The XRD patterns
shown in Figure 4 reveal that the plain TiO2 hollow
microspheres are mainly composed of pure anatase phase,
which is consistent with results reported previously.68 The CuO
(3 wt %)-incorporated sample demonstrates a similar pattern,
and no Cu species peak was observed. This phenomenon was
also observed for Cu-incorporated TiO2 materials when Cu
contents were lower than 3 wt %.48,49,51 Diffraction peaks for
copper species were not detected, implying that they are highlydispersed or it is due to the low Cu(II) concentration and
extremely small CuO clusters.49,51 In addition, this result also
suggests that the incorporation of Cu with low loading does not
affect the crystalline structure of the TiO2 hollow microspheres.
Interestingly, with a high CuO loading (e.g., 10 wt %), a peak
located at ca. 35.6° is clearly observed, as shown in Figure S1
(Supporting Information), implying the presence of crystalline
CuO.