Plasmonic Materials and Their Applications in Surface Enhanced Raman Spectroscopy and Solar Hydrogen Production

Abstract

This project focused on the development of novel plasmonic materials for (i) the low threshold detection of adulterants in food and beverage systems using Surface Enhanced Raman Spectroscopy (SERS); and (ii) solar hydrogen production from biofuels. Chapter 1 and 2 provide background context for each of these studies, whilst chapter 3 describes key methodologies used in the multidisciplinary research. 3D SERS substrates were prepared using 3-dimensionally (3D) ordered macroporous (3DOM) MO2 (M = Si, Ti, Si1-x-TixO2 or Zr) thin films using the colloidal crystal template technique. The 3DOM MO2 films, comprising a face-cubic-centred (f.c.c) array structure of submicron sized macropores, possessed angle dependent structural colour and pseudo photonic bandgaps (PBGs) at visible wavelengths along to [111] direction. After decoration with Au nanoparticles at various Au loadings, these Au/3DOM MO2 films were successfully used for solvent refractive index sensing (both the Au localised surface plasmon resonance and the [111] PBGs red-shifted linearly with solvent refractive index) and SERS detection of melamine via aqueous solutions. All Au/3DOM MO2 demonstrated excellent SERS activities with SERS enhancement factors (EFs) up to 1010 for aqueous melamine detection, enabling the detection limit as low as 10-6 M. SERS enhancements of these substrates decreased in the order Au/TiO2 ≈ Au/ZrO2 > Au/Si-TiO2 > Au/SiO2. The enhanced SERS responses in melamine detection can be explained by the presence of more SERS hotspots (with strong metal support interaction) in Au/TiO2, Au/ZrO2 and Au/Si-TiO2. Hydrophobicity and hydrophilicity of Au/3DOM MO2 may also play a role (i.e. hydrophobicity would decrease upon Au deposition, especially for Au/ZrO2). Different pH dependent forms of melamine and the formation of melamine-cyanuric acid complex (at pH = 1) could be discerned in the SERS spectra. Coadsorbed chloride appeared to be important for the protonated melamine adsorption in Au/3DOM MO2 films at low pH. Results confirm the Au/3DOM MO2 thin films are a robust and versatile optical sensing platform. For the solar hydrogen production work, a comprehensive series of M/TiO2 (M = Pd, Au or Pd-Au) photocatalysts using Degussa P25 TiO2 (85 wt.% anatase + 15 wt.% rutile) as the support phase were prepared using deposition-precipitation with the Urea (DP-Urea) method. Calcination, H2 reduction or galvanic reduction were used to generate Au/TiO2, Pd/TiO2 and Pd-Au/TiO2 photocatalysts. TEM, XRD, EDS, XPS and XRF confirmed the presence of metal nanoparticles for each photocatalyst. A high metal dispersion is found in Pd/TiO2 and Pd-Au/TiO2 systems, consistent with a strong Pd-TiO2 metal support interaction. Photoluminescence (PL) data revealed that all metal co-catalysts effectively suppressed the electron-hole (e--h+) pair recombination in TiO2. Photocatalytic H2 tests in various alcohol-water mixtures showed that the rate of H2 activity depended on the metal co-catalyst; co-catalyst type and the sacrificial reagents with the activity decreased in the following order: Pd-Au/TiO2 >Pd/TiO2 > Au/TiO2. Optimum co-catalyst loadings were 0.9 wt.% for Au/TiO2, 0.5 wt.% for Pd/TiO2, and 0.125 wt.% Pd-0.25 wt.% Au/TiO2 (H2 reduction). The effectiveness of each metal for promoting photocatalytic H2 production mainly depended on the work function of the metal, extent of the metal support interaction, as well as M-H bond strength. Pd having the highest work function (Φ = 5.6-5.7 eV), compared to Au metal (Φ = 5.4 eV), formed a more efficient Schottky junction with TiO2, whilst density of valence d-states present on Pd was effective in trapping and accepting photogenerated electrons in TiO2. Bimetallic Pd-Au/TiO2 photocatalysts demonstrated superior H2 production rates compared to monometallic based photocatalysts, showing a positive synergistic effect between Pd and Au nanoparticles on H2 production rates, which decreased in the order glycerol > 1,2-ethanediol > methanol > ethanol for all M/TiO2 photocatalysts. Physical properties of alcohols such as alcohol polarity, polarizability, viscosity, and the exponential of potential separation change between the VB of TiO2 and alcohols also affect the rate of H2 activities. Results of this PhD study support global research efforts relating to food security and the development of a sustainable H2 economy.