1T-Phase MoS2 Nanosheets on TiO2 Nanorod ... - ACS Publications

Apr 22, 2017 - ABSTRACT: A novel three-dimensional (3D) photoanode with exfoliated MoS2 nanosheets on TiO2 nanorod arrays (TiO2 NAs) was successfully ...
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Research Article pubs.acs.org/journal/ascecg

1T-Phase MoS2 Nanosheets on TiO2 Nanorod Arrays: 3D Photoanode with Extraordinary Catalytic Performance Yuxi Pi, Zhen Li, Danyun Xu, Jiapeng Liu, Yang Li, Fengbao Zhang, Guoliang Zhang, Wenchao Peng,* and Xiaobin Fan* State Key Laboratory of Chemical Engineering, School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin University, No. 135, Yaguan Road, Jinnan District, Tianjin 300354, China S Supporting Information *

ABSTRACT: A novel three-dimensional (3D) photoanode with exfoliated MoS2 nanosheets on TiO2 nanorod arrays (TiO2 NAs) was successfully fabricated by combining hydrothermal and drop-casting methods. The influences of the different phases (1T and 2H) of the MoS2 nanosheets on the photoelectrochemical (PEC) performances of such a TiO2 NAs/MoS2 3D system have been systematically investigated. Parallel experiments revealed that the TiO2 NAs/1TMoS2 composite with optimized 1T-MoS2 loading exhibited the highest photoelectric conversion efficiency, which was about 440 and 93% higher than those of the TiO2 NAs and the TiO2 NAs/2H-MoS2 counterpart, respectively. The enhanced catalytic performances of the TiO2 NAs/1T-MoS2 composite can be attributed to the superior conductivity of 1T-MoS2 and the strong interaction between TiO2 NAs and 1T-MoS2. The photogenerated holes can therefore transfer from the TiO2 NAs to the 1T-MoS2, thus leading to enhanced charge separation efficiency. KEYWORDS: Photoelectrochemical, Water splitting, MoS2, TiO2, 3D system

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layer nanosheet, which is highly interesting for many fields, including photocatalytic hydrogen production applications.25−31 In particular, different types of TiO2 /MoS 2 composites with superior photocatalytic activities have been explored in the past few years. For example, Zou et al. reported MoS2 nanosheets on the surface of TiO2 nanosheets that showed excellent photocatalytic H2 evolution rate.32 Fang et al. demonstrated that TiO2@MoS2 composite exhibited enhanced photocatalytic and photocurrent performances.33 However, the reported TiO2/MoS2 composites were classical powder assemblies, difficult for recycling use or requiring additives/ binders to prepare the electrodes. Compared to the common TiO2 nanocrystals, one-dimensional (1D) ordered TiO2 nanostructures offer more efficient charge transfer, improved ion diffusion, and increased active facet exposure.34 Modifying the 1D TiO2 nanostructure framework by coating MoS2 nanosheets on the top surface should therefore provide us more promising photoeletrodes.35−37 Considering that MoS2 has two main phases (octahedral 1T phase and trigonal prismatic 2H phase,38 it might be also interesting to probe the influences of these phases on the PEC performances of the hybrid systems. In this study, we develop a simple method for the fabrication of binder-free photoanode by coating 1T- or 2H-MoS2 nanosheets on the surface of TiO2 NAs grown on FTO

INTRODUCTION Due to increasing global demand for energy and environmental protection, great efforts have been devoted to developing clean energy, especially hydrogen energy.1−3 Photoelectrochemical (PEC) water splitting has been considered a powerful tool in transforming solar light into hydrogen fuel since the 1970s.4−8 However, the challenging task in the PEC process is designing visible-light semiconductors to trap major fractions of solar energy with effective charge separation at electrode/electrolyte interfaces. Thus, it is crucial to optimize the semiconductor photoelectrode for a wider spectrum of absorption, a matching energy band to H2/O2 evolution potential, and a better crystallinity for efficient charge transfer.9−12 TiO2, as one of the most significant semiconductor materials, is widely used as photocatalysts and photoelectrodes due to its strong optical absorption, favorable band edge positions, and abundant availability.13−20 However, the energy unitization efficiency of TiO2 under visible light is very low, attributed to its wide band gap (∼3.2 eV) and the rapid recombination of photogenerated electron−hole pairs. Methods were therefore developed to increase its photoactivity, such as metal/nonmetal element doping,21 quantum dots deposition,22,23 and heterojunctions construction with other semiconductors.24 Despite these progresses, facile, scalable, cheap and environmentally friendly strategies are still desired to synthesize TiO2-based composites with enhanced performance as the photoelectrodes for efficient solar hydrogen generation. Molybdenum disulfide (MoS2) has a layered structure similar to that of graphite and can be exfoliated to be a single- or few© 2017 American Chemical Society

Received: February 18, 2017 Revised: April 5, 2017 Published: April 22, 2017 5175

DOI: 10.1021/acssuschemeng.7b00518 ACS Sustainable Chem. Eng. 2017, 5, 5175−5182

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Synthesis of TiO2 NAs/MoS2. Utilizing a drop-casting method, the TiO2 NAs/1T-MoS2 composite was synthesized. Typically, a predetermined amount of exfoliated 1T-MoS2 nanosheets were deposited on the TiO2 NAs by drop-casting, and the samples were dried at ambient temperature. TiO2 NAs/2H-MoS2 composite was prepared by annealing TiO2 NAs/1T-MoS2 under nitrogen atmosphere at 500 °C for 30 min. Systematic studies revealed that the content of MoS2 had a crucial effect on the PEC performance of these composites (Figure S1) and that the optimal loading amount of MoS2 nanosheets should be ∼0.1 mg of TiO2 NAs per 1 cm2. Characterization. The samples were characterized by X-ray diffraction (XRD) (Bruker-Nonius D8 FOCUS diffractometer), scanning electron microscopy (SEM) (FEI NOVA NanoSEM430), energy dispersive X-ray spectroscopy (EDX) (FEI NOVA NanoSEM 430), Raman (Renishaw inVia reflex, Raman spectrometer with 532 nm laser excitation), diffusion reflectance UV−visible (UV−vis) spectra (Unico UV-2800), four-point probe meter (RST-9 4 PROBES TECH), and X-ray photoelectron spectroscopy (XPS) (PHI5000VersaProbe). PEC Measurements. PEC characterization was carried out on an electrochemical workstation (CHI 660E, Chenhua, Shanghai, China) under AM 1.5 G illumination (100 mW/cm2) provided by a solar simulator (Microsolar300, PerfectLight). The as-prepared TiO2 NAs based samples were used directly as working electrodes, leaving an active area of ∼1 cm2. The counter electrode and the reference electrode were platinum foil and Ag/AgCl (saturated KCl), respectively, and 0.5 M Na2SO4 aqueous solution purged with N2 was used as the electrolyte during measurements. The electrochemical impedance spectra (EIS) for each photoanode were recorded at corresponding open-circuit potential,14,39 with the frequency ranging from 100 kHz to 0.01 Hz and the modulation amplitude of 10 mV. Mott−Schottky plots were taken at 60, 80, and 100 Hz with an amplitude of 5 mV at each potential. An optical power meter (CELNP2000, CEAULICHT) was used to obtain the incident photon to current conversion efficiency (IPCE).

(Scheme 1). This 3D binder-free photoanode consists of a core of TiO2 nanorods and MoS2 shells. Surprisingly, TiO2 NAs with Scheme 1. Schematic Sketch of Synthesis Route to Fabricate TiO2 NAs, TiO2 NAs/1T-MoS2, and TiO2 NAs/2H-MoS2 Composites on FTO Glass

a metallic 1T MoS2 composite exhibited a strikingly enhanced PEC performance compared to those of pure TiO2 NAs and TiO2 NAs/2H-MoS2 counterpart. The mechanism of the improved PEC performance of TiO2 NAs/1T-MoS2 composite was also investigated.

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EXPERIMENTAL SECTION

Preparation of the Seed Solution. The FTO glass substrate was cut to a dimension of 1 cm × 6 cm and cleaned by sonicating in acetone, ethanol, and deionized water in sequence for about 30 min. Then, the substrate was dried in air at room temperature. A solution containing 100 mL of ethanol and 10 mL of titanium(IV) butoxide was prepared for seeding the growth of the nanorods. Sequentially, each substrate was spin-coated with 10 drops (∼0.4 mL) of the seed solution over the conductive surface of the glass. The substrate was then annealed at 500 °C for 30 min in air. Synthesis of TiO2 Nanorod Arrays. TiO2 nanorod arrays (TiO2 NAs) were synthesized by a simple hydrothermal approach. Typically, 30 mL of deionized (DI) water was mixed with 30 mL of concentrated hydrochloric acid (36−38 wt %) to reach a total volume of 60 mL in a 100 mL Teflon-lined stainless-steel autoclave. The mixture was stirred at atmosphere conditions for 10 min. One mL titanium butoxide was then added into the above-mentioned solution with stirring for 20 min. One piece of FTO substrate was placed at an angle against the wall of the Teflon-liner with the conducting side facing down, which can avoid the possible influence of particle deposition. The autoclave was then sealed and kept in an oven at 180 °C for 4 h to facilitate the growth of the nanorods. After the autoclave was cooled to room temperature, the sample was taken out and rinsed extensively with DI water and ethanol, respectively, and subsequently annealed at 500 °C for 30 min in nitrogen atmosphere prior to further studies. Synthesis of Exfoliated 1T-MoS2 Nanosheets. The 1T-MoS2 nanosheets were prepared by sonication-assisted lithium intercalation according to our previous studies with slight modification.31 In a typical experiment, 1 g of bulk MoS2 powders was dispersed in the nBuLi/hexane solution (10 mL, n-butyllithium, 2.5 M solution in hexanes) with sonication for 3 h in a 50 mL Schlenk flask. Excess nBuLi was recovered by dilution with hexane, and the intercalated samples were retrieved by centrifugation and washed three times with 20 mL of hexane to remove excess lithium and organic residues. Standard air-free techniques were employed by using a Schlenk line under the protection of inert gas. Exfoliation was achieved by immediately sonicating the freshly intercalated samples in water (200− 300 mL) for about 30 min, and the exfoliated samples were purified by centrifugation and washed three times with DI water.

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RESULTS AND DISCUSSION The crystal structures of the obtained samples were examined by X-ray diffraction (XRD), as shown in Figure 1. Eliminating the peaks arising from the FTO conductive glass, all the diffraction peaks that appear upon pure TiO2 nanorod arrays (TiO2 NAs) are attributed to the tetragonal rutile phase (Powder Diffraction File (PDF) No. 76−1940, Joint Committee on Powder Diffraction Standards (JCPDS),

Figure 1. XRD patterns of TiO2 NAs, pure 1T-MoS2 nanosheets, pure 2H-MoS2 nanosheets, TiO2 NAs/1T-MoS2 composite with 0.1 mg of 1T-MoS2 loading, and TiO2 NAs/2H-MoS2 composite with 0.1 mg of 2H-MoS2 loading. 5176

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ACS Sustainable Chemistry & Engineering 2004).40 As for the pure 1T-MoS2 and 2H-MoS2 nanosheet samples, XRD results show that the crystal structures of them are similar. To be specific, the detected peaks of them can be assigned to the (002), (100), and (110) planes in hexagonal phase MoS2 (a = b =0.316 nm, c = 1.230 nm, PDF No. 37− 1492, JCPDS, 2004).41 In the case of TiO2 NAs/1T-MoS2 and TiO2 NAs/2H-MoS2 composites, all diffraction peaks of rutile TiO2 are still present, suggesting that 2D-MoS2 nanosheets loading do not change the crystal phase of TiO2, while no obvious peaks for MoS2 could be detected in these composites due to the small percentage and weak intensity of the MoS2. The morphologies and microstructures of the samples were characterized by scanning electron microscopy (SEM). Figure 2a,b show the top- and cross-section view SEM images of the

Typical TEM and HRTEM images for the 1T-MoS2 nanosheets and 2H-MoS2 nanosheets were shown in Figure S4. Individual Mo (red dot) atoms can be observed for the 1TMoS2 nanosheets (inset in Figure S4b), while the Mo (red dot) and S (green dot) atoms in 2H-MoS2 nanosheets was arranged in the shape of honeycomb. The phase difference can also be confirmed by the UV−vis spectra for them. As shown in Figure S5, 1T-MoS2 nanosheets show featureless absorption from the UV region to the NIR region, while the 2H-MoS2 nanosheets show characteristic A, B, and C excitonic features. Specifically, the A and B excitonic peaks arising from the K point of the Brillouin zone can be observed between 600 and 700 nm, and the C-exciton appears at around 420 nm.30,38 Raman and XPS spectra were also tested for the phase characterization of the MoS2 nanosheets (Figures S6 and S7). To investigate the phase compositions in the anodes, phonon spectra of TiO2 NAs, TiO2 NAs/1T-MoS2, and TiO2 NAs/2HMoS2 composites were measured by Raman scattering (Figure 3). The phonon vibrational modes of the TiO2 NAs at 143,

Figure 3. Raman spectra of pure TiO2 NAs, TiO2 NAs/2H-MoS2, and TiO2 NAs/1T-MoS2 composites.

235, 445, and 608 cm−1 correspond to the B1g, multiphoton process, Eg, and A1g modes of rutile TiO2, respectively.42 For TiO2 coating with 2H- or 1T-MoS2 nanosheets, the Raman peaks of TiO2 at 143 cm−1 are shifted to lower wavenumbers slightly. These redshifts of the Raman peaks indicate that a surface strain generates in the coated MoS2 nanosheets and that intimate interactions exist between MoS2 and TiO2.43 The Raman peaks at 381 and 406 cm−1 appear in TiO2 NAs/2HMoS2 composite, ascribed to the E12g and A1g vibration modes of 2H-MoS2 (Figure S6), respectively.31,44 Moreover, in TiO2 NAs/1T-MoS2 composite, a decrease in the E12g intensity as well as the emergence of the characteristic J1 peak of 1T-MoS2 (Figure S6)31,38 can be also observed, despite the fact that the 1T-MoS2 signals were significantly suppressed. To study the chemical states of MoS2 in the two composites, XPS analysis was carried out. Consistent with our previous study,30,31 the binding energies of Mo 3d3/2 and Mo 3d5/2 peaks in the pure 1T-MoS2 locate at 231.9 and 228.7 eV, respectively (Figure 4a). After loading onto the TiO2 NAs, however, the Mo 3d 3/2 and Mo 3d 5/2 peaks of the TiO 2 NAs/1T-MoS 2 composite obviously shifted to lower binding energies by 1.31 and 1.16 eV, respectively. For transition metal dichalcogenide like MoS2, it is known that injected electrons will be transfer to the d orbital of the transition metal center,45

Figure 2. SEM images of samples: top (a) and cross-sectional (b) views of as-formed TiO2 NAs; TiO2 NAs/1T-MoS2 composite at (c) low and (d) high magnification; TiO2 NAs/2H-MoS2 composite at (e) low and (f) high magnifications.

as-grown TiO2 NAs. Highly ordered and large-scale TiO2 nanorods are vertically aligned on the FTO substrate. The average diameter of the nanorods is about 200 nm (Figure 2a), and the mean length is about 3 μm (Figure 2b). The nanorods with smooth surfaces have a rectangular cross section. As seen in Figure 2c, the exfoliated 1T-MoS2 nanosheets with lateral sizes at the micrometer scale are uniformly deposited on the top of TiO2 NAs. The uniform distribution of MoS2 on TiO2 NAs can also be confirmed by EDX mapping of Ti, O, Mo, and S elements (Figure S2). Selected SEM and corresponding mapping images on a larger scale were shown in the Supporting Information, and uniform distribution of MoS2 can also be observed in the selected range (Figure S3). Note that the highresolution SEM image (Figure 2d) reveals the semitransparent properties of the crumpled MoS2 nanosheets, attributed to its exfoliated 2D structure. Similar morphology is also observed in the TiO2 NAs/2H-MoS2 counterpart (Figure 2e,f). 5177

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improved IPCE in the UV and visible regions compared to that of the TiO2 NAs. Electrochemical measurements were carried out to evaluate the PEC performance of the fabricated photoanodes. The photocurrent densities versus applied voltage of the as-prepared photoanodes are recorded in the dark and under AM 1.5 G illumination (100 mW/cm2 ). Dark scan linear sweep voltammagrams (LSV) from −0.2 to +1.2 V were performed as a control (Figure S10). To our delight, with irradiation, the maximum photocurrent density of TiO 2 NAs/1T-MoS 2 composite with 0.1 mg of 1T-MoS2 loading is ∼2.4 mA/cm2. This value is 3.4 and 1.7 times those of the pristine TiO2 NAs (0.7 mA/cm2) and the TiO2 NAs/2H-MoS2 counterpart (1.4 mA/cm2), respectively (Figure 6a). Additionally, photocurrent Figure 4. XPS spectra of Mo 3d, S 2s for (a) 1T-MoS2 nanosheets and TiO2 NAs/1T-MoS2 composite and (b) 2H-MoS2 nanosheets and TiO2 NAs/2H-MoS2 composite.

resulting in the shift of the binding energies of Mo 3d3/2 and Mo 3d5/2 peaks to lower binding energy.31,46 Therefore, the observed peak shift here indicates that the electrons are injected from TiO2 NAs to the MoS2 nanosheets. This result can be further supported by the shift of the Ti 2p3/2 and Ti 2p1/2 peaks in Figure S8. As a comparison, the Mo 3d3/2 and Mo 3d5/2 peaks of the TiO2 NAs/2H-MoS2 counterpart only shift by 0.88 and 0.89 eV, respectively (Figure 4b), indicating the relatively weak electron transfer for the TiO2 NAs/2H-MoS2 composite. UV−vis diffuse reflectance spectra (Figure 5a) show that the absorption band edge of the TiO2 NAs/1T-MoS2 composite

Figure 6. (a) Variation of photocurrent density versus applied potential, (b) photocurrent density under chopped AM 1.5 G illumination (100 mW/cm2), (c) photoconversion efficiency as a function of applied potential, and (d) transient current densities with light on/off every 20 s at an external bias of 0.8 V measured from TiO2 NAs, TiO2 NAs/2H-MoS2 composite with 0.1 mg of 2H-MoS2 and TiO2 NAs/1T-MoS2 composite with 0.1 mg of 1T-MoS2 in the dark and under AM 1.5 G illumination (100 mW/cm2).

Figure 5. (a) UV−vis absorption spectra of pure TiO2 NAs, TiO2 NAs/1T-MoS2, and TiO2 NAs/2H-MoS2 composites. (b) Corresponding plot of transformed Kubelka−Munk function versus photon energy.

obviously shifts to longer wavelength compared to that of the TiO2 NAs indicating that the combination of TiO2 and 1TMoS2 can effectively broaden the light absorption in visible light region. The corresponding Kubelka−Munk-transformed reflectance spectra are shown in Figure 5b, where the slopes of the tangents on horizontal axis are band gap energies. The calculated Eg is estimated to be only 2.96 eV for the TiO2 NAs/ 1T-MoS2 composite, which is smaller than those of pure TiO2 NAs (3.12 eV) and the TiO2 NAs/2H-MoS2 counterpart (3.01 eV). The narrower bandgap of TiO2 NAs/1T-MoS2 composite means the separation of photoelectrons from vacancies can be effectively promoted; thus, it may have a better sunlightharvesting ability and an enhanced PEC water-splitting performance.33,35,47,48 Moreover, to investigate whether the broadened absorption of the composite could be used for the excitation of electrons to oxidize water, IPCE was calculated (Figure S9). We found that both of the composites showed

density of the samples were also measured under chopped AM 1.5 G illumination (100 mW/cm2) (Figure 6b). Similar to Figure 6a, the photocurrent under chopped irradiation was in the order of TiO2 NAs/1T-MoS2 > TiO2 NAs/2H-MoS2 > TiO2 NAs. The strong interaction at TiO2 NAs/1T-MoS2 interface, the metallic nature of the 1T-MoS2, as well as the relatively smaller band gap should be responsible for such enhancement. In addition, the suitability of the TiO2 NAs/MoS2 3D architecture as a potential water-splitting photoanode can be determined using theoretical analysis of the PEC measurements. Applied bias to PEC hydrogen generation efficiency (ABPE) can be calculated according to the following equation:6,9,49,50 5178

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ACS Sustainable Chemistry & Engineering ABPE (%) =

|Jph | × (1.23 − |Vapp|) Plight

× 100%

Vapp is the applied potential, which is obtained as Vapp = Vmeas − Vaoc

(1) 51

(2)

where Vmeas is the measured potential versus RHE, and Vaoc is the open-circuit potential under the same illumination condition. Jph and Plight are the photocurrent density obtained under the applied bias and the power intensity of the illumination, respectively. The calculated ABPE values are shown in Figure 6c. The maximum efficiency of 0.15% is observed for the pristine TiO2 NAs at +0.51 V versus Ag/AgCl. Under the same conditions, the TiO2 NAs/2H-MoS2 counterpart has a maximum efficiency of 0.42%. Remarkably, for the TiO2 NAs/1T-MoS2 composite, the highest efficiency (0.81%) is about 93 and 440% higher than those of the TiO2 NAs/2HMoS2 and TiO2 NAs, respectively. Moreover, the morphologies of TiO2 NAs/MoS2 composite after electrochemical measurements were also characterized (Figure S11). Very few changes could be found compared to Figure 2, indicating the good structure stability of TiO2 NAs/MoS2 during the PEC process. The stability of the photoanodes could also be proved by a long time PEC activity test, which will be discussed later (Figure S12). To examine the photoresponse of the photoanode overtime, the transient photocurrents were recorded at a fixed bias of +0.8 V (vs Ag/AgCl) with light on/off cycles at 100 mW/cm2. Figure 6d illustrates the current−time (I−t) characteristics for three different electrodes of pristine TiO2 NAs, TiO2 NAs/2HMoS2, and TiO2 NAs/1T-MoS2 composites. To our delight, TiO2 NAs/1T-MoS2 composite displays desired photoelectric performance with fast response time and stable photostability. To be specific, the TiO2 NAs/1T-MoS2 composite exhibits a photocurrent density of 1.91 mA/cm2, which is significantly higher than those of TiO2 NAs (0.42 mA/cm2) and the TiO2 NAs/2H-MoS2 counterpart (1.12 mA/cm2). In addition, the stability of TiO2 NAs, TiO2 NAs/2H-MoS2, and TiO2 NAs/1TMoS2 photoanodes was measured under continuous (12 h) irradiation. As shown in Figure S12, compared to the bare TiO2 NAs and TiO2 NAs/2H-MoS2, the TiO2 NAs/1T-MoS2 photoanode exhibited better stability. A photocurrent of ∼1.79 mA/cm2 (>92% of the starting current) for the TiO2 NAs/1T-MoS2 can be obtained after 12 h PEC test, while this percentage is only ∼87% for the TiO2 NAs and ∼82% for TiO2 NAs/2H-MoS2. The electrochemical impedance spectroscopy (EIS) is carried out to better understand the improved charge-transfer properties of the TiO2 NAs/1T-MoS2 electrode. The Nyquist plots show that the overall charge-transfer resistance (Rct) of the TiO2 NAs/1T-MoS2 electrode is the smallest not only in the dark (Figure 7a) but also under light illumination (Figure 7b). To be specific, for the TiO2 NAs/1T-MoS2 electrode, the resistance Rct fitted by Z-View software is only 168.2 Ω, which is significantly lower than those of TiO2 NAs/2H-MoS2 (507.6 Ω) and TiO2 NAs (1115 Ω). This result further demonstrates that the incorporation of 1T-MoS2 nanosheets can greatly improve the charge transfer across the electrode/electrolyte interface, resulting in significant improvement in the PEC performance. In order to determine the conductivity type of the obtained samples, the flat-band potentials (VFB) of FTO, TiO2 NAs, 2H-

Figure 7. EIS response of TiO2 NAs, TiO2 NAs/2H-MoS2 and TiO2 NAs/1T-MoS2 composites: (a) in the dark and (b) under AM 1.5 G illumination (100 mW/cm2).

MoS2, and 1T-MoS2 nanosheets were measured by the Mott− Schottky method (Figure S13). It is reported that the nanostructured 2H MoS2 can show either p- or n-type conductivities, depending on the different preparation methods.52 Considering the positive slopes of Mott−Schottky plots (Figure S13c), the 2H-MoS2 nanosheets here should be an n-type semiconductor. As the VFB is about 0.3 V below the conduction band edge (CB) for undoped n-type semiconductor,53−55 the CB of TiO2 NAs and 2H-MoS2 nanosheets are estimated to be −0.51 and −0.17 V vs NHE, respectively. Note that the optical band gap of TiO2 NAs was determined as 3.12 eV (Figure 5), and that of the 2H-MoS2 nanosheets converted from their 1T counterparts was ∼1.71 eV.31 On the basis of the above results, the energy band levels of the TiO2 NAs, 1T-MoS2, and 2H-MoS2 systems were presented in Figure 8. The Fermi level of FTO is lower than the CB of TiO2 NAs,

Figure 8. Energy diagram for the TiO2 NAs, 1T MoS2, and 2H MoS2 systems.

while the Fermi level of 1T-MoS2 and the valence band (VB) of 2H-MoS2 nanosheets are higher than the VB of TiO2 NAs. With irradiation, the photoexcited electrons can be therefore easily transferred from the conduction band (CB) of TiO2 NAs to FTO and then to the counter electrode Pt for the H2 generation. In contrast, the photogenerated holes in the VB of TiO2 will flow to the 2H- or 1T-MoS2 nanosheets for the water oxidation reaction.56 This electron and hole transfer process will increase the charge separation during the PEC test, and allows us to obtain photoanodes with high activities. On the basis of above results, the highly efficient PEC water splitting of the TiO2 NAs/1T-MoS2 composite can be explained by the proposed mechanism below (Figure 9). In 5179

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strategy provides a flexible and straightforward route for design and preparation of nanocomposites based on functional semiconducting nanostructures with TiO2 nanorod arrays, promising for new opportunities in energy and environment applications, including photocatalysts and other photovoltaic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00518. Transient current densities with light on/off from TiO2 NAs/1T-MoS2 and TiO2 NAs/2H-MoS2 composite with different loadings of MoS2; SEM image and EDS spectrum of TiO2 NAs/1T-MoS 2 composite and corresponding quantitative EDS mapping of Ti, O, Mo, and S; TEM and HRTEM images of pure 1T- and 2HMoS2 nanosheets; absorption spectra of pure 1T- and 2H-MoS2 nanosheets in aqueous solution; Raman spectra of pure 2H- and 1T-MoS2 nanosheets; photocurrent density versus applied potential of TiO2 NAs, TiO2 NAs/2H-MoS2, and TiO2 NAs/1T-MoS2 composites in the dark; XPS spectra of 1T-MoS2 nanosheets, TiO2 NAs/1T-MoS2 composite, 2H-MoS2 nanosheets, and TiO2 NAs/2H-MoS2 composite; IPCE spectra of TiO2 NAs, TiO2 NAs/2H-MoS2, and TiO2 NAs/1TMoS2 photoanodes; Mott−Schottky plots of bare FTO substrate, TiO2 NAs, 2H-MoS2 nanosheets, and 1TMoS2 nanosheets (PDF)

Figure 9. Schematic illustration of the mechanism of the photoelectrochemical performance.

detail, with light irradiation, the electrons in the VB of TiO2 are excited to the CB to generate electron−hole pairs. The photogenerated electron−hole pairs can quickly recombine within the TiO2 NAs, resulting in inferior hydrogen generation efficiency for the pure TiO2 sample. The extension of reactive electron and hole lifetimes via charge carrier separation is the key in photocatalysis.57 By loading 2D MoS2 nanosheets on the surface of TiO2 NAs, the photogenerated holes may be transferred from TiO2 to MoS2 through the intimate interfacial contacts, leading to enhanced charge separation efficiency and decreased recombination rates of the electron−hole pair. Then, the excited electrons transfer to the counter electrode and participate in the reduction of water to form H2 under the applied electrical field. It is worth noting that the mobility of electrons is usually much higher than that of the holes.58 This discrepancy should also contribute to separation of electrons and holes, and the photoexcited electrons can move to the counter electrode under the applied bias. As shown above, the TiO2 NAs/1T-MoS2 composite shows much better PEC performance than the 2H phase counterpart. This result can be explained by the easier transfer of the photogenerated holes from TiO2 NAs to the 1T-MoS2 nanosheets, owing to the larger impetus (Figure 8) and better electrical conductivity of 1T-MoS2. The electrical conductivities of MoS2 nanosheets were measured by a four-point probe method. An electrical conductivity of 3500 S/m was obtained for 1T-MoS 2 nanosheets, while 2H-MoS2 nanosheets showed an electrical conductivity of only 1.43 S/m. These results are also consistent with the fact that 1T-MoS2 is metallic, whereas 2H-MoS2 is semiconducting.59,60 Moreover, stronger interaction was present in the case of TiO2 NAs/1T-MoS2 composite, which was supported by the larger energy shift of the Mo 3d3/2 and Mo 3d5/2 peaks in TiO2/1T-MoS2 composite (Figure 4).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yang Li: 0000-0003-3003-9857 Wenchao Peng: 0000-0002-1515-8287 Xiaobin Fan: 0000-0002-9615-3866 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by the National Natural Science Funds (No. 21676198) and the Program of Introducing Talents of Discipline to Universities (No. B06006).

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CONCLUSIONS In summary, a novel 3D TiO2 NAs/MoS2 photoanode for photoelectrochemical (PEC) water splitting was successfully fabricated by a facile method. Exfoliated MoS2 nanosheets were uniformly coated on the top surface of TiO2 NAs. The influences of the different phases (1T and 2H) of the MoS2 nanosheets on the PEC performances of such a TiO2 NAs/ MoS2 3D photoanodes have been systematically investigated. Parallel experiments revealed that the TiO2 NAs/1T-MoS2 photoanode with optimized 1T-MoS2 loading exhibits the highest photoelectric efficiency, which was about 440 and 93% higher than those of the TiO2 NAs and the TiO2 NAs/2HMoS2 counterpart, respectively. The difference was attributed to the enhanced charge mobility of 1T-MoS2 nanosheets and the fastest charge separation with lower recombination chance of TiO2 NAs/1T-MoS2 photoanode. It is revealed that this

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