The principal challenge lies in designing a system that completes the two necessary half reactions—the oxidation of water and the reduction of either H+ or carbon dioxide—efficiently in visible, rather than UV, light. In visible light, the overall reaction proceeds closer to the thermodynamic potential of the generated fuel and has limited over-potential. The system, then, needs to exploit the time scale, molecular configuration, and electronic potential of electron transfers between the light absorber, catalyst, and reactants to generate fuel efficiently. This leads to constraints both on the individual components and the overall architecture of the artificial photosynthetic system. For example, in all current proposals for visible light driven fuel formation, much like in a plant, the light absorber and catalyst are two, spatially separated components. One reason for this is that the initial charge separation step has to isolate electrons and holes long enough for chemical reactivity to take place.
The systems under development, with promising initial efficiencies, are truly heterogeneous in the sense that they take advantage of both molecular and materials components, which means that one has to be well versed in the nature of discrete molecular orbital states, the density of states in a solid, and how these two worlds interact. Most of the candidate catalysts, and some of the light absorbers, have transition metal centers with active d-levels that define a particular redox potential and enable high turnover rates. Therefore, the spectroscopic tools (transient optical spectroscopy, photoemission, and transient x-ray spectroscopy) will be geared to accessing the oxidation state and electronic/structural configuration of these d-levels. Collaboration with more synthetically orientated groups will explore a range of both molecular and materials based light absorbers and catalysts.
Transient Optical Spectroscopy
Transient optical spectroscopy is a pump-probe technique in which an initial laser light pulse creates an excited state of the system, and a subsequent, probe pulse measures its spectrum and lifetime. Since the light absorber has a well-defined spectrum at visible wavelengths, transient optical spectroscopy is ideally suited for identifying the excited states of the light absorber and the associated electron transfer rates to and from the catalysts and reactants. In particular, we are looking to quantify quantum efficiencies of either the half-reactions or the full reaction based on electron transfers between these components. We will also investigate the fundamental Marcus parameters (reorganizational energy, free energy change, and overlap integrals) underlying these electron transfer rates. The primary lab equipment will be a Ti:Saph Regen laser system with ~100fs pulses and ~mJ pulse powers. While the initial focus will be on the electronically excited state, the lab will have laser frequencies spanning to the infrared, providing access to the excited state structural degrees of freedom.
Photoemission ejects an electron from the valence band and therefore directly accesses the potential and orbital character of the hole involved in the first step of any oxidative process. Furthermore, since photoemission is a surface sensitive technique—reaching at most the first couple of unit cells, it is naturally sensitive to the catalytic properties of materials. In some cases, these surface states may be the same as bulk states, and in others they may diverge. Materials that look different based on bulk properties could have the same photoemission features at the surface that enable catalysis. Other questions in catalysis can be addressed by measuring the full band structure through angle-resolved photoemission. To what degree is catalysis aided by extended band like states needed to transport electrons to the catalytic site? Finally, while photoemission has traditionally been done in an ultra high vacuum environment, in recent years capabilities have been extended to ambient pressure—which means that the effects of reactants on catalytic sites can be studied in situ.
Transient X-ray Spectroscopy
In its static form, x-ray absorption spectroscopy (XAS) is widely used to obtain element specific local structural and electronic information. It allows for the direct identification of the oxidation state, the coordination environment, and bond distances by measuring the absorption in the vicinity of K-edges (originating from occupied core s-levels) and L-edges (originating from occupied core p-levels). In recent years, transient x-ray spectroscopy has been developed—extending XAS into a time-resolved experiment, whereby a visible light laser pulse pumps the system into an excited state, subsequently probed by an x-ray pulse. In the near future, we are investigating the structural changes that enable a long excited state lifetime of a particular light absorber. In the farther future, we plan to use this technique to study photo-driven catalysis. Transient XAS is uniquely able to isolate the electronic and structural configuration of a catalyst's active site by combining the chemical specificity of x-rays with a pump-probe technique. One major, longer term application of this technique would be to follow the intermediate steps of water oxidation on a catalyst's surface.
Diagram of Beamline 6 at the Advanced Light Source used for ps and fs transient x-ray spectroscopy and diffraction.