Category Solar Energy Conversion

Ensemble Calculations of Structural Trajectories: Coupling Molecular Dynamics Simulation in Data Analysis

Solar energy conversion functions often depend on dynamic structures in so­lution or on a supporting matrix where a transiently appearing dynamic structure could evolve into a precursor for catalytic intermediates. Such dy­namic structures are implicitly depicted by the Debye-Weller factor in the conventional XAS data analysis in Equation (12.1), without specific description of the structural origin. In many homogeneous photochemical reactions, metal complexes interact with solvent molecules to form transient dynamic solvated structures, such as dynamic bonding between the catalyst molecule and the solvent or substrate molecules. These dynamic structures may well be the precursor or transition states in catalytic reactions, but were unfortunately obscured in the conventional data analysis...

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Normal Mode Analysis Coupled Structural Analysis

As the time resolution improves, experimental observation approaches direct visualization of nuclear motions during the photoexcitation, which provides the basis for structural control of light-matter interactions in solar energy conversion. The excited state to final photochemical product trajectories are often initiated by vibrational motions. When photons add energy to molecules, their energy will be dissipated among different vibrational modes. Therefore, carrying out a normal mode analysis (NMA) of the excited state will provide a guide to how the nuclear motions were launched at the very beginning of the photon-matter interaction...

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Extracting Multiple Transient Structures in XTA Data Analyses

As shown in Equation (12.2), the accuracy of simultaneously and independently obtaining fractions of the transient species and their corresponding transient spectra can be challenging. Current approaches using MIA and MXAN have accomplished the first steps. The requirement is to combine these approaches with accurate potential energy surfaces around metal centers, which by itself is already difficult. Since MXAN and MIA or similar approaches take advantage of the structurally sensitive multiple scattering features in a relatively narrow energy window and with varying combined nuclear coordinates, they succeeded in extracting multiple species and their fractions simultaneously...

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Excited State Structural Dynamic Calculations for Transition Metal Complexes Involved in Solar Energy Conversion

Increasing numbers of groups are now engaging in calculations of excited state structural dynamics from first principles in isolated molecular systems and with relatively small molecules of light elements.162 165 However, only very limited calculations of excited state structural dynamics with heavy elements are available; these calculations provide not only excited state structures but also dynamics, as well as potential energy surfaces around the metal centers, en­abling accurate XANES calculations. This is not uniquely for XTA analyses, but for the whole field of XAS including steady-state measurements.

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Detecting Transient Structures with Low Concentrations in Photocatalytic or Irreversible Processes and Rephasing the Coherence of Nuclear Motions in an Ensemble

Most current XTA studies are on the transient structures of excited states that are directly generated by photons through ground to excited state transitions. In solar energy conversion processes, such as photocatalysis, the most inter­esting structures to be determined are often those resulting from the excited state, such as a photocatalyst-substrate complex and other intermediate structures produced in subsequent transformations. In order to find the structure/property correlations of the catalysts, a detailed knowledge and direct observation of reaction trajectories is necessary.

As illustrated by Figure 12.11, a photoinitiated chemical reaction starts from the light absorption/excited state formation, followed by its transformation to other intermediates and finally the product...

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Visualization of Fundamental Events in Photon-Matter Interactions: Capturing the Transition States

Solar energy conversion processes start from photon-matter interactions and a photon triggered chemical reaction almost always generates changes in nuclear geometry of those involved. The availability of high repetition rate, intense and femtosecond hard X-ray pulses could enable the visualization of nuclear geometry evolution in many light-induced reactions in solar energy conversion processes, including ‘‘the transition state’’ (TS)152 at the top of the potential barrier crossing trajectory between reactants and products. It is believed that a TS has ‘‘no lifetime’’ or a barrier crossing time of a fraction of the vibrational period...

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Future Research and Development

A decade of development of XTA has transformed transient structural de­termination from a fantasy to somewhat routine measurements. However, its contributions to solar energy research as well as our fundamental under­standing of photon-matter interactions need further advances in different areas as outlined below.

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Photoinduced Interfacial Electron Transfer (Heterogeneous Electron Transfer)

Since their discovery in early 1990s, dye sensitized solar cells (DSSCs) have been extensively investigated because of their potential in wide commercial applications for conversion of sunlight to electricity. The key interaction of light and matter in DSSCs is light induced interfacial charge transfer from transition metal complex dye sensitizers to semiconductor nanoparticles...

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