How mobile are dye adsorbates and acetonitrile molecules on the surface of TiO2 nanoparticles? A quasi-elastic neutron scattering study

Motions of molecules adsorbed to surfaces may control the rate of charge transport within monolayers in systems such as dye sensitized solar cells. We used quasi-elastic neutron scattering (QENS) to evaluate the possible dynamics of two small dye moieties, isonicotinic acid (INA) and bis-isonicotinic acid (BINA), attached to TiO2 nanoparticles via carboxylate groups. The scattering data indicate that moieties are immobile and do not rotate around the anchoring groups on timescales between around 10 ps and a few ns (corresponding to the instrumental range). This gives an upper limit for the rate at which conformational fluctuations can assist charge transport between anchored molecules. Our observations suggest that if the conformation of larger dye molecules varies with time, it does so on longer timescales and/or in parts of the molecule which are not directly connected to the anchoring group. The QENS measurements also indicate that several layers of acetonitrile solvent molecules are immobilized at the interface with the TiO2 on the measurement time scale, in reasonable agreement with recent classical molecular dynamics results

Importance of Polaronic Effects for Charge Transport in CdSe Quantum Dot Solids

We developed an accurate model accounting for electron-phonon interaction in colloidal quantum dot supercrystals that allowed us to identify the nature of charge carriers and the electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping

Multiscale modelling of intermolecular charge transfer in dye sensitised solar cells

Quantum chemistry based simulations allow us to explore the length and time scales which are experimentally inaccessible. In particular, these simulations bring a unique perspective on processes governed at the nanoscale by electronic interactions such as charge transfer. In this thesis, I present a framework for the multiscale simulation of hole transfer between dye molecules tethered on (101) TiO2 surfaces as in Dye Sensitized Solar Cells (DSSC). At the molecular level, I use methods derived from ground state density functional theory to calculate the reorganization energy (λ_tot) including ionic solvent effects, and electronic coupling (J_ij) distributions representing the conformational disorder of a dye monolayer. At the nanoscale, I use the semi-classical non adiabatic Marcus's equation to calculate the rate of hole transfer in the high temperature limit from λ_tot and J_ij. At the macroscopic scale, I calculate hole diffusion coefficients from kinetic Monte Carlo (KMC) simulations and validate my results by comparing with experimental data, when available. I find that the polar electrolytes used in DSSC contribute to 80% of the total reorganization energy of hole exchange. By including the effect of structural rearrangement of the dyes on various timescales, I show that large amplitude fluctuations of the tethered dyes at the microsecond timescale may enable charges to escape configurational traps. However, the analysis of Quasi Elastic Neutron Scattering (QENS) data on dye sensitised TiO2 nanoparticles suggests that the dyes are immobile between tens of picoseconds and few nanoseconds. This implies that the hypothesised dynamical rearrangement of the dye monolayer at the microsecond timescale originates from the collective motion of the molecule and its neighbours. These findings suggest that charge transport across disordered dye monolayers is enabled by the structural rearrangement of the molecules while the low measured diffusion coefficients (~10^-8 cm^2.s^-1) arise from the high polarity of the medium.Open Acces

Computational Optimization of Electric Fields for Improving Catalysis of a Designed Kemp Eliminase

Here we report a computational method to improve efficiency of a de novo designed Kemp eliminase enzyme KE15, by identifying mutations that enhance electric fields and chemical positioning of the substrate that contribute to free energy stabilization of the transition state. Starting from the design that has a kcat/KM of 27 M-1 s-1, the most improved variant introduced four computationally targeted mutations to yield a kcat/KM of 403 M-1 s-1, with almost all of the enzyme improvement realized through a 43-fold improvement in kcat, indicative of a direct impact on the chemical step. This work raises the prospect of computationally designing enzymes that achieve better efficiency with more minimal experimental intervention using electric field optimization as guidance