3388
J. PhyS. Chem. 1983, 87,3368-3370
Effect of the Synthetic Preparation on the Photochemical Behavior of Colloidal CdS J. P. Kuczynskl, 6. H. MlIoravljevlc,+and J. K. Thomas' Department of Chemistry, University of Notre "8,Notre &me, Indiana 46556 (Re&&:
May 6, 1983)
The luminescence properties of a CdS powder dispersion and colloidal CdS samples synthesized via several different procedures were studied both at room temperature and 77 K. At room temperature it was found that the luminescence lifetimes of all the samples were very short (T < 1ns). However, at 77 K the luminescence lifetime of the colloidal samples exists for much longer times ( T > 0.5 ps) in contrast to the powder dispersion where the emission lifetime is less than 1 ns. The emission wavelength maxima for colloidal CdS depends on the excitation light intensity while no such effect was observed for the polycrystalline powder dispersion. The luminescence properties and the efficiency of photoinduced electron transfer are discussed in light of the crystallinity of the samples as determined by X-ray diffraction analysis.
Introduction In recent years numerous studies have been conducted on the use of colloidal semiconductors in interfacial electron transfer reactions promoted by visible In this regard much attention has been focused on Ti02 colloids since this semiconductor is stable with respect to anodic dissolution.1° However, the bandgap of Ti02 is relatively large which results in poor spectral response to visible radiation. The focus of the research has therefore shifted toward CdS, a semiconductor with a much smaller bandgap and enhanced spectral response in the visible region." Several studies have concentrated on the photophysical and photochemical properties of colloidal CdS; the luminescence spectrum, efficiency of various quenchers, and the quantum yield for hydrogen production have all been rep~rted?*~Jl However, there are significant differences in the published resulta, the reasons for which remain unclear. With respect to this problem we report results related to the influence of structure on the photophysical and photochemical properties of colloidal CdS. Experimental Section Materials. Cadmium sulfide (99.99+ ?& pure, Aldrich Chemical Co.), cadmium chloride (J.T. Baker Chemical Co.), sodium sulfide (Mallinckrodt),sodium dodecyl sulfate perfluoroheptanoic acid (Min(B.D.H. Chemicals, LM.), nesota Mining and Manufacturing Co.), and carboxymethylcellulose (Hercules) were used as received. Methyl viologen (Aldrich Chemical Co.) was recrystallized from methanol three times prior to use. CdS samples were prepared via three different procedures: (1)a suspension of the crystalline powder was accomplished by sonication in a sodium dodecyl sulfate (SDS, 3 X M) solution; (2) a CdS colloid was synthesized by stoichiometric addition of Na2S to a CdC12/SDS (3 X M) solution in an ice water bath by slow precipitation; (3) a CdS colloid was prepared by very rapid precipitation from equimolar quantities of CdC1, and Na2S preheated to 70 "C in the M SDS followed by sonication. presence of 3 X Instruments. Pulsed irradiation studies were performed with a 337-nm beam (8mJ energy, 6-ns pulse width) from a Lambda Physik XlOO laser or with 337-nm light (30-pJ energy, 120-ps pulse width) from a P.R.A. Nitromite N2 laser. The short-lived transients produced were monitored by fast spectrophotometry (response I 1ns) and the data were captured by a Tektronix 7912 A digitizer with sub-
'
On leave from the Boris Kidrich Institute for Nuclear Science, Radiation Chemistry Department, 11001 Belgrade, P.O. Box 522, Yugoslavia.
0022-3654/83/2087-3368$0 7.5010
sequent processing by a 4052 A minicomputer. Steadystate irradiation studies were carried out with an Oriel 150-W Xe arc lamp with appropriate filters for wavelength selection. Absorption and emission spectra were recorded on a Perkin-Elmer 552 spectrophotometer and a PerkinElmer MPF 44B spectrofluorimeter, respectively. Quantum yields were determined by using the 488nm line from a Spectra Physiks argon ion laser and a Scientech light meter. X-ray diffraction data were obtained with a Diano X-ray diffractometer using Cu K a radiation.
Results and Discussions Figure 1 shows the luminescence spectra of a CdS dispersion and two colloidal samples (precipitated in an ice bath and at 70 "C). The spectra were recorded at 77 K with steady-state irradiation. The different Stokes shifts observed in each system indicates that the emission arises from different energy states suggesting that some difference exists between the samples. The luminescence decays, at the emission maxima, at 77 K for the same samples in Figure 1are shown in Figure 2. This figure illustrates that the luminescence decay of the polycrystallinepowder dispersion is very fast ( T < lo* s) and essentially follows the laser pulse, in excellent agreement with previous obser~ations.~J~ However, the luminescence decay profiles of both colloidal samples are much slower with lifetimes approaching 1w.The emission decays do not follow a simple exponential nor can they be described by a power dependence on time as previously (1) Fender, T. H. J . Phys. Chem. 1980,84,1485. (2) Kalyanasundaram, K.; Borgarello, E.; Gritzel, M. H e l a Chem. Acta 1981, 64, 362. Gritzel, M. Acc. Chem. Res. 1981, 14, 376, and references therein. (3) Kraeutler, B.; Bard, A., J. Am.Chem. SOC.1978,100,4317. Izumi, I.: Fan. F. F.: Bard. A. J.Phvs. Chem. 1981.85.218. Bard. A. Ibid. 1982. . . 86, 172, and'references theiein. (4) Thomas, J. K. Acc. Chem. Res. 1977,10, 133. Chem. Reu. 1980, 80,283. (5) Henglein, A. Ber. Bunsenges. Phys. Chem. 1982,86,201. J.Phys. Chem. 1982,86, 2291. (6) Nakato, Y.;Tsumura, A.; Tsubomura, M. Chem. Phys. Lett. 1982, 85,387. (7) Sreva, E. F.; O h ,G. R.; Hair,J. R. J. Chem. SOC., Chem. Common. 1980, 401. (8) Darwent, J. R.; Porter, G. Chem. Commun. 1981,4,145. (9) Kuczynski, J.; Thomas, J. K. Chem. Phys. Lett. 1982, 88,445. (10) Sato, S.; White, J. M. Chem. Phys. Lett. 1980, 72,83. Kawai, T.; Sakata, T. Ibid. 1980, 72, 83. Kawai, T.; Sakata, T. Ibid. 1980, 72, 87. (11) Duonghong,D.; Ramsden, J.; Gr&tzel,M. J.Am. Chem. SOC.1982, 104,2977. Kalyamwundarum, K.; Borgarello,E.; Duonghong, D.; Gritzel, M. Angew. Chem., Int. Ed. Engl. 1981,20,987. (12) Rossetti, R.; Brus, L. J. Phys. Chem. 1982,86, 4470.
0 1983 American Chemical Society
The Journal of Physical Chemlstty, Vol. 87, No. 18, 1983 3309
Letters
4c
I
I
30
5 00
600
700
800
WAVE LENGTH ( n m )
I
I
20
["I
20 Flgure 3. X-ray diffraction patterns of the precipitates of the CdS powder dispersion (lower trace) and the colloid prepared at 70 OC (upper trace).
Flgure 1. Steady-state luminescence spectra at 77 K: (a) CdS dispersion; (b) CdS COHold precipttated at -0 OC (Ice bath); (c) CdS colldd precipitated at 70 OC. See text for details.
\A
a'
\
fa I
100
200 300 TIME ( n s )
400
500
Flgure 2. Time-resolved iumlnescence decays of (a) CdS dlspersion; (b) CdS colold prepared at -0 O C ; (c) CdS colloid prepared at 70 OC. All spectra obtained at 77 K at the emisslon peak maximum. P.R.A. Nitromite N2 laser used as excttation source.
suggested for molecular solids.13 Since both colloids were prepared with the same reagents it seems unlikely that the observed differences in the emission spectra and decay profiles arise from impurities. The colloidal samples differ in their synthetic procedures; i.e., the temperature at which the precipitation was performed and the rate at which the reagents were combined. Both of these factors strongly influence crystal growth and morphology. Taken in conjunction with the luminescence properties these factors suggest that a structural change could be responsible for the observed differences in the emission spectra and transient decays. In order to check such a possibility we conducted an X-ray diffraction analysis. Shown in Figure 3 are the X-ray diffraction patterns of the polycrystalline CdS powder and the precipitate of the colloid prepared at 70 OC. The interplanar spacing d values calculated from 28 peak values, as well as the peak intensities, correlate well with the published data for crystalline CdS powder.13 For the powder, the three most intense peaks are quite distinct and separate. The peaks for the colloidal sample, however, have merged into essentially one broad peak exhibiting slight shoulders on either side of the maximum. (The diffraction pattern for the precipitate of the CdS colloid prepared at -0 OC looks very similar). Even though the peaks have coalesced, the X-ray analysis (13) Debye, P.; Edwards, J. 0. J. Chem. Phye. 1952,20,263. (14) Powder Diffraction File, ASTM 1967.
A '
I
I
Y
800 WAVE LENGTH ( n m ) Flgure 4. Lumlnescence spectra of the CdS coiloid prepared at - 0 O C obtained at 77 K with varying excitation source intensities: high intensity from Lambda Physik N, laser (A);moderate intensity from and steady-state, low-Intensity (150-W P.R.A. Nitromite N, laser (0); Xe arc lamp) excitation from a fluorimeter (soild line).
400
500
600
700
indicates that the colloidal samples possess crystalline structures. Peak broadening suggests that the colloids consist of small microcrystallites and/or that the crystal lattice is perturbed by a nonstoichiometric geometry or incorporation of a large number of defect sites. Figure 4 shows the luminescence spectra of colloidal CdS synthesized at -0 "C obtained under different excitation light intensities at 77 K. Increasing exciting light intensity results in a noticeable blue shift in the emission peak maximum; the CdS powder dispersion does not exhibit such behavior. The difference is attributed to the large number of defects incorporated into the colloidal particle or upon the particle surface. These defects serve as either electron or hole trapping centers located within the bandgap such that recombination at these centers occurs at longer wavelengths thandirect conduction band-valence band transitions. The number of such sites, however, is limited. At high excitation intensities these trapping sites become saturated and the majority of the luminescence subsequently arises from direct transitions between the conduction and valence bands. As a result, the emission maximum shifts toward the blue. The efficiency of photoinduced electron transfer from the CdS samples to methyl viologen, MV2+, was determined by measuring the quantum yield for MV+ production. The quantum yield for both CdS colloids was 8 X while the measured value for the dispersion was 3 X
J. P h p . Chem. 1983,87,3370-3372
3370
The particle sizes for the colloidal samples were on the order of 750 A in radius whereas the sizes for the dispersion averaged 10000 A in radius. Several other procedures of colloid preparation were utilized, among them, precipitation in the presence of a perfluoro surfactant and carboxymethylcellulose. All of these colloids exhibited similar properties to CdS colloids prepared at room temperature.
Conclusions The luminescence properties, as well as the X-ray data, exhibit significant differences between the CdS crystalline powder dispersion and colloidal samples indicating that the colloids are structurally modified. Numerous colloids prepared by precipitation from various cadmium salts and several sulfides in the presence and absence of stabilizers do not show “pure” crystalline behavior. Intercalation of ions (e.g., the counterion of the cadmium salt, Cd2+and/or Ss) into the CdS structure produces defect sites which lead to red shifts in the emission maxima and longer-lived luminescence.
The efficiency of electron transfer from the irradiated CdS samples is dependent on the particle sizes, Le., surface area/g of CdS. Preliminary results indicate that the electron transfer efficiency increases as the surface area is increased. Further research is necessary in order to understand the correlation between bulk structure and surface properties of colloidal particles with the observed photochemical behavior. Particle size distribution, excitation spectra, adsorption isotherms of various electron acceptors, and surface analysis of the CdS particles, as well as other related work, are presently in progress.
Acknowledgment. J. K. thanks the donors of the Petroleum Research Foundation, administered by the American Chemical Society, for support of this work. B.H.M. acknowledges partial support from the National Science Foundation (CHE 82-01226) and the Army Research Office (DAA 6 29-80-K-0007). Registry No. CdS, 1306-23-6; Na2S, 1313-82-2; CdCl,, 10108-64-2;SDS, 2386-53-0.
Reactive Infinite Order Sudden Rate Constants for F
+ H2(v=OJ=O)
-
H
+ HF(v’)
J. Jelllnek,+ Department of Chemlcal Physics. Weizmann Instltute of Sciences, Rehovot, Israel 76100
M. Baer, Applied Mathemetlcs, Soreq Nuclear Research Center, Yavne, Israel 70600
and D. J. Kourl’ Department of Chemistry and D e p a m n t of Physlcs, Unlverslty of Houston-Unlverslty (Recelvd: June 10, 1983)
Park, Houston, Texas 77004
-
State-selected and total reactive rate constants for the F + H2(u=0,j=O) HF(u=1,2,3) + H,reaction have been recalculated from the most recent reactive infinite order sudden (RIOS) approximation cross sections obtained with the l-average choice of 1. Comparisons with classical trajectory and experimental results are made. The RIOS results, although larger than any others obtained with the Muckerman 5 (M5) potential, are still smaller than any of the experimental results.
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Recently, we presented results of calculations of stateselected rate constants for the F + H2(u=Oj=O) HF(u=1,2,3) + H reaction using cross sections obtained with the 1-average version of the reactive infinite order sudden (RIOS) appr0ximation.l However, subsequently, it was found that a minor error had caused the cross sections we used to be somewhat smaller than in fact the l-average RIOS predicts., The state-selected rate constants obtained with the earlier values of the cross sections were somewhat low compared to classical trajectory results. In addition, an estimate of the thermally averated rate constant was well below experimental results. The present paper contains the F + H2 rate constants obtained with the correct l-average RIOS cross sections. Chaim Weizmann Fellow. Present address: Department of Chemistry and James Franck Institute, University of Chicago, Chicago, IL 60637.
The theoretical expression for the rate constant for the reaction F + H2(uo,j,) HF(u) + H (1) -+
is given by
(2)
In this expression, f,( T ) is the fraction of F + H2 collisions (1) V. Khare, D. J. Kouri, J. Jellinek, and M. Baer in ‘Potential Energy Surfaces and Dynamica Calculations”,D. G. Truhlar, Plenum, New York, 1981, pp 475-93. (2) The corrected integral and differential cross sections were published in M. Baer, J. Jellinek, and D. J. Kouri, J. Chem. Phys., 78,2962, (1983).
0022~3654/83/208~-337~~0~.50/0 0 1983 American Chemical Society
