Chauhan Saurabh, Watson David F
Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, USA.
Phys Chem Chem Phys. 2016 Jul 27;18(30):20466-75. doi: 10.1039/c6cp03813a.
Colloidal semiconductor quantum dots (QDs) exhibit excitonic and surface states, both of which may participate in charge-transfer processes relevant to solar energy conversion. To explore this inherent complexity of the charge-transfer mechanisms of QDs, we used steady-state and time-resolved emission measurements to characterize excited-state electron transfer (ET) from core-only CdSe QDs and core/shell CdSe/ZnS QDs to TiO2 nanoparticles (NPs). Core-only QDs transferred electrons from both excitonic and surface states to TiO2 with rate constants of ET (ket) of approximately (1-3) × 10(8) s(-1) and (4-7) × 10(7) s(-1), respectively. Efficiencies of ET (ηet) from excitonic and surface states were approximately 71-82% and 64-76%, respectively. Thus, trapping of electrons lowered their potential energy but did not greatly affect the efficiency of their transfer to TiO2. Photogenerated holes were transferred from core-only CdSe QDs to adsorbed 3-mercaptopropionic acid (MPA), which linked the QDs to TiO2. We characterized core/shell CdSe/ZnS QDs as alternatives to core-only QDs. The ZnS shell eliminated the undesirable trapping of electrons and transfer of photogenerated holes to MPA. We measured ket of approximately (1-3) × 10(8) s(-1) and ηet of approximately 66-85% for ET from excitonic states of core/shell CdSe/ZnS QDs to TiO2 NPs. The insensitivity of ket to the presence of the ZnS shell may have arisen from increased cross-linking of core/shell QDs to TiO2. Our results highlight the involvement of surface states in excited-state ET processes of core-only QDs and, for the heterostructures reported here, the improved performance of core/shell CdSe/ZnS QDs relative to core-only CdSe QDs.
胶体半导体量子点（QDs）表现出激子态和表面态，这两者都可能参与与太阳能转换相关的电荷转移过程。为了探究量子点电荷转移机制的这种内在复杂性，我们使用稳态和时间分辨发射测量来表征从仅含核心的CdSe量子点和核壳结构的CdSe/ZnS量子点到TiO2纳米颗粒（NPs）的激发态电子转移（ET）。仅含核心的量子点将激子态和表面态的电子转移到TiO2，电子转移速率常数（ket）分别约为(1 - 3)×10(8) s(-1)和(4 - 7)×10(7) s(-1)。激子态和表面态的电子转移效率（ηet）分别约为71 - 82%和64 - 76%。因此，电子的捕获降低了它们的势能，但对它们转移到TiO2的效率影响不大。光生空穴从仅含核心的CdSe量子点转移到吸附的3-巯基丙酸（MPA），MPA将量子点与TiO2连接起来。我们将核壳结构的CdSe/ZnS量子点作为仅含核心量子点的替代物进行了表征。ZnS壳消除了电子的不良捕获以及光生空穴向MPA的转移。我们测量了从核壳结构的CdSe/ZnS量子点的激子态到TiO2 NPs的电子转移的ket约为(1 - 3)×10(8) s(-1)，ηet约为66 - 85%。ket对ZnS壳的存在不敏感可能是由于核壳量子点与TiO2之间交联增加所致。我们的结果突出了表面态在仅含核心量子点的激发态电子转移过程中的作用，以及对于本文报道的异质结构，核壳结构的CdSe/ZnS量子点相对于仅含核心的CdSe量子点性能的提升。
