Advanced Electrocatalysts for Hydrogen Evolution Reaction Based on

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Advanced electrocatalysts for hydrogen evolution reaction based on core shell MoS2/TiO2 nanostructures in acidic and alkaline media Aneela Tahira, Zafar Hussain Ibupoto, Raffaello Mazzaro, Shuji You, Vittorio Morandi, Marta Maria Natile, Mikhail Vagin, and Alberto Vomiero ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02119 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Energy Materials

Advanced Electrocatalysts for Hydrogen Evolution Reaction based on Core Shell MoS2/TiO2 Nanostructures in Acidic and Alkaline Media Aneela Tahiraa, Zafar Hussain Ibupotoa, b*, Raffaello Mazzaroa,c, Shuji Youa, Vittorio Morandic, Marta Maria Natiled, Mikhail Vagine,f, Alberto Vomieroa,g* a

Division of Material Science, Department of Engineering Sciences and Mathematics, Luleå

University of Technology, 97187 Luleå, Sweden b

Dr. M.A Kazi Institute of Chemistry University of Sindh Jamshoro, 76080 Sindh, Pakistan

c

Istituto per la Microelettronica ed i Microsistemi, Consiglio Nazionale delle Ricerche (IMM-

CNR), Sede di Bologna, 40129 Bologna, Italy d

Istituto di Chimica della Materia Condensata e Tecnologie per l’Energia, Consiglio Nazionale

delle Ricerche (ICMATE-CNR) and Dipartimento di Scienze Chimiche, Università di Padova, 35131 Padova, Italy e

Laboratory of Organic Electronics, Department of Science and Technology, Linköping

University, 60174 Norrköping, Sweden f Department of Physics, Chemistry and Biology, Linkoping University, 58183 Linköping, Sweden g

Department of Molecular Sciences and Nanosystems, Ca’ Foscari University Venice, Via Torino

155, 30170 Mestre (Ve), Italy

Abstract Hydrogen production as alternative energy source is still a challenge due to the lack of efficient and inexpensive catalysts, alternative to platinum. Thus, stable, earth abundant and inexpensive catalysts are of prime need for hydrogen production via hydrogen evolution reaction (HER). Herein, we present an efficient and stable electrocatalyst composed of earth abundant TiO2 nanorods decorated with molybdenum disulfide thin nanosheets, a few nanometers thick. We grew rutile TiO2 nanorods via hydrothermal method on conducting glass substrate, and then we nucleated the molybdenum disulfide nanosheets as top layer. This composite possesses excellent hydrogen evolution activity both in acidic and alkaline media at considerably low overpotential (350mV and 700 mV in acidic and alkaline media, respectively) and small Tafel slopes (48 and 60 mV/dec in acidic and alkaline conditions, respectively), which are better than several transition metal dichalcogenides, such as pure molybdenum disulfide and cobalt 1

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diselenide. A good stability in acidic and alkaline media is reported here for the new MoS2/TiO2 electrocatalyst. These results demonstrate the potential of composite electrocatalysts for HER based on an earth abundant, cost effective and environmentally friendly materials, which can also be of interest for a broader range of scalable applications in renewable energies, such as lithium sulfur batteries, solar cells and fuel cells. Corresponding authors: Alberto Vomiero *, Zafar Hussain Ibupoto * Email address: [email protected], [email protected]

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Introduction Hydrogen is a potential candidate for substituting fossil fuels because it is an environmentally friendly energy carrier source [1, 2]. Despite the broad range of applicability, the clean, inexpensive and scalable production of hydrogen is still a challenging task. Up to now, several attempts have been taken to demonstrate novel techniques for hydrogen production [3]. Among the reported methods, hydrogen evolution reaction (HER) based on electrocatalysts is known to be one of the most effective and promising [4]. The most demanding aspect of the HER process is to have an efficient electrocatalyst with improved stability, durability and hydrogen production efficiency. To date, Pt-based electrocatalysts are the only efficient and stable one for HER, but Pt scarcity and high cost makes them unsuitable for large-scale exploitation for hydrogen production and other related applications [5,6,7]. For this reason, extensive efforts have been devoted to explore alternatives [8-11], and the development of non-precious electrocatalysts with performance and efficiency similar to that of Pt based electrocatalysts are of utmost importance. To date, various types of earth-abundant transition metal-based compounds have proven as promising alternatives, including metal sulfides, selenides, phosphides, carbides, and their composites [10, 11]. However, most of the catalysts exhibit poor performance compared to Ptbased electrocatalysts, and the majority are fabricated through complicated multi-step techniques, which increase the cost of the final products. Extended development has been done for HER catalysts based on layered structures of transition metal dichalcogenides, including molybdenum disulfide (MoS2) both in its crystalline and amorphous forms [12, 13, 14, 15] as well as in molecular mimics’ state [16]. However, none of them showed good HER performance, mainly due to the low density of reactive active sites and/or poor electrical conductivity of the catalyst [17, 18, 19]. These findings indicate the importance to develop new MoS2 structures with high density of active sites and/or its composites or heterostructures, which can boost its catalytic performance [20]. Besides MoS2, TiO2 is known as excellent anode for photocatalytic water splitting activated by UV light [208] and has been extensively investigated in different photocatalytic reactions because of its chemical stability, cost effectiveness, environmental compatibility, and the possibility of tuning its electronic energy gap by suitable doping [21-27]. In this work, we aim at developing a two-step method to create MoS2 nanosheets with a large number of active edges and high specific surface by decorating single crystalline, aligned, TiO2

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nanorod arrays, which combine the outstanding catalytic properties of MoS2 and the excellent charge transport properties of the TiO2 nanorod array. We synthesized the TiO2/MoS2 bi-layered structure on conducting glass substrate via hydrothermal method and obtained highly stable electrocatalyst towards hydrogen production. To date, no report on composite TiO2/MoS2 heterostructures appeared in the literature for HER, demonstrating high electrocatalytic activity and stability in harsh acidic and alkaline conditions in electrochemical water splitting. The obtained results of HER are significant and considerable in the panorama of nonprecious electrocatalysts and provide a promising platform for large-scale production of efficient, cost effective and environmentally friendly electrocatalysts. Results and discussion Morphology and structure The morphology and structure of the prepared TiO2/MoS2 samples has been investigated by SEM, TEM and XRD analysis, and compared to the properties of pristine TiO2 and MoS2 samples. Low magnification SEM micrographs of the TiO2/MoS2 sample and pristine TiO2 is displayed in Figure S1, showing a compact array of nanorods and some flower-like nanorods overgrowth. The energy dispersive X-ray spectra (EDS, Figure S1) performed on the same area confirms the deposition of a MoS2 layer on the TiO2 nanostructured substrate. Figure 1 reports a detail of the nanorods morphology for the pristine TiO2 nanorods and the TiO2/MoS2 sample. The TiO2 nanorods are few microns in length and 50 to 150 nm in diameter. The deposition of the MoS2 layer results in a conformal film composed of a few nanometers thick nanosheets, completely covering the TiO2 nanorods and resulting in a core shell nanostructure, as confirmed by TEM analysis. The EDS map for MoS2/TiO2 (Figure S2) clearly indicates the uniform distribution of Ti, O, Mo, and S within the composite structure. Several studies report that high density of exposed active edges is mandatory for high HER activity in nanostructured MoS2 [28]. In the present case, being the MoS2 deposited as a shell for TiO2 nanorods, we expect a high density of active edges to favor the HER. XRD analysis (Figure 2Error! Reference source not found.a) indicates that the TiO2 nanorods exhibit rutile phase (JCPDS card no 96-900-9084, a slight overlap with some reflections from the FTO substrate does not impair the clear assignment of the phase). The higher intensity of the peak at 36.1 (101 direction), compared to the JCPDS card, suggests the growth of the nanorods along

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this preferential orientation (as confirmed by TEM). Reflections from the sample after the growth of MoS2 layer indicate still the presence of TiO2 rutile phase. No reflections are recorded in the XRD pattern from the MoS2 layer, which is likely to be amorphous. Raman spectroscopy indicates that the pure TiO2 sample is homogeneous and all the Raman peaks at 240, 440 and 607 cm-1 are in good agreement with the rutile phase of TiO2 (Figure 2b) in agreement with XRD. The TiO2/MoS2 and pure MoS2 samples show very rough background with a weak Raman peak at around 408 cm-1, which can be assigned to molybdenite. It is probably related to poor crystallization of MoS2 as observed also in XRD analysis. Irreversible laser induced crystallization was observed in TiO2/MoS2 sample as the Raman intensity from both rutile and molybdenite were enhanced with increased laser power. Figure 3 shows the HR-TEM characterization of the TiO2/MoS2 sample. As expected, the sample exhibits large elongated single crystal structures, with diameter around 200-300 nm and 1-4 microns long (Figure 3a). The crystal structure is compatible with Rutile phase (Figure 3b) and the surface of the nanorod is covered by a continuous layer of semi-amorphous material. Some 0.61 nm fringes can be recognized (Figure 3c), compatible to the (0,0,2) reflection of MoS2 lattice. The fringes are broad and wrinkled, due to the small crystal domains of the MoS2 structure. The combined STEM analysis and EDS mapping at the interface between one of these extruded amorphous areas and the TiO2 nanorod (Figure 3d and Figure S3) clearly shows that the nanorod is surrounded by MoS2. The thickness of the MoS2 shell ranges from 10 to 15 nm, depending on the area of sample observed. The TEM results are in full agreement with SEM, XRD and Raman studies. In summary, a layer of aligned, single crystal TiO2 rutile nanorod s is conformal covered by a densely packed layer of thin amorphous MoS2 shell. No spurious contamination is detected, within the sensitivity of EDS. The results of XPS analysis are reported in Figure 4 and Figure S4. The shape and binding energy observed for Ti 2p and O1s core levels of pure TiO2 nanorods agree with those reported in the literature for TiO2 (Table 1 and Figure S4). The higher O/Ti ratio (2.8) is due to the surface hydroxylation of nanorods, as confirmed by a shoulder around 531.2 eV on the O 1s peak. The XPS spectra of MoS2 and TiO2/MoS2 composite nanostructures on FTO are reported in Figure 3, while binding energies are summarized in Table 1. Beyond the adventitious carbon, the only species observed at the surface of both the as prepared samples were Mo, S, O. No signal due to

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titanium was detected on TiO2/MoS2 suggesting that the surface of TiO2 nanorods is homogeneously coated by the MoS2 layer. The Mo 3d XPS spectrum of pure MoS2 on FTO consists of two doublets, as well evidenced by the fitting. The contribution at 226.2 eV is due to the S 2s core level and it was not considered for the quantitative calculations. The doublet at lower binding energies (BEs) with Mo 3d5/2 and 3d3/2 at 229.0 and 232.2 eV, respectively, is characteristic for Mo (IV) in MoS2. The second doublet at higher BEs with Mo 3d5/2 and 3d3/2 at 232.8 and 236.0 eV, respectively, is ascribable to Mo(VI) in MoO3. [29] Concerning the Mo 3d core level of TiO2/MoS2 composite sample, besides the Mo (IV) and Mo(VI) doublets mentioned above, one additional doublet with Mo 3d5/2 and 3d3/2 at 230.8 and 233.9 eV, respectively, is evident. Weber et al. [30] attributed similar contribution to Mo(V) present in oxysulfide intermediate phase forming during the sulfidation of MoO3 to MoS2. Similar values for Mo(V) were also observed by Ming et al. [31]. It is noteworthy that BEs of Mo 3d in TiO2/MoS2 are shifted at slightly higher values than those observed for pure MoS2. This could be due to the interaction with TiO2 surface. The atomic percentages of these three molybdenum states are also reported in Table 2. The S 2p XPS spectra in Figure 4 consist of two doublets with S 2p3/2 BEs around 161.7-162.0 eV and 163.5-163.6 eV respectively, (see also Table 1) suggesting the presence of two different divalent sulfide ions. Several Authors [30, 32] attributed these two contributions to the existence of terminal sulfide S2- and bridging disulfide S22- ligands. Moreover, on the pure MoS2 on FTO, a contribution relative to the presence of SO42- species is also evident at higher BEs (168.7 eV) [33]. The O 1s core level of pure MoS2 on FTO and TiO2/MoS2 composite nanostructures, respectively (Figure S4), are characterized by a contribution centered at 531.3 eV, which denotes a higher hydroxylation degree than pure TiO2 nanorods. The atomic percentages reported in Table 2 indicates that the S/Mo ratios observed for MoS2 and TiO2/MoS2 composite (2 and 1.8, respectively) are in good agreement with the expected values. Both EDS and XPS studies have confirmed the successful deposition of Mo and S elements in the composite structure. Ti is not detected by XPS due to the dense packing and thickness of the layered MoS2 structure. Importantly, Mo exists in various oxidations states such as (IV, V and VI) as confirmed by XPS study. Hydrogen evolution reaction (HER)

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The morphological and compositional characterization suggest that the composite system may benefit of the high density of sharp active edges of the MoS2 for efficient HER activity, which may result in small Tafel slopes, low overpotential and good exchange current densities. To monitor the catalytic activity of various electrocatalysts, the systematic electrolysis of water in 0.5M H2SO4 and 1M KOH electrolytes saturated with high purity N2 gas was performed in a three-electrode set up at room temperature. To prevent the high resistance offered after the growth of TiO2 nanorods on FTO substrate in harsh acidic conditions, the electrocatalysts were scratched from FTO substrates and were used for the modification of a glassy carbon electrode (GCE) and used as efficient catalysts. Figure 5a shows the polarization curves for various electrocatalysts in acidic media. The TiO2/MoS2 composite for HER exhibits very low overpotential (approximately 350mV) compared to the pristine MoS2 and TiO2 electrocatalysts. All the potentials are reported against reversible hydrogen electrode (RHE) after iR correction. Interestingly, the GCE shows a significant catalytic activity at higher potential. In 1 M KOH (Figure 5b), the TiO2/MoS2 composite system shows relatively high overpotential for achieving 10 mA/cm2 current density in alkaline media, compared to acidic environment, but the overpotential is lower, compared to pristine MoS2 and TiO2. The enhanced performance of TiO2/MoS2 electrocatalysts is due to the TiO2 nanorods which provide high surface where MoS2 is largely exposed with more density of active sites and consequently a dominant HER performance is demonstrated at TiO2/MoS2. Additionally, an amorphous layer MoS2 on TiO2 nanorods as confirmed by XRD analysis works well for this kind of applications. Indeed, the amorphous surface of MoS2 can contain more unsaturated sulfur (S22−) ligands and higher density of active edge sites, compared to a well crystallized film, which are favorable for the hydrogen evolution reaction. An important figure of merit for HER electrocatalysts is their Tafel slope, which measures the HER kinetics and can be estimated from the linear region of the Tafel plots (Figure 5c) for acidic media. The Tafel slope can be obtained through the Tafel equation:

η  a  b log j

Where a is a constant, b is the Tafel slope, and j is the current density.

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In acidic environment, a Tafel slope of 48 mV/dec was obtained for the TiO2/MoS2 heterostructures, which is much lower than several reported and cost effective HER catalysts under similar electrolytic conditions [34,35]. Mainly three reaction steps are involved in HER in acidic conditions [36]. The first one is known as the elementary step, generally termed as Volmer reaction; the second step is the evolution of H2 gas (desorption) (Heyrovsky reaction), and the third step is the recombination step (Tafel reaction). The Tafel slope is a primary and inherent property of the catalyst, which indicates what is the ratelimiting step involved in the HER. Therefore, it is very important to measure such parameter. With high adsorption of hydrogen (Hads) approximately equal to 1, Pt-based HER mechanics are typically following the Volmer Tafel mechanism (step 1 and 3), the step two is considered the rate limiting step at low overpotentials as verified by the obtained Tafel slope of 30 mV/decade [37]. TiO2/MoS2 composite system has a Tafel slope of 48 mV/dec, which indicates that the electrochemical process proceeds according to the Heyrovsky reaction, i.e. mainly electrochemical desorption of hydrogen gas. The Tafel slope of 32 mV/dec in Pt/C indicates the Volmer mechanism, in which the rate-determining step is the recombination of hydrogen atoms. In 1 M KOH electrolyte, the Tafel slopes are found to be 40, and 60 mV/dec for Pt/C and TiO2/MoS2, respectively (Figure 5d). The kinetics in the alkaline medium is governed by the Heyrovsky reaction, where electrochemical desorption is the rate limiting step of the process for the TiO2/MoS2 composite system. The catalytic performance in terms of Tafel slopes of our composite system in acidic environment is much better than the best catalysts in the literature based on MoS2 [37], WS2 [38], WS2(1-x)Se2x [39], and transition metal dichalcogenides of the first row, e.g. CoSe2 [40], NiSe2 [41], and CoS2 [42], testifying its outstanding HER performance. The cyclic voltammetry was used to calculate the active electrochemical surface area against Ag/AgCl reference electrode as shown in Figure 6. The active surface was estimated by taking the average current density versus the scan rate. The slope of the linear fitting corresponds to the electrochemical surface area. The measured active surface area is 0.01085 F/cm2 for MoS2/TiO2, which is larger than 0.00193 F/cm2 for pristine MoS2 and 1.79375E-4 F/cm2 for the TiO2 nanorods. The surface area of the TiO2/MoS2 is larger than the one of MoS2 and TiO2 most probably due to the core shell structure of the composite, in which a thin layer of MoS2 is deposited on an array of TiO2 nanorods. Such geometry might induce larger exposure of MoS2 lateral sides, simultaneously

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reducing the probability of agglomeration and closely packed structure for MoS2 sheets during the electrochemical measurements. The low specific surface area of the pristine TiO2 is somehow expected, since the surface-to-volume ratio of such material is low and does not allow to obtain a high available surface. The low specific surface area for the pristine MoS2 is most likely due to its layered and compact structure, which is the result of the strong agglomeration during the electrochemical measurements. The high active surface for the composite electrocatalyst is responsible for the superior HER performance, compared to the pristine MoS2 and TiO2 nanorods. We monitored the stability for MoS2/TiO2 composite electrocatalyst in acidic and alkaline media via chronopotentiometry (Figure 7a, b). It is worth to observe that after a certain time the electrocatalyst maintains the potential at fixed current density of 10 mA/cm2. These findings indicate that the composite electrode is highly stable for at least 18 hours in acidic media and at least 12 hours in alkaline media. After these time intervals, the stability experiment was stopped with no potential drop. These results confirm the long-term stability for the MoS2/TiO2 electrocatalyst. Electrochemical impedance spectroscopy was utilized to quantify the surface phenomena and the kinetics of HER on the developed catalysts (Figure 8a). Pristine TiO2 showed an order of magnitude smaller values of total capacitance (Figure 8a) visible at low frequencies in comparison with pristine MoS2 and TiO2- MoS2 composite. Consistently, the capacitance of TiO2 film Figure 8b visible as ordinate value of transition towards the saturation [43] was an order of magnitude lower than estimated for MoS2 and TiO2- MoS2 composite. These effects probably illustrate the porosity-enhanced morphology attained on a composite film in comparison with pristine materials. The quantification of impedance data was carried out by the simplest equivalent circuit comprising two RC elements (inset in Figure 8c) developed for an electrode covered with a damaged (porous) coating [44,45]. Since the boundary between the layers is not ideally smooth, due to increased surface roughness of the porous film, a quantitative analysis of the electrode impedance response requires a more complicated, distributed circuit model featuring constant phase elements (CPE) rather than pure capacitors. The equivalent circuit providing the best fit consists of the solution resistance Rs and two combined R-CPE units (I and II). A single set of parameters has been used to simultaneously fit the real and imaginary parts of the impedance over the frequency range from 1.25 Hz to 100 kHz. A value of the fitting quality parameter 2 of  0.001 obtained for all spectra indicates a very good fit. The first R-CPE unit showed smaller RC values than the second one 9



illustrating the faster kinetics of the first process. Importantly, the TiO2-MoS2 composite film showed a significantly smaller value of RII (8,2 k) in comparison with pristine films (106.5 k and 20.2 k for TiO2 and MoS2, respectively), which motivates the assignment of RII to charge transfer resistance of faradaic phenomenon and illustrates the fastest HER rate on composite (Figure 8d). The composite film revealed a transitional value of HER rate between the two pristine films. The rest of the parameters showed a non-monotonous trends in transition between pristine and composite films. Conclusions and perspectives In summary, a novel MoS2-decorated TiO2 nanorod composite system is developed by hydrothermal method as efficient electrocatalyst for hydrogen evolution reaction both in acidic and alkaline media. The peculiar structure of MoS2 layers conformal decorating the single crystal TiO2 rutile nanorod array results in high density of active edges and fast charge transport, leading to excellent HER activity at low overpotential and small Tafel slope (48 mV/dec and 60 mV/dec in acidic and alkaline environment, respectively). The measured Tafel slopes are very close to the benchmarking Pt/C and indicate that electrochemical desorption is the rate limiting step for the HER process in both acidic and alkaline media. The porosity enhancement as well as the HER rate increase by means of composite material synthesis were quantified with impedance spectroscopy. The composite system is highly stable under operating conditions, for at least 18 hours in acidic media and at least 12 hours in alkaline media, testifying the effectiveness of the applied strategy to overcome the stability issue in HER for inorganic materials in highly acidic media. Overall, we demonstrated an effective strategy to produce an electrocatalyst with superior performances compared to most of the HER electrocatalytic materials reported in the literature. In addition, the cheap and scalable production method is very promising for the practical implementation of this technology at large scale. Furthermore, this composite system can be exploited as ideal model for electrocatalytic reactions in other fields close to water splitting, such as fuel cells, lithium sulfur batteries and supercapacitors.

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Experimental section Advanced catalysts synthesis. The functional catalysts were fabricated on fluorine doped tin oxide (FTO) glass substrate by hydrothermal method. In typical synthesis, first FTO substrate was sonicated in acetone and followed by ethanol and distilled water. Then FTO substrate was dried by the N2 gas at room temperature. Prior to the growth of TiO2 nanorods, FTO substrate was functionalized with the seed layer of TiO2 nanoparticles using dip coating method and seed particles were obtained by dissolving 1 g of titanium butoxide in 10 mL ethanol, then FTO substrate was annealed at 500 oC in air for 20 min. The precursors were used as received and the growth solution for the fabrication of TiO2 nanorods was prepared by mixing 1 gram of titanium butoxide (TTIP, 6 mL n-butanone, and 6 mL 37% HCl and solution was stirred for 20 mints. Then cleaned FTO substrate was, kept in the bottom of Teflon vessel by exposing the conducting side towards to the growth solution. Then precursor solution was transferred to Teflon vessel of 100 mL capacity and sealed in stainless steel autoclave. Afterwards, the autoclave was kept in preheated electric oven at 180 oC for 40 to 60 min. Then autoclave was naturally cooled down at room temperature and prepared product was washed several times with distilled water dried at room temperature. The growth of MoS2 on FTO substrates decorated with TiO2 nanorods was also carried out by hydrothermal method using 170 mg of ammonium phosphomolybdate hydrate and L-cysteine as sulfur source in 50 mL of distilled water at 200 oC for 24 h. After that a black colored MoS2 product on TiO2 nanorods was successfully formed and were used for further experiments. At the same time, MoS2 nanostructures were also grown on bare FTO substrate for comparative study in the targeted study of electrocatalysts based hydrogen evolution reaction. Structural characterizations. Transmission electron microscopy (TEM) measurements were performed with a TEM/STEM FEI Tecnai F20 instrument, equipped with a Fischione Scanning Transmission Electron Microscopy (STEM) detector and an EDAX energy dispersive X-Ray spectroscopy (EDS) detector, and operated at 200 keV. Scanning electron microscopy (SEM) images were obtained with a (FEI MagellanTM 400 HR-SEM) at an accelerating voltage of 25 keV and the EDS spectra by selecting the targeted region of SEM image. X-ray diffraction (XRD) was performed on a PANalytical X'Pert instrument.

The Raman spectra of as-prepared TiO2,

TiO2/MoS2 and MoS2 were recorded with 50 objective under 532 nm laser excitation using Senterra Raman spectrometer. The laser power was fixed to 0.2 mW to avoid laser heating.

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Surface characterization. X-Ray photoelectron spectroscopy (XPS) spectra were recorded with a Perkin Elmer Φ 5600ci Multi Technique System. A standard Al Kα radiation working at 250 W was used to record both extended spectra (survey at 187.85 eV pass energy, 0.5 eV·step−1, 0.05 s·step−1) and detailed spectra (for Ti 2p, Mo 3d, S 2p, O 1s and C 1s, at 23.50 eV pass energy, 0.1 eV·step−1, 0.1 s·step−1). The standard deviation in the BE values of the XPS line is 0.10 eV. The atomic percentage, after a Shirley-type background subtraction [46] was evaluated by using the PHI sensitivity factors [33]. The peak positions were corrected for the charging effects by considering the C 1 s peak at 284.8 eV and evaluating the BE differences [47]. Electrochemical measurements. 10 mg of, MoS2, TiO2, MoS2/TiO2 and 20% Pt/C catalysts and 500 µl of 5 wt% Nafion solution were dispersed in 2.5 mL of distilled water in separate vessels and sonicated for 30 mints in order to get well dispersed ink. The catalysts were deposited on glassy carbon electrode (GCE) drop casting method used as working electrode in the electrolysis of water in 0.5 M H2SO4 and 1M KOH saturated with N2 gas. The catalyst ink was deposited on glassy carbon electrode of 3 mm in diameter by drop casting method using 10 μl of the catalyst ink. Linear sweep voltammetry (using Solartron analytical CH Instruments) at the scan rate of 5 mVs−1 was used to record the polarization curves of the various catalysts. The silver-silver chloride electrode was used as the reference electrode, and a large platinum plate (0.91 cm2 in size) was used as the counter electrode. A 2-compartment electrochemical cell was used for electrochemical measurements, consisting in the working electrode in one compartment, and the counter and reference electrodes into another compartment. Both compartments were containing the same alkaline electrolyte and were separated by a proton exchange membrane (PEM). In the applied setup, the reference electrode Ag/AgCl saturated with 3m KCl was positioned in a plastic tube close to the working electrode, while the counter electrode was at far distance from the working electrode in the other compartment of the cell, separated by the PEM. AC impedance experiment was performed under conditions of η = 0.3 V from 1000 kHz – 0.1 Hz with an AC voltage of 10 mV. In electrochemical experiments Ag/AgCl electrode was used as the reference and it was calibrated with respect to reversible hydrogen electrode (RHE). In the electrolyte solution E (RHE) = E (Ag/AgCl) + E All the potentials are presented in the current results against RHE except cyclic voltammetry for the calculation of electrochemical surface area

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Acknowledgments R.M. and A.V. acknowledge the Knut & Alice Wallenberg Foundation (project number 2016.0346) and the Kempe Foundation (project number JCK-1606) for the financial support. A.V. acknowledges the European Union’s Horizon 2020 research and innovation programme under grant agreement No 654002 for partial financial support. References (1) Shen, Y.; Lua, A. C.; Xi, J.; Qiu, X., Ternary Platinum–Copper–Nickel Nanoparticles Anchored to Hierarchical Carbon Supports as Free-Standing Hydrogen Evolution Electrodes.

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ASSOCIATED CONTENT Supporting Information Available: **Complementary SEM, STEM, EDS mapping and XPS**

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Figures Error! Reference source not found.

Figure 1: SEM images for the (a) TiO2 nanorods, (b) MoS2–decorated TiO2 nanorods at high magnification. Scale bar 200 nm.

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Figure 2

Figure 2: (a) The distinctive XRD patterns for pure TiO2 nanorods on FTO and TiO2/MoS2 nanorods. (b) Raman spectra of as prepared TiO2, TiO2/MoS2 and MoS2 samples. Black stars (*) and blue circles (o) refer to Raman peaks from rutile and molybdenite, respectively.

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Figure 3

Figure 3: (a) Low resolution TEM micrograph of MoS2 covered TiO2 nanorod and Selected Area Diffraction performed on the same nanorod displaying a single diffraction pattern (Rutile (1,2,1) Zone Axis). (b) High Resolution TEM micrograph of the core of MoS2 covered TiO2 nanorod: inset showing the FFT of the image. (c) HR-TEM image at the interface between TiO2 nanorods and MoS2 layer, displaying the main features of Rutile and Molybdenite phases. (d) STEM-EDS profile of the edge of a decorated rutile nanorod and (inset) distribution of Ti and Mo. Scale bar in (c) and (d) equal to 20 nm, unless stated otherwise.

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Figure 4

Figure 4: Mo 3d (left panel) and S 2p (right panel) XPS spectra of MoS2 (green line) and TiO2/MoS2 nanorods (blue line). Fitting of Mo3d and S 2p XPS peaks is reported. Spectra are normalized with respect to their maximum and baseline.

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Figure 5

Figure 5: (a) Linear sweep voltammetry polarization curves for different catalysts at the scan rate of 5 mV /s in 0.5 M H2SO4 saturated with N2 gas. (b) LSV curves for the various catalysts in 1 M KOH saturated with N2 gas. (c) Tafel plots HER in 0.5 M H2SO4. (d) The corresponding Tafel plots for HER in 1 M KOH.

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Figure 6

Figure 6. Cyclic voltammetry at various scan rates for measuring the electrochemical surface area against Ag/AgCl as reference electrode in 1M KOH at room temperature. (a) Cyclic voltammetry runs for the TiO2 at various scan rates and (b) the corresponding linear fitting for the TiO2. (c) Cyclic voltammetry runs for the MoS2 at various scan rates and (d) the corresponding linear fitting

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for the MoS2. (e) Cyclic voltammetry runs for the TiO2/MoS2 at various scan rates and (f) the corresponding linear fitting for the TiO2/MoS2

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Figure 7

Figure 7: (a) Chronopotentiometry stability for MoS2/TiO2 in 0.5M H2SO4, for 18 hours. (b) Chronopotentiometry stability for 12 hours in 1 M KOH for MoS2/TiO2

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Figure 8

Figure 8: Impedance experiment was performed under conditions of η = 0.3 V from 100 kHz – 0.1 Hz with an amplitude of 10 mV in 0.5 M H2SO4. (a-b) The effect of film composition on total capacitance. The frequency dependencies of the total capacitance calculated from impedance spectra acquired on pristine TiO2 and MoS2 (black and red, respectively) and a composite TiO2MoS2 films (blue). Intensity (c) and phase (d) Bode plot of impedance spectra acquired on pristine (TiO2 and MoS2 as black and red, respectively) and a composite films (blue); dots: experimental points; solid lines: fitting with the equivalent circuit (Inset).

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Table 1. XPS peak positions (binding energy, eV) of TiO2, MoS2 and MoS2/TiO2.

TiO2 MoS2

MoS2/TiO2

Ti2p 458.8 464.6 --

Mo 3d --

S 2p3/2 --

O 1s 530.1

229.0 (Mo(IV) in MoS2) 232.2

161.7 (S2- in MoS2) 163.6 (S22- in MoS2) 168.7 (S in SO42-)

531.8

232.8 (Mo(VI) in MoO3 236.0 229.5 232.6

162.0 (S2- in MoS2) 163.5

230.8 233.9 233.0 236.1

Table 2. XPS atomic composition (%) of MoS2 and MoS2/TiO2. Samples MoS2 TiO2/MoS2

Mo 33.7 [69 % Mo(IV); 31 % Mo(VI)] 36.1 [67 % Mo(IV); 7 % Mo(V); 26 % Mo(VI)]

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S 66.3 63.9

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