Enhanced photo-assistant electrocatalysis of anodization TiO2 nanotubes via surrounded surface decoration with MoS2 for hydrogen evolution reaction

Applied Surface Science 433 (2018) 197–205

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Enhanced photo-assistant electrocatalysis of anodization TiO2 nanotubes via surrounded surface decoration with MoS2 for hydrogen evolution reaction Yuanyuan Tian, Ye Song, Meiling Dou, Jing Ji ∗ , Feng Wang ∗ State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 1 August 2017 Received in revised form 23 September 2017 Accepted 30 September 2017 Available online 2 October 2017 Keywords: Nanotube array structure V-TiO2 @MoS2 Photo-assistant electrocatalytic property Anodization HER

a b s t r a c t A highly ordered TiO2 nanotube array covered with MoS2 is fabricated through a facile anodization of a metallic Ti followed by electrochemical deposition approach. The morphologies characterization of v-TiO2 @MoS2 indicate that a whole scale of 1D TiO2 nanotube uniformly covered with the MoS2 layer inside and outside, and the pathway inside the TiO2 nanotube is kept flow-through. The as-synthesized vTiO2 @MoS2 hybrid exhibits higher efficient and stable visible light activities than that of either pure TiO2 nanotubes or nv-TiO2 @MoS2 nanostructures. By electrochemical measurements such as linear sweep voltammetry(LSV) and electrochemical impedance spectroscope (EIS) under light illumination or in dark, we find that the v-TiO2 @MoS2 hybrid shows markedly enhanced photoelectrochemical performance. Furthermore, we compare the electrocatalytic behavior of v-TiO2 @MoS2 under illumination in H2 SO4 /Lactic acid within Na2 S/NaSO3 solution. The results show that the photo-assistant electrocatalytic activity in acidic environment is much better than in alkaline environment. The highly directional and orthogonal separation of charge carriers between TiO2 nanotubes and MoS2 layer, together with maximally exposed MoS2 edges, light harvesting and junctions formed between TiO2 and MoS2 is supposed to be mainly responsible for the enhanced photo-assistant electrocatalytic activity of v-TiO2 @MoS2 . © 2017 Elsevier B.V. All rights reserved.

1. Introduction The utilizing of solar energy to hydrogen fuel from water via photocatalysis has been considered to be an effective strategy for tackling the global energy and environmental crisis. Generally, the semiconductors generate charge-carrier (electron−hole) pairs due to solar light harvesting [1–3] and suitable energy band alignment for reduction of H+ to H2 [4–9] and therefore be widely used for photocatalysis of HER. Among the numerous candidates, titanium dioxide (TiO2 ) is often employed as a photocatalyst [10] for self-cleaning and solar energy conversion [11–13] for its chemical stability and biological benign [14–17]. However, the wide band gap of pure TiO2 (∼3.1 eV) results in only ∼5% of the entire solar spectrum absorbed, and is very inefficient for visible solar light harvesting. To improve the photoelectrocatalytic activities under visible light, many investigations have developed to reduce the TiO2

∗ Corresponding authors. E-mail addresses: [email protected] (J. Ji), [email protected] (F. Wang). https://doi.org/10.1016/j.apsusc.2017.09.259 0169-4332/© 2017 Elsevier B.V. All rights reserved.

bandgap by doping [18–20] or band gap engineering [21], aiming to broaden its light harvesting window to visible range [22,23] and to construct suitable electronic heterojunction for charge separation and transportation [24–26]. As most photocatalytic reactions are carried out in the electrolyte, it is necessary to consider the nature of the semiconductor/electrolyte interface, which determines both energetics of phase boundaries and kinetics of the reaction. Therefore, optimizing crystal structures (polymorphs and faceting) [27–29] or morphologies is another effective strategy to enhance the photocatalytic performance of TiO2 . Among a plethora of crystal structures and morphologies of TiO2 such as nanowires, nanosheets, and nanotubes [22,30–33], anatase TiO2 nanotube have been prepared with different synthesis strategies, and show advantages of 1D nanostructures [34–43]. Therein, the vertically oriented anatase TiO2 nanotube arrays prepared by a facile electrochemical anodization method exhibits the highest photocatalytic activity due to high specific surface and excellent electron transportation [32,44,45]. Recently, layered MoS2 , a well known p-type semiconductor [25,46–50], has been reported the band gap (Eg) of ∼ 1.9 eV [51,52] on going transition from direct to indirect band gap semiconductor, which opens a possibility for visible light

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adsorption. Coupling with TiO2 , a regular n-type semiconductor, to form 1D MoS2 /TiO2 nanotube hybrid, the as prepared hybrid composite not only creates the p-n heterojunction between MoS2 and TiO2 , thus enhancing the separation of photoinduced electrohole pairs, but also broadens its light harvesting window to visible range in the quest of visible light photocatalysis. More importantly, MoS2 with a layered structure is considered a promising electroatalyst for HER due to its similar free energy of adsorbed H (GH ) on crystalline MoS2 (010) edge sites with those of Pt [53,54]. Together with the desirable designation of the architectural structure morphology enabling synergistic impacts on photocatalytic activity, 1D MoS2 /TiO2 nanotube composite is anticipated to perform a superior integrated performance of photocatalytic and electrochemical activity. In this work, a highly ordered TiO2 nanotube array covered with MoS2 was fabricated through a facile anodization of a metallic Ti substrate followed by electrochemical deposition approach. In contrast to conventional composite prepared from 1D TiO2 nanotubes in which the introduction of foreign species is preferentially deposited only at top surface of the TiO2 nanotubes and blocked the tube openings [55], the composite prepared with this approach presents a whole scale of 1D TiO2 nanotube uniformly covered with the MoS2 layer inside and outside, and the pathway inside the TiO2 nanotube is kept flow-through. This characteristic of the asprepared hybrid structure favors for both light harvesting and band gap engineering. More meaningful, 1D TiO2 nanotube wrapped with MoS2 layer not only maximally expose the MoS2 edges, but also offers enlarged interfacial contact area between 1D MoS2 /TiO2 nanotube and electrolyte. Together with the highly directional and orthogonal separation of charge carriers between 1D TiO2 nanotubes and MoS2 layer, which is exploited for a fast transport, the as-synthesized composites showed a superior electrocatalytic activity for HER by assistance of visible light. In addition, directly anodic 1D TiO2 on Ti substrates is binder free, and was chosen to be a cathode. Different from classical powder assemblies, requiring additives/binders to prepare the electrodes, the as prepared composite provides a facile way for recycling use. Due to advantageous directional charge transfer, increased active facet exposure the interfacial contact, and sufficient electrochemical reaction of individual nanostructures, the as-synthesized composites integrate electro- and photocatalysis toward HER. 2. Experimental 2.1. Materials Lactic acid, sulfuric acid, potassium chloride, sodium sulfite anhydrous and hydrofluoric acid aqueous solution was obtained from Sinopharm Chemical Reagent Co., Ltd. Ammonium tetrathromolybdate and Sodium sulfide nonahydrate was obtained from J&K. Ti foil (200*100*0.2 mm, 99.7% purity) was purchased from ZhongNuo Advanced Material (Beijing) Technology Co., Limited. Deionized water (DI water) (Resistivity > 18.2M cm−1 ) was prepared by a pure water equipment (TTL-6B). All chemical reagents were used as received. 2.2. Preparation of anodic TiO2 nanotube array wrapped with MoS2 (labeled TiO2 @MoS2 ) An electrochemical anodization and a followed electrochemical deposition were preferred to prepare the TiO2 @MoS2 nanotube array structure. Firstly, Ti foil (40*20*0.2 mm) were ultrasonically washed in ethanol, acetone and DI water for 15 min each, respectively, and then dried in a nitrogen atmosphere prior to anodization. Then the highly ordered TiO2 nanotubes were prepared by anodic

oxidation in a HF aqueous electrolyte (0.8vol.%), following that described by previous work [56]. In a typical reaction, Ti foils were immersed in a HF aqueous solution (0.8vol.%), and suffered from a constant 20 V potential for 1hr at room temperature in a twoelectrode electrochemical cell supplied with a DC power. After anodic oxidation, the as-prepared products were washed with DI water, and dried in a N2 stream. For preparation of TiO2 @MoS2 nanotube array, the as prepared anodic TiO2 nanotube array was immersed in aqueous solution composed of 2 mM (NH4 )2 MoS4, 2 mM Na2 S• 9H2 O and 0.1MKCl, and then vacuumed for 30 min. The above pretreated system was electrodeposited for 10–30 min, and the film obtained was washed with DI water. The as prepared product was annealed at 400 ◦ Cfor 2hr under the argon atmosphere to obtain TiO2 @MoS2 nanotube array electrode (labeled v-TiO2 @MoS2 ). For comparison, the TiO2 @MoS2 nanotube was prepared by immersing in the aqueous precursor solution without vacuuming (labeled nvTiO2 @MoS2 ). Another control sample of Ti foil covered with MoS2 (labeled MoS2 /Ti) is prepared by similar process with that of TiO2 @MoS2 nanotube arrays, except that the anodization process is rule out. 2.3. Characterization Scanning electron microscopy (SEM) was conducted using a JEOL-6701F field emission scanning electron microscope at 10 ␮A of scanning current and 5 kV of accelerating voltage. High resolution transmission electron microscopy (HRTEM) images are obtained from a JEOL JSM-3010 instrument which was operated at 200 kV. X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max-2500B2+/PCX system which is operated at 40 kV and 30 mA using Cu K␣ radiation with a scan rate of 4◦ min−1 . X-ray photoelectron spectroscopy (XPS) was conducted by an ESCALAB250. The optical properties were investigated by UV–vis diffuse reflectance spectra using a UV–vis spectrophotometer (Shimadzu UV2450, Japan) with an integrating sphere attachment. Photoluminescence measurements (PL) were conducted on a Hitachi F-4500 FL spectrophotometer with an excitation light at 410 nm induced from a Xe laser source. PL decay dynamics measurement was carried out by fluorescence lifetime spectrometer (FLS980 Edinburgh Instruments). 2.4. Electrochemical measurements We performed the electrochemical experiments in a threeelectrode cell which was made of quartz. A Pt wire and calomel (Hg/Hg2 Cl2 , SCE) were employed as the counter and reference electrodes, respectively, and the as prepared catalysts were used as the working electrode which has an area of 0.16 cm2 . Linear sweep voltammetry (LSV) with scan rate of 10 mV s−1 was conducted in 0.5 M H2 SO4 /lactic acid (pH = 0.52) and 0.5 M Na2 S/NaSO3 (pH = 12.8) solution, respectively, using an electrochemical workstation (CHI660E, Chenhua, China). Both of lactic acid and Na2 S function as a sacrificial agent. Prior to the test measurements, N2 was introduced into the electrolyte solution to eliminate the dissolved oxygen. A 300W Xe arc lamp (CEL-HXF300) was employed as a light source. The integrated visible light intensity was 150 mW/cm2 . IPCE was measured at 0 V versus RHE with a monochromator in the range of 380−700 nm. The light intensity was measured with a photometer (Newport, 840-C, USA). The typical monochromatic photon power density in the IPCE measurements was 0.7 mW/cm2 . The IPCE is expressed as IPCE = (1240I)/(␭ Jlight ), where I represents the measured photocurrent density, ␭ represents the incident light wavelength, and Jlight represents the recorded incident light power density.

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Fig. 1. SEM images of top view (inset) and cross-section for (a) anodized pristine TiO2 nanotube, (b) nv-TiO2 @MoS2 nanotube arrays of 10 min deposition time, (c)vTiO2 @MoS2 nanotube arrays of 10 min deposition time, (d) v-TiO2 @MoS2 nanotube arrays of 20 min deposition time, (e)v-TiO2 @MoS2 nanotube arrays of 30 min deposition time, and (f) MoS2 /Ti of 10 min deposition time.

EIS measurements were carried out at −0.15 V vs. RHE in the frequency range of 10−1 to 105 Hz. Mott−Schottky (MS) curveswere recorded on an electrochemical workstation (CHI 660E, Chenghua, China) with a frequency of 104 Hz in 0.50 M H2 SO4 /lactic acidand 0.50 M Na2 S/NaSO3 solution respectively. 3. Results and discussion 3.1. Structure characterization Fig. 1a–f showed typical SEM images of the anodic TiO2 nanotube, nv-TiO2 @ MoS2 , v-TiO2 @MoS2 samples synthesized with different deposition time, and MoS2 /Ti. Fig. 1a demonstrated that TiO2 nanotube layer was uniformly grown in-situ on Ti foil substrate. The regularly arranged pore structure of the film is uniformly distributed, and the pore sizeis ∼ 110 nm. The cross-sectional film is viewed to be consist of well-arranged nanotubes of about 400 nm in length. Fig. 1b show the top-view and cross sectional SEM images of the highly ordered nv-TiO2 @MoS2 (10 min) nanotube arrays. The sample of nv-TiO2 @MoS2 is prepared without treatment of vacuum pumping for MoS2 precursor prior to electrodepostion process. After decoration with MoS2 , the well-ordered pores struc-

ture still exist in nv-TiO2 @MoS2 arrays, but some MoS2 stack and accumulate around the tube mouth and randomly decorated on the outer walls of tube. In comparison with the rough surface on open mouth of anatase nv-TiO2 @MoS2 arrays, the morphologies of the v-TiO2 @MoS2 arrays conducted with vacuum pumping MoS2 precursor display a distinctly clean top surface, and the open tube mouth of TiO2 nanotube arrays is clearly observed. On closer observation, the average tube diameter decreased to ∼ 90 nm due to partially filling of MoS2 in the TiO2 nanotubes, as shown in Fig. 1c inset, and the outer walls of self organized TiO2 nanotubes are also uniformly decorated by a thin MoS2 layer. When the deposition time is increased to 20 min, an uneven surface structure of the MoS2 deposited TiO2 nanotube arrays was observed in stead of the well-ordered pores structure (Fig. 1d inset). Owing to the gradually deposition of MoS2 inside and outside, the empty space inside and outside TiO2 nanotube were fully stuffed with MoS2 . With the deposition time increasing to 30 min, MoS2 accumulated at top of the TiO2 arrays, besides filling TiO2 nanotubes, displaying more smooth surface than that obtained with 20 min deposition time (Fig. 1e inset). v-TiO2 @MoS2 arrays were further characterized by TEM. The TEM images shown Fig. 2a–c confirmed the ordered TiO2 array

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Fig. 2. TEM images of top view of v-TiO2 @MoS2 arrays of deposition time of (a) 10 min, inset is the HRTEM image of v-TiO2 @MoS2 arrays, (b) 20 min, (c) 30 min, and (d)cross-section forv-TiO2 @MoS2 arrays of deposition time of 10 min.

tubular structures partially or fully filling of MoS2 . Fig. 2d clearly showed that the MoS2 was distributed around both the inner and outer surface of TiO2 nanotubes in a loose flocculating form. The inset in Fig. 2a is a HRTEM image of the sample. The observed lattice spacing of 0.24 nm is attributed to the (004) plane of anatase (JPCDS 21–1272), and 0.61 nm fringes inside and outside the nanotube is ascribed to the (002) planes of MoS2 respectively, (JPCDS 65-0160). The MoS2 crystal fringes for sample of 10 min deposition time are not detectable due to its deep trap in the TiO2 tubes. The composition and structure of the MoS2 deposited TiO2 nanotube with and without annealing were also characterized by XRD technique, as shown in Fig. 3. The peaks at ∼15◦ is indexed to (002) planes in the hexagonal phase MoS2 (JCPDS card No. 650160). The diffraction patterns at 25.3◦ comes from the natase TiO2 phase (JCPDS card No. 21–1272), which is ascribed to (101) facet of anatase TiO2 . Although the (002) facets belonging to MoS2 can be clearly visualized in the HRTEM investigation, the peak corresponded to (002) plane of the MoS2 (14.2◦ ) is not obviously detected probably due to the less degree of crystallinity. The detailed information of the as-prepared v-TiO2 @MoS2 nanotube arrays can be further obtained from XPS analysis (Fig. 4). As shown in Fig. 4a, the survey XPS spectra indicate that the main elements are Ti, O, S, and Mo of the v-TiO2 @MoS2 nanotube arrays. A typical two peaks of high resolution XPS spectrum of the Mo3d (Fig. 4b) is located at 232.8 and 229.7 eV, which are attributed to Mo (+4)3d3/2 and Mo (+4)3d5/2 , respectively [57–59]. In addition, Mo6+ was observed at 236.1 and 233.2 eV for Mo(6 + ) 3d3/2 and Mo(6 + ) 3d5/2 , respectively, which are due to the presence of MoO3 . As illustrated in Fig. 4c, two peaks are observed at 464.5 and 458.8, which can be ascribed to Ti 2p1/2 , Ti2p3/2 , respectively. The S2p peak is split into two peaks at 162.2 eV for S2p1/2 and 163.8 eV for S 2p3/2 transition, respectively (Fig. 4d).

Fig. 3. XRD patterns of the anneal-v-TiO2 @MoS2 , v-TiO2 @MoS2, and nv-TiO2 @MoS2 . The deposition time is 10 min.

3.2. Optical property Fig. 5a shows UV−visible absorption spectra of pure Ti foil, MoS2 /Ti, TiO2 , nv-TiO2 @MoS2 and v-TiO2 @MoS2 nanotube array heterostructure. For the pristine TiO2 , an obvious absorption at wavelength shorter than 400 nm is ascribed to the intrinsic band gap absorption of ∼3.2 eV. After the MoS2 deposited into the TiO2 nanotubes, the enhanced absorption in the visible light region, compared to pure TiO2 nanotubes, is clearly observed. For the v-TiO2 @MoS2 nanotube arrays heterostructure (deposition time of 10 min), the revealed absorption edges are located at about

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Fig. 4. (a) XPS spectrum of the v-TiO2 @MoS2 nanotube arrays, and high resolution XPS spectrum of (b) Mo3d, (c) Ti2p, and (d) S2p.The deposition time of MoS2 is10 min.

Fig. 5. (a) UV−visible absorption spectra, and (b) PL spectra of the as prepared samples. The deposition time of MoS2 is 10 min.

500 nm and 700 nm, which are consistent with the literature [60]. In comparison with nv-TiO2 @MoS2 , the absorption intensity for v-TiO2 @MoS2 nanotube arrays is much strong due to the flowthrough structure which allows more TiO2 nanotubes exposed to light (Fig. 5a). It is clear that the absorption in the visible light region shifts to right (visible range) with increased amount of MoS2 (Fig.S1). For v-TiO2 @MoS2, the high absorption in the visible range ( > 500 nm) is contributed by MoS2 . The MoS2 loading on TiO2 nanotube exhibits a mixed property of both MoS2 and TiO2 . As the PL emission derived of the recombination of free charge carriers, the PL spectra of Ti, MoS2 /Ti, nv-TiO2 @MoS2 and vTiO2 @MoS2 were measured by fluorescence spectroscopy at an excitation wavelength of 410 nm to investigate the influence of the cocatalyst on the interfacial electron transfer between TiO2 and MoS2 . Remarkably, Fig. 5b showed that the emission intensity of the v-TiO2 @MoS2 heterostructure is much weaker than Ti, MoS2 /Ti and nv-TiO2 @MoS2 , implying that MoS2 loaded on TiO2 can effectively hinded the recombination of electrons and holes under illumination. Efficient charge separation is further evaluated by PL decay dynamics [61–63]. We further analyzed the change of photoelec-

tron transfer behavior of v-TiO2 @MoS2 nanotubes by monitoring the emission decay, as shown Fig.S2. From Fig.S2, we can see that the fluorescence decay of the v-TiO2 @MoS2 nanotubes is faster than that of pure TiO2 , MoS2 /Ti and nv-TiO2 @MoS2 . The TiO2 , MoS2 /Ti and nv-TiO2 @MoS2 exhibited emission decays with average lifetimes of 3.23, 3.58 and 3.16 ␮s, respectively, while the average time of the v-TiO2 @MoS2 decreased to 2.07 ␮s. The reduction of decay time could be attributed to the well organized architecture and the charge transfer from MoS2 to TiO2 nanotubes thereof. In order to further evaluate the efficiency of photogenerated charge carrier, the photocurrent responses of v-TiO2 @MoS2 structure at different bias (vs. RHE) under illumination of Xe lamp were tested. For comparison, the controlling experiments for TiO2 nanotube, nv-MoS2 /TiO2 , and MoS2 /Ti structures were also conducted. (Fig. 6) The transient photocurrent (chronoamperometric) curves of Ti, MoS2 /Ti, nv-TiO2 @MoS2 and v-TiO2 @MoS2 were conducted at bias of −0.2 Vand 0.4 V vs. RHE, respectively, in 0.5 M H2 SO4 /Lactic acid solution as shown in Fig. 6.

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Fig. 6. On-off J−t curves of as prepared samples at bias vs.RHE of (a)-0.2 V, and (b) 0.4 V in 0.5 M H2 SO4 /lactic acid solution.

In general, all the as prepared samples except TiO2 nanotubes show remarkable stable photocurrent under light illumination at −0.2 V, and the sample of v-TiO2 @MoS2 exhibits the maximum net photocurrent density of ∼5mAcm−2 , which is much higher than that of MoS2 /Ti and nv-TiO2 @MoS2 (∼1.1 and 2.5 mA cm−2 , respectively), whereas, little photocurrent was detected for sample of pristine TiO2 nanotubes (Fig. 6a). However, when the photoelectrodes biased at anodic potentials (0.4 V vs. RHE), photocurrents for all photoelectrodes except TiO2 show initial photocurrent spikes at the moment the light was turned on, and then overshoots to offer a cathodic transient when the light is turned off. The main feature of the photocurrent transientsis the characteristic of almost complete recombination which originated from the fact that the electron and hole current have different relaxation times and opposite sign [64–66]. In comparison with the photocurrents biased at cathode, the photocurrents biased at anodic potential (0.4 V) are much small. Only in the pristine TiO2 nanotubes the steady photocurrent is obtained which shows the characteristic of n-type semiconductor. The v-TiO2 @MoS2 composites for deposition time of 20 min and 30 min exhibit similar behaviors with that of 10 min (Fig.S3). The photo-responded behaviors can be explained by the band bending of the semiconductor. For n-type electrode, normally, excess negative charge accumulate at the surface of semiconductor (accumulation layer), and then they deplete from the solid into the solution, leaving behind a positive excess charge in the depletion layer. When the electrons are depleted below the intrinsic level, the space charge region goes into inversion layer in which the electric fields associated with these charge distributions result in upward curvature of the bands which is beneficial to produce anodic photocurrent. For p-type semiconductors, similar considerations apply, other than mobile positive holes and the immobile negatively charged within the depletion layer. Consequently, the electric fields in inversion layer lead to downward curvature of bands in favor of generation of cathodic photocurrent. Although the flat band value for v-TiO2 @MoS2 is −0.08 V (Fig.S5a), implying n-type [67–71], it is much small. Herein, upward curvature of bending is easily driven to flat and further inverted into downward at much negative bias potential (e.g. −0.2 V vs. RHE), where the photoinduced electrons can be easily driven to the surface in which they are scavenged by H+ , thus producing cathodic photocurrent. The remarkable and steady photocurrent for v-TiO2 @MoS2 derives of both intimate heterojunctions created between MoS2 and TiO2 and the downward bending at the interface of electrolyte facilitating the separation of photoinduced electron-hole pairs in the composites. Furthermore, we investigated the photo-response of the vTiO2 @MoS2 in 0.5 M Na2 S/NaSO3 solution(Fig.S4).With the bias potential varied from 0.3 to −0.3 V vs. RHE, the characteristic of photocurrent changed from anodic to cathodic. It was observed that the anodic transient photocurrents (biased at more than 0.2 V)

are much more stable than those in 0.5 M H2 SO4 /lactic acid, but the cathodic transient photocurrents (biased at less than −0.1 V) are 100 times smaller as compared with the behaviors of transient photocurrent in acidic environment, and the cathodic photocurrent can’t reach a stable stationary. In addition, little photocurrent is observed at bias potential 0.1 V vs. RHE. The features of the photocurrent transients mentioned above can be ascribed to the influence of applied bias potential on the energy banding. When the electrode is forward biased at −0.1 or −0.2 V, the forward bias voltage overcomes the barrier potential, and the electric fields result in the depletion region to narrow, leading to little upward or almost flat curvature since the flat band potential of −0.23 V (Fig.S5b) is close to applied forward bias. Herein, the observed cathodic transient currents is attributed to proper energy band alignment between TiO2 and MoS2 rather than the band bending at the interface between the catalyst and electrolyte. As the potential fall short of −0.23V(eg. −0.3V), the remarkably increased photocurrent is obtained due to the fact that the forward bias potential goes beyond flat band and results in downward curvature of the bands, where the photo-responded behavior is similar with that of p-type semiconductor. Accordingly, one acquired an additional energy input assisting the holes driven to the counter electrode, and H+ is reduced by photo-induced electrons, thus producing cathodic photocurrent. For ease of understanding, we graphically demonstrated the mechanism analysis about band bending as shown in Fig.S11. We also carried out IPCE measurement at an applied voltage of 0 V vs. RHE. The IPCE action spectra for TiO2 , MoS2 /Ti, nv-TiO2 @MoS2 and v-TiO2 @MoS2 electrodes are shown in Fig. 7. In contrast to pristine TiO2 nanotube electrode, the photocurrent action spectra obtained for MoS2 /Ti, nv-TiO2 @MoS2 and v-TiO2 @MoS2 electrode show similar trends. More importantly, v-TiO2 @MoS2 substantially enhanced IPCE in the entire testing wave-length region, which displays the highest IPCE value, and is in accordance with the photocurrent results. 3.3. Electrochemical measurments Under illumination, cathodic polarization curve of the vTiO2 @MoS2 was recorded to explore the photo assistant electrocatalytic activity for HER performance in acidic and alkaline electrolyte, respectively (Fig. 8a and b). For comparison, the HER activities of the as-prepared TiO2 nanotubes, MoS2 /Ti, and nvTiO2 @MoS2 were investigated in dark or under light illumination. In addition, the cathodic polarization curves of the v-TiO2 @MoS2 of different deposition time are also shown in Fig.S6. Generally, in both of cases, acidic and alkaline, polarization curves in Fig. 8a and b showed that the v-TiO2 @MoS2 electrocatalyst exhibited higher HER performance under illumination than the

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Fig. 7. IPCE spectra of as prepared samples in 0.50 M H2 SO4 /lactic acid electrolyte at 0 V vs. RHE.

other controllable samples. Using the over potential at 10 mAcm−2 as the standard, the over potential of v-TiO2 @MoS2 is170 mV, higher than Pt, but lower than the MoS2 /Ti nanotube(183mV)and the nv-TiO2 @MoS2 (189 mV). For purposed of clarity, the current densities of the catalysts are depicted with column charts (Fig. 8c and d). It was observed that the photo-assistant electrocatalytic activity in acidic environment is by far the better than in alkaline environment. The striking differences between the electrocatalytic behavior under illumination in acidic and alkaline solution resulted from varying flat-band potentials of the electrode which has been shown to vary with pH[72,73] and the interrelation of flat band with the applied bias potential. In acidic solution, the negative biased

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potential (cathodic polarization) promote the separation of photoinduced electron-hole pairs as it is more negative than its flat band (-0.08 V), consistent with the results obtained from thetransient photocurrent (chronoamperometric) tests. The overwhelming performance of v-TiO2 @MoS2 nanorod array structure for HER also originates from its higher electrochemically active surface area (ECSA). The EDLC value of the v-TiO2 @MoS2 is 4.95mF cm−2 , bigger than the values of its control samples (1.03, 4.34, and 4.60mFcm−2 for TiO2 , MoS2 /Ti and nv-TiO2 @MoS2 , respectively), as shown in Fig.S7 and Fig.S8. Those results indicated that v-TiO2 @MoS2 had the largest effective electrochemical area, and thus had more active sites. Nyquist plot is carried out by EIS to estimate the surface kinetics of the as prepared catalysts, as shown in Fig. 9. The diameter of the fitted semicircle is the charge transfer resistance (Rct ), which means the resistance of electron transport. In a Nyquist plot, the overlaying Nyquistplots of v-TiO2 @MoS2 displayed a narrower semicircle diameter than that of TiO2 nanotubes, MoS2 /Ti, and nv-TiO2 @MoS2 . In either case, in dark or under illumination, the Rct values of binary catalyst are found to be much lower than unary catalysts(nvTiO2 @MoS2 for 7.01 and 5.89, and v-TiO2 @MoS2 for 4.69 and 4.19 under light illumination and in dark, respectively) (Table 1). The significantly reduced Rct of v-TiO2 @MoS2 shows faster HER kinetics and better catalytic charge-transfer impedance. Nyquist plots of the as prepared catalysts conducted in 0.5 M Na2 S/NaSO3 solutionat potential of −0.4 V vs. RHE are shown in Fig.S9. Additionally, operational stability is also an important criterion for a HER catalyst. To assess the stability of TiO2 @ MoS2 nanotube, a current density vs. time (I−t) curve was recorded over a longer reaction time (Fig.S10) at −0.15 V (vs. RHE). As shown in Fig.S10, TiO2 @ MoS2 nanotube electrode showed a photocurrent of ∼0.3 mA in the initial stage of photo-irradiation, then decreased to 0.2 mA gradually and maintained the constant photocurrent for more than 10000s.

Fig. 8. CathodicJ−V curves of as prepared samples in (a) 0.5 M H2 SO4 /Lactic acid, (b) 0.5 M Na2 S/NaSO3 solution. The column charts of the differences in current density of the catalysts in (c) 0.5 M H2 SO4 /Lactic acid, and (d) 0.5 M Na2 S/NaSO3 solutionat the potential of −0.2 V vs. RHE.

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Table 1 Charge transfer resistance (Rct ) of as-prepared samples with or without illumination. Sample Act / D/L

Ti

TiO2

MoS2 /Ti

nv-TiO2 @MoS2

v-TiO2 @MoS2

0·5MH2 SO4 /Lactic acid

8855/6981

1163/1893

5.59/4.13

7.01/5.89

4.69/4.19

with integration of electro- and photocatalysis provided a new avenue toward the photoelectrocatalytic H2 production. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2017.09. 259. References

Fig. 9. Electrochemical impedance spectroscopy studies of the as prepared samples in 0.5 M H2 SO4 /Lactic acid solutionin dark and under simulated solar light, respectively, at potential of −0.15 V vs. RHE.

Undoubtedly, the significantly improved photo-assistant electrocatalytic activity of v-TiO2 @MoS2 resulted from its precise design on structure where a high quality heterojunction between TiO2 and MoS2 is created. Since TiO2 has a slightly higher conduction band position [37] than MoS2 [74], photogenerated electrons are easily driven to the MoS2 . For such structures, the highly directional and orthogonal separation of charge carriers between 1D TiO2 nanotubes and MoS2 layer allowed the photo generated electrons transport in MoS2 which is exploited for main active sites for electrocatalysis of HER. Consequently, v-TiO2 @MoS2 showed the superior integration of photo-and electrocatalytic activity for HER, especially, in acidic environment. 4. Conclusion A highly ordered TiO2 nanotube array covered with MoS2 was fabricated through a facile anodization of a metallic Ti substrate followed by electrochemical deposition approach. The uniquely designed structure offers not only maximally expose the MoS2 edges, but also intimate contacts between TiO2 and MoS2 . The as-synthesized v-TiO2 @MoS2 hybrid exhibits higher efficient and stable visible light activities, further markedly enhanced electrochemical performance under illumination, than that of either presitine TiO2 nanotubes or nv-TiO2 @MoS2 nanostructures. Furthermore, the electrocatalytic behavior of v-TiO2 @MoS2 under illumination in H2 SO4 /Lactic acid and Na2 S/NaSO3 solution indicated that the photo-assistant electrocatalytic activity in acid environment is by far the better than in alkaline environment. Using the over potential at10 mAcm−2 as the standard, the over potential of v-TiO2 @MoS2 is only 170 mV. The highly directional and orthogonal separation of charge carriers between 1D TiO2 nanotubes and MoS2 layer, together with maximally exposed MoS2 edges, light harvesting and the junctions formed between TiO2 and MoS2 is supposed to be mainly responsible for the enhanced photo-and electrocatalytic activity of v-TiO2 @MoS2 . The v-TiO2 @MoS2 hybrid

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