Surface engineering and photophysics of InP/ZnS quantum dots for photocatalytic application

Photocatalytic H₂ evolution from water splitting by using quantum dots is an encouraging way to tackle the energy shortage. On the other hand, conversion of CO₂ to chemical fuels by photocatalytic approach is also regarded as a promising solution to simultaneously address both energy crisis and environmental issues, high-efficient photocatalytic materials are therefore developed and investigated. Among them, Re(bpy)(CO)₃L (bpy = 2,2’-bipyridine, L = halogen atom) and their derivative attract tremendous attention on photocatalytic reduction CO₂ applications due to 1) ultralong lifetime of excited triplet metal-to-ligand charge transfer (³MLCT)state allows photoinduced electrons transfer to catalytic site efficiently, 2) transition metal center can fix CO₂ molecule onto itself easily, 3) band alignment can be tuned by modification of bpy moiety where can also store one-electron-reduced species. However, transition metal complex photocatalyst suffers from insufficient visible light absorption and difficult multi-electron accumulation due to inevitable triplet-triplet annihilation. In this case, we employed heavy-metal free colloidal quantum dot InP/ZnS as photosensitizer to achieve robust light absorption at the visible range, multi-electron transfer process, finally realized photocatalytic conversion of CO₂ to 8-electron product methane. In this context, surface engineering and typical photophysical properties affected by the introduction of InP/ZnS QDs for photocatalytic applications were systematically investigated in this thesis for understanding the excited dynamics of heterostructure photocatalytic systems involving InP/ZnS QDs.
In the first work of thesis, we studied the photoinduced charge carriers dynamics in InP/ZnS QDs engineered by a series of inorganic ligands (S²ˉ, C^lˉ, and PO4³ˉ) and organic ligand (oleylamine) by time-resolved spectroscopies (transient absorption (TA) spectroscopy, time-resolved midinfrared (TRIR) spectroscopy and time-resolved photoluminescence (TRPL)) Complementary with TA and TRPL, we confirmed the ultrafast hole transfer from InP/ZnS to surface ligand. Strikingly, QDs capped with S²ˉ exhibited fast hole injection time and long-lived hole lifetime at S²ˉ determined to be 4.2 ps and >4.5 ns respectively. S²ˉ ligand also can remove electrostatically attached surfactants (NMF) to compensate for the spatial charge redistribution and balance the ionic radii and net charge to create an optimal condition for charge transfer. Such conditions are beneficial to prohibit excellent photocatalytic H₂ evolution (213 μmol/mg within 10 h), which elucidates the crucial role of surface ligand in the photocatalytic activities of colloidal QDs.
In the second work, we introduced InP/ZnS as photosensitizer and linked with Re(bpy)(CO)₃Br photocatalyst by covalent bond. Possible multi-electron donation within the specific QD/transition metal complex hybrid photocatalyst system was confirmed when multiple photocatalysts were attached to one QD. After basic structural characterization, the time-dependent density functional theory (TD-DFT) calculation presented an unconventional electronic structure, photoexcited electrons directly resided on the bpy moiety of Re-catalyst, and exhibited efficient exciton delocalization and separation. Transient absorption and infrared spectroscopy revealed the ultrafast multi-electron transfer on two-attachment mode to be shorter than one picosecond, while excited holes were delocalized on QD with an ultralong lifetime, which guaranteed excellent multi-electron donation during photocatalytic reactions. Accordingly, such multiple photocatalyst attachment exhibited significant catalytic activity on photocatalytic reduction of CO₂ to 8-electron product methane. This work provides a new strategy to control photocatalytic products via multi-electron donation pathway tuned by attached molecules.
In the last work, we employed the same QD/transition metal complex structure but modified the multi-electron transfer driving force by tuning the band alignment of QDs, Re-catalyst covalently tethered to QDs with three different sizes were therefore prepared due to the electronic structure of QDs is size-tunable. Similar to the second work, after basic characterization, transient absorption demonstrated the smallest QDs exhibited efficient multi-electron transfer to be short than one picosecond due to the large electron transfer driving force, ultrafast multi-electron injection prevented the Auger recombination and annihilation process. Finally, photocatalytic reduction of CO₂ to methane can only be achieved by the smallest QDs with two catalysts attachment mode. This work demonstrated that the driving force dominates the multi-electron transfer process and consider as a guidance for engineering the photocatalytic performance of CO₂ reduction, as well as the product selectivity.