Carbon quantum dots (CQDs) were considered as desirable up-conversion luminescent materials. However, the reason leading to their up-conversion luminescence was not clear. In this paper, CQDs decorated ZnSn(OH)(6) was successfully synthesized by facile in situ hydrothermal method. Under NIR light irradiation, ciprofioxacin degradation efficiency of sample was 37.4%. Subsequently, reasons for the up conversion effect of CQDs were investigated to some extent. An amorphous carbon layer around CQDs was observed in composite (ZnSn(OH)(6)@CQDs@C) by HR-TEM and elemental mapping images. To study the effect of amorphous carbon layer on up-conversion performance, individual CQDs decorated ZnSn (OH)(6) (ZnSn(OH)(6)@CQDs) was also fabricated by compositely coating. ZnSn(OH)(6)@CQDs had no up conversion luminescence under 980 nm laser light excitation and its photocatalytic activity was negligible, implying amorphous carbon layer played a crucial role for realizing of up-conversion luminescence. A comparison of the FTIR spectra of two composites, ZnSn(OH)(6)@CQDs@C sample revealed greatly enhanced surface oxidation degree, polarity and hydrophilicity. Surface state of ZnSn(OH)(6)@CQDs@C composite was controlled by adjusting hydrothermal time, and the results confirmed that up conversion performance had a close relationship with surface states of samples. This work could provide a new insight into understanding the up-conversion effect of CQDs. (C) 2018 Elsevier Inc. All rights reserved.