J. Phys. Chem. 1992, 96, 6144-6152
6744
Acknowledgment. This work was supported by NASA Lewis Research Center, Brookpark, OH. Registry No. CO, 630-08-0; Ni, 7440-02-0; KOH, 1310-58-3; C02, 124-38-9; C03’-,3812-32-6; ”C, 9118-70-1.
References and Notes (1) Adzic, R. R. In Modern Aspecfs ofElecfrochemistry;White, R. E., Bockris, J. OM., Conway, B. E.,Us.Plenum ; Press: New York, 1990 Vol. 21. (2) Parsons, R.; VanderNoot, T. J . Elecfroanal. Chem. 1988, 257, 9. Iwasita-Vielstich T. In Advances in Electrochemical Science and Engineering; Tobias, C., Gerischer, H., Eds.; Wiley: New York, 1990; Vol. 1. (3) For a general review in the area of CO electrooxidation see: Beden, B.; Lamy, C.; de Tacconi, N. R.; Arvia, A. J. Electrochim. Acfa 1990, 35, 691. (4) Clavilier, J.; Faure, R.; Guinet, G.;Durand, R. J. Electroanal. Chem. 1980,107,205. Hourani, M.; Wieckowski, A. J. Electroanal. Chem. 1987, 227, 259. Zurawski, D.; Rice, L.; Hourani, M.; Wieckowski, A. J . Electroanal. Chem. 1987, 230, 221. (5) Chang, S-C.; Weaver, M. J. J . Phys. Chem. 1991, 95, 5391. (6) Wagner, F.; Ross, P. J . Elecfroanal. Chem. 1988, 250, 301. (7) The Ni(lI1) crystal (Cornell Laboratory) was a thin wafer circular in shape with a surface geometrical area of 0.785 cm2.
(8) Netzer, F. P.; Madey, T. E. J . Chem. Phys. 1982, 76, 710. (9) Hubbard, A. T.; Stickney, J. L.; Rosasco, S.D.; Soriaga, M. P.; Song, D. J. Electroanal. Chem. 1983, 150, 165 and references therein. Wagner, F. T.; Ross, Jr. P. N. J. Electroanal. Chem. 1983, 150, 141 and references therein. Homa, A: S.;Yeager, E.;Cahan, B. D. J. Electroanal. Chem. 1983, 150, 181 and references therein. J. L. Stickney, J. L.; C. B. Ehlers, C. B.J . Vac. Sci. Technol. A 1989, 7, 1801. Wagner, F. T.; Moylan, T. E.J. Electrochem. Soc. 1989,136,2498. Kamrath, M.; Zurawski, D.; Wieckowski, A. Langmuir. 1990.6, 1683 and references therein. Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J . Electroanal. Chem. 1989,264,291. Leung, L.-W. H.; Gregg, T. W.; Goodman, D. W. Rev.Sci. Instrum. 1991,62, 1857. (10) Wang, K.; Eppell, S.J.; Chottiner, G.S.;Schenon, D. A.; Reid, M. A. submitted to Rev. Sci. Instrum. (1 1) Schrebler-Guzman, R. S.;Vilche, J. R.; Arvia, A. J. Corros. Sci. 1987, 18, 765. Schrebler-Guzman, R. S.;Vilche, J. R.; Arvia, A. J. J . Electrochem. Soc. 1978, 125, 1578. (12) Visintin, A.; Chialvo, A. C.; Triaca, W. E.;Ania, A. J. J . Elecrrwnal. Chem. 1987, 225, 227 and references therein. (13) Zurawski, D.; Wasberg, M.; Wieckowski, A. J . Phys. Chem. 1990, 94. 2076. (14) Berry, G.M.; Bothwell, M. E.;Michelhaugh, S.L.; McBride, J. R.; Soriaga, M. P. J. Chim. Phys. 1991,88, 1591. (15) Yau, S.-L.; Gao, X.;Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J. Am. Chem. SOC.1991, 113,6049.(16) Kita, H.; Shimazu, K.; Kunimatsu, K. J . Electroanal. Chem. 1988, 241, 163.
Multinuclear Magic-Angle Spinning and Double-Rotation NMR Study of the Synthesis and Assembly of a Sodalite Semiconductor Supralattice R. Jelinek, B. F.Chmelka, Materials Sciences Division, Lawrence Berkeley Laboratory, and Department of Chemistry, University of California, Berkeley, California 94720, and Department of Chemical and Nuclear Engineering, University of California, Santa Barbara, California 93106
A. Stein,+and G.A. O h * Advanced Zeolite Materials Research Group, Lash Miller Chemical Loboratories, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada MSS 1 A1 (Received: December 10, 1991; In Final Form: March 2, 1992)
29Simagic-angle spinning, 27Aland Z3Namagic-angle spinning, and double rotation NMR provide structural and electronic details for the newly developed sodalite semiconductor quantum supralattices. The evolution of the nucleation and crystallization processes of sodium halide sodali- is monitored, and the appearance of anionempty sodalitecages is detected. Subtle changes in the electronic and quadrupolar interactions of sodium and aluminum nuclei occur upon loading chloride, bromide, and iodide into the sodalite cages. The NMR results suggest that the development of electronic coupling occurs throughout the lattice in mixed-halide chloro,iodosodalites. A preference for silver exchange of sodium cations in halide-containing cages, over hydroxideantaining and anionempty sodalitecavities, is detected. Extraction of the isotropic chemical shift and quadrupolar contributions to the sodium resonances is achieved by performing the NMR experiments at two magnetic field strengths. The parameters obtained indicatea change in the charge distribution around the sodium nuclei upon exchanging approximately onequarter of the extraframework Na+ cations with silver, which parallels other data pointing to the onset of a semiconductor supralattice within the sodalite matrix.
Introduction The Federovian cuboctahedral structure of sodalites enables them to host insulator, semiconductor, or metal cluster guests of uniform size and shape.’ This unique quality confers upon this class of microporous solids potential uses as advanced materials, which may include semiconductor quantum expanded metal or superconducting array^,^ and redox-active supralattices in erasable highdensity optical data storage and processing media! Single-crystal and Rietveld powder X-ray diffraction methods are pivotal in providing atomically precise structural details of solids that display long-range order. Solid-state NMR on the other hand can address structural questions of a more local nature, To whom correspondence should be addressed. ‘Current address: Bayer AG, D-5090 Leverkusen, Germany.
yielding invaluable information on the environment of atomic species in materials having short-range order or amorphous configurations. The use of high magnetic fields, combined with spatial-averaging sample reorientation NMR techniques like magbangle spinning (MAS) and double rotation (DOR), enables one to obtain a wealth of structural information on solid lattices, including open-framework structures like zeolites. Numerous studies have used MAS to examine structural aspects of the framework nuclei 29Si5and 27Al.6 23NaNMR studies concentrating upon the charge-balancing cations have been conducted as weL7 The recently developed DOR technique is additionally useful in probing fine structural details when observing quadrupolar nuclei.* This work concentrates on the application of 29SiMAS, 27Al and 23NaMAS, and DOR, complemented by X-ray diffraction
0022-3654/92/2096-6744$03.00/00 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6145
A Sodalite Semiconductor Supralattice
TABLE I: Experimental Panmeters Used in the NMR M-lllelltS
resonance
nucleus 29Si 2 7 ~ 1
23Na
freuuencv 99.35 130.31 132.28
90’ pulse
reference material
TMS dilute aqueous Al(NO)3 0.1 M NaCl
length, LLS 11 13 11
and mid- and far-infrared (IR) measurements, in probing intimate structural details of the sodium halosodalite system [unit cell composition Na8X2(A16Si6012), where x = c1, Br, I], and its silver-exchanged derivatives Na8-pAgd(,(A&Si6Ol2),a semiconductor quantum supralattice ~ t r u c t u r e .Previous ~ ~ ~ ~ works have investigated the structural and electronic properties of these materials, including anion and cation distribution and motion inside the cages, using X-ray diffraction and optical spectroscopy.1° Far-IR studies on sodalite structures were conducted as well.” Issues addressed in this study include the process of nucleation and crystallization of the parent sodium sodalite materials, cation and anion arrangements throughout the framework, and the transformation of the sodium parent materials into a semiconductor supralattice upon silver ion exchange. In addition, better insight is sought toward detecting and understanding possible electronic coupling between the contents of adjacent sodalite cages, an important concept when considering further technological and scientific applications of these materials, such as in quantum electronic, nonlinear optical and photonic devices. Experimental Section
The various parent sodium halosodalites and the silver-exchanged samples were synthesized according to a procedure described elsewhere.9J2 Room temperature, high-resolution powder X-ray data were collected on a Phillips PW 1051 diffractometer using Ni-filtered Cu Ka radiation (A = 1.54178 A, Ka2 stripped, 40 mA, 45 kV) with a liquid nitrogen cooled solid-state Ge detector. Infrared measurements were carried out on a Nicolet SDX or a Nicolet 2OSXB FT-IR spectrometer with far-IR capability (650-30 cm-’). All spectra were obtained with 4-m-Iresolution by madding 250-500 interferograms (far-IR range) or 100 interferograms (mid-IR range). The spectra were baseline co~ected by subtracting a linear ramp from the observed results. NMR measurements were carried out at 11.7 T, using a Chemagnetics CMX-500 spectrometer and MAS probehead. Additional experiments were carried out in a 9.4-T field using a Bruker AM-400 spectrometer. DOR spectra in both fields were obtained using a home-built probe described elsewhere;I3 the spinning speed was 5 kHz for the inner rotor and 500-700 Hz for the outer one. In the MAS measurements the samples were spun at 5-6 kHz. Experimental parameters used for the observed nuclei are given in Table I. The accuracy of peak positions relative to the external reference was f0.3 ppm. Fifty scans using 15-s delays were accumulated in the 29Siexperiments. A total of 5WlOOO acquisitions were averaged at room temperature, in the 27Aland 23NaMAS and DOR experiments, using 0.5-s delays between short, -30°, pulses. All spectra were zero-filled to 2K data points, with 50-Hz Lorentzian broadening.
Results .ad Discussion Sodalite Crystallization. The crystallization process of the sodalite structure involves the intermediate formation of a NaA zeolite phase,14 though several studies conclude that no zeolite A is formed in the presence of chloride salts.15 We investigated the evolution of the NaBr-sodalite crystallization and the possible appearance of different species during the synthesis. The bromosodalite synthesis was interrupted at different times, and the structural details of the synthesized materials were examined. For reaction products recovered after 0.5-150 h, selected powder XRD patterns are shown in Figure 1. Within 0.5 h, Figure la, all sodalite reflections are already observed, but they are still very weak and broad. The 0.54 XRD pattern also features a broad background due to an amorphous phase, as well as additional reflections due to zeolite A as the major crystalline phase. The
45
35
25
02e
15
5
Figure 1. XRD powder patterns of products from N a B d a l i t e synthesis, sampled after (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 4 h, (e) 7.8 h, (f) 13
h, (g) 153 hr. -89.2 I
-60
40
.loo
.I20
-140
PPm
Figure 2. 29SiMAS spcctrum of NaBr-sodalite sampled after 0.5 h.
NaA zeolite reflections decrease with time, Figure lb-d, and disappear completely within 4 h; the amorphous background is no longer apparent by this time as well. The sodalite pattern continues to sharpen with further reaction time and, as shown in Figure Id-g, reaches its final form within 4-8 h. The NMR data support a nucleation m c d ” involving NaA zeolite as a chemical intermediate and provide further structural and quantitative information. The 2?3i MAS sptctrum at t = 0.5 h synthesis time, Figure 2, displays two partially resolved resonances. The lowest intensity peak, at -86.1 ppm, corresponds to the NaBmodalite structure,16while the rcsonance at -89.2 ppm is assigned to the Si(OAl), tetrahedra in NaA ~eo1ite.l~More information is providedb y 2 7 A l MAS experimentsshown in F* 3. Upon comparison with the fully crystalline material, the prominent downfield peak at 62.2 ppm in Figure 3b-e is assigned to Al(OSi)4 species in the NaBr-sodalite framework. This shift is close to the reported value of 61.0 ppm for 27Alin sodalite hydrate and 65.0 ppm for NaCl-sodalite,18 the difference can be traced to structural and electronic sensitivity of the framework toward the encapsulated cluster (see discussion in the following section). The upfield peak at 58.7 ppm coincides with the known
6746 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Jelinek et al. 6.8
62.2
A 153 h
; 2 Sh
fd)
(h)Dehydrated
(a)
NaA zeolite
L 70
60
50
40
30
PPm
Figure 3. 27A1MAS spectra of (a) NaA zeolite, and products from N a B M l i t e synthesis sampled after (b) 0.5 h, (c) 2 h, (d) 8 h, (e) 153 h.
20
A
10
0
-10
-20
ppm
Figure 4. 23Na MAS spectra of (a) NaA zeolite, and products from N a B M l i t e synthesis sampled after (b) 0.5 h, (c) 2 h, (d) 8 h, (e) 153 h.
position for four-coordinated aluminum in NaA zeolite,18whose 27AlMAS spectrum is shown in Figure 3a. The quantitative transformation of the NaA phase into sodalite is followed as a function of the synthesis time: after 2 h, the downfield powder pattern predominates, Figure 3c, as NaBr-sodalite becomes the major component. A weak NaA zeolite remnant is still observed in the 27Al spectrum at this time but disappears completely thereafter. The 23NaMAS results, shown in Figure 4, feature a noticeable differencefrom the 27AlMAS data. The intensity of an upfield resonance near -2.5 ppm decreases with time in the early stages of the reaction, Figure 4b,c, but in contrast to the 27AlMAS data, this peak d a s not disappear later. 23Na DOR experiments
20
10
0
.10
.20
PPm
Figure 5. 23Na MAS spectra of (a) hydrated NaBr-sodalite and (b) dehydrated NaBr-sodalite.
conducted on the series produced identical results,19confirming that we do not observe a quadrupolar-broadened signal, but rather a resonance from a distinct 23Nasite. As shown in Figure 4e, the upfield peak is still present even after more than 100 h, at which the XRD and 27Almeasurements indicated a final equilibrium condition. A single 23Naresonance is detected at -1.8 ppm in pure NaA zeolite, as shown in Figure 4a, downfield from the -2.5 ppm resonance observed throughout the synthesis series. This appears to reflect a sodium environment in sodalite which is somewhat different from that in NaA zeolite. The peak at -2.5 ppm disappears upon vacuum dehydration [4 h at 110 “C],as shown in Figure 5. A similar result is produced upon dehydration of NaBr-sodalite with anion-free cavities intentionally introduced into the lattice.19 The sodium signal from the anion-free cages in the dehydrated sample is spread over a large frequency range and thus is not observed in the examined spectral window. This effect is probably due to an increase in the electric field gradients around the Na+ cations?O in the absence of anionic species or water molecules in the cavity. In addition, dehydration might cause a large distribution of the sodium environments inside the anion-empty sodalite cage. In the presence of water molecules, however, the sodium ions exhibit fast motion, so that the line is motionally narrowed. The observation of two separate sodium resonances indicates the akence of Na+ exchange between sodalite cages in the hydrated sample at room temperature. The downfield peak in Figure Sa, at around 7 ppm, is therefore assigned to Na+ cations in Na4Br cages, and the upfield peak at -2.5 ppm to sodium in Na3.nH20cavities. The 23Naspectra lead us thus to conclude that a certain number of “defect” cages, which do not contain anions, are produced during the synthetic procedure. These cages give rise to the 23Naresonance at -2.5 ppm. Integration of the peak intensities in the 23NaMAS spectrum of the synthesized material in Figure Sa indicates that 10% anion-free cages are produced. This side product in hydrothermally prepared sodalites is not detected by XRD and may have been overlooked in previous studies. Smeulders et a1.21have hypothesized that halide anions diffuse out of the sodalite cages, leaving behind Nad3+clusters, whereas our study suggests that the Na3*nH20cages are apparently already present after the initial synthetic route. A later report conducted by nutation analysis found evidence for water molecules inside the sodalite cages.22 Water molecules, however, do not fit into cages occupied by halide ions due to spatial restrictions, but nontheless are present in the halide-free cavities. This finding thus supports our observation
-
A Sodalite Semiconductor Supralattice
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6747 61.2 I
(A)
Sodolite Coge (b)
(B)
NaI-sod
63.2 I
n
NaBr-sod
64.5 I
NaC1-sod
Sodolite
Figure 6. (A) Sodalite cage showing a single cuboctahedron, a central anion, and four cations in the six-ring sites. Each comer represents a TO4 unit (T = Si or Al). (B) Sodalite framework, emphasizing the close packing of cages.
of the production of sodalite cages which do not contain halide anions in the synthesis of halosodalites. Incidentally, it has recently been found22that the anion-free cages arise from extraction of NaOH during washing of the crude product with water, from defect hydroxide anion-containing cages produced in the assynthesized materials. Hakwodalites. Figure 6 shows a schematic picture of a sodium halide cluster inside the sodalite cage. The monovalent sodium cations are tetrahedrally arranged around the anion, which is located in the center of the cavity. The anionic species encapsulated within the sodalite framework in this study, namely, I-, Br-, and C1-, have different radii (2.06, 1.82, 1.67 A, respect i ~ e l y ~and ~ ) ,Pauling electronegativityvalues. These properties can be invoked to explain perturbations in the structure of the aluminosilicate framework and the electronic properties of the charge-balancing Na+ cations. Previous 29SiMAS of sodalite studies have correlated 29Sichemical shifts with Si-0-A1 ang l e ~ ,reflecting ~ ~ * ~ structural ~ and electronic differences due to the presence of the various anions. 27AlDOR spectra of the anionloaded samples are shown in Figure 7 and exhibit similar sensitivity to structural features of the sodalite framework upon loading various anions. As expected, we observe a shielding effect on the framework atoms as the charge-attracting capacity; that is, the Pauling electronegativity of the anions decreases,and the size of the anion increases. NaIsodalite, for example, gives rise to an aluminum resonance at 61.2 ppm, Figure 7c, whereas NaClsodalite, which contains the smallest and most electronegativeanion among the three anions examined, produces a peak at 64.5 ppm, Figure 7a. The trend toward a more shielded 27Alenvironment reflects also a widening of the Si-0-AI bond which may be ascribed to the larger size of I- compared to C1-. Sodium exhibits a high sensitivity toward the anionic species as shown in Figure 8. C1- and Br--containing samples feature single Gaussian-shaped 23NaMAS peaks, Figure 8a,b, while the MAS spectrum of NaIsodalite shows a substantially broader pattern, Figure 8c. Application of DOR averages out the anisotropic quadrupolar broadening and enables one to obtain isotropic 23Nashifts for NaBrsodalite and NaIsodalite, as shown in Figure 8d,e, respectively. No apparent differences in the resonance line widths and positions were observed between the MAS and DOR spectra of NaClsodalite. The Na+ cations are 4-fold coordinated, being within bonding distance from the halide ion and three framework oxygens, Figure 6. Thus, the electro-
Ppm Figure 7. 27Al DOR spectra of (a) NaClsodalite; (b) NaBrsodalite; and (c) NaIsodalite. The asterisks indicate spinning sidebands. 6.2
I
20
10
NaC1-sod
0
-10
-20 6.9
I
NaBr-sod
A
\ n- ’ c (
I . .
20
10
0
-10 -20
20
..1
. . . . 1 . . . . 1
10
0
....I
. .
-10 -20
PPm PPm Figure 8. 23Na NMR spectra of various sodium halosodalites: (a) NaClsodalite, MAS;(b) N a B d a l i t e , MAS; (c) NaI-sodalite, MAS; (d) N a B d a l i t e , DOR; (e) NaI-sodalite, DOR. The asterisks indicate spinning sidebands.
negativity difference between the framework oxygens and the anion has probably a significant effect in determining the charge distribution around the encapsulated sodium, and thus electronic shielding and quadrupolar effects. The calculated values are A ( x ( 0 ) - x(X)) = 0.28, 0.48, and 0.78 for X = C1, Br, and I, respectively, where x is the Pauling electronegativity. Under these circumstances, an asymmetric distribution of charge is expected in the iodide cavities, consistent with the quadrupolar broadening in Figure 8c, as more negative charge is drawn toward the framework oxygens. On the other hand, a less asymmetric sodium environment is expected in the C1- cages, for example, as the
Jelinek et al.
6148 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
TABLE I[: "Na Resonance Positioua at 11.7 aod 9.4 T,aod Calculated NMR Parameters for Different Halosadaiites'
35
40
45
6dl1.7T), PPm 6.2 6.9 3.4
NaClsodalite NaBr-sodalite NaI-odalite
6d9.4Th PPm 6.1 6.0 0.1
L,b, ppm 6.3 8.5 9.3
CQ, MHz
-0 1 1.9
O28
'The isotropic chemical shift, &+., and quadrupolar coupling constant, CQ,were calculated using eq 1 and 2. q was taken as 0.67, providing &13%accuracy for the calculation of CQ. A
I
4
iv
I
I
I
I
230 I80 I 3 0 80 750
700 650 WAVENUMBER
Figure 9. X-ray diffraction patterns of (a) 1:4 physical mixture of NaClsodalite and NaIsodalite, respectively; (b) NaC1l,lIo,q-sodalite, chemically synthesized, and (c) mid-IR spectra of (i) NaCI-sodalite; (ii) NaIsodalite; (iii) 1:l physical mixture of NaClsodalite and N a I s o dalite; and (iv) chemically synthesized NaC11.8610.1,sodalite.
WAVENUMBER
230 180
I30
80
WAVEN U M B E R
Figure 10. (A) Series of far-IR spectra of physical mixtures of NaCIscdalite/NaIdalite in the following ratios: (a) 1:O; (b) 1O:l; (c) 1:l; (d) 3:7; (e) 0 : l . (B) Series of far-IR sptctra of the chemically synthesized materials: (a) N a C l d a l i t c ; (b) NaC11,9610,Msodalite; (c) NaC1,,9b,l-sodalite;(d) NaC1l,lIo,q-sodalite;(e) NaIsodalite.
for example, might produce a decrease in the AI-O-Si angle. Consistent with the above arguments, the deshieldmg effect upon the aluminum and silicon nuclei appears related to higher electron density around the framework oxygen atoms and correspondingly to more shielded Na" cations. Mixed Hakoddites. An interesting and closely related class of materials are the mixed halosodalites Na8Cl2,IP4alite. It is important to assess and understand the organization of the anions within the sodalite lattice, as this affects the electronic band where Biso,obs is the observed resonance position and 8cs,iM)is the of the material. Models of mixed anion distribution isotropic chemical shift. The isotropic quadrupolar shift, b ~ , ~ ~structure , include (1) an ordered array, in which a higher-order unit cell is related to the magnetic field strength and the quadrupolar would be created (manifested by extra diffraction lines in the parameters by powder XRD pattern); (2) a solid solution of anions within the lattice; (3) a segregated physical mixture of chloro- and iodosodalite microcrystallites (detected as a superposition of pure NaClsodalite and N a I d l i t e spectra using the spectroscopic methods in this study); and (4) a mixture of chloride and iodide where I is the nuclear spin, 7 is the asymmetry parameter, CQ domains smaller than -50 A, which is below the detection limit = 2 q Q / h is the quadrupolar coupling constant, and vo is the of XRD. resonance frequency. The calculated parameters are presented Figures 9 and 10 show powder XRD patterns, mid-IR and in Table 11. Recently published quadrupolar coupling constants,% far-IR spectra of pure chloro- and iodosodalites, chemically obtained through analysis of 23Na MAS spectra of satellite prepared NaICl-sodalites, and physical mixtures of the pure transitions, are in close proximity to the values reported here. The materials. The XRD pattern of the chemically synthesized maisotropic shifts and quadrupolar coupling constants indeed indicate terial in Figure 9b shows no extra diffraction lines corresponding more shielding, and less charge asymmetry, of the sodium electo integer multiples of the unit cell edge, thus eliminating the tronic environment, as the electronegativity of the anion increaseti, ordered-array model from consideration. This observation is and the charge polarization between the anion and the framework merent from electron microscopy results obtained for the mineral oxygens becomes less. The size of the halide may additionally nosean?' a sodalite in which Na4.Hz0 and Na4.S04 units form have an electronic effect on the sodium cations. A smaller halide, electron density moves toward the more electronegative chloride from the framework. Further experiments in a lower, 9.4-T, magnetic field, enable one to extract the isotropic chemical shift and quadrupolar values using (1) ho,obs = &s,iso + 6Q.iso
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6749
A Sodalite Semiconductor Supralattice 61.2
Na-23
6.2
NaBr-cages
A1-27
I
I
(a)
63.8
5.6
I
I
I
62.8
3.7
I
I
20
10
0
-10
-20
PPm 61.5
n
3.4 0.3
/ 20
10
*I I*
\ 0
Figure 12. 23NaDOR spectra of sodium bromohydroxysodalite before and after silver exchange: (a) Na,Brl,2(0H)o.s-sodalite; (b) Na6Ag,Brl,2(0H)o,8-sodalite. The asterisks indicate spinning sidebands.
I
-10
-20
70
60
50
40
30
PPm PPm Figwe 11. 23Naand 27AlDOR spectra of mixed-halide sodalites: (a) 5 4 physical mixture of NaCl-sodaliteand NaI-sodalite, respectively; (b) NaCll,9J,,l-sodalite;(c) NaCb,51,,podalite;(d) NaC11.1b,94ite.The asterisks indicate spinning sidebands.
an incommensurate superlattice structure. The XRD pattern of the physical mixture, Figure 9a, stands in contrast to the single-phase, narrow-peak pattern of the chemically prepared sample, Figure 9b. The pattern of the physically mixed sample features broad peaks that are composed of the corresponding separate chloro- and iodcmdalite signals. Similar interpretation is apparent from the mid-IR spectra in Figure 9c, where the results Stem to exclude the formation of separate NaClsodalite and NaIsodalite microcrystallites. Three distinct framework, vs(T-O), vibrational modes are observed for pure NaClsodalite and NaIsodalite shown in Figure 9c, i and ii, respectively. v,(T-O) modes of the chemically prepared sample, Figure 9c, iv, give rise to three sharp bands in the 650-740-cm-I region, while a clear superposition of NaCl-sodalite and NaIsodalite bands is observed in the spectrum of the physical mixture, Figure 9c, iii. Intriguingly, the far-IR spectra of a series of chemically synthesized NaIClsodalites shown in Figure 10B feature new cation/anion-coupled translational modes (200-50 cm-’) that are different from the cation and anion modes detected in the physically prepared samples, shown in Figure 10A. This observation is explained in terms of coupling between the Na41 and Na4Cl clusters formed in the chemically synthesized material.lO.ll 27Aland UNa DOR spectra of mixed halide sodalitesare shown in Figure 11. Physically mixed samples of NaCl- and NaIsodalite, shown in Figure 1 la, feature two distinct resonances, at 6.2 and 3.2 ppm for 23Naand at 64.2 and 61.2 ppm for 27Al, in the anticipated positions for uncorrelatcd NQI and Na4Clcages. Chemically prepared mixed-halide samples, on the other hand, exhibit peaks at different positions from the physical mixture. When small amounts of chloride are displaced by iodide in the NaCl-sodalite lattice, Figure 11b, both aluminum and sodium resonances are shifted in the direction of the 23Na position in NaIsodalite. The same effect is observed for NaI-sodalite resonances upon incorporation of C1- as shown in Figure 1IC;the
resonance position can be considered as a weighted sum of the anions in the sodalite. Moreover, extended Hiickel molecular orbital (EHMO) calculations on two adjacent sodalite cages containing Na& clusters (X = C1 and I) indicate that the charge on Na+ increases as iodide replaces chloride in neighboring cavities.12 As mentioned earlier, an additional contribution to the shielding of the sodium nuclei upon addition of I- anions may arise from an increase in the size of the unit cell. A significantly different NMR result is obtained for the chemically synthesized NasCll Io,9-sodalite. The 27AlDOR resonance at 61.5 ppm,Figure 1Id, points to a shielded aluminum environment, farther upfield than what would be expected from a sample with roughly equivalent chloride and iodide compositions. The corresponding 23Naspectrum features a peak at around 3 ppm and a distinguishable shoulder at around 0 ppm. The two signals can be assigned to 23Na in chloride and iodide cages, respectively, though both resonances are shifted noticeably to higher field. This shielding effect cannot be traced entirely to a change in the framework dimensions to accomodate chloride and iodide, since this does not explain the extraordinary upfield shift of the 23Naresonances from sodium cations in both cavities. This result might indicate electronic coupling between clusters throughout the aluminosilicate framework. Intercavity coupling may alter the electron density functions within the cages, placing higher average charge around the sodium nuclei. Silver Exchange: F o r a u h of Sodim/Sihrer Bnwosodrlite. Ion-exchanging the sodium bromosodalite system with silver is known to produce semiconductor supralattice structures? For the present study, silver exchange with the sodium cations inside the cages was carried out progressively, yielding N a a - & B r sodalite. The sodium environments, as well as framework structural praperties, were examined at various stages of exchange. In addition, the exchange process was studied in sodalites intentionally containing known amounts of defects, specifically OHor anion-free cages, yielding Na~-figpOH,Br2-,-sodalite or NaE,,,Ag,[ ],Br2-,-sodalite, respectively. In these two classes of compounds we detected a clear preference for silver exchange into halidacontaining cages, and not into hydroxide-containing cavities or halide-free ones. As shown in Figure 12, the sodium resonance associated with the sodium bromide cavities diminishes upon silver exchange, whereas the intensity of the sodium resonance in hydroxide-containingcages remains essentially constant. This similarly holds true for anion-empty sodalite cavities. Figure 13a-c clearly shows significantly fewer sodium cations in halidacontaining cages, relative to those in anion-free environments,
6750 The Journal of Physical Chemistry, Vol. 96, No. 16, 1992
Jelinek et al.
6.2
6.9
I
I
Na-l I cages
I
I \* (C)
6.2
I
20
10
0
PPm
-10
.20
20
10
0
.10
-20
PPm
Figure 13. 23NaDOR spectra of sodium bromosodalites, which contain halide-free cages, before and after silver exchange ([ ] = empty cage): (a) Na725BrI25[ lo,5-sodalite; (b) Na625AglBrl251 Io7,sodalite; (c)
I
(d) Na6 2Br02[I I ,-soda,Iite; (e) Na,Ag, 25BrI2 5 [ lo ,,-sodalite. Na4 2Ag2Bro2[ l1,sodalite. The asterisks indicate spinning sidebands.
for higher silver loadings. Such preferential Ag+ exchange into cavities with halide anions persists even when the cages containing Br- are in a very low abundance, Figure 13d,e. Preferred silver exchange into halide-containing cavities is manifested at samples with high silver loading, Figure 13. Powder XRD determined structures of encapsulated Nab&@ clusters10 indicated that the A g B r internuclear distance lengthens with increasing the number of Ag+ in the cage, because the covalency of the Ag-Br bond decreases. Thus, the silver preference cannot be assigned solely to the stronger covalent bond which forms between the silver and halides than that between silver and hydroxide.28A possible explanation is the higher hydration energy of Na+ compared to Ag+ (-406 kJ/molZ8and -339 kJ/m01,~’ respectively) which induces more effective complexation between Na+,rather than Ag+, and hydroxide or water. Thus, Na+ cations may experience more favorable “solvation”in the halidefree and OH--containing cages than Ag’ cations, as the two types of cavities host imbibed water molecules. Previous studies of Na8-@gpBr2-sodalitepoint to a statistical distribution of silver atoms w t l n the sodalite lattice, rather than creation of aggregates distributed inhomogeneously through the material?-’O The 23Naresonance associated with the bromide cages is affected by silver loading in a way which is consistent with the above description, Figure 14. We observe, for example, symmetrical peaks in the 23NaDOR spectra, whose centers of mass move upfield with silver loading. This indicates a different average chemical environment rather than a superposition of mixed phases. Further information can be extracted from the peak positions; whereas the halide-xge sodium peak is shifted slightly downfield as the silver content is increased from 0 to 2.7 atoms/unit cell, Figure 1 4 a 4 , an abrupt jump, from -7 to 5.7 ppm, is observed when four and more Ag+ are loaded per unit cell, Figure 14e,f. In addition, broadening of the peaks is observed. Application of 23NaDOR in a 9.4-T magnetic field enables us, using q s 1 and 2, to estimate the isotropic chemical shift and quadrupolar contributions to the observed resonance position, and the results are shown in Table I11 and Figure 15. While the isotropic chemical shift of the 23Nanuclei remains essentially
20
10
0
-10
.20
PPm
F i i 14. 23NaDOR spectra of sodium bromosodalite in different stages of silver exchange: (a) Na,Br2-sodalite; (b) Na7,2A&.8Br2sodalite;(c) Na6,1Agl,9Br2-aodalite;(d) Na5,~Ag2,,Br2sodalite; (e) Na4A&Br2-sodalite; ( f ) Na2,8Ag5,2Br2aodalite.The halide-free defect cages present in the sodalite framework, scc text, are not accounted for in the notation. The asterisks indicate spinning sidebands.
TABLE Uk uNa Resonance Positions at 11.7 and 9.4 T, and Clrlerdrtd NMR Parameters for Various Silver-Exchanged NaBr-Sodrlitea‘ A+! per unit cell 0 0.8 1.9 2.7 4 5.2
b(l1.7T), PPm 6.9 7.2 7.0 7.6 5.1 5.7
bOd9.4T), PPm 6.0 6.0 6.1 6.2 3.6 3.6
ba..,b, ppm 8.5 9.3 8.3 10.1 9.5 9.5
CQ, MHz 1 1.1 1 1.1 1.5 1.5
The isotropic chemical shift, ba,iro, and quadrupolar coupling constant, CQ,were calculated using eqs 1 and 2. q was taken as 0.67, providing &13% accuracy for the calculation of CQ.
unchanged, Figure 15b, the quadrupolar interaction seems to increase when the silver loading is raised to beyond two atoms per unit cell, Figure 15c. In this context, a more asymmetricdistribution of charge around the Na+ nuclei is consistent with greater electron density being directed toward the framework oxygens upon silver loading. This effect likely originates in the localization of electron density in covalent AgBr bonds of Na.+,,Ag,Br clusters, which is known to diminish with Ag+ Therefore, the ‘effective” electronegativity of the Br- decreases with Ag+ loading, causing consequently a more asymmetric electronic environment around the remaining Na+ cations. It seems, however, that the change in the quadrupolar interaction experienced by the Na+ is not gradual, but somewhat abrupt beyond the one silver per cavity loading threshold, Figure 1%. This “step” might signal the de-
The Journal of Physical Chemistry, Vol. 96, No. 16, 1992 6751
A Sodalite Semiconductor Supralattice
t
(a)
I
A
Observed shift
7
F
t
\
L
5t
t . (b)
l0I L I Imtmpic Chemical Shift
L
1.2
1.0
1
w
0
/
/
2 4 6 Number of Ag+ per U.C.
Figure 15. (a) Observed Z3NaDOR resonance positions, (b) isotropic chemical shifts, and (c) quadrupolar coupling constants as a function of the amount of silver exchanged in a unit cell of NaBr-sodalite. The isotropic chemical shifts were calculated from eq 1 using results from DOR experiments at two magnetic field strengths; the quadrupolar coupling constants were calculated using eq 2, with 7 taken equal to 0.67 providing an accuracy of *13% for the calculation of the quadrupolar
coupling constant. velopment of intercavity electronic coupling. Indeed, previous optical reflectance, X-ray diffraction, and infrared results have indicated the possible formation of an expanded AgBr semiconductor supralattice throughout the sodalite structure when silver and bromide loadings are sufficiently high?-10*'2Recent 3sCland *'Br NMR results30 show a profound difference in the anion resonance positions between coupled and uncoupled Na&,,Ag,Br clusters, thus offering support for this interpretation. Moreover, it should be noted that no change in the resonance position is observed in the NaAgBr[]sodalite series, Figure 13a-q materials for which electronic coupling between the halide cages might not occur as they contain a high abundance of anion-empty cavities. The positions of the 23Naresonance for sodium in halide cages in those samples remain essentially the same for various degrees of silver exchange. 29Siand 27AlNMR experiments could detect no structural differences in the framework of the sodalites as a function of silver loading. This result stands in contrast to the sensitivity of 27Al NMR to different halides inside the sodalite cavities. Stronger space-filling effects are likely present for the different halides, compared to Na' and Ag' which possess almost identical ionic radii, 1.13 and 1.14 A, respectively.23The lack of apparent change in the NMR spectra of the framework nuclei suggests also that a relatively weak interaction exists between the silver and the framework atoms, consistent with the notion of a strong interaction between the silver and the anions inside the cavities. In addition, electronic couplings between adjacent cages do not alter significantly the environment of the framework atoms.
Conclusions This study demonstrates the usefulness of high-resolution solid-state NMR in addressing structural and electronic questions concerning the preparation and assembly of sodalite semiconducting supralattices. Information about cation-anion distributions, effects of defect cavities, and intercavity electronic coupling is obtained by performing high-resolution MAS and DOR experiments on the 29Siand 27Alframework atoms, and extraframework 23Nacations in the sodalite cages. The additional use of two magnetic fields provides accurate estimates of t k isotropic chemical shifts and quadrupolar contributions to the resonances. Following insertion of various halide anions into the sodalite cages, 27Aland, particularly, 23NaNMR yield subtle information on the distribution of charge inside the cavities. Na' quadrupolar interactions and electronic shielding increase as the charge polarity between the halide and framework oxygen increases, while the framework S i U A l bond angle increases. Examination of mixed-anion structures indicates that structural and electronic modifications occur within the sodalite framework. A unique shielding effect is observed for sodium in a sodalite lattice that hosts almost equal amounts of chloride and iodide; this is believed to signal the formation of electronic coupling between sodium halide clusters throughout the material. Useful chemical information is additionally provided on the process of silver exchange into the sodalite cavities. A preference is detected for silver/sodium exchange in halide-containing cages, compared to OH--containing or halide-free cavities. Changes in the electronic environment of the sodium nuclei are observed throughout the silver-exchange process. An abrupt increase in the quadrupolar interaction is detected upon loading about two Ag+ per unit cell which corroborates other data pointing to the onset of a semiconductor supralattice within the sodalite matrix. These are pivotal issues in understanding the compositional dependence of band structures in this interesting class of advanced materials.
Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials and Chemical Sciences Division, US.Department of Energy, under Contract No. DE-AC03-76SF00098. R.J. acknowledges useful conversations with Professor A. Pines. B.F.C. acknowledges support as a NSF-NATO postdoctoral fellow at the Max-Planck-Institut fiir Polymerforschung,Maim, Germany. G.A.O. acknowledges the financial support of the Natural Science and Engineering Research Council (NSERC) of Canada's Operating and Strategic Grants Programmes, Alcan, Canada, and Optical Recording Corp., Toronto. A S . thanks NSERC for a 1967 Science and Engineering Postgraduate Scholarship.
References and Notes (1) Ozin, G. A.; Kuperman, A,; Stein, A. Angew. Chem. 1989,101, 359.
( 2 ) O h , G. A.;
Kirkby, S.;Meszaros, M.; Ozkar, S.;Stein, A.; Stucky, G. D. In Materials for Nonlinear Optics; ACS Symposium Series 455, Marder, S.R., Sohn, J. E., Stucky, G. D., Eds.; American Chemical Society:
Washington, DC, 1991; pp 554-581. (3) Zhu, Y.;Huang, D.; Feng, S.Phys. Rev. B 1989, 40, 3169. ( 4 ) Stein, A.; Ozin, G. A,; Stucky, G. D. J. Soc. Photogr. Sci. Technol. Jpn. 1990,53, 322. ( 5 ) Fyfe, C. A.; Thomas, J. M.; Klinowski, J.; Gobbi, G. C. Angew. Chem. 1983. 22.259. Fvfe. C. A.: Grondev. . H.: . Fene. ". Y.: Kokotailo. G. T. J. Am. Chem. Sbc. 1990; 112, 8812. ( 6 ) Lippmaa, E.; Samoson, A.; Magi, M. J . Am. Chem. Soc. 1986, 108,
1730.
(7) Tijink, G. A. H.; Janssen, R.; Veeman, W. S.J. Am. Chem. Soc. 1987, 109, 7301. (8) Jelinek, R.; Chmelka, B. F.; Wu, Y.;Grandinetti, P. J.; Pines, A.; Barrie, P. J.; Klinowski, J. J . Am. Chem. SOC.1991, 113, 4097. Wrenn Wooten, E.; Mueller, K. T.; Pines, A. Acc. Chem. Res. 1992, 25, 209. ( 9 ) Stein, A.; Ozin, G. A.; Stucky, G. D. J . Am. Chem. SOC.1990, 112, 904. (10) Stein, A.; MacDonald, P. M.;Ozin, G. A,; Stucky, G. D. J . Phys. Chem. 1990, 91, 6943. Stein, A.; Mezsaros, M.; Macdonald, P. M.; Ozin, G. A.; Stucky, G. D. Adu. Mater. 1991, 3, 306. Stein, A.; Ozin, G. A.; Macdonald, G. M.; Stucky, G. D.; Jelinek, R. J. Am. Chem. SOC.1992,114, 4901. (11) Godber, J.; Ozin, G. A. J. Phys. Chem. 1988, 92,4980. (12 ) Stein, A. Ph.D. Thais, University of Toronto, November 1991. Ozin, G. A.; Stein, A. In Advances in the Synthesis and Reactivity of Solids;
6752
J. Phys. Chem. 1992,96,6752-6755
Mallouk, T., Ed.;JAI Press Inc.: Greenwich, CT, in press. (13) Wu, Y.; Sun, B. Q.; Pines, A.; Samoson, A.; Lippmaa, E. J. Magn. Reson. 1990, 89, 291. (14) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Res: London, 1982. (15) Breck, D. W. Zeolite Molecular Sieves; R. E. Kieger Publishing Co.: Malabar, 1984. (16) Weller, M. T.; Wong, G. J. J . Chem. Soc., Chem. Commun. 1988, 1103. (17) Engelhardt, G.; Fahlke, B.; Migi, M.; Lippmaa, E. Zeolites 1983,3, 292. (18) Fyfe, C. A.; Gobbi, G. C.; Hartman, J. S.; Klinowski, J.; Thomas, J. M.J . Phys. Chem. 1982,86, 1247. (19) Jelinek, R. Ph.D. Thesis,in preparation. (20) Kundla. E.; Samoson, A.; Lippmaa, E. Chem. Phys. Lett. 1981,83, 229. (21) Smeulders,J. B. A. F.;Hefni, M. A.; Klaasen, A. A. K.; de Boer, E.; Westphal, U.; Geismar, G. Zeolites 1987, 7 , 347.
(22) (a) Jansscn, R.; Breuer, R. E. H.; de Boer, E.; Geismar, G. Zeolites 1989,9,59. (b) Engelhardt,J.; Felsche, J.; Sieger, P. J. Am. Chem. Soc. 1992, 114, 1173. (23) Shannon, R. D. Acta Crystallogr. 1976, ,432, 751. (24) Ramdas, S.; Klinowski, J. Nature 1984, 308, 521. ( 2 5 ) Klinowslti, J. Prog. Nucl. Magn. Reson. Spectrosc. 1984, 16, 237. Engelhardt, G. In Recent Advances in Zeolite Science; Klinowski, J., Barrie, P. J., Eds. Stud. Surf.Sci. Card. 1989, 52, 151. (26) Nielscn, N. C.; Bildsoe, H.; Jacobscn, H. J.; Norby, P. Zeolites 1991, 11, 622. (27) Hassan, I.; Buseck, P. R. Am. Mineral. 1989, 74, 394. (28) Handbook of Chemistry and Physics, 66th ed.; CRC Press: Boca Raton, FL, 1985-86. Gaydon, A. G. Dissociation Energies and Spectra of Diatomic Molecules, 3rd ed.; Chapman and Hall: London, 1968. (29) Cotton, F. A.; Wilkiwn, G. Advanced Inorganic Chemistry;4th ed.; Wiley: New York, 1980; p 255. (30) Jelinek, R.; Stein, A.; Ozin, G. A. J . Am. Chem. Soc.,submitted for publication.
Determination of Sublimation Pressures of a C6dClo Solid Solution C.Pan,? M. S.Chandrasekharaiah,*D.Agan,* R.H. Hauge,*.t and J. L.Margrave*pt.* Department of Chemistry and Rice Quantum Institute, Rice University, Houston, Texas 77251, and The Houston Advanced Research Center, 4800 Research Forest Drive, Wbodlands, Texas 77381 (Received: December 20, 1991; In Final Form: March 26, 1992)
The sublimation pressures in equilibrium with a polycrystalline C@/C70solid solution have been measured with a quartz crystal microbalance (QCM) and by transpiration methods, in the temperature range 772-857 and 806-929 K, respectively. The results from the two independent methods show good agreement. The solid solution was found to have a total vapor pressure of 8.1 X lo4 Torr at 800 K. It is estimated that the total vapor pressure of the C@/C70solid solution could reach 1 atm at ca. 1523 K. The analyses of the compositions of C, and C70 in the solid and vapor phases also reveal that Cso is more volatile than C70.
Introduction
The fullerenes Cm and C70 have recently stimulated intense interest and activity with regard to their chemical and physical properties.'-s However, descriptions of the thermodynamic behavior of C@and C70 remain incomplete. The fact that C@and C70 are relatively volatile has led to a variety of proposed a p plications which require sublimation of the mixtures.@ Because of the cost of separating C60and C70, it is likely that mixtures will often be used in many applications. Here we report the sublimation pressure studies of a polycrystalline mixture of Cm and C70 with a quartz crystal microbalance and classic transpiration methods. Recmt work by Sun et al.Io in the study of C,/C, solid solution formation has revealed that C, and C70 mixtures, if prepared from the toluene solution, can form a solid solution over a Cso mole fraction range of ca. 70% and 824, in contrast to the whole composition range of the solid solution when prepared by a sublimation method. The X-ray diffraction patterns of our CW/C70 mixture samples were found to be close to those of the C6O/C70 solid solutions, and the mole fraction of the Cs0 falls within the solid solution composition range. The transpiration method is one of the oldest and most versatile ways of studying heterogeneous equilibria involving solids and gases. This technique has been extensively described by Kubaschewski and Evans," Margrave,'* Richardson and Alcock,13 Merten and Bell,'4and SchBfer.Is In the transpiration experiment, an inert gas,a camer, is passed over a condensed sample at a flow rate sufficiently low for equilibrium conditions to be established. The vapor of the sample is transported by the carrier gas to some point downstream from the sample and is collected and analyzed to determine the vapor pressures of components in the sample. *To whom correspondence should be addressed. Rice University. *The Houston Advanced Research Center.
0022-3654/92/2096-6752$03.00/0
The quartz crystal microbalance (QCM) has proved to be a useful tool in conjunction with the Langmuir free evaporation technique for measuring the vapor pressures of solids and liquid~.'"'~ Its high mass sensitivity allows the measurement of vapor pressures several decades lower than other methods. By employing a Knudsen effusion cell, we show here that this modified technique can also be applied to the measurement of vapor pressuns. It is noteworthy that there are two major requirements that have to be met: (1) the sticking coefficient of the vapor on the QCM must be near unity, and (2) the condensation of impurities from the sample container and the heating device must be negligible. The Knudsen effusion method has been widely used in the measurement of thermodynamic quantities of high-temperature equilibria. A molecular beam, characteristic of the equilibrium vapors in the Knudsen cell, is formed by the species effusing from the cell orifice. This molecular beam can then be coupled either with a mass spectrometer or with a deposition monitor (e.g., QCM). In this experiment, the sample is loaded into a Knudsen cell which is resistively heated, The cell should be aligned with the QCM surface so that a fraction of molecules evaporated from the sample, represented by F, can reach the QCM surface. This fraction is dependent on the geometry of the experimental setup. The equilibrium flux of the effusing molecules, J, (molecules/ (cm2*s)),is related to the Knudsen vapor pressure Pk by the Knudsen-Langmuir equation J, = ( 2 ~ m k T ) - ~ / ~ P , (1) where m is the molecular weight, k the Boltzmann constant, and T (K) the sample temperature. In a Knudsen effusion cell, the Knudsen vapor pressure Pk is a close approximation of the equilibrium vapor pressure P,, as given by the following equation
Pk = P,[1
+ (BWB/A)(l/CYL - 1)]-'
0 1992 American Chemical Society
(2)
