(375h) Porous Nanowire Assemblies On Graphene for Dye-Sensitized Solar Cells

Tremendous interest exists towards synthesizing nanoassemblies for dye-sensitized solar cells (DSSCs) using earth abundant and friendly materials with green scalable approaches.[1] Traditional approaches towards nanowires prevent multi-metallic nanowires, give low surface areas, and give a lack of surface chemistry for decorating nanoparticles. Supercritical CO₂ (scCO₂) is an enabling solvent to overcome several of these challenges  for the synthesis of nanomaterials and nanodevices due to its low viscosity, high diffusivity, and “zero” surface tension [2].  This work focused on synthesizing titanium dioxide porous and metal-doped titanium dioxide nanowires from graphene surfaces using scCO₂. [3,4]. Graphene was examined as a catalyst mat for DSSC assemblies while quantum dots were decorated on the TiO₂ nanowires.

The results showed that undoped and doped (Fe, Zr, various quantum dots) TiO₂ nanowires were grown on the surface of graphene sheets using a sol-gel method in scCO₂,whichhelped exfoliate graphene sheets. TiO₂ nanowires less than 40 nm (when scCO₂ was used as a solvent) were uniformly decorated on the graphene sheets, in which surface -COOH functionalities acted as a template for nanowire growth. A variety of bi and trimetallic nanowires were synthesized, giving enhanced optical properties and photoefficiencies in DSSCs. Fe and Zr doped TiO₂ nanowires grown on graphene showed smaller crystal sizes, gave higher visible absorption and surface areas, and higher efficiency solar cells (>4% efficiency).[5]  HRTEM images confirmed formation of porous nanowires with high aspect ratios and high metal dispersion using XPS. EELS mapping showed high dispersion of bimetallic and multi-metallic wires that could be connected directly to graphene assemblies.  Higher surface areas of bimetallic and multi-metallic nanowires on graphene were found to  enhance the electron-hole separation while reducing the band gap of the nanowires making them a potential for DSSCs. [3]. Theoretical band structures of assemblies were studied using the Vienna ab-initio Simulation Package (VASP) based on the Density Functional Theory (DFT).  Functionalized graphene sheets decreased significantly the band gap but increase the binding energy values due to carboxylate adsorption sites, hence enhancing DSSC performance.[5]

 1)      Oudel, P., & Qiao, Q. (2012). One dimensional nanostructure/nanoparticle composites as photoanodes for dye-sensitized solar cells. Nanoscale, 4(9), 2826-2838. 

2)       R. Sui, Paul A. Charpentier*, Synthesis of Metal and Silicon Oxides Nanostructures by Direct Sol-Gel Reactions in Supercritical Fluids, Chem. Rev., (2012), 112(6), 3057-3082..

3)      Nasrin Farhangi, Yaocihuatl Medina-Gonzalez , Rajib Roy Chowdhury, and Paul A. Charpentier, Using Supercritical CO₂ for growing TiO₂/Graphene Nanoassembles: Synthesis and Photocatalytic Effect, Nanotechnology (2012), 23(29), 294005/1-294005/12.

4)      Nasrin Farhangi, Rajib Roy Chowdhury, Yaocihuatl Medina-Gonzalez, Madhumita B. Ray and Paul A. Charpentier Visible light active Fe doped TiO2 nanowires grown on Graphene using Supercritical CO₂, Applied Catalysis B: Environmental 110 (2011) 25– 32.

5)      Ayissi, Serge; Charpentier, Paul; Farhangi, Nasrin; Wood, Jeffery; Palotás, Krisztián; Hofer, WernerInteraction of Titanium Oxide Nanostructures with Functionalized Graphene Sheets: A DFT Study, J. Phys Chem. C (2013) Submitted.