J. Phys. Chem. 1988, 92, 2583-2587 mol-', whereas that of tert-butyl alcohol is 193.7 kcal mol-'. Therefore, the proton affinity of the cluster would only have to be a few kcal mol-' higher in order to bring about a change in fragmentation pattern. However, the same is not true for the secondary alcoholacetaldehyde combination. The proton affinities of 2-propanol and 2-butanol are 191.2 and 191.1 kcal mol-', respectively, whereas that of acetaldehyde is 186.6 kcal mol-'. Based on these values, there should be no change in reaction pathway in going from (ROH),.CH3CHOH+ to ROH. CH3CHOH+.
4. Conclusion The fragmentation patterns of a series of alcohol in clusters have been studied by using the presence of metastable peaks to determine pathways. In many instances, the decomposition routes lead to products which are identical with those found in alcohol ion-molecule reactions. In particular, the frequent occurrence of an H 2 0elimination reaction appears to be a feature common to both ion-molecule and ion cluster reactivity. Structures where the bonding is dominated by electrostatic, e.g., ion-dipole, interactions would appear to offer the best description of the reaction intermediates in the water elimination processes.26 It is conceivable that some of the reaction products considered above may provide a clue as to the step(s) which lead to the formation of protonated clusters following ionization. One of the most common products, particularly in the secondary and tertiary alcohols, is a cluster-bound protonated aldehyde or ketone, Le., (ROH),.CH3HOH+ or (ROH),.(CH,),COH+. In the primary alcohols the equivalent product ion, (ROH),CH20H+, is observed only for alcohols larger than ethanol. Each of these fragments is the product of an a-cleavage reaction, which is the most fre(26) Bowen, R. D,.; Williams, D. H. J. Am. Chem. SOC.1980, 102, 2752.
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quently observed process following the electron impact ionization of an isolated alcohol molecule. In all those case where n > 1, the above ions are observed to decompose to give the corresponding protonated alcohol, (ROH),H+. On the basis of this evidence, we would like to propose the following mechanism for the formation of protonated alcohol clusters
-
-
+ e ((ROH),+)* (ROH),.R'OH+ + R" (24) where R' + R" = R and R" is lost through an a-cleavage reaction (ROH),.R'OH+ (ROH),H+ + R'O (25) (ROH),
-
where R'O is either an aldehyde or a ketone. Indirect support for this proposal comes from the observation that C H 2 0 H + has been shown to contribute to the formation of protonated methanol via the r e a c t i ~ n ' ~ . ' ~ CH20H'
+ CH30H
-
CH30H2'
+ CH2O
(26) There appears to be no evidence of an equivalent C H 3 0 H . C H 2 0 H + ion in, for example, the high-pressure ion-molecule studies on methanol? It may be that the bound intermediate (the cluster analogue of reaction 26 is only stable in the higher alcohols. Our experimental results show that, for most alcohols, the above mechanism might not account for the formation of the protonated m = 1 ion, ROH2+. However, their presence could be accounted for with the reaction (ROH),H+
-
ROH2+
+ ROH
(27)
Acknowledgment. We thank the SERC for the award of an equipment grant. Registry No. C H 3 0 H , 67-56-1; H,CCH,OH, 64-17-5; H3CCH2CHZOH, 71-23-8; H3C(CH2),0H, 71-36-3; H,C(CH,),OH, 71-41-0; H,CCH(OH)CH, 67-63-0; H3CCH(OH)CH,CH,, 78-92-2; (CHJ3C(OH), 75-65-0; (CH3)2C(OH)CH2CH3, 75-85-4.
Molybdenum Oxide Structure on Silica-Supported Catalysts Studied by Raman Spectroscopy and Extended X-ray Absorption Fine Structure Spectroscopy Noriyoshi Kakuta,*t Kazuyuki Tohji, and Yasuo Udagawa Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444, Japan (Received: September 22, 1987)
Raman spectroscopy and EXAFS (extended X-ray absorption fine structure) spectroscopy have been used to investigate the formation of molybdenum oxide on silica-supported catalysts. The existence and the structure of interaction species at low loadings are elucidated by both spectroscopies. The Raman data of the catalyst at higher loadings confirm the previously reported observations, that is, the presence of Moo3 phase after the calcination at 773 K, while EXAFS spectra clearly demonstrate that the major molybdenum species is different from MOO,crystallite. The reason of the apparent contradiction is discussed.
Introduction Alumina-supported molybdenum oxide catalysts are well-known as the hydrodesulfurization catalyst. Various spectroscopic methods have been applied in order to identify active species and to understand why a small amount of additives has a profound effect on catalytic activities. In spite of the abundance of the work on alumina-supported Mo catalyst, silica-supported Mo catalysts have attracted much less attention. One reason is that the catalytic activity was considered to be much lower compared with the alumina-supported catalysts. Recently, however, several research groups'-3 published that silica-supported Mo catalysts have high catalytic activities for alcohol synthesis and photochemical re-
'
Present address: Department of Materials Science, Toyohashi University of Technology, Tempaku, Toyohashi, Aichi 440, Japan.
actions. In these reactions the reactivity is reported to be very sensitive to the structure and valence of the supported Mo species. Ono et ale4reported that at low loadings most of the supported Mo atoms are in tetrahedral coordination and these Mo ions play a significant role in the photoreaction, but the octahedrally coordinated Mo ions, which are formed at high loadings, are not active in this reaction. The local structure of Mo is thus of great interest. Raman spectroscopy is effective in detecting the presence of the crystalline as well as amorphous oxides. Consequently, aluminum-supported Mo catalysts have been extensively studied by (1) (2) (3) (4)
Tatsumi, T.; Muramatsu, A.; Tominaga, H. Chem. Lett. 1984, 685. Ogata, A.; Kazusaka, A.; Enyo, M. J . Phys. Chem. 1986, 90, 5201. Yang, T.-J.; Lunsford, J. H. J . Cutul. 1987, 103, 55. Ono, T.; Anpo, M.; Kubokawa, Y. J . Phys. Chem. 1986, 90, 5201.
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Raman spectroscopy.>18 On the other hand, Raman spectroscopic study on silica-supported Mo catalysts is scarce and little is known about the structure of surface species.“*1p-21From Raman spectral study Knozinger et aI.l9 observed the existence of two molybdenum oxide species; an “interaction species” at low loadings and a “free Moo3” at high loadings. The former is supposed to be a plyanion chemically interacting with the support surface and the latter a small Moo3 crystallite still having a weak interaction with the support. Although Raman spectroscopy is a sensitive tool for the characterization of catalysts, the assignment of bands and the correct interpretation are not straightforward and the conclusion derived must be substantiated by other means. EXAFS spectroscopy gives a direct determination of the local environment of a selected element through the coordination number and interatomic distances. It is applicable to amorphous materials which give only a diffuse XRD pattern. A combined use of these two methods is expected to give a more thorough picture of the structure of the molybenum species in the catalyst. In this paper an elucidation of the identification of Mo species formed on silica with different concentrations before and after calcination stage is attempted by Raman and EXAFS spectroscopic study. Before the calcination the existence of two kinds of species, which are precursors of the two species after the calcination, is found. Raman spectra obtained after the calcination confirm the observation by Knozinger et aI.,l9 that the spectrum is identical with that of MOO,, but EXAFS spectra clearly indicate that not all species formed at high loadings are Moo3 crystallites. The contradiction between the results derived from these two methods is discussed in relation with a possible catalyst structure.
Kakuta et al.
cm-1
Figure 1. Raman spectra of reference materials.
Experimental Section Catalysts were prepared by impregnating silica gel (Kieselgel 60,465 m2/g, Merck) with an aqueous solutions of (NH4),Mq02, (Katayama) at pH 7-8 and then dried at 373 K overnight. The catalysts were calcined at 773 K under air for 5 h. Five molybdenum catalysts containing 0.5, 1.0, 5.0, 10, and 20 wt % Mo were prepared. X-ray diffraction (XRD) analysis was carried out about the dried as well as the calcined catalyst by using the Rigaku Geigerflex 2027 diffractometer. Silicon powder was used as a reference for relative intensity measurements. The 514.5-nm lines of the argon ion laser (Lexel, Model 95-2) was used as the excitation source and was adjusted to 30 mW (measured at the sample position) and focused by a cylindrical lens to avoid local heating and resultant decomposition. The Raman spectra were obtained with a powder sample containing glass. The spectrometer used was a Spex 14018 double monochromator coupled to a photomultiplier (HTV R649) for detection
10 wt% 5 wt% 1 wt% 0.5 wt%
I / 1100 900 I
( 5 ) Brown, F. R.; Makovsky, L. E. Appl. Spectrosc. 1977, 31, 44. (6) Brown, F. R.; Makovsky, L.E.; Rhee, K. H. J. Carol. 1977,50, 162.
(7) Medema, J.; VanStam, C.; DeBeer, V. H.:Konings, A. J. K.; Koningsberger, D. C. J . Catal. 1978, 53, 386. (8) Knozinger, H.; Jeziorowski, H.J . Phys. Chem. 1978, 82, 2002. (9) Jeziorowski, H.; Knozinger, H.J . Phys. Chem. 1979, 83, 1166. (10) Zingg, D. S.; Makovsky, L. E.; Tischer, R. E.; Brown, F. R.; Hercules, D. M. J . Phys. Chem. 1980, 84, 2898. (I 1) Cheng, C. P.; Ludowise, J. D.; Schrader, G. L. Appl. Spectrosc. 1980, 34, 146. (12) Dufresne, P.; Payen, E.; Grimblot, J.; Bonnelle, J. P.J . Phys. Chem. 1981, 85, 2344. (13) Schrader, G. L.; Cheng, C. P.J . Catal. 1983, 80, 369. (14) Stencel, J. M.; Makovsky, L. E.; Diehl, J. R.; Sarkus. T. A. J . Caral. 1985, 95, 414. ( 1 5 ) Leyrer, J.; Zaki, M. I.; Knozinger, H. J . Phys. Chem. 1986, 90,4775. (16) Payen, E.; Kasztelan, S.; Grimblot, J.; Bonnelle, J. P. J . Raman Spectrosc. 1986, 17, 162. (17) Castellan, A.; Bart, J. C. J.; Vaghi, A,; Giordano, N. J . Caral. 1976, 42, 162. (18) Wang, L.; Hall, W. K. J . Catal. 1982, 77, 232. (19) Jeziorowski, H.; Knozinger, H.; Grange, P.; Gajardo, P. J . Phys. Chem. 1980,84. 1825. (20) Cheng, C. P.; Schrader, G. L. J . Caral. 1979, 60, 276. (21) Stencel, J. M.; Diehl, J. R.; D’Este, J. R.; Makovsky, L. E.; Rodorigo, L.; Marcinkowska, K.; Addnot, A,; Roberge, P.C.; Kaliaguine, S.J . Phys. Chem. 1986, 90, 4139.
I
MOOS
I
1
”
700
J
”
500
300
100
c m-1 Figure 2. Raman spectra of dried Mo/SiO, catalysts.
with slit width of 500 wm. A computerized data-acquisition system was constructed to collect and store the data. The scanning rate was 1 cm-I/s, and the data were accumulated by repeated scanning. The frequencies reported are accurate to f 2 cm-’. All experiments were performed at room temperature. EXAFS measurements were carried out by using a doublecrystal in-laboratory EXAFS system which will be described elsewhere.22 In short, X-rays from a high-power X-ray generator were monochromatized by Ge(440) and LiF(220) dispersing crystals. Incident and transmitted beam intensity are detected simultaneously by a semitransmitting ionization chamber and a germanium solid-state detector. The X-ray source with a tungsten target was operated at 40 kV and 180 mA. The spectra were taken at least twice to ensure the reproducibility. The analysis was made as has been described p r e v i o ~ s l y . ~ ~ ~
~~
~~
(22) Tohji, K : Udagawa, Y.; Kawasaki, T.; Mieno, H Reu Sci. Instrum., in
press
Raman and EXAFS Studies of Molybdenum Oxide Structure
1 Mo-0
.-c>I
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2585
i
I
20 w t %
I
EQ
-cE
20 wt%
yd-J
Q
x
0
2
4 6 8 Distance
8
2
5
5wt%
w lwt%
,
Figure 3. Fourier transforms of EXAFS spectra of dried Mo/Si02 catalysts.
Results In order to facilitate the description of the Raman spectra, the spectra of the reference compounds, namely, M a 3 and aqueous solution of (NH4)6M07024,are shown in Figure 1. Dried Catalyst. Figure 2 shows the Raman spectra of dried catalyst containing from 0.5 to 20 wt % Mo. At the lowest loading studied, that is, 0.5 wt %, two bands are observed: a sharp band at 882 cm-I and a broad one at 795 cm-’. This spectrum is different from any of the several molybdenum reference compounds studied. With increasing Mo concentration, other bands at 956 and 943 cm-’ appear and increase in intensity. At higher loadings than 10 wt % these two bands dominate, and the spectra are almost identical with that of aqueous solution of (NH4)6M07024 except for the low-frequency region where the scattering from silica gel dominates. These results indicate the existence of two kinds of species: one at low loadings that is different from heptapolymolybdate and the other at high loadings that resembles to free heptapolymolybdate ion. The Fourier transforms of the EXAFS spectra, the radial structure function, of the dried catalysts with 1 and 20 wt % loadings are shown in Figure 3 as are those of (NH4)6M07024. Always only one peak corresponding to the M o - 0 distance is observed. In (NH4)6M07024,each Mo is octahedrally coordinated by six oxygen atoms and seven MOO, octahedra join together by sharing edges.24 Because these octahedra are severely distorted, Mo-O distances range from 1.650 to 2.475 A, the average being 2.085 A. The peak position of the radial structure function, which is about 1.2 A (phase factor uncorrected), is unusually short. This large phase shift as well as the broad feature should be due to the interference between the contributions to EXAFS oscillation from many M o - 0 distances and is not due to experimental artifact. This can be justified from the observation that in Na2Mo04, which has regular tetragonal form, the peak position is observed at 1.41 A, which agrees well with the literature25 and corresponds reasonably to the crystallographically determined value of 1.76 A. A detailed analysis using curve-fitting techniques seems to be impossible about heptamolybdate because of the complicated structures and was not attempted. Since the radial structure (23) Tohii, K.; Udagawa, Y.; Tanabe, S.; Ueno, A. J . Am. Chem. SOC. 1984, 106, 612. (24) Shimao, E. Bull. Chem. SOC.Jpn. 1967,40, 1609. (25) Parham, T. G.; Merrill, R. P. J . Cutul. 1984, 85, 295.
1100
900
700 500 c m-1
300
100
Figure 4. Raman spectra of calcined Mo/Si02 catalysts.
functions for 20 wt % catalyst and heptamolybdate in Figure 3 are almost identical, however, it can be concluded that Mo atoms are in distorted hexagons in the catalyst. In the 1 wt % catalyst the radial structure function is similar to that of heptamolybdate but the Raman spectra of the molybdate formed are obviously different from that of heptamolybdate as is shown in Figure 2. The identification of this molybdate species needs more study, but it can be said that most of Mo atoms are in a distorted hexagonal structure at low loadings, too. Calcined Catalyst. The Raman spectra of calcined Mo/Si02 catalysts are shown in Figure 4. From a comparison with Figure 2 it can be noticed that the 0.5 wt % sample does not change at all by calcination, suggesting this species to be extremely stable. Samples with higher loadings, however, do change by calcination, as expected, and with increasing concentration not only three sharp bands at 669, 822, and 998 cm-I but also many peaks in the low-frequency region, which are characteristic of the bulk Moo3, become prominent. The 20 wt % catalyst has peaks at the same frequency as and intensity similar to Moo3. The intensity of these peaks becomes much weaker with decreasing Mo contents. The radical structure functions of the EXAFS spectra of the calcined samples are shown in Figure 5 as are those of Moo3. It is known that the Mo atom is located in a distorted octahedron in Moo3.% There are five different Mc-0 distances ranging from 1.67 to 2.34 A, and oxygen atoms can be classified into two groups: four 0 atoms with shorter distances (avera e 1.82 A) and two atoms with longer distances (average 2.30 ). Reflecting this, the radial structure function of M o o 3 has two barely separated peaks shorter than 2 A (phase factor uncorrected), as well as another peak at 3.25 A, which corresponds to the Mc-Mo distance (average 3.66 A). T h e EXAFS oscillation of 20 wt % catalyst is distinctly different from that of pure Moo3. The oscillation is much simpler and decays rapidly with increasing k (wave vector of the photoelectron), which means the contribution from light elements is dominant. As a result, in the radial structure function the peak height due to Mo-Mo is lower than that of MOO,. With decreasing concentration, the peak due to Mo-Mo becomes much lower and the catalyst with 5 wt % loading shows only one peak which corre-
1
(26) Kihlborg, L. Ark. Kemi 1963,21, 357.
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The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 Mo- Mo
I
Pi
M\-o
1”’
.i
-I
Kakuta et al.
/I
21 22 23 24 25 26 21 20 29
I 0
1
2
4
6
8
8
Distance Figure 5. Fourier transforms and EXAFS oscillations of EXAFS spectra of calcined Mo/Si02 catalysts: a, 1 wt %; b, 20 wt %; c, Mo03. sponds to octahedrally coordinated Mo-0, like the dried stage. The dominant species at higher loadings is, therefore, not MOO, crystallite which is suggested by Raman spectra. XRD was also studied in order to see whether the species have long-range order or not, and the results are shown in Figure 6 . The XRD pattern of the 20 wt % catalyst is similar to that of MOO,, but the intensity is lower than the reference material which is a physical mixture of MOO, and S O 2with the same concentration. A comparison of the intensities indicates that about 20% of the total supported molybdenum crystallizes as MOO, on 20 wt % catalyst, indicating that the rest, 80%, is in the X-ray amorphous state. In the 10 wt % catalyst the intensity becomes much weaker, and the catalyst with lower loadings did not show any XRD pattern. In passing, no distinct XRD pattern was observed with all the dried catalyst. Chan et aLZ7have reported that the intensity of the Raman bands of the crystalline WO, is much higher than that of the surface tungsten oxide species. The XRD results suggest that a similar phenomenon occurs on Mo catalysts, too.
Discussion As is shown in Figure 2, for the dried catalyst with higher loadings than 10 wt %, the Raman as well as the EXAFS spectra are almost the same as that of heptamolybdate anion. It should be mentioned that Cheng and SchraderZ0observed much more bands than we did in the same system. Although the reason for the discrepancy is not clear, the existence of higher polymers like MosO~~“may be responsible because of the different pH employed in their impregnation solution. In the dried catalyst at low loadings, Raman bands due to heptapolyanion are not observed, but EXAFS results show that the environments of Mo atoms are similar to that of heptapolyanion. The observed species are very stable and do not change at all by calcining the sample a t 773 K, indicating a very strong interaction with the support. Tetrahedrally coordinated Mo at low loadings has been postulated by several authors,12+18~z0 but the present EXAFS results do not show any evidence that the Td species dominate. A picture that can be derived from the results of Raman and EXAFS spectroscopy is as follows: there is a specific site on the surface of the silica that interacts strongly with the molybdate (27) Chan, S . S . ; Wachs, I. E.; Murrell, L. L. J . Cutul. 1984, 90, 150.
0
28
Figure 6. X-ray diffraction patterns of calcined Mo/Si02 catalysts: a, 10 wt %; b, 20 wt %; c, Moo3 S O 2 (20 wt %).
+
ion and the anion is anchored at the site. At high loadings the sites are saturated and Mo atoms exist as heptamolybdate which has almost no interaction with the support. The weakly adsorbed heptamolybdate species are formed above 5 wt % loadings, and they are converted to MOO, by calcination at 7 7 3 K. Ng et aLZ8suggested that the specific site is the surface hydroxyl and the concentration is determined by the surface hydroxyls on Mo/TiOz catalyst. They also pointed out that the MOO, are formed prior to monolayer coverage on SiOz and A1203catalysts and it is due to the heterogeneity of the support used. Assuming the surface area of 20 A* per molybdate group,6>l2the monolayer coverage is reached at about 20 wt % in the present catalyst if the surface area does not change by the impregnation and the sites are uniformly distributed. Thus, the MOO, formation above 5 wt % suggests that the silica gel surface used is also heterogeneous as reported in the literature^.^-'^-^^ The interaction between the site and the ion makes the considerable vibrational frequency change in the Raman spectra at low loadings, but the local environments of Mo deduced from EXAFS do not change very much. In addition, the peak intensity which reflects the coordination number around Mo atom does not show obvious differences in Figure 3. These imply that the Mo atom of the strongly interacted species at low loadings is also octahedrally coordinated by oxygen atoms. The calcination at 7 7 3 K has no effect on the catalyst at low loadings, because the Raman spectra are the same. Knozinger et al.I9 observed that two kinds of species exist after calcination: one is the “interaction species” at low loadings and the other is a “free MOO,” at high loadings. Our results about the catalyst at low loadings confirm their conclusions and add just that the “interaction species” is formed already in the dried stage; it is stable and does not change by calcining at 7 7 3 K. The structures of the interaction species may depend on the properties of the supports employed because the observed frequency is somewhat different. Similarities between the Raman and EXAFS spectra of the dried and calcined catalysts with low loadings indicate that Mo atoms are still octahedrally coordinated. The molybdenum species in the calcined catalysts with higher loadings, however, needs more consideration. Both Knozinger et al.I9 and Cheng et aLZ0observed the formation of “free MOO,” or “MOO, phase”. Raman spectra obtained in this work indeed support the existence of MOO, crystallite, but the analysis of (28) Ng, K . Y. S.;Gulari, E. J . Cutal. 1985, 92, 340.
J. Phys. Chem. 1988, 92, 2587-2591 EXAFS spectra gives a contradictory result; not all the species formed are MOO, crystallite, because of the Mo-Mo coordination number being much smaller in the catalyst, as is shown in Figure 5. The layered structure is often pointed out as a model of surface molybdenum species. This cannot, however, explain the small coordination number of Mo-Mo observed by EXAFS, because the coordination number in this model is 6 , which is the same as that of bulk. XRD results give a clue to explain the contradiction between the Raman and the EXAFS results, the existence of large amounts of X-ray amorphous species. These are in mixture with MOO, crystallite. In order to elucidate these X-ray amorphous species, we attempt to compare the scattering intensity of the Raman bands of crystallite MOO, with ammonium heptamolybdate, assuming that the X-ray amorphous species exist as heptapolyanion-type oxide. The physical mixture of the same Mo contents was used for Raman measurement. The peak intensity of the 822-cm-’ band of MOO, is about 7 times stronger than that of the 946-cm-I band of ammonium heptamolybdate. The result indicates that the polymolybdate species should be detectable with 20 wt % catalyst by Raman spectroscopy if the X-ray amorphous oxides, which account for about 80% of the supported Mo, exist as one kind of
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oxide such as heptapolyanion. However, Raman spectra do not show any bands except for the bands due to MOO, as is shown in Figure 4, suggesting that the X-ray amorphous species are not one kind of heptapolyanion. Most likely, they are a mixture of several kinds of Mo oxides. According to the EXAFS results, as is shown in Figure 5, Mo atoms are octahedrally coordinated by six oxygen atoms in X-ray amorphous species. Thus, we can conclude that, in addition to MOO, crystallites, there are many kinds of octahedrally coordinated Mo species on the 20 wt % catalyst. Finally, the above observation can be summarized in the following. At low loadings, the “interaction species” is formed already in the dried stage and it is stable and does not change by calcining at 773 K. At high loadings, free heptamolybdate is formed first and many kinds of Mo oxides exist after calcination at 773 K. One is MOO, crystallites and the others are the various kinds of octahedrally coordinated Mo oxide clusters.
Acknowledgment. We thank T. Kadowaki of the Research Institute for Catalysis, Hokkaido University, who kindly measured the surface area of silica gel. Registry No. Molybdenum oxide, 11098-99-0.
Photochemistry of Colloidal Semiconductors. 25. Quenching of CdS Fluorescence by Excess Positive Holes Ani1 Kumar, Eberhard Janata, and Arnim Henglein* Hahn-Meitner-Institut Berlin, Bereich Strahlenchemie, 1000 Berlin 39, Federal Republic of Germany (Received: September 22, 1987)
Colloidal CdS dissolves anodically (to yield CdZ+,SOPz-,and SO:- ions) when it is attacked by OH radicals produced by y-irradiation. The fluorescence intensity of CdS slightly decreases during the first stages of this dissolution, but drastically increases in the later stages as the particles become very small. This increase is attributed to the removal of surface sites at which the radiationless recombination of charge carriers takes place. Pulse radiolysis experiments were carried out in which the changes in fluorescence intensity upon the attack of a colloidal particle by one or a few OH radicals were studied. The OH radicals inject holes into the surface of the colloidal particles, and these holes decrease their ability to fluoresce. One excess hole introduced in this way has 50-100% quenching efficiency. The optical absorption of the deposited holes was also observed. Several holes cannot be stored on a particle as they rapidly react with each other. The longer the lifetime of fluorescence the more efficient is the quenching. The holes injected by OH radicals slowly leave the particles which then become fluorescent again. The consequences of this effect in photocatalysis are pointed out. A mechanism of quenching is discussed, in which the red fluorescence is attributed to the interaction of the electron generated by light absorption with various defect sites on the surface, trapped holes of the S- type acting as radiationless recombination centers.
Introduction It was shown in the first papers of this series that colloidal cadmium sulfide in aqueous solution fluoresces, and that this fluorescence is quenched by certain The quenchers used were stable molecules or ions, such as methylviologen or T1’. Numerous reports on the fluorescence of colloidal CdS have appeared in the meantime4 in which similar quenchers were ap(1) Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 301-305. (2) Henglein, A. J . Phys. Chem. 1982,86, 2291-2293. (3) Henglein, A. Top. Cur?. Chem. 1987, 143, 113-180. (4) Rossetti, R.; Brus, L. J . Phys. Chem. 1982,86,4470-4472. Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. E. J. Phys. Chem. 1986, 90, 3393-3399. Duonghong, D.; Ramsden, J. J.; Gratzel, M. J . Am. Chem. SOC.1982, 104, 2977-2985. Ramsden, J. J.; Gratzel, M. J . Chem. SOC., Faraday, Trans. 1 , 1984, 80, 919-933. Ramsden, J. J.; Webber, S. E.; Gratzel, M.J . Phys. Chem. 1985,89,2740-2743. Serpone, N.; Sharma, D. K.; Jamieson, M. A,; Gritzel, M.; Ramsden, J. J. Chem. Phys. Leu. 1985, 115, 473-476. Kuczynski, J. P.; Milosavljevic, B. H.; Thomas, J. K. J. Phys. Chem. 1984, 88, 980-984. Tricot, Y.-M.;Fendler, J. H. J . Phys. Chem. 1986, 90, 3369-3374.
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plied. In the present paper, an unusual quenching effect is investigated, Le., the influence exerted by excess positive holes on the fluorescence of the colloidal particles. Such excess holes can be injected from OH radicals generated radiolytically.5 Experiments of this type are of interest with respect to our understanding of the reactions of the charge carriers produced by light absorption in small semiconductor particles. In many fluorescence studies, strong laser flashes have served as exciting light. Under these conditions, holes may be accumulated on the particles which in turn may influence the fluorescence. This often makes the results of such studies not comparable to those obtained at low light intensities. The main aim of the present work is to show how the damage produced in the attack of a small CdS particle by the OH radical influences its ability to fluoresce. Moreover, under conditions ~~
( 5 ) Baral, S.; Fojtik, A,; Weller, H.; Henglein, A. J . Am. Chem. SOC.1986, 108, 315-318.
0 1988 American Chemical Society
