J. Phys. Chem. 1984, 88, 5556-5560
5556
Photoluminescence of Zinc Oxide Powder as a Probe of Electron-Hole Surface Processes Masakazu Anpo* and Yutaka Kubokawa Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan (Received: May 31, 1984)
The photoluminescence of ZnO powder exhibits a sharp (-380 nm) and a broad (-500 nm) peak, both emissions being quenched by added oxygen. The features of quenching together with ESR measurements suggest that the quenching is a surface phenomenon, arising from trapping photoformed electrons as well as producing a weak adsorption complex on the surface. The intensity of both emissions is changed by pretreatments at various temperatures in oxygen or under vacuum. The former exhibits a vibrational fine structure at 77 K, the energy separation of which is similar to the energy of the Zn-0 vibration. From these results, it is concluded that the emission around 380 nm arises from annihilation of bound excitons, [Zn+-O-],*. The emission around i.e., electron-hole pairs, which is represented by the reverse process of [Zn2+-02-], =i 500 nm appears to be associated with the presence of oxygen anion vacancies near the surface. ESR measurements show that some photoformed electron and hole pairs separate from each other and are trapped as the Zn+ ions and 0-anion radicals, respectively, which play a significant role in the photoinduced surface reactions on ZnO.
Introduction The studies of the photocatalysis and photoelectrochemical reactions on semiconductors have received great attention in connection with the utilization of solar energy.’ However, there seem to be few studies made on the primary processes in the photoreactions, e.g., photoformation of holes and electrons, their separation, and their capture at specific sites on the surface. Most studies in this field appear to be confined to the analysis of reaction products and their yields.’ It seems to be important to characterize the excited states of the catalysts by means of various techniques used in the study of photochemistry. It is well-known that the absorption of a photon corresponding to the fundamental absorption band of an oxide leads to the formation of electron and hole pairs, i.e., excitons, which undergo radiative decay, and light is emitted as photoluminescence. This process is represented as [MetI+-02-] [Me(?l-l)+-O-1* ‘hv‘ since the upper part of the valence band of metal oxides belongs only to the 2p state of 02-ions.2 Tench and Pott showed first that the photoluminescence studies of oxides are very useful in the studies of the surface structure and the excited states of the oxides, because of photoluminescence’s high sensitivity and nondestr~ctivity.~Recently, the authors have investigated the photoluminescence as well as photoinduced surface reactions on transition-metal oxides supported on porous Vycor glass.4 According to the work of Henglein,5 the intensity of photoluminescence of CdS as well as its photodegradation in the liquid phase is seriously affected by added methylviologen. Such effects have been attributed to its high efficiency for trapping photoformed electrons. Bard et al. have shown that the photoluminescence quenching, photoinduced oxygen adsorption, and photodecomposition observed with colloidal ZnS particles are very sensitive to surface effects and concluded that the photolu~~
~~
(1) (a) Bard, A. J. J. Photochem. 1979,10,59. (b) Tanaka, K.; Blyholder, G. J. Phys. Chem. 1972, 76, 1394 and earlier series. (c) Bickley, R. I. Catalysis (London) 1982,5, 308. (d) Kubokawa, Y.; Anpo, M. Shokubai 1981,23, 89. (e) Anpo, M.; Kubokawa, Y. Hyomen Kagaku 1983,6200. ( 2 ) (a) Zakharenko, V. S.; Cherkashin, A. E.; Keier, N. P.; Koshcheev, S . V. Kinet. Katal. 1975,16,182. (b) Morrison, S . R.“The Chemical Physics of Surfaces”; Plenum Press: New York, 1977. (c) Che, M.; Tench, A. J. Adu. Catal. 1983, 31, 77. (3) Tench, A. J.; Pott, G. T. Chem. Phys. Lett. 1974, 26, 590. (4) (a) Anpo, M.; Tanahashi, I.; Kubokawa, Y. J. Phys. Chem. 1980,84, 3440. (b) Anpo, M.; Tanahashi, I.; Kubokawa, Y. J. Phys. Chem. 1982,86, 1. (c) Anpo, M.; Tanahashi, I.; Kubokawa, Y. J. Chem. Soc., Faraday Trans. 1 1982. 78.2121. (d) Anuo, M.; Kubokawa, Y. J. Catal. 1982, 75,204. (e) Anpo, M.; Kubokawa, Y: Rep. Asahi Glass Found Ind. Technol. 1983, 42, 99. ( 5 ) Henglein, A. J . Phys. Chem. 1982, 86, 2291.
0022-3654/84/2088-5556$01 .50/0
minescence can be used as a probe of electron-hole surface processes.6 A similar conclusion has been obtained by Brus et al., who investigated the effect of the surface-adsorbed species on the photoluminescence quantum yields of CdS colloids.’ These results showed that the measurement of photoluminescence provides valuable information about the nature of their excited states and the mechanism of photocatalytic reactions on them, as would be expected from the development of the organic photochemistry using these techniques.* It has been reported that ZnO, which has been prepared in an appropriate way, shows photoluminescence having the emission - . ,? at around 380 nm and ca. 500 nm on excitation with bandgap irradiation? However, its details are unclear at present. Although it is well-known that ZnO exhibits a high activity for the photocatalytic oxidation of hydrocarbons, carbon monoxide, alcohols, etc.,l*’Ono studies of its photoluminescence have been carried out in conjunction with the photocatalytic reactions. Studies along this line have been undertaken in the present work.
Experimental Section The catalyst was Kadox-25 ZnO powder (surface area, 2.0 m2/g by the BET method; particle size, 0.11-0.17 pm by scanning electron microscope). About 0.3 g of the sample powder was compressed to form a pellet of 15-mm diameter at 500 kg/m2 pressure. The sample pellet was introduced into a quartz cell equipped with an electric furnace and window section. The sample could be moved vertically between the window section and the furnace section, so that its temperature could be changed from 77 to ca. 1200 K. The cell was connected to a conventional vacuum system (ca. torr available). A standard thermal treatment of the samples was performed as follows. The sample was slowly heated to 727 K under vacuum, and the O2at 100 torr was admitted. After cooling in O2 to room temperature, the sample cell was evacuated at the same temperature until torr was reached. Details of the apparatus and procedures were described previously! The photoluminescencespectra were measured by using a Shimazu RF-501 spectrofluorophotometer with filters to eliminate scattered light in the temperature range of 77-300 K. In the case of the measurements of lifetimes of the emission, ZnO samples were excited at 300 K using a N, laser with a (6) Baker, W. G.; Bard, A. J. J . Phys. Chem. 1983, 87, 4888. (7) Rossetti, R.; Brus, L. J . Phys. Chem. 1982, 86, 4470. (8) Hercules, D. M. “Fluorescence and Phosphoresence Analysis”; Wiley: New York, 1966. (9) (a) Nicoll, F. H. J . Opt. SOC.Am. 1948, 38, 817. (b) Heiland, G.; Mollow, E.; Stockman, F. Solid State Phys. 1959, 8, 191. (10) (a) Zakharenko, V. S.; Cherkashin, A. E.; Keier, N. P.; Gerasimova, G. F. Kinet. Katal. 1975, 16, 174. (b) Pichat, P.; Herrman, J. M.; Disdier, J.; Mozzanaga, M. N. J . Phys. Chem. 1979,83, 3122.
0 1984 American Chemical Society
Electron-Hole Surface Processes on ZnO Powder
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5557
4001
,& 4490
3901
h 50
450
400
500
473
573 673 773 873
Degassing ternperoture , K
Figure 3. Effect of the degassing temperature of the ZnO sample upon A,, of its emissions at 300 K.
550
t
excitation, 300nm
5t
Wavelength, nm
Figure 1. Photoluminescence spectrum of ZnO powder at 300 K. The ZnO sample was degassed at 673 K for 1 h. PW
373
573
973 , K Figure 4. Effect of the degassing temperature of the ZnO sample upon the intensity of its emissions at 300 K. The sample was degassed for 1 Degassing
Wavelength
, nm
Figure 2. Excitation spectra of the emissions and absorption spectrum of ZnO powder at 300 K. The excitation spectrum of the ultraviolet emission (a) was monitored at 390 nm (excitation beam slit width, 43.0 nm), and the excitation spectrum of the visible emission (b) was monitored at 520 nm (excitation beam slit width, 45.0 nm).
773
Temperature
h at each temperature. 20
excitation, 300 nrn -admission
15
of O2 4.3torr
T
ineversible
nanosecond pulse width. ESR measurements were carried out at 77 and 300 K using a JES-ME-1 (X band) spectrometer. Details of these measurements were described el~ewhere.~.” Results 1 . Photoluminescence Properties of Zinc Oxide Powder. As shown in Figure 1, the photoluminescence of ZnO consists of an ultraviolet emission (A, at 380 nm) and a visible emission (Arnm at ca. 500 nm). The former is very intense and sharp, while the latter is rather broad and weak, being in good agreement with the earlier reporkg Figure 2 shows the excitation spectrum of the emissions. It is clear that both emissions are observed when ZnO is excited with light having energy larger than the bandgap of ZnO (3.3 eV = 370 nm). The decrease in the intensity of the excitation spectrum in the range of wavelengths less than 360 nm is attributable to a decrease in the fraction of the oxide activated by UV irradiation, owing to the increase in the absorption coefficient in this region.’* The effects of the degassing temperatures of ZnO on the photoluminescence were investigated. Figure 3 shows that with increasing degassing temperature A, of the ultraviolet emission is scarcely or not at all affected, while A,, of the visible emission is shifted toward a longer wavelength. As shown in Figure 4, with increasing degassing temperature the intensity of the visible emission scarcely changes up to 873 K and then slightly increases. 88,
(1 1) Anpo, M.; Fujii, T.; Suzuki, S.; Kubokawa, Y .J. Phys. Cbem. 1984,
2512.
(12) Bube, R.H. “Photoconductivity of Solids”; Wiley: New York, 1960.
0
IO
20 Time
30 0
I 20
40
60
. rnrn
Figure 5. Effect of the addition of oxygen upon the emissions of ZnO at 300 K. On the other hand, the ultraviolet emission drastically changes with increasing temperature. The intensity increases up to 673 K, showing a maximum, and then decreases. Hauffe et al. also reported that both the wavelength and the intensity of the visible emission scarcely change by doping with Li02.13 The lifetimes of these emissions depended upon the pretreatments of the samples etc. such as the degassing temperature, the reduction with Hz, Although the results will be reported in the forthcoming paper, the lifetime of the sample degassed at 673 K was around 1.0 1.1s at 300 K, being in agreement with the results in the l i t e r a t ~ r e . ~ ~ 2. Quenching of the Photoluminescence with Oxygen. The effects of addition of O2 on the intensity of the emission were investigated, because of its efficiency for electron capturing. As (1 3) Poerffler, W.; Hauffe,
K. J . Catal. 1964, 3, 156
5558 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
I
Anpo and Kubokawa
b I
-
J
*t
373 473
. , \
360 380 400 420 Wavelength, n m
Figure 6. Fine structure of the ultraviolet emission of ZnO powder at 77 K (excitation, 300 7.0 nm; emission slit width, 0.8 nm).
*
shown in Figure 5 , on introduction of O2onto ZnO both emissions markedly decrease in intensity. The intensity recovers with evacuation of the sample at 300 K, though its recovery is partial. The ESR spectrum indicated that addition of O2 at 300 K upon the previously degassed ZnO samples led to the appearance of the ESR signal due to adsorbed 02.This signal was assigned to the 02-anion radicals, since its shape and the g values are in good agreement with those of;he 0, anion radicals adsorbed on Zn0.14 UV irradiation of ZnO in the presence of O2 led to the growth of the ESR signal of the 02-anion radicals together with the appearance of a new ESR signal. Apart from the new signal which will be discussed in the near future, the results suggest that the reversible quenching might be associated with the formation of weak adsorbed oxygen complex. While the irreversible quenching is attributable to the formation of the 0, anion radicals on the surfaces, the 0, species are rather stable and remain on the ZnO surfaces after evacuation a t 300 K. They were only removed by evacuation at about 423 K. Oster and Yamamoto also observed that the oxygen molecules suppress the photoluminescence as well as the photoconductivity of ZnO, attributing it to trapping of electrons by 02,resulting in the formation of 02-anion radi~a1s.l~ Henglein5 and Bard et a1.6 have also observed that added oxygen quenches the photoluminescence of CdS or ZnS, attributing it to the formation o the 02-, anion radicals under UV irradiation. The presence of the 0, species before UV irradiation due to charge transfer from ZnO to oxygen will bring about a lowering of the level of electrons in ZnO, which is expected to result in the change in the photoluminescence intensity. In fact, it has been found that the intensity of the photoluminescence of ZnO containing adsorbed oxygen species increases with increasing degassing temperature, Le., with decreasing amount of the 0, anion radicals. It has been established with Pt-loaded T i 0 2 that in the case of Pt-loaded metal oxides photoformed electrons are easily transferred from oxides to Pt particles, suggesting that Pt can act as a scavenger of photoformed electrons. The following results are in accord with such an expectation. When ZnO was loaded with Pt, the intensity of the ultraviolet emission decreased, its lifetime being shortened from 3.0 to 0.8 ps a t 300 K. 3. Assignment of the Emission. 3.1 Ultraviolet Emission. Figure 6 shows the vibrational fine structure of the ultraviolet emission observed at 77 K. No fine structure is observed with the visible emission. The energy separation of the vibrational bands in the system, i.e., 420, 620, and 560 cm-', seems to be in agreement with the energy of the vibration of the Zn-0 groups, since the metal-oxygen bond stretching vibration of ZnO is ob(14) (a) Lunsford, J. H.; Jayne, J. P . J. Chem. Phys. 1966, 44, 1487. Lunsford, J. H. Carol. Reu. 1973, 8, 135. (b) Che, M.; Tench, A. J. Adu. Catal. 1983, 32, 1. (15) Oster, G.; Yamamoto, M. J . Appl. Phys. 1966, 37, 823.
573
Temperature,
673 K
Figure 7. Effect of the calcination temperature of ZnO sample in oxygen upon the intensity of its emissions at 300 K. The sample was degassed at a temperature lower than the calcination temperature by ca.50 K after heating in oxygen.
served in the range of 530, 500, and 440 cm-l by IR measurements.16 Figure 7 shows the effects of the calcination temperature in oxygen on the photoluminescence of ZnO prereduced by degassing at 873 K. With increasing calcination temperature, ultraviolet emission increases markedly in intensity around 600 K, while visible emission increases slightly. Kokes showed that a t 600 K the diffusion of the interstitial zinc ion, Zn+, under the influence of the electric field provided by oxygen adsorbed on ZnO becomes rapid enougn to produce a new [Zn2+-02-],pair on the s ~ r f a c e s . ' ~Those results together with the results shown in Figures 6 and 7 suggest that the emission observed around 380 pair nm is associated with the presence of the surface [ZnZ+-O2-], sites. The 02-ions constituting the [Zn2+-02-],pair sites are not normal lattice oxygen ions but have some characteristics of adsorbed species. The existence of adsorbed 02-ions on ZnO has been proposed by Stone et a1.,18 who showed those species are involved in the oxidation of alcohols. Furthermore, in the action spectrum for the photoconductivity of Ti02, Iwaki et al.I9 have observed a maximum in the region of the energy lower than the bandgap, such as 415 nm. This peak has been attributed to the presence of the specific 02-ions on the surfaces by Cunningham et aL20 The correlation between the emission at 380 nm and the [Zn2+-02-],pair sites is supported by the change in the intensity of this emission with increasing temperature up to 673 K (Figure 4). According to the work of Tanaka and Blyholder,21who investigated the oxygen species adsorbed on ZnO using TPD and ESR techniques, two peaks are observed around 473 and 573 K, being attributed to the 0, and the 0-anion radicals, respectively. This suggests that owing to the desorption of the 02-and/or 0species, the concentration of 02-(,)"species,Le., [Zn2+-02-],pair sites, will increase up to the degassing temperature of about 600 K. Above this temperature the concentration of the [Zn2+-02-], pair sites will decrease owing to the desorption of the 02-(,) species. Thus, the results shown in Figure 4 are explicable. Accordingly, it might be concluded that the emission around 380 nm arises from the process
.S [z~+-o-I,* hu'
[~n2+-02-1,
where the subscript s refers to the surface. Since Zn+ and 0- can be regarded as localized electrons and holes, respectively, the ~~~
~~
(16) Karninori, 0.; Yamaguchi, N.; Sato, K. Bunseki Kagaku 1967,16, 1050. (17) Kokes, R. J. J. Phys. Chem. 1962, 66, 99. (18) Barry, T. I.; Stone, F. S . Proc. R. SOC.London, $era A 1960, 225, 124. (19) Iwaki, T.; Miura, M. Bull. Chem. SOC.Jpn. 1971,44, 1154; 1973,46, 1631 ....
(20) Cunningham, J.; Doyle, B.; Leahy, E. M. J . Chem. SOC.,Faraday Trans. I 1979, 75, 2000. (21) Tanaka, K.; Blyholder, G. J . Phys. Chem. 1972, 76, 3184.
Electron-Hole Surface Processes on ZnO Powder reverse process results in the annihilation of excitons, Le., recombination of electrons and holes. 3.2 Visible Emission. Since bandgap irradiation is necessary for appearance of the visible emission, there is no doubt about participation of photoformed electrons and holes in the emission. Van Craeynest et a1.22 concluded that the visible emission is generated by the recombination of electrons from the conduction band with trapped holes, i.e., trapping of photoformed electrons by the oxygen ion vacancies in ZnO, since it is expected that the anion vacancies lie 2.7 eV below the bottom of the conduction band, Le., that such a trapping of photoformed electrons is accompanied by the emission, its energy being similar to the observed value (-2.5 eV). Furthermore, according to the calculation using a semicontinuum model, the energy level of the excited state of the oxygen vacancies is about 2.49 eV,23in agreement with the A,, value. With increasing degassing temperature, the intensity of the visible emission increases and its A,, shifts toward a shorter wavelength. Such a feature suggests that visible emission is associated with the anion vacancies, since the concentration of anion vacancies increases with increasing degassing temperature. Thus, the above assignment of the visible emission is supported. It is seen that the excitation spectrum of the visible emission shows another peak at about 470 nm (2.75 eV), being in good agreement with the above assignment.
Discussion 1 . Excited States of ZnO Powders. It is well-known that on bandgap irradiation of an oxide the formation of electron-hole pair (excitons) takes places prior to the production of free electrons and hole^.^^^^^ The significance of such electron-hole states in the photocatalysis has been shown with highly dispersed metal oxides on support^.^ The photocatalytic activity of the supported metal oxides is closely associated with the structure of the charge-transfer excited complex [Me("')+-O-] * formed on the surfaces. According to the work of Bard et al.,25 with small semiconductor particles such as TiOz and ZnO, UV irradiation brings about the e--h+ formation as a L-M (ligand-metal) charge transfer and not the formaton of free electrons and holes, since the thickness of the space charge layer formed on small particles is not sufficient to separate photoformed e--h+ pairs. Steinbach has also suggested that the excited states of semiconductor particles can be adequately approximated by electron-hole pair states rather than free electrons and holes.26 The above consideration together with those described in the preceding papersz7 supports the conclusion just described above that the excited states of ZnO powder are characterized by the e--h+ pairs, Le., [Zn+-O+], complex, being similar in character to the charge-transfer excited complex [Me("-*)+-O-] * observed with the highly dispersed metal oxide on supports.4 Thus, the appearance of the vibrational fine structure in the photoluminescence of ZnO powder is explicable. It is worth noting that with ZnO single crystals a similar vibrational fine structure has been observed in the emission near the band edge (380 nm = 3.2 eV), being attributed to annihilation of bound excitons.28 The fine structure of the phosphorescence of VzOs supported 3 transition is the on porous Vycor glass showed that the 0 strongest.4a The Franck-Condon principle suggests that the nuclear distance of the V=O complex will become longer in the charge-transfer excited state. In fact, photoformation of C 0 2 , i.e., photoreduction of Vz05supported on porous Vycor glass with CO molecules, proceeded very easily. A similar correlation is
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(22) Van Craeynest, F.; Maenhout-Van der Vorst, W.; Dekeyser, W. Phys. Status Solid 1965, 8, 841. (23) Wei, W. F. Phys. Rev. B Solid State 1977, 15, 2250. (24) Kuczynski, J.; Thomas, J. K. J . Phys. Chem. 1983,87, 5498. (25) Ward, M. D.; Bard, A. J. J . Phys. Chem. 1982, 86, 3599. (26) Steinbach, F. Fortschr. Chem. Forsch. 1972, 25, 117. (27) (a) Anpo, M.; Aikawa, N.; Kodama, S.; Kubokawa, Y . J . Phys. Chem. 1984,88, 2569. (b) Anpo, M.; Aikawa, N.; Kubokawa, Y . J . Phys. Chem., in press. (28) Weiher, R. L.; Tait, W. C. Phys. Rev. 1968, 166, 791.
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5559 2.0214
1
2t
/
ID
' i p . . 10
20 30
U V irradiation time ,min
Figure 8. ESR spectrum of photoformed O-(,)anion radicals on ZnO powders at 77 K and its growth with UV irradiation time a t 77 K.
-
observed with the photocatalytic activity of unsupported oxides such as ZnO. As shown in Figure 6,29the 0 1 transition is the strongest in the case of ZnO, suggesting that there is little or no difference in the nuclear distance of the [Zn2+-OZ-], complex between the excited [Zn+-O-],* and the ground [Zn2+-02-], states. In fact, Volodin et al.30detected the COz- anion radicals resulting from the reaction 0-, CO C02-. Nevertheless, no evolution of COz was observed up to the temperature of about 370 K. Cunningham et al.31also observed that the photoassisted release of atomic oxygen from ZnO requires an activation energy. Comparison between the results with V205supported on porous Vycor glass and the ZnO powder system suggests not only the close correlation between the photoreactivity of metal oxides and the structure of their excited states but also the validity of the model of electron-hole pairs for the special sites on ZnO powders. 2. Electron-Hole Pairs [ 1 W e ( ~ ' ) + -*0and ] the Species Formed by Electron and Hole Trapping. It is expected that the e--h+ pairs [Zn+-O-],* make no direct contribution to the electric conductivity of oxides as well as to the appearance of ESR signals. In the cases where photoconduction or a photoinduced new ESR signal appears, dissociation of the e--h+ pairs [Zn+-O-],* will take place. Tench and Gerischer have observed that the action spectrum for the photocurrent of ZnO exhibits a maximum around 380 nm and concluded that irradiation of 380 nm brings about formation of the charge-transfer excited states, Le., excitation states which dissociate at specific sites to form free electrons and holes; thus photoconduction results.32 In connection with this problem ESR measurements were carried out with ZnO powders at 77 K in vacuo under UV irradiation. New ESR signals having g values of 1.961 (A species) and gll= 2.003 and g , = 2.021 (B species) appeared, their intensity increasing with UV irradiation time (Figure 8). Both signals are attributable to Zn+ ions (A species) and O-,,) species (B species), r e s p e ~ t i v e l y . ~In ~ ~view ~ ~ of - ~the ~ results reported by Tench and G e r i s ~ h e rit, ~appears ~ that the [Zn+-O-],* pair formed under UV irradiation dissociates at specific sites to form a free electron or hole, the remaining one being trapped. Thus, species will result. formation of Zn+ ions and O-,,)
+
-
(29) The vibrational fine structure of ZnO (Figure 6) is not a popular one, which is seen in the emission of gaseous molecule. However, the strongest transition could be assigned to the 0 1 transition, since the energy separation between the 0 0 and 0 1 transition bands is in agreement with that obtained by IR measurements,16 as described above. (30) Volodin, A. M.; Cherkashin, A. E. Kinet. Katal. 1979, 22, 979. (31) Cunningham, J.;Finn, E.; Samman, N. Faraday Discuss. Chem. SOC. 1974, No. 58, 160. (32) Tench, D. M.; Gerischer, H. J . Electrochem. SOC.1977,124, 1612. (33) Volodin, A. M.; Cherkashin, A. E. Kinet. Katal. 1981, 22, 598. (34) Wong, N. B.; Tarrit, Y . B.; Lunsford, J. H. J . Chem. Phys. 1974,60, 2148. (35) In the presence of O2the ESR signal due to O-(sl was not observable. The 02anion radicals and a new ESR signal, probably due to Oj-anion radicals, were observed. These results suggest that the photoformed O-(slanion radicals would be near and/or surfaces of ZnO powder."
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- -
J . Phys. Chem. 1984,88, 5560-5563
5560
It has been reported by a number of workers that the O-,,) anion radicals formed by trapping photoformed holes exhibit a high reactivity toward various reactions such as oxidation of carbon monoxide and hydrocarbons and dehydrogenation of alcohols.lb-e It seems to be a very important problem to clarify the difference species which compose the in reactivity between the O-,,) charge-transfer excited complexes, [Me("-')+-O-] *, and the O-,,) anion radicals stabilized on specific surface site^.^,^^ It has been (36) Kazansky, V. B. "Proceedings of the 6th International Congress on Catalysis, July 1976, London"; Bond, G. C., Wells, P. B., Tompkins, F. C., Eds.; Chemical Society: London, 1977; Vol. 1, p 50.
shown that a close existence of photoformed electrons and holes is necessary for fission of the C=C bond which is an important step in the photoinduced metathesis reaction of alkenes over Moo3 supported on porous Vycor g l a s ~ ~and ~ gin~ the photocatalytic hydrogenation of alkenes or alkynes with H 2 0over TiOz p0wder.2~ Further study appears to be necessary for complete understanding of this problem. The results obtained in the present work suggest that measurements of the photoluminescence from the surface region of oxides is very useful in the studies of their excited states, the information of which is a prerequisite for understanding of the photocatalytic action of metal oxides. Registry No. ZnO, 1314-13-2; 02,7782-44-7; 02-,11062-77-4.
Isotope Shifts and Force Field for Carbon Tetrachloride in a Krypton Matrix Llewellyn H. Jones,* Basil I. Swanson, and Scott A. Ekberg Los Alamos National Laboratory, University of California, Los Alamos, New Mexico 87545 (Received: June 13, 1984)
The high-resolution infrared spectrum of the v 3 and v 1 + v4 modes of carbon tetrachloride of natural isotopic abundance in a krypton matrix displays a multitude of absorption peaks due to the several isotopic species and the presence of two dominant sites, one of which shows site symmetry splitting. We have assigned all of the observed transitions and from the isotope shifts have estimated a quadratic force field for each site (one assumed to have Tdsymrnetry and one assumed to have C3, symmetry). The difference in force fields is slight for the two sites. The results are in agreement with a Urey-Bradley force field estimated earlier without the benefit of isotope shifts.
Introduction TABLE I: Raman Peaks for vl, v2, and v4 of CC14 in a Krypton Matrix Because of the many isotopic species present in CCl, of natural frequency, cm-I isotopic abundance the infrared and Raman spectra are quite complex. A number of infrared and Raman studies were made 3 5 c 1 37c1 obsd" calcd manv vears and amroximate force fields were c a l c ~ l a t e d ~ ~ ~ v 1 Mode by u%g a Urey-Bradle;'potential field. The infrared spectrum 4 460.2 460.2 of v 3 and v 1 + v4 of CC14in an argon matrix was reported by King; 1 3 457.1 457.1 however, the resoltuion was not adequate to make complete as2 2 454.0 454.0 signments for the various isotopic species. Recently we have 3 1 450.9 450.8 observed high-resolution spectra in the region of the v 3 mode of v2 Mode CC14in a krypton matrix.8 A large number of peaks are observed 4 218.8 218.8 due to the many isotopic species, the presence of two dominant 1 217.4 217.3 3 sites one of which shows site symmetry splitting, and Fermi 2 2 215.8 215.8 resonance of v 1 v4 with vj. We have been able to assign all of 3 1 214.3 214.3 this fine structure and determine many isotope shifts. Such inv4 Mode formation should be useful in estimating a more quantitative 4 313.6 (45)b 313.7 (45) quadratic force field than previously r e p ~ r t e d . ~We , ~ have also 3 1 E 312.4 (36) 312.5 (39) studied the Raman spectra of CC14 isolated in a krypton matrix 2 2 Bz 311.4 (10) 311.3 (10) under moderate resolution. In this article we wish to report the 1 3 A, 309.8 (28) 309.9 (32) assignments of the observed fine structure and the resulting es2 2 Ai 309.8 (28) 309.5 (32) timate of a quadratic force field for comparison with those derived 3 1 A, 309.8 (28) 309.1 (32) 2 B1 308.0 (13) 307.9 (10) 2 from a Urey-Bradley potential f ~ n c t i o nwithout ~,~ the use of 1 3 E 306.8 ( 5 ) 306.7 (4) isotopic data. "Any differences for the two sites are not resolvable at the resolution used. bNumbers in parentheses are relative intensities based on 45 for (1) Langseth, A. Z.Phys. 1931, 72, 350. c35c1,. (2) Schaefer, C.; Kern, R. Z. Phys. 1932, 78, 609. (3) Plyler, E. K.; Benedict, W. S. J. Res. Natl. Bur. Stand. (US.)1951, Experimental Section 47., _202. _For infrared studies a matrix of Kr/CC14 = 10000 was prepared (4) Madigan, J. R.; Cleveland, F. F. J . Chem. Phys. 1951, 19, 119. as described previously.8 For Raman studies the concentration (5) Long, D. A.; Spencer, T. V.; Waters, D. N.; Woodward, L. A. Proc. R . Soc. London,Ser. A . 1957, 240,499. was increased to Kr/CC14 = 200 and the matrix was deposited (6) Shimanouchi, T. In "Physical Chemistry, an Advanced Treatise"; on a copper block at 10 K. The scattering was observed from the Henderson, D., Ed.; Academic Press: New York, 1970; Vol. IV, p 283. 5145-A line of an argon ion laser by using a Spex double (7) King, S.T. J . Chem. Phys. 1968, 49, 1321. monochromator at about 0.5-cm-' resolution. The observed (8) Jones, L. H.; Swanson, B. I. J . Chem. Phys. 1984, 80, 3050.
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0022-3654/84/2088-5560$01.50/0
0 1 9 8 4 American Chemical Society
