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Article

TiO2 Nanotubes/Ag/MoS2 Meshy Photoelectrode with Excellent Photoelectrocatalytic Degradation Activity for Tetracycline Hydrochloride

by
Tingting Li
1,2,
Zhuhong Wang
1,2,
Chaochao Liu
1,2,
Chunmin Tang
1,2,
Xinkai Wang
1,2,
Gongsheng Ding
1,2,
Yichun Ding
1,2 and
Lixia Yang
1,2,*
1
College of Environmental and Chemical Engineering, Nanchang Hangkong University, Nanchang 330063, China
2
Key Laboratory of Jiangxi Province for Persisitent Pollutants Control and Resources Recyle, Nanchang Hangkong University, Nanchang 330063, China
*
Author to whom correspondence should be addressed.
Nanomaterials 2018, 8(9), 666; https://doi.org/10.3390/nano8090666
Submission received: 7 August 2018 / Revised: 24 August 2018 / Accepted: 25 August 2018 / Published: 27 August 2018
Browse Figures
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Abstract

:
A novel type of TiO2 nanotubes (NTs)/Ag/MoS2 meshy photoelectrode was fabricated with highly oriented TiO2 nanotube arrays grown from a Ti mesh supporting Ag nanoparticles and three-dimensional MoS2 nanosheets. In this structure, Ag nanoparticles act as bridges to connect MoS2 and TiO2 and pathways for electron transfer, ensuring the abundant production of active electrons, which are the source of •O2−. The TiO2 NTs/Ag/MoS2 mesh can be used as both photocatalyst and electrode, exhibiting enhanced photoelectrocatalytic efficiency in degrading tetracycline hydrochloride under visible light irradiation (λ ≥ 420 nm). Compared to unmodified TiO2 NTs, the improved photoelectrocatalytic activity of the TiO2 NTs/Ag/MoS2 arise from the formation of Z-scheme heterojunctions, which facilitate the efficient separation of photogenerated electron-hole pairs through the Schottky barriers at the interfaces of TiO2 NTs–Ag and Ag–MoS2.

Graphical Abstract

1. Introduction

Over recent years, the presence and fate of antibiotics in the water environment has attracted considerable social attention due to threat they pose to human health and the safety of the ecological environment [1,2,3]. Tetracycline hydrochloride (TC·HCl) is the second most popular antibiotic, which has been widely used in clinical treatment and animal husbandry [4]. Backhaus et al. [5] found that TC·HCl was most toxic to aquatic microorganisms. TC·HCl cannot be biodegraded in traditional water treatment processes, and may hinder the removal of other organic pollutants [6]. Therefore, various methods have been employed to eliminate TC·HCl in the water environment, including advanced oxidation/reduction processes, adsorption, microbial degradation, and photocatalytic degradation [7,8,9,10]. Among them, semiconductor-based photocatalytic degradation is considered as an effective, energy-saving and sustainable technology to degrade TC·HCl under solar irradiation and ambient conditions [11,12].
A large number of semiconductor photocatalysts, such as modified TiO2 [6,13], BiWO6 [14], BiOI [15], Ag3PO4 [16], and Ag/AgIn5S8 [11] have been developed for photocatalytic degradation of TC·HCl. Although satisfactory removal efficiency of TC·HCl has been achieved with these photocatalysts, the addition of powdery chemicals results in difficult separation, poor reutilization, and extra pollution. In recent years, TiO2 nanotubes (NTs) fabricated by anodization of Ti foil have been widely studied due to their high photocatalytic activity, excellent chemical stability, and cyclic utilization. However, during photocatalysis, the reverse side of Ti foil cannot be excited by light, leading to deficient application of the catalyst. Moreover, TiO2 NTs grown vertically on Ti foil have the tendency to scatter incident light and cause the loss of photons. Furthermore, TiO2 NTs with lengths of more than 10 microns tend to peel away from Ti substrate due to the weak adhesion force on Ti foil, which is unfavorable for the photocatalysis. As a result, Ti mesh was selected in this study as an ideal alternative to solve these disadvantages. Unlike planar Ti foil where nanotubes are grown vertically in two-dimensional (2D) arrays, the TiO2 NTs/Ti mesh exhibit three-dimensional (3D) arrays grown from a grid of fine titanium wires [17]. Therefore, the loss of photons attributed to scattering effects in the liquid can be remarkably minimized because the nanotubes grown surrounding the Ti wires can absorb reflected light from different directions [18]. Compared with Ti foils, TiO2 NTs formed over Ti meshes can harvest more light. Moreover, anodic TiO2 NTs/Ti mesh can be used as both photocatalyst and electrode. Under light irradiation, TiO2 NTs/Ti mesh can serve as photocatalyst to generate photogenerated electrons (e−)-holes (h+) pairs, which are prone to recombine rather than participate in redox reaction of target pollutants [19]. Since TiO2 NTs/Ti mesh can also work as anode, the photogenerated electrons produced in TiO2 NTs can rapidly transfer to the positive pole of external circuit under the assistance of external electric field, which effectively restrains the recombination of e−-h+ pairs. As a result, the quantum efficiency of photoinduced carriers is enhanced, and the final photoelectrocatalytic efficiency is greatly improved [20].
However, the application of TiO2 NTs in photocatalysis is restricted by its low visible light utilization due to the wide band gap of TiO2 (3.2 eV for anatase and 3.0 eV for rutile). Various attempts have been made to improve the absorption efficiency of visible light, such as transition metal cations doping [21], nonmetal anions doping [22], surface modification with noble metal [23], and semiconductor heterojunctions [24]. In terms of the modified noble metal, silver nanoparticles (Ag NPs) with localized surface plasmon resonance (LSPR) effect is one of the most suitable candidates for assistant photocatalysts due to its low cost and high activity [25]. Furthermore, MoS2 is a graphene-like layered transition metal dichalcogenides with an appropriate band gap of 1.17 eV, which is small enough to narrow the band gap of TiO2 to extend its wavelength response range to the visible region [26]. As a result, the coupling of a wide band gap semiconductor (TiO2 NTs) with two suitable narrow band gap ones (Ag and MoS2) leads to the formation of TiO2 NTs/Ag/MoS2 heterojunctions, which will be highly effective in improving the utilization of solar light, promoting interfacial charge transfer, and thus enhancing photoelecrocatalytic activity for removing TC·HCl.
In this study, a conductive Ti mesh bearing highly ordered and oriented TiO2 NTs was employed as solid substrates for the assembly of Ag nanoparticles and MoS2 nanosheets through photochemical reduction and hydrothermal methods. The Ag nanoparticles and MoS2 nanosheets were uniformly distributed on the top surface of TiO2 NTs rather than filling in the nanotubes, thus allowing these tubular channels open to the environment, which was beneficial for effective separation of photogenerated e−-h+ pairs. Under visible irradiation and applied voltage, the resulting TiO2 NTs/Ag/MoS2 mesh worked as photoelectrode and exhibited superior photoelectrocatalytic activity for the degradation of TC·HCl. In addition, a possible photoelectrcatalytic degradation mechanism was proposed by identifying reactive species involved and electron spin resonance (ESR) spectra.

2. Experimental Section

2.1. Preparation of TiO2 NTs, TiO2 NTs/Ag, TiO2 NTs/MoS2 and TiO2 NTs/Ag/MoS2 Meshy Photoelectrode

TiO2 NTs meshy photoelectrode was prepared via electrochemical anodic oxidation. Titanium meshes (99.8%) with the size of 1 cm × 4 cm and a thickness of 0.1 mm were ultrasonically cleaned with acetone, ethanol, and deionized (DI) water. A two-electrode electrochemical cell was employed for the anodization using Ti mesh as the working electrode and platinum foil as the counter electrode. The Ti mesh was anodized under 30 V for 8 h in the electrolyte of N,N-dimethyl sulfoxide containing 2 wt % HF. After anodization, the samples were washed with DI water and then annealed at 500 °C in air atmosphere for 3 h to convert amorphous TiO2 to anatase phase.
TiO2 NTs/Ag meshy photoelectrode was fabricated by photochemical reduction method. In a typical process, the TiO2 NTs/Ti mesh substrate was soaked in 0.006 M AgNO3 aqueous solution under ultrasonication for 30 min and then dried in air. The mesh was then immersed in 0.1 M methanol solution and irradiated under the 300 W Xe lamp for 30 min to reduce adsorbed Ag+ to Ag0.
TiO2 NTs/MoS2 meshy photoelectrode was synthesized by a facile hydrothermal reaction. Typically, 10 mg of sodium molybdate (Na2MoO4·2H2O) and 30 mg of thioacetamide (C2H5NS) were dissolved in 20 mL DI water to form a transparent solution. The resulting solution was transferred into a 50 mL Telfon-lined stainless steel autoclave, and the TiO2 NTs/Ti mesh substrate was vertically immersed in the solution. Subsequently, the autoclave was heated in an oven at 200 °C for 24 h. After cooling down to room temperature, the product was washed with DI water and dried at 80 °C for 12 h. The as-prepared sample was annealed at 450 °C in N2 atmosphere for 2 h to obtain the highly crystalline MoS2.
TiO2 NTs/Ag/MoS2 meshy photoelectrode was prepared through a similar hydrothermal process and calcination using the TiO2 NTs/Ag mesh as substrate.

2.2. Characterization

The morphologies and microstructures of the products were characterized by field emission scanning electron microscopy (FESEM, S-4800, Hitachi Tokyo, Japan) and transmission electron microscopy (TEM, JEOL3010, JEOL Ltd., Akishima, Tokyo, Japan). The crystalline phase of the samples was determined by X-ray powders diffraction (XRD) using a Bruker D8 X-ray diffractometer (Bruker, Billerica, MA, USA) with CuKα (λ = 0.15418 nm) in the 2θ range of 10° to 80°. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250, Thermo Scientific, Waltham, MA, USA) was employed to examine the surface properties and chemical composition of the samples. The Brunauer–Emmett–Teller (BET) surface area was measured using a TriStar II 3020 Surface Area and Porosity Analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). The UV-vis diffuse reflection spectra (DRS) were collected on a U-3900H spectrophotometer (Hitachi, Tokyo, Japan) equipped with integrating sphere accessory, using BaSO4 as reference. The photoluminescence (PL) spectra were recorded with fluorescence spectrophotometer (Hitachi, F-7000, Tokyo, Japan) at an excitation wavelength of 380 nm.

2.3. Photoelectrochemical Measurements

The photocurrent measurements were carried out using an electrochemical workstation (CHI660C, ShanghaiChenhua Co., Ltd., Shanghai, China) with a standard three-electrode configuration with the as-prepared meshy photoelectrode as working electrode, a KCl-saturated calomel electrode (SCE) as reference electrode, and a platinum foil as counter electrode. A 300 W Xe lamp (PLS-SXE300UV, Beijing Trusttech Co. Ltd., Beijing, China) with a 420 nm cut-off filter was employed as the source of visible light (λ ≥ 420 nm). The time-dependent photocurrent response was measured in an electrolyte containing 0.5 mol/L Na2SO4 at a fixed bias of 0 V versus SCE under chopped visible-light irradiation (light/dark cycles of 50 s).
Electrochemical impedance spectroscopy (EIS) measurements were recorded using the same three-electrode configuration at an AC voltage magnitude of 0 mV with the frequency range of 105–1 Hz. The supporting electrolyte was a solution containing 0.5 mol/L Na2SO4, 2.5 mmol/L potassium hexacyanoferrate (III) (K3[Fe(CN)6]), and 2.5 mmol/L potassium ferrocyanide (K4[Fe(CN)6]).

2.4. Photoelectrocatalytic Degradation Experiments

Photoelectrocatalytic removal of TC·HCl with the meshy photoelecrodes was performed in a photoelectrochemical reactor. The as-prepared meshy photoelectrodes with an effective geometrical area of 4.0 cm2 was used as working electrode, a platinum foil as counter electrode, and 80 mL of 10 mg/L TC·HCl solution as target pollutant. A DC power (Array 3646A, Taiwan, China) supplied electricity with a bias voltage of 0.1 V. Prior to irradiation, the solution was magnetically stirred in the dark for 60 min to establish the adsorption–desorption equilibrium. Then, the samples were evaluated by degradation of TC·HCl solution under visible irradiation (λ ≥ 420 nm). In every 30 min time interval, 3 mL of the solution was removed, and the concentration changes of TC·HCl were analyzed by checking the absorbance at the characteristic adsorption peak of 358 nm.

3. Results and Discussion

3.1. Morphological and Structural Characterization

The photographs of four meshy photoelectrodes based on Ti mesh substrates are shown in Figure 1. The TiO2 NTs meshy photoelectrode (Figure 1a) exhibited a silver color; after being deposited with Ag nanoparticles, it turned into light blue with a metallic luster (Figure 1b). After being decorated by MoS2 nanosheets, the resulting TiO2 NTs/Ag/MoS2 meshes appeared black (Figure 1c,d), which is beneficial for improving the capability in harvesting light.
The morphologies of as-prepared meshy photoelectrodes were characterized by SEM. Figure 2a exhibits that the pure TiO2 NTs had vertically oriented tubular structure with a bamboo-like appearance. The nanotubes were 40–50 nm in diameter and 4–5 μm in length. The TiO2 NTs/Ag depicted in Figure 2b shows that Ag nanoparticles were evenly distributed on top surface and the tube walls of the TiO2 NTs (Figure 2b), which acted as nucleation centers for the in situ growth of MoS2 nanocrystalline. As shown in Figure 2c, MoS2 nanosheets were grown over the TiO2 NTs/Ag well-constructed morphology, which interconnected with each other closely to form three-dimensional networks. Here, the anatase TiO2 NTs after thermal annealing treatment was highly conductive [27]. The TiO2 NTs could serve as backbone for the in situ growth of MoS2 nanosheets with random elastic defomations and distortion edges, which constructed a stable hybrid configuration with a lot of exposed active sites [28]. The EDS spectrum (Figure 2d) of the TiO2 NTs/Ag/MoS2 confirmed the presence of O, S, Ti, Mo, and Ag elements. Quantitative analysis of EDS gave the deposited composite a possible composition of MoS2.
The microstructures of the resulting TiO2 NTs, TiO2 NTs/Ag and TiO2 NTs/Ag/MoS2 were further identified by TEM and HRTEM (JEOL Ltd., Akishima, Tokyo, Japan) observations. As depicted in Figure 3a, the unmodified TiO2 NTs were highly ordered and transparent with diameter of 50 nm and wall thickness of 10 nm, in agreement with the SEM observations in Figure 2a. TEM image in Figure 3b confirmed that Ag nanoparticles were uniformly and densely loaded on both the internal and external walls of the TiO2 NTs without blocking at the top openings (Figure 3b). The uniform distribution of Ag nanoparticles enabled the fast transfer of the photogenerated electrons, which contributed to reduce the combination probability of photogenerated e−-h+ pairs on the TiO2 NTs and enhance the quantum efficiency. The TEM image in Figure 3c exhibits that the decoration of Ag and MoS2 had no significant influence on the tubular structure of TiO2 NTs, indicating a good combination between the MoS2 and TiO2 crystals. Due to the inherent resistance of TiO2, the deposition of foreign species appeared preferentially on the outside rather than the inside of nanotubes [26]. Few-layer MoS2 nanosheets surrounded the tube openings and the intertubular gaps between the TiO2 NTs, leaving a large portion of tube surface accessible to the outer environment; this facilitated the exposure of active sites for absorbing and decomposing target molecules. The selected area electron diffraction (SAED) pattern (inset in Figure 3c) suggested that the TiO2 NTs that combined the Ag nanoparticles and MoS2 nanosheets was an ideal hybrid. The typical HRTEM image of the TiO2 NTs/Ag/MoS2 is shown in Figure 3d. The well-bounded lattice spacing of 0.35 nm corresponded to the (101) plane of anatase structure, which was the major exposed lattice plane of anatase TiO2 [27]. Besides, the (002) lattice plane of hexagonal 2H-MoS2 with a lattice spacing of 0.62 nm was also observed. To further confirm the distribution of Ti, O, Mo, S, and Ag elements on the TiO2 NTs/Ag/MoS2, element mappings are supplied in Figure S1. The Ti and O mappings showed obvious bright edge and dark inside, demonstrating a distinct tubular structure. The color signals of Mo, S, and Ag mappings were evident, suggesting that MoS2 nanosheet and Ag nanoparticles were homogeneously deposited on TiO2 NTs.
Figure 4 depicts the XRD patterns of the meshy photoelectrodes: the TiO2 NTs, TiO2 NTs/Ag, and TiO2 NTs/Ag/MoS2 composites. The diffraction peaks of pure TiO2 NTs (Figure 1a) were indexed to (101), (004), (200), (105), (211), (204), (220), and (301) crystal planes of anatase phase (JCPDS No. 21–1272). A major peak located at 25.3° can be ascribed to (101) plane, which is the most thermodynamically stable crystal face of anatase TiO2. However, the characteristic peaks assigned to Ag0 (JCPDS No. 65–2871) were not observed in the diffraction peaks of the TiO2 NTs/Ag (Figure 1b), probably due to its low content, poor crystallinity, and high dispersity. As for the TiO2 NTs/Ag/MoS2 (Figure 1c), the characteristic peaks emerged at 14.4, 33.5, and 35.8 eV, which could be assigned to the (002), (101), (102) lattice plane of the hexagonal phase 2H-MoS2 (JCPDS card No. 37–1492) [29]. As reported, the growth of MoS2 along the (002) plane might be inhibited by the pure TiO2 NTs during the hydrothermal process, so the assembly of ultrathin MoS2 layers results in the formation of nanosheets rather than nanoflowers [27,30].
To further elucidate the surface elemental composition and chemical states present in the TiO2 NTs/Ag/MoS2, surface analysis was performed using XPS technique. The full survey spectrum, shown in Figure 5a, revealed that the composite was composed of O, Ti, Mo, Ag, and S elements. As shown in typical high-resolution XPS spectrum of Ag 3d (Figure 5b), the Ag 3d 3/2, and Ag 3d 5/2 peaks were identified at 369.3 eV and 375.3 eV, and the splitting of the 3d doublet was 6.0 eV. This binding energy indicated that Ag mainly existed in the Ag0 state on the TiO2 NTs [25]. The Ti 2p peaks located at 460.2 and 465.9 eV (Figure 5c) with a spin-orbital doublet splitting (Ti 2p3/2-Ti 2p1/2) of 5.7 eV, which indicated an oxidation state of Ti4+ in TiO2 [31]. In Figure 5d, the Mo 3d spectrum shows two primary peaks at 230.2 and 233.3 eV, which can be assigned to the doublet Mo 3d 5/2 and Mo 3d 3/2 of Mo 4p [32]. The S 2p spectrum showed two primary peaks at 162.9 and 164.1 eV (Figure 5e), which can be attributed to the S 2p3/2 and S 2p1/2 orbitals of divalent sulfide ions (S2−) of MoS2 [33]. For the TiO2 NTs/Ag/MoS2 composites, the fitted peaks shifted to negative higher energies of ≈1.5 eV, suggesting the electronic interaction among MoS2, Ag, and TiO2 [23,27]. The XPS results further confirmed the MoS2 and Ag were successfully incorporated into the TiO2 NTs, which are in good agreement with the XRD results.
The Brunauer–Emmett–Teller specific surface area of the as-synthesized meshy photoelectrodes were measured by the nitrogen adsorption and desorption isotherms. As exhibited in Figure 6, the isotherms of the samples could be classified to a type-IV isotherm (based on IUPAC recommendation) with a H3 hysteresis loop at P/P0 > 0.6 (BDDT classification). In comparison to the BET surface area of the TiO2 NTs (21.6 m2/g), the BET surface area of the TiO2 NTs/Ag, TiO2 NTs/MoS2, and TiO2 NTs/Ag/MoS2 increased gradually (29.1 m2/g, 34.5 m2/g and 40.2 m2/g). The larger BET surface area and porous framework of the TiO2 NTs/Ag/MoS2 can provide more active sites and improve the flow rate of both target molecules and the entry and multireflections of visible light, which can be favorable to the enhancement of photocatalytic activity.

3.2. Photoelectrochemical Measurements

The UV-vis diffuse reflectance spectra of the as-prepared meshy photoelectrodes are depicted in Figure 7a. The pure TiO2 NTs only responded to the ultraviolet light. After being modified by Ag or MoS2, not only was the UV-vis light adsorption of TiO2 NTs obviously enhanced but a red shift of the absorption edge to the visible region was also observed. The TiO2 NTs/Ag/MoS2 displayed the strongest optical absorption intensity in the light range of 400–700 nm. This is because the black MoS2 is beneficial for improving the response to the visible light. Furthermore, the enhanced visible-light absorption is also caused by the metal-like local surface plasma resonance effect, which arises from collective oscillations of excess electrons on the edge of MoS2 [34].
The photoluminescence (PL) spectrum was employed to evaluate the recombination rate of the photogenerated e−-h+ pairs. As shown in Figure 7b, the pure TiO2 NTs exhibited a distinct emission peak centered at 450 nm, which can be ascribed to the electrons trapped at shallow level defects [35]. The ternary TiO2 NTs/Ag/MoS2 exhibited the lowest PL intensity, indicating that the electron-hole recombination of self-trapped excitation in TiO2 NTs was greatly suppressed by the introduction of Ag and MoS2.
To further verify the separation rate of photogenerated charge carriers, the photocurrent responses of the four meshy photoelectrodes were determined. As shown in Figure 8a, the photocurrent densities promptly increased or decreased when light was turned on or off. The lowest photocurrent density was recorded over the TiO2 NTs due to fast recombination of e−-h+ pairs. The TiO2 NTs/Ag/MoS2 exhibited the highest photocurrent density of 0.25 mA cm−2, which can be ascribed to the efficient separation of the photogenerated e−-h+ pairs on the interfaces of TiO2 NTs–Ag and Ag–MoS2.
Electrochemical impedance spectra (EIS) was performed to study the electrochemical properties of the as-prepared meshy photoelectrodes. As depicted in Figure 8b, the arc radius on the Nyquist plot of the TiO2 NTs/Ag/MoS2 was the smallest among the four meshy photoelectrodes under visible light irradiation, indicating that the introduction of Ag and MoS2 effectively decreased the resistance of the TiO2 NTs and consequently accelerated the electron transfer velocity on the meshy photoelectrode.

3.3. Photoelectrocatalytic Activity of the Meshy Photoelectrodes

The photoelectrocatalytic (PEC) activities of the as-prepared samples were evaluated by the degradation of TC·HCl under visible light irradiation. As shown in Figure 9a, the degradation of TC·HCl was negligible in dark and direct photolysis. After irradiation for 120 min, 60.1%, 68.3%, 78.1%, and 97.2% of TC·HCl were removed by the TiO2 NTs, TiO2 NTs/Ag, TiO2 NTs/MoS2, and TiO2 NTs/Ag/MoS2 meshy photoelectrodes, respectively. Figure 9b exhibits the UV-vis spectra of TC·HCl with respect to irradation durations in the presence of the TiO2 NTs/Ag/MoS2 meshy photoelectrode. The intensity of characteristic peaks of TC·HCl diminished gradually as irradiation time increased and nearly disappeared within 120 min. These results demonstrate that TC·HCl can be effectively degradated by TiO2 NTs/Ag/MoS2 under visible light.
The PEC activity of the TiO2 NTs/Ag/MoS2 meshy photoelectrode was evaluated by applying different voltage of 0.05 V, 0.1 V, 0.2 V, 0.3V, and 0.5 V. As depicted in Figure S2a, the TiO2 NTs/Ag/MoS2 showed the highest degradation efficiency with the applied voltage of 0.1 V. Figure S2b shows the removal efficiency of TC·HCl under different degradation processes, including electrochemical catalysis (EC) at 0.1 V bias potential, photocatalysis (PC), and PEC. The PEC removal efficiency of TC·HCl was much higher than that in the PC and EC processes. With assistance of both light excitation and applied bias potential, the electron density on the TiO2 NTs/Ag/MoS2 was much thicker than those in EC and PC processes, which ensured the effective degradation of TC·HCl and achieved the highest catalytic efficiency.

3.4. Study on Catalyst Stability and Photoelectrocatalytic Mechanism

The results of the recycle experiments of the TiO2 NTs/Ag/MoS2 are exhibited in Figure 10. There was a slight loss of 8% in PEC activity after five runs for the degradation of TC·HCl. Additionally, the XRD pattern of the fresh and used TiO2 NTs/Ag/MoS2 during the PEC process in Figure S3 revealed that the crystal phase had not changed significantly. This suggests that the TiO2 NTs/Ag/MoS2 meshy photoelectrode possesses excellent stability and reliability for PEC degradation of TC·HCl.
In order to explore the predominant active species in the PEC process, 1,4-benzoquinone (BQ), ethylenediamintetraacetic acid disodium (EDTA-2Na) and tert-butyl alcohol (TBA) were applied as the scavengers of superoxide radical (•O2−) [36], hole (h+) [37] and hydroxyl radical (•OH) [38], respectively. As illustrated in Figure 11, the addition of BQ exhibited strong inhibition on the PEC activity of the TiO2 NTs/Ag/MoS2, suggesting that •O2− radicals played an important role in the degradation of TC·HCl. When EDTA-2Na and TBA were added, the degradation efficiency of TC·HCl decreased from 96.2% to 52.0% and 68.1%, respectively, indicating that h+ and •OH radicals are minor reactive species.
The spin-trapping ESR technique (with DMPO) was employed to confirm •OH and •O2− radicals generated over the TiO2 NTs/Ag/MoS2 (Figure S4). Under visible light irradiation, the signals of DMPO-•OH were demonstrated by quartet lines with peak intensity of 1:2:2:1. In addition, four characteristic peaks of DMPO-•O2− were observed under visible light irradiation, but no such signals were detected in the dark. The results indicate that the TiO2 NTs/Ag/MoS2 meshy photoelectrode can be excited by visible light to produce e−-h+ pairs. Moreover, the charge separation is efficient enough to allow photogenerated electrons or holes to react with adsorbed O2 or H2O molecules to generate active radicals such as •OH and •O2−.
As a recent paper depicted [39], there are two mechanisms related to the charge transfer path across the interface in TiO2-based heterojunctions, including a double-charge transfer and a direct Z-scheme mechanism. The double-charge transfer mechanism favors charge separation at the expense of a decrease in the potential energy of photogenerated electrons and holes. The Z-scheme mechanism supports the electrons transfer across the interface, which could provide a way for preserving photostability while maintaining a high reduction potential. In this work, on the basis of above experimental data and theoretical analysis, the Z-Scheme mechanism for the PEC degradation of TC·HCl over the TiO2 NTs/Ag/MoS2 meshy photoelectrode is proposed in Scheme 1. According to a previous report [22], the conduction band (CB) and valance band (VB) potentials of MoS2 are −0.12 eV and +1.78 eV, respectively. The CB and VB potentials of TiO2 are −0.29 eV and +2.91 eV, respectively. Under visible light irradiation, the photogenerated electrons (e−) of MoS2 are excited from its VB to CB, creating equal amount of holes (h+) on the valence band (R1). However, TiO2 cannot be excited by visible light due to its wide band gap (3.2 eV). The electrons from the extra circuit inject into the CB of TiO2 NTs, transfer to metallic Ag and then to the VB of MoS2, and finally combine with the holes located at VB of MoS2 (R2). In such a way, the electrons left in the CB of MoS2 with more negative potential value facilitate the formation of •O2− (R3). The •O2− radicals further participate in a side reaction to further generate •OH (R4,5,6). These active radical species of •O2−, •OH, and h+ have high oxidizability, which could take part in the PEC degradation of TC·HCl (R7). During these charge transfers, the metallic Ag could act as a cross-linking bridge for the formation of Z-scheme heterojunction, which facilitates the efficient separation of photogenerated electron-hole pairs and leads to a higher degradation efficiency.
Nanomaterials 08 00666 i001

4. Conclusions

TiO2 NTs/Ag/MoS2 meshy photoelectrode was successfully prepared by decorating Ag nanoparticles and MoS2 nanosheets on the anodic oxidized TiO2 NTs/Ti mesh via photoassisted reduction and hydrothermal process. This composite showed remarkable photoelectrocatalytic activity and excellent stability for the PEC degradation of TC·HCl under visible light irradiation. The enhanced PEC activity can be attributed to the strong visible light adsorption, fast electron transfer velocity, and efficient separation of photogenerated electron-hole pairs. Since the TiO2 NTs/Ag/MoS2 mesh is solid and conductive, it can be used as electrode in photovoltaic devices, which opens up new prospects in environmental and energy applications, such as PEC degradation of pollutants, solar hydrogen evolution, and lithium ion batteries.

Supplementary Materials

The following are available online at https://www.mdpi.com/2079-4991/8/9/666/s1, Figure S1: Element mapping pictures of TiO2 NTs/Ag/MoS2, Figure S2: Degradation efficiencies of TC·HCl under visible light irradiation: (a) with bias potential of 0.05 V, 0.1 V, 0.2 V, 0.3V, and 0.5 V; (b) direct photolysis, electrical catalysis, photocatalysis, and photoelectrocatalysis, Figure S3: XRD patterns of TiO2 NTs/Ag/MoS2: (a) before; (b) after PEC degradation of TC·HCl, Figure S4: ESR spectra of radical adducts trapped by DMPO in TiO2 NTs/Ag/MoS2 dispersions in dark and under visible light irradiation: (a) DMPO-•OH; (b) DMPO-•O2-.

Author Contributions

T.L. and L.Y. conceived and designed the experiments; Z.W., C.L., C.T., X.W., and G.D. carried out the experiments, gathered and analyzed the data; Y.D. provided professional English guidance; T.L. and L.Y. wrote the paper.

Funding

This research was funded by the National Natural Science Foundation of China (51608250, 51468044, 51478214 and 51720105001), Distinguished Young Scientists program of Jiangxi Province (20162BCB23040, 20162BCB23042), the Natural Science Foundation of Jiangxi Province (20171BAB206049), and Department of Education Fund of Jiangxi Province (GJJ170607).

Conflicts of Interest

The authors declare that they have no conflict of interest.

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Figure 1. Digital pictures of (a) TiO2 nanotubes (NTs); (b) TiO2 NTs/Ag; (c) TiO2 NTs/MoS2; and (d) TiO2 NTs/Ag/MoS2 mesh.
Figure 1. Digital pictures of (a) TiO2 nanotubes (NTs); (b) TiO2 NTs/Ag; (c) TiO2 NTs/MoS2; and (d) TiO2 NTs/Ag/MoS2 mesh.
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Figure 2. SEM images of (a) TiO2 NTs; (b) TiO2 NTs/Ag; and (c) TiO2 NTs/Ag/MoS2; (d) EDS analysis of TiO2 NTs/Ag/MoS2.
Figure 2. SEM images of (a) TiO2 NTs; (b) TiO2 NTs/Ag; and (c) TiO2 NTs/Ag/MoS2; (d) EDS analysis of TiO2 NTs/Ag/MoS2.
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Figure 3. TEM images of (a) TiO2 NTs; (b) TiO2 NTs/Ag; and (c) TiO2 NTs/Ag/MoS2 (inset: SAED pattern); (d) HRTEM image of TiO2 NTs/Ag/MoS2.
Figure 3. TEM images of (a) TiO2 NTs; (b) TiO2 NTs/Ag; and (c) TiO2 NTs/Ag/MoS2 (inset: SAED pattern); (d) HRTEM image of TiO2 NTs/Ag/MoS2.
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Figure 4. XRD patterns of (a) TiO2 NTs; (b) TiO2 NTs/Ag; and (c) TiO2 NTs/Ag/MoS2.
Figure 4. XRD patterns of (a) TiO2 NTs; (b) TiO2 NTs/Ag; and (c) TiO2 NTs/Ag/MoS2.
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Figure 5. X-ray photoelectron spectroscopy (XPS) spectra of TiO2 NTs/Ag/MoS2: (a) survey, (b) Ag 3d, (c) Ti 2p, (d) Mo 3d, and (e) S 2p.
Figure 5. X-ray photoelectron spectroscopy (XPS) spectra of TiO2 NTs/Ag/MoS2: (a) survey, (b) Ag 3d, (c) Ti 2p, (d) Mo 3d, and (e) S 2p.
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Figure 6. Nitrogen absorption–desorption isotherms of TiO2 NTs, TiO2 NTs/Ag, TiO2 NTs/MoS2, and TiO2 NTs/Ag/MoS2.
Figure 6. Nitrogen absorption–desorption isotherms of TiO2 NTs, TiO2 NTs/Ag, TiO2 NTs/MoS2, and TiO2 NTs/Ag/MoS2.
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Figure 7. (a) UV-vis diffuse reflectance spectra; (b) PL spectra of TiO2 NTs, TiO2 NTs/Ag, TiO2 NTs/MoS2, and TiO2 NTs/Ag/MoS2.
Figure 7. (a) UV-vis diffuse reflectance spectra; (b) PL spectra of TiO2 NTs, TiO2 NTs/Ag, TiO2 NTs/MoS2, and TiO2 NTs/Ag/MoS2.
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Figure 8. (a) Photocurrent responses; (b) electrochemical impedance spectroscopy (EIS) Nyquist plots of TiO2 NTs, TiO2 NTs/Ag, TiO2 NTs/MoS2, and TiO2 NTs/Ag/MoS2.
Figure 8. (a) Photocurrent responses; (b) electrochemical impedance spectroscopy (EIS) Nyquist plots of TiO2 NTs, TiO2 NTs/Ag, TiO2 NTs/MoS2, and TiO2 NTs/Ag/MoS2.
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Figure 9. (a) The photoelectrocatalytic (PEC) efficiencies of TC·HCl under different conditions; (b) UV-vis absorption spectra for the degradation of TC·HCl on the TiO2 NTs/Ag/MoS2.
Figure 9. (a) The photoelectrocatalytic (PEC) efficiencies of TC·HCl under different conditions; (b) UV-vis absorption spectra for the degradation of TC·HCl on the TiO2 NTs/Ag/MoS2.
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Figure 10. Cycling runs in the PEC degradation of TC·HCl over the TiO2 NTs/Ag/MoS2.
Figure 10. Cycling runs in the PEC degradation of TC·HCl over the TiO2 NTs/Ag/MoS2.
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Figure 11. The PEC efficiencies of TC·HCl with different scavengers over the TiO2 NTs/Ag/MoS2: no scavenger; with 1 mM ethylenediamintetraacetic acid disodium (EDTA-2Na); with 1 mM tert-butyl alcohol (TBA); and with 1 mM 1,4-benzoquinone (BQ).
Figure 11. The PEC efficiencies of TC·HCl with different scavengers over the TiO2 NTs/Ag/MoS2: no scavenger; with 1 mM ethylenediamintetraacetic acid disodium (EDTA-2Na); with 1 mM tert-butyl alcohol (TBA); and with 1 mM 1,4-benzoquinone (BQ).
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Scheme 1. Schematic illustration of proposed mechanism of the photogenerated electrons and holes transfer in the TiO2 NTs/Ag/MoS2 interface.
Scheme 1. Schematic illustration of proposed mechanism of the photogenerated electrons and holes transfer in the TiO2 NTs/Ag/MoS2 interface.
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MDPI and ACS Style

Li, T.; Wang, Z.; Liu, C.; Tang, C.; Wang, X.; Ding, G.; Ding, Y.; Yang, L. TiO2 Nanotubes/Ag/MoS2 Meshy Photoelectrode with Excellent Photoelectrocatalytic Degradation Activity for Tetracycline Hydrochloride. Nanomaterials 2018, 8, 666. https://doi.org/10.3390/nano8090666

AMA Style

Li T, Wang Z, Liu C, Tang C, Wang X, Ding G, Ding Y, Yang L. TiO2 Nanotubes/Ag/MoS2 Meshy Photoelectrode with Excellent Photoelectrocatalytic Degradation Activity for Tetracycline Hydrochloride. Nanomaterials. 2018; 8(9):666. https://doi.org/10.3390/nano8090666

Chicago/Turabian Style

Li, Tingting, Zhuhong Wang, Chaochao Liu, Chunmin Tang, Xinkai Wang, Gongsheng Ding, Yichun Ding, and Lixia Yang. 2018. "TiO2 Nanotubes/Ag/MoS2 Meshy Photoelectrode with Excellent Photoelectrocatalytic Degradation Activity for Tetracycline Hydrochloride" Nanomaterials 8, no. 9: 666. https://doi.org/10.3390/nano8090666

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