Development of Novel Au/TiO2 Photocatalysts for Solar Hydrogen Production

Abstract

Concerns about the future availability of fossil fuels for electricity generation and transportation, together with the negative environmental impacts of fossil fuel use in these applications, motivates the search for alternative renewable energy technologies. Efficient harnessing of solar energy to either directly generate electricity (using photovoltaic devices) or to generate energy carriers such as hydrogen (using photoelectrochemical or photocatalytic systems) is widely viewed as the best approach for achieving future energy supply security. The development of a sustainable Hydrogen Economy, in which H2 will replace fossil fuels for electricity generation and transportation, hinges on the discovery of simple, low cost and sustainable technologies for H2 manufacture, distribution and storage. This thesis research supports the growth of a sustainable H2 Economy, and is aimed at the development of efficient Au/TiO2 photocatalysts for solar H2 production from alcohol-water mixtures. The initial aim of this research project was to examine the effect of Au loading and TiO2 support composition on the activity of Au/TiO2 photocatalysts for H2 production from ethanol-water and methanol-water mixtures under UV excitation (365 nm, 6.5 mW cm-2). A series of Au/TiO2 photocatalysts (Au loadings = 0-10 wt.%) are prepared by the deposition-precipitation with urea method, using Degussa P25 TiO2 (85 wt.% anatase, 15 wt.% rutile) as the support phase. TEM analyses showed that all of the photocatalysts comprised supported Au nanoparticles (average size 4-5 nm) located at interfacial sites between TiO2 particles. UV/Vis, XRD, XPS and photoluminescence measurements confirmed that the supported Au was in metallic form. H2 production tests were carried out under liquid slurry conditions, with the photocatalysts suspended in ethanol/water mixtures or methanol/water mixtures of different composition (ranging from pure water to pure alcohol). In the ethanol-water system, the highest hydrogen production rates (~34 mmol g-1 h-1, quantum yield = 22 %) were achieved at an ethanol:water ratio of 80:20 and Au loadings of 0.5-1 wt.%. For the methanol:water system, the optimum Au loading was the same, but the best methanol:water stoichiometry was 50:50. The optimal H2 production rates in these systems correspond to ~14 L kgCatal -1 min-1, comparable to the requirements of a 1 kW PEM fuel cell (15 L of H2 min-1). Results confirm that Au nanoparticles serve as cathodic sites for H2 production during reaction (by accepting electrons photoexcited in TiO2), whilst the presence of alcohol is necessary for achieving high H2 production rates by acting as a sacrificial hole scavenger and proton source. The rates of H2 production were enhanced by 20-40 % under simultaneous UV and visible excitation (both of intensity comparable to that in sunlight), due to stimulation of the Au localised surface plasmon resonance (LSPR) at 560-580 nm. The exact mechanism for this enhancement is unclear, though could be due to localised heating effects at the photocatalyst surface. The high hydrogen production rates achieved in the Au/TiO2 system when P25 TiO2 was used as the support phase suggested a likely synergistic interaction between the three components phases (anatase, rutile and gold). To explore this synergy, and the extent to which it promoted H2 production, pure anatase and rutile fractions were isolated from P25 TiO2 by selective chemical dissolution and then functionalized with Au nanoparticles (3 wt.% loading). H2 production rates in ethanol-water mixtures (under UV and UV/visible light) for the Au/anatase and Au/rutile photocatalysts were substantially lower than those determined for an Au/P25 TiO2 photocatalyst at the same gold loading. EPR studies suggest that in P25 TiO2 electrons migrate from the conduction band of rutile to anatase lattice traps across interfacial surface sites. Electron transfer from interfacial sites to Au is strongly dependent on the location of the gold nanoparticles on the TiO2 support. Synergistic electron transfer between the TiO2 polymorphs and Au nanoparticles is responsible for the higher rates of H2 production realised in the Au/P25 TiO2 system. Three phase interfacial sites, involving anatase, rutile and gold nanoparticles, are identified as catalytic ‘hot-spots’ for H2 production from alcohol-water mixtures. TiO2 inverse opals comprise a face-centred cubic (fcc) array of air spheres in a TiO2 matrix. Due to their three dimensional periodic structure, with periods on the length scale of visible light, these materials possess photonic band gaps (PBGs) which can be used to control and manipulate the flow of light. In particular, the ability of TiO2 inverse opals to reduce the group velocity of light at the PBG edges and thus suppress spontaneous emission (i.e. electron-hole pair recombination) makes these materials attractive as photocatalysts. Here, six different TiO2 inverse opal photonic crystals with PBGs along the [111] direction at 300, 345, 407, 491, 563 and 692 nm were fabricated in the form of powders and thin films by the colloidal crystal template technique. Colloidal crystals (synthetic opals) composing monodisperse PMMA colloids arranged on a fcc lattice were used as sacrificial templates. The PMMA colloidal crystal templates and their TiO2 inverse opal replicas all displayed angle dependent structural colour, as is typical for photonic crystals. The optical properties of these materials were consistent with photonic band structure calculations by the planewave expansion method based on their geometric structure and composition. The photocatalytic properties of the TiO2 inverse opals were first evaluated through gas-phase ethanol decomposition experiments under UV excitation. Slow photon photocatalytic enhancement was observed for the TiO2 inverse opal with a PBG along the [111] direction at 345 nm, due to overlap between the red edge of the PBG and the TiO2 absorption edge at 388 nm. At the edges of the PBG, electromagnetic radiation travels with a strongly reduced group velocity, which in this particular case served to both enhance momentum transfer between incident photons and the semiconductor, as well as suppress electron-hole pair recombination in TiO2. Decoration of the TiO2 inverse opals with gold nanoparticles greatly enhanced their activity for photocatalytic H2 production from ethanol-water mixtures, by the same mechanism seen in the Au/P25 TiO2 system. Results guide the development of next generation semiconductor photocatalysts for H2 production from biofuels.