ADSORPTION AND INCORPORATION OF OXYGEN BY LEAD SULFIDE AT ROOM TEMPERATURE

The difficulty in obtaining clean PbS surfaces, because of the slow outgassing of CO2 noted previously,1 has been overcome by the use of a final rapid sublimation. The present work shows clearly that an extensive chemisorption of oxygen occurs on polycrystalline PbS films at 25° and that the oxygen is largely retained at the surface. The evolution of SO2 after oxygen sorption is variable and substantially below stoichiometric. The total uptake of oxygen at room temperature is suggested to be the sum of three processes: the sorption and ionization of oxygen by capture of carrier electrons from the n-type PbS film, the chemisorption of oxygen on the PbS in amounts that approach one oxygen atom per Pb atom in the surface, and the incorporation of oxygen into the lattice by replacement of a sulfur atom followed by a partial evolution of SO2. This last process does not bear a fixed relationship to the oxygen sorption and apparently is controlled by the surface state of the PbS itself. The time dependence of the oxygen sorption and the change in resistance of the PbS film were suitably expressed by the Elovich equation. Extrapolation of the slow sorption process observed at reduced pressures to atmospheric conditions indicates that appreciable surface oxidation would occur for polycrystalline films in a few minutes time at normal room conditions.