Carbon quantum dots with high quantum yield prepared by heterogeneous nucleation processes

Homogeneous and heterogeneous nucleation processes are employed to synthesize carbon quantum dots (CDs) by a solvothermal method, with o-phenylenediamine used as the precursor. For the heterogeneous process, titanium dioxide (TiO₂) nanoparticles were employed as the nucleation seeds. The ultraviolet-visible (UV–vis) and photoluminescence (PL) properties of CDs prepared by homogeneous (CDs) and TiO₂-induced heterogeneous (T-CDs) methods are compared herein. The experimental results show that, in comparison to the CDs, the n-π * transition peak in the UV–vis absorbance spectra of T-CDs red-shifts slightly and becomes broad, implying smaller and multiple band gaps of the T-CDs. The PL spectra demonstrate that the emission wavelengths of CDs and T-CDs are about 530 and 496 nm, respectively, which are independent of the reaction conditions. By tuning the operation conditions, the maximum quantum yield (QY) obtained by the CDs is 18 %, while a value of 68 % can be accomplished by the T-CDs. If N-doped TiO₂ nanoparticles were utilized as the nucleation seeds, an even higher QY (85.9 %) could be achieved. Fourier-transform infrared spectroscopy and x-ray photoelectron spectroscopy analysis demonstrated that the T-CDs contain a higher percentage of N atoms and a lower fraction of pyrrolic state N, which is inferred to be the reasons causing the shorter emission wavelength and higher QYs of the T-CDs.