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ESRSTUDIESOF ZnOz AND ZnO
Electron Spin Resonance Studies on Zinc Peroxide and on Zinc Oxide Obtained from a Decomposition of Zinc Peroxide
by R. D. Iyengar*l and V. V. Subba Rao Center for Surface and Coatings Research, Lehigh University, Bethlehem, Pennsylvania
18015
(Received February
4, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
Zinc peroxide prepared by the interaction of H2Oz on zinc oxide has been found t o be esr active. Apparently the activity arises from the H2Oz used which can give rise to species such as 02- or HO2. Evidence is presented to indicate a reaction between diamagnetic peroxide ions (022-) and water vapor, leading to the formation of superoxide ions (02-). Zinc oxide formed on decomposition of the peroxide shows greater thermal stability compared to other preparations and does not readily lead to the formation of 02- species at the surface.
Zinc peroxide @noz)is looked upon in many ways as a transition between ionic and covalent peroxides2 and a consideration of its electronic structure (Figure 1) leads to the belief it is a diamagnetic oxide. However, during the course of an investigation, it was found that Zn02 prepared by the interaction of ZnO and concentrated H20zwas paramagnetic and esr active. Experiments were, therefore, carried out on the ZnOz to trace the origin of the esr activity and identify the species responsible. Also, properties of ZnO formed from the decomposition of ZnO2 were investigated. The present notes describes some unexpected but interesting observations made during the course of such a study.
Experimental Section Materials. Two Zn02 samples were used in the investigation. One of them, furnished by the New Jersey Zinc Co., was prepared by the reaction of a ZnSOl solution with an ammoniacal solution of HzOz followed by drying at 90" and a t 140". The second sample was prepared by the interaction of ZnO (in the form of a paste) with 30% HzOZover a period of 2 weeks. The material obtained was filtered and dried a t 140" for 2 hr. While a majority of experiments were done with the second sample, both samples showed very similar, if not identical, behavior. Apparatus. The vacuum outgassing of samples was carried out in a conventional vacuum system provided with a mercury diffusion pump and a liquid nitrogen trap. Esr studies were made with a Varian spectrometer (V-4500) with TE-104 mode dual cavity operating at about 9.5 kMHz. The exact frequency could be measured using a Hewlett-Packard wavemeter. 1 ,1Diphenylpicrylhydrazyl (dpph) was used as a standard for g measurements. Except where mentioned, all spectra were recorded at liquid nitrogen temperature.
Results The esr spectrum of an "as is" sample of zinc peroxide is shown in Figure 2a. For comparison the spectrum 02-species, obtained by adding oxygen at Torr a t 25" to a sample of pure ZnO (SP-500; N. J. Zinc Co.) vacuum outgassed for 2 hr at 500", is shown in Figure 2b. On outgassing Zn02 for 2 hr at 150", the intensity of the spectrum increased (Figure 3 4 . However, additional increase in the outgassing temperature (Figures 3b and c) lowered the intensity of the spectrum. Vacuum treatment a t 200" for 2 hr eliminated the signal altogether. Since zinc peroxide decomposes completely around 200°, its behavior, following outgassing at temperatures above 300", should be similar to that of ZnO. Contrary to expectations, Zn02 vacuum treated at 400" (Figure 3d) indicated the total absence of any esr signal around g = 1.96 (characteristic of Zn and/or oxygen vacancies). At still higher temperatures for the vacuum treatment (500", 2 hr), the esr spectrum revealed the formation of a new, intense, symmetric signal (AH(peak to peak) = 2.5 G) at g = 2.002 (cf. g,(free electron) = 2.0023) besides a much smaller signal at g = 1.96 (Figure 3e). Exposure a t room temperature of this sample to oxygen ( 5 to 100 Torr) produced approximately a 10% decrease in the intensity of the g = 2.002 signal and no changes in the g = 1.96 signal. Further increase in oxygen pressure produced no further changes. Outgassing a t 25" of the sample exposed to oxygen produccd no new signals. On heating the peroxide sample at 500" for 3 hr in a flowing stream of oxygen, the esr spectrum showed only (1) To whom correspondence should be addressed at SherwinWilliams Chemicals, Research Center, 10909 Cottage Grove Avenue, Chicago, Ill. 60628. (2) N. G. Vannerberg in "Progress in Inorganic Chemistry," Vol. 4, F. A. Cotton, Ed., Interscience, New York, N. Y., p 125.
The Journal of Physical Chemistry, Vol, 75, N o . $0,1971
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R. D. IYENGAR AND V. V. SUBBA RAO
(40'2-
(4 0;
Figure 1. Electronic structure of (a) peroxide ion and (b) superoxide ion.
I
I
2.049 2.009
Figure 3. Esr ($-band) spectra of zinc peroxide following vacuum treatment: (a) a t 150" for 2 hr, signal level 10, 70 mW; (b) a t 200" for 5 min, signal level 50, 70 mW; (c) a t 200" for 30 min, signal level 50, 70 mW; (d) a t 400' for 2 hr, signal level 100, 70 mW; and (e) a t 500' for 2 hr, signal level 100, 70 mW. All spectra recorded a t - 196'.
2.003
Figure 2. Esr (2-band) spectra of (a) zinc peroxide "as is" (not outgassed), and (b) 0 2 on zinc oxide obtained following exposure of vacuum-treated ZnO (prepared by vapor phase oxidation of Zn) to oxygen a t Torr. Both spectra a t -196'; signal level 25; 70 mW.
a minor signal at g E 1.96 (Figure 4a). Removal of excess oxygen (by outgassing for 5 min at 25") increased slightly the intensity of this signal. Additional vacuum outgassing at 500" for 2 hr enhanced the intensity of the (g E 1.96) signal by more than 100% (Figure 4b). Torr) t o this sample However, addition of oxygen left the spectrum essentially unchanged (Figure 4c) and did not lead to the formation of the triplet characteristic of 0 2 - on ZnO (cf. Figure 2b). On exposing ZnOz,outgassed under vacuum for 2 hr at 150", to water vapor ( 5 Torr), the esr spectrum showed an increase in the intensity of the triplet by about 50% (Figure 5b). No new signals were observed. Room temperature outgassing to remove the water vapor restored more or less the original intensity (Figure 5c). Addition of a second dose of water vapor again produced an increase in intensity but only by about 25%. Soaking an '(as is" sample of ZnOz in water for over 12 hr followed by drying at 100" for 48 hr also showed an increase in the intensity but not t o the extent earlier observed (Figure 5d). However, exposure of a sample of Zn02, previously vacuum heated at 200" for more than 2 hr, to water vapor at 25", did not produce the triplet. The Journal of Physical Chemistry, Vol. 76, N o . 20,1971
1
I
Figure 4. Esr (2-band) spectra of zinc peroxide. (a) heated in a stream of oxygen a t 500' ( 3 hr), cooled in oxygen to 25', sampIe sealed with oxygen in tube, signal level 200, 70 mW; (b) sample from (a) vacuum treated at 500' for 2 hr, Torr) and signal level 200, 70 mW; (c) oxygen added re-outgassed a t 25' for 5 min, signal level 200, 70 mW. All spectra recorded a t - 196".
Discussion If the superoxide ions
(02-), formed during the preparation, occupy regular lattice positions in the pyrite type of lattice reported for Zn02,2p3the ear spectra obtained should be similar to those published for alkali metal superoxide^^,^ where 0 2 - ions occupy regular
(3) R. D. Iyengar, V . V. Subba Rao, and A. C. Zettlemoyer, Surface Sci., 13, 251 (1969). (4) J. E. Bennett, D. J. E. Ingram, and D. Sohonland, Proc. Phys. SOC.London, Ser. A , 69, 556 (1956). (5) J. E. Bennett, B. Mile, and A. Thomas, Trans. Faraday SOC., 64, 3200 (1968).
309 1
ESRSTUDIES OF ZnOz AND ZIlO
between the peroxide ions, present close to the surface, and water, such as 3OZ2-
Figure 5. Esr (2-band) spectra of zinc peroxide: (a) vacuum treated at 150" (2 hr); (b) sample from (a) following addition of water vapor (5 Torr); (c) sample b outgassed a t 25" for 30 min; (d) soaked in water for 12 hr and dried at 100" for 48 hr. All spectra recorded at - 196" at the same signal level and microwave power.
lattice positions6-D and are located in an environment of axial symmetry. Thus, for NaOz the reported4 g values are gx = gY = (gI) = 2.002 and gz(= 911) = 2.175. A comparison of these values with the g values obtained from spectra in the present study shows no similarities whatever, indicating no regular arrangement for 0 2 - ions. On the other hand, the triplet observed with Zn02 is quite similar to that observed for 0 2 - on ZnO except for the fact that the g component on the low-field side (gl) is broader than observed for the latter. This broadening, associated with the absorption on the lowfield side, is probably caused by the existence of an array of sites where the crystal field environments for the paramagnetic species are different and can cause a spread of g values and consequently broaden the peak. The effect of such broadening should be greatest for the peak observed at the lowest magnetic field where the deviation from the free spin value (9, = 2.0023) is the largest. This is also in accordance with our observations. It seems that the observed triplet in the esr spectrum of ZnOz could very well arise from an excess of hydrogen peroxide used in the preparation which can yield species such as 0 2 - or HOz. The adsorption of such species on small particles of ZnOz resulting during the preparation is quite possible; however, the observation that oxygen added to the sample (maintained at liquid nitrogen or room temperature) causes no broadening or a reduction in the intensity of the esr signals argues in favor of the paramagnetic species (arising from HzOz)being trapped within the bulk or present in the form of an addition compound. Such addition compounds have been known to exist.lOsll The increase in intensity observed on soaking in water or exposure to water vapor is suggestive of a reaction,
+ 2H20
202-
+ 40H-
(1)
wherein superoxide ions are formed. Removal of water from the hydroxyl ions by vacuum treatment a t 150" can lead to reformation of Oz2- and can account for the observed decrease in intensity. Heating the sample of ZnOz under vacuum a t 200" or above can be expected to bring about the decomposition of not only the HzOz associated with the sample but of the ZnOz as well. The triplet in the esr spectrum is thus eliminated. Exposure of the sample following the heat treatment at or above 200" to water vapor does not lead to the formation of the triplet since there is no ZnOz and as such 02-formation cannot occur as suggested in eq 1. Previous s t ~ d i e s ~ have ~ ' ~ shown - ~ ~ that ZnO, following vacuum treatment a t temperatures above 350°, yields an intense and broad signal (g Si 1.96) which has been variously attributed in the past to conduction electrons,12 Zn+ ions in shallow donor bands,lB and oxygen ion vacancies with trapped electrons. l7 Iyengar and coworkers15observed that the broad signal at g = 1.96 was resolved into two components under certain conditions such as in the presence of an adsorbed layer of tert-butyl hydroperoxide. The two components with g values of 1.961 and 1.965 were attributed by these authors to the formation of interstitial Znf ions and oxygen vacancies with trapped electrons, respectively. These observations have been generally confirmed by the recent work of Setalra, et uZ.l8 However, according to Sancier,19both the components of the g = 1.96 signalin the esr spectrum of ZnO probably arise from conduction electrons, and the two slightly different g values are indicative of the presence in ZnO of two different environments for these electrons. Admittedly, while there are differences in the expressed views pertaining t o the identity of the species responsible for the two components, it is clear that the initial formation of (6) V. Kassatochkin and V. Kotov, J . Chem. Phys., 4, 458 (1936). (7) V. Kassatochkin and V. Kotov, Zh. Tekn. Fiz., 7, 1468 (1937). (8) V. Kassatochkin, Dokl. Akad. Nauk SSSR, 47, 199 (1945). (9) I. A. Kazaranovskii, Zh. Fiz. Khim., 14, 320 (1940). (10) S. 2. Makarov and L. V. Ladeinova, Zh. N e o r g . Khim., 1, 2708 (1956). (11) S. 2. Makarov and L. V. Ladeinova, Izv. Akad. Nauk SSSR Otd. Khim. N a u k , 139 (1957). (12) R. J. Kokes, J . Phys. Chem., 66, 99 (1962). (13) J. Schneider and R. Rauber, 2. Naturforsch. A, 16, 712 (1961). (14) K. M. Sancier, J . Catal., 5 , 314 (1966). (15) M. Codell, J. Weisberg, H. Gisser, and R. D. Iyengar, J . Phys. Chem., 72, 2460 (1968). (16) K. A. Muller and J. Schneider, Phys. Lett., 4, 2288 (1965). (17) P. H. Kasai, Phys. Rev., 130, 989 (1963). (18) M. Setaka, 8 . Fujieda, and T. Kwar, Bull. Chem. SOC.J a p . 43, 2377 (1970). (19) K. M. Sancier, Surface Sci,. 21, 1 (1970). The Journal of Physical Chemtktry, Vol. 76,No. 80,1971
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M. G. DEBACKER AND JAMES L. DYE
the g E 1.96 signal of the two components therein begins with the loss of oxygen from the sample either by a thermal or photochemical means. Since ZnOz decomposes to ZnO completely around 200°, its behavior following outgassing at temperatures above 200" should be similar to that of ZnO. However, the total absence of formation of the g G 1.96 signal in samples outgassed under vacuum at 400", and the difficulty in inducing such a signal even after vacuum treatment a t higher temperatures, suggests that the oxygen loss from the sample does not readily occur. It is possible that incorporation of certain cationic impurities into the lattice structure of ZnO can bring about a change in its surface properties and render the removal of oxygen from it more difficult. HoKever, the method of preparation of ZnOz revealed no specific impurity likely to be incorporated into the lattice. The reasons, if any, for the need for more drastic outgassing conditions to regenerate the donor defects (possibly) oxidized and destroyed by the HzOz used in the preparation, are therefore not clear.
The symmetric signal formed at g = 2.002 when ZnO2 is vacuum treated at 500" for 2 hr is irreversibly reduced in intensity (10%) by the added oxygen. The signal is not observed in Zn02 samples subjected to oxygen treatment a t 500" for 3 hr. In the past, such signals on ZnO in the vicinity of g = 2.002 have been attributed to (a) 0- ions,20(b) bulk defectsja( G ) an electron trapped is an ensemble of oxygen atoms,21(d) a species such as (ZnOz)-v-l where v is an integer,22and (e) electrons in mechanically induced shallow traps.lg A positive identification of this signal from observations made in the present study has not been possible.
Acknowledgment. This research was supported by the Advanced Research Projects Agency, Department of Defense, monitored by the Naval Research Laboratory under Contract No. NONR 610(09). (20) AM.Sedaka and T. Kwan, Bull. Chem. SOC.Jap., 38, 1414 (1965). (21) Y . Fujita and J. Turkevich, Discuss. Faraday SOC.,4, 407 (1966). (22) H. Ueda, Can. J. Chem., 46, 891 (1968).
Metal-Ethylenediamine Solutions. Extinction Coefficients and Equilibria1 by Marc G. DeBacker and James L. Dye* Department of Chemistry, Michigan State University, East Lansing, Michigan 48828
(Received March 16, 1971)
Publication costs assisted by the U. S. Atomic Energy Commis&n
The V band, characteristic of the optical absorbance of solutions of sodium metal in ethylenediamine, was produced by the reaction of cesium metal solutions with solutions of sodium salts. By keeping the cesium concentration in excess and using flow techniques, problems caused by decomposition were minimized. The absorbance of the V band was a linear function of the initial concentration of the sodium salt and yielded a n oscillator strength of 1.9 jZ 0.2 and an extinction coefficient of 8.2 i. 0.3 X lo4 M-l cm-I based upon the stoichiometry Na-. Combination of these results with pulse-radiolysis studies yielded an oscillator strength of 0.88 jZ 0.12 and an extinction coefficient of 2.0 0.3 x lo4 M-1 cm-1 for the solvated electron in ethylenediamine. Other equilibria involving M+ and e- were examined.
Introduction The spectra of mctal solutions in amines and ethers have been studied extensively2-l1but these studies have failed to provide quantitative values of the molar extinction coefficients. The major obstacle has been the instability of solutions, especially in spectral cells of large surface-to-volume ratio. Catalysis of the decomposition by the container walls seems to be a major limiting factor.1° Another source of difficulty has been the confusion over band assignments which was ultimately traced to the ease of contamination by sodium from the Pyrex container.6 The Journal of Physical Chemistry, Vol. 76, No. 20,1971
There has been general agreement that the broad absorption band in the infrared is characteristic of the (1) This work was supported by the U. S. Atomic Energy Commission. (2) R. R. Dewald and J. L. Dye, J . Phys. Chem., 68, 121 (1964). (See this paper for earlier references.) (3) M. Ottolenghi, K. Bar-Eli, H. Linschitz, and T. R. Tuttle, Jr.,
J . Chem. Phys., 40, 3729 (1964). (4) M. Ottolenghi, K. Bar-Eli, and H. Linschitz, ibid., 43, 206 (1965). ( 5 ) L. R. Dalten, J . D. Rynbrandt, E. M. Hansen, and J. L. Dye, ibid., 44, 3969 (1966). (6) I. Hurley, T. R. Tuttle, Jr., and S. Golden, ibid., 48, 2918 (1968).
