A comparison of TiO₂ and ZnO photocatalysts for dye degradation and H₂ production

Abstract

The human population on Earth is currently increasing at a rate of 228,000 per day, creating huge challenges regarding: (1) clean and sustainable energy production, (2) pollution prevention and abatement and (3) water treatment. This research project directly targets these issues, through the development of TiO₂ and ZnO photocatalysts for H₂ production from alcohol-water mixtures and aqueous dye degradation. Titanium dioxide (TiO₂) and zinc oxide (ZnO) are two of the most widely studied semiconductor photocatalysts, yet few comparative studies of the photocatalytic activity of TiO₂ and ZnO nanoparticles have been reported, motivating the current investigation. Two different TiO₂ photocatalysts were investigated in this work; the commercially available Degussa P25 TiO₂ (a 6:1 Anatase:Rutile mixture, 48 m2g-1) and batches of monodisperse, submicron sized TiO₂ colloids synthesized by the hydrolysis of titanium(IV) tetraisopropoxide (TTIP) in the presence of a structure directing agent (typically dodecylamine, DDA). The synthesis of the latter involved the addition of TTIP to vigorously stirred solutions of the structure directing agent (typically dodecylamine, DDA) in a methanol:acetonitrile:H₂ O solvent mixture. The H₂ O:TTIP ratio was varied from 3.0-12.0, which yielded batches of monodisperse TiO₂ colloids with diameters 940-490 nm, respectively. The use of other structure directing agents (other alkyl amines with shorter or longer alkyl chain lengths than DDA) yielded polydisperse TiO₂ colloids. The monodisperse colloids prepared using DDA were collected by centrifugation, washed, then redispersed in ethanol. Gravitational sedimentation resulted in the formation of amorphous TiO₂ colloidal crystals, comprising TiO₂ colloids arranged on a face-centred cubic (fcc) lattice. The amorphous TiO₂ colloids contained about 30-40 wt.% DDA. Calcination of the amorphous TiO₂ colloids at 500 oC yielded low surface area anatase TiO₂ colloids (10 m²g⁻¹), whilst hydrothermal treatment at 160 oC in ethanol-water mixtures gave high surface area mesoporous TiO₂ colloids (~100 m²g⁻¹). Calcination of the TiO₂ colloids at 750 oC gave rutile TiO₂ colloids. The anatase and rutile colloidal crystals behaved as photonic crystals, possessing pseudo photonic bandgaps (PBGs) along the [111] direction at near-IR (NIR) wavelengths. The PBG position was dependent on the diameter of the TiO₂ colloids, the TiO₂ solid volume fraction and the TiO₂ phase, consistent with a modified Bragg's law expression. ZnO nanoparticles with a wide range of morphologies were synthesized using reported literature methods, which used a number of different zinc precursors (zinc nitrate, zinc sulphate, and zinc acetate), bases (NaOH and NH3) and Zn2+:base concentration ratios. Depending on synthesis conditions, these methods yielded ZnO sheets, ZnO hexagonal platelets, ZnO polyehdra or ZnO rods and rod aggregates. For each Zn salt, the product morphology was dependent on the pH and the Zn²⁺:base concentration. The photocatalytic activity of the various TiO₂ and ZnO products for aqueous methylene blue (MB) photo-oxidation were evaluated in aqueous buffered solutions at pH 6 and pH 10, respectively, under a UV flux comparable to that present in sunlight. P25 TiO₂ showed the highest photocatalytic activity for MB photo-oxidation, followed by the anatase TiO₂ colloids in aqueous buffered solutions at pH 6. Whereas, ZnO showed the highest activity in aqueous buffered solutions at pH 10. Amongst the ZnO nanoparticles, the ZnO nanosheets and hexagonal platelets showed the highest activity, which suggest that the crystal habit, in particular the fraction of exposed (0001) crystal faces, of the ZnO nanoparticles strongly influences the photocatalytic activity. The TiO₂ and ZnO photocatalysts were subsequently decorated with Au or Pd nanoparticles, and the photocatalytic activity of the resulting M/TiO₂ and M/ZnO photocatalysts (M = Au, Pd, with metal loadings 0.5 wt.% or 1 wt.%) evaluated for H₂ production from alcohol-water mixtures under UV irradiation. An alcohol:water volume ratio of 10:90 was used in all experiments, and the alcohols (sacrificial hole scavengers) tested were methanol, ethanol, ethylene glycol, 1,2- propanediol, and glycerol. In the absence of alcohol, low H₂ production rates were realised for all photocatalysts, indicating that both M/TiO₂ and M/ZnO photocatalysts are not particularly active for water-splitting under UV. For the M/TiO₂ photocatalysts, H₂ production rates increased dramatically in the presence of the alcohols, with the Pd/P25 TiO₂ photocatalysts affording the highest activities. The highest H₂ production rates were achieved for the Pd/P25 TiO₂ photocatalysts in glycerol-water mixtures (0.5 wt.% Pd/P25 TiO₂ = 39.4 mmol g⁻¹ h⁻¹ , 1.0 wt.% Pd/P25 TiO₂ = 41.4 mmol g⁻¹ h⁻¹). H₂ production rates for the M/TiO₂ photocatalysts decreased in the order glycerol > ethylene glycol ~ 1,2-propanediol > methanol ~ ethanol. Results suggest number of OH groups and α-hydrogens on the alcohol, and in turn alcohol polarity and polarizability, strongly influence photocatalytic H₂ production rates on M/TiO₂ surfaces. By comparison, M/ZnO photocatalysts displayed poor activity for H₂ production, which may be due to the photocatalytic instability of ZnO under the applied testing conditions. Results in this thesis guide the development of improved photocatalysts for dye degradation and photocatalytic H₂ production. Data demonstrates that P25 TiO₂, owing to its unique phase composition and synergistic charge transfer across rutile-anatase heterojunctions, exhibits superior photocatalytic activity to both TiO₂ colloids and various morphological forms of ZnO in these reactions. Further research is required to explore the intermediates produced during these reactions to provide a better mechanistic understanding or reaction processes.