Fig. 3. XRD patterns of the Cu₂O–Cu catalysts synthesized by using 5 M and 7 M NaOH, respectively (a); XPS spectra of the Cu₂O–Cu catalyst synthesized under the optimum condition (5 M NaOH, 10 mA laser energy), including XPS full spectrum (b), Cu 2p spectrum (c), and Cu LMM Auger spectrum (d), respectively.