Polarized Raman and optical absorption spectra of the mordenite single crystals containing sulfur, selenium, or tellurium in the one-dimensional nanochannels

22 March 1996

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 251 (1996) 230-234

Polarized Raman and optical absorption spectra of the mordenite single crystals containing sulfur, selenium, or tellurium in the one-dimensional nanochannels V.V. P o b o r c h i i a,b a Institute for Materials Research, Tohoku University, Sendai 980-77, Japan b Ioffe Physico-Technical Institute, St. Petersburg 194021, Russian Federation

Received 26 September 1995; in final form 9 January 1996

Abstract

Polarized Raman and optical absorption spectra of the natural mordenite single crystals containing adsorbed species of sulfur, selenium, or tellurium within one-dimensional channels (diameter of the channel ca. 0.7 nm) have been studied. The bands in the Raman and absorption spectra, which are polarized along the channel direction, are attributed to the helical chains. Experimental evidence is shown for the assignment of other bands to the ring-like molecules S6, S 8, Se6, and probably Te 6.

1. Introduction

Recent interest in the zeolite-confined materials is associated with the possibility of preparing a variety of low-dimensional systems inside zeolite pores. Mordenite-confined chalcogens Se, S, and Te are good candidates for the generation of one-dimensional polymeric chains inside channels, which are directed along the c-axis of the mordenite crystal and possess an elliptical cross-section of 0.67 x 0.7 nm (Fig. 1). The structure of the mordenite-confined cbalcogens, especially Se, was examined in a number of works by Raman scattering [1-6], EXAFS [7,8], electron microscopy [9], and optical absorption spectroscopy [6-8,10-13]. However, this problem is not completely solved at the present time. One of the reasons for this is a shortage of experimental studies

on single crystals. Most studies have been performed with powder samples with ca. 1 /xm size crystallites. In all the previous studies, starting with the work reported in Refs. [3,10,11 ] and including recent works [5,6,13], all the observed phenomena had been considered with the assumption that the mordenite-confined selenium only form chains. In this work, another point of view is proposed and experimentally supported. The present work is devoted to study of the polarized Raman and absorption spectra of the natural mordenite single microcrystals (the maximal sizes being ca. 20 x 50 X 300 ~ m along the a-, b-, and c-axes, respectively), containing S, Se, or Te (crystals: M - S , M-Se, and M - T e , respectively). Polarized Raman spectra of M - S and M - T e are presented for the first time, and a new interpretation

0009-2614/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PII S 0 0 0 9 - 2 6 1 4 ( 9 6 ) 0 0 0 4 5 - 0

V.V. Poborchii / Chemical Physics Letters 251 (1996) 230-234

a=18.13

b=20.29 ---I

c=7.50

Fig. 1. Cross-section of mordenite in the plane normal to the main channel direction and the shape of the mordenite single crystal.

of the Raman spectra of M - S e is reported. The most important result of this work is the first experimental evidence for the existence of the ring-like molecules of S, Se, and Te in the mordenite channels. A way of distinguishing between the features of the chains and rings in the Raman and optical absorption spectra, and for extracting the real absorption spectra of the one-dimensional chalcogen chains from the absorption spectra of M - S , M-Se, and M - T e crystals, is shown.

2. Experimental Natural mordenite crystals with the approximate unit cell formula of the framework Ca2Na4AlsSi40096 have been used. Chalcogens were adsorbed into the mordenite channels at a temperature of ca. 500°C during several days after the hydration of the mordenite. Optical microscopy study shows that the sulfur and selenium are distributed homogeneously along the mordenite crystal, but the content of the tellurium changes gradually from the highest concentration at the edges of the crystal to the lowest concentration in the middle part of the crystal. This effect is associated with the low rate of diffusion of tellurium in the mordenite channels because of the large Te atom size. It is not so easy to perform Raman measurements of microcrystals such as mordenite using only macrooptical Raman devices. In this work, Raman spectra have been studied using the microoptical equipment as well as the traditional macrooptical technique. Usage of a microoptical Raman device consisting of a microscope optically connected with a double or triple monochromator, allowed to find easily microcrystals, to choose high-quality crystals

231

or high-quality part of a crystal, and by using a microobjective, to collect effectively the llight scattered from the few microns area excited by the laser microprobe. Triple Dilor-Z, and double DFS-24 monochromators were used. The 647.1 nm line of a Kr laser was used for the excitation of the ~pectra of M - S e and M-Te. The 514.5 nm line of an Ar laser was used for the excitation of the spectrum of M - S and M-Se. The power of the incident light ~vas 1-20 mW and the size of the light probe was 10-30 /xm. It is well known that Raman spectra 0f zeolite matrices, which are excited by visible light, are much weaker than the spectra of the adsorbed chalcogens. The bands of the mordenite vibrations were weaker than the noise level in Raman spectra of M-Se, M-S, and M-Te. Studying the optical absorption spect~-a of the microcrystals with sizes similar to the sizes of the examined mordenite crystals is practically impossible, if one uses a traditional macrooptical device. Optical absorption spectra of mordenite crystals containing chalcogens have been studied wi!h the microspectrophotometer consisting of a LUMAM microscope equipped with two quartz micrOobjectives and optically connected with a single mofiochromator MDR-4. The light probe size was ca. 10 /xm. Mordenite crystals were immersed i n t o the glycerol-water mixture with a refraction index close to that of the mordenite to avoid surface light scattering.

3. Results and discussion Raman spectra of M - S for the polari~ ations aa, bb, and cc ('aa' means that the polarizat!ons of the incident and scattered light are parallel to ]the a-axis of the mordenite crystal) are shown in Fig i 2. The cc spectrum of M - S displays clearly the bands of the molecules S 6 (206, 265, 484 c m - l ) , S 8 (160, 225, 245, 484 cm- i ), and polymeric sulfur S, (275, 410, 456 cm - l ) according to the references [114,15]. The bands at 265 cm -l (S6) and at 275 cm -l (S,,) overlap, giving a doublet structure. The bands of the S 6 and S s molecules are displayed in iall of the polarizations (aa, bb, cc). The bands o 4 the polymeric chain are displayed only in the cc spectrum. It

232

V.V. Poborchii / Chemical Physics Letters 251 (1996) 230-234 160 225

Z [-

z

484

225 160 1 ; 2061245

--

100

484 bb

300

500

RAMAN SHIFT, cm-1 Fig. 2. Polarized Raman spectra of the M-S single crystal excited with the 514.5 nm line of an Ar laser.

should be noted that the bands of the S 8 molecule are slightly shifted to the higher energies compared to the positions of the bands of the S 8 molecule [14]. This fact means that the S 8 molecules in the channels are slightly pressed by the mordenite framework. Optical absorption spectra of M - S are presented in Fig. 3a. Absorption of the light polarized parallel to the c-axis (EIIc) is stronger than that of the light polarized parallel to the b-axis (Ellb). The spectrum E]]b can be described by the absorption of S 6 and S 8 molecules, the peak position at ca. 4.5 eV can be associated with the S 8 absorption band [16]. The spectrum E[[c is determined not only by absorption of S 6 and S 8 molecules, but also by the polymeric chain absorption, which gives the band at the lowenergy side of the S 6 and S 8 absorption. Thus, the polymeric sulfur chain contributes only to E[[c absorption spectra and cc-polarized Raman spectra of the M - S crystal. The high activity of the sulfur chain vibrations in the cc-polarized Raman spectra with 514.5 nm excitation is associated with the resonant enhancement due to the E[[c absorption of the chain. It is not so difficult to extract the Raman spectra of the sulfur chain from the spectrum of M - S , because the bands of the chain, S 6, and S 8 molecules are basically known. In fact, the bands at 456, 410, and 270 cm -1, and the broad low-frequency acous-

tic-like band, all of which are cc-polarized, should be attributed to the chain vibrations. The problem of how to extract the absorption spectrum of the chain from the absorption spectra of M - S , is quite complicated. However, if we suppose that the contribution of the S 6 and S 8 molecules to the EIIb spectrum of M - S is roughly equal to that to the EIIc spectrum, we can consider the difference between these two spectra as a contribution of the chains. This difference spectrum is displayed in Fig. 3b. The spectrum is in agreement with the absorption spectrum of polymeric sulfur [ 16]. Let us consider now Raman spectra (Fig. 4) and absorption spectra (Fig. 3) of M - S e from the same point of view we used for M - S . Raman spectra of M - S e (Fig. 4) have been explained [4-6] with the assumption that only one type of Se species is situated in the mordenite channels. The polymeric chain structure, consisting of the chain-like and ring-like fragments, was proposed. However, this assumption should be revised. Similar to the case of M - S , we can expect that

-~

L5

M-Te - M-Se .....

Ud

<

1.0

M-S

\'~i/b /

i

/ .t"

0

l

0.5

I •

/

i~. I* ' ii e

- -

~ 1.5 rT~ 121 <

1.0

b

0.5

2.0

3.0

4.0

PHOTON ENERGY, eV Fig. 3. Polarized absorption spectra of the M-S, M-Se, and M-Te single crystals for the polarizations EIIc and El[b (a); corresponding difference spectra between Ellc and EIIb absorption spectra.

V.V. Poborchii / Chemical Physics Letters 251 (1996) 230-234

r--.-.-

3O0

200

100

R A M A N SHIFT, cm-1 Fig. 4. Polarized Raman spectra of the M - S e single crystal excited with the 514.5 nm line of an Ar laser (top) and with the 647.1 nm line of a Kr laser (other spectra). For the 647.1 nm excitation, the intensity of the cc-spectrum is an order-of-magnitude higher than the intensities of the spectra for aa- and bb-polarizations.

the Se chains in M - S e contribute only to cc-polarized Raman spectra and E[Ic absorption spectra. With this assumption, only the 256 cm-1 band and the low-frequency acoustic-like band can be attributed to the chain vibrations. The bands at 104, 135, and 274 cm-~ should be attributed to another Se species. This point of view is supported by the dependence of Raman spectra on the wavelength of excitation (Fig. 4). The 256 c m - t band on the one hand, and the 104, 135, and 274 c m - l bands on the other hand show different dependencies. At 514.5 nm excitation, the latter bands become weaker compared to the 256 cm-1 band. It is easy to propose the species which can be responsible for these bands. The same bands have been found recently in Raman spectra of the chabazite, containing selenium (Ch-Se) [17]. The spectrum of Ch-Se [17] is similar to the aa spectrum of M-Se. Both spectra can be assigned to the Se 6 ring molecule described in [18]. The diameter of this molecule, 0.65 nm, is slightly smaller than the chabazite cavity sizes of 0.67 × 0.67 x 0.7 nm and the mordenite channel cross-section of 0.67 × 0.7 nm. Thus, the S e 6 molecule can be stabilized inside the chabazite cavity and mordenite channel. The polarization dependence of Raman spectra of M - S e corresponds to the Se 6 molecule oriented by

233

the three-fold axis along the b-axis of the rnordenite crystal. In fact, the bands at 104 cm -~ and 135 cm -~, which can be assigned to the E g and Ag bond-bending modes, should be less active, when the incident and scattered light beams are polarized parallel to the three-fold axis of the molecule, the Eg mode being forbidden in this geometry, i The A s symmetric bond-bending mode is not forbidden, but it is much more active for the polarizations of the incident and scattered light beams parallei to each other and perpendicular to the three-fold axis of the molecule. Polarized absorption spectra of M - S e can be treated in the same manner as absorption Ipectra of M-S. The main contribution to the absorption is due to the chains for Ellc and due to the Se 6 molecules for Ellb. The absorption anisotropy for iM-Se is higher than that for M-S. Probably, this rOeans that the ratio of the total number of Se atoms in the chains to the total number of Se atoms in We rings is higher than the same ratio for S atoms in M - S e and M - S , respectively. This comparison is not absolutely correct because Se 6 molecules are oriented in the channel in contrast to the S 6 and S 8 molecules, which basically have no definite orientation. However, the data on absorption spectra of M - S e [6] show roughly the same absorption for Ella ]and Ell& We can conclude that the contribution o~ the Se 6 molecules to the Ell c absorption spectrum iis similar to that to Ella and Ellb spectra. Thus, the] contribution of the Se 6 molecules to the Ellc ~bsorption spectrum is quite weak. As a first approximation, we can ignore this contribution. At high energies, the contribution of the S e 6 molecules is not iso small, and probably the difference between EIIcl and EIIb spectra of M - S e (Fig. 3b) gives more detailed information about the spectrum of the one-dimensional Se chains. The spectra of M - T e have also been s~died (Fig. 5). Unfortunately, only the bands due to !the bondstretching modes have clearly been obsdrved. The polarization dependence of the bands is isimilar to that in Raman spectra of M-Se. The c&polarized band is displayed at 172 cm-~. This ban~ is similar to the 256 cm- ~ band in the spectrum of l~l-Se. The aa-, bb-, and cc-active bands are situa~d at 185 cm- 1. This band is similar to the 274 cm* l band of M-Se. The band at 172 cm- 1 should be aitributed to

234

V.V. Poborchii / Chemical Physics Letters 251 (1996) 230-234

185

e~

Acknowledgements The author is grateful to Professor V.N. Bogomolov for supplying natural mordenite and to the International Science Foundation (Grant R4P300) for the partial support.

aa bb

z cc

100

200

References

RAMAN SHIFT, cm-1 Fig. 5. Polarized Raman spectra of the M-Te single crystal excited with the 647.1 nm line of a Kr laser, the intensity of the cc-spectrum being an order-of-magnitude higher than the intensities of the other spectra.

the one-dimensional Te chain, and the band at 185 c m - l should be attributed to some kind of ring-like molecules, probably the 're 6 molecule with a structure similar to the Se 6 molecule. The Te 6 molecule should be slightly pressed in the mordenite channel. Absorption spectra of M - T e (Fig. 3) can be treated in the same manner as absorption spectra of M - S e .

4. Conclusion The helical chains and ring-molecules (56, 58, Se 6, Te 6) coexist in the mordenite channels containing adsorbed chalcogens. The chains contribute to absorption spectra only for Ell c polarization. Correspondingly, the resonance enhancement of Raman spectra of the chains is working for cc-polarization, and so Raman bands of the chains are active in this polarization. Ring molecules absorb the light for all the possible polarizations, and so the molecules' vibrations are active for aa-, bb-, and cc-polarizations. The Se 6 molecules were found to be oriented by the three-fold axis along the b-axis of the mordenite. The ratio between the concentrations of chains and rings is higher for M - S e and M - T e than that for M - S . However, this ratio is stable for all M - S e and M - T e crystals (for M - S crystals some variations have been observed). It can be associated with some kind of regular arrangement between chains and rings inside channel. Study of this problem is now in progress.

[1] V.N. Bogomolov, V.V. Poborchii, S.G. Romanov and S.I. Shagin, J. Phys. C 18 (1985) L313. [2] V.N. Bogomolov, V.V. Poborchii, S.G. Romanov and S.I. Shagin, Proc. of the Soviet Conference Stekloobraznyje Poluprovodniki, Leningrad, 1985, p. 107. [3] V.N. Bogomolov, V.V. Poborchii, S.V. Kholodkevich and S.I. Shagin, JETP Letters 38 (1983) 532. [4] V.V. Poborehii, Proc. of the 1st Japanese-Russian Meeting Material Design Using Zeolite Space, Kiryu, Japan, 1991, p. 1. [5] V.V. Poborehii, J. Phys. Chem. Solids 55 (1994) 737. [6] V.V. Poborchii, M.S. Ivanova and S.S. Ruvimov, in: Studies in surface science and catalysis, Vol. 84. Zeolites and related microporous materials: State of the art 1994, eds. J. Weitkamp, H.G. Karge, H. Pfeifer and W. HSldrich (Elsevier, Amsterdam, 1994), p. 2285. [7] K. Tamura, S. Hosokawa, H. Endo, S. Yamasaki and H. Oyanagi, J. Phys. Soc. Japan 55 (1986) 528. [8] J.B. Parise, J. MacDougall, N. Herron, R. Farlee, A.W. Sleight, Y. Wang, T. Bein, K. Moiler and L.M. Moroney, lnorg. Chem. 27 (1988) 221. [9] O. Terasaki, K. Yamazaki, J.M. Thomas, T. Ohsuna, D. Watanabe, J.V. Sanders and J.C. Barry, Nature 330 (1987) 58. [10] V.N. Bogomolo~;, S.V. Kholodkevich, S.G. Romanov and L.S. Agroskin, Solid State Commun. 47 (1983) 181. [11] L.S. Agroskin, V.N. Bogomolov, A.I. Gutman, A.I. Zadorozhnii, L.P. Rautian, S.G. Romanov, Pis'ma Zh. Eksp. Teor. Fiz. 31 (1980) 617. [12] Y. Nozue, T. Kodaira, O. Terasaki, K. Yamazaki, T. Goto, D. Watanabe and J.M. Thomas, J. Phys. Condens. Matter 2 (1990) 5209. [13] Y. Katayama, N. Koyama and K. Tsuji, AlP Conf. Proc. (USA), No. 309, Part 2 (1994) 1511. [14] A.T. Ward, J. Phys. Chem. 72 (1968) 4133. [15] J. Berkovitz and W.A. Chupka, J. Chem. Phys. 47 (1967) 4320. [16] B. Meyer, T.V. Oommen, D. Jensen, J. Phys. Chem. 75 (1971) 912. [17] Yu.A. Bamakov, V.V. Poborchii and A.V. Shchukarev, Phys. Stat. Sol. 37 (1995) 847. [18] K. Nagata, K. Ishibashi and Y. Miyamoto, Japan. J. Appl. Phys. 20 (1981) 463.