Abstract
High surface area, self-organized nanoporous ZrO2 arrays with perfect adhesion to the Zr substrate were synthesized by anodization in an aqueous electrolyte containing (NH4)2SO4 and NH4F. The obtained semiconductor materials were tested as photocatalysts for decolorization of the methyl red (MR) as a model azo dye pollutant. It was demonstrated that as-synthesized anodic ZrO2 anodic layers are already crystalline and, therefore, do not require further thermal treatment to provide a high photocatalytic performance. However, photocatalytic efficiency could be improved by annealing at a relatively low-temperature of 350 °C. Higher annealing temperatures caused a gradual drop of photocatalytic activity. The photocatalytic behavior was correlated with the crystal phase transformation in anodic ZrO2. It was found that higher photocatalytic activity was observed for the tetragonal phase over the monoclinic phase (predominant at elevated temperatures). It results from the optimal and complex electronic structure of annealed ZrO2 with three different energy states having absorption edges at 2.0, 4.01 and 5.28 eV.
Highlights
Zirconium dioxide (ZrO2 ) is found to be a multipurpose material for special applications
The second anodizing step induces higher current densities than those observed for the first anodizing step. Such an increase in the current density is characteristic for two-step anodizations, since the growth of pores at the second step occurs much more on the pre-patterned Zr surface formed in the first anodizing step
The current–time curves demonstrate an exponential decay of current density with time, which is typically observed for hard anodization conditions and indicates a predominance of oxide formation over oxide dissolution [28,29,32]
Summary
Zirconium dioxide (ZrO2 ) is found to be a multipurpose material for special applications. Since its high biocompatibility and low cytotoxicity, it is a promising material for biomedical implants [5,15,16,17]. ZrO2 is an n–type semiconductor which makes it a potential candidate for photo- (PC) and photoelectrocatalytic (PEC) applications [18,19,20,21,22]. The properties of ZrO2 are often compared with TiO2 (benchmark photocatalyst) as they belong to the same IV-group transition metals oxides [23]. ZrO2 is characterized by a wider band gap (∼5 eV) as well as a conduction band shifted towards more-negative (−1.0 V vs NHE) and valence band towards more-positive (4.0 V vs NHE)
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