J . Chern. Soc., Faraday Trans. I, 1988, 84(5), 1287-1300 Photophysical and Photochemical Properties of CdS with Limited Dimensions Rodney D. Stramel, Takashi Nakamura and J. Kerry Thomas* Chemistry Department, University of Notre Dame, Notre Dame, IN 46556, USA Laponite, porous Vycor glass and molecular sieves, faujasite X and sodalite, have been used to prepare small particles of CdS with limited dimensions. Laponite limits Done dimension of the CdS particles by the interplaner spacing of 11.5 A, Porous Vycor glass limits two dimensions of the CdS particles to < 40 A. Molecular sieves were used to limit the particle size to the size of the cavity; however, the particles constructed were larger than the cavity dimensions. The absorption onset and main emission wavelength increase with increasing Cd2+ concentration.The spectral properties of PbS are also reported. Methyl viologen and Cu2+ quench two different emission bands arising from CdS on laponite in solution. Methyl viologen quenches the high-energy emission, thought to arise from exciton recombination, while Cu2+ efficiently quenches a low-energy emission thought to arise from S2- deficiencies. Methyl viologen shows dynamic-type kinetics, while the static- type kinetics observed from Cu2+ gives an estimate of the particle size. The spectroscopic properties of the particles are studied as a function of cadmium concentration and discussed in terms of particle size. During the past three years there has been significant increase in interest in the photophysics and photochemistry of colloidal semiconductors.Much of this work deals with non-aqueous systems or aqueous systems in which the colloidal particles are stabilized by large amounts of surfactant. The electron-hole pairs produced on illumination can be exploited to study electron transfer at the solid-liquid interface. The size of the colloid greatly influences the spectral properties as compared to single crystals or large particles. Typically the absorption spectrum onset shifts from 520 nm (2.4 eV) to higher energy and the spectra become structured. The luminescence arising from these particles is also shifted to higher energy. These unique spectral properties are said to result from quantum-mechanical effects which affect the energy level of the conduction band. Indeed small particles of CdS,lP7 ZnS,8 PbS,g and various other material^^^-'^ have been prepared in either aqueous or non-aqueous systems.The spectral properties have also been explained by quantum-mechanical calculations. 14-17 Several others have indicated that CdS may be prepared conveniently in constrained systems such as cellulose,1s p~lyurethane,'~ Nafion2' and porous Vycor glass.21 It has been shown that the particles located in these systems exhibit unique and enhanced spectral properties. The spectral properties of CdS incorporated into soda-lime silicate glass was studied in 192622 and further investigated in 1946.23 The data reported indicate that the samples contained very small CdS aggregates, confirmed by a blue shift in both the absorption onset and luminescence band.The unique spectral properties observed were explained by polarization of S2- by Cd2+. In this paper we extend this work, i.e. prepare and study the spectral properties of very small particles of CdS and PbS in restricted and constrained environments. This is accomplished by precipitation of CdS and PbS in porous Vycor glass, zeolites or on clay surfaces. The dimensions of the CdS particles are limited by the nature of the support, 12871288 Photophysical and Photochemical Properties of CdS a long single dimension in porous Vycor glass, a two-dimensional thin sheet on synthetic clay and three small dimensions in the cage of the zeolite. Experiment a1 The materials used were as follows: cadmium sulphide (99.99 + YO pure, Aldrich Chemical Co.), cadmium chloride (99.999 YO Aldrich Chemical Co.), cadmium perchlorate (G.Frederick Smith Co.), lead perchlorate (Aldrich Chemical Co.), hydrogen sulphide (Matheson) and copper sulphate (J. T. Baker Chemical Co.), methyl viologen (N,N'-dirnethyL4,4'-bipyridinium dichloride) (Aldrich Chemical Co.), which was recrystallized three times from methanol, laponite (Laporte Industries, England) faujasite X, sodalite (Exxon Research Co.) and porous Vycor glass no. 7930 (Corning Glass Works). Cadmium or Lead Sulphide incorporated in Vycor Glass Incorporation of CdS or PbS into porous Vycor glass was accomplished by soaking pre- cut sections of the prepared glass in a stirred solution of Cd(ClO,), or Pb(CIO,), for 24 h. The sample was then rinsed and vacuum dried. The glass was rapidly immersed in a solution of isopropyl alcohol saturated with H,S.The CdS on the surface of the glass was removed with dilute HC1. The isopropyl alcohol was removed from the glass pores under vacuum. The glass sample was kept in the dark. Cadmium or Lead Sulphide on Laponite Laponite is a synthetic hectorite with a cation exchange capacity of 0.8 mmolg-l for monovalent cations. Laponite is easily dispersed and solutions up to 20 g dm-3 are transparent. Sodium is the exchangeable cation and can be replaced by most other inorganic and organic cations. To a solution of colloidal laponite a solution of Cd(ClO,), or Pb(ClO,), was added. The solution was stirred for several hours and dried using a rotoevaporator. The powder was dried overnight at 110 "C and stored over Linde MS 5A until further use.The sulphides of the cation-exchanged Laponite were prepared by exposing the dry Laponite powder to a stoichiometric amount of H2S in an oxygen-free environment, and allowing them to react in the dark. After several hours (4-24 h) of reaction time the samples were purged with nitrogen and studied. Cadmium or Lead incorporated in a Zeolite Matrix Preparation I Typically 5 g of Na faujasite X was mixed with 5 g Cd(NO,), in 100 cm3 aqueous ammonia. The sample was filtered and washed with aqueous ammonia, dried at 100 "C for 1 h to remove the water and at 300 "C for 1 h to remove the NH,. This procedure was repeated. The sample is 90 % exchanged with Cd'' in sites I, 1', I1 and I11 [fig. 2(A), see later]. Preparation 2 Typically 5 g of Na faujasite X in 100 cm3 water was adjusted to pH 7.0 with HCl.To the solution 5 g of Cd(NO,), or Pb(NO,), was added and stirred for 30 min. The sample was filtered and dried at 100 "C for 1 h then at 300 "C for 1 h. This procedure was repeated. The exchanged cations are the same for preparation 1.R. D. Stramel, T. Nakamura and J. K. Thomas 1289 Cadmium incorporated into Sodalite The exchange of cations in sodalite is carried out using a molten salt. Typically 5 g of sodalite was mixed with 5 g Cd(NO,),, placed in a Teflon bomb and heated at 100 "C overnight. After cooling, the slurry was washed with water, filtered and dried at 100 "C for 1 h. The sulphides of the cation-exchanged zeolites were prepared by exposing the dry sample to a stoichiometric amount of H,S in an oxygen-free environment. After a reaction time of several hours (sometimes at elevated temperatures) the samples were purged with nitrogen and studied immediately.Instrumentation A Lambda-Physik N, laser with pulse duration 8 ns, energy 6 mJ and wavelength 337.1 nm (main line) was used to study the emission decay. The short-lived transients produced were monitored by fast spectrophotometry (response < 1 ns) and the data captured by a Tektronix 7912A digitizer with subsequent processing by a 4052 minicomputer. Steady-state absorption and emission spectra were recorded on a Perkin- Elmer 552 spectrophotometer and a Perkin-Elmer MPF44B spectrofluorimeter, interfaced to a 405 1 minicomputer through an analogue- to-digital converter.X-Ray diffraction data were obtained with a Diano X-ray diffractometer using Cu Kcc radiation. Results and Discussion CdS on Laponite The reflectance absorption spectra of CdS-laponite powder is shown in fig. 1(A). At high concentration, 0.5 mmol CdS (g laponite)-l, an absorption onset at 510 nm is observed, suggesting large particles. As the concentration is decreased, the absorption onset exhibits a blue shift, indicative of smaller particles. The emission spectra of the samples are shown in fig. 1(B). At high concentration, 0.5 mmol CdS (g laponite)-l, the spectrum is broad, spanning 250 nm, possibly owing to a large distribution of particle size. The emission peak at 560-580 nm is thought to arise from e-/h+ pair recombination on the surface of CdS.,' As the exchanged concentration is decreased, the emission spectra exhibit a blue shift and become narrower.These data, at low concentration of CdS, are similar to that reported for small colloidal particles.l-' The X-ray diffraction sgectra of the cadmium-exchanged laponite had a (001) interplaner spacing of 1 1.5 A corresponding to a single layer of water absorbed between the sheets. This is the same spacing obtained in a sample of laponite treated in the same manner as the Cd2+-exchanged samples. This indicates that the absorbed Cd2+ does not effect the interplanar spacing. On exposure of the samples to H,S the interplanar spacing again did not change, indicating thoat one of the dimensions of the CdS particle is restricted by the clay sheets to 11.5 A. However, no diffraction pattern is observed for CdS.This result suggests that either the CdS formed is to small to diffract X-rays or that it is amorphous in nature. The time-resolved luminescent decays are shown in fig. 1 (C). The emission decays do not follow a simple exponential dependence on time. At long wavelengths (low energy) the luminescent lifetimes are long (several hundred nanoseconds), while that at short wavelengths (high energy) the luminescent lifetimes are short (tens of nanoseconds). It is suggested that the luminescence arises from the electron-hole pair recombination on the surface of the particle. Short-wavelength luminescence arise from small particles. The e-/h+ pairs produced on excitation quickly recombined on the surface. Long-1290 Photophysical and Photochemical Properties of CdS 0.4 - (A1 8 .O - n I 0.0' 120 160 200 0 40 80 Time/ns 0.5 I I I c, .- 9 6 - !a E -5 - v A c,- .- c.' I ' 0.0 ' ' ' " a . ' Fig. 1. (A) Reflectance absorption spectra of CdS on laponite (mmol g-') : (a) 0.05, (6) 0.1, (c) 0.2, ( d ) 0.3 and (e) 0.5. (B) Emission spectra of CdS on laponite (mmol g-l) (a) 0.05, (b) 0.1, (c) 0.2, ( d ) 0.3 and (e) 0.5. (C) Plot of In(intensity) us. time for the luminescent decay at different wavelengths (in nm). 6.0 4.0 2.0 0.0 wavelength luminescence arises from large particles. The e-/h+ pairs have a longer distance to diffuse before they recombined giving rise to a long-lived luminescence. A simple calculation to determine the number of CdS-laponite particles shows that the CdS particles must b: small. If we assume that the laponite particle has the dimensions 700 x 700 x 10 A (from electron microscopy), a volume of 4.9 x cm3 per particle is obtained; given that the density of laponite is 1.2 g a value of 2.45 x 1017 particles of laponite per gram of laponite is obtained.It is establishedz4 that the cation exchange capacity of laponite is 0.8 mmol g-l for monovalent cations; these sites are clustered in islands, and the concentration of the islands was determinedz5. 26 as ca. 0.07 mmol (g laponite)-l or 3.25 x lo1' islands (g laponite)-l, which gives 133 islands per particle. Table 1 lists the number of CdS-laponite particles and CdS-islands. Based on this calculation, the CdS particles must contain only a few molecules of CdS and are very small. If it is assumed that the CdS is oply located on the faces of the clay sheets and not on the edges a total area of 9.8 x lo5 A' per laponite particole is calculated, from which the average distance between the islands is calculated as 86 A.This implies that there is little interaction between islands of CdS.R. D. Stramel, T. Nakamura and J. K . Thomas Table 1. [CdS]/laponite particle ratio CdS CdS /mmol (g laponite)-' /laponite particle CdS/island 0.0 I 0.05 0.1 0.2 0.3 0.5 24 122 245 490 735 1224 0.18 0.92 1.85 3.68 5.52 9.2 1291 Fig. 2. (A) Absorption spectra of CdS on laponite colloid: (b) 5 g dm-3 of 0.1 mmol g-', (c) 2.5 g dmW3 of 0.2 mmol g-l, ( d ) 1.25 g dmP3 of 0.3 mmol g-' and (e) 1 g dm-3 of 0.3 mmol g'. (B) Emission spectra of CdS (mmol g-l) on laponite (5 g dm-3) colloid: (a) 0.05, (b) 0.1, (c) 0.2 and ( d ) 0.3.1292 Photophysical and Photochemical Properties of CdS 400 4 80 560 640 720 800 wavelength/nm 10.0) 1 , I I 40 0 4 80 560 640 720 800 wavelength/nm Fig.3. Change in emission spectra with time. (A) 5 gdm-3 colloid of 0.05 mmol CdS (g laponite)-' at time (a) 5 min, (b) 7 h, ( c ) 27 h and ( d ) 72 h after preparation of the colloid. (B) 5 g dmP3 colloid of 0.3 mmol CdS (g 1aponite)-' at time (a) 5 min, (b) 6 h, (c) 19 h and ( d ) 72 h after preparation of the colloid. CdS on Colloidal Laponite The solid samples of CdS on laponite were suspended in water, making a transparent colloidal solution that scatters very little light. The absorption spectrum of these solutions are shown in fig. 2(A).For comparison, the concentration of CdS is normalized. The spectra are red-shifted when compared with the spectra of the dry powders. All spectra have absorption onsets well below 520 nm; as the concentration of CdS is decreased the absorption onset blue shifts. The emission spectra of these solutions are shown in fig. 2(B). Water has a significant effect on the emission spectra. All emission peaks observed on the dry powder areR. D. Stramel, T. Nakamura and J. K. Thomas 1293 [MVz'l/ 10" rnol drn-3 Fig. 4. Quenching of the 470 nm emission from a 5 gdm-3 solution of 0.05 mmol CdS(g laponite)-' by MV2+. quenched by water, as expected if the luminescence arises from the surface of CdS, where water can interact with the charge carriers. However, there are two peaks observed at ca.470 and 680 nm. It is well established that the emission at 680 nm is due to sulphur defi~iencies.~~ The peak at 470 nm is thought to arise from exciton recombination from small particles,' and is not affected by water. Luminescence lifetimes were < 6 ns at all wavelengths. The ratio and intensity of these two peaks change with time as shown in fig. 3. No change in absorption spectra was observed. The peak remaining at 470 nm indicates that there are small particles which do not grow in size owing to the protection of the laponite layers. Quenching of CdS Luminescence by the Methyl Viologen Dication, MV2+ Fig. 4 shows the effect of MV2+ on the emission intensity of CdS. The emission at 470 nm is efficiently quenched by MV2+. Since MV2+ is located between the layers28 it can efficiently quench the 470 nm band.The quenching of the emission band at 680 nm is less efficient. The emission at 680nm from a 5 g dm-3 solution of 0.3 mmol CdS(g laponite)-l is only 12 % quenched with 5 x mol dm-3 MV2+, after which flocculation occurs. This quenching behaviour can be explained as follows: The MV2+ is found between the clay layers; owing to the negative charge of the clay, it competes with the photogenerated holes for the electrons, a process which leads to a decrease in luminescence intensity. The emission at 680 nm is inefficiently quenched either because of unfavourable energetics or because the MV2+ is located in a region (between the clay layers) where favourable charge transfer cannot occur. A linear plot of ln(Zo/Z) us.[MV2+] for quenching of the 470 nm emission indicates dynamic-type kinetics, suggesting that the MV2+ remains mobile. Quenching by Cu2' Fig. 5 shows the effect of Cu2+ on the emission intensity. The emission at 680 nm is efficiently quenched by Cu2+, while the emission peak at 470 nm is inefficiently quenched.1294 Photophysical and Photochemical Properties of CdS 1 .a 0 0.5 [Cu2+]/IO" mol dm-3 [c~~+]/lO-~ mol dm-3 Fig. 5. (A) Quenching of the 680 nm emission from a 5 g dm-3 solution of 0, 0.05 mmol CdS (g laponite)-' and A, 0.3 mmol CdS (g laponite)-l by Cu2+. (B) Poisson plot of the quenching data : 0, slope = 5.6 x lo5 dm3 m o t ' and A, slope = 4.34 x lo5 dm3 mol-l. In solution Cu2+ first comes in contact with a face of a clay particle where it is strongly attracted to the CdS particle, forming an insoluble sulphide (insolubility product Ksp = on which it is strongly absorbed.Here it serves as a hole trapping site that ultimately leads to a decrease in luminescence intensity. The emission at 470 nm is inefficiently quenched by Cu2+, indicating that Cu2+ cannot quench the exciton emission. A linear plot of ln(Io/I) vs. [Cu"] for quenching of the 680 nm emission indicates static-type kinetics. From the Poisson kinetic equation In (Z,,/I) = [Q]/[S] a site concentration can be calculated. For a 5 g dmP3 solution of 0.05 mmol CdS (g laponite)-l, [S] = 1.78 x lop6 mol dm-3, and for 0.3 mmol CdS (g laponite), [S] = 2.3 x lop6 mol dmP3, these correspond to 140 oand 652 CdS site-', respectively. Assumipg the volume of molecular CdS is 50 A3, total volumes of 7000 and 32600 A3 can be calsulated for 0.05 and 0.3 mmol (g lapon$e)-', respectively. This gives radii of 15 and 32 A for a disc with height of 10 or 11.9 A and 19.8 A for a sphere.Comparing these calculated radii with those published for the expected absorption onset of small colloidal particles,l6. l7 the values correspond quite to with the absorption onset of dry powders. It is suggested that the red shift in the absorption spectrum upon suspending the samples in water is due to a decrease in energy on the surface owing to the interaction with water. This is not observed in colloidal samples because either they are prepared in organic solvents or the surface is coated with stabilizers in aqueous solution.CdS incorporated in Porous Vycor Glass The absorption spectra of CdS incorporated in porous Vycor glass are shown in fig. 6(A). The spectra are dependent on the concentrationoof CdS. Because of the nature of porous Vycor glass, two dimensions are limited to 40 A, while the third long dimension is dependent on the CdS concentration. At high CdS concentration (0.01 mol dm-3 CdS) the absorption spectrum shows a very sharp onset at 520 nm (2.4 eV), which correlates well with the spectrum exhibited by large crystals of CdS as well as the spectrumR. D. Stramel, T. Nakamura and J . K. Thomas 1295 300 350 400 450 500 550 600 w avelength/nm 100.0 80.0 n c c1 .r( 60.0 .!a W x Y .- 40.0 Y .5 20.0 0 .o 400 500 600 70 0 800 w avelength/nm Fig. 6. (A) Absorption spectra of CdS in porous Vycor glass: (a) (b) (c) and ( d ) mol dm-3.(B) Emission spectra of CdS in porous Vycor glass (a) (b) (c) and ( d ) mol dm-3. previously published for 5 x mol dm-3 CdS in Vycor glass.21 Fig. 6 also shows the effect of decreasing the concentration of CdS in Vycor glass. As the concentration of CdS is decreased, the absorption onset shifts toward the blue and attains an absorption tail. Fig. 6(B) shows the emission spectra of CdS in porous Vycor glass. Oxygen does not effect either the shape or the intensity of the spectra. At mol dm-3 CdS the emission spectra shows two peaks located at 480 and 580 nm. As the concentration is increased, both peaks increase in intensity and the 480 nm peak exhibits a red shift. The higher- energy peak is associated with direct e-/h+ pair recombination from the exciton bands.The 580 nm peak is associated with e-/h+ pair recombination on the surface. This emission band is not observed in colloidal systems owing to the interaction of the charge carriers with the aqueous phase. On addition of water to the porous Vycor glass the emission band at 580nm is quenched and a new band appears at 680nm which is attributed to a sulphide deficiency emission. The lifetime of the emission is short (< 6 ns) in all samples.1296 ( A ) Photophysical and Photochemical Properties of’ CdS 10.0 8.0 h m +I .I E: 6.0- rd I 1 1 I (8) - - - Fig. 7. (A) Absorption spectra of CdS on (a) sodalite, (b) faujasite X (preparation 2) and ( c ) faujasite X (preparation 1). (B) Emission spectra of CdS on (a) sodalite, (b) faujasite X (preparation 2) and (c) faujasite X (preparation 1). CdS on Zeolite Powder To investigate further the photophysical properties of CdS in constrained systems, CdS was prepared in zeolites, specifically faujasite X and sodalite.Sodalite is composed of SiO, and A10, tetrahedra linked by shared corners. Faujasite is composed of sodalite cages joined tetrahedrally through four of the eight hexagonal faces to give hexagonal prisms. Synthetic sodium faujasite with %/A1 ratios between 1 .O and 1.5 and between 1.5 and 3.0 are called X and Y zeolites, respectively. The faujasite X in this study had Si/Al = 1.19. The sodium cations in these zeolites are exchangeable and can be replaced with almost any other inorganic or organic cation as size permits.The diameter of the pores is 0.66 nm in sodalite, 0.66 nm in faujasite X I (b-cages) and 1.3 nm in faujasite X I1 (supercages); thus small particles may be grown inside the cages. The sodium cations were 90% exchanged with cadmium cations.R. D. Stramel, T. Nakamura and J. K. Thomas 1297 Table 2. Temperature dependence on the onset of absorption onset of CdS particles sample onset 298 K onset 77 K A/eV CdS/ laponi tea CdS/porous Vycor glass 5 x mol dm-3b 540 nm (2.29 eV) 510 nm (2.43 eV) -0.14 mol dm-3a 520 nm (2.38 eV) 495 nm (2.51 eV) -0.13 mol dmP3 a 505 nm (2.46 eV) 492 nm (2.52 eV) -0.08 no temperature dependence 1 0-4 mol dm-3 a no temperature dependence CdS/zeolites" no temperature dependence Aldrich CdS" 525 nm (2.36 eV) 500 nm (2.48 eV) -0.12 CdS colloidd no temperature dependence CdS/nafion" 512 nm (2.42 eV) 495 nm (2.5 eV) - 0.08 a This work.Ref. (21). ' Ref. (20). Ref. (33). The absorption spectra of CdS in faujasite X and sodalite are shown in fig. 7(A). Again the spectral properties are different from those of bulk CdS and colloidal CdS. The CdS particles are confined to the cages and thus limited in size. Preparation of small metallic particles of Pt, Pd and Ni29 indicate that the particles size are less than 4 nm in diameter and the zeolite cage remains intact. No X-ray diffraction pattern was observed which could arise from CdS, indicating either small particles or amorphous particles. No change in the zeolite diffraction pattern was observed before or after formation of CdS. The emission spectra are shown in fig. 7(B).A single broad band is observed in both systems. This suggests that the emission arising from the CdS in the 0.66 nm diameter cages in sodalite is the same as that arising from the 1.3 nm diameter cages and 0.66 nm diameter cages in faujasite. The lifetimes of the luminescence of all samples at all wavelengths were < 6 ns. Data reported on the size of Pt particles in faujasite X indicate a decrease in particle size with increasing ion exchange. In all samples a single particle size of narrow distribution was obtained. Based on this information and the spectral data obtained in this study, it is suggested that the particles size of CdS $re similar in both faujasite and sodalite and are probably on the order of 4.0 nm (40 A).Nature of the Systems The three systems discussed in this paper limit the dimensions of the CdS particle. Laponite limits one dimension of theoCdS particle through the interplanar spacing of the clay sheets to not more than 11.5 A, while the other two dimensions depend on the concentration of the CdS. Although it is not possible to determine the size of the CdS particles on the dry powder in these studies, the size of the particle may be obtained in solution through kinetic oanalysis. The sizes obtained for 0.05 and 0.3 mmol CdS (g laponite)-l, 15 and 32 A, respectively, correlate well with the predicted absorption onsets of small colloidal particles when compared to the spectra of dry powders, indicating that most of the particles are protected by the laponite layers.As the temperature is lowered to 77 K the expected shift in the main absorption band ( - 5.0 x loP4 eV K-l) is not observed, indicating that the CdS particles do not possess the photophysical properties of large CdS crystals. It is well d o c ~ m e n t e d ~ ~ ~ ~ ~ that the pobes in porous Vycor glass are honeycombed with tubes which have a diameter of 40 A ; therefore CdS prepared in this system has1298 Photophysical and Photochemical Properties of CdS 220 320 420 520 620 720 220 320 420 520 620 720 wavelengt h/nm 220 320 420 520 620 720 wavelength/nm Fig. 8. (A) Reflectance absorption spectra of PbS on laponite (mmol g-'): (a) 0.01, (b) 0.05, (c) 0.01 and (d) 0.03. (B) Absorption spectra of PbS on Laponite colloid (6) 5 g dm-3 of 0.05 mmol PbS g-l, (c) 2.5 g dmP3 of 0.1 mmol PbS g-l, and (d) 0.83 g dm-3 of 0.83 mmol PbS g-l.(C) Absorption spectra of PbS in porous Vycor glass (a) (b) (c) loP3 and (d) mol dm-3. two dimensions limited to < 40 A and a third dimension dependent on the concentration qf CdS. In fact that amounts of CdS used in this study limit the two dimensions to 4 40 A. The long dimension along the tube might be of some significant length; once again, in these studies it is not possible to determine them. As the temperature is lowered to 77 K, the absorption spectrum shifts to the blue in sample of mol dm-3; however for lo-* mol dm-3 no shift is observed. These data, along yith the data of CdS on laponite, suggest that a minimum of one long dimension > 64 A is needed to obtain the optical properties of large semiconducting crystals.Poreparation of CdS in zeolite supports should have limited the particle growth to 13 A in diameter. oHowever, data on small metal particles indicate that particle size can be as large as 40 A without destroying the zeolite cages. As the temperature is lowered to 77 K no shift in the absorption band is observed, indicating that the particles do not possess the photophysical properties of large CdS crystals. Once again it was not possible to determine the particles size in this support. Table 2 lists the photophysical properties of the CdS particle from this and other studies. andR. D. Stramel, T. Nakamura and J . K. Thomas 1299 Lead Sulphide in Constrained Media The reflectance absorption spectra of PbS prepared on laponite are shown in fig.8(A). All spectra have absorption onsets well below that expected for large PbS crystals (0.37 eV).32 The powders are orange in colour. As the concentration is decreased, the absorption onset shows very little blue shift; however, discrete absorption bands begin to appear in the absorption spectra. No luminescence was detected by our instruments on excitation of the samples. Others have reported shifts in the absorption spectra onset from PbS prepared in acetonitrile ;13 however, no discrete absorption bands are visible in the published spectra. Luminescence has also been reported from this sample. It is suggested that laponite protects the small particles of CdS from further growth. Further evidence came from PbS laponite colloidal suspensions.The solid samples of PbS on laponite were suspended in water, and once again, transparent colloidal solutions were obtained. The absorption spectra of these solutions, with [PbS] normalized, are shown in fig. 8 (B). As with the CdS-laponite, the spectra are red-shifted when compared with the spectra of the dry powders. At high concentrations these spectra have tails that extend beyond 750 nm (the limit of our instrument); however, the absorption bands remain present in the spectra, indicative of small PbS particles. No change in absorption spectra was observed over 72 h. Fig. 9(C) shows the absorption spectra of PbS in porous Vycor glass. At high concentrations the glass is black and transparent. As the concentration of PbS is decreased, the absorption onset shifts to the blue and absorption bands begin to appear in the absorption tail.The spectral data of PbS in these constrained systems are consistent with that of CdS. Conclusions The studies presented in this paper tend to illustrate the usefulness of the various matrices in limiting the dimensions of CdS and PbS. The photophysical properties of the small particles are dependent on the environment. 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