Abstract
WO3-decorated TiO2 nanotube arrays were successfully synthesized using an in situ anodization method in ethylene glycol electrolyte with dissolved H2O2 and ammonium fluoride in amounts ranging from 0 to 0.5 wt %. Anodization was carried out at a voltage of 40 V for a duration of 60 min. By using the less stable tungsten as the cathode material instead of the conventionally used platinum electrode, tungsten will form dissolved ions (W6+) in the electrolyte which will then move toward the titanium foil and form a coherent deposit on the titanium foil. The fluoride ion content was controlled to determine the optimum chemical dissolution rate of TiO2 during anodization to produce a uniform nanotubular structure of TiO2 film. Nanotube arrays were then characterized using FESEM, EDAX, XRD, as well as Raman spectroscopy. Based on the FESEM images obtained, nanotube arrays with an average pore diameter of up to 65 nm and a length of 1.8 µm were produced. The tungsten element in the samples was confirmed by EDAX results which showed varying tungsten content from 0.22 to 2.30 at%. XRD and Raman results showed the anatase phase of TiO2 after calcination at 400 °C for 4 h in air atmosphere. The mercury removal efficiency of the nanotube arrays was investigated by photoirradiating samples dipped in mercury chloride solution with TUV (Tube ultraviolet) 96W UV-B Germicidal light. The nanotubes with the highest aspect ratio (15.9) and geometric surface area factor (92.0) exhibited the best mercury removal performance due to a larger active surface area, which enables more Hg2+ to adsorb onto the catalyst surface to undergo reduction to Hg0. The incorporation of WO3 species onto TiO2 nanotubes also improved the mercury removal performance due to improved charge separation and decreased charge carrier recombination because of the charge transfer from the conduction band of TiO2 to the conduction band of WO3.
Highlights
Mercury is one of the earliest known metals and has been used by humankind for more than 2300 years [1,2]
Activated carbon is the most effective adsorbent for mercury removal but it is too expensive for large-scale treatment [7]
Fluoride content of 0 wt % produced no nanostructures as presented in Figure 1a, where only an oxide layer of TiO2 was observed from the FESEM micrograph
Summary
Mercury is one of the earliest known metals and has been used by humankind for more than 2300 years [1,2]. Mercury can be successfully removed from a high concentration solution by membrane filtration, precipitation, ion exchange, and other methods. Activated carbon is the most effective adsorbent for mercury removal but it is too expensive for large-scale treatment [7]. A major limitation of TiO2 is its large band gap of 3.20 eV which only allows it to utilize about 2%–3% of the solar light that reaches the earth [10]. In this present study, tungsten trioxide (WO3 ) was doped onto the TiO2 nanotubes using an in situ anodization method in order to improve the photocatalytic ability of TiO2. The influence of fluoride ion content on the growth of WO3 -TiO2 nanotubes was studied, which is important in tailoring the desired length, pore size, and wall thickness of the nanotubes for a high aspect ratio (length/pore size) to achieve effective mercury removal
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