Photocatalytic Activities Enhanced by Au-Plasmonic Nanoparticles on TiO2 Nanotube Photoelectrode Coated with MoO3
Li et al. Nanoscale Research Letters
Photocatalytic Activities Enhanced by Au- Plasmonic Nanoparticles on TiO Nanotube 2 Photoelectrode Coated with MoO 3
Chia-Jui Li 1
Chuan-Ming Tseng 0
Sz-Nian Lai 1
Chin-Ru Yang 1
Wei-Hsuan Hung 1
0 Department of Materials Engineering, Ming Chi University of Technology , New Taipei City , Taiwan
1 Department of Material Science and Engineering, Feng Chia University , Taichung , Taiwan
Although TiO2 was formerly a common material for photocatalysis reactions, its wide band gap (3.2 eV) results in absorbing only ultraviolet light, which accounts for merely 4% of total sunlight. Modifying TiO2 has become a focus of photocatalysis reaction research, and combining two metal oxide semiconductors is the most common method in the photocatalytic enhancement process. When MoO3 and TiO2 come into contact to form a heterogeneous interface, the photogenerated holes excited from the valence band of MoO3 should be transferred to the valence band of TiO2 to effectively reduce the charge recombination of photogenerated electron-hole pairs. This can efficiently separate the pairs and promote photocatalysis efficiency. In addition, photocurrent enhancement is attributed to the strong near-field and light-scattering effects from plasmonic Ag nanoparticles. In this work, we fabricated MoO3-coated TiO2 nanotube heterostructures with a 3D hierarchical configuration through two-step anodic oxidation and a facile hydrothermal method. This 3D hierarchical structure consists of a TiO2 nanotube core and a MoO3 shell (referred to as TNTs@MoO3), as characterized by field emission scanning electron microscopy and X-ray photoelectron spectroscopy.
Metal oxide; Core-shell structure; Plasmonic nanoparticles; Photocatalysis reaction
Background
Rapid technological development has been accompanied
by an increased demand for energy. Consequently, research
into alternative energy sources has become popular over
the past decade, with many scientists focused on renewable
energy sources with low carbon emissions and minimal
environmental impact. These include solar energy [
1, 2
],
geothermal heat [
3, 4
], tides [
5
], and various forms of
biomass [
6, 7
]. Photocatalytic water splitting, as the most
direct method for achieving the goal of clean and
renewable energy [8], is also the most investigated method of
directly converting solar energy into chemical energy.
Some common means of promoting energy conversion
efficiency include increasing the reaction area, catalyst
deposition, and compositing with secondary materials; for
example, synthesizing specific microstructures [
9–11
],
depositing Pt as a catalyst [
12, 13
], and combining two
different metal oxides [
14–16
].
TiO2 nanotube (TNT) arrays have received
considerable attention for their large surface area, robust
photocatalytic activity, and vectorial charge transfer properties
[
17–19
]. However, the practical application of TiO2 is
restricted by its wide band gap (3.2 eV). This results in
absorbing only UV light, which accounts for 4% of total
sunlight, greatly limiting its photocatalytic activity in the
visible light region. In addition, the high recombination
rate of TiO2 lowers the efficiency of photocatalytic
activity. To solve these problems, many studies have focused
on extending the absorption edge of TiO2 into the
visible light region, including doping with nitrogen or
other nonmetals [
20, 21
], surface modification with
noble metals [
22, 23
], and coupling with
narrow-bandgap semiconductors [
14–16
].
Molybdenum trioxide (MoO3) is a p-type metal oxide
semiconductor with a high work function and excellent
hole conductivity; therefore, it is widely used in organic
solar cells and organic light-emitting diodes [
24, 25
].
MoO3 has a band gap of approximately 2.8 eV, with 20–
30% ionic character and the capacity to absorb both UV
and visible light [26]. The valence and conduction band
positions of MoO3 are both lower than those of TiO2.
Hence, a heterojunction between TiO2 and MoO3 might
enhance photocatalytic activity by decreasing the charge
recombination and promoting the charge transfer
process [
27
]. Under visible light irradiation, the holes
excited from the valence band of MoO3 should be
transferred to the valence band of TiO2, to reduce the charge
recombination of photogenerated electron–hole pairs.
Plasmonic photocatalysis has recently facilitated the
rapid enhancement of photocatalytic efficiency under
visible light irradiation [
28, 29
]. A surface plasmon is a
surface electromagnetic wave on the metal–dielectric
interface, widely used in optical, chemical, and biological
sensing for the high sensitivity of its resonant waves. The
surface plasmon resonance effect is confined to the metal
surface to form a highly enhanced electric field [30].
When the particular resonance frequency of plasmonic
metal nanoparticles matches that of the incident photon,
strong electric field forms near the surface of (...truncated)