4720
J. Phys. Chem. 1995,99, 4720-4732
New Procedures for the Preparation of CdS and Heterogeneous Cr/CdS Phases in Hybrid Xerogel Matrices. Pore Structure Analysis and Characterization Kyung Moon Choi, John C. Hemminger, and Kenneth J. Shea* Department of Chemistry, University of Califomia, Imine, California 9271 7 Received: November 2, 1994; In Final Form: December 24, 1994@
A CdS phase dispersed in a porous poly( 1,4-~henylene)-bridgedsilsesquioxane (PPS) xerogel was prepared from a sol-gel solution containing Cd2+ ions. Both wet and dried Cd2+-doped gels and xerogels were exposed to a sulfide source to produce “low-dimensional” microcrystalline CdS phases dispersed in the polysilsesquioxane xerogel. The CdS phase was characterized by EDAX, electron diffraction, UV, XPS, and fluorescence analyses. The doping procedure produces xerogels with surface areas and average pore sizes that were somewhat lower and smaller that the undoped materials, but in all cases an open cell porous xerogel structure was retained. Related technology has been used to prepare dispersed heterogeneous phases consisting of intimate mixtures of CdS and chromium metal in a porous polysilsesquioxane xerogel. In this case, the chromium metal precursor, a sol-gel processable aryl tricarbonyl chromium(0) complex, is homogeneously incorporated into the xerogel matrix. A microcrystalline CdS phase was independently prepared by successive diffusion of Cd2+and S2- ions in the dried xerogel. This was followed by heat treatment under vacuum (120 “C, < 1 mmHg) to liberate chromium atoms, which produced the mixed Cr/CdS phase in the xerogel matrix. TEM images show domains rich in microcrystalline CdS and Cr/CdS clusters in the xerogel. The Cr and CdS crystalline phases in these xerogels were identified by both electron diffraction and EDAX experiments.
Introduction Semiconductorparticles, including CdS, PbS, ZnS, CdSe, and ZnO, have been prepared in a variety of matrices including polymers,’-3 micelles,4s5 glassy and zeolite^.^ Nano-sized semiconductor clusters dispersed in rigid, porous materials are particularly attractive as candidates for use as catalystsI0 or for the fabrication of optical devices8 More complex formulations that include nanocomposites of both metal and semiconductor particles (for example, Pt-coated TiOz,” Ptcoated CdS,’* and Rh-coated CdSI3) have been prepared for use as photocatalysts. There are a number of strategies for controlling the growth of nano-sized semiconductor particles. Particle growth is influenced in part by diffusion of reagents, the number of nucleation sites, stabilization of growing particles by surface functionality, the boundary constraints of the growth matrix, and the opportunity for equilibration or “ripening” of particles formed under kinetic growth conditions. Summaries of these findings are a~ai1able.I~ In this paper we focus on the preparation and characterization of semiconductor and nano-composites of semiconductor/ transition metal phases in amorphous, porous xerogels. An analogy may be made to the use of microcrystalline zeolites and porous aluminosilicates as a matrix for growing nano-sized semiconductor particles.Is For example, Zhang et al.I4reported that the pore size of an aluminoborosilicate film, which could be controlled by the aging time of the sol-gel process, controlled the particle size of ZnS clusters. These authors also observed that when the ZnS-doped film was annealed (100-600 “C), a coarsening of the semiconductor particle size was observed. The mean particle size was larger than the mean pore diameter of undoped annealed films. They suggested that the pore walls did not effectively constrain particle growth under these conditions or that the ZnS particles became elongated in the unconstrained direction parallel to pore channels. The pore @
Abstract published in Advance ACS Absrracrs, March 1, 1995.
0022-365419512099-4720$09.00/0
Bridged Polysilsesquioxane
Figure 1. Schematic depiction of a bridged polysilsesquioxane.
model for these materials, interstitial voids between aggregated colloidal aluminosilicate particles and microcrystalline materials, such as zeolites, is better defined compared to, for example, porous amorphous materials such as the polysilsesquioxanes that are the focus of this paper. The materials used in the present study, bridged polysilsesquioxanes, are a family of hybrid xerogels prepared from molecular building blocks.l6-Is They may be viewed as molecular composites of inorganic oxides and organic network polymers (Figure 1). Many representatives have high surface areas and a relatively narrow distribution of pore sizes that range from the high micropore to low mesopore domain (15-100 A). The pore size is “controlled” by both choice of molecular building block and processing conditions. We have been 0 1995 American Chemical Society
CdS Phases in Hybrid Xerogel Matrices exploring the use of these xerogels as a template for particle growth within the pores of these amorphous materials. In previous studies, we utilized porous polysilsesquioxane xerogels as a matrix for the growth of "quantum-sized" CdS particles." These amorphous materials provide a matrix for the growth of semiconductor particles. The particle size of the semiconductor roughly correlates with the average pore diameter (N2, BET analysis) of the xerogel. Furthermore, the variable organic component of the molecular building block (Figure 1) allows introduction of organometallic groups which, subsequent to sol-gel processing and xerogel formation, can serve as a precursor to highly active transition metal particles dispersed in the rigid porous matrix. We have demonstrated this approach with the preparation of nano-sized particles of Cr metal (< 100 A) dispersed in a porous polysilsesquioxane xerogel.I8 In this paper, we report preparation of novel, low-dimensional CdS semiconductor phases in hybrid xerogels that are produced by first incorporating Cd2+ ions in the sol-gel solution. The sulfide source is introduced both before and after drying of the xerogel, a variation that gives rise to CdS phases with somewhat different morphologies. Importantly, we find that inclusion of Cd2+ ions during the sol-gel step produces more uniformly doped materials with higher loadings while still retaining the nano-sized characteristics of the CdS phase. We also describe the preparation of mixed nanophases containing both CdS and Cr metal dispersed in a porous polysilsesquioxane matrix. These materials are related to bimetallic catalysts (Pd/Ag)'9,20and CdS co-colloids2' prepared for use as photocatalysts and colloidal dispersions of "metalcapped" semiconductor particles.22
Experimental Section A. Preparation of Monomers. 1,4-Bis(triethoxysilyl)benzene (M-1) was prepared from tetraethoxysilane (TEOS) and 1,Cdibromobenzene (Aldrich) by a procedure described previously.'6 Chromium tricarbonyl (triethoxysily1)benzene (M-2) was synthesized from chromium hexacarbonyl and (triethoxysily1)benzene in THF/dibutyl ether (1/12 v/v) under a nitrogen atmosphere. The product was purified from Si02 flash column chromatography by using a diethyl ethedpetroleum ether ( l / l O volume ratio) as an eluent. A detailed procedure has been described elsewhere.I8 B. Preparation of X-1, X-la, X-2, and X-3. Poly(l,4phenylene)-bridged silsesquioxane was prepared by sol-gel chemistry from 1,4-bis(triethoxysilyl)benzene (4.0 mmol, 1.6 g) in 10 mL of THF solution by adding a solution of acid catalyst (4 N HCl, 0.4 mL). After shaking, the solution was transferred to a polyethylene bottle and sealed. Gelation occurred in 2 days. After aging for 3 more days, the gel was broken into small pieces and stirred in water (1 h) and then air-dried for 2 days. The material was denoted X-1. A solution of 0.05 M Cd(N03)~in THF was used for the preparation of X-la, X-2, and X-3. 1,4-Bis(triethoxysilyl)benzene (4.0 "01, 1.6 g) was added to a 0.05 M Cd(N03)2 THF (10 mL) solution. Then acid catalyst (4 N HC1, 0.4 mL) was added and the solution capped. The mixture gelled after 2 days. For the preparation of X-la and X-2, after aging for 3 more days, the gel was rinsed several times with water. For X-la the wet gel was then air-dried for 2 days. For X-2, the wet gel was then immediately treated with 0.1 M Na2S aqueous solution (50 mL). A yellow glassy material was obtained. The xerogel (X-2) was isolated by filtration and then air-dried for 2 days. For the preparation of X-3, following the aging (total time 5 days), the gel containing cadmium ions was washed with water and then air-dried for 3 more days. A 0.1 M Na2S solution
J. Phys. Chem., Vol. 99, No. 13, 1995 4721 was added to the dry gel. A yellow coloration formed immediately. This xerogel was washed with water several times with monitoring of the Na and S peaks by EDAX, and then samples were submitted for elemental analysis. The material was denoted X-3. Preparation of the xerogels is represented schematically in Figure 2. C. Preparation of X-4, X-5, and X-6. Somewhat different sol-gel and processing conditions are employed for the preparation of Cr-containing xerogels. Poly( 1,4-~henylene)-bridged silsesquioxane was prepared from 1,4-bis(triethoxysilyl)benzene (5.0 "01, 2.02 g) in 23 mL of THF solution by adding an m 0 H catalyst (15 M m O H , 1.9 mL). Gelation occurred in 5 h. After aging for 3 more days, the gel was broken to small pieces and soaked in a series of solvents (THF, CHzClz, EtOEt, toluene, and CCL) for 2 h each. After air drying for 3 days, the gel was ground to a particle size of 100-150 pm and then dried under a high vacuum ( e1 " H g ) at room temperature for 10 h. This reference xerogel was denoted X-4. A mixture of 1,4-bis(triethoxysilyl)benzene (4.9 "01, 1.97 g) and chromium tricarbonyl (triethoxysily1)benzene(0.1 mmol, 0.037 g) was prepared in a 25 mL flask, which was then filled to 23 mL with THF. Ammonium hydroxide (15 M N b O H , 1.9 mL) was then added. After shaking, the solution was transferred to a polyethylene bottle and capped. The mixture gelled after 5 h. The same solvent and drying treatment used for X-4 was followed, resulting in a green glassy material. After crushing, the green glassy powder was heated at 120 "C under a high vacuum (< 1 " H g ) for 24 h. This xerogel was labeled x-5. For the preparation of xerogel X-6, the green glassy pcjwder, prior to heating, was soaked in a 0.05 M CdC12 solution (EtOW H20, v/v 1/4) for 3 days and then washed with water. A solution of 0.1 M Na2S was added to the dried xerogel, giving rise to a yellow-green glass. The yellow-green glass was heated under a high vacuum at 120 "C for 10 h to produce the CdS/Cr phase. This material was denoted X-6. A schematic for the preparation of X-5 and X-6 is shown in Figure 3. D. Elemental Analysis. Elemental analysis of CdS and Cr was performed by Galbraith Laboratories, Inc., Knoxville, TN. E. Porosity Measurements. Pore structure analysis of the polysilsesquioxane xerogel was performed from the NZadsorption-desorption isotherms using a Micromeritics (ASAP 2000) porosimetry analyzer. Before measurements, all samples were degassed at 80 "C under high vacuum for 10 h. Sample amounts of approximately 50 mg were used. The surface area, volume of sorbent (V,,,),and BET constant (C) were calculated by both BETz3 and L a n g m ~ i rmethods ~~ using the Micromeritics software. In the BET plot, PO and P are the saturated and equilibrium vapor pressure of gas. V is the volume of gas absorbed. The average pore diameters of xerogels were calculated by the BJH25method from ZV, and CAP (cumulative pore volume and the cumulative pore area, respectively). This data is summarized in Tables 1 and 2. F. TEM, EDAX, and Electron Diffraction. Powdered samples of xerogels were deposited on a plasma-etched amorphous carbon substrate supported copper grid. TEM dark-field images were obtained with a Philips TEM (CM 20/STEM) electron microscope by controlling the objective aperture centered over the [1111 reflections. The energy-dispersive X-ray diffraction (EDAX) pattern of particles was also obtained by an EDAX analyzer (Philips TEM-EDAX, PV 9800). For the electron diffraction, the camera length was calibrated experimentally with a gold standard, and a X-ray spectrum analyzer at 200 kV was used. G. Auger and XPS Measurements. A fragment of X-3
4722 J. Phys. Chem., Vol. 99, No. 13, 1995
(Et0)sSi
0 -
Si(OEt)3
Choi et a].
4NHCl
I
4.0 mmol M - 1 in THF *
~+ioL:$\
M-1
x-1 1)4 N
HCI /
4.0 mmol M - 1 / 0.05 M Cd(NO,), in THF
b)
0
(EtO)3Si
Si(OEt)3
M-1
2) Wash with water, Dry : X-la 3)Add Na,S before dried : X-2 Add Na$ after dried : X-3
X-2 or X - 3 Figure 2. Scheme for the preparation of (a) 1.4-phenylene-bridged silsesquioxane xerogel X-1 and (b) CdS-doped xerogels X-2and X-3.
was mounted on the sample holder by using double-sided tape. Auger and XPS measurements of CdS particles doped in the xerogel were performed with a photoelectron spectrometer (VG Scientific LTD ESCALABMK 2). XPS measurements were obtained with an aluminum anode X-ray beam (AI K a = 1486.6 eV). The XPS and Auger measurements were obtained under an ultrahigh vacuum (UHV) of 5.1 x Torr. An ion gauge controller (VG Scientific IGC 27) was used to monitor the pressure. A computer program for the data acquisition and manipulation was used. The photoelectron energy was calculated using a computer-interfaced electron analyzer. Results were obtained under the following conditions: analyzer energy = 100 eV; step size = 0.5 eV. H. UV Measurements for Particle Size Analysis. The W absorption edge was fitted by the Brus method.'" The yellow glassy xerogel doped with colloidal CdS was soaked for 7 days in 1,2-dichlorobenzene, a solvent with a refractive index (n = 1.55 1 at 20 "C) similar to that of the xerogel. The absorption spectrum of xerogel doped with CdS in dichlorobenzene was obtained with a Hewlett-Packard (8452A) diode array spectrophotometer using a microsampling UV cuvette designed for solid samples. The particle size of CdS was calculated by using the following equation.26
where me (0.19) and ?nh (0.8) are the effective masses of the electron and the hole, and d and E are the diameter and the dielectric constant of the semiconductor. A band-gap energy
of bulk CdS (2.58 eV)17 was used to calculate the UV blueshift term (AE) by reducing particle size. I. Laser Fluorescence Spectroscopy. Small pieces of X-3 were mounted on the observation glass slide of a Zeiss Optical Microscope (Axioscope). A Supergraphite Ion Laser (CR-2000 K) was used at an excitation wavelength of 476.5 nm. The fluorescence generated was graded by a Chromex imaging spectrograph (250 IS), and its intensity was measured with a CCD detector (Princeton Instruments CCD-1024 EUV). An excitation wavelength of 476.5 nm was used along with a 520 nm filter, 200 pm slit width, and a collection time of 1 s.
Results and Discussion In previous studies, dried polysilsesquioxane xerogels were used as a matrix for the growth of quantum-sized CdS particles by successive treatments with aqueous solutions of CdC12 and Na2S.I7 TEM images revealed xerogels homogeneously dispersed with CdS particles. Average CdS particle size varied with the choice of amorphous silsesquioxane matrix. For example, with phenylene-bridged xerogels, 60 f 15 A CdS particles were produced, and with hexylene xerogels, 90 f 20 8, CdS particles were produced. In the present study, we examine the consequences of including Cd'+ ions in the solgel solution. This work was motivated by a desire to achieve a more uniform distribution of Cd2+ions in the matrix, to obtain higher loadings of semiconductor particles in the matrix, and to observe what changes, if any, in morphology on both the semiconductor phase and the xerogel matrix would result. Earlier workers have established that the addition of metal ions and chelating agents can influence the kinetics of both hydrolysis
J. Phys. Chem., Vol. 99, No. 13, I995 4723
CdS Phases in Hybrid Xerogel Matrices
I
Sol-gel functionality
I
Met ai-c 1u ster precursor M-2
ce
1 1) 0.05 M CdCI, / 0.1 M Na2S 2) 120"C, < 1 mmHg
Figure 3. Preparation of polysilsesquioxanes doped with Cr (X-5)and Cr/CdS phases (X-6).
and condensation steps in metal oxide-forming reactions.27 Indeed, the new procedures reported in the present work produced a number of significant changes in all three aspects. Each of these effects is discussed in turn. Effect of Cd2+Ions on Xerogel Morphology. In previous work, CdS particles were produced in dried xerogels by successive treatment with aqueous solutions of Cd2+ ions and S"- ions.I7 The resulting pore structure of these doped xerogels was not evaluated. In the present study, a reference material, X-1, was prepared by the standard sol-gel formulation that did not include Cd2+ ions. In addition, three new formulations that included (0.05 M) Cd2+ ions in the sol-gel solution were used. The first formulation (X-la) was dried after washing with water. The second (X-2) formulation was treated with a source of S2- ions while the silsesquioxane was still a wet gel. The third (X-3) material was treated with sulfide ions afer drying to a xerogel. For both X-2 and X-3, exposure to a sulfide source produced an immediate yellow coloration in the material. Xerogel X-3 was then washed with water several times to remove unreacted sodium and sulfur ions. The washing process was monitored
by EDAX until the Na peak disappeared. After drying, X-3 was analyzed and found to contain 0.93 and 0.8 wt % of Cd and S, respectively. The results indicate nonstoichiometry. The X-3 was then soaked in water for 10 days, dried, and analyzed. The X-3 contained 1.19 and 0.24 wt % of Cd and S,respectively. These loadings are approximately twice as large as those obtained previously. The stoichiometry of Cd and S is 1/1 within experimental error. Aside from the addition of Cd2+ions and the Na2S treatment, X-2 and X-3 were processed identically to X-1. Pore structures of dried samples of all three xerogels were analyzed by N2 adsorptioddesorption methods. The results of the analyses are shown in Figure 4 and summarized in Table 1. Surface areas of X-1 obtained by both BET and Langmuir methods are 754 and 942 m2/g, respectively. Figure 4 shows the N2 adsorptiondesorption isotherm plots for X-1, X-la, and X-3. All four materials have relatively high internal surface areas, a characteristic of aryl-bridged silsesquioxane xerogels. 16.2R Interestingly, the surface areas of xerogels X-1 and X-2 are essentially identical (754 m2/g vs 741 m2/g). In contrast, the surface area of X-3 is approximately one-third lower (501 m2/
4724 J. Phys. Chem., Vol. 99, No. 13, 1995
Choi et al.
TABLE 1: Pore Structure Analysis of X-1, X-la, X-2, and X-3 As Determined by Nitrogen AdsorptiodDesorption compound
parameters
BET
x-1
surface area (m2/g) V, (cm3/g) at STP CV, (ads.) (cm3/g) Up(ads.) (m2/g) d (ads.) (A) CV, (des.) (cm3/g) Up(des.) (m2/g) d (des.) (A) surface area (m2/g) V, (cm3/g) at STP CV, (ads.) (cm3/g) Up(ads.) (m2/g) d (ads.) (A) CV, (des.) (cm3/g) Up(des.) (m2/g) d (des.) (A) surface area (m2/g) V,,, (cm3/g) at STP CV, (ads.) (cm3/g) Up(ads.) (m2/g) d (ads.) (A) CV, (des.) (cm3/g) Up(des.) (m2/g) d (des.) (A) surface area (m2/g) V, (cm3/g) at STP ZVp (ads.) (cm3/g) Up(ads.) (m2/g) d (ads.) (A) CV, (des.) (cm3/g) CAP (des.) (m2/g) d (des.) (A)
754.22 173.25
X-la
x-2
x-3
BJH
0.0632 44.881 56.32 0.0676 34.349 78.72 570.79 131.12 0.0766 55.450 55.3 0.0585 49.901 46.9 741.41 170.31 0.0770 58.206 52.95 0.075 1 52.871 56.82 50 1.46 115.19 0.0550 39.299 56.00 0.0464 37.527 49.46
TABLE 2: Pore Structure Analysis of X-4, X-5, and X-6 As Determined by Nitrogen Adsorptioflesorption compound
parameters
BET
x-4
surface area (m2/g) V , (cm3/g) at STP CV, (ads.) (cm3/g) Up(ads.) (m2/g) d (ads.) (A) CV, (des.) (cm3/g) U P (des.) “/g) d (des.) (A) surface area (m2/g) V,,, (cm3/g) at STP CV, (ads.) (cm3/g) Up(ads.) (m2/g) d (ads.) (A) CV, (des.) (cm3/g) Up(des.) (m2/g) d (des.) (A) surface area (m2/g) V,,, (cm3/g) at STP ZV, (ads.) (cm3/g) Up(ads.) d (ads.) (A) CVp (des.) (cm3/g) CAP (des.) (m2/g) d (des.) (A)
1121.56 257.64
X-5
X-6
BJH
0.4883 407.21 47.96 0.4005 327.11 48.98 963.97 22 1.44 0.4169 343.33 48.57 0.3282 25 1.74 52.15 574.45 131.96 0.0761 64.003 47.55 0.0580 40.20 57.78
g). A possible explanation for this observation is that the pressure of Cd2+ ions, during drying of the xerogels, but not during gelation, results in reduced surface areas. The sulfide treatment following gelation and aging should remove most of the Cd2+ ions from the wet gel. Divalent Cd2+ ions can coordinate several SiOH groups, facilitating their condensation or drawing polymer silsesquioxane chains closer together. This manifests itself during the drying of the xerogel rather than during the gelation and aging stages. Consistent with this is the observation that there is a gradual decrease in the average
pore size of the xerogels, from 78 8, in X-1 to 50 8, in X-3. If Cd2+“templating” is responsible for a more highly condensed, tighter xerogel, a smaller average pore volume would be expected. Additional support of this analysis is the observation that if the Na2S treatment of dried-Cd2+-containing xerogel is omitted (as with X-la), there is no CdS precipitation and the resulting xerogel has a surface area of 570 m2/g (BET) and an average pore diameter of 47 8, (BJH, desorption average pore diameter). Thus, it would appear that the difference in xerogel morphology between X-2 and X-3 arises from the presence or absence of Cd2+ ions during drying to a xerogel. CdS precipitation prior to drying does not have a sign@cant influence on the resulting morphology. Figure 5 shows a summary of porosity results obtained from different sol-gel processing conditions with and without added salt. Cadmium Sulfide Morphology. Doping CdS in dried xerogels by successive treatment with aqueous solutions of Cd2+ ions and sulfide ions produced discrete CdS particles.” In contrast, the TEM image of xerogel X-3 is notable by the absence of discrete particles. Instead, the xerogel fragments contain what appears to be a continuous dark phase dispersed throughout the otherwise featureless polysilsesquioxane matrix. We note that the CdS loading is higher in X-2 and X-3 than previous doped materials, which may account for some of these differences (i.e., a superimposition of many small-sized particles). Although the undoped (X-1, X-la) and doped (X-2, X-3) materials showed clear differences, verification of the composition of these features was achieved by EDAX, electron diffraction, fluorescence spectroscopy, and XPS. As one of the darker regions was scanned, the EDAX spectra revealed both Cd and sulfur in addition to the elements that comprise the polysilsesquioxane matrix and the sample grid. The electron diffraction pattern arising from one of these dark regions is indicative of polycrystalline CdS of cubic structure. Thus, the apparent “continuous” CdS domain (the dark feature in Figure 6) is a composite of microcrystalline CdS with a crystal morphology more characteristicof nano-sized ( ~ 9 8,) 0 particles. The opportunity to produce high loadings of apparently nanosized semiconductor particles in a porous glassy matrix may be useful for the preparation of heterogeneous catalysts. To further characterize the CdS phases, XPS and Auger spectra of X-3 were recorded (Figure 7). The sample gave rise to what appears as a single sulfur peak at 164.6 eV. Previous XPS studies of CdS have assigned the S2p peak of sulfur in bulk CdS at 162 eV.29 Surface oxidation or other forms of oxidized sulfur (S042-) have peaks in the range of 168 eV.29A high-energy shift in the XPS sulfur S2ppeak due to the quantumsize effort has been previously reported.29 In consideration of the reagents and manner of synthesis of the CdS particles, we can conclude from the XPS/Auger spectra that the binding energy of the sulfur (2p) peak at 164.6 eV is consistent with sulfur atoms that are part of nano-sized particles. It is also interesting that the absence of a 168 eV peak suggests there is little if any oxidized forms of sulfur which have been previously observed in colloidal dispersions of nano-sized CdS.29 Although no special precautions were taken to exclude air, the mesoporous polysilsesquioxane matrix may provide a barrier that retards oxidation of these high surface area particles. Additional characterization of the CdS phase was achieved from the UV spectra and fluorescence emission of CdS-doped samples. Figure 8a shows the W absorption edge plot obtained from X-3, and Figure 8b and c shows the fluorescence emission obtained from both bulk single-crystalline CdS and CdS-doped xerogel X-3. The UV absorption edge plot was obtained from
CdS Phases in Hybrid Xerogel Matrices
J. Phys. Chem., Vol. 99,No. 13, 1995 4725
Isotherm Plot +ads, *des 300 280 260
1
260 240 220 -
ii 140 120
40 20 0 180
-0.0
0:l
0:2
0.3
0.4
0:5
0.6
0.7
0.8
0.9
1D
Relative pressure (P/PQ) Figure 4. Nitrogen adsorption-desorption isotherm plots of (a) X-1, (b) X-la, and (c) X-3.
X-3 soaked in dichlorobenzene for 7 days to achieve transparency. The absorption of CdS was analyzed according to the Brus method,26 and the x-intercept of the plot was determined to be 2.48 eV. The diameter of the CdS particles in X-3 was calculated using eq 1 to be 45 A. Bulk CdS shows a fluorescence maximum peak at 705 nm. In contrast, X-3 shows a fluorescence maximum peak Amax at 668 nm. Previously, O'Neil et aL30 reported that the fluorescence of aqueous colloidal suspensions of CdS with a particle diameter of 45 8, shows a peak maximum at 655.6 nm. This
blue shift from bulk CdS in the fluorescence emission of X-3 is consistent with a nano-size effect. Thus, both W and fluorescence analyses indicate that the CdS phase prepared in X-3 exhibits the spectroscopic characteristics of colloidal CdS, with diameters in the range 45-50 A. An average pore diameter of X-3 obtained by porosity (N2, BJH) analysis (49 A) is in agreement with the spectroscopic results. Thermal Stability of CdS-Doped Materials. Unlike colloidal suspensions and polymer-dispersed composites of semiconductor particles, bridged polysilsesquioxanes exhibit con-
4726 J. Phys. Chem., Vol. 99, No. 13, 1995
Choi et al.
SA: 754 $/g Dia: 78.7 A
WET GEL
2 M CdC12 H20,THF L
E l
WATER PROCESS DRY
SA: 570.80m2/g Dia: 46.9 A
WET GEL
I
SA: 741 .40m2/g Dia: 56.8 A DRY
SA: 501.5 m*/g Dia: 49.5
Figure 5. Schematic summarizing the influence of CdS doping and added Cd2+ ions on the porosity of xerogels.
w
b
Figure 6. TEM images of polysilsesquioxane doped with colloidal CdS phase.
siderable thermal stability. This permits us to observe the effect of temperature annealing on the semiconductor phase. The thermal stability of the CdS phases in xerogel X-3 was chosen for study. Powders of X-3 were annealed at 350 "C for 5 h; the electron diffraction patterns were compared with samples that were not heated. Figure 9 shows the electron diffraction pattern of CdS prepared in X-3 treated with and
without heating at 350 "C. Prior to heating, a polycrystalline CdS pattern with a cubic structure was observed. After heating, the same sample showed a single-crystalline CdS pattern also of cubic structure. The spots in Figure 9 were identified by using a relationship of rd = Lil (r, a radius of each circle; d, lattice parameter; L, camera length; A, wavelength of electron beam at 200 kV = 0.025 A). The "annealing" at 350 "C for 5
J. Phys. Chem., Vol. 99, No. 13, 1995 4727
CdS Phases in Hybrid Xerogel Matrices
I I 1.47 1.26 1.05
I
'
0.84 ' 0.63
'I
'
0.42 0.21
0
'
1150
1035
920
805
690
575
460
345
230
115
0
Binding Energy (eV)
Max. 408.2 eV
1.44. 1.28
.
0.32
-
388.00 400.40 402.80 405.20 407.80 410.00 412.40 414.80 417.u) 419.60 4 L.00
Binding Energy (eV)
Max. 164.6 cV 0.90
'
156.00 157.60 159.20 160.80 162.40 164.00 165.60 167.20 168.80 170.40 172.00
Binding Energy (eV)
Figure 7. Auger and XPS spectra of X-3: (a) full spectra; (b) Cd (3d); (c) S (2p).
h obviously did not produce any change in crystal morphology. The matrix apparently did not permit aggregation of CdS, or if it did, the aggregation did not result in change in crystalline morphology. The difference in polycrystallineand single-crystal diffraction patterns may simply reflect differences in the size of the sampling domain.
Mixed Nanophases of Cadmium Sulfide and Chromium Metal. The incorporation of a zero-valent aryl chromium tricarbonyl monomer, M-2, into the sol-gel formulation provides an opportunity, following processing and drying, to create small ('100 A) particles of chromium metal dispersed in the ~ e r o g e l . ~The ' method is quite general for the preparation of a
4728 J. Phys. Chem., Vol. 99, No. 13, 1995
a> 21 0
Choi et al.
t
190 170
2
*
% 2
c b
150
130 110
W
90 70
sa -2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
4.2
Ea (eV)
4
Wavelength (nm)
.5
Figure 8. (a) UV absorption edge plot of CdS in X-3. Least-squares plot was fitted using data (*). Laser fluorescence spectra of (b) bulk single-crystalline CdS and (c) colloidal CdS in X-3.
variety of transition metal particles. Semiconductor particles “capped” with transition metals have been explored for use as photo catalyst^.^^ We have examined the “intemal doping”
approach as a means to produce intimate mixtures of nanosized CdS and Cr domains in a porous xerogel matrix. We combined the intemal doping procedure for incorporating
J. Phys. Chem., Val. 99, No. 13, 1995 4729
CdS Phases in Hybrid Xerogel Matrices
a)
Figure 9. Electron diffraction pattern of CdS in X-3(a) before and (b) after the heating process. Part (a) shows a CdS polycrystalline pattern with cubic structure. Part (b) shows a CdS single-crystalline pattern of cubic structure. Each spot in (b) is identified by its { 1 1 1 ) and (220) planes with a zone axis of [ 1 101.
a)
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Figure 10. (a) TEM image and (b) EDAX pattern of X-6doped with heterogeneous Cr/CdS phases in two distinct domains of the TEM image. the transition metal precursor and the conventional CdS-doping procedures described previously.'* The method is summarized in Figure 3. Characterization by TEM of xerogel X-5 (Cr doped) has been described elsewhere.'* Small (< 100 A) irregularly shaped Cr clusters are found in the xerogel matrix
followed heating of dried xerogels under vacuum at 120 "C. The TEM image of xerogel X-6 is shown in Figure 10. The features of this material differ significantly from xerogels doped individually with either CdS or Cr. In the TEM image, there are numerous features not observed in amorphous xerogel X-1.
4730 J. Phys. Chem., Vol. 99, No. 13, 1995
Choi et al.
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120 "C
