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• BPQDs were prepared by a liquid exfoliation combined with solvothermal method. • The photocatalytic activity of BPQDs was investigated by degradation of dyes. • The decolorization efficiency of IC can get to 98.22% (≤23 min) under sunlight. A metal-free nanocatalyst was proposed to be applied in the photocatalysis of dye degradation based on black phosphorus quantum dots (BPQDs). BPQDs were prepared by a liquid exfoliation combined with solvothermal method. BPQDs show excellent catalytic performance with higher photocatalytic efficiency and shorter degradation time than those for most of previous works. The morphological, structural, and optical properties and functional groups for BPQDs were characterized by numerous techniques. They are transmission electron microscope (TEM), X-ray diffraction (XRD), Raman, fluorescence and UV–vis absorption spectrophotometry, respectively. To investigate the photocatalytic activity of BPQDs, indigo carmine (IC) was employed as a model dye. The decolorization efficiency of IC can get to 98.22% in the sun. Furthermore, the degradation time is 23 min in the sun, which is shorter than other reports. The degradation rate constant is 0.214 min−1 which is about 10 times higher than most of previous works. More importantly, the species after degradation were confirmed by mass spectrum, which own much lower carbon content than IC. The present BPQDs as nanocatalysts may be explored to degrade more organic molecules in environmental wastewater. Scheme 1. Synthesis of BPQDs and the degradation process of the organic dyes. [Display omitted] [ABSTRACT FROM AUTHOR]