Langmuir 1989, 5, 58-66
58
has been no critical discussion of this type for superficial mixtures. To fix this term, we take the minimal thickness of the layer for Lo,i.e., Lo 6 A, when xpo = 0. The results are shown in Figure 8, where we have also plotted the variations in Q l p 6 , calculated by using the regular solution model Qff = 1 - -xp" At (16)
RT
At being a parameter which characterizes the differences in interactions between the different pairs. As with the quasi-chemical hypothesis,17 there is disparity between these values and the experimental results. This confirms the challenge represented by aqueous superficial mixtures, which will require more experimental and theoretical investigations. Registry No. Ethanol, 64-17-5.
Formation and Photophysical Properties of CdS in Zeolites with Cages and Channels X. Liu and J. K. Thomas* Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received March 24, 1988. In Final Form: August 5, 1988 The formation and photophysical properties of CdS in zeolites with cages and channels, such as the zeolites sodalite, A, X,chabazite, L, ZSM-6, and offretite, were investigated. The resulte show that CdS clusters form in the biggest cages of the zeolites with cageg (sodalite cage in sodalite, a-cage in zeolite A, and eupercage in zeolite X)and in the main channels of the zeolite with channels. The sizes of the cluetera are constrained by the sizes of the cages. The size of CdS formed in the zeolites with channels is more uncertain due to the ease of migration of CdS along the channels. The Cd2+cations located in the small cages may or may not be used for CdS formation, depending on the amount of Cd2+cations already located in the big cages. If Cd2+cations are deficient in the big cages, the Cd2+cations in the small cages will migrate out into the big cages; otherwise, they will remain intact. Absorption spectra of the CdS formed in these two kinds of zeolite systems show different features, indicatb different particle size and distributions of CdS. Emission spectra are similar for both systems, broad and s 6A ,ureless. The maximum position of emission changes with changing excitation wavelength.
Introduotion Preparation and photophysical and photochemical investigations of semiconductors of small particles have become a very active research area in recent This is not only because of the significantly different photophysical and photochemical properties of the semiconductors compared to bulk but also because of the importance of understanding such materials. ~~~~~
~
~~
~~
~~
(1) (a) Thomas, J. K.J. Phys. Chem. 1987,91,267. (b) Kuczyneki, J.; Thomas, J. K.Langmuir 1985,1,158. (c) Kuczynski, J.; Thomas, J. K. J.Phys. Chem. 1983,87,5498; 1985,89, 2720. (d) Kuczynski, J.; Milosavljevic, B. H.; Thomas, J. K.J . Phys Chem. 1984,88, 980. (e) Lianos, P.; Thomas, J. K. Chem. Phys. Lett. 1986,125, 299. (2) (a) Brue., L. New J. Chem. 1987,11,123. (b) Rosetti, R.; Nakahara, S.; Brus, L. E. J . Chem. Phys. 1983, 79,1086. (c) Brus, L. E. J . Chem. Phys. 1984,80,4403. (d) Rosetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1984,80,4464. ( 3 ) (a) Fojtik, A.; Weller, H.; Koch, V.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1984,88,969. (b)Weller, H.; Fojtik, A.; Henglein, A. Chem. Phys. Lett. 198S, 117,484. (c) Bard, S.; Fojtik, A.; Weller, H.; Henglein, A. J.Am. Chem. SOC.1986,108,375. (d) Spanhel, L.; Haase, M.; Weller, H.; Henglein, A. J. Am. Chem. SOC.1987,109, 5649. (4) (a) Ramsden, J. J.; Webber, S. E.; Griitzel, M. J . Phys. Chem. 1985, 89,1740. (b) Ramsden, J. J.; GrBtzel, M. J . Chem. SOC.,Faraday Trans. I 1984,80,919. (c) Duong, H. D.; Ramsden, J.; Griitzel, M. J.Am. Chem. SOC. 1982,104, 2977. (5) (a) Nozik, A. 3.; Williams, F.; Nenadovic, M. T.; Fbjh, T.; Micic, 0.I. J . Phys. Chem. 1985,89,397. (b) Nedeljkovic, J. M.; Memadovoc, M. T.; Micic, 0. I.; Nozik, A. J. J. Phys. Chem. 1986, 90,12. (6)Parise, J. B.; MacDougau, J. E.; Herron, N.; Farlee, R.; Sleight, A. W.; Wang, Y.; Bein, T.; Moller, K.; Moroney, L. M. Znorg. Chem. 1988, 27, 221. (7) Tamura, K.; Hosokawa, S.; Endo, H.; Yamasaki, S.; Oyanagi, H. J . Phys. SOC.Jpn. 1986, 55, 528. (8) Stramel, R.; Thomas, J. K. J. Colloid Interface Sci. 1986,110,121. (9)Wang, 2.;Herron, N. J. Phys. Chem. 1987, 91, 257.
A large body of work has been carried out in systems such as aqueous solution, alcohol, micelles, reversed micelles, clays,1-6JoJ1and zeolite^^^^^^ in order to understand changes in the photophysicalproperties of semiconductors as a function of size, for example, the blue shift of the absorption threshold of semiconductors with decreasing particle size. Among these systems, zeolites, a crystalline aluminosilicate containing regular three-dimensional channels or cavities with molecular dimensions, provide uniformly constrained systems that are able to grow semiconductors with defined particle size. Much more attention has been paid to CdS,1-6a semiconductor with a band gap of 2.42 eV, owing to its ease of experimental handling and potential applications in conversion of solar energy. CdS in zeolite systems is prepared by the procedure of ion exchange of zeolites with an aqueous solution of Cd2+salts followed by reaction with H2S gas or aqueous Na2S solution. This procedure allows CdS to form in the zeolite cavities or channels, and the particle size of CdS is constrained. However, several characteristic features of zeolite structures, which affect CdS formation, must be considered. First, any type of zeolite normally contains one or more than one kind of cage. For instance, zeolite sodalite contains only one kind of cage named for sodalite cage. Zeolite A contains three kinds: a sodalite cage, an a-cage, and a double fourmembered ring (D-4); faujasite also contains three, a so(IO) Kraeutler, B. K.; Bard, A. J. J.Am. Chem. SOC.1978,100,4317. (11) Anpo, M.; Aikawa, N.; Kubokawa, Y.; Che, M.; Jonis, C.; Giamello, E. J . Phys. Chem. 1985, 89, 5017.
0743-7463/89/2~05-0058$01.50/0 0 1989 American Chemical Society
Langmuir, Vol. 5, No. 1, 1989 59
Properties of CdS in Zeolites
,J
ollretlte
-Sodalite
1 I 1
ZSM-5
!
Table I. Structure Parameters of the Zeolites Used in This Study" entry aperture, zeolite main cage or channel diameter, A A sodalite sodalite cage 6.6 2.1 6.5 2.1 zeolite A sodalite cage 4.1 a-cage 11 2.1 zeolite X sodalite cage 6.6 7.4 13 supercage chabazite chabazite cage 6.7-10 3.7-4.2 2.1 zeolite L cancrinite cage 2.6 7.1 7.1 channel along [001] offretite gmelinite cage 3.6-5.2 channel along [001] 6.4 ZSM-5 channel along [OlO] 5.4-5.6 5.4-5.6 5.1-5.5 channel along [ 1001 5.1-5.5
kept at this temperature for 2 h, and then taken back down to room temperature. The H2S gas was introduced into the tube at 1atm at a flow rate of -20 cm3/min. The reaction was carried out at room temperature for 1 h, and then the products were purged with Nzfor 4-12 h in order to remove the residual H2S. dalite cage, a supercage, and a double six-membered ring The absorption spectra were recorded immediately after the (D-6), etc., as shown in Figure 1. These different cages sample preparation. are not all available for the formation of semiconductors 3. Characterization of Zeolite Structures before and after CdS Formation. a. X-ray Diffraction. X-ray diffraction was such as CdS. This is due to the common procedure of carried out on a Diano X-ray diffractometer operating at 45 KV preparation of CdS, where the S% anions or H2Smolecules and 30 mA and at a scanning speed of 2"/min. (-3.6 8, in diameter for the anions'O) are too big to penb. Infrared Spectroscopy. Infrared spectra were recorded etrate into the small cages through the six-membered ring on an IBM FT-IR/32 spectrometer equipped with an IBM mi(2.1 8, in diameter"). Second, the distribution of cations crocomputerwith a resolution of 2 cm-'. The KBr wafer technique such as Cd2+in the different cages is another factor that was used. affects the CdS formation. The Cd2+cations located inside c. Chemical Analysis. The Cd2+content was analyzed by the small cages cannot be used for the formation of CdS using the standard polarographic technique operating in the unless they migrate out from these cages into the big cages. scanning range -0.45 to -0.90 V and 15 pA F.S. sensitivity. d. ESR. ESR spectra of Cu2+-containingsamples were reIn the present paper, we present some detailed studies corded at room temperature on a Varian ESR spectrometer. The of CdS formation in a number of zeolites with different field was calibrated by using DPPH as a standard. cavities and channels and discuss the photophysical 4. Absorption and Emission Spectroscopy. a Absorption properties of the CdS formed in these constrained systems. Reflectance Spectroscopy. Front surface reflectance spectra were recorded with a Perkin-Elmer552 W-vis spectrophotometer Experimental Section coupled to a Model 561 chart recorder. The specular reflectance 1. Chemicals. a. Zeolites. The zeolites used are as follows: accessory has a fixed angle of incidence of 6O. BaSO,, coated on sodalite (containing NazS),prepared by following Barrer's proan aluminum plate, was taken as a standard background. cedures;'2 zeolite A, Nal~lzSilz0a~21Hz0, synthesizedaccording b. Emission Spectroscopy. Emission data were obtained to Break;" zeolite X, Na,&3ilMOw-205HZ0, Exxon products; with a Perkin-Elmer MPF-44B spectrofluorimeter,interfaced to zeolite Chabazite, Exxon products; zeolite L, Kg[Al&7072], a Model 056 recorder,at liquid nitrogen temperature. The samples synthesized according to Breck;ll ZSM-5, Na6.6A15.5Siso.5019z, were held in an ESR tube and evacuated to -lo* Torr overnight synthesized with TPA as a template and calcined at 550 "C before measurement. overnight; Offretite (Grace product), calcined at 550 "C, 23 h. b. Reagents. CdClZ,Fisher Scientific, 99.3%; HzS gas, Du Results Pont, used without further purification; NazS, Fisher Scientific; Before the results are presented, it is necessary to deHzO, deionized; NaCl, Fisher Scientific. scribe briefly the structures of the zeolites used. Figure 2. CdS Preparation. a. Ion Exchange. The above zeolites 1 shows the cages and channels found in these zeolite were ion-exchanged with an aqueous solution of CdClz(0.1 m/L, structures, and Table I lists the parameters of these cages sample to solution ratio is 1:200) at room temperature while and channels: pore aperture, cage diameter, etc. The stirring for 48 h. The products were filtered, washed with deionized water, and dried at 50 "C in air. The supernatant liquid location and distribution of cations in these zeolites can was collected in a 1-L volumetric flask for Cd2+analysis. The be found in the literature15 for the original form of the amount of Cd2+in the products was calculated by subtraction zeolite, but for the Cd2+-exchangedzeolites, insufficient from the amount of Cd2+added. The reverse ion exchange of data are available except for zeolites X and A.16J' HowCd2+-exchangedzeolites was carried out by using an aqueous ever, the literature survay states that divalent cations solution of NaCl at room temperature overnight, followed by occupy preferably the cation sites in the smaller cages separating,washing, and drying at 50 "C in air; such samples were rather than those in the bigger cages or ~hanne1s.l~ The ready for the preparation of CdS. migration of divalent cations toward the smaller cages b. CIS Formation. The Cd2+-exchangedzeolites were placed during dehydration often occurs if the zeolites are partially in a quartz tube with a fritted glass disk. The thickness of the sample bed was about 1 cm, and the sample was loosely packed. ion exchanged. Upon adsorption of guest molecules, the The temperature was increased slowly to 200 "C with N2flowing, reverse processes could take place, depending on the ex-
Figure 1. Building cages found in zeolites with cages and schemetic representation of the channels found in zeolites with channels.
(12) Moeller, T. Inorganic Chemistry, A Modern Introduction; Wiley: New York, 1982. (13) Breck, D. W. Zeolite Molecular Series; Wiley: 1974. (14) (a) Barrer, R. M.; Cole, J. F.; Sticher, H. J. Chem. Soc. A 1968, 2475. (b) Barrer, R. M.; Cole, J. F. J . Chem. SOC.A 1970, 1516.
(15) Mottier, W. J. Compilation of Extra-framework Sites in Zeolites; Butterworth Scientific: Guildford, England, 1982. (16) Calligaris, M.; Nardin, G.; Randoccio, L.; Zangrandre, E. Zeolites 1986, 439.
(17) McCusker, L. B.; Seff, K. J. Phys. Chem. 1981,85, 166.
Liu and Thomas
60 Langmuir, Vol. 5, No. 1, 1989
perimental conditions and the strength of the interactions between the cations and the guest molecules. The CdS formation in zeolite matrices involves all these processes. 1. CdS Formation in Sodalite. The sodalite structure only contains sodalite cages with entry apertures of 2.1 A. This aperture is too small to allow H2S or S2-to penetrate under ordinary experimental conditions. Therefore, in order to prepare CdS inside the sodalite cages of sodalite, the S2-anion must be introduced into the sodalite cages during the formation of sodalite. Barrer et al.14found that Na2S salt could be occluded inside sodalite cages during sodalite formation from a mixture of kaolinite, NaOH, Na2S.9H20, and water a t 90 “C. The Na2S-containing sodalite is pale-blue, indicating the occlusion of Na2S. The number of S2-anions occluded in each sodalite cage is 1 or 2 (S2-or 522-1, depending upon the amount of Na2Sused. The Na2S-containingsodalite obtained by the direct hydrothermal synthesis was then ion-exchanged with an aqueous solution of Cd2+to form CdS in the sodalite cages. The ion exchange was accompanied by a color change from pale blue to pale yellow while CdS forms. The amount of Cd2+ cations after two ion exchanges with an aqueous solution of CdC1, reached -1.2 Cd2+per unit cell (uc). The Na+ cations which can be ion-exchanged come from two parts: one being the charge-balancing Na+ for the negative framework and the other being from the Na2S occluded. The ion exchange was affected by the occluded Na2S and also by the CdS formed. According to the composition of the unit cell of the sodalite (Na,&&,15A&Si6OZ4)and the number of sodalite cages per unit cell (2 sodalite cages per unit cell), the maximum number of Cd2+ cations per sodalite cage is 2. Therefore, CdS formed after the two Cd2+ion exchanges at most is in the form of CdS, Cd2S, CdS2, and (CdS)2if the Na+ cations are homogeneously distributed. Recently, Liu and Thomasla found the appearance of a new band in the IR spectra of Cd2+-exchangedzeolites with sodalite cages and pointed out that the new band resulted from the Cd2+cations located inside the sodalite cages. The preceding IR band is not observed in supercages and was used to monitor the Cd2+locations between the big cages such as the supercage in zeolite X and Y and the smaller sodalite cages. IR studies of Cd2+-exchanged Na2S-containingsodalite indicate that Cd2+is observed in this system, i.e., some Cd2+do not react with H2S. CdS has no absorption band in the mid-IR spectrum, so the IR band of the Cd2+-exchangedNa2S-containingsodalite implies that some sodalite cages do not contain S2- or S22anions, which is consistent with the low S2-content in the as-synthesized Na2S-containing sodalite, 0.15 S2-/uc. Absorption and Emission Spectra of CdS in Sodalite. Figure 2A shows the reflectance spectrum of the as-synthesized Na2S-containingsodalite and the Cd2+-exchanged Na2S-containingsodalite, which has a weak absorption below 450 nm. This results from the occluded Na2S and Na2S2inside the sodalite cages. Below 300 nm the absorption is assigned to S2-,and the absorption between 300 and 450 is due to S22-.17 The absorption increases for the whole spectral range upon ion exchange of this sample with an aqueous solution of CdC12. The absorption below 300 nm increases much more rapidly than that above 300 nm. This can be assigned to the formation of CdS and Cd2S (absorption below 300 nm) and CdS2and CdzS2(absorption above 300 nm). A weak and broad emission peak centered at 4-40nm was observed at 77 K (Figure 2b) with an excitation wavelength (18)Liu, X.; Thomas, J. K. C h e n . Phys. Lett. 1988, 144, 286.
NqS-Socblite
n K
600 WAVELENGTH mrn)
400
Figure 2. (A) Absorption reflectance spectra of (1)NazS-containing sodalite and (2) CdS-Na2S-containing sodalite. (B) Emission spectra of Naps-containingsodalite and Cd2+-NapScontaining sodalite with an excitation wavelengths of (1)310, (2) 320, and (3) 340 nm. I
1250
lo00 750 Wavenumbers c 1-1
500
Figure 3. Infrared spectra of Cd2+-exchangedzeolite (Cd4,2-A) and CdS-containing zeolite A (CdS-Cd,,-A). of 340 nm for the Cd2+-exchangedNa2S-containing sodalite. Under identical conditions, no emission was observed for the as-synthesized Na2S-containing sodalite, which indicates that the emission is due to the formation of CdS. Changing the excitation wavelengths from 300 to 350 nm did not change the position of the maximum emission. 2. CdS Formation in Zeolite A. Zeolite A contains one sodalite cage and one a-cage per unit cell. Cd2+exchange of zeolite A with an aqueous solution of CdC1, creates a Cd2+-exchanged zeolite A with 4.2 Ca2+/uc, Cd4,2-A. Most of the Cd2+cations are located inside the sodalite cages after dehydration according to McCusker and Seff s Cd2+-exchangedsingle-crystal structural determination.” IR studies of the Cd2+-exchangedzeolite A support the single-crystal studies and exhibit the new band in its spectrum, indicating that Cd2+is located inside the sodalite cage. After H2S is introduced into the Cd2+-exchangedzeolite A under the conditions mentioned in the Experimental Section, CdS is formed inside the a-cages. This is supported by JR studies before and after CdS formation. The new band in the IR spectrum of Cd2+-exchangedzeolite A disappeared after CdS formation (see Figure 3), indicating that the Cd2+cations originally located inside the
Langmuir, Vol. 5, No. 1, 1989 61
Properties of CdS in Zeolites A
A
a 400 Wavelength
300
1250
500 inrn)
:"\ I !
\
f
I
lo00 750 Wavenumbers crn-1
500
Figure 5. Infrared spectra of Cd2+-exchangedzeolite X (Cd%-X), Na+reverse-exchanged Cd,-X (Cd19-X),and their correspondmg CdS-containing products.
B
\
Wavelength (nrn)
Figure 4. (A) Absorption reflectance spectrum of CdS-Cd4.2-A. (B)Emission spectra of CdS-Cd4.2-Awith an excitation wavelengths of (1) 310, (2)337, (3)350, (4)360, and (5) 380 nm. sodalite cage migrate into the a-cages, there forming CdS. Dehydration of Cd2+-exchangedzeolite A did not remove the new band from the IR spectrum, indicating that migration of Cd2+occurs during the CdS formation but not during the dehydration. Dehydration does not cause Cd2+ to migrate from sodalite cages to a-cages. The interaction between S2- and Cd2+is responsible for the observed Cd2+ migration. Because of the migration of Cd2+cations from sodalite cages to a-cages, all the Cd2+cations are available for the CdS formation inside the a-cages; therefore, the average number of CdS in each a-cage is 4.2. Absorption and Emission of CdS in Zeolite A. The absorption reflectance spectrum of CdS-zeolite A is given in Figure 4A. From the figure, it can be seen that there are two absorption maxima centered at 270 and 300 nm. In addition to these two peaks, a weak absorption tail is also observed in the spectrum with an onset at 460 nm. This weak absorption tail results from the structural damage of zeolite A during the CdS formation, which creates big pores in the structure. IR studies show a broad shoulder around 856 cm-' (see the arrow in Figure 3) due to structure damage. The very consistency of the weak absorption band in IR spectrum with the weak absorption
tail in reflectance spectrum indicates that the extent of the damage is not serious, and the amount of CdS in the form of bigger particles is quite low. The intensity of the peak centered at 300 nm, with onset around 380 nm, increases rapidly before reaching its maximum. This peak is due to CdS formed inside the a-cages and is sensitive to particle size. In CdS-Cd4.2-A this peak corresponds to particles with 4.2 CdS molecules. The peak centered at 270 nm is insensitive to particle size and is observed for many CdS-zeolite samples and even in the bulk CdS powder. Therefore, it is evident that this peak is not due to blue shift of the CdS with smaller sizes but is more likely due to intrinsic nonbonding transitions between valence and conduction bands.2 Emission spectra were measured at 77 K. At room temperature, the emission was quite weak. Figure 4b shows the emission spectra of the CdS-Cd4.2-A at 77 K with varying excitation wavelength. The emission is broad and structureless, and the maximum position changes toward longer wavelength as the excitation wavelength increases. 3. CdS Formation in Zeolite X. Zeolite X contains three kinds of cages: a double six-membered ring, a sodalite, and a supercage. For CdS formation, only the supercage can be used. Ion exchange of zeolite X with an aqueous solution of CdC12creates Cd2+-exchangedzeolite X with 36 Cd2+cations per unit cell (Cd,-X). The studies of reverse ion exchange of Cd,-X with an aqueous solution of NaCl indicate the locations of these Cd2+cations: 19 Cd2+cations in the sodalite cages and 17 in the supercages. The Cd2+cations in the supercage can be easily removed by the reverse ion exchange of the Cd2+-exchangedzeolite X with an aqueous solution of NaC1,16 producing a Cd2+-exchangedzeolite X with 19 Cd2+located only in the smaller sodalite cages (Cdlg-X). Dehydration caused no significant amount of Cd2+cations in the sodalite cages to migrate into the supercages, even though no Cd2+cations were present in the supercages. Upon CdS formation, the Cd2+cations inside sodalite cages of Cda6-X did not migrate into the supercages to form CdS with H2S, as shown in the IR spectra in Figure 5. The absorption band at 932 cm-I was still evident, showing that no migration of the Cd2+cations from sodalite cages to supercages takes place. However, upon CdS formation in the Cdlg-X in which no Cd2+ cations are located inside supercages, the Cd2+cations located in the sodalite cages do migrate out and form CdS in the su-
Langmuir, Vol. 5, No. 1, 1989
Liu and Thomas
,
300
400 Wavelength
500 (nm)
6bO
Figure 7. ESR spectra of Cu2+cations in Cd2+and Cu2+coexchanged zeolites X at different treatments. (A) Hydrated Cd2+,Cu2+-exchanged zeolite X (Cd41Cul-X) and hydrated Na+ reverse-exchangedCdllCul-X (CdmCul-X): g, > gl, (gl = 2.27 and gll = 2.00) for both of them. (B)Dehydrated CdllCul-X: gll > g, kI1 = 2.35 and g, = 2.06). (C) After Cd(Cu)S formation: gll > g, (gll = 2.33 and g, = 2.05).
1250
lo00
750
500
Wavenumbers crn-1
520
640 Wavelength (nm) Figure 6. (A) Absorption reflectance spectra of CdS-containing zeolites X CdS-Cds6-X and CdS-Cd19-X. (B) Corresponding emission spectra of CdS-Cd,-X and CdS-Cd19-Xwith excitation wavelengths of (1) 330, (2) 350, (3) 370, and (4) 390 nm. 10
percages. This event is proved by the disappearance of the IR absorption band at 932 cm-', see Figure 5. In these two samples, therefore, similar amounts of Cd2+cations are available for CdS formation, as the Cd2+cations remain in the sodalite cages in Cd,,-X, while the Cd2+cations migrate from the sodalite cages to supercages in Cdlg-X. Absorption and Emission of CdS in Zeolite X. Absorption spectra of both CdS-Cd,-X and CdS-Cdlg-X are very similar, as shown in Figure 6A, which is consistent with the above concept of the available Cd2+ for CdS formation. Both samples have onsets around 430 nm and reach maxima centered at about 330 nm; also, the peak centered at 270 nm is observed for both samples. The profile of the absorption of CdS in both samples is similar to what has been observed in CdS-zeolite A in that the intensities increase rapidly to the maximum. Emission features of both samples are also very similar (see Figure 6B). A broad structureless peak is observed at 77 K, and the maximum position of the emission changes toward longer wavelength (from 500 to 570 nm) as the excitation wavelength increases (from 330 to 390 nm). A t room temperature, very weak emission can be observed for both samples. 4. CdS Formation in Zeolite X in the Presence of Trace Amounts of Cu2+Cations. Trace amounts of Cu2+ cations may be introduced into zeolite X during ion ex-
Figure 8. Infrared spectra of Cd2+,Cu2+-exchanged zeolites X and their corresponding Cd(Cu)S-containingproducts. change of zeolite X with Cd2+. A zeolite X sample obtained in this study contained 41 Cd2+/uc and 1 Cu2+/uc, i.e., Cd41Cu1-X. The Cu2+cations present in CdllCul-X were characterized by ESR spectroscopy, which showed that the Cu2+cations were present in the form of trigonal-bipyramidal symmetry with g, > gll (see Figure 7A).18 Reverse ion exchange of CdIICul-X with Na+, which removed 21 Cd2+cations from the sample and created CdpCul-X, did not affect the profile of the ESR spectrum, indicating that Cu2+cations are located somewhere inside the sodalite cages, which is confirmed by IR studies that showed that the Cd2+cations removed from the sample were located in the supercages. Dehydration of CdIICul-X caused change of the symmetry of Cu2+: g,, > g,, as shown in Figure 7B, suggesting that water molecules are coordinated to the Cu2+cations. Dehydration of CdzoCul-X showed the same phenomenon. Upon CdS formation achieved by passing H2S gas through dehydrated CdllCul-X and CdzoCul-X, two phenomena were observed (i) migration of Cu2+cations from the sodalite cages to the supercages, with the color of the samples tending toward pale brown, indicating the formation of CuS, and (ii) a decrease of the ESR signal, as shown in Figure 7C. The ESR signal decrease during the Cd(Cu)S formation is probably due to strong dipolar interactions of Cu2+,as in the case of CuO where Cu2+is undetectable in ESR,lgor to the reduction of du2+to Cu', which is a cation with no ESR signals. The behavior of (19)Karge, H.G.; Ziotek, M.; Lanieeki, M. Zeolites 1987, 197.
Properties of CdS in Zeolites
Langmuir, Vol. 5, No. 1, 1989 63
A
1
I
B
I , L1, 300
400 Wavelength
500
800
mmi B
Figure 9. (A) Absorption reflectance spectra of Cd(Cu)SCd&l-X and Cd(Cu)S-Cd&ul-X. Arrows indicate the excitation wavelengths for emission measurements. (B) Emission spectra of samples in A excited at X indicated by the arrows, i.e., (1)310, (2) 337, (3)350, and (4)382 nm. Cu2+upon Cd(Cu)S formation is different from that of Cd2+. Figure 8 shows changes of the IR spectra of CdllCul-X upon the reverse ion exchange with Nat and upon the Cd(Cu)S formation. Absorption and Emission of Cd(Cu)S in Zeolite X. Absorption studies in Figure 9A are in accord with the ESR results of Cu2+migration out of the sodalite cage to form CuS in the supercage. The absorption spectra of both samples showed long tails with onsets at about 540 nm. For Cd(Cu)S-CdllCul-X, the absorption reaches a maximum around 320 nm, and the peak at 270 nm is also observed. For Cd(Cu)S-Cd&ul-X, however, a very well resolved peak is seen at 350 nm. The peak around 300 nm is present only as a shoulder and overlaps the peak centered at 270 nm. Emission features of both Cd(Cu)S-zeolite X samples are completely different from that observed for the CdSzeolite X samples. Firstly, at room temperature, Cd(Cu)S-CdllCul-X fluoresces while Cd(Cu)S-CdzoCul-X does not (in the case of CdS-zeolite X, both CdS-Cdw-X and CdS-Cd19-X fluoresce). Secondly, at 77 K, the emission of Cd(Cu)S-CdllCul-X is much stronger than that of Cd(Cu)S-CdzoCul-X. (Both CdS-Cd,,-X and CdS-Cd19-X show similar emission intensities.) Thirdly, the maximum position of the emission is at longer wavelengths (570 and -590 nm, respectively) for both samples of Cd(Cu)S-zeolite X, compared to that for CdS-zeolite X, Figure 9B. Finally, the positions of the emission maxima of both samples do not shift with increasing excitation wavelengths from 310 to 380 nm, in contrast to those of the samples of CdS-zeolite X. (20) Herman, R. G. Znorg. Chern. 1979, 18,995. (21) Cheatnoy, N.; Harris, T.D.;Hull, R.;Brus, L.E.J. Phys. Chern. 1986, 90,3393.
Wavelength (nml
Figure 10. (A) Absorption reflectance spectrum of Cd2+-chabazite. Arrows indicate the excitation wavelengths used for emission measurementa. (B) Emhion spedra of sample in A with excitation wavelengths of (1) 306, (2) 337,(3)350,(4)370,and (5) 390 nm.
5. CdS Formation in Chabazite. Chabazite has a chabazite cage and double six-membered rings, as shown in Figure 1. The Cd2+ion excnahge for this zeolite was not efficient, and only about 0.2 Cd2+per unit cell was reached on ion exchange. By comparison to the Ca2+location in chabazite,lS it is suggested that Cd2+cations are located inside the chabazite cages. Absorption and Emission of CdS in Chabazite. Absorption of CdS-chabazite is similar to that observed for other CdSzeolites with cages; the onset is at about 420 nm, and the intensity rapidly approaches the maximum at 305 nm. The peak centered at 270 nm is also observed. Figure 10A shows the absorption spectrum. The emission spectrum of this sample also shows a broad structureless band. The maximum change in the emission with excitation wavelength is different than that exhibited by other Cd2+-zeolite samples. Figure 10B shows the spectra at different excitation wavelengths. At room temperature, no emission is observed. 6. CdS Formation in Zeolites with Channels. Three zeolites with channels were chosen for CdS formation: zeolite L, offretite, and ZSM-5. The channel profiles along a stated direction are shown in Figure 1. Zeolite L and offretite have one main channel along the c direction, and ZSM-5has two channels with similar dimensions along a and b. Cd2+ion exchange of zeolite L and offretite gave partially ion-exchanged Cd2+-L and Cd2+-offretite. The Cd2+contents are 1.2 Cd2+/ucfor zeolite L and 0.7 Cd2+/uc for offretite, respectively. Cd2+-exchangedZSM-5 has 1.3 Cd2+/ucdue to its high framework Si/A1 ratio (Si/Al = 16.5). The location of Cd2+ in these Cd2+-exchanged zeolites is not exactly known. Cd2+may locate in the main
64 Langmuir, Vol. 5, No. 1, 1989
Liu and Thomas
B c
!
400
500 6W Wavelength (nmi
1.. Wavelength i n m )
Wavelength inm)
Figure 11. (A) Absorption reflectance spectra of CdS-zeolite L (top), Cds-offretite (middle),and CdS-ZSM-5 (bottom). Arrows indicate the excitation wavelength used for emission measurements. (B)Emission spectra of the samples in A CdS-zeolite L (left), CdS-offretite (middle),and CdS-ZSM-5 (right) with excitation wavelengths of (1) 310, (2) 337, (3) 350, (4) 370, (5) 390, and (6) 410 nm. channels, in the wall of the main channels, in smaller cages such as cancrinite cages, in double six-membered rings in zeolite L, and in gmelinite cages and double six-membered rings in offretite. Upon the passage of H2Sthrough these dehydrated Cd2+ zeolites, it is likely that CdS particles form in the big main channels and migrate along the channels to form big CdS particles. Absorption and Emission of CdS Formed in Zeolites with Channels. Figure 11A shows the absorption spectra of CdS-zeolite L, CdS-offretite, and CdS-ZSM-5. The spectra show the similar features for all three samples, in contrast to the CdS formed in the zeolites with cages (compared with the corresponding figures). The spectra have two steps, the first of which has rather low intensity and covers a broad range of wavelengths. For CdS-zeolite L, the intensity rises at about 650 nm and reaches the first step at about 440 nm. Below 440 nm, the intensity increases further and reaches a maximum centered at 270 nm. For CdS-offretite, the intensity starts at 450 nm, reaches the first step a t 340 nm, and increases further to reach a maximum at 240 nm. The CdS-ZSM-5 absorbs at 530 nm, has the fiist step at 320 nm, and finally reaches a maximum at 240 nm. Emission spectra of these three samples are also quite similar, as shown in Figure 11B. The position of the emission maximum shows no change with increasing excitation wavelength below 340 nm, while above 370 nm the emission maximum increases with increasing excitation wavelengths. Discussion 1. CdS Formed i n Zeolites witd Cages. It is clear that the big cages of the zeolites with cages provide the space for CdS formation regardless of where the Cd2+ cations are located. Migration of Cd2+cations from the smaller cages to the big cages may occur, depending upon the amount of Cd2+cations present in the big cages. If the
number of Cd2+cations present in the big cages is large, then the Cd2+cations located in the smaller cages will not migrate into the big cages, thus forming CdS, but will still be present in the form of cations compensating the negative charge of the aluminosilicate framework. Dehydration does not cause Cd2+cations in the smaller cages to migrate into the big cages, but the reaction formingCdS from H2S and Cd2+ cations causes migration. This is very well demonstrated in the systems of CdS-zeolite A, CdSCdls-X, and Cd(Cu)S-CdzoCul-X. The particle sizes of the CdS formed in these zeolites are limited by the sizes of the cages: a-cage in zeolite A, sodalite cages in sodalite, and supercages in zeolite X. The cage size limitation prevents migrating of Cd2+cations in the smaller cages to the big cages to form big particles of CdS in the big cages if certain CdS particles have already formed in the big cages, as seen in the cases of CdS-Cd,-X and Cd(Cu)SCddlCU1-X. In the case of CdS-sodalite, the size of the CdS particle is limited by the number of S2-anions occluded, and the article is at most composed of two CdS molecules. For the CdS-Cd4.2-A, the average number of CdS molecules per a-cage is 4.2. If these four CdS molecules form a cluster with a cubane structure in the a-cage, the diameter of the cubane along the [lll]direction will be approximately 1 0 A,which is the right size for the CdS cluster to stay in the a-cage (11 A). So the number of CdS molecules in the CdSa-cage is about 4. However, for CdS-Cd,-X, Cdl,-X, Cd(Cu)S-Cd*lCul-X, and Cd(Cu)S-Cd&U1-X, the average number of CdS per supercage is 2.0-2.5. This number of CdS molecules is not enough to fill the space of a big supercage (13 A in diameter). Absorption spectra of these samples also indicate that the particle sizes of CdS in the supercages are bigger than those in the a-cages (see Figures 4A, 5A, and 9A). Combining these results together, seems to imply that CdS molecules only fully occupy some supercages and that other supercages are empty. According to Breck,13the maximum amount of adsorbed H$
Properties of CdS in Zeolites
Langmuir, Vol. 5, No. 1, 1989 65
6
A
4501
CC-L
I 330
350 400 Excitation Wavelength cnm)
B
6
ocds-L cds-2%-5
A
OCdS-Gf fret ite
" 300
350 O
400
L
Excitation Wavelength (nm)
Figure 12. (A) Plot of the change of the maximum position of emission of Cd2+in the zeolites with cages as a function of excitation wavelength. (E) Plot of the change of the maximum position of emission of CdS in the zeolites with channels as a function of excitation wavelength.
at a pressure of 400 Torr is -16 H2S molecules per supercage. Therefore, it can be concluded that the number of CdS molecules per supercage is in the range 4-16. The question arises as to why CdS molecules solely occupy some supercages and form particles and why Cd2+cations in the smaller cages do not migrate out and form CdS particles in the empty supercages. The answer of these questions is not clear at present but is probably due to the blocking of CdS particles of other supercages and, as a consequence, the prevention of diffusion of H2S in the whole sample. The intensity of absorption spectra of CdS in the zeolites with cages rises quickly to the wavelength maximum and covers a narrow range, indicating that the particles (or clusters) of CdS in these matrices are rather uniform. The different features of CdS in the zeolites with channels will be discussed later even though the Cd2+content is quite low in these samples. Emission measurements support the above argument. The emission maximum position is consistent with the size of the cages: sodalite cage < a-cage < supercage. Figure 12A summarizesthe change of the CdS emission maximum in these samples with excitation wavelength. The emission maximum changes linearly with increasing excitation wavelength for CdS-Cd4.2-A, CdS-CdS6-X, and CdSCd19-X, but for CdS-chabazite, there is first a decrease and then an increase with increasing excitation wavelength.
A minimum of emission maximum is observed at an excitation wavelength of 360 nm. The emission spectrum of CdS depends on many factors such as particle size, lifetime of the excited states, the separation of electrons and holes, Franck-Condon factors, and any nonradiative processes. The position of the emission maximum does depend on the particle size: the bigger the particles, the longer the emission wa~elength.~?~ However, the profile of the emission does not depend on the particle size; different sizes of CdS particles give the same emission profile. These phenomena have been already observed by Brus et al.19 They pointed out that the distribution of excited states is intrinsic and not extrinsic; in other words, it is not the distribution of the sizes that cause the distribution of emitting states. For CdS in the zeolites with cages, the particles are rather uniform, proving that the particle sizes are really not responsible for the broad emission. In previous studies the absorption spectrum of CdS showed progressive blue shifts with decreasing particle size. The absorption spectra were broad and increased in the UV, but features indicative of maxima increasingly appeared upon decreasing particle size. The broadness of the spectra was attributed to a nonhomogeneous distribution of particle sizes. Therefore, intrinsic factors must be the origin for the observed emission features. Considering that CdS clusters in zeolites only contain a few molecules (smaller in particle size than those in colloidal systems), separation of states in the valence band and conduction band will be even larger compared to those suggested for colloidal CdS. The lifetimes of the excited states are extremely short at room temperature, 7 < s, with the emission decay following the time profile of the laser pulse. Therefore, Franck-Condon factors will play an important role in the emission spectra. As a consequence, the emission maximum changes with increasing excitation wavelength, in contrast to what has been observed in colloidal CdS systems. The change in emission intensity as temperature decreases to 77 K indicates that nonradiative effects, strong coupling between the excited states, and the lattice phonons cause the emission at room temperature to be very weak. 2. CdS in Zeolites Formed in the Presence of Cu2+. The presence of Cu2+during CdS formation in zeolite X affects the absorption and emission of CdS. The absorption spectra becomes broader than those observed in the absence of Cu2+. The emission is enhanced in the case of Cd(Cu)S-Cd4,Cul-X and decreased in the case of Cd(Cu)S-CdzoCu,-X even though similar amounts of Cd2+ and Cu2+ in both samples are used to form Cd(Cu)S. Another phenomenon observed is that the position of the emission maximum does not change with increasing excitation wavelength in contrast to the CdSzeolites X. The emission maximum of Cd(Cu)S-Cd4,Cul-X (570 nm) is smaller than that of Cd(Cu)S-Cd&u,-X (590 nm). Addition of guest cations into CdS can cause two opposite effects on the emissions, depending on the existing state of the guest cations. For Cu2+cations, if they are located on the external surface of CdS, the emission of CdS will be quenched. In contrast, if they are present in the internal particle as a lattice element, the emission will be enhanced. Such effects explain the data observed with the Cd(Cu)S-Cd,--X and Cd(Cu)S-CdaoCul-X samples. For the former, CuS is present as an entity inside the particles, and for the latter, CuS is present as a quencher on the external surface of CdS. The different features for these two samples must be due to the different migration mechanisms. In Cd(Cu)S-Cd4,Cul-X, Cd2+located in the supercages forms CdS along with Cu2+from the sodalite
66
Langmuir 1989,5,66-70
cages, but in Cd(Cu)S-CdzoCul-X, as no Cd2+or Cu2+is present in the supercages, CdS and CuS may form separately and do not mix as in the case of Cd(Cu)SCd4lC~1-X. 3. CdS Formed in Zeolites with Channels. The CdS formed in zeolites with channels displays different absorption features from CdS formed in zeolites with cages. The intensity of the absorption spectra of CdS in zeolites with cages increases rapidly, and the spectra cover a rather narrow range of wavelengths, while the intensity of the absorption spectra of CdS in zeolites with channels increases slowly, and the spectra cover a wide range of wavelengths. These spectral features indicate a broad distribution of CdS particles in these systems compared to those in the zeolites with cages. Considering the specific structures of the zeolites with channels, it is not difficult to understand why the CdS in this kind of system displays broad particle size distribution. In zeolites with cages, the cages have rather narrow apertures for the entry and migration of molecules: a sodalite cage has a diameter 6.6 A, but the entry is only 2.1 A;an a-cage in zeolite A has a diameter 11 A with an entry of only -4 A; and the supercage in zeolite X has a diameter of 13 A,but the entry is only 7.4 A. Once CdS particles begin to form in such cages, further migration of particles through the narrow aperture aggregate to form long particles is impeded. However, in zeolites with channels, more freedom is offered to the CdS particles to aggregate along the channels, as the channels and the entry have similar dimensions and bigger particles may be formed. As a consequence (see Figure l),freedom of movement along the channels provides an opportunity for the CdS to form a wide distri-
bution of different particles, giving rise to the characteristic absorption spectra. The emission spectra of CdS in the zeolites with channels are similar to those observed in zeolites with cages. Figure 12B shows the changes of the maximum position of emission with increasing excitation wavelength. Conclusion CdS formation in zeolites with cages and channels is affected by the sizes of the cages and channels and by the distribution of Cd2+cations among these cages. In some cases, the Cd2+ cations located in the small cages will contribute to the CdS formation, but this depends on how many Cd2+cations are located in the big cages. If the number Cd2+cations in the big cages is low, then Cd2+ cations located in the small cages will migrate out into the big cages. The number of CdS formed in the different cages is 1 or 2 in a sodalite cage, 4 in a a-cage, and 4-16 in a supercage, respectively. The CdS formed in zeolites with channels display a distribution of particles with different sizes due to the freedom CdS migration along the channels. Absorption spectra show different features for CdS formed in zeolites with cages and in zeolites with channels. The former covers a rather narrow range of wavelength, and the latter covers a wider range, as the particles of CdS are smaller and of a narrower size distribution in the former than in the latter. Acknowledgment. We thank the National Science Foundation for support of this work. Registry No. CdS, 1306-23-6.
Metal/n-ZnO Interaction: Effect of the Surrounding Atmosphere on IR Transparency F. Boccuzzi,* A. Chiorino, G. Ghiotti, and E. Guglielminotti Dipartimento d i Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, via P. Giuria 7, 10125 Torino, Italy Received March 2, 1988. I n Final Form: June 14, 1988 IR spectra of three different metaJ/ZnO samples (metal = Cu, Pt, Ru) under vacuum, in hydrogen, and in oxygen are discussed in comparison with those of pure ZnO. In all the reduced samples in vacuo, an electron transfer from the ZnO donor centers to the metal particles has been put in evidence. The growth of a broad absorption in the presence of H2 is interpreted as due to the repopulation of the ZnO donor levels as a consequence of the spillover of H atoms from the metal to ZnO, where they are adsorbed in a protonic form. The differences observed between the three examined samples depend on the different activity of the metals in dissociating H2. O2restores the IR transparency with production of water. The analysis of these phenomena leads to some information on the mechanism of H2sensihg on metal/ZnO systems. Introduction Metal-semiconductor oxide systems are widely studied in the last years because of the applications that they have in catalysis and photocatalysis and as gas sensors. An important factor that may have a role in the properties of these systems is the electron transfer between metal particles and the support, induced by a difference in the Fermi energy levels. Moreover, the surrounding atmosphere (vacuum, hydrogen, oxygen, etc.) can be very important with respect to a change in the properties of the 0743-7463/89/2405-0066$01.50/0
metal semiconductor systems and in particular their electric and optical properties. In fact, on different systems of this kind an atmosphere of H2 induces a strong increase in the electric conductivity.' In this paper we illustrate the effect of the treatments and of the atmosphere on the IR transparency of three different M/ZnO systems (M = Cu, Pt, and Ru), in order to have spectroscopic information that can be related to (1) Aapnes, D. E.; Heller, A. J. Phys. Chem. 1983,87, 4919.
0 1989 American Chemical Society
