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Overview

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.

A main 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 to enhance the lifetime of charge separated species, which inherently requires an understanding of fundamental electronic structure and underlying dynamics. In collaboration with the Osterloh group (UC Davis), the transient charge separation and recombination kinetics in several intriguing systems were explored. Experimental effort to date was directed toward exploring two categories of 2-D substrates as useful photocatalysic systems: (1) exfoliated niobiate [H_1-xCa₂Nb₃O₁₀]^x- (x = 0-0.15) and titanate A₂Ti₄O₉ (A = K, H) nanosheets and (2) CdSe nanoribbons. Both groups of materials are demonstrated to exhibit photochemical hydrogen evolution under irradiation with ultrafast and visible excitation light.

Three goals were identified in the original PRF proposal:

1) Resolving nanosheet photodynamics after direct excitation with UV and blue-light excitations,

2) Resolving the photodynamics of secondary inorganic nano-materials as possible light-absorbing components in multi-component systems and

3) Resolving the dynamics of constructed two- and three-component systems from materials explored in Goal 1 and Goal 2.

Effort toward all three goals has been accomplish this past year as outlined below resulting in one published manuscript and two currently in preparation.

Results

Exfoliated niobate and titanate nanosheets: The ultrafast (<100fs) 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 are probed directly via their broad absorption bands as resolved with the existing ultrafast laser setup. To characterize the long-time (10ns 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 resolved a 43-ms decay time for the electron-hole recovery dynamics, which are demonstrated to be appreciably slower than the recovery dynamics of 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 are argued to originate either from deep trap sites on the sheets or from more effective electron-hole separation because of the micrometer dimensions of the 2D lattice. This effort resulted in one published manuscript.

The influence of the micrometer dimensions of the 2D lattice on the electron-hole recovery dynamics of HCa₂Nb₃O₁₀ nanosheets were explored in both the fs and ms time regime. We identified the role of edge trap sites vs. surface trap sites on recovery dynamics by modifying the ratio of trap sites by via variable sonication times. This effort resulted in a manuscript in preparation.

Photodynamics of CdSe nanopaticles and nanoribbons: The influence of morphology on the photodynamics of metal sulfides were explored and compared to the metal oxide nanosheets studied above. CdSe nanoribbons show catalytic activity for photochemical hydrogen evolution from aqueous Na₂S/Na₂SO₃ solution under irradiation with ultraviolet and visible light. The transient kinetics for CdSe nanoparticle and nano-ribbons in water and in 0.1 M Na₂S/ Na₂SO₃ (as a hole scavenger) solution were characterized. We demonstrated that the addition of the hole scavenger leads directly to an increase of trapped electrons that is ascribed to the inhibition of recombination dynamics caused by the photo-reduction of holes by Na₂S/ Na₂SO₃. This enhancement of the trapped electron lifetimes with the sacrificial electron donor increases the photogenerated H₂ evolution rate. The ultrafast photodynamics and hydrogen generation capacity of CdSe nanoparticles and nanosheets are currently in progress. This effort resulted in a manuscript in preparation.

3. CdSe/MoS₂ co-catalyst dynamics: The study of multi-component systems under Goal 3 was initiated with materials that have MoS₂ combined to the CdSe nanosheets explored above. It was demonstrated that increasing the MoS₂ relative amount increased the photocatalytic activity before reaching a plateau and then eventually suppressing activity. We have just started to explore the underlying enhancement and quenching mechanisms in this two-component system.

Summary and Second year effort

The goals for the next year in the grant include completion of the two manuscripts in preparation discussed above and to continue the exploration of the CdSe/MoS₂ co-catalyst photodynamics. The extension into exploring two-component materials with metal oxides will be emphasized with the goal of resolving clear vectoral charge separation and recombination dynamics.

The development and understanding of nano-scale dynamics afforded by this ACS-PRF grant was leveraged toward the submission, as a co-PI, of a large scale NSF (SOLAR) proposal concentrating on the use of nanoparticles for photovoltaic applications. The development of efficient photovoltaic and photocatalysic materials involves similar underlying themes of generating and maintaining persistent charge separation (either for REDOX chemistry in photocatalysis or for charge extraction in photovoltaics). Understanding the basis for one has potential for advancing the other field.