Fig. 6. Absorption and photoluminescence (PL) emission spectra of the CDs washed for six times: H-CDs_(w) (a), P-CDs_(w) (b) and R-CDs_(w) (c) under excitation light of different wavelength (the inset legends); XRD patterns of the three CDs (d); CD_(w) is for the six-times-washed CDs.