Abstract:
The projected depletion of fossil fuels in the near future motivates the development of a sustainable hydrogen (H2) economy. Realisation of the H2 economy hinges on the discovery of simple, low cost technologies for H2 manufacture, distribution and storage. With respect to H2 manufacture, noble metal modified titania (TiO2) photocatalysts such as Pd/TiO2 and Au/TiO2 display excellent H2 production rates from ethanol-water mixtures under UV fluxes comparable to those present in sunlight. This research targeted the development of efficient TiO2-based photocatalysts for H2 production from ethanol-water mixtures. The approach involved both photonic band gap (PBG) engineering and optimisation of synergies between TiO2 and added co-catalysts (Au, Pd or Pd-Au nanoparticles systems). Further optimisation of photocatalyst performance were sought through the optimisation of TiO2 crystal size, TiO2 phase composition and co-catalyst loading in the Pd-Au/TiO2 system. Photonic crystals are materials which have a periodically modulated refractive index in 1, 2 or 3-dimensions, with periods on the length scale of visible light. In order to get a handle on how to PBG engineer TiO2 for photocatalytic H2 production, an investigation into the origins of iridescence (pseudo PBGs at visible wavelengths) in some common insect species found around the Auckland region was undertaken. Examples included the Steelblue ladybird (Halmus chalybeus), the Green Bottle fly (Lucilia caesar), the Oak Leaf Miner moth (Phyllonorycter messaniella), the Golden Plusia moth (Thysanoplusia orichalcea) and the chrysalis of the Monarch butterfly (Danaus plexippus). Additionally, male peacock display feathers (Pavo cristatus) were studied and used as a direct comparison to published literature to confirm the viability of our analysis methods. All specimens were characterised by optical light microscopy, cross-sectional Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FT-IR) and UV-Vis reflectance. The cuticle of the Steelblue ladybird demonstrated a 1-D photonic crystal at the exocuticle formed by 140 nm thick chitin lamellae. The male peacock feather barbules contained 2-D arrays of melanin rods (diameters 120-150 nm) wrapped in a keratin sheath. The reflected colour of the feather barbules red-shifted as the diameter of the melanin rods increased. The gold wing scales of the two moth species contained complex 3-D architectures wherein chevron type diffraction gratings were observed on the wing scales. The gold colour of the surface nipples on the Monarch butterfly chrysalis resulted from reflection of light over a broad range of wavelengths from chitin sheets arranged in a ‘chirped’ multilayer stack, a structure also seen in the exocuticle of the Green Bottle fly. The optical properties of the Steelblue ladybird cuticle, Green Bottle fly cuticle and the peacock feather barbules were all consistent with a modified Bragg’s law expression, which considers refraction and diffraction in the photonic crystal architecture. The optical properties of all gold reflecting structures examined in this thesis require a more complex model to explain the optical properties (beyond the scope of this thesis). The natural photonic crystal studies led to the successful fabrication of a series of 3-dimensionally ordered macroporous (3DOM) Zirconia (ZrO2) thin films and powders by the colloidal crystal template technique. The films showed pseudo photonic band gaps along the [111] direction in the UV-Vis-NIR region. The optical properties of a 3DOM ZrO2 material could be tailored by changing the diameter of PMMA colloids in the colloidal crystal template. SEM, TEM and XRD analyses revealed that 3DOM ZrO2 photonic crystals comprised a face-centered cubic array of spherical macropores (diameters ranging from 150-400 nm, respectively) within a matrix of nanocrystalline tetragonal ZrO2. The specific surface areas of all samples were 30-51 m2 g-1, and independent of the macropore size. The optical properties of the 3DOM ZrO2 thin films were comprehensively characterised and were found to be in excellent accord with a modified Bragg’s law expression that considers both refraction and diffraction of light in the 3DOM architectures, and also agreed with photonic band gap structure calculations. The PBGs red-shifted on immersion in organic solvents, with the magnitude of the shift being directly proportional to the refractive index of the solvent. Solvent refractive index sensing with a precision of ~0.005 is demonstrated. Preliminary contact angle measurements for water droplets on the 3DOM ZrO2 film gave angles of 130º, indicating the 3DOM ZrO2 surface is superhydrophobic, this is entirely attributable to the rough periodic surface structure of the films on the sub-micron length scale. The next stage of research explored Pd/TiO2, Au/TiO2 and Pd-Au/TiO2 photocatalysts. Initially, Degussa P25 TiO2 was used as the support phase in order to identify the optimum co-catalyst combination and loadings for photocatalytic H2 production from ethanol-water mixtures. A series of Pd-Au/P25 TiO2 photocatalyst powders (Pd, Au or Pd + Au weight loadings = 0-4.00 wt.%) were prepared by the deposition precipitation with urea method wherein both Pd2+ and Au3+ were then simultaneously deposited on the TiO2 support. Samples were then activated under a H2 flow at 500 ºC for 2 h to form Pd, Au or Pd-Au nanoparticles. UV-Vis, XRD, XPS and TEM suggest Pd-Au alloy nanoparticles may have formed on the TiO2 surface. Photocatalytic H2 production tests were carried out in ethanol-water mixtures (80:20 and 2:98 EtOH:H2O ratios) under UV-fluxes comparable to that of sunlight. Results showed that a 0.25 wt.% Pd-0.25 wt.% Au/P25 TiO2 photocatalyst gave the highest photocatalytic H2 production rates of 68 mmol g-1 h-1 (at 80:20 EtOH:H2O) and 25 mmol g-1 h-1 (at 2:98 EtOH:H2O). These are amongst the highest H2 production rates yet reported at the two EtOH:H2O ratios and UV fluxes used in this study. A series of 3DOM TiO2 photonic crystal films and powders with PBGs along the [111] direction at UV and visible regions were then successfully fabricated using the colloidal crystal template technique. The as-prepared samples comprised a face-centred cubic array of macropores in a nanocrystalline anatase TiO2 matrix. Pd-Au/3DOM TiO2 photocatalysts were prepared using 3DOM TiO2 supports (anatase form), with PBGs along the [111] direction at 345 nm, 441 nm, 560 nm and 640 nm. The former sample showed the highest rate of photocatalytic H2 production in EtOH:H2O mixtures, which is tentatively attributed to a slow photon photocatalytic amplification mechanism wherein the reaction rate is enhanced through overlap of the 3DOM TiO2 PBG on fcc (111) or (200) planes and the electronic absorption edge of TiO2 (~385 nm). This overlap enhances light absorption by increasing the effective absorption pathlength and also serves to supress electron-hole pair recombination via inhibiting spontaneous emission. Calcining the 3DOM supports at higher temperatures before Pd and Au deposition also positively enhanced the rate of H2 evolution, which can be attributed to optimisation of TiO2 crystallite size and phase composition. A calcination temperature of 650 °C was optimal, yielding a TiO2 phase composition similar to P25 TiO2. At higher temperatures, the transformation of anatase to rutile adversely affects photocatalytic performance. Results guide the development of new and improved semiconductor photocatalysts for solar H2 production from H2O and biofuels. Based on results presented, the Pd-Au/P25 TiO2 system should be further explored and optimised as it has great promise due to the very low metal loadings needed to achieve high rates of H2 production.