AmericanChemicalSociety.com

Background: Solar energy is the only inexhaustible energy source abundant enough to satisfy all the energy needs of our planet, but is only practical if an extensive solar-based infrastructure can be deployed and operated in an environmentally friendly manner. Developing the sophisticated devices of this infrastructure that efficiently harnesses solar energy is one of the greatest scientific, technological, economic, and moral challenges of our time, and requires collaborative effort from researchers with expertise from a wide range of scientific and technological areas. Support from the PRF-G was used to initiate the PI’s effort in applying time-resolved (specifically ultrafast) spectroscopic measurements to characterize the photoinitiated dynamics that underlying the efficiency of multi-component water-splitting catalysts constructed with nanometer-scale inorganic materials. The ultimate goal is to understand and apply the information on the rapid dynamics of charge separation, transfer and eventual recombination toward developing novel materials capable of coupling incident solar radiation into useful hydrogen fuels.

An important issue limiting photocatalysis of water as a potential source of hydrogen fuel is the inability to maintain charge separation of electrons and holes for a long enough period of time to allow the required redox chemistry to occur. The first step in overcoming this issue is to develop catalytic materials that enhance the lifetime of charge separated species, which inherently requires an understanding and characterization of the fundamental electronic structure and underlying dynamics of potential samples. In collaboration with the Osterloh group (UC Davis), the transient charge separation and recombination kinetics in several intriguing systems were explored.

First year results: The first year of support, the primary and secondary dynamics of exfoliated niobate and titanate nanosheets were resolved. The (<100 fs) electron-hole generation and subsequent recombination kinetics of TBA2-Ti4O9 nanosheets were resolved with transient absorption spectroscopic measurements on suspensions in water. The resulting trapped holes and electrons trapped in mid gap states were probed directly by their broad visible absorption bands as resolved with the existing ultrafast laser setup. To characterize the secondary, long-time (10 ns to ms) recombination kinetics, we constructed a transient photolysis setup that used excitation light from the ultrafast setup or a separate Nd:YAG laser. This new setup was used to resolve a 43-ms decay time for the electron-hole recovery dynamics in the TBA2-Ti4O9 nanosheets, which exhibit appreciably slower recovery dynamics than suspended TiO2 nanocrystals (a common UV photocatalyst) with a 68-ns time constant. The 6 orders of magnitude slower kinetics observed for the TBA2-Ti4O9 nanosheets originate energetically- either from deep trap sites or kinetically- hinder kinetics due to the increases range of mobility for the micrometer dimensions of the 2D lattice. This effort resulted in one published manuscript in Chemistry of Materials.

Second year results: The second year effort concentrated on the primary photodynamics of CdSe nanomaterials, specifically zero-dimensional nanoparticles (NP) and two-dimensional nanoribbons (NR). Although we previously demonstrated that the CdSe NRs exhibited photocatalytic H2 generation capacity [Chem. Commun. 2206-2208 (2008)], we observed that the CdSe NPs exhibited a 15-fold greater capacity in aqueous Na2S/Na2SO3 solution under irradiation with visible light (> 400 nm). This was the impetus to explore the femtosecond photodynamics of both systems in more detail in water, which had not previously been measured. For the CdSe NRs, we observed a long living, spectrally-broad emissive bound exciton population that persists for >10ps before trapping to lower energy states resulting in non- or weakly emissive excitons. This originates from the unique low-defect (aka traps) CdSe NR material. In contrast, typically NPs exhibits excitons with stimulated emission that persists for <1ps if at all. We demonstrated that the addition of the hole scavenger weakly increases trapped electrons populations and inhibits recombination dynamics between the holes and electrons, which in turn increases the photogenerated H2 evolution rate. However, the added HS- scavenger main influence is more substantial with the long time kinetics (>20 ns) to result in the magnitude change of H2 generation capacity. The broad, long living emission population in the CdSe NRs could have an important impact in developing tunable or mode-locked nanolasers.

In contrast to the 2-D CdSe NRs, the photodynamics of the water solubilized CdSe NPs exhibit significant HS- dependent femtosecond dynamics. This is described within the context of a rapid redox reaction coupled to multi-particle Auger dynamics to generate high energy trapped charges, which facilitate H2 generation. Interestingly, the experimental synthesis protocol for making the citrate-ligated water-solubilized CdSe NPs generate samples that exhibit low frequency oscillations (0.1 cm-1) in the transient signals. These are particularly exciting as they are two low frequency for acoustic or optical phonon quantum beats and may be ascribed to electronic quantum beats between the fine splitting of the lowest energy electron state. If correct, phase coherence is preserved in these room temperature systems over long timescales, which have only been observed in <10K samples. Further exploration of these oscillations and their influence from the synthesis protocol is underway. Two manuscripts (as a series) are currently in preparation: one concentrating on the CdSe NRs and the other on the CdSe NPs.

Greater Impact: Support from the PRF was leveraged into a continuing effort for out laboratory in resolving and understanding how the primary and secondary dynamics underlying the photoexcitation of inorganic nanoparticle systems can be coupled into useful function. The experimental results from the PRF grant enabled the successful funding of a multi-laboratory National Science Foundation SOLAR proposal that investigates the photodynamic processes in inorganic photovoltaic systems. Many of the photoinduced dynamics in these systems is qualitatively similar to the dynamics in the photocatalysts studied above. Results from the PRF were also leveraged in the submission of two NSF proposals, currently under review, for investigating organic photovoltaic dynamics.

One postdoctoral researcher and three graduate students were supported by PRF funds during the funding period. One, of which, has graduated and a second graduate student will graduate in June 2011. Support by the PRF was an important component in developing the energy related effort and we are appreciative for its receipt.