MXene hybrid nanoflowers with enhanced electrocatalytic activity for hydrogen evolution

Journal Pre-proof A facile method to produce MoSe2/MXene hybrid Nanoflowers with enhanced Electrocatalytic activity for hydrogen evolution

Jia-Jun Huang, Xiao-Qing Liu, Fei-Fei Meng, Lan-Qi He, JunXi Wang, Jia-Cheng Wu, Xi-Hong Lu, Ye-Xiang Tong, Ping-Ping Fang PII:

S1572-6657(19)30995-6

DOI:

https://doi.org/10.1016/j.jelechem.2019.113727

Reference:

JEAC 113727

To appear in:

Journal of Electroanalytical Chemistry

Received date:

16 September 2019

Revised date:

30 November 2019

Accepted date:

4 December 2019

Please cite this article as: J.-J. Huang, X.-Q. Liu, F.-F. Meng, et al., A facile method to produce MoSe2/MXene hybrid Nanoflowers with enhanced Electrocatalytic activity for hydrogen evolution, Journal of Electroanalytical Chemistry(2019), https://doi.org/ 10.1016/j.jelechem.2019.113727

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© 2019 Published by Elsevier.

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A Facile Method to Produce MoSe2/MXene Hybrid Nanoflowers with Enhanced Electrocatalytic Activity for Hydrogen Evolution Jia-Jun Huang, Xiao-Qing Liu, Fei-Fei Meng, Lan-Qi He, Jun-Xi Wang, Jia-Cheng Wu, Xi-Hong Lu, Ye-Xiang Tong and Ping-Ping Fang*

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MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab

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of Low-carbon Chem & Energy Conservation of Guangdong Province, School of

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E-mail: [email protected]

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Chemistry, Sun Yat-Sen University, Guangzhou 510275, People’s Republic of China

Abstract

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Keywords: MXene; MoSe2; nanoflowers; HER; electrocatalysis

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Transition metal dichalcogenides are considered as one of the most attractive candidates for hydrogen evolution reactions but their inferior conductivity is utterly disappointing. Herein, by virtue of a facile, scalable hydrothermal method, MoSe2 is in-situ grown on the ultrathin titanium carbide substrate (MXene), generating a novel three-dimensional nanoflower with freestanding petals for electrocatalytic hydrogen evolution reaction. The ingenious introduction of the highly-conductive MXene as the skeleton for the MoSe2 growth significantly facilitates the charge/ion transport and a mountain of electrochemical active sites are exposed thanks to its unique three-dimensional architecture. Benefiting from these advantageous properties, the MoSe2/MXene hybrid nanoflowers exhibit a low onset potential of 61 mV vs. RHE

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for HER at acidic environment and around 6-fold current density increase is observed at -300 mV compared with MoSe2. Notably, they display long-term cycling viability, experiencing negligible decay after 2,000 cycles. This work paves the way for future applications of MXene-based nanomaterials in other clean-energy-conversion reactions.

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1. Introduction Hydrogen, a thriving clean energy source, is recognized as one of the most

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potential alternatives to the finite fossil fuels taking advantages of its high

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energy density and nonpolluting nature. With this regard, electrochemical water

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splitting for hydrogen production has been extensively investigated as an

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effective approach for next-generation energy conversion [1-3]. Currently, the

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Pt-based materials are the most ubiquitous and active electrocatalysts for the hydrogen evolution reaction (HER) but their further widespread applications

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are badly impeded by the notorious high cost and low earth-abundance [4, 5]. Therefore, the exploration of high-performance HER electrocatalysts consisting of inexpensive and earth-abundant elements has been one of the grandest targets in renewable energy researchfield in recent years. Transition metal dichalcogenides (TMDs) (MX2: M = W, Mo; X = S, Se, Te), composed by non-precious metals, are extensively proposed as alterative electrocatalysts of noble metals for HER because they possess good electrocatalytic performance in acidic conditions and are more suitable for large-scale industrial applications considering their large abundance and relatively low price [6-8]. Yet, the TMDs

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suffer from their intrinsic poor conductivity which is substantially unfavorable for efficient charge/electron transport and further injures their electrocatalytic activity for HER. In order to address aforementioned issues, great efforts are dedicated to the rational hybridization of TMDs with highly-conductive substrates and scaffolds

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such as carbon nanotubes [9-11] and graphene [12-14]. Recently, MXene, a new graphene-like two-dimensional (2D) material described by a chemical

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formula of Mn+1XnTx (n = 1, 2 or 3; M is Ti, V, or Cr; X is C or N; T x is the

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surface functional group), which has great potential in not only HER but also

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NRR [15], is demonstrated as an advantageous additive or support to alleviate

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the resistance of the semi-conductive TMDs and further enhance their

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electrochemical performance [16-20]. This should be ascribed to its ultrathin feature as well as favorable metallic conductivity. Besides, MXene generally

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possesses rich surface chemical groups (typically -O, -OH, -F or -Cl) which can act as a functionalized substrate for the nucleation and thereby anchoring other active materials. For instance, Attanayake et al. recently reported a benign HER electrocatalyst comprising vertical MoS2 grown on Ti3C2 MXene nanosheets. The unique perpendicular architecture of such nanohybrids considerably decreased the electrical resistance yet increased catalytic active sites, leading to a low onset potential of ~95 mV vs. RHE for hydrogen production [21]. Taking account to the poor oxygen resistance of ultrathin Ti 3C2 MXene, Wu and his coworkers developed a carbon nanoplating strategy to prevent the structural

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degradation of MoS2/Ti3C2 composite, which endowed the MoS2/Ti3C2 @C nanocatalyst with both admirable energy storage ability and attractive HER properties [22]. In fact, the combination of MXene with TMDs for hydrogen production remains at its infant stage and current research interest is mainly focused on MoS2 among the limited reported literatures.

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In this present work, we developed a simple hydrothermal method to synthesize three-dimensional (3D) MoSe2/MXene hybrid nanoflowers with

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nanoscale petals, and further demonstrated their capability to function as an

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advanced electrocatalyst for hydrogen evolution. Currently, edge sites of TMDs

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are commonly considered as the well-known active centers for HER. Their

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unique 3D structure provided capacious spaces for ion/electron transport and

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ensured large active edge site exposure of MoSe2. Meanwhile, the MoSe2 outer layers effectively hinders the oxygen corrosion of the ultrathin MXene

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substrate. Such specific architecture endowed the MoSe2/MXene hybrid nanoflower with good conductivity and high stability. When tested in acidic conditions, the MoSe2/MXene hybrid nanoflowers exhibited obviously superior HER electrocatalytic activity in contrast to their single-component counterparts. Specifically, they displayed a low onset potential of- 61 mV vs. RHE and a high current density of 10 mA cm−2 was achieved at relatively low overpotential of 180 mV vs. RHE. Besides, such extraordinary HER catalytic activity was also accompanied by excellent long-term cycling viability (no decay after 2,000 cycles). The cost-effective electrocatalysts embedding

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non-precious metals may hold the promise as a new type of promising nanocrystals for various energy-conversion applications.

2. Experimental section 2.1 Chemicals and materials Unless stated, all the reagents are of analytical grade and were used without

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further purification. Ammonium molybdate tetrahydrate and ethylenediamine monohydrate were purchased from Tianjin Damao Chemical Reagent Factory and

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TCI Shanghai Co Ltd., respectively. Selenium dioxide was purchased from Shanghai

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Macklin Biochemical Co Ltd. Ethanol was offered by Beijing Chemical Works. All

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(resistivity of 18.2 MΩ cm at 25 °C).

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solutions were made using ultrapure water from a Millipore purification system

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2.2 Preparation of Ti3C2 MXene nanosheets

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Briefly, Ti3C2 MXene was obtained by selectively etching the Al layer in the Ti3AlC2 MAX phase with 48% HF acid. Typically, 1.0 g of Ti3AlC2 powder was slowly added into 10 ml of 48% aqueous HF solution under vigorous stirring, and then kept stirring at 35 °C for 24 h. Subsequently, the black suspension was continuously washed by distilled water until the pH reached 6, and then dried at 60 °C in the vacuum oven. The as-obtained products were dispersed into water under the ultrasonic treatment for 2 h. Finally, the black supernatant containing the suspension of the Ti3C2 (∼1.0 mg mL−1), was collected for further use. 2.3 Preparation of MoSe2/MXene hybrid nanoflowers

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A group of MoSe2/MXene hybrid nanoflowers were achieved via a facile hydrothermal method. Here, the synthesis procedure of the hybrid nanoflowers with optimal HER performance was taken as an example to illustrate the general experimental details. Specifically, 0.17 mmol ammonium molybdate tetrahydrate, and 2.4 mmol SeO2 powder were dissolved in 15 mL previous MXene supernatant under

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ultrasonic treatment for 30 min. 25 mL of ethanediamine were then added into the homogeneous mixture with vigorous stirring for 15 min at room temperature. The

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resultant solution was transferred into a 40 mL Teflon-lined stainless-steel autoclave,

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maintained at 200 °C for 20 h, and cooled down to room temperature. The obtained

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black precipitates were collected by centrifugation at 8000 rpm for 7 min, washed

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with distilled water and ethanol at least 3 times, and then dried at 60 °C in the air. To

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obtain the final products, the above collections were annealed at 500 °C for 1 h in flowing N2 atmosphere. MoSe2/MXene hybrid nanoflowers with varied Mo:Ti ratios

and MXene.

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were also prepared using the same procedure with different dosing ratios of Mo salts

2.4 Preparation of MoSe2 nanoflowers The experimental protocol for MoSe2 nanoflower synthesis was exactly identical to the one we used for Mo10/Ti fabrication except that 15 mL MXene supernatant was substituted by 10 mL ultrapure water. 2.5 Characterization SEM images were acquired using a GeminiSEM 500 (Carl Zeiss Microscopy

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GmbH, Germany). TEM images were obtained on a Tecnai F30 (FEI, The Netherlands). XRD patterns were acquired via D-MAX 2200 VPC with Cu Kα radiation (40 kV, 20 mA). XPS measurements were conducted on Thermo fisher Scientific K-Alpha+ with monochromatic Al Kα radiation (12 kV, 6 mA) as the excitation source. Raman spectra were acquired via a laser micro-Raman spectrometer

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(Renishaw inVia) and a He-Ne laser of 532 nm was employed as the incident light.

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ICP-AES measurements were carried out via Optima 8300 (PerkinElmer).

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2.6 Electrode preparation and electrochemical measurements a standard

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All the electrochemical measurements were conducted in

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three-electrode system using a CHI 760 electrochemical workstation (CH Instruments, China). Typically, 4 mg of catalyst and 30 μL Nafion solution (5 wt%) were dispersed

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in 2 mL of water/ethanol mixture solution (volume ratio of 3:1) under sonication for

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30 min to form a homogeneous ink. Then 10 μL of the catalyst ink (containing 20 μg of catalyst) was loaded onto a glassy carbon electrode with 3 mm diameter (catalyst loading 0.28 mg cm-2). Linear sweep voltammetry with a scan rate of 50 mV s-1 was carried out in 0.5 M H2SO4 using a Ag/AgCl (in 3 M KCl solution) electrode as the reference electrode, and the glassy carbon electrode with catalysts as the working electrode, a platinum sheet as the counter electrode. The potentials were calibrated to the reversible hydrogen electrode (RHE) based on the formula 𝐸𝑅𝐻𝐸 = 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.196 𝑉 + 0.0591 pH . Electrochemical impedance spectroscopy (EIS) were conducted in the frequency range from 100 kHz to 0.01 Hz.

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3. Results and discussions

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Figure 1. The schematic illustration of synthetic route of MoSe2/MXene hybrid nanoflowers.

The preparation procedure of MoSe2/MXene hybrid nanoflower is schematically

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illustrated in Figure 1. Here, we chose the representative and most-widely studied

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MXene of Ti3C2 as the scaffold for MoSe2 growth. Briefly, the stacked laminar Ti3C2 (Figure 2a) was firstly obtained by removing Al atoms of commercial bulk Ti3AlC2 powders with HF acid etching. A liquid exfoliation process was then conducted to prepare well-separated 2D Ti3C2 nanosheets with the lateral size of about several hundreds of nanometers (Figure S1). The as-synthesized ultrathin Ti3C2 nanosheets contain plenty of negatively charged oxygen/fluorine functional groups on the active surface, which favor efficient electrostatic adsorption of charged ions, making it an ideal template for subsequent heterogeneous nucleation of MoSe2. Under hydrothermal conditions, the resultant MoSe2/MXene hybrid composite assembled as interconnected 3D nanoflowers with freestanding petals. In fact, by altering the

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addictive amount of Ti3C2 substrate, we obtained a series of MoSe2/MXene hybrid nanomaterials with different component ratios (the content ratios of MoSe2 to Ti3C2 ranging from 2:1 to 50:1) and their elemental distributions analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) are summarized in Table S1. It is worthy to mention that all the hybrid samples preserved the flower-like

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architecture and no notable morphology difference was observed. Moreover, to highlight the advantageous characteristic of hybrid nanocomposites, pure MoSe2

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sample was also prepared for control experiments using a similar hydrothermal

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method reported in the literature [23]. (See experimental details in the SI) Similarly,

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the MoSe2 also maintained the 3D-nanoflower shape (Figure S2).

Figure 2. (a) SEM image of the stacked laminar Ti3C2 after HF etching; (b, c) SEM images of the Mo10/Ti hybrid nanoflower at different magnifications; (d) HRTEM image; (e) HAADF image; and (f) corresponding mapping images for Mo, Se, Ti, C elements of the Mo10/Ti hybrid nanoflower.

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To shed some light on the fine nanostructure of the hybrid materials, a series of characterizations were carried out on a typical MoSe2/MXene hybrid nanocomposite (denoted as Mo10/Ti hybrid nanoflower) of which the molar ratio of Mo and Ti is 10:1. The morphology investigation by scanning electronic microscopy (SEM) imaging (Figure 2b-c) reveals that the product appears as 3D interconnected flowers which are

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made up by a mountain of 4 nm-thick freestanding petal-like nanosheets. These ultrathin, isolated vertical nanopetals are prone to promote large space for hydrogen

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evolution. Figure 2d presents the high-resolution transmission electron microscopy

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(HRTEM) image of the Mo10/Ti hybrid nanoflower, in which the interlayer distance of

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ca.0.28 nm and ca.0.23 nm corresponds to the enlarged (100) and (103) lattice planes

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of MoSe2, respectively. Figure 2e-f shows the high-angle annular dark field (HAADF)

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image and the corresponding elemental mapping images by energy dispersive X-ray spectroscopy (EDS). The presence and uniform distribution of Mo, Se, Ti and C

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elements proves that every individual nanoscale petal is composed by both Ti3C2 and MoSe2. And the ratio of Ti and C in the elemental mapping images might be attributed to the molecules adsorbed on the surface during hydrothermal synthesis and the following carbonization during subsequent calcination, similar to the MXene materials previously reported [15]. Note that the incorporation of metallic MXene into the semi-conductive MoSe2 is very likely to enhance the electrical conductivity while the unique 3D structure of the nanohybrids would provide a large specific surface area with increased predominant active centers. These two beneficial factors are supposed to simultaneously facilitate the mass diffusion and charge/ion transport process at the

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electrode/electrolyte interface during HER test.

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Figure 3. (a) XRD patterns and (b) Raman patterns of Mo10/Ti, MoSe2, and Mxene; (c) High-resolution XPS spectra of Ti 2p of Ti3C2 MXene. High-resolution XPS spectra of (d) Ti 2p, (e) Mo 3d and (f) Se 3d of Mo10/Ti.

The successful adhesion of MoSe2 to the Ti3C2 MXene nanosheets in the hybrid is also demonstrated by XRD spectrum comparison of MoSe2, Ti3C2 and Mo10/Ti hybrid nanoflower. As shown in Figure 3a, the pure MoSe2 displays four diffraction peaks at ca. 31.4°, 37.8°, 55.9° and 56.9° which are attributed to the (100), (103), (110) and (008) planes of its 2H phase (JCPDS no. 29-0914;

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space group P63/mmc, a = b = 3.287 Å, c = 12.925 Å). The introduction of Ti3C2 does not change the general XRD pattern of 2H-MoSe2 but leads to the occurrence of a weak yet characteristic MXene peak at ca. 25° in the Mo10/Ti hybrid nanoflower. Furthermore, the peak intensity gradually increases with the increased content of Ti3C2 in the heterojunction samples (Figure S3a),

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validating again the coexistence of the two different components. In additon, by comparing with the XRD patterns of pure MoSe2 and Mxene (Figure S3b), the

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peak at about 4 degree can be attributed to the distance between the layers. And

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the value of distance between the layers is 2 nm, which was calculated by the

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Bragg's Law. The characteristic features of MoSe2 and Ti3C2 are also visualized

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by the Raman spectroscopy investigation of the Mo 10/Ti hybrid nanoflower

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(Figure 3b). Specifically, the two peaks at 400 and 600 cm-1 are correlated to the Ti-C vibrations [24] while the other two characteristic peaks at 233.8 and

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281.5 cm-1 are assigned to the A1g and E12g of MoSe2, respectively [25]. The higher peak intensity of A1g compared to E12g may be attributed to the edge-terminated MoSe2 sheets, similar as those reported in the literatures [26]. In order to get more insightful information concerning the surface chemical composition and chemical bonding information of the Mo 10/Ti hybrid nanoflower, X-ray photoelectron spectroscopy (XPS) investigation was thenperformed. As expected, C, Ti, Mo and Se were all detected in the elemental analysis of its XPS spectrum (Figure S4a), signifying that MoSe2 nanosheets were coupled to the ultrathin Ti3C2 nanosheets. Figure 3c presents

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the XPS spectra of the Ti3C2 MXene, in which the two predominant peaks at ca.455.0 and ca.460.1 eV are attributed to Ti–C (2p3) and Ti–C(2p1), respectively. The Ti 2p spectrum can be deconvoluted into four peaks centered at 452.5, 455.8, 460.1 and 465.3 eV, corresponding to Ti(II), Ti(III), Ti(IV) (2p3) and Ti(IV) (2p1) [27]. Apparently, for the Mo10/Ti hybrid nanoflower, the peak related to the high-valence Ti(IV) (2p1) almost vanishes and the peak of

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Ti(IV) (2p3) becomes weaker than that of the Ti3C2. In addition, all the peaks of

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Ti 2p experience a negative shift compared to the Ti 3C2, revealing the Ti

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element presents lower valance state in the composite (Figure 3d). When it

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comes to the Mo and Se elements, distinct disparities are detected before and

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after the introduction of the Ti3C2. To be specific, in the Mo10/Ti nanohybrid,

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the two major peaks at 229.1 and 232.3 eV sample are assigned to Mo 3d5/2 and Mo 3d3/2 (Figure 3e) while two peaks located at 54.5 and 55.3 eV (Figure 3f)

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are ascribed to Se 3d5/2 and Se 3d3/2 [28, 29]. Yet, in the pure MoSe2 control sample, the XPS peaks for Mo 3d5/2 and Mo 3d3/2 lies in 228.6 and 231.7 eV (Figure S4b), and those attributed to Se 3d5/2 and Se 3d3/2 locate at 54.1 and 55.3 eV (Figure S4c). It is thus obvious that the incorporation of Ti 3C2 and MoSe2 results in positive shifts for both Mo and Se, suggesting that both Mo and Se perform the higher valance state in the nanocomposite. These detailed information indicates that MoSe2 coverages are tightly grown on the Ti3C2 scaffold via specific chemical interactions between the species of Ti(IV) (2p1), Ti(IV) (2p3), Mo(IV) and Se, in which the electrons are transferred from MoSe 2

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to Ti3C2. Considering the fact that no peaks indicative of new species are found in the XRD pattern of Mo10/Ti hybrid nanoflower, such electron transfer process may only occur right at the shallow interface between Ti3C2 and MoSe2. The as-observed valence state variation is desired to specifically offer

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more electrochemical active sites and enhance the structural stability.

Figure 4. (a) LSV curves of Ti3C2 MXene, MoSe2 and different MoSe2/MXene hybrid

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rate: 50 mV s-1.

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nanoflowers; (b) LSV curves of Mo10/Ti hybrid nanoflower initially and after 2,000 cycles; scan

The electrocatalytic activity of various MoSe2/MXene hybrid nanoflowers toward HER was then evaluated in 0.5 M H2SO4 solution by recording their linear sweep voltammogram (LSV) curves with a typical three-electrode system. These hybrid nanoflowers with the MoSe 2/Ti3C2 content ratios of 2:1, 4:1, 20:1 to 50:1 are denoted as Mo2/Ti, Mo4/Ti, Mo20/Ti and Mo50/Ti for short. For comparison, similar electrochemical measurements were also performed on their single-component counterparts, namely the MoSe2 nanoflower and the ultrathin Ti3C2 MXene nanosheets. The LSV curves in Figure 4a & Figure S5

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illustrate that the HER performance of the hybrid nanomaterials invariably overmatches

their

single

component

counterparts,

highlighting

the

advantageous properties of the hybridization strategy, and the HER performance parameters of various samples are summarized in the Table S2. In addition, the electrocatalytic activity of the MoSe 2/MXene hybrid nanoflower is

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highly dependent on the MoSe2/Ti3C2 ratios. Specifically, with the increase of the MoSe2/Ti3C2 ratios from 2:1 to 50:1, the catalytic activity first increase and

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then decrease, reaching the best HER performance on the Mo 10/Ti hybrid

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nanoflower. It has an admirable onset potential of ca. 61 mV vs RHE, which is

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substantially lower than that of MoSe2 nanoflowers (~200 mV) and the 2D

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Ti3C2 (~380 mV). Around 6-fold current density increase is also observed at

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-300 mV compared with MoSe2. More importantly, such distinct HER performance outstrips plenty of recently reported MXene-based electrocatalysts

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[21, 30], TMDs nanostructures [31-34] and their hybrids with MXene as well as carbonaceous materials including carbon cloth [35], graphene [36-38], and glassy carbon [39, 40] (Table S4). Additionally, as the cathodic current density rises sharply under more negative potentials, Mo 10/Ti hybrid nanoflower could deliver a relatively high current density of 10 mA cm−2 at an exceptionally low overpotential of ca. 180 mV, greatly surpassing that of the MoSe 2 (350 mV). The greatly-improved HER activity of Mo10/Ti hybrid nanoflower should be contributed by its specific 3D architecture, optimized conductivity and the unique electronic structure of the MoSe2/MXene interface. Long-term stability

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is

a

significant

and

indispensable

criterion

for

high-performance

electrocatalysts. To assess the durability of the Mo 10/Ti hybrid nanoflowers, continuous cycling tests were conducted in an acidic environment. As shown in Figure 4b, the negligible difference is noticed between the polarization curves measured before and after 2,000 cycles, verifying the fascinating stability of our

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Mo10/Ti hybrid nanoflower electrocatalyst.

Figure 5. (a) Tafel plots of MoSe2, MXene and Mo10/Ti hybrid nanoflower; (b) Current due to double-layer charge/discharge plotted against CV scan rate; (c) Nyquist plots of the MoSe2, MXene and Mo10/Ti composites; (d) Mott-Schottky plots of MoSe2 and Mo10/Ti hybrid nanoflower.

To better comprehend the superior activity of the Mo 10/Ti hybrid nanoflower, we firstly compare the Tafel slope of various nanocatalysts which is closely associated with the rate-limiting step of the HER. The smaller value

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corresponds to lower overpotential required for hydrogen evolution. As shown in Figure 5a, the Tafel slopes of these two control samples are identical (200 mV dec−1) whereas the nanohybrid displays an obviously decreased Tafel slope of ca. 91 mV dec−1, suggesting that the nanocomposites are more favorable for quick charge transfer. Cyclic voltammograms (CVs) of various samples at

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different scan rates were then recorded to extract the electrical double-layer capacitance, Cdl, an important parameter commonly used to determine the

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electrochemical surface areas (ECSAs). We can see that the addition of metallic

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Ti3C2 with a particular high Cdl value (236 µF cm-2) to the MoSe2 substrate

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results in an evident increase of Cdl from 33 µF cm-2 for MoSe2 to 44 µF cm-2

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for the Mo10/Ti hybrid nanoflower (Figure 5b). These results suggest that more

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electrocatalytic active sites are introduced to the nanohybrid surface and further boost the HER efficiency.

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Electrochemical impedance spectroscopy (EIS) measurements were also carried out to investigate the charge transfer kinetics for HER and the Nyquist plots are presented in Figure 5c, whose corresponding parameters are shown in the Table S3. By virtue of the high conductivity of Ti3C2, the Mo10/Ti hybrid nanoflower show considerably lower charge-transfer resistance of 16.6 Ω compared to that of the pure MoSe2 (242.4 Ω). To go further, Mott-Schottky curves are plotted to evaluate how the Ti3C2 addictive affects the flat band potential (EFB) and the charge carrier density (ND) of the MoSe2 support [41].

Journal Pre-proof The EFB and ND of samples were derived from intercept and the slope of 1/C 2 vs. E plot based on the following equation [42]: 1 𝐶2

2

= 𝑞ƐƐ

0 𝑁𝐷

[(𝐸 − 𝐸𝐹𝐵 ) −

k𝐵 𝑇 𝑞

（1）

]

where C is the capacitance, ε is the relative permittivity of MoSe2, ε0 is the vacuum

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permittivity, q is elementary electric charge. In Figure 5d, the positive slopes of the MoSe2 and Mo10/Ti hybrid nanoflower reflect their n-type semiconductor nature [43].

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Yet, the EFB of the Mo10/Ti hybrid nanoflower experiences a positive shift in

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comparison with pure MoSe2, indicative of a corresponding fermi level shift towards

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the conduction band with dense carrier density [41, 42, 44]. Meanwhile, Mo10/Ti

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hybrid nanoflower shows an extremely smaller slope, signifying a high ND at the

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contact interface. All in all, the hybrid nanostructure could effectively facilitate the charge transfer process, enhance the diffusion pathway for electrons/ions in the

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capacitive layer, and therefore boost the HER performance.

4. Conclusions

We have designed 3D interconnected MoSe2/MXene hybrid nanoflowers with ultrathin petals as advanced electrocatalysts for efficient HER. The strategic introduction of Ti3C2 skeleton to the MoSe2 gives the nanocatalysts a series of beneficial properties including large ECSA, high conductivity as well as intrinsic stability.

Benefiting

from

these

benign

characteristics,

the

as-fabricated

MoSe2/MXene hybrid nanoflower function as a robust HER catalyst which delivers

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obviously optimized HER performance compared with their single-component counterparts and other hybrid materials reported recently, as evidenced by their low onset overpotential of 61 mV and high current densities at relatively low overpotential. Besides, the MoSe2/MXene hybrid nanoflower exhibits remarkable cycling stability and long-term viability. The strategy to optimize the electrocatalytic properties

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reported herein is also applicable to other energy-conversion chemical reactions

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driven by TMDs.

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Conflicts of interest

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There are no conflicts to declare.

Acknowledgements

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This work was supported by the National Key Research and Development

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Program of China (2016YFA0202604), the National Natural Science Foundation of China (21405182, 21802173 and 21773315), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2017). Pearl River S&T Nova Program of Guangzhou (201710010019). Natural Science

Foundation

of

Guangdong

Province

(2019A1515011117,

2018A030310301). Special Funds for the Cultivation of Guangdong College Students' Scientific and Technological Innovation Climbing Program and “Climbing Program” Special Funds.

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Author Contributions J.J. Huang did the synthesis, characterization, electrochemical catalysis experiments and analyzed the data; X.Q. Liu analyzed the data and wrote the paper; F.F. Meng, L.Q. He, J.X. Wang, J.C. Wu helped with the characterization; X.H. Lu and Y.X. Tong helped with the analysis of the data; P.P. Fang conceived the idea, analyzed the data, wrote the paper, supported and supervised the whole work.

Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which

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may be considered as potential competing interests:

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Graphical abstract

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Highlights 1. A facile hydrothermal method is developed to synthesize a series of MoSe2/MXene hybrid nanoflowers. 2. The HER performance of the nanoflowers is dependent on the composition ratio. 3. Around 6-fold current density enhancement is achieved at -300 mV compared with

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MoSe2.