"Electrochemically Wired” Semiconductor Nanoparticles: Toward Vectorial Electron Transport in Hybrid Materials and

1. e- hυ≥ Eg Cat. 2 H+ (aq) H2 (g) h+ Thiophene Functional Acid Ligands Porous Sol-Gel Matrix CdSe Nanoparticles e- e-PEDOT Nanowire Array MV++ ITO = Glass ITO CdSe - 2.5 e-PEDOT MV+• - 3.0 h+ - 3.5 ODA - 4.0 - 4.5 - 5.0 - 2.0 - 5.5 - 1.5 - 6.0 - 1.0 - 0.5 0.0 0.5 1.0 1.5 "Electrochemically Wired” Semiconductor Nanoparticles: Toward Vectorial Electron Transport in Hybrid Materials and Solar-Assisted Hydrogen Production” Neal R. Armstrong, Jeffrey Pyun, Dominic McGrath, Zhiping Zheng, Scott Saavedra Department of Chemistry, University of Arizona, Tucson, Arizona 85721 nra@u.arizona.edu; jpyun@email.arizona.edu Preliminary Photoelectrochemistry NanoMaterials OUR LONG TERM VISION: Synthesis of Photocatalytic Nanoparticles Objectives: Synthesis and functionalization of semiconductor nanocrystals, organic dendimers and heterodimeric nanocrytals with electron-rich conjugated thiophene precursors Efb ~ - 0.35 V I. CdSe Semiconductor Nanoparticles B. Dendritic Ligands for Nanocrystals A. Functional Nanocrystals: Ligand Exchange Thiophene Functionalized PAMAM Dendrimers Photo-assisted (filtered 150 W Xe arc lamp) reduction of methyl viologen (MV++, 1mM in acetonitrile with 0.1 M TBAHFP) on a hybrid platform. CdSe(ODA, see below) nanoparticles were spin-cast from toluene solution onto electrochemically grown PEDOT (e-PEDOT) on ITO. Electrochemically “wired” semiconductor nanoparticles (SC-NP) inclusions in microporous sol-gel (MPSG) environments Thiophene Functionalized CdSe Nanocrystals PAMAM/Thiophene Functionalized CdSe Nanocrystals Schematic representation of photocatalytic reduction of MV++. Ligand 4 E (eV) V vs NHE THF PEDOT 0.0 WORK FROM PRIOR DOE SUPPORT WHICH MOTIVATES THIS EFFORT: CdSe-NP - 2.95 eV ** PAMAM Dendrimer Capped CdSe Nanocrystals Hexadecylamine (HDA) Capped CdSe Nanocrystals MV++/MV+. Pyridine Capped CdSe Nanocrystals Thiophene Capped CdSe Nanocrystals - 3.5 eV †† The templated electrochemical growth of poly(3,4-ethylenedioxythiophene) (PEDOT) into porous sol-gel films (50:50 TEOS:MTES) on ITO was demonstrated, along with PEDOT-mediated electron transfer to ferrocene modified PAMAM dendrimers encapsulated within these sol-gel matrices (up to 20% of the ferrocene-decorated PAMAM dendrimers could be “wired” to the ITO substrate). • Heterodimeric Nanostructures: • Introduction of Catalytic Sites • for H2 Evolution C. Variation of Nanocrystal Morphology - 4.15 eV * 3-6 nm Noble Metal Nanoparticle D = 7 nm • 5.05 eV * CdSe Quantum Dot CdSe Nanorod Length = 60 nm Width = 7 nm - 5.5 eV † 20 nm 8 nm ** Value determined by Eº’ + Eg * Value determined from E º’ †UPS Data (Valence Band Onset) †UPS Data + Eg Au nanotips CdSe Nanorice Frontier orbital energies estimated electrochemically • Work in progress: • “Wiring” of SC-NPs to PEDOT/PEDOTCA-like columns. • Characterization of frontier orbitals in SC-NPs wired to conductive substrates (Au, ITO) using X-ray and UV photoelectron spectroscopies • Condutive-tip AFM to measure i/V properties of wired SC-NPs on nanometer length scales 60 nm 8 nm “Conducting Polymer Growth in Porous Sol-Gel Thin Films: Formation of Nanoelectrode Arrays and Mediated Electron Transfer to Sequestered Macromolecules,” Walter J. Doherty III, Neal R. Armstrong, S. Scott Saavedra, Chemistry of Materials, 17, 3652-3660 (2005). CdSe Nanorods Students/Postdocs supported by this effort: Clayton Shallcross; Mike Szalai; Yinglan Wu; Dr. In-Bo Shim (postdoc). Research Support: Department of Energy, Office of Science Hydrogen Fuel Initiative -- Basic Energy Sciences: Solar Hydrogen Production Leveraging Support: DE-FG02-02ER15378 – “Biomimetic Energy Transduction: Artificial Photosynthesis in a Stabilized Lipid Membrane Coupled to a Semiconductor”(Scott Saavedra, P.I.)