Effect of Ion Exchange on Photoluminescence of Layered Niobates

The sharp and strong Raman bands (880 cm-1 for K4Nb6O17 and 955 cm-1 for KNb3O8) assigned to short Nb−O bonds which stuck out into interlayers were ...
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J. Phys. Chem. 1996, 100, 17323-17326

17323

Effect of Ion Exchange on Photoluminescence of Layered Niobates K4Nb6O17 and KNb3O8 Akihiko Kudo* Department of Applied Chemistry, Faculty of Science, Science UniVersity of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan

Tadayoshi Sakata Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan ReceiVed: July 3, 1996; In Final Form: August 6, 1996X

Layered niobates K4Nb6O17 and KNb3O8 showed bright blue emission at 77 K. The luminescent properties were changed by exchanging potassium ions at interlayers with other cations. The luminescent intensity of NH4+-exchanged K4Nb6O17 was higher than that of native K4Nb6O17. In contrast, for H+-exchanged K4Nb6O17 and KNb3O8, the luminescent intensities decreased and the emission spectra red-shifted compared with their native layered niobates. The sharp and strong Raman bands (880 cm-1 for K4Nb6O17 and 955 cm-1 for KNb3O8) assigned to short Nb-O bonds which stuck out into interlayers were observed for the native layered niobates. The Raman bands were decreased by H+ exchange corresponding to the luminescent intensity. These results indicated that characteristics of the short Nb-O bonds were changed by cation exchange at interlayers, resulting in the change in the energy state of niobate layers. Thus, it was demonstrated that photophysical properties of niobate layers could be controlled by substances at interlayers.

Introduction Photochemistry of ion-exchangeable layered compounds has recently been studied extensively.1 Among the compounds, layered alkali niobates have unique properties. CdS-loaded K4Nb6O17 shows photocatalytic activities for hydrogen evolution from aqueous K2SO3 solutions under visible light irradiation.2 Visible light photolysis of hydrogen iodide proceeds using ruthenium-complex-sensitized layered niobate particles.3 Emission spectra and the lifetime of luminescence of Ru(bpy)32+ intercalated in some layered titanates and niobates are different from those in aqueous solutions showing guest-guest and hostguest interaction.4 In these processes, layered titanates and niobates work as mediators for electron transfer reactions and materials that serve as microspaces of interlayers. However, photoexcitation of layered compounds themselves is not necessary. On the other hand, Ni-loaded K4Nb6O17 photocatalysts efficiently decompose water into hydrogen and oxygen in a stoichiometric amount.5 Some layered niobates and titanoniobates such as KNb3O8 and CsTi2NbO7 show high photocatalytic activities for hydrogen evolution from aqueous methanol solutions as well as K4Nb6O17.5a,6 K4Nb6O17 catalysts show activity for a photoinduced oxygen isotope exchange reaction.7 Methyl viologen intercalated in layered titanates and niobates is photochemically reduced.8,9 Photoexcitation processes of titanate and niobate layers directly take part in these photocatalytic reactions. Therefore, it is important to study the photophysical properties of layered titanates and niobates, for example, luminescence, which is the principal process, in order to understand their photochemistry. Sanz-Garcia et al. have reported cathode luminescence of K4Nb6O17 by electron beam radiation.10 However, luminescence studies that showed properties attributed to layered structures had not been reported. The authors have preliminary reported that KNb3O8 and K4Nb6O17 show bright blue photoluminescence at 77 K, and their emission spectra red-shift with H+ exchange.11 Blasse et al. have recently * E-mail: [email protected] X Abstract published in AdVance ACS Abstracts, October 1, 1996.

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observed a long decay constant (3.4 ms) for photoluminescence of KNb3O8.12 In the present study, effects of interaction between niobate layers and ions at interlayers on photoluminescence of niobate layers themselves were investigated. The factors affecting photoluminescence of ion-exchanged layered niobates are discussed on the basis of Raman measurements and a configurational coordinate diagram. Experimental Section Layered potassium niobates were prepared by melting mixtures of Nb2O5 (Wako Pure Chemical, 99.9%) and K2CO3 (Kanto Chemical) at 1443 K (1 h) for K4Nb6O17 and 1473 K (10 h) for KNb3O8 in a platinum crucible.13 Excess K2CO3 (20-30 mol %) was added to compensate for the loss due to volatilization and to get good crystallinity. The structures of synthesized crystals were confirmed by X-ray diffraction. Ionexchange reactions were carried out by suspending layered potassium niobate powder (1.5 g) in aqueous AlCl3, CaCl2, and NH4Cl solutions (0.5 M). The degree of H+ exchange was controlled by changing the concentration and temperature of aqueous HCl and HNO3 solutions and was determined by using ICP (Seiko Instruments, SPS1500VR plasma spectrometer) and EDX (Philips, EDAX-PV9900) instruments. FT-IR (JEOL, JIR7000) equipped with a Raman unit (RS-RSU200) was used for Raman measurements. The Raman spectra were obtained using radiation of 1064 nm from a Nd:YAG laser. Powdered samples were used for luminescence measurements at liquid nitrogen temperature (77 K) using a quartz cell and a fluorometer (Spex, Fluoromax, light source; 150 W Xe lamp, photomultiplier tube; R928P). Results Figure 1 shows emission spectra of some ion-exchanged K4Nb6O17 at 77 K. Native K4Nb6O17 showed blue luminescence with a maximum around 420 nm. It is noteworthy that twodimensional niobate sheets showed efficient luminescence. The © 1996 American Chemical Society

17324 J. Phys. Chem., Vol. 100, No. 43, 1996

Figure 1. Emission spectra of cation-exchanged K4Nb6O17 at 77 K: (a) native; (b) NH4+; (c) Ca2+; (d) Al3+; (e) H+. The degrees of NH4+, Ca2+, Al3+, and H+ exchange to K+ were determined to be 34, 20, 58, and 68% by measuring amounts of eluted K+ using ICP measurements, respectively. However, H+ may partly be changed for NH4+-, Ca2+-, and Al3+-exchanged derivatives.

Kudo and Sakata

Figure 4. Raman spectra of H+-exchanged K4Nb6O17: degree of H+ exchange, (a) 0%; (b) 17%; (c) 66%; (d) 68%; (e) 92%.

Figure 5. Emission spectra of H+-exchanged KNb3O8 at 77 K: degree of H+ exchange, (a) 0%; (b) 18%; (c) 30%; (d) 54%; (e) 81%. Figure 2. Emission spectra of H+-exchanged K4Nb6O17 at 77 K: degree of H+ exchange, (a) 0%; (b) 17%; (c) 66%; (d) 68%; (e) 92%.

Figure 6. Excitation spectra of H+-exchanged KNb3O8 at 77 K: degree of H+ exchange, (a) 0%; (b) 18%; (c) 30%; (d) 54%; (e) 81%. Figure 3. Excitation spectra of H+-exchanged K4Nb6O17 at 77 K: degree of H+ exchange, (a) 0%; (b) 17%; (c) 66%; (d) 68%; (e) 92%.

emission intensity was increased when K+ was exchanged with NH4+, which possessed smaller polarizing power than K+. In contrast, the emission spectra red-shifted and the intensities were drastically decreased when K+ was exchanged with Ca2+, Al3+, and H+, which had larger polarizing power. Thus, it was found that the polarizing power of cations at interlayers affected the energy state of niobate layers, resulting in a change in luminescent properties. Figures 2 and 3 show the effect of the degree of H+ exchange on emission and excitation spectra of K4Nb6O17 at 77 K. The emission wavelength red-shifted and the intensity was decreased with an increase in the degree of H+ exchange. The luminescence was hardly observed for nearly 100% H+-exchanged K4Nb6O17. In excitation spectrum, the onsets were almost the same as each other except for the 17% H+-exchanged one. Kim et al. have reported that the onsets of diffuse reflection spectra of K4Nb6O17 and KNb3O8 hardly changed even after H+ exchange.3b The effect of H+ exchange on excitation spectra was the same as that on the diffuse reflection spectra. Figure 4 shows Raman spectra of native and H+-exchanged K4Nb6O17. The sharp and strong band was observed at 880

cm-1 for native and 17% H+-exchanged K4Nb6O17. The intensity of this band was decreased and a new weak band at 940 cm-1 arose as the degree of H+ exchange increased up to 68%. At 92% H+ exchange, only weak and broad bands were observed. Jehng et al. have investigated Raman spectra of various niobates including layered perovskite oxides.14 According to their results, niobates with highly distorted NbO6 units have short Nb-O bonds with high bond orders (>1) and show Raman bands due to a stretching mode around 850-1000 cm-1. Moreover, a Raman band due to the short Nb-O bond of KCa2Nb3O10 was shifted by H+ exchange from 930 to 965 cm-1. K4Nb6O17 consists of highly distorted NbO6 units due to the two-dimensional structure and the Nb-O bonds which stick out into interlayers are short (∼ 1.70 Å),15a as observed in KCa2Nb3O10. Therefore, the sharp and strong Raman band at 880 cm-1 for K4Nb6O17 was assigned to the stretching mode due to the short Nb-O bonds with a high bond order (doublebond-like). A decrease in the sharp Raman band intensity corresponded to that of luminescence. Here, the sharp Raman band was also observed for NH4+-exchanged K4Nb6O17, whose luminescent intensity was increased compared with native K4Nb6O17. Figures 5 and 6 show the effect of the degree of H+ exchange on emission and excitation spectra of KNb3O8 at 77 K. Native KNb3O8 showed blue luminescence with a maximum around

Photoluminescence of Ion-Exchanged Layered Niobates

J. Phys. Chem., Vol. 100, No. 43, 1996 17325

Figure 7. Raman spectra of H+-exchanged KNb3O8: degree of H+ exchange, (a) 0%; (b) 18%; (c) 30%; (d) 54%; (e) 81%.

450 nm even at 300 K.11 The emission wavelength red-shifted and the luminescent intensity was decreased as the degree of H+ exchange increased, as for the case of K4Nb6O17. In excitation spectra, the onsets were around 327 nm and hardly changed with H+ exchange, agreeing with the results of their diffuse reflection spectra.3b Figure 7 shows Raman spectra of native and H+-exchanged KNb3O8. The sharp and strong Raman band was observed at 955 cm-1 for native and 18% H+-exchanged KNb3O8. This band is also assigned to the stretching mode of short Nb-O bonds (∼1.73 Å) which stuck out into interlayers.15b The intensity of the band was decreased and a new sharp band appeared at 980 cm-1 as the degree of H+ exchange increased. A decrease in the Raman band intensity at 955 cm-1 corresponded to that in luminescent intensity, which was also the case for K4Nb6O17. Raman bands due to short Nb-O bonds of K4Nb6O17 and KNb3O8 in the present study and of KCa2Nb3O10 as reported14 shifted to higher wavenumber by H+ exchange. In general, bond orders of the short Nb-O bonds should decrease due to hydrogen bonding by H+ exchange and, therefore, their Raman bands should have shifted to lower wavenumber. However, such a shift was not observed in the present study. The reason is not clear at the present stage. Discussion Blasse et al. have examined luminescence of a series of titanates and niobates, which indicated that the excitation energy was shifted to the lower side and the Stokes shift was decreased by delocalization of excitation energy.16 For K4Nb6O17 and KNb3O8, excitation wavelengths were longer and the Stokes shifts were smaller compared with niobates with isolated NbO6 units, as in CaNb2O6.17 This fact suggests that the excitation energy in K4Nb6O17 and KNb3O8 is delocalized in several NbO6 units at least. However, it is not clear if the excitation energy is delocalized in niobate layers in a long range. On the other hand, Blasse et at. have shown that the emission center of KNb3O8 is the localized niobyl bonds (the short Nb-O bonds) and that the excitation energy comes from corner-sharing niobate octahedra.12 It has also been reported that short metal-oxygen bonds affect the luminescent properties of titanates and niobates.16g Both niobate layers of K4Nb6O17 and KNb3O8 consist of edgeand corner-sharing NbO6 units and have short Nb-O bonds which stick out into interlayers.15 In native K4Nb6O17 and KNb3O8, K+ ions coordinate to the oxygen in the short Nb-O bonds, while exchanged cations do so in ion-exchanged derivatives. Luminescent properties are probably changed by a change in the bond order in the short Nb-O bond due to the difference of polarizing power of exchanged cations. Let us discuss the effect of H+ exchange on the luminescent properties qualitatively using a simple configurational coordinate

Figure 8. Configurational coordinate diagrams: (a) native layered niobates; (b) H+-exchanged derivatives.

diagram16c for luminescence from the short Nb-O bonds in niobate layers (Figure 8). The bond orders of the short Nb-O bonds which stick out into interlayer are more than 1 due to high distortion. The bond order in H+-exchanged derivatives is smaller than that in native ones because hydrogen bonding is formed with the oxygen atom of the Nb-O bond. Therefore, a force constant, k, in the eq 1 for the short Nb-O bond in native layered niobates is larger than that in H+-exchanged derivatives.

U ) 1/2k∆x2

(1)

where U and ∆x represent potential energy and displacement, respectively. In general, k in the ground state is larger than that in the excited state. If the difference in equilibrium bond distance between Nb and O atoms for native layered niobates is smaller than that for H+-exchanged derivatives, ∆Q is smaller than ∆Q′, as shown in Figure 8. The energy splitting between ground and excited states for native layered niobates is larger than that for H+-exchanged derivatives (∆U > ∆U′) because the interaction between Nb and O for the former is stronger than that for the latter. Thus, ∆U becomes smaller and ∆Q larger for H+exchange derivatives compared with native ones, resulting in red-shifts in emission spectra (EM > EM′), as shown in Figures 2 and 5. On the other hand, excitation spectra (EX) depend on k, ∆U, and ∆Q. The changes in k, ∆U, and ∆Q would cancel each other resulting in little difference in excitation spectra, as shown in Figures 3 and 6. The discussion above is based on the energy diagram of localized Nb-O bonds which stick out into interlayers. However, the excitation energy is probably delocalized more or less judging from the onset of excitation spectra and the Stokes shift mentioned above. The number of short Nb-O bonds whose oxygen atoms are terminal and unshared is one per two NbO6 units for K4Nb6O17 and two per three for KNb3O8.15 Therefore, it is reasonable that the energy states of the local Nb-O bonds can largely contribute to the whole energy states of niobate layers. In general, photocatalytic activities for hydrogen evolution are remarkably increased by H+ exchange, suggesting that photogenerated electrons readily transfer to H+ at interlayers.18 Electron trapping by H+ at interlayers can contribute to quenching of luminescence for H+-exchanged derivatives. A decrease in the luminescent efficiency for H+-exchanged layered niobates may be explained by taking account of a decrease in ∆E in Figure 8 (∆E > E′), which is attributed to the structural change of niobate layers, especially short Nb-O bonds. However, the quenching seems to be mainly due to the electron trapping by H+ at interlayers rather than an increase in the probability of nonradiative transition due to the decrease in ∆E.

17326 J. Phys. Chem., Vol. 100, No. 43, 1996 The strong Raman band still remained for low H+-exchanged K4Nb6O17 (17%) and KNb3O8 (18%), although the luminescence intensities were drastically decreased. These results could suggest that delocalized excitation energy (or electrons) is trapped by H+ exchanged at interlayers. Electrons could move in the two-dimensional niobate layers more or less due to the delocalization mentioned above, with the result that quenching of the luminescent intensity does not have to be linear to the number of H+-exchanged sites. Conclusions K4Nb6O17 and KNb3O8 arose as a new type of blue luminescent material. The energy states of their two-dimensional niobate layers could be changed by ion exchange, which affected the bond order of the short Nb-O bonds sticking out into interlayers. This phenomenon will be interesting from the point of view of effects of substances at interlayers on energy states of oxide layers, namely, host-guest interaction in twodimensional materials. Acknowledgment. The support by Tokuyama Science Foundation and by Grants-in-Aid for Scientific Research (07740543) from the Ministry of Education, Science, and Culture is greatly acknowledged. The authors thank Dr. Ohki for SEM-EDX measurements and Dr. Wakiya for ICP measurements at Tokyo Institute of Technology, and Professor Koide and Mr. Baba for Raman measurements at Science University of Tokyo. References and Notes (1) Ogawa, M.; Kuroda, K. Chem. ReV. 1995, 95, 399, and references therein. (2) (a) Yoshimura, J.; Kudo, A.; Tanaka, A.; Domen, K.; Maruya, K.; T. Onishi Chem. Phys. Lett. 1988, 147, 401. (b) Yoshimura, J.; Tanaka, A.; Kondo, J.; Domen, K. Bull. Chem. Soc. Jpn 1995, 68, 2439. (3) (a) Kim, Y. I.; Salim, S.; Huq, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1991, 113, 9561. (b) Kim, Y. I.; Atherton, S. J.; Brigham, E. S.; Mallouk, T. E. J. Phys. Chem. 1993, 97, 11802.

Kudo and Sakata (4) Nakato, T.; Kusunoki, K.; Yoshizawa, K.; Kuroda, K.; Kaneko, M. J. Phys. Chem. 1995, 99, 17896. (5) (a) Domen, K.; Kudo, A.; Shinozaki, A.; Tanaka, A.; Maruya, K.; Onishi, T. J. Chem. Soc., Chem. Commun. 1986, 356. (b) Kudo, A.; Tanaka, A.; Domen, K.; Maruya, K.; Aika, K.; Onishi, T. J. Catal. 1988, 111, 67. (c) Kudo, A.; Sayama, K.; Tanaka, A.; Asakura, K.; Domen, K.; Maruya, K.; Onishi, T. J. Catal 1989, 120, 337. (d) Domen, K.; Kudo, A.; Tanaka, A.; Onishi, T. Catal. Today 1990, 8, 77. (e) Sayama, K.; Tanaka, A.; Domen, K.; Maruya, K.; Onishi, T. Catal. Lett. 1990, 4, 217. (f) Sayama, K.; Tanaka, A.; Domen, K.; Maruya, K.; Onishi, T. J. Catal. 1990, 124, 541. (g) Sayama, K.; Tanaka, A.; Domen, K.; Maruya, K.; Onishi, T. J. Phys. Chem. 1991, 95, 1345. (6) Sekine, T.; Yoshimura, J.; Tanaka, A.; Domen, K.; Maruya, K.; Onishi, T. Bull. Chem. Soc. Jpn. 1990, 63, 2107. (7) Metrat, G.; Courbon, H.; Pichat, P. J. Phys. Chem. 1992, 96, 5491. (8) Kameyama, A.; Domen, K.; Maruya, K.; Endo, T.; Onishi, T. J. Mol. Catal. 1990, 58, 205. (9) (a) Nakato, T.; Miyata, H.; Kuroda, K.; Kato, C. React. Solids 1988, 6, 231. (b) Nakato, T.; Kuroda, K.; Kato, C. J. Chem. Soc., Chem. Commun. 1989, 1144. (c) Nakato, T.; Kuroda, K.; Kato, C. Chem. Mater. 1992, 4, 128. (d) Nakato, T.; Ito, K.; Kuroda, K.; Kato, C. Microporous Mater. 1993, 1, 283. (e) Nakato, T.; Kuroda, K.; Kato, C. Catal. Today 1993, 16, 471. (10) Sanz-Garcia, J. A.; Dieguez, E.; Zaldo, C. Phys. Status Solidi A 1988, 108, K145. (11) Kudo, A.; Sakata, T. Chem. Lett. 1994, 2179. (12) Blasse, G.; Tol, F. v. Solid State Commun. 1995, 95, 465. (13) Nassau, K.; Shiever, J. W.; Bernstein, J. J. Electrochem. Soc. 1969, 116, 348. (14) Jehng, J.-M.; Wachs, I. E. Chem. Mater. 1991, 3, 100. (15) (a) Gasperin, M.; Bihan, M. T. L. J. Solid State Chem. 1982, 43, 346. (b) Gasperin, P. M. Acta Crystallogr. 1982, B38, 2024. (16) (a) Blasse, G.; Bril, A. Z. Phys. Chem. N. F. 1968, 57, 187. (b) Blasse, G. Struct. Bonding 1980, 42, 1. (c) Blasse, G. Prog. Solid State Chem. 1988, 18, 79. (d) Alarcon, J.; Blasse, G. Phys. Status Solidi A 1993, 135, 627. (e) Hamoumi, M.; Wiegel, M.; Blasse, G. J. Solid State Chem. 1994, 108, 410. (f) Wiegel, M.; Hamoumi, M.; Blasse, G. Mater. Chem. Phys. 1994, 36, 289. (g) Bouma, B.; Blasse, G. J. Phys. Chem. Solids 1995, 56, 261. (17) Wachtel, A. J. Electrochem. Soc. 1964, 111, 534. (18) (a) Domen, K.; Kudo, A.; Shibata, M.; Tanaka, A.; Maruya, K.; Onishi, T. J. Chem. Soc., Chem. Commun. 1986, 1706. (b) Domen, K.; Ebina, Y.; Sekine, T.; Tanaka, A.; Kondo, J.; Hirose, C. Catal. Today 1993, 16, 479.

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