The Surface Groups Regulation And Properties Research Of Carbon Quantum Dots

Carbon quantum dots(CQDs) constitute a fascinating class of photoluminescencenanocarbons that comprise discrete, quasispherical nanoparticles with sizes below10nm.They are attracting considerable attention as nascent quantum dots, particularly for theirlow-toxicity，good chemical inertness and favorable biocompatibility. In general the size,structure and surface state will affect the properties of the CQDs, but a growing number ofstudies have shown that under a certain size and specific synthetic conditions, surface groupsare the key factor that influence the performance of CQDs. At present the effects of thesurface groups on the properties of CQDs have not systematically understand and study. Inthis work, we prepared CQDs containing different surface groups and explored O, N and Cleffects on the properties of them.The amino modified CQDs (N-CQDs) was successfully prepared using ammonia asnitrogen source. Compared with the unmodified CQDs, the fluorescence intensity of N-CQDswas significantly enhanced. By changing the reaction temperature, the nitrogen source as wellas the nitrogen source types, the best preparation condition of N-CQDs was obtained that isusing ammonia as nitrogen source and heated at200oC to modify the prepared carbonquantum dots. The fluorescence intensity enhancement arising from the change of surfacegroups was determined throught the test of the absorption, fluorescence and infrared ofN-CQDs.The oxygen-contained CQDs were synthesized by heating various carbon sources inHNO3solutionat reflux. The experimental results showed that the sizes and photoluminescentbehaviors of CQDs can be tuned by HNO3concentration and reaction time. The pure carbonquantum dots were found with photocatalytic performance through photocatalytic performance test. After sodium borohydride reduction of the prepared CQDs thephotoluminescence intensity of the CQDs were improved but contrary to the photocatalyticefficiency. Therefore, the different types of surface groups could be the important reasons forthe photoluminescence and photocatalytic properties changes of the CQDs.The CQDs were synthesized by hydrothermal treating with ethylene glycol as the carbonsource. Then ammonia and sulfoxide chloride were used to modify the prepared CQDs, theCQDs containing O, N and Cl were obtained.Transmission electron microscopy showed thatthe particle size of the modified CQDs had no significant difference. Photoluminescence andphotocatalytic test results indicated that the coexistence of O and N radicals in CQDs resultsin strong photoluminescence while CQDs containing O and Cl radicals showed highphotocatalytic activity, Thses results proved that different surface groups attributed to thedifferences of photoluminescence and photocatalytic properties.By combining the theoretical and experimental results a model was established toexplain the dependency of surface groups and CQDs properties using band gap bending. Thecarbon-oxygen bond, carbon-nitrogen bond and carbon-chlorine bond on the CQDs surfacewill form different band gap bending, the direction and degree of the band gap bending willinfluence photogenerated carriers separation, which leads to the differences betweenphotoluminescence and photocatalytic properties.