Photocatalytic Activities Enhanced by Au-Plasmonic Nanoparticles on TiO2 Nanotube Photoelectrode Coated with MoO3

Nanoscale Research Letters, Oct 2017

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.

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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)


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Chia-Jui Li, Chuan-Ming Tseng, Sz-Nian Lai, Chin-Ru Yang, Wei-Hsuan Hung. Photocatalytic Activities Enhanced by Au-Plasmonic Nanoparticles on TiO2 Nanotube Photoelectrode Coated with MoO3, Nanoscale Research Letters, 2017, pp. 560, Volume 12, Issue 1, DOI: 10.1186/s11671-017-2327-y