The Role of Metal Electrode and Nanoparticle Surface Functionalization in a Hybrid Biological/Organic Device for Hydrogen Production

Open Access
Author:
Grimme, Rebecca Ann
Graduate Program:
Chemistry
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 23, 2009
Committee Members:
  • John H Golbeck, Thesis Advisor
Keywords:
  • photocatalytic hydrogen production
Abstract:
The generation of H2 by solar energy conversion is a promising way to supply the world’s energy needs as a means to supplant fossil fuels. All photocatalytic systems contain 3 components: a photochemical module that harnesses the sun’s energy, a catalytic module that generates H2, and a means of linking the two modules together. To this end a hybrid biological system has been designed that incorporates a dithiol molecular wire to connect the terminal [4Fe-4S] cluster of Photosystem I (the photochemical module) with either a Pt or Au nanoparticle (the catalytic module). The sulfhydryl group at one end of the molecular wire allows for the functionalization the nanoparticles via a metal-sulfide (either Au-S or Pt-S) bond and the other sulfhydryl group can chemically rescue the [4Fe-4S] cluster, of Photosystem I, thereby generating a strong coordination bond. The use of sacrificial electron donors enables this bioconjugate system to evolve H2 when continuously illuminated. In an effort to maximize H2 production, the pH and ionic concentration of the solution, the mobility of the electron donor, the length and degree of saturation of the molecular wire, and the intensity of the light were systematically investigated. Optimal conditions included a solution buffered at pH 6.0, cross-linked plastocyanin, rebuilt spinach PS I, and the use of 1,4-benzenedithiol to connect PS I to the Pt nanoparticle. Illumination of this optimized Photosystem I/dithiol molecular wire/nanoparticle bioconjugate at a light intensity of 70 μE m-2 s-1 generated H2 at a rate of 312 μmol H2 mg Chl-1 h-1 over the course of 24 hours. The success of these studies has led to the belief that a functional device that utilizes these PS I/molecular wire/nanoparticle bioconjugates could be constructed. Unfortunately, inherent inefficiencies exist within this system due to the use of sacrificial electron donors and the reliance on diffusion chemistry for delivery of electrons to PS I. By directly linking the bioconjugates to an electrode surface, a cathodic half-cell would be created that would be capable of generating H2 when illuminated. Initial studies have been conducted to look at the interactions between PS I and a gold electrode surface. Ellipsometric measurements and atomic force microscope imaging have revealed that PS I generally maintains its native structure and forms relatively dense monolayers on both –hydroxyl and –carboxyl terminated alkanthiol-capped gold surfaces. This technology could easily be transitioned into a device that incorporates the PS I/molecular wire/nanoparticle bioconjugates for photocatalytic H2 production.