Fig. 6. ADF-STEM images and the corresponding EELS spectra taken from several regions of the specimen showing the existence of different Cu-species after 60 seconds of the reaction. ADF-STEM images (a–d, top), TEM images (e–h, middle) and EELS spectra of O–K edge (i, bottom) from (a), (b) and (c), respectively, where no O signals were detectable from (d). EELS spectra of Cu–L edge (j, bottom) from (a), (b), (c) and (d), respectively. In view of the fine structures exhibited in the EELS spectra, it is confirmed that (a) is CuO, (c) is Cu₂O and (d) is Cu(0). In (d), the Cu/O atomic ratio is estimated to be <2, indicating a mixture of Cu(0) and Cu₂O.