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

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. Electronic supplementary material The online version of this article (10.1186/s11671-017-2327-y) contains supplementary material, which is available to authorized users.


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][10][11], depositing Pt as a catalyst [12,13], and combining two different metal oxides [14][15][16].
TiO 2 nanotube (TNT) arrays have received considerable attention for their large surface area, robust photocatalytic activity, and vectorial charge transfer properties [17][18][19]. However, the practical application of TiO 2 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 TiO 2 lowers the efficiency of photocatalytic activity. To solve these problems, many studies have focused on extending the absorption edge of TiO 2 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][15][16].
Molybdenum trioxide (MoO 3 ) 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]. MoO 3 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 MoO 3 are both lower than those of TiO 2 . Hence, a heterojunction between TiO 2 and MoO 3 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 MoO 3 should be transferred to the valence band of TiO 2 , 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 the metal. Furthermore, tunable interactions between incident visible light and excited plasmonic nanoparticles are achieved by controlling their sizes and shapes, as well as the dielectric constant of the surrounding environment [31][32][33].
In the present work, we first synthesized MoS 2 coating on the surface of TNTs through a hydrothermal method. MoS 2 was then oxidized to MoO 3 through a simple annealing process (Scheme 1). This process enabled high coverage of MoO 3 nanoscale particles with a highly ordered structure. To further enhance the photocatalytic water-splitting performance, we introduced a surface plasmon resonance (SPR) effect.

Fabrication of the TiO 2 Nanotubes
The TNTs were fabricated by a two-step anodic oxidation method. Prior to the anodic oxidation process, the titanium foil was cut to size and placed in acetone, then ethanol, then deionized (DI) water, and then subjected to ultrasonic vibration for 5 min. Anodic oxidation was carried out using a conventional two-electrode system with the Ti foil as an anode and a carbon rod as a cathode. All electrolytes consisted of 0.3 wt% ammonium fluoride (NH 4 F) in ethylene glycol (C 2 H 6 O 2 , EG) solution with 5 vol% water. All processes were carried out at room temperature.
In the first step of anodic oxidation, the Ti foil was anodized at 60 V for 30 min; the as-grown nanotubes were subsequently removed in 1 M HCl by ultrasonic vibration. The same Ti foil then underwent a second anodic oxidation process at 60 V for 30 min. After both steps were completed, the prepared TNTs were washed with ethanol and DI water. The TNTs were annealed in air at 450°C for 4 h at a heating rate of 2°C/min to form the anatase TNTs.

Synthesis of TNTs@MoO 3 Core-Shell Structure
The TNTs@MoO 3 core-shell structure was synthesized with a hydrothermal method and a simple annealing process. MoS 2 nanosheets were synthesized by the following procedures: 0.12 g of sodium molybdate (Na 2 MoO 4 ·2H 2 O) and 0.24 g of thioacetamide (TAA) were dissolved in 80 mL of DI water under vigorous stirring for 15 min. Subsequently, the transparent solution and as-grown TNTs were transferred into a 100-mL Teflon-lined stainless steel autoclave, which was sealed and heated to 200°C at a heating rate of 3°C/min and Scheme 1 Charge separation at the interface of the TiO 2 -MoO 3 composite held for 24 h. After the autoclave was cooled to room temperature, the prepared TNTs@MoS 2 were washed with DI water. The TNTs@MoS 2 were annealed in air at 450°C for 4 h with a heating rate of 2°C/min to form the TNTs@MoO 3 core-shell structure.

Deposition of Au Nanoparticles
The plasmonic cocatalyst photoelectrodes (Au/TNTs @MoO 3 ) were fabricated with the prepared TNTs @MoO 3 cocatalytic core-shell structure through the hydrothermal method, followed by the standard sputtering deposition of Au nanoparticles.

Characteristic Analysis and Photocurrent Measurements
The microstructures and morphologies of the samples were examined using field emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS). To confirm the bonding energy of the developed TiO 2 , MoS 2 , and MoO 3 photoelectrodes, Xray photoelectron spectroscopy (XPS) was employed. Finally, the photocatalytic reaction was measured in 1 M NaOH solution by operating three terminal potentiostats at room temperature under 532-nm laser irradiation with a 1-mm diameter spot size. Figure 1 shows the SEM images and EDS mapping of the prepared samples. Figure 1a-c shows the SEM images of the TNTs, TNTs@MoS 2, and TNTs@MoO 3 . The SEM image of TNTs obtained by two-step anodic oxidation of Ti foil in 0.3 wt% NH 4 F contained in ethylene glycol solution (Fig. 1a) exhibited uniform pore size (100-120 nm). After the core-shell structure was formed with MoS 2 covered through the hydrothermal method, the porous structure of TNTs was not blocked to reduce the active reaction sites (Fig. 1b). Subsequently, the TNTs@MoO 3 core-shell structure was formed by a simple annealing process in the tube furnace (Fig. 1c). Figure 1d shows the SEM image and EDS mapping of Au/TNTs@MoO 3 , providing clear information about the Ti, O, Mo, and Au. The uniform deposition of the island-like Au nanoparticles, observable on top of the TNTs@MoO 3 , facilitated the generation of the SPR effect.

Results and Discussion
XPS was used to investigate the chemical states of the TNTs@MoO 3 after conversion from TNTs@MoS 2 through a simple annealing process (Fig. 2). Three characteristic peaks of Ti and O can be observed in Fig. 2a, b. The binding energies at the Ti2p1, Ti2p3, and O1s peaks are 464.6, 458.9, and 530.4 eV, respectively. In Fig. 2c, a Mo3d3 peak at 231.6 eV and Mo3d5 peak at 228.9 eV can be identified, indicating the chemical composition of MoS 2 in the TNTs@MoS 2 . In addition, a weak peak appearing at approximately 226 eV is the signal peak of S2s. The Mo3d3 and Mo3d5 peaks in Fig. 2d with binding energies of 235.6 and 232.6 eV are ascribed to Mo 6+ in MoO 3 . Therefore, the XPS investigations confirm that the red shift of the spectrum reflects the conversion of the Mo element valence from tetravalent to hexavalent. The photocatalytic water-splitting performance of the prepared photoelectrodes was measured under 532-nm laser irradiation. Figure 3a, b shows the photocurrent response (I-V curves) of TNTs@MoO 3 and Au/ TNTs@MoO 3 . According to the results, TiO 2 @MoO 3 exhibits a higher photocurrent because of the enhanced charge separation rate at the TiO 2 @MoO 3 heterogeneous interface (shown in Fig. 3a). Furthermore, with the integration of Au nanoparticles, Au/TNTs@MoO 3 presented a photocurrent response approximately 1.5 times higher than  Figure 3c shows the I-T curves of the TNTs, TNTs@MoO 3 , and Au/ TNTs@MoO 3 at the bias voltage of 0 V. As shown in Fig. 3c, the photocurrent response was higher again in the Au/TNTs@MoO 3 structure compared to the TNTs@MoO 3 photoelectrode without the application of bias voltage. The photocurrent response of Au/TNTs@MoO 3 could be enhanced through the simple SPR effect.
To further investigate the photocatalytic activity of the prepared photoelectrodes, we also examined the extended photocurrent responses and electrochemical impedance spectroscopy to understand the photocurrent stability and the charge transfer at the photoelectrode-electrolyte interfaces (Fig. 3d, e). The extended stability of the photoelectrode with the optimal performance, Au/TNTs@MoO 3 , was examined under 532nm laser irradiation for approximately 1.5 h (Fig. 3d). At the applied voltage of 0.8 V, the photocurrent remained at 57% of its initial value. Figure 3e shows the Nyquist plots of all three tested photoelectrodes under 532-nm laser irradiation recorded at a DC potential of 1.23 V versus RHE and an AC potential frequency range of 10 6 -1 Hz with an amplitude of 1 V under 532nm laser irradiation. According to the results, smaller semicircle diameters can be observed in the Au/ TNTs@MoO 3 sample, indicating a lower transport impedance for charge carriers. The formation of a heterogeneous interface between TiO 2 and MoO 3 is confirmed to facilitate charge transfer and enhance photocatalytic activity through the excellent carrier conduction properties of the Au nanoparticles.

Supporting information
In the supporting information (Additional file 1) we performed the Raman spectra analysis of MoS 2 layer, the related thickness and average pore size of SEM images of TNTs, and the enhancement mechanism of the system.
In this study, we successfully fabricated a TNTs@MoS 2 core-shell heterostructure by a two-step anodic oxidation process and a facile hydrothermal method to form a TNTs@MoO 3 core-shell structure through a simple annealing process. According to the results, a MoO 3 coating on a photoelectrode can enhance its utilization of photons in the visible region. Moreover, with the integration of plasmonic Au nanoparticles, a significant improvement in the water-splitting photocurrent was observed compared to pure TiO 2 nanotubes under visible light irradiation. The energy band engineering of the TNTs@MoO 3 heterostructure favors charge transfer and suppresses photogenerated electron-hole pair recombination between MoO 3 and TiO 2 , leading to enhanced photocatalytic activity.