Monna Rozana

Photocatalytic performance of freestanding tetragonal zirconia nanotubes formed in H2O2/NH4F/ethylene glycol electrolyte by anodisation of zirconium

ZrO2 nanotubes (ZrNTs) were produced by anodisation of zirconium foil in H2O2/NH4F/ethylene glycol electrolyte. The as-anodised foils were then soaked in the anodising electrolyte for 12 h. Soaking weakens the adherence of the anodic layer from the substrate resulting in freestanding ZrNTs (FS-ZrNTs). Moreover, the presence of H2O2 in the electrolyte also aids in weakening the adhesion of the film from the foil, as foil anodised in electrolyte without H2O2 has good film adherence. The as-anodised FS-ZrNTs film was amorphous and crystallised to predominantly tetragonal phase upon annealing at >300 °C. Annealing must, however, be done at <500 °C to avoid monoclinic ZrO2 formation and nanotubes disintegration. FS-ZrNTs annealed at 450 °C exhibited the highest photocatalytic ability to degrade methyl orange (MO), whereby 82% MO degradation was observed after 5 h, whereas FS-ZrNTs with a mixture of monoclinic and tetragonal degraded 70% of MO after 5 h.

Effect of KOH added to ethylene glycol electrolyte on the self-organization of anodic ZrO2 nanotubes

ZrO2 nanotube arrays were formed by anodizing zirconium sheet in ethylene glycol (EG) and EG added to it KOH (EG/KOH) electrolytes. The effect of KOH addition into EG electrolyte to the morphology of nanotubes and their crystallinity was investigated. It was observed that the tubes with diameter of ∼80 nm were formed in EG electrolyte with <0.1 vol % water, but the wall smoothness is rather poor. When KOH was added into EG, the wall smoothness of the nanotubes improve, but the diameter of tubes is smaller (∼40 nm). Despite smoother wall and small tube diameter, the degradation of methyl orange (MO) on the tubes made in EG/KOH is less compared to the tubes made in EG only. This could be due to the less tetragonal ZrO2 presence in the tubes made in EG/KOH.

Formation of Anodic Oxide Nanotubes in H2O2 - Fluoride Ethylene Glycol Electrolyte as Template for Electrodeposition of α-Fe2O3

Anodic oxidation of titanium (Ti), zirconium (Zr) and niobium (Nb) foils in fluoride ethylene glycol (EG) added to it 1 H2O2 as oxidant was done to produce oxide film with nanostructures at 40 V. Whilst arrays of aligned nanotubes were successfully formed on the surface of Ti and Zr respectively, anodic Nb2O5 was found to consist of nanoporous structure with pore size of ~ 20 nm. Despite long nanotubes were formed on both Ti (2 μm) and Zr (3 μm), the surface of the nanotubes suffered from severe dissolution, thinning the wall and collapsing them. Well defined, ordered surface structure of the nanotubes is required as they will be used as template for subsequent deposition of nanoparticles. This was achieved when Ti anodised in 5 ml H2O2 fluoride EG. With excess H2O2 etching at the surface occur more uniformly forming homogenous surface structure. α-Fe2O3 were then electrodeposited on this surface at-3 V from chloride solution and the mode of formation is believed to be due to electrogeneration of base at the surface of the TiO2.

Characterizations and photoelectrochemical properties of Fe2O3 and ZrO2 nanotubes formed by anodic oxidation process

Anodization of Zr and Fe foils in fluoride ethylene glycol produced surface oxide with nanotubular structure. After heat treatment, surface oxide on Fe foil crystalized to predominantly α-Fe2O3 (hematite) and on Zr foil tetragonal/monoclinic ZrO2 was produced. Anodization of both Zr and Fe for anodic nanotubes formation were done in ethylene glycol/NH4F/KOH electrolyte at 50 – 60 V. The anodized foils were used as photoanode in a photoelectrochemical (PEC) cell. At 0.5 V vs Ag/AgCl, photocurrent of a PEC cell comprising of α-Fe2O3 photoanode was 0.22 mA cm−2. PEC with ZrO2 photoanode displayed smaller photocurrent of 0.02 mA cm−2 at 0.5 V in 1M NaOH electrolyte.