Chemical Dynamics of Photocatalysis:
How does light drive energy-storing chemical reactions?
Photocatalysis involves absorption of light by electronic resonances in materials and molecules, followed by a series of intricate energy transfer, charge transfer, bond breaking and bond formation processes. The final outcome is that energy from light is stored in chemical bonds. Photocatalysis is the source of almost all bio-energy on Earth. In light of the emerging need for sustainable energy, understanding and harnessing sunlight driven photocatalysis is one of the most important scientific challenges of the present time.
To successfully design efficient artificial light harvesting systems, it is necessary to understand the elementary steps of photocatalytic reactions. Many of these elementary steps occur on the ultrafast time-scale (femtoseconds to picoseconds). We will employ state-of-the-art ultrafast laser spectroscopy to initiate electronic excitations in materials and molecules of interest for light harvesting and follow the chemistry that is triggered by them in real time. We will study two general classes of photocatalytic reactions.
Heterogeneous Photocatalysis at the Interface of Semiconductors and Molecules:
Many surfaces and interfaces are potent catalysts. Interaction of a molecule with a semiconductor surface can facilitate photocatalytic reactions in the molecule. While many examples of such reactions are known, understanding the intricate elementary steps of these reactions still remains challenging. For example, splitting of water into hydrogen and oxygen gas on the surface of TiO2 is known for a few decades. However, its detailed mechanism is still under debate. Such reactions hold great promise for the future of solar light harvesting and sustainable energy.
We will study the dynamics of photoreactions in molecules adsorbed on semiconductors by surface-specific, short pulse laser spectroscopy methods. With ultrashort (~30 fs) visible or ultraviolet pulses, we will generate excited electrons and holes in the semiconductor. Then we will track the motion of charges across the interface and study the ensuing chemical reactions in the adsorbed molecules by monitoring molecular vibrations with ultrashort (<100 fs) IR laser pulses. A surface specific nonlinear spectroscopy method known as sum-frequency generation will be used to study the molecular vibrations at the interface. We will piece together a time-line of events (with femtosecond resolution) on how charges are generated in the semiconductor, how they migrate, and how they trigger chemical reactions in the adsorbed molecules. We hope to identify the crucial steps in this process and with close collaboration with material scientists and synthetic chemists, propose and implement ways to make them more efficient.
Figure 1. The scheme for the time resolved surface sensitive nonlinear optical technique. The UV-visible pulse initiates a photochemical reaction by generating electrons and holes in a semiconductor photocatalyst. The charges traverse the interface and start redox reactions. The reactions are followed in time domain with sum frequency generation (SFG) spectroscopy, which is a surface sensitive method with chemical specificity.
Homogeneous Photocatalysis on the Molecular Scale:
Many photocatalytic reactions are facilitated by molecular catalysts without the need for a surface. Most biological photoreactions, including photosynthesis, belong to this category. Photocatalysts avoid formation of high energy reaction intermediates by appropriately coupling the motion of excited electrons and nuclei. An example of such coupling is correlated motion of electrons and protons known as proton coupled electron transfer (PCET). Although PCET is observed in many natural and artificial processes, its microscopic dynamics on the ultrafast time scale presents theoretical and experimental challenges.
We will employ ultrafast spectroscopy methods to understand the elementary steps of such reactions. Initiating an electronic excitation in a molecule with a visible or UV short pulse, we will monitor the ensuing chemical reactions with IR pulses. The simplest form of such a measurement is known as visible-pump-IR-probe spectroscopy. It can also be extended to 2D-visible-IR spectroscopy (in analogy to 2D heteronuclear NMR) to identify spectral correlations between electronic and vibrational degrees of freedom. We will apply these methods to a variety of model chemical systems, and biological enzymes.
Parallel to the experiments, we will collaborate with theoretical chemistry groups in the department to understand excited state dynamics in photocatalysts. The work will also incorporate collaborative efforts from materials sciences and inorganic synthesis. Later, this research will be extended to hybrid materials, where synthetic and natural photocatalysts are coupled to semiconductor interfaces. The vision of this work is to deliver ultrafast mechanistic knowledge to the fields of synthesis and materials sciences for designing new light harvesting photocatalytic materials and molecules.
Figure 2. (a) Scheme for ultrafast measurment of PCET. A pump pulse initiates the reaction in the donor which results into transfer of an electron and proton to their respective acceptors. The visible probe pulse monitors the arrival of the electron on the acceptor. The IR pulse monitors the non-equilibrium vibrations in the proton acceptor which are caused by proton transfer. (b) Proposed concurrent ultrafast visible-IR spectroscopy. (c) The cartoon traces shows whether electron transfer occurs concurrently with vibrations associated with proton transfer.
- Microscopic Quantum Coherence in Photosynthetic Light Harvesting Antenna, J. Dawlaty, A. Ishizaki, A. De, G. Fleming, Philosophical Transactions of Royal Society A (in press).
- Mapping the Spatial Overlap of Excitons in a Photosynthetic Complex via Coherent Nonlinear Frequency Generation, J. Dawlaty, D. Bennett, V. Huxter, G. Fleming, Journal of Chemical Physics, 135, 044201 (2011).
- Ultrafast Multidimensional Spectroscopy: Principles and Applications to Photosynthetic Systems, G. Schlau-Cohen, J. Dawlaty, G. Fleming, IEEE Journal of Selected Topics in Quantum Electronics, 99, 1 (2011).
- Measurement of Ultrafast Carrier Dynamics in Epitaxial Graphene, J. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, M. G. Spencer, Appl. Phys. Lett., 92, 042116 (2008).
- Measurement of the Optical Absorption Spectra of Epitaxial Graphene from Terahertz to Visible, J. Dawlaty, S. Shivaraman, J. Strait, P. George, M. Chandrashekhar, F. Rana, M. G. Spencer, D. Veksler, Y. Chen, Appl. Phys. Lett., 93, 131905 (2008).
- Ultrafast Optical-Pump Terahertz-Probe Spectroscopy of the Carrier Relaxation and Recombination Dynamics in Epitaxial Graphene, P. George, J. Strait, J. Dawlaty, S. Shivaraman, M. Chandrashekhar, F. Rana, M. Spencer, Nano Letters, 8, 12, 4248 (2008).
- Carrier Recombination and Generation Rates for Intravalley and Intervalley Phonon Scattering in Graphene, F. Rana, P. George, J. Strait, J. Dawlaty, S. Shivaraman, M. Chandrashekhar, M. Spencer, Phys. Rev. B, 79, 115447 (2009).
- High frequency measurements on an AlN/GaN-based intersubband detector at 1550 and 780 nm, D. Hofstetter, E. Baumann, F. Giorgetta, J. Dawlaty, P. George, F. Rana, F. Guillot, E. Monroy, Appl. Phys. Lett., 92, 231104 (2008).
- Force-Gradient Detected Nuclear Magnetic Resonance, S. R. Garner, S. Kuehn, J. Dawlaty, N. E. Jenkins, and J. A. Marohn, Appl. Phys. Lett., 84, 5091 (2004). 1
- Effects of Noise on Parameter Recovery from Raman Spectrograms, J. Dawlaty, D. Ulness, Journal of Raman Spectroscopy, 32, 211 (2001).