O-18-EXCHANGE WITH THE SUBSTRATE DURING O-18 OXIDATION OF CU PARTICLES SUPPORTED ON O-16-OXIDIZED SI(100)

An O-16(2)-oxidised Si(100) surface, covered with 15 nm Cu particles, was oxidised in O-18(2) and subsequently reduced in H-2. After reducti on, there still was a significant amount of O-18 present in the sample . In this paper we concerned ourselves with revealing the origin of th is remaining O-18. By performing some strategic experiments we found t hat there was no significant contribution to the residual O-18 in the sample due the following processes: oxidation of the substrate to a fu rther extent, the remain of an O-18-adlayer on the sample after reduct ion, and reactions of reduction products with the substrate, The remai ning O-18 was caused by self-diffusion of oxygen atoms: maximum entrop y was achieved by mixing O-16 atoms in the substrate with O-18 located inside the copper particles, We drew this conclusion from the fact th at O-18-exchange also occurred during vacuum annealing of an oxidised sample, which is impossible in the case of the other proposed mechanis ms, Also, O-18 could be extracted from a metallic Cu/SiO2 sample throu gh O-16(2)-oxidation, annealing of the sample in vacuo, and subsequent reduction of the sample.