.; Kim, W. Y.; Kamat, P. V. J. Phys. Chem. B 1997, 101, 7480. (26) Kumar, A.; Jain, A. K. J. Mol. Catal. A: Chem. 2001, 165, 265. (27) Choi, J. K.; Yeo, K. C.; Yoon, M. J.; Lee, S. J. J. Photochem. Photobiol. A 2000, 132, 105.
