Research Article Cite This: ACS Catal. 2018, 8, 1683−1689
pubs.acs.org/acscatalysis
MoS2 Quantum Dots as Efficient Catalyst Materials for the Oxygen Evolution Reaction Bishnupad Mohanty,†,‡ Mahdi Ghorbani-Asl,*,§ Silvan Kretschmer,§ Arnab Ghosh,∥,⊥ Puspendu Guha,∥,# Subhendu K. Panda,∇ Bijayalaxmi Jena,‡ Arkady V. Krasheninnikov,*,§,○,¶ and Bikash Kumar Jena*,†,◆ †
CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, India Department of Chemistry, Utkal University, Bhubaneswar 751004, Odisha, India § Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, 01328 Dresden, Germany ∥ Institute of Physics, Bhubaneswar 751005, India ⊥ Department of Physics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India # Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400085, India ∇ CSIR-Central Electrochemical Research Institute, Karaikudi 630003, India ○ Department of Applied Physics, Aalto University School of Science, P.O. Box 11100, 00076 Aalto, Finland ◆ Academy of Scientific & Innovative Research, New Delhi 110001, India ¶ National University of Science and Technology MISiS, 119049 Moscow, Russia
ACS Catal. 2018.8:1683-1689. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/14/18. For personal use only.
‡
S Supporting Information *
ABSTRACT: The development of an active, earth-abundant, and inexpensive catalyst for the oxygen evolution reaction (OER) is highly desirable but remains a great challenge. Here, by combining experiments and first-principles calculations, we demonstrate that MoS2 quantum dots (MSQDs) are efficient materials for the OER. We use a simple route for the synthesis of MSQDs from a single precursor in aqueous medium, avoiding the formation of unwanted carbon quantum dots (CQDs). The as-synthesized MSQDs exhibit higher OER activity with a lower Tafel slope in comparison to that for the state of the art catalyst IrO2/C. The potential cycling of the MSQDs activates the surface and improves the OER catalytic properties. Density functional theory calculations reveal that MSQD vertices are reactive and the vacancies at the edges also promote the reaction, which indicates that the small flakes with defects at the edges are efficient for the OER. The presence of CQDs affects the adsorption of reaction intermediates and dramatically suppresses the OER performance of the MSQDs. Our theoretical and experimental findings provide important insights into the synthesis process of MSQDs and their catalytic properties and suggest promising routes to tailoring the performance of the catalysts for OER applications. KEYWORDS: MoS2, quantum dots, electrocatalysis, oxygen evolution reaction, first-principles calculations, defects
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comparison to the HER, the OER involves a multistep (proton coupled with four electrons) oxidation process and gives sluggish electrode kinetics.6 The state of the art Ir- and Rubased catalysts give a higher rate for the OER process, but the scarcity and high cost of these materials limit their practical applications.7,8 Therefore, the search for new and efficient catalysts to accelerate this inherently sluggish kinetics by decreasing the overpotential and increasing the rate of conversion is of great importance, and a great deal of effort
he concerns over the hasty depletion of fossil fuels, the cumulative increase in energy consumption, and the associated ecological issues have compelled society to explore new energy resources, especially for automotive industries. In combination with the energy harvested from the sun and wind, water is the best renewable resource to produce fuel and minimize the effects of fossil fuels on the environment.1−3 It is no surprise that the process of water splitting to hydrogen (H2) and oxygen (O2) has attracted substantial attention as a clean and eco-friendly energy source. The hydrogen evolution reaction (HER) is important for the generation of the cleanest fuels, while the oxygen evolution reaction (OER) is equally important for fuel cells and metal−air batteries.4,5 In © 2018 American Chemical Society
Received: September 17, 2017 Revised: December 15, 2017 Published: January 4, 2018 1683
DOI: 10.1021/acscatal.7b03180 ACS Catal. 2018, 8, 1683−1689
Research Article
ACS Catalysis
morphology (Figure 2). As is evident from Figure 2B, the assynthesized MSQDs are rather narrow in size distribution.
has recently been focused on transition-metal-based electrocatalysts, such as metal oxides,9,10 phosphides,11−14 sulfides,15−17 and selenides,18−20 for the HER and OER due to their high catalytic stability and abundance. At the same time, two-dimensional (2D) transition-metal dichalcogenides (TMDs) such as MoS2, WS2, and MoSe2 have recently received an enormous amount of attention21−24 in the context of catalysis due to their morphology and reduced dimensionality, as well as chemical properties. Specifically, MoS2 has been extensively studied as a unique electrocatalyst for the HER.25,26 The activity of MoS2 and other TMDs toward the OER has also been investigated. 27,28 It has been demonstrated that sulfur-terminated edges of MoS2 flakes are catalytically very active.29 Therefore, nanostructures such as quantum dots (QDs) of MoS2 must be favored as efficient materials over their bulk counterparts. The electrochemical HER on MoS 2 QDs (MSQDs) has been thoroughly studied,30,31 but the performance of the MSQDs toward the OER has not yet been explored. Moreover, the synthesis of MSQDs is normally done in the presence of carbon (organic) solvents.32 This leads to the formation of unavoidable carbon QDs (CQDs) and ultimately may interfere with the properties and performance of MSQDs. In this report, a single-step hydrothermal synthesis technique has been developed using a single precursor ((NH4)2MoS4), avoiding the carbon source (organic) solvents, as schematically shown in Figure 1. The resultant MSQDs exhibit excellent
Figure 2. (A, B) TEM and (C, D) HRTEM images of MSQDs. The inset in (A) is the selected area electron diffraction pattern of MSQDs. The inset in (B) shows the statistical analysis of size distribution of the as-synthesized MSQDs.
Most of the particles have sizes of less than 5 nm with very low aggregation. The HRTEM image shows a lattice spacing of 2.3 Å which is assigned to the (110) plane of a hexagonal pattern of MoS2.34 The selected area electron diffraction pattern indicates that the MSQDs are crystalline in nature.35 The atomic valence state and chemical composition measurement were carried out by X-ray photoelectron spectroscopy (XPS) (Figure S1). The high-resolution XPS of the Mo 3d region in MSQDs was deconvoluted into two main intense peaks at 232 and 235 eV that correspond to the Mo 3d5/2 and Mo 3d3/2 signatures, thus revealing the presence of mixed valences of +4 and +5 states of Mo. A similar observation has been documented by the Shaijumon group.30 This indicates that the edges of the MSQDs are slightly oxidized, usually being in contact with water and the atmosphere. The above conclusion has been confirmed by the findings of the peak at 168 eV, which points to the presence of S−O bonds.36 Raman spectroscopy experiments were carried out to compare the spectra of the as-synthesized MSQDs with those of the pristine MoS2 (Figure 3A). The pristine MoS2 shows two strong characteristic peaks at around 379.3 and 404.5 cm−1 associated with the in-plane 1 E2g and out-of-plane A1g vibrational modes of the Mo−S bond, respectively.37 The frequency, intensity, and width of the peaks observed for 1E2g and A1g vibrational modes reflect the layer thickness of the MoS2.38 The as-synthesized MSQDs show 1E2g and A1g vibrational modes at around 380 and 403.3 cm−1, respectively. Interestingly, it has been observed that the MSQDs exhibit a red shift of 1E2g and blue shift of A1g vibrational modes, in comparison to the pristine MoS2. This confirms that the as-synthesized MSQDs consist of a few layers of MoS2. This is in agreement with similar observations reported by Lee et al.39 The intensity ratio of the A1g and 1E2g modes was derived to deduce the texture information on the assynthesized MSQDs. The A1g/1E2g ratio of MSQDs has a higher value of 3.32 in comparison to the pristine MoS2 (2.36), which evidently reflects the higher edge-terminated structure.40
Figure 1. Schematic representation of the MSQD synthesis. Ammonium tetrathiomolybdate was used as a single precursor for MoS2 growth.
electrocatalytic activity toward the OER due to reactive sites, including vertices and defects at MS2 edges, as confirmed by our first-principles calculations. The surface of MSQDs was activated by potential cycling to explore the performance. For comparison, the MSQDs have also been synthesized in the presence of an organic solvent (DMF) to produce a mixture of MoS2 and carbon QDs (MSQDs@CQDs) under similar synthesis conditions. The electrocatalytic activity has been analyzed to reveal the influence of the unavoidable CQDs on the OER performance of sole MSQDs. A single precursor, ammonium tetrathiomolybdate, was used as the source for both Mo and S. Hydrazine was utilized as the reducing agent for the formation of MoS2. The possible reaction in the formation of MoS2 from (NH4)2MoS4 in the presence of hydrazine is presented in eq 1.33 2(NH4)2 MoS4 + N2H4 → 2MoS2 + N2 + 2(NH4)2 S + 2H 2S
(1)
Details of the synthesis process are given in the Supporting Information. After the formation of MSQDs, high-resolution TEM (HRTEM) was used to characterize their sizes and 1684
DOI: 10.1021/acscatal.7b03180 ACS Catal. 2018, 8, 1683−1689
Research Article
ACS Catalysis
MSQDs to charge recombination through defect states that exist mostly on the edges of the QDs.45 Having characterized the synthesized MSQDs, we proceed to the study of their OER properties. The catalyst was modified over a glassy-carbon (GC) working electrode. The OER polarization curves were recorded by linear sweep voltammetry (LSV) at a scan rate of 5 mV/s in 1 M KOH. A control experiment on GC was also performed for verification. Benchmark experiments on the catalyst IrO2/C and pristine MoS2 were carried out for comparison. All of the polarization curves were iR corrected, and the LSV plots are shown in Figure 4A. The IrO2/C catalyst exhibited higher OER activity
Figure 3. (A) Raman spectrum of pristine MoS2 (the inset shows the Raman spectrum of MSQDs), (B) UV−visible spectrum of MSQDs (the inset shows the images before and after irradiation of UV light from a 365 nm lamp). (C) PL spectra of MSQDs excited at different wavelengths. (D) Fluorescence lifetime spectra at different emission wavelengths.
The UV−visible spectrum of the as-prepared MSQDs gives a single peak near the UV region at 300 nm (Figure 3B). The MSQD solution displays blue fluorescence on irradiation with a 365 nm UV lamp (inset of Figure 3B). This absorption peak at 300 nm was assigned as the signature of the excitonic features of MSQDs.41 The small MSQDs cause a quantum confinement effect, which leads to an increase in the band gap. The gap was estimated to be around 3.73 eV, which is much higher than that of bulk MoS2 (1.2 eV) and monolayer MoS2 (1.9 eV).42 The high uniformity of size and thickness of MSQDs would produce an inherent photoluminescence (PL) spectrum. Therefore, the PL spectra of MSQDs dispersions were recorded at different wavelengths (Figure 3C). The increase in excitation wavelength led to a red shift in the luminescence emission spectra, and it was observed over wavelengths ranging from nearly 350 to 550 nm. The obtained broad peak may be attributed to a direct band-edge recombination. The excitation-dependent luminescence indicates polydispersity in the lateral dimension of the MSQD dispersions.43 The red shift in the emission is likely due to the formation of deeper trap states for the uncompensated sulfide ions and metal ions on the surface of the QDs. The excitation at 320 nm shows a maximum peak intensity at 400 nm, and the peak intensity was observed to decrease with an increase in the excitation wavelength. This gives a particle size distribution which agrees well with the TEM observations.44 To understand the nature of the recombination processes of the MSQDs, the fluorescence lifetime spectra were taken at different emission wavelengths. All fit well to a third-order exponential decay profile with an average reduced weighted residual χ2 value of
