title: Donor-Acceptor Interaction Determines the Mechanism of Photoinduced Electron Injection from Graphene Quantum Dots into TiO2: π-Stacking Supersedes Covalent Bonding.
abstract: Interfacial electron transfer (ET) constitutes the key step in conversion of solar energy into electricity and fuels. Required for fast and efficient charge separation, strong donor-acceptor interaction is typically achieved through covalent chemical bonding and leads to fast, adiabatic ET. Focusing on interfaces of pyrene, coronene, and a graphene quantum dot (GQD) with TiO2, we demonstrate the opposite situation: covalent bonding leads to weak coupling and nonadiabatic (NA) ET, while through-space π-electron interaction produces adiabatic ET. Using real-time time-dependent density functional theory combined with NA molecular dynamics, we simulate photoinduced ET into TiO2from flat and vertically placed molecules and GQD containing commonly used carboxylic acid linkers. Both arrangements can be achieved experimentally with GQDs and other two-dimensional materials, such as MoS2. The weak through-bond donor-acceptor coupling is attributed to the π-electron withdrawing properties of the carboxylic acid group. The calculated ET time scales are in excellent agreement with pump-probe optical experiments. The simulations show that the ET proceeds faster than energy relaxation. The electron couples to a broad spectrum of vibrational modes, ranging from 100 cm-1large-scale motions to 1600 cm-1C-C stretches. Compared to graphene/TiO2heterojunctions, the molecule/TiO2and GQD/TiO2systems exhibit energy gaps, allowing for longer-lived excited states and hot electron injection, facilitating charge separation and higher voltage. The reported state-of-the-art simulations generate a detailed time-domain, atomistic description of the interfacial charge and energy transfer and relaxation processes, and demonstrate that the fundamental principles leading to efficient charge separation in nanoscale materials depend strongly and often unexpectedly on the type of donor-acceptor interaction. Understanding these principles is critical to the development of highly efficient photovoltaic and photocatalytic cells.
authors.affiliation: College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education, Beijing Normal University , Beijing, 100875, P. R. China.
authors.forename: Run
authors.initials: R
authors.lastname: Long
authors.affiliation: Kimika Fakultatea, Euskal Herriko Unibertsitatea and Donostia International Physics Center, 20018 Donostia, Euskadi, Spain.|IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Euskadi, Spain.
authors.forename: David
authors.initials: D
authors.lastname: Casanova
authors.affiliation: College of Chemistry, Key Laboratory of Theoretical & Computational Photochemistry of Ministry of Education, Beijing Normal University , Beijing, 100875, P. R. China.
authors.forename: Wei-Hai
authors.initials: WH
authors.lastname: Fang
authors.affiliation: Department of Chemistry, University of Southern California , Los Angeles, California 90089, United States.
authors.forename: Oleg V
authors.identifier: 0000-0002-5140-7500
authors.initials: OV
authors.lastname: Prezhdo