On The Energy Transfer And Electron Transfer In The System Of Carbon Quantum Dots/Dye Molecules

In recent decades nanomaterials have attracted great attention,showing excellent performances in many fields.Thanks to their large surface area,nanomaterials differ greatly in their properties from the common bulk materials and even exhibit unexpectedly physical/chemical properties in some cases.In order to obtain more excellent properties,scientists have spent much time on their syntheses and adjustments;however,there lacks in-depth understanding on the related microscopic mechanisms.Ultrafast spectroscopy opens a new window for the investigations of nanomaterials,enabling mechanistic insights into a variety of interactions involved.In the fields of semiconductor quantum dots-based photocatalysis and solar cells,the understanding of exciton quenching kinetics is instructive for the improvement of material performances.Currently,most of the studies on exciton quenching kinetics are focused on either energy transfer(EnT)or electron transfer(ET)processes;nevertheless,the interrogations of the relationship between the two processes remain limited.In the major part of this thesis,we employed steady-state absorption and photoluminescence(PL)spectroscopy as well as ultrafast transient absorption(TA)spectroscopy to examine the EnT/ET processes and their relationship in a typical system of carbon quantum dots/rhodamine B molecules(denoted CQDs/RhB).The two different CQDs(o-CQDs and m-CQDs)were synthesized from two precursors,i.e.,o-phenylenediamine and m-phenylenediamine,respectively.By virtue of the different spectral overlaps between the PL emission of the two CQDs and the absorption of RhB,the efficiencies of EnT from the donor CQDs to the acceptor RhB should be different for the two systems.On the basis of analyses using Forster model,we found that the EnT efficiency in o-CQDs/RhB(73.2%)is more than twice that in m-CQDs/RhB(33.5%).Notably,given the energy relationships among o-CQDs,m-CQDs,and RhB,the ET process can also take place from the two CQDs to RhB.On the basis of analyses of the ultrafast TA results,we found that the ET behavior is not pronounced at all in the o-CQDs/RhB system while quite remarkable in the m-CQDs/RhB system.Therefore,we infer that in the EnT/ET-coexisting system of quantum dots/dye molecules the two processes manifest an effect of"one wanes and the other waxes".We have also investigated the charge carrier dynamics in graphitic carbon nitride(g-C3Na)with different degrees of polymerization,by means of ultrafast spectroscopy.Our collaborators synthesized a new type of g-C3N4 that features a better photocatalytic performance with a smaller degree of polymerization.Our ultrafast spectroscopy characterizations provided a plausible interpretation:With reducing the degree of polymerization,some sort of new defect states would be introduced into the g-C3N4 system;the emergence of these new defect states that are capable of capturing and accumulating photoexcited electrons would lead to the promotion of carriers separation and hence the improvement of photocatalytic performance.