Oxygen species adsorbed on zinc oxide

Volume 8. number 2

CHEMICAL

OXYGEN

SPECIES

PHYSICS LETTERS

ADSORBED

.ON

15 January 1971

ZINC

OXIDE

A. J. TENCH and T. LAWSON Ckemistry

Division,

A.E.R.E.,

Haneell.

Received 13 November

Berkshire.

UK

1970

Oxygen enriched with ” 0 has been used to identify 0; adsorbed on partially reduced 2110. The unpaired electron interacts with both oxygen nuclei equally and the hyperfine tensor is Axx = SO. Ayy = 0 and A,,= 15 gauss.

The oxygen species adsorbed on oxides are not well known since little direct evidence of their identity has been available. Many of the physical properties of non-stoichiometric zinc oxide, containing excess zinc, such as conductivity and luminescence are influenced by the chemisorption of oxygen. In the past, a number of ESR investigations of the nature of the adsorbed species have been made with conflicting results. Recently, we have used oxygen Iabelled with 170 to identify some paramagnetic oxygen species adsorbed on surfaces [I] and we wish to report the identification of Oq on ZnO. SampIes of ZnO were prepared by thermal decomposition of the oxalate in air followed by heating overnight, in vacua at 5OO’C. Addition of low pressures of 1602 to the ZnO samples destroyed some of the signal at g = 1.96 and gave a well defined ESR spectrum at 77OK with g2 = 2.0090 and g3 = 2.0024, in agreement with earlier work [2]; g1 showed fine structure with a range of values from 2.0519.to 2.0424. This spectrum can be interpreted as a single species in several weLl defined sites and the various gl values are determined by the local crystal field, indicating a number of slightly different adsorption sites. .A similar experiment was repeated using oxygen enriched to 58 atom % with I70 which gave a well defined hyperfine structure (fig. 1). This spectrum has-been interpreted ati the cum of an eleven line spectrum from (170170)and a six line spectrum from (170160)- born centred about g3 (& ) to ether with a three line spectrum from (re 01 8 0))‘. A-more detailed z+alysis indicates some. poorly resoiv-edhjrperfine .‘._struc&re about &(&&and the sp&um can be fitted by + ashmetric: hype+fine tensor .of t&e ^ .. : _.

formA,=80&2, A,,,,=O*5andAL,,=15&4 gauss where the zz direction is along the molecular axis. The value for Av,, has been estimated from the line width of the Lrne at g2 since no hyperfine structure was resolved. This tezsor is in good agreement with that reported for 1708 on MgO [l]; assuming that the principal values are positive this indicates a spin density of 0.02 in the 2s orbital of each oxygen nucleus, the remaining electron density being in the 2~7;~ orbit&. The two oxygen atoms in the molecular ion appear to have identical hyperfine splitting constants within experimental error; that is, the unpaired electron must interact equtiy with both oxygen nuclei. This indicates that both oxygen nuclei in the moiecular ion experience a very similar environment. There are a number of possible configurations for such an ion adsorbed at a surface and we suggest that the equivalence of these oxygen nuclei means that the ion is adsorbed with the molecular axis parallel to the surface plane on both ZnO and MgO. This configuration would minimise any polarisation

of the ion due to the surface.

The hyperfine tensor described above does not agree with that reported recentLy by Code11 et al, [3 ] for oxygen on ZnO. Unfortunately, in their case the assignments of the lines are not clear because of the poor signal ta noise ratio. Their splitting c&&ant of 34.6 gauss is not easily reconciled with our data and th& a+ecies

observed by Codeli must arise -from oxygen in a different_environment from that described in this paper. Measurements on the relative line intensities for our data show that the ratios of the (170170)ties to those of the (170160)lines “e close to the theoretical predictions for the 02 ion and contributions from other adsorbed 177

-‘Volume 8,‘nunibel’

2

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CHEM!CAL PHYSIC: LETTE+

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Fig. 1. The first derivative FkR spectrum of 1702 on ZnO at 77OK. The’field increases from left to right and the ..gain’ and medutation have been increased to show the outer lines. No attempt haa been made to insert levels for the lines about El.

_.. species appear to-be insignificant. ’ Experiments on heating 2x10 with pre-adsorbed f-702 or %2 inaicste that exchange of Og with oxygen ions frbm the lattice does not occur to a significant extent at temperatures of lUO°C or less, tit some exchange becomes apparent after heating at 200°C. On some samples, a new asymmetric signal det’eloped in the temperature *ange 225O to 275OC with typical gvalues of 2:0145, 2.0129, 2.0092, and 2.0024. This new

than one site, but no hyperfine splitting from 170 could be detected.

R.EFERENCEjS [l]A.J.Tench and P.Holroyd, Chem. Commun. (1969) 471; A.J. Tenth and R.L. Nelson, 3. Chem. Phya. 44

(1966) 1714. [2] J.H.Lunsford and J.P.Jayne. J. Chem. Phys. 44 (l?66) 1487. [3] M.Codell, J.Weisberg, B.Gisser and R.D.Iyenga~, J. +I. Chem. Sot. 91 (1969) 7763.

signal disap$eared after heating at‘temperaturqs above 300°C and has been tentatively ascribed

to an 0” ion’ adsorbed on the surface at more

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