功函数测量研究氧与银和银-钯合金表面的相互作用

在超高真空条件下, 采用电子束阻挡势技术测量固体表面功函数连续变化, 并与AES, TDS等手段相配合, 研究了氧在Ag和Ag-Pd合金表面的吸附和脱附动力学。结果表明, 在Ag和Ag-Pd合金表面的两种不同的氧吸附态, 具有相反的电荷转移效应, 未解离的分子态吸附降低了表面功函数, 而解离的原子态氧吸附使表面功函数明显升高。与纯银相比, 氧在银钯合金表面吸附具有较小的粘附系数和较大的偶极矩。银钯合金组成变化时功函数和AES的连续测量表明, 表面结构从无序向有序转变和表面银偏析均为功函数降低过程。

INTERACTION OF OXYGEN WITH Ag AND Ag-Pd ALLOY BY WORK FUNCTION MEASUREMENT

The adsorption and desoption of oxygen on Ag and Ag-Pd alloy surfaces have been investigated by AES, TDS and by the continuous measurement of work func- tion change. Two kinds of different adsorbed bonding configurations have been revealed: The adsorbed atomic oxygen increases the surface work function, which is due to the transfer of electron from metal to adsorbate, forming the O_(ads)~(-δ) species. For the saturated surface of Ag and Ag-Pd alloy the dipole moment per adsorbed oxygen is 0.24 and 0.34 Debye respectively. The adsorption of molecular oxygen leads to a decrease of work function, which can be explained with the electron feedback from the metal surfaces to molecular oxygen species. Work function change as a function of the exposure of oxygen was also obtained. From these data an approximate experimental sticking coefficient has been derived by using the expression: S=K·P~(-1)·dΔφ/dt. It is shown that the sticking coefficient of oxygen on Ag-Pd alloy surface is slightly less than that on Ag surface, which implies that the alloying of Ag and Pd decreases the surface activity. A continuous change of surface composition for Ag-Pd alloy has been realized by using argon ion bombardment and subsequently annealing. A conclusion has been reached that both the transformation of the surface structure from disorder to order and the surface enrichment of silver during the annealing of alloy result in the decrease of surface work function.