In a typical procedure, 25 mg bulk boron powder was directly added into 50 ml dimethylformamide (DMF) solvent to form a suspension with the initial concentration of 0.5 mg/ml. The suspension was firstly sonicated by 700 W probe ultrasonication for 3 h at 5 °C, then centrifuged at 6000 rpm for 30 min to remove unexfoliated boron particles. Next, the obtained light brown dispersions in DMF were centrifuged at 15000 rpm for 1 h to concentrate the as-exfoliated boron sample. After that, the collected boron samples were further treated by high energy ball milling (Nanjing University Instrument Plant, QM-3SP2) with a rate of 500 rpm for 24 h. Finally, the resultant boron/DMF solution was centrifuged successively at 10000 rpm for 30 min and 13000 rpm for 60 min to obtain the final BQDs product.

Typically, the as-prepared BQDs dispersion was further centrifuged at 15000 rpm for 1 h. The resultant solid product was collected, followed by drying under vacuum at 60 °C for 8 h. 3.5 mg of the solid BQDs product was then redispersed in 4 mL of deionized water (DIW) by 15 min sonication under a constant temperature of 10 °C. The obtained BQDs/DIW dispersion was used to determine the corresponding absorption spectra.

The morphology and microstructure of the samples were characterized via scanning electron microscope (SEM; Sirion, FEI, Netherlands), HRTEM (Tecnai G2 F30) and AFM (Dimension Edge, Bruker, America) equipped with an energy-dispersive X-ray spectrometer (EDS; Genesis 7000, EDAX Inc., USA). The elemental compositions were analyzed via X-ray photoelectron spectroscopy (XPS; AXIS-Ultra instrument, Kratos Analytical, England) with a monochromatic Al Kα X-ray beam (225 W, 15 Ma, 15 kV). The UV-Vis diffuse reflectance spectra (DRS) of the samples were measured with the diffuse reflectance accessory of UV-Vis spectrophotometer (UV-2550; Shimadzu, Kyoto, Japan), in which BaSO₄ was used as a background between 200–1200 scopes.