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(4) S. Yamashita, H. Ono, and 0. Toyama, Bull. Chem. SOC. Jpn., 35, 1849 (1962). (5) S. Malkin and E. Fischer, J. Phys. Chem.,66, 2482 (1962); D. Gegiou, 90, 3907 (1968); K. Muszkat, and E. Fischer, J . Am. Chem. SOC., D.Gegiou, K. A. Muszkat, and E. Fischer, ibid., 90, 12 (1968). (6) J. Ronayette, R. Arnaud, P. Lebourgeois, and J. Lemaire, Can. J . Chem., 52, 1848 (1974); J. Ronayette, R. Arnaud, and J. Lemaire, ibid., 52, 1858 (1974). (7) L. 8. Jones and G. S. Hammond, J . Am. Chem. Soc., 87, 4219 (1965). (8) E. Fischer, J. Am. Chem. SOC.,90, 796 (1968). (9) P. S. Engei and C. Steel, Acc. Chem. Res., 6, 275 (1973). (IO) H. Rau, Angew. Chem., Int. Ed. Engl., 12, 224 (1973). (11) W. S. Struve, Chem. Phys. Lett., 46, 15 (1977). (12) D. I. Schuster, M. D. Goldstein, and P. Bane, J . Am. Chem. SOC., 99, 187 (1977). (13) G. S. Hartley, Nature(London), 140, 281 (1937); J . Chem. Soc., 633 (1938).
J. R. Harbour and M. L. Hair (14) C. G. Hatchard and C. A. Parker, Proc. R. SOC.London, Ser. A , 235, 18 (1956). (15) A. A. Lamola and G. S. Hammond, J . Chem. Phys., 43, 2129 (1965). (16) For the importance of light monochromaticity in the calculation of 4 values using relationship I see, e.g., G. M. Wyman, Mol. Photochem., 6, 81 (1974). (17) A. R. Horrocks, A. Kearvell, K. Tickle, and F. Wilkinson, Trans. Faraby Soc., 62, 3393 (1966). (18) P. Bortolus, G. Bartocci, and U. Mazzucato, J . Phys. Chem., 79, 21 (1975); G. Bartocci, U. Mazzucato, and P. Bortolus, J. photochem., 6, 309 (1976-1977). (19) R. Arnaud and J. Lemaire, Can. J . Chem., 52, 1868 (1974). (20) E. Amouyai and S. Monti, to be published. (21) A. Kellmann, J . Phys. Chem., 81, 1195 (1977). (22) L. D. Fogel and C. Steel, J . Am. Chem. Soc., 98, 4859 (1976). (23) N. C. Baird and J. R. Swenson, Can. J . Chem., 51, 3097 (1973). (24) M. S. Gordon and H. Fischer, J . Am. Chem. Soc., 90, 91 (1968).
Radical Intermediates in the Photosynthetic Generation of H202with Aqueous ZnO Dispersions John R. Harbour" and Michael L. Hair Xerox Research Centre of Canada Limited, Mississauga, Ontario L5L 1J9, Canada (Received September 18, 1978) Publication costs assisted by Xerox Research Centre of Canada
The technique of spin trapping has been applied to a study of the photosynthesis of hydrogen peroxide in aqueous zinc oxide dispersions. In additive-freesystems, the hydroxyl radical was detected whereas in systems containing either formate or oxalate, the .CQ; radical was observed. The measurement of oxygen uptake was also accomplished on these same systems. Comparison of the number of radicals with the amount of H2Q2formed and of the quantum efficiencies determined by both electron spin resonance and oxygen uptake strongly suggest that these radicals are major participants in the mechanism of hydrogen peroxide photosynthesis. A mechanism is in fact suggested which is consistent with the observation of these radical species. It appears that additives such as formate or oxalate function by acting at least partially as hydroxyl radical scavengers. The conversion ratio (defined as the number of hydrogen peroxide molecules formed per one molecule of oxygen consumed) is introduced and is observed to be a function of initial oxygen concentration in the additive free systems. This reaction also proceeds in the presence of a surfactant.
the reaction nor whether the radicals produced are of Introduction sufficient number to be major participants in the overall I t is well known that when ZnO powder is dispersed in H202synthesis. I t is the goal of this paper to determine an oxygenated aqueous medium it will generate hydrogen whether radicals are generated in zinc oxide dispersion peroxide (H202)upon illumination with light of wavelength upon illumination, to identify the specific radicals, and to less than 380 nm.ld5 Addition of certain compounds such determine their concentrations. The technique of spin as formate, oxalate, and phenol greatly increase the trapping7 was employed for radical detection and repreamount of H202produced. The quantum efficiency of this sents an extension of our earlier work on radical detection reaction with additives usually approaches its proposed in pigment dispersions.s-10 maximum of 0.5. This high quantum efficiency coupled Systems containing only ZnO, water, and O2 have been with the absence of side reactions has made this system a model for the study of heterogeneous p h o t o c a t a l y ~ i s . ~ ~ ~studied as well as systems to which compounds such as formate or oxalate have been added. In both of these cases (In fact, the use of the term photocatalysis is apparently it is known that the oxygen which is incorporated into the inaccurate, as discussed later.) H202comes from dissolved molecular O2and that light of Various mechanisms have been proposed for these reenergy 23.2 eV (band gap energy) is r e q ~ i r e d .A~ limiting actions. In fact recent studies by Dixon and Healy6 have concentration of H202of M has been observed in the cast doubt on whether this system is photocatalytic. They additive-free case and this increases to M when argue that photocorrosion is occurring with interstitial zinc formate or oxalate is added.* Carbon dioxide is liberated ion (Zn') being oxidized to Zn2+followed by hydrolysis. when formate or oxalate is present. Nevertheless all the mechanisms which have been proposed to date invoke radical intermediates. However, the Experimental Section only evidence for radical involvement is indirect, through Zinc oxide powder was obtained from Fisher Scientific, observation of a ZnO photoinduced polymerization of both BDH Chemicals (Analar), and Ventron (ultrapure). All methyl methacrylate and acrylonitrile.2 This observation the powders were effective in the photosynthesis of H2Oz does not define which particular radicals are involved in 0022-3654/79/2083-0652$0 1.OO/O
1979 American Chemical Society
Photosynthetic Generation of H202with Aqueous ZnO
The Journal of Physical Chemistry, Vol. 83, No.
I
12
1
1
-
Time(sec
0 -
I
0
2
4
6
8
I
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I
Figure 2. (A, top) The .OH adduct ESR spectrum. (B, bottom) The .OH adduct signal intensity as a function of illumination. The magnetic field is fixed at the position indicated by the arrow in Figure 2a..
Time (min )
Figure 1. O2concentration as a function of illumination and addition of catalase (as indicated by the arrow at 4.8 min).
and in the generation of .OH radicals. Quantitative data on quantum efficiency and number of radicals were obtained using the Analar ZnO. Distilled water was redistilled from an all-glass apparatus. The 5,5-dimethyl-1pyrrolinyl-1-oxy (DMPO) was synthesized' and purified prior to use by bulb-to-bulb distillation on a vacuum system. All other chemicals were used as received. ESR spectra were obtained on a Varian E12 electron spin resonance spectrometer. The samples were illuminated in situ using a Hanovia Model 997B-1 KW Hg-Xe lamp i n a Schoeffel Model LH 151N lamp housing in conjunction with either a Corning CS 0-52 or 0-51 filter. 4-Hydroxy-2,2,6,6-tetramethylpiperidinooxy (Eastman) was used as a concentration standard. The quantum efficiency 9 was determined using the ESR technique by measuring the ratio of the rate of trapped .COP-radical production to the number of photons per second incident on the cell. The radical concentration was determined by double integration using a Nicolet 1180 computer directly interfaced to the spectrometer and the light intensity was measured using a Alphametrics Model dc 1010 radiometer and a Model PllOOS silicon light probe with both CS 0-51 W/cm2). and 7-37 filters (1100 X O2 uptake was measured with a US1 Model 53 oxygen monitor coupled to a constant temperature bath. These dispersions were illuminated with a quartzline 300-W projection lamp using a CS 0-52 filter. The quantum efficiency measurement for H20zgeneration in this system employed both the CS 0-51 and 7-37 filters (670 X W/cm;'). The total amount of H 2 0 2formed was determined by adding catalase to the system after illumination. This enzyme catalyzes the decomposition of H202to H 2 0 and O2 which is directly measured with the monitor: 2H209
catalase
2Hz0
+02
Since one molecule of O2is evolved from the decomposition molecules, the amount of HzOzpresent is twice of two H202 the amount of O2 evolved upon addition of catalase. Figure 1illustrates this technique for a ZnO dispersion containing formate. Results Additive Free Systems. Irradiation of a ZnO dispersion containing the spin trap DMPO results in the ESR spectrum shown in Figure 2A. This spectrum has been previously identified as that of the hydroxyl radical ad-
ZnO,
HzO
Light o f f
1 I
I2
I
I
I
I
84
60
36
Tirne(sec )
Figure 3. Signal intensity of the .OH adduct as a function of illumination. Field position is as indicated in Figure 2a.
duct.ll This implies that .OH radicals are being generated and subsequently trapped by the DMPO (eq 1). By fixing
I
0
0
the field a t the position (a) shown in Figure 2, the time dependence of the radical concentration was determined. This is also reproduced in Figure 2. It is evident that, the intensity peaks with time. This probably is due to a photoinduced destruction of the radical adduct catalyzed by the ZnO since the signal intensity levels off when illumination is blocked. The kinetic curves shown in Figure 3 reveal the dependence of signal intensity with dissolved oxygen concentration. In a nitrogen purged system, the intensity peaks a t 20 s and decays away quickly. However, in both the air- and oxygen-saturated cases the curves peak a t longer times (30 s in air and 50 s in 0,) and exhibit significant radical concentrations even after 1min. With 02, a high radical concentration is maintained up to 100 s. This suggests that .OH radicals are being continuously generated in the presence of oxygen. The use of signal intensity to determine radical concentrations is not practical in these cases since O2 is known to broaden the spectral lines (Le., the apparent radical concentration in the N2 purged case deduced from signal intensity allone would overestimate the number of radicals). Nevertheless,
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J. R. Harbour and M. L.
The Journal of Physical Chemistry, Vo/. 83, No. 6, 1979
Hair
assigned to the C02-adduct. The formation of the C02-radical can be accounted for by either of two mechanisms: a direct reaction with -OH (eq 3 and 5) or by direct oxidation by the hole (eq 4 and 6) (see Discussion). HCOO- + .OH HOH .Cop(3) + HCOO- C02- + H+ (4)
+
+
+
-0
\
0
0-
/ / C-C
+ H+ + .OH ---+
0
a significant amount of hydroxyl adduct is formed in the N2 purged case. The O2 concentration in solution in this system is estimated to be -2 X 10-6 M even after N2 purging and this probably accounts for the initial burst of hydroxyl radical production. Oxygen uptake measurements show that hydrogen peroxide is formed in these closed systems and that a M is reached. This limiting concentration of -1 X is significantly higher than the 1 X 10-5M limiting H202 concentration reported by previous workers in open ~ y s t e m s .In~ addition, it is interesting to follow the ratio of H z 0 2formed to O2 consumed, defined here (eq 2) as the
"202 f o r m e d / 0 2
consumed
= CR
/
I c-c\
DMPO.
(2)
conversion ratio (CR). In the system shown in Figure 1 (containing formate) the value determined for CR approaches unity. However, in the systems containing no additives, the CR ratio is found to be less than one (typically between 0.2 and 0.5) and to decrease with the amount of O2 initially present. For an oxygen-saturated dispersion, CR was equal to 0.20 whereas for the airsaturated case CR increased to 0.43. Lowering the concentration by a factor of 4 further increased the CR to 0.58. These experiments used ZnO dispersions which were illuminated until approximately 65% oxygen uptake occurred, but similar results were obtained for a constant period of illumination (2 min). The hydroxyl adduct concentration was measured by doubly integrating the spectrum and comparing the value to a standard (see Experimental Section). The .OH adduct concentration was found to be about 6 X M. This concentration is only half that of the H 2 0 2 actually generated, but it represents only a minimum since (i) there is not 100% trapping and the trapping efficiency is unknown, (ii) the radical is known to be unstable during illumination (Figure 2), and (iii) there is no mixing occurring in the ESR cell and therefore equilibrium is not obtained. However, this concentration of hydroxyl radicals is sufficiently high to suggest that it does play a major role in the overall process. Additive Systems. Previous workers have demonstrated that additives such as formate and oxalate will increase the overall yield of H202synthesis in these systems4 In addition the formation of carbonate (i.e., liberation of COZ) in the same yield as H 2 0 2demonstrates that these particular additives are chemically active.lJ2 When formate or oxalate (at -0.1 M) was added to the ZnO dispersion (containing DMPO), a different lightinduced radical signal was generated (Figure 4) with no evidence of formation of the .OH adduct which was observed in the additive-free system. This new signal which has a N = 15.8 G , abH = 19.1 G, and g = 2.0058 is readily
(5)
0-
\
The ESR spectrum of the .COP- adduct of
+ ,COi
'0 -0
Figure 4.
(H,O) t CO,
/
t Q -+
co, t .co;
(6)
0
These reactions are found to be oxygen dependent and the kinetic curves are similar to those obtained for the additive-free case (Figure 3). Again, the O2 purged system continues generating .CO, for longer times than either the N2 or air-saturated cases. The concentration of the C02-adduct as determined from the ESR data is -2 X M. As discussed earlier this is a minimum value but is again sufficiently close to the H 2 0 2concentrations (-8 X M) to consider this radical as a major participant in the reaction mechanism. The conversion ratios obtained with either formate or oxalate present are close to unity. The CR shows no dependence on oxygen concentration and this is to be contrasted with the additive-free case where CR decreased with increasing oxygen content. It should be noted that the rates of O2 uptake and H 2 0 2formation in both the additive free and the additive case were not significantly reduced by the presence of the spin trap DMPO. Quantum Efficiency. If the conversion ratio approaches unity then the quantum efficiency (a) of H202production can be determined from the initial rate of oxygen uptake and should yield a maximum value. When formate is present this is very nearly correct. Using this technique a value of = 0.14 was determined for the air-saturated case and this can be compared to a value of = 0.5 which is generally considered to be the maximum value obtainable in H z 0 2 photosynthesis. Reported literature values range from 0.2 to 0.5l for oxygen-saturated systems. The quantum efficiency was also determined from the ESR kinetics. In this case a value of = 0.04 was obtained, but again, for reasons discussed earlier (photodecomposition, unknown trapping efficiency, and insufficient mixing) this value is less than maximum. In addition, the initial quantum efficiency determination by ESR follows the rise in intensity of an absorption line. This line is broadened by the oxygen initially present and therefore the intensity measured underestimates the real rate and a, since the intensity standard was measured in a system considerably depleted of oxygen. Relatiue Rates. The rate of O2uptake (and hence HzOz synthesis) is increased by a factor of 3 when additives such as formate are present. A similar relative rate was determined from the ESR studies by comparing the initial rates of formation of the COz- and -OH radical signals. In the ESR method, the value of the relative rate is dependent on the relative trapping rates of DMPO for .CO2and .OH (which are unknown) but the agreement with the O2 uptake measurements is consistent with the fact that the .OH and C02-radicals are intermediates in the H202 photosynthesis. Superoxide. In previous work on the photoexcitation of other particulate photoconductors we have shown that
Photosynthetic Generation of H,02 with Aqueous ZnO
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
TABLE I I _ _ -
Reduction 0 , -1 e- -+ (O;), (O;), t H+ + (HO,.), (HO,.), t e -+ (HO,-), (HO;), t H' + (H,O,), + H,O, or 2(HO2.)s -* (HzO,)s + 0 , Oxidation OH- t @ - + . O H OH + surfacelimpurities -+ oxidized product Zn-OH + 8 -+ Zn2++ .OH For Additives HCO; + .OH -+ H,O t .CO; or HCO; t @-+ CO; + H'
Limiting (H,O,), t .OH -+ (HO, )s t H,O
(9) (10) (11) (12)
(13) (14) (15) (16) (3) (4) (17)
the superoxide anion, 02-,is readily detected by the spin trapping technique. In the present series of experiments this radical was never observed even though the DMPO O I ]; adduct was observed when a ZnO dispersion containing excessive H20zwas illuminated. Its formation can be accounted for by eq 7 and 8. It must be concluded Hi02 *OH H20 + HO2* (7)
+
HOy
+ DMPO
4
-+
DMPO { O J
+ Ht
(8)
therefore that this species would be trapped )under the conditions of our experiment, and its nonobservance indicates that its presence in solution is unlikely. Effect of Surfactant. When suspending oxide particles in a fluid, it is common practice to adsorb a surfactant on the surface to increase the stability of the suspension. The anionic surfactant Aerosol OT (AOT) readily adsorbs on the surface of ZnO to give a good dispersion in aqueous media O2 uptake measurements on this system show that H 2 0 2 is still photogenerated but a t a slower rate. In addition the CR was determined to be only 0.2. The ESR resultcj show a factor of 40 reduction in magnitude of the .OH adduct signal. These results are not unexpected since the hydroxyl. radicals produced a t the interface see a high concentration of surfactant and will readily undergo a hydrogen abstraction reaction with the surfactant. p H and Temperature. Although a thorough quantitative study of this reaction as a function of pH and temperature has not been made, it was determined that the reaction proceeds readily over the temperature range -5-40 "C. The variation of pH within a range of pH 2-8 did not significantly alter the results.
Discussion These results can be incorporated into a general mechanism for the production of H 2 0 2from illumination of ZnO dispersions (see Table I for a summary). For the additive-free case, the absorption of a photon (A 5380 nm) leads to the formation of an electron (e-)-hole (e) pair.16 It is well established3 that the oxygen in the HzOzcomes only from dissolved O2 and it is proposed that the electron reduces adsorbed molecular oxygen (indicated by the subscript s) to superoxide anion (eq 9). (HOJ, is then reduced by a second electron to generate H202(eq 11and 12). A surface dismutation could also conceivably occur (eq 13) and both of these reactions give a CR = 1, amax = 0.5 and require two H+ to be used for each Hz02produced. For the oxidation half of the synthesis, an oxidizable substance must be present. Rubin et a1.l and Vail et a1.12 have postulated that impurities may be involved in the
655
additive-free system while Dixon and Healy6 suggest that interstitial Zn' may be the effective reductant. In order to account for the hydroxyl radical formation detected in our experiments, we suggest two possible reactions. The first involves oxidation of OH- (i.e., HZ0)las in eq 14 which maintains a constant pH since two OH- are oxidized !while two H+ are taken up per one HzOz formed. The -OH radicals initially formed must then react with either the surface and/or the product or impurities present in the system (eq 15). It is also possible that a "surface hydroxyl group" or lattice oxygen ( 0 9 is oxidized by the hole thus accounting for .OH production (eq 16). The anodic photodissolution of ZnO has been studied by Gerischer17who has shown that both the oxidatilon of H 2 0 (E"' = 0.8 V) and the oxidation of ZnO ( E O ' = 0.7 a t pH 7) are thermodynamically possible since the redox level of the e approaches E"' = +2.6. Hence either or both of the above reactions could be operative. These results indicate that ZnO is (1)converting solar energy to chemical energy by reducing O2to H 2 0 2through the oxidation of water, or (2) undergoing a photochemical oxidation during the photosynthesis of H202. Previous workers have reported a limiting concentration of 1 X M Hz02in the additive-free case using an open system. To account for this, a reaction between the hydroxyl radical and adsorbed H20zis proposed (eq 17). The effect of this is to dynamically limit H z 0 2formation. It is interesting to note that the H 2 0 zconcentrations determined in our closed system are substantially higher ( 1X M) than those reported for open systems with oxygen purging and relatively long illumination (hours). Our data indicate that the CR decreases under these conditions and this may account for the low limiting vdues. It is surprising that the CR should vary and that v,alues less than one can be observed. This could, however, be easily accounted for either by direct photoreduction of adsorbed H 2 0 2or by a catalytic decomposition of (adsorbed) HzOP Both equations would give a CR value less than one. When formate or oxalate was added to the system (Le., the additive system) only the C02-radical was detected. This radical is known to result from reaction of the hydroxyl radical with either formate or oxalate,lsJg hence the role of this additive may simply be that of an hydroxyl radical scavenger (eq 3 and 5). Alternatively, the photogenerated hole may give direct oxidation of the formate. The scavenging of hydroxyl radicals would prevent eq 17 from occurring and therefore increase the rate of O2uptake and Hz02synthesis in agreement with the experimental observation. This scavenging would also increase the limiting concentration of H z 0 2and this is in fact observed: The limiting concentration of H202is increased from 1 X to 1X M after addition of formate a t which point a direct reduction of HzOzmay become limiting. The fact that additives can act as hydroxyl radical scavengers has an impact on attempts to correlate oxidizable substrates on the basis of redox level. The present results suggest that such a correlation should also include a measure of the ability of the additive to react with the hydroxyl radicals (i.e., undergo hydrogen abstraction). C 0 2 evolution is observed with both formate and oxalate additives. We suggest that this results from a further oxidation of COz- by a hole. The reaction of C02-with O2 seems to be unimportant as a CO2/H2O2ratio of 1 is experimentally observed and no Oz- was observed in solution.
-
Acknowledgment. We thank Professor James R. Bolton and the University of Western Ontario for the use of the
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The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
ESR facilities. In addition, we also thank Dr. Alan McIntosh for assistance in the computer determination of radical concentrations and John Tromp for making the oxygen uptake measurements.
Zehe et al. (8) (9) (10) (11) (12)
References and Notes (1) T. R. Rubin, J. G. Calvert, G. T. Rankin, and W. MacNevin J . Am. Chem. Soc., 75, 2850 (1953). (2) M. C. Markham and K. J. Laidler, J . Phys. Chem., 57, 363 (1953). (3) J. G. Calvert, K. Theurer, G. T. Rankin, and W. MacNevin, J . Am. Cbem. Soc., 76, 2575 (1954). (4) T. Freund and W. P. Gomes, Catal. Rev., 3, 1 (1969). (5) M. D. Archer, J . Appl. Nectrochem., 5, 17 (1975). (6) D. R. Dixon and T. W. Healy, Aust. J . Chem., 24, 1193 (1971). (7) E. G. Janzen, Acc. Cbem. Res., 4, 31 (1971).
(13) (14) (15) (16) (17) 1181 (19)
J. R. Harbour and M. L. Hair, J . Phys. Chem., 81, 1791 (1977). J. R. Harbour and M. L. Hair, J . Phys. Chem., 82, 1397 (1978). J. R. Harbour and M. L. Hair, Photochem. Photoblol., in press. J. R. Harbour, V. Chen, and J. R. Bolton, Can. J . Chem., 52, 3549 (1974). C. B. Vail, J. P. Holmquist, and L. White, J . Am. Chem. Soc., 76, 624 (1953). 0. P. Chawla and R. W. Fessenden, J. Phys. Cbem., 79, 2693 (1975). R. Livingston and H. Zeldes, J . Chem. Phys., 44, 1245 (1966). E. G. Janzen and J. I-Ping Liu, J . Mag. Reson., 9, 510 (1973). H. Gerischer, J. Nectroanal. Chem., Interfacial Nectrochem., 58, 263 (1975). H. Gerischer in "Solar Power and Fuels", J. R. Bolton, Ed., Academic Press, New York, 1977. R. 0. C. Norman and P. R. West. J . Chem. SOC.6 . 389 119691. 2. C. DraganiE, M. M. CosaniE, and M. T. NenadoviE, J. f b y s . Chem:, 71, 2390 (1967).
Matrix Reactions of Molecular Oxygen with Indium and Gallium Atoms' Michael J. Zehe," Denis A. Lynch, Jr.,2bBenuel J. Kelsall, and K. Douglas Carlson" Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 106 (Received October IO, 1978)
Frozen inert gas matrices of Nzand Ar containing O2 and In or Ga atoms have been prepared by matrix isolation techniques and studied by infrared absorption spectroscopy. Analyses of the spectra have shown that these reagents react to produce metal superoxide molecules, M'OY, which are similar to those of the alkali and alkaline earth metals. The stretching modes of these species absorb in the regions of 1080-1090 cm-l for vl(Al), 330-380 cm-' for u2(Al), and 270-290 cm-' for u3(B1). In addition to the superoxides,various aggregate species are produced. One of these has been identified as the superoxide dimer, which apparently has an 02M-M02 structure of D2d symmetry. Another aggregate has been identified as a rhombic MOzMspecies which is formed by the addition of a metal atom to the superoxide. Small quantities of the suboxide dimer (InzO)zalso were detected in these matrices. The evidence indicates that this species was formed by the addition of an In atom to each oxygen of the In021n(Dzh)species. This dimer is of interest because it is readily formed by aggregation of the suboxide monomer in experiments involving the vaporization of the condensed oxide systems.
Introduction The vapor phases of the sesquioxides of Al, Ga, In, and T1 have been extensively studied by mass spectrometry and matrix isolation ~ p e c t r o s c o p y . ~At - ~ typical ~ vaporization temperatures, these vapors consist primarily of the metal suboxide molecules, M20, along with generally smaller concentrations of the gaseous metal atoms and trace quantities of various complex oxide species. The infrared spectra of these vapors deposited in frozen inert gas matrices exhibit a strong absorption band, a moderately intense band, and several weak absorption bands. The strong absorption band been well characterized as the antisymmetric stretching vibration of the M 2 0 specie^.^^^ Some of the other bands have been given tentative and sometimes controversial assignment^.^!^ The evidence is now reasonably conclusive that the moderately intense band and probably one of the weak bands represent vibrational modes of a suboxide dimer, (MzO)2.9J0This dimer species apparently is produced by aggregation of the monomer in the matrix, rather than by direct isolation from the vapor phase. In the course of investigating the nature of these two bands, Lynch7 carried out experiments to explore whether the metal atoms or a possible O2 impurity in the matrix might be responsible for producing the carrier species. These experiments consisted of depositing metal atoms and molecular oxygen in the matrix either along with or in the absence of the vapors from the condensed metal oxides. Marino and Whites have published a brief report on similar experiments for the A1-0 system which were carried out with identical objectives. 0022-3654/79/2083-0656$0 1.OO/O
With the possible exception of the A1 system,s these added metal and O2 reagents had no apparent affect on the infrared bands normally obtained with the vaporized oxides. On the other hand, the spectra obtained with these reagents exhibited various new infrared bands, and these were established as belonging to species produced in the matrix by reaction of O2 with the metal atoms. Furthermore, the infrared spectra of these species were found to have remarkable similarities with those reported by Andrews and for the matrix reactions of O2 with the atoms of the alkali and alkaline earth metals. The purpose of this article is to describe more recent and more detailed experiments on the matrix reactions of O2 with the atoms of the In and Ga metals.
Isolation Experiments Molecular species were prepared by simultaneously depositing In or Ga atoms and O2 in freezing inert-gas matrices. The deposition substrate was a CsI disk maintained a t 16 K by a closed-cycle helium refrigerator (Cryogenic Technology, Inc.). The metal atoms were produced by vaporizing the condensed metal from graphite Knudsen effusion cells. The metals were extensively outgassed a t high temperatures prior to the deposition. The O2 was deposited along with the matrix gas from stored mixtures prepared with known concentrations. Both N2 and Ar were used as matrix gases. These were doped with 0.25-20 mol % Oz and deposited a t rates between 0.5 and 3.0 mmol/h. The metal atoms were simultaneously deposited over periods of 5-30 h a t rates of 0.2-2 pmol/h. These deposits had matrix gas-to-metal 8 1979 American Chemical Society
