Environmental Molecular Sciences Laboratory

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Crystal face influence on hematite-organic ligand interactions at iron oxide/water interfaces studied using nonlinear

Date: 
Monday, December 17, 2018
Principal Investigator: 
Amanda Mifflin
Lead Institution: 
University of Puget Sound
Closed Date: 
Monday, September 30, 2019
Project ID: 
50664
Abstract: 

The proposed work will examine the interfacial phenomena of the organic ligand acetohydroxamic acid (aHa) at the hematite/water interface and the molecular structure at organic/aqueous interface using the nonlinear optical spectroscopy techniques second harmonic generation (SHG) and sum frequency generation (SFG) and molecular simulation methods. A variety of hematite crystal faces and electrolyte compositions will be explored. Experiments will focus on absolute surface charge densities and surface pKa values for the hematite/water interfaces, as well as monitoring the change in the aHa signal as the aHa is exposed to iron oxide/water interfaces at varying pH and aHa concentrations. These studies will provide direct evidence of surface coverage, structure and kinetics of complexation and dissolution. Polarization and phase measurements will be conducted to examine adsorbate surface orientation. Molecular dynamics simulations and quantum chemistry simulations will be used to gain molecular-level insight into the adsorption of aHa to hematite surfaces. Results from such experiments on aqueous organic ligand-iron oxide interfaces will provide crucial information about biogeochemical iron and carbon cycling, leading to improved global models.
The SHG experiments will be conducted at the University of Puget Sound using thin-film hematite samples fabricated by EMSL. The SFG experiments will be conducted in the Environmental Spectroscopy Laboratory at EMSL, also using thin-film hematite samples fabricated by EMSL. This work is dependent on access to the Deposition and Microfabrication and Nonlinear Spectroscopy capabilities at EMSL. Hematite thin films (~20 nm thick) will be deposited onto transparent substrates using Pulsed Laser Deposition (PLD) for use in the SHG and SFG experiments (various crystal faces of hematite will be fabricated). PLD is not available at the University of Puget Sound. The thin film is required to avoid absorption of the laser light and burning of the sample while maintaining optical reflectivity. The SFG experiments must be conducted at EMSL, as the Environmental Spectroscopy Laboratory houses the most convenient SFG spectrometers to the University of Puget Sound, and the only high-resolution SFG spectrometer currently available for use in the United States.