J . Phys. Chem. 1987, 91, 257-260 zero for asymmetric IR modes such as v,(CH3) and 6,(CH3). Consequently, these modes should not be seen by IR spectroscopy if the surface-dipole selection rule is strictly operating. Greenler et al. have carried out calculations concerning the validity of the surface-dipole selection rule for metal particles of various sizes20 and have concluded that the rule should indeed become “weakened” as particle size decreases and metallic character becomes diminished. This weakening effect should begin for metal particles approaching 20 in diameter.20 Under the preparation conditions used in our studies, Pt and Rh surfaces contain very small average metal particle diameters (19 and 28 A, respectively), possibly leading to the relaxation of the surface-dipole selection rule on these surfaces (see Table 11), permitting the observation of the asymmetric C-H modes. In contrast, for the Pd and Ru surfaces, with relatively large average particle sizes (84 and 66 A, respectively), one does expect the surface dipole selection rule to be in operation. Indeed, these Pd
257
and Ru surfaces exhibit no ethylidyne asymmetric modes, in agreement with expectations.20 In summary, the 300 K reaction of C2H4with Pt, Rh, Pd, and Ru supported on alumina has been shown to form the ethylidyne surface species in all four cases. Other minority hydrocarbon species are also present. Pt and Rh surfaces exhibit I R band intensities roughly 2-3 times greater than those for Pd and Ru, depending on the IR mode being compared. The reasons for this are discussed and are at least partially due to dispersion differences for the various metal surfaces. The surface-dipole selection rule t and Rh surfaces containing particles appears to be weakened on P of 20-30-A average diameter, as evidenced by rather strong IR intensities for asymmetric IR modes of adsorbed ethylidyne.
Acknowledgment. We acknowledge with thanks the full support of this work by the Science Research Laboratory and the 3M Central Research Laboratories.
Optical Properties of CdS and PbS Clusters Encapsulated in Zeolitest Y. Wang* and N. Herron* E. I . du Pont de Nemours and Company, Central Research and Development Department, Experimental Station, Wilmington, Delaware 19898 (Received: September 16, 1986; In Final Form: October 31, 1986)
We report optical properties of semiconductor clusters, CdS and PbS, encapsulated in zeolites. At a low loading level of CdS in zeolite Y , isolated clusters with size a i k , A. J. J. Phys. Chem. 1986, 90, 12. (7) Ramsdtn, J. J.; Webber, S. E.; Gratzel, M. J . Phys. Chem. 1985, 89, 2740.
(8) Enea, 0.;Bard, A. J. J . Phys. Chem. 1986, 90, 301. (9) Fox, M. A. Acc. Chem. Res. 1983, 16, 3 14. (10) Kreibig, U. Z . Phys. B: Condes. Matter Quanta 1978, 31, 39. (1 1) If the arrangement of semiconductor clusters is highly ordered as in a crystal structure, then the semiconductor/zeolite composite is a true superlattice structure. This type of superlattice is grown three-dimensionally, different from the traditional superlattice or multiple quantum well structures which grow along one dimension only. There is no fundamental reason why this type of three-dimensionally grown superlattice structure cannot be built with zeolite as the templates. Of course, optimum growth conditions have to be found and certain lattice matching requirements have to be met, just as in the growth of a traditional superlattice. Our preliminary X-ray diffraction data show that most of the CdS clusters are indeed well-ordered. Data refinement is in progress.
0 1987 American Chemical Society
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The Journal of Physical Chemistry, Vol. 91, No. 2, 1987
Letters
CdS/ZEOLITE Y
PbS/zeolites 1.10E+06,
1 80E+05~
Lorenz-Mie,
M
e=30
/ 1
:8,OOE+O5+
Y-
+ QI
n
t
CdS/Y, 7 . 4 w t z
l.OOE+OS-
6 ,O O E + O 5 CdS/Y.
8.00E+04+
1.1
C
wtz
~
0
Lorenz-Mie,e=JO
\
1
PbS/Y
,..
+-
?6,00E+040 L?
n
0
4 00E+04-
Z.O0E+O4-
\
\
\\ O.OOE+OO
O.OOE+OO
200
200
300
400 500 600 W a v e l e n g t h , nm
700
800
Figure 1. Absorption spectra of CdS in zeolite Y compared to that of
micron size CdS powder and the calculated Lorenz-Mie spectrum (with medium dielectric constant assumed to be 30). The absorption coefficients are normalized to the volume concentration of CdS. internal zeolite cavities. Different superlattice structures can be built by choosing different zeolites as the templates. This Letter communicates our initial results in this area. Experimental Section Two different zeolites, mordenite and Y, were used. Zeolite mordenite has a unidirectional channel of 7-A diameter while zeolite Y has 13-& tetrahedral symmetry cages interconnected by 8-A windows and 5-A cages interconnected by 3-A windows.I2 Both the CdS and PbS semiconductors may be prepared in an identical manner. Here we briefly describe the preparation procedure for CdS in zeolite Y. Zeolite Y in its sodium cation form was first ion-exchanged to the cadmium cation form by treating with aqueous cadmium nitrate solution at pH 5. This was followed by calcination at 400 O C in flowing dry oxygen. Hydrogen sulfide was then flowed over the sample at controlled pressure and flow rate for 1 h, followed by evacuation to remove excess hydrogen sulfide. Numerous variations in the above scheme have been tried, and we have found that the optical properties and stoichiometry of the semiconductors are sensitive to the preparation conditions. Samples used in this study have 1:l metal to sulfur ratio a t low loading level (1 wt %) and excess Cd at high loading level. The concentration of Cd and S atoms are determined by atomic absorption method. X-ray powder diffraction patterns for these materials show that zeolite crystallinity is maintained through the synthetic procedure, and detailed analysis of the powder data is in progress. Most notable is that no peaks for the bulk semiconductor phase are evident in the powder data, indicating that the size of the semiconductor particles is too small to be detectable which in turn argues for the encapsulation inside the zeolite pores.* ESCA data also show there is no significant CdS outside the zeolite pores. The absolute absorption coefficients of these semiconductor clusters were obtained through the diffuse reflectance theory of Kubelka-Munk.I3 Both diffuse reflectance of the diluted (with pure zeolite) sample and scattering coeffcient of the zeolite were measured independently. The absorption coefficient is normalized to volume concentration of the semiconductor. Details of the experimental procedures will be described in a future publication. (12) Zeolite Molecular Sieves; Breck, D. W., Ed.; Krieger Publishing Co.: Malabar, FL, 1984. ( 1 3 ) Reflectance Specrroscopy; Kortum, G . , Ed.; Springer-Verlag: New York, 1969.
300
400
500
600
700
I
800
wavelength, n m
Figure 2. Absorption spectra of PbS in zeolite Y and zeolite M compared to the calculated Lorenz-Mie spectrum. Absorption coefficients are normalized to the volume concentration of PbS.
Results and Discussion Absorption Spectra. Absorption spectra of zeolite-encapsulated CdS and PbS clusters are shown in Figures 1 and 2, along with the small-particle Lorenz-Mie spectra calculated by using bulk optical constants of CdSI4 and PbS.15 It is clear that the cluster spectra are totally different from the bulk spectra. In general, the cluster spectra are greatly shifted to the highenergy side and possess weaker oscillatory strength compared to the bulk. For low loading CdS/Y (1 wt %), the normal band-gap transition (around 500 nm) virtually disappears. Instead, an absorption peak shows up near 280 nm. The intensity of this peak is sensitive to sample preparation conditions. We tentatively assign this spectrum to CdS clusters of size
