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Photocatalytic technology has been considered to be an ideal approach to solve the energy and environmental crises, and TiO₂ is regarded as the most promising photocatalyst. Compared with bare TiO₂, TiO₂ based p-n heterojunction exhibits a much better performance in charge separation, light absorption and photocatalytic activity. Herein, we developed an efficient method to prepare p-type TiO₂ quantum dots (QDs) and decorated graphitic carbonitrile (g-C₃N₄) nanocomposites, while the composition and structure of the TiO₂@g-C₃N₄ were analyzed by X-ray diffraction, Fourier transform infrared spectroscopy ,thermogravimetric analysis, transmission electron microscopy, X-ray photoelectron spectroscopy and UV-visible diffuse reflectance spectroscopy characterizations. The characterization results reveal the surface decorated TiO₂ quantum dots is decomposed by titanium glycerolate, which exhibits p-type conductivity. The presence of p-n heterojunction over interface is confirmed, and photoluminescence results indicate a better performance in transfer and separation of photo-generated charge carriers than pure semiconductors and type-II heterojunction. Moreover, the synergy of p-n heterojunction over interface, strong interface interaction, and quantum-size effect significantly contributes to the promoted performance of TiO₂ QDs@g-C₃N₄ composites. As a result, the as-fabricated TiO₂ QDs@g-C₃N₄composite with a p/n mass ratio of 0.15 exhibits improved photo-reactivity of 4.3-fold and 5.4-fold compared to pure g-C₃N₄ in degradation of organic pollutant under full solar spectrum and visible light irradiation, respectively.