ZnO And MoSexOy Modified TiO2 Nanotubes For Photoelectrochemical And Sensing Applications
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Date
2018-09
Authors
Ng, Siow Woon
Journal Title
Journal ISSN
Volume Title
Publisher
Universiti Sains Malaysia
Abstract
One-dimensional (1D) anodic self-organized TiO2 nanotube layers have attracted
considerable scientific and technological interest over a decade. The major drawbacks
are their rather wide band gap energies (3.0–3.2 eV) with high photoactivity only in the
ultraviolet (UV) spectral region (l 390 nm, 5% of the solar spectrum), relatively
low conductivity and high carrier recombination rate. Furthermore, as a high aspect
ratio (HAR) nanostructure, the deposition of a uniform secondary coating within the
TiO2 nanotube layers remains a challenge. To overcome these critical issues, present
work is intended (i) to synthesize anodic 1D TiO2 nanotube layers as an independent
semiconductor and for the incorporation of secondary materials; (ii) to produce homogeneous
and conformal ZnO and MoSexOy coatings by atomic layer deposition (ALD)
within TiO2 nanotube layers; (iii) to form heterostructure in order to enhance charge
transport efficiency, increase light absorption and extend the functional range to the visible
spectral region; and (iv) to evaluate UV photodetection responses, ethanol sensing
responses at low temperatures, photocurrent generation and photocatalytic degradation
of methylene blue (MB). The synthesis of the TiO2 nanotube layers was carried
out by a conventional electrochemical anodization of Ti sheet in fluorinated organic
electrolytes. A low bias, visible-blind, UV photodetector was constructed in a sandwich
structure comprising of ITO/electrolyte/TiO2 nanotubes/Ti for the first time to
investigate the photodetection in UV-A, B and C spectral regions. The highest sensitivity
was credited to 5 mm TiO2 nanotube layer, with sensitivity 850, responsivity
740 mA/W and short rise and decay time less than 1.5 s. Synergistic effects arising from the deposited materials and TiO2 core layer strongly influenced the photoelectrochemical
(PEC) properties and substantially enhanced the photocurrent conversion
efficiencies. The ZnO (19 nm) coated nanotube layers presented 80–95% incident
photon-to-electron conversion efficiencies (IPCE) between 305–375 nm with the aid
of 2 V bias. This value is among the highest values reported for ZnO/TiO2 combination.
The MoSexOy (2 nm) coated layers demonstrated up to 40-fold higher photocurrent
than the blank nanotube layers at 470 nm. As an environmental application, the
MoSexOy coatings have shown to be an effective photocatalyst for MB degradation
with significantly accelerated photocatalytic degradation rates at 2.259 102 min1.
It is also for the first time, MoSexOy is synthesized by ALD and deposited within HAR
nanostructure. Finally, the ZnO coatings yielded an improved ethanol sensing response
up to 11-fold compared to the blank nanotube layers in operating temperatures of 100–
200 C with high stability, durability, and reproducibility. Overall, the current work
demonstrated that anodic self-organized TiO2 nanotube layer is functional as a sensing
layer for UV and ethanol detection; and a photocatalyst in PEC and photodegradation
activities.
Description
Keywords
Nanotechnology , Photocatalysis