Publication: Nanoporous anodic oxide formation on niobium and niobium-titanium alloys for photoreduction of hexavalent chromium
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Date
2023-09-01
Authors
Nurhaswani Binti Alias
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Abstract
Niobium pentoxide (Nb2O5) nanoparticle, a semiconductor-based compound, has
demonstrated significant potential in reducing toxic heavy metal ions such as hexavalent
chromium, Cr(VI) to trivalent chromium, Cr(III), comparable to titanium oxide (TiO2).
However, the used of this nanoparticle, can create secondary pollutant which is time
consuming for the separation step and not economical. The immobilization photocatalyst
is one of the best options to address this problem. In this project, the Nb2O5 has been
grown on a supported Nb substrate using the anodization procedure and employed right
away as a photocatalyst in Cr(VI) solution. The elimination of Cr(VI) from the system is
quick and complete when this photocatalyst is used, allowing for repeated uses. The
anodization was conducted in fluorinated ethylene glycol (EG) electrolyte at varying
anodization parameters included applied voltage, NH4F concentration, anodization time,
and electrolyte temperature to optimize the morphology and structural properties of
Nb2O5. Anodization of Nb foil at 60 V and 30 min produced weak crystalline
orthorhombic Nb2O5 with “pore-in-pore” nanoporous structure. No crystallite Nb2O5
formed when anodization occurred <60 V. Prolonging the anodization time resulted in
high dissolution of Nb oxide which are undesirable for the application. The annealing
process increased the crystallinity while also influencing the surface and optical
properties. When compared to the as-anodized sample, the Nb2O5 annealed sample has a narrow optical band gap, but poor charge separation. As-anodized Nb formed at 60 V and
70 °C electrolyte temperature achieved the highest Cr(VI) reduction (100% reduction
within 75 min, surface area: 4 cm2
) compared to annealed sample (100% reduction within
120 min, surface area: 4 cm2
), but annealed sample has better cyclability performance
(92% of Cr(VI) reduction remained after the fourth cycle). Due to the high recombination
rate of photogenerated electron-hole pairs of Nb2O5 and TiO, the TiO2-Nb2O5 mixed
oxides were synthesized as well by anodization of TiNb binary alloys in fluorinated EG
electrolyte. The TiNb binary alloys was fabricated at different Nb content (wt. %) to
evaluate the effect of different composition on morphology and structural properties of
mixed oxides TiO2 and Nb2O5 after the anodization. Vertically aligned nanotubular oxide
structures were successfully formed on the surface of Ti and TiNb binary alloys pellets as
substrate, except on pure Nb, which exhibited the nanopores morphology. The Nb
composition in the TiNb binary alloys has an effect on the oxide formation; increasing the
Nb content (up to 40 wt.%) resulted in thicker oxide, with sample Ti-40Nb forming the
longest nanotubes. Ti-60Nb outperformed all other TiNb binary alloys compositions in
photoreduction of Cr(VI) in UV with 100% reduction within 120 minutes (surface area:
1.33 cm2
). Furthermore, the TiNb mixed oxide exhibits improved photoactivity properties
when compared to TiO2 or Nb2O5 alone, owing to excess electrons in TiO2's conduction
band that are transferred from Nb2O5 to Cr(VI)