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Amorphous molybdenum sulfide quantum dots: an

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Sep 22, 2016 - used different tungsten phosphide nanomaterials for HER purposes at ... synthesized amorphous molybdenum sul de quantum dots through a ...

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Cite this: J. Mater. Chem. A, 2016, 4, 15486

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Amorphous molybdenum sulfide quantum dots: an efficient hydrogen evolution electrocatalyst in neutral medium† Diptiman Dinda,‡a Md. Estak Ahmed,‡b Sumit Mandal,a Biswajit Mondalb and Shyamal Kumar Saha*a To overcome the limitations of active edges, electrical conductivity and the surface area of MoS2 nanosheets, in the present work, we have successfully synthesized amorphous MoSx quantum dots with a larger number of edge atoms using a simple chemical reaction via a bottom-up approach. Structural and chemical characterizations are carried out by TEM, XRD, Raman and XPS measurements. XPS and EDX analyses indicate a larger number of unsaturated ‘S’ atoms in these ultrafine amorphous quantum dots. We have used this material as an efficient electrocatalyst for the hydrogen evolution reaction (HER) in neutral medium. The material shows a remarkably low overpotential (65 mV) towards HER compared

Received 19th July 2016 Accepted 6th September 2016

to that of other crystalline MoS2 quantum dots or nanomaterials. The origin of such a low onset potential is the presence of more unsaturated sulfur (S22) ligands and enhanced active edge sites. It also shows

DOI: 10.1039/c6ta06101j

very high catalytic activity as well as good stability after 12 h of hydrogen generation in neutral water

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medium.

Introduction One of the most important challenges in the world is the supply of fuel due to increasing energy demands for the up-growing civilizations. Hydrogen is one of the most promising energy carriers that can meet the ever increasing energy demand.1 Intensive research has been carried out over the past few years to develop new systems capable of producing hydrogen from water.2–5 The hydrogen evolution reaction (HER) using water splitting technology is considered as a sensible process for this purpose.6,7 Until now, platinum (Pt) and its alloys are the most effective electrocatalysts for HER, but the high cost and scarcity of these metals largely prevent their practical applications.8,9 Therefore, it is important to explore efficient alternative electrocatalysts that are cheap and easily processable for the future generation of hydrogen as an alternative energy source.

a

Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India. E-mail: [email protected]

b

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

† Electronic supplementary information (ESI) available: Comparison tables of the onset potential and exchange current density, optical measurements, LSV experiment in different media, calculation of TOF and TON, faradic efficiency, LSV curve of a bare ITO plate, modied Tafel plot at the lower overpotential region, electrochemical measurements along with Nyquist plots on a Cu foil substrate, XPS and SEM analysis of the sample aer the HER process and different characterizations of amorphous nanoparticles along with stability curves are given. See DOI: 10.1039/c6ta06101j ‡ Authors contributed equally to this work.

15486 | J. Mater. Chem. A, 2016, 4, 15486–15493

Although nature's hydrogenase enzymes have produced hydrogen from a neutral (pH 7) solution effectively, they suffered from a lower number of active sites and poor stability in solution.10 This limits their practical use. In most of the cases, strong acids or bases have been frequently used to generate hydrogen efficiently. These create lots of environmental and handling issues. Therefore, an exceedingly active and stable electrocatalyst for the production of hydrogen from neutral water medium is highly desirable. Apart from Pt, a very few non-noble metal compounds have been investigated as catalysts for this purpose.11–16 Recently, Prof Sun's group have used different tungsten phosphide nanomaterials for HER purposes at pH 7.17–19 Aer the discovery of graphene, recent advances in 2-dimensional (2D) materials have opened up a new horizon for a novel class of low-dimensional systems. Interesting physical and chemical properties along with excellent potential in electronics, optoelectronics and energy conversion applications make these materials a subject of topical interest to the scientists.20–23 Among those, molybdenum sulde (MoSx), chalcogenide derivatives of molybdenum have proven to be very promising materials in the designing of transistors, topological insulators, batteries, optical sensors and catalysts.24–27 Since the hydrogen binding energy of molybdenum sulde is close to that of Pt-group metals, their composites have been regarded as the promising substituent in the HER process.28–33 However, the limitations of active edges, electrical conductivity and the surface area of MoSx nanostructures have hindered their use in the bulk production of hydrogen. Accordingly, exhaustive

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research has been focused on the achievement of more active sites at the edges and higher surface area which are crucial to the catalytic activity toward HER. There have been many reports focusing on the preparation of layered-type molybdenum sulde nanostructures; however, reports on the synthesis of quantum dots are relatively few.34–36 Compared to nanosheets, quantum dots have a higher surface area with more edge atoms, which are very useful in designing advanced HER catalysts.37–41 Besides this, the sulfur content in MoSx materials also plays an important role in the HER process. The unsaturated sulfur atoms on the surface of MoSx materials take part in the discharge reaction and adsorb more hydrogen ions leading to the quick formation of hydrogen taking electrons from the electrode. Earlier, Hu's group had reported that amorphous MoS3 particles with S22 ligands show superior catalytic activity towards HER.42 Recently, Li's group has also explained the higher HER efficiency of the MoSx materials due to the presence of bridging S22 or apical S2 in their amorphous states.43,44 Therefore, designing of amorphous MoSx nanocatalysts with more active sites, superior electrical conductivity and better stability is still a big challenge for HER performance especially in neutral pH medium. To explore this, in the present work, we have successfully synthesized amorphous molybdenum sulde quantum dots through a simple chemical reaction via a bottom-up approach. We have been able to control their size below 4 nm. We have characterized it by different microstructural analysis techniques like TEM, XPS, RAMAN and XRD analysis. The XPS study reveals that the material contains an intense S 2p3/2 peak for S22 ligands besides the traditional S2 peak which symbolizes more unsaturated S atoms on its surface. We have successfully used these amorphous ultrane quantum dots as an efficient electrocatalyst for HER in neutral medium. It shows a very low onset overpotential (h) of 65 mV towards HER compared to that of other crystalline MoS2 quantum dot materials. The presence of more unsaturated (S22) sulfur ligands and the enhancement in the active edge sites give such a low onset potential value.45,46 The material also shows very high catalytic activity along with excellent stability aer 12 h of hydrogen production. We have also studied charge transfer resistance for the system by electrochemical impedance spectroscopy during HER. It gives an Rct of 604 U at 70 mV overpotential and gradually decreases to 64 U with an increase in the potential conrming more catalytic activity of the material for improved HER activity. Finally, such a material with more exposed surface areas and unsaturated S22 ligands will pave a new pathway to pursue efficient electrocatalysts for energy production and conversion in the near future.

Results and discussions Microstructural analysis To obtain structural information, we have performed the XRD analysis of the as-synthesized material. From Fig. 1(a) it is seen that no sharp peaks are observed for the crystalline phase, indicating the formation of an amorphous phase. Very broad peaks at 2q values 20 , 34 and 57 reveal the growth of

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Journal of Materials Chemistry A

Fig. 1 (a) XRD patterns and (b) Raman spectra of amorphous molybdenum sulfide quantum dots.

amorphous molybdenum sulde nanoparticles by a bottom-up chemical approach. Raman spectra as shown in Fig. 1(b), exhibit the typical characteristic peaks of molybdenum sulde nanoparticles. Two broad peaks at 384 and 409 cm1 are attributed to the in-plane vibration (E2g) of Mo and S atoms and the out-of-plane vibration (A1g) of S atoms respectively. Both the peaks are slightly blue shied compared to their crystalline phase, possibly due to more structural defects and disorders in their amorphous phase.47 XPS analysis is also performed to measure the binding energy of bonded elements, from which we can estimate the various chemical states of those elements. Fig. 2(a) shows the characteristic peaks of Mo and S atoms in low range XPS spectra. The binding energy of Mo 3d5/2 and 3d3/2 peaks are observed at 229 and 232.2 eV, respectively, indicating the +IV oxidation state of Mo in the material. The S 2s peak at 226.2 eV is also visible in the Mo region. To elucidate the nature of ‘S’ ligands, we have also analyzed the high resolution ‘S 2p’ peak of the sample. All the spectra are quite similar to the reported amorphous molybdenum sulde materials compared to those of crystalline MoS2 nanoparticles. From Fig. 2(b), we can see that the S 2p peak is the best tted with two doublets of S 2p3/2 energy states. One doublet appeared at 161.9 eV, corresponding to the S2 ligand and the other at 163.9 eV for the bridging disulde S22 ligand or apical S2 ligand.48 Besides these, two peaks at 163.2 and 164.8 eV with

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evolution reaction in argon saturated pH 7 buffer solution (containing 100 mM of Na2HPO4$2H2O and 100 mM of KPF6 as the supporting electrolyte). To investigate the electrocatalytic HER activity of this material, linear sweep voltammetry (LSV) measurements are conducted using a conventional three electrode system. To do this, we have prepared a working electrode with our sample. Fig. 4(a) shows the polarization curve with a sweep rate of 2 mV s1. When the onset potential is about 0.07 V vs. RHE, a large cathodic current starts to increase along with the gas bubbling on the surface of the electrode and thereaer, the cathodic current rises rapidly at more negative potentials. The as-synthesized material exhibits quite higher HER activity at a very low onset overpotential (h) of 65 mV compared to that of the conventional MoSx-based nanostructures in the literature at pH 7 as given in Table S1.† For a further study of it's HER activity, current densities at higher potentials are tted to the Tafel equation (h ¼ a + b loghji; where b is the Tafel slope). From Fig. 4(b), it is seen that the catalyst shows a Tafel slope of 73.9 mV dec1, which is comparable to the reported values for bare MoS2 nanoparticles. A comparatively higher Tafel slope suggests that the Volmer–Heyrovsky mechanism is operative during this HER process following the reactions given below;49 H2O + e-catalyst / H-catalyst + OH (Volmer reaction) H2O + e-catalyst + H-catalyst / H2-catalyst + catalyst + OH (Heyrovsky reaction) Fig. 2 (a) XPS spectra of Mo 3d and (b) deconvoluted XPS spectra of S 2p in amorphous molybdenum sulfide quantum dots.

low intensity appeared for the S 2p1/2 of S2 and S22 ligands, respectively. Therefore, this intense S 2p3/2 peak for the S22 ligand indicates that more unsaturated S ligands, which are associated with the enhanced HER activity, are present in the assynthesized amorphous MoSx material. TEM measurements are carried out to analyze the size and structural property of the quantum dots. Fig. 3(a) shows the homogeneous distribution of the quantum dots over a large area. From the high resolution TEM (HRTEM) images as shown in Fig. 3(b), a narrow size distribution of these quantum dots between 1.2 and 4 nm with no lattice fringes is observed. The inset shows the size distribution curve of these quantum dots with an average size of 2.3 nm. The SAED pattern in Fig. 3(c) clearly shows the formation of amorphous quantum dots. The EDX elemental analysis given in Fig. 3(d) also conrms the formation of molybdenum sulde quantum dots with a high S/Mo atomic ratio of 2.6. Such uniformly distributed quantum dots with an extremely small size contain structural disorder and more active sites which make the material a potential candidate for the HER process. HER activity Now, we have used these amorphous molybdenum sulde quantum dots as an electrochemical catalyst for the hydrogen

15488 | J. Mater. Chem. A, 2016, 4, 15486–15493

where, the e-catalyst represents metal-bound electrons, and H-catalyst and H2-catalyst represent a hydrogen atom and a hydrogen molecule adsorbed to the surface metal atoms of the catalyst, respectively. To compare the catalytic efficiency, we have also measured it's HER activity under acidic as well as basic conditions. We have carried out LSV experiments in 0.5 M H2SO4 and 1 N NaOH solutions. Under acidic conditions, our material shows very promising HER activity with a very high current density above 5 mA cm2. But, the catalytic activity in alkaline medium is not so impressive due to the absence of protons in the solutions. We have attached their activity in Fig. S3 and S4 of the ESI.† Since the present study aims at exploring the electrolytic HER property at pH 7, we have carried out all the catalytic experiments in this neutral medium. Besides this, another important parameter is the exchange current density, j0, one of the widely used parameters to evaluate the HER activity. It is found to be 8.71 mA cm2 for our system, which is quite comparable to other MoS2-based nanocomposites as given in Table S2.† This reects the abundance in the number of active edge sites in the material to adsorb more hydrogen ions on its surface.46 Apart from that, other useful parameters to estimate its inherent catalytic activity is the turnover frequency (TOF). TOF is the number of H2 molecules evolved from an active site per second. The calculated TOF for this material is 2.83 s1 at 480 mV overpotential. This high value of TOF in our sample strongly suggests the presence of bridging S22 or apical S2 ligands that contribute such superior catalytic

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Fig. 3 (a) Low-resolution TEM images, (b) high-resolution TEM images including the size distribution curve in the inset, (c) SAED pattern and (d) EDX analysis (inset shows the elemental mapping) of amorphous molybdenum sulfide quantum dots with a particle size