Electron-Rich Ligands and Exchange Procedures

1. E (eV) V vs NHE PEDOT 0.0 e- CdSe(NP) hυ≥ Eg -2.0 -2.5 -2.95** Cat. -1.5 -3.0 MV++/MV+• -3.5†† -1.0 -3.5 2 H+ (aq) -4.15* H2 (g) -0.5 -4.0 h+ 0.0 -4.5 1.0 -5.0 -5.05* 1.5 -5.5 Porous Sol-Gel Matrix -5.5† 2.0 -6.0 CdSe Nanoparticles No CdSe-NPs Before Crosslinking e-PEDOT Nanowire Array After Crosslinking ITO Glass e- 180 s MV++ ITO e-PEDOT 120 s CdSe 65 s MV+• h+ 55 s 45 s 20 s 5 s 3-6 nm Noble Metal Nanoparticle D = 7 nm ProDOT-CA CdSe Nanorod Length = 60 nm Width = 7 nm ProDOT-CA 20 nm CdSe CdSe CdSe THF 60 °C 8 nm = ODA Nanoparticles are prepared with ODA ligand shells that provide solubility in non-polar solvents. Ligand-exchange is used to displace ODA with pyridine, followed by exchange with a new carboxylic acid terminated 2,3-propylene-dioxy-thiophene (ProDOT-CA). The resulting nanoparticles are capped with a dense shell of electroactive groups, capable of cross-linking with themselves (to form an oligomeric ProDOT shell), or with an intersecting PEDOT or ProDOT chain growing up from an ITO electrode. 60 nm 8 nm Photoelectrochemistry of CdSe Nanoparticle Thin Films Covalently Linked to Electron-Rich Thiophenes R. Clayton Shallcross, Gemma D. Ambruoso, In-Bo Shim, Jeffrey Pyun, Neal R. Armstrong Department of Chemistry, University of Arizona Tucson, Arizona 85721 clayshal@email.arizona.edu, jpyun@email.arizona.edu, nra@email.arizona.edu Trying to save the world… … one electron at a time. Our Goal: New Pathways to Photoelectrochemical Hydrogen Production Determination of Orbital Energies Frontier Orbital Energies UPS (He II) Data Sustainable photoelectrochemical reactions at II-IV semiconductors are difficult due to well known photo-corrosion reactions, and “back reactions” which short circuit the intended redox target. We have developed a new approach which “wires” CdSe nanoparticles to an electron-rich poly-(thiophene) (PEDOT) which will ultimately be embedded in a porous sol-gel environment.1 This thin film architecture provides the possibility of fast hole capture, isolation of the photoactive nanoparticles and vectoral electron transfer to mitigate corrosion and back reactions. Ultimately, the platform will include heterodimeric structures for efficient hydrogen generation from protic solutions. 1 = Doherty, et.al., Chemistry of Materials, 2005, 17, 3652-3660. vacuum level shift:  1.1 eV Clean Au CdSe-Py *Eo’ (fc/fc+•) ** Eo’ + Eg (Tauc method) † High KE edge (see right) † † HOMO + Eg (Tauc method) Sample prepared by self-assembly of pyridine-capped CdSe onto 1,6-hexanedithiol-functionalized Au foil. CdSe Nanoparticle Synthesis / Photoelectrochemistry of Crosslinked Films Electrochemical cross-linking of the ProDOT-CA capped NPs leads to a clear photoelectrochemical enhancement of ET to an electron acceptor (MV++). Several synthetic strategies have been evaluated for the preparation of a variety high quality II-IV semiconductor nanoparticles (SC-NPs). The methods of Peng and coworkers2 appear to give best results, using air-stable reagents, which provide particles of variable sizes, narrow size distributions, and high quantum yields and crystallinity. 2 = Peng et al. J. Am. Chem. Soc. 2002, 124, 2049-2055. Schematic representation of photoassisted reduction of methyl viologen (MV++) via CdSe-NPs crosslinked to a PEDOT substrate. Electron-Rich Ligands and Exchange Procedures Materials in Progress: Variable Morphology CdSe Quantum Dots CdSe Nanorice CdSe Nanorods Research Support: Department of Energy/Basic Energy Sciences (Solar Hydrogen Program) National Science Foundation (Chemistry) Materials Characterization Program, State of Arizona