(3aa) Photo/Electrochemical Charge Transfer Processes: Fundamentals to Devices for Energy Scavenging and Storage

         Today, semiconductor electrochemistry plays vital roles in several technological processes such as energy scavenging by photoelectrochemical solar cells to energy storage by Li ion batteries.  Improvements in device performance require understanding the physics of the semiconductor excitonic transitions to the chemistry of the interfacial phenomena. The proposed research project is highly interdisciplinary in nature, which spans fundamental studies of interfacial phenomenon, spectroscopic techniques in ultrafast time scale, which probes exited state transitions, and synthesis of nanostructure to fabrication of photoelectrochemical solar cells and Li ion batteries.  Specific projects are discussed below.

         “Electrochemical doping” is recently discovered phenomenon in which the oxygen redox couple in adsorbed water films tends to pin the Fermi level in semiconductor at the electrochemical potential of the redox couple.^1,2  The effect, which was first noted with undoped diamond,makes it conductive. Electrochemically mediated charge transfer process is a very general phenomenon that has often been unrecognized. Since its discovery in diamond, the effect has been observed in other technologically important materials such as in single-walled semiconducting carbon nanotubes, graphene³; gallium nitride and zinc oxide.⁴  The effect can occur whenever semiconductors or other solids are exposed to humid air.  We propose to explore this effect in metal oxide and nitride nanowires.  Novel surface conducting devices and sensors that exploit this effect would be fabricated by microfabrication technique.

          Photocatalytic and photoelectrochemical activity especially in nanostructure is strongly influenced by the surface electronic properties of the semiconductor.  These properties in turn depend on the nature of defect states and band edge positions of the semiconductor with respect to the redox couple.  Thus by controlling these properties one can tailor the performance of photoelectrodes.  The proposed studies will couple the synthesis and characterization of inorganic nanowires with fundamental studies of energy absorption and transport, for high performance solar cells and Li ion batteries.   Using “band edge engineering,” novel nanowire based hybrid electrode would be designed that are stable against degradation. 

1.  Chakrapani, et al. Science 2007, 318, 1424-1430.

2.   Maier, et al.   Phys. Rev. Lett. 2000, 85, 3472-3475.

3.   Sque, et al. Phys. Stat. Sol. (A) 2007, 204, 3078-3084.

4.   Chakrapani, et al. J. Am. Chem. Soc. 2008, 130, 12944-12952.