2734
Oxygen Adsorption on Zinc Oxide'
by H. Saltsburg and D. P. Snowden General Atomic Division of General D y n a m i c s Corporation, J o h n J a y H o p k i n s Laboratory for Pure and Applied Science, S a n Diego, California (Received March 87, 1964)
A recent publication by Glemza and Kokes2 dealing with the observation of transient conductivity changes in ZnO following limited oxygen adsorption has been criticized by Peers3 with respect to the proposed mechanism, and an alternative has been presented. Specifically, Peers has suggested that surface effects are the reason that the transient, slow reversal of the direction of change of the d.c. conductivity is observed, following oxygen exhaustion in the system; he proposes that bulk-to-surface diffusion processes are basically responsible for the observed effect. Transient effects of this type have been under study in this laboratory in the 02-Ni0 system.4a In view of the suggestion that surface effects dominate the d.c. conductivity of ZnO, preliminary experiments which we have carried out on the 02-Zn0 system a t 200-300" are relevant. Our experimental technique employs the simultaneous measurement of frequency-dependent conductivity and gas adsorption, the latter being carried out in the presence of a helium carrier gas into which reactive gas plugs are injected so as to produce pressure pulses over the loosely powdered sample in the conductivity Bulk diffusion effects are minimized due to the limited gas-solid contact time (-2-5 sec). The frequency-dependent conductivity of ZnO samples obtained from various sources has been measured. The conductivity increases with frequency, probably due to increasing participation of the whole of the sample a t higher frequencies. However, no simple frequency-dependent structure in the conductivity is seen which could be interpreted on the basis of a simple model of the structural inhomogeneities of the powder such as that described by K o ~ p s . ~ Pulsed oxygen adsorption on ZnO a t 200-300" produces a transient conductivity change which is characterized by a very sharp decrease,followed by a rather slow increase in conductivity. I n essence, the solid sees a pressure pulse (at partial pressures of 7.6 or 76 nim. of oxygen) for a few seconds, and then the reactive gas is swept away and the reversal begins. This is the type of observation made by Glemsa and Kokes except that the time scale is shortened by an order of magnitude in the decay and considerably more than that in the exposure. The Journal of Physical Chemistry
NOTES
Qualitatively, the same decay seen a t d.c. is seen a t all frequencies from lo3 to 1O1O C.P.S. The magnitude of the relative conductivity change ( u - u i n l t ) / u l n i t varies slowly with frequency, decreasing less than a factor of 10 between lo3and 1O'O c.P.s., while over this same range the conductivity itself increases by a factor of >lo4 (Fig. 1). Consequently, it is clear that a similar electronic process occurs throughout the sample and that the changes seen a t d.c. are not due to surface effects. This result is in agreement with the e.s.r. measurements of Kokes.6 Diffusive effects of the type envisioned by Peers are not consistent with these results; and further, in the experiments involving pulsed adsorption, the time scale permitted is an order of magnitude lower than in Kokes' work and the effects are also observed a t lower temperatures, making the diffusion arguments more unlikely. Belenkii and Alkhazov? in experiments on the interaction of O2 and CO with Fe203,identical with those of Glemza and Kokes, were able to exhibit the dependence of the over-all time scale of the change and reversal on the initial quantity of the reactive gas, also tending to rule out diffusion effects, although they offered an explanation similar to that of Peers. Further evidence that the changes observed are true transient electronic effects in the solid and not structural artifacts is gained by examining in a d.c. measurement the photoresponse of ZnO during decays caused by pulsed-oxygen adsorption. It is found that the decay is rapidly and irreversibly quenched by photons with energies greater than that of the optical absorption edge (in powders X
