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Cu S-MoS2 Nano-Octahedra at the atomic scale: Using a Template to Activate the Basal Plane of MoS2 for Hydrogen Production Pradipta Sankar Maiti, Anal Kr Ganai, Ronen Bar-Ziv, Andrey N. Enyashin, Lothar Houben, and Maya Bar Sadan Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018
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Chemistry of Materials
Cu2-xS-MoS2 NanoNano-Octahedra at the atomic scale: Using a Template to Activate the Basal Plane of MoS2 for Hydrogen Production Pradipta Sankar Maitia, Anal Kr Ganaia, Ronen Bar-Ziva,b, Andrey N. Enyashinc,d, Lothar Houbene, and Maya Bar Sadana* a b c d e
Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva, Israel. Nuclear Research Center Negev, Beer-Sheva, Israel Institute of Natural Sciences and Mathematics, Ural Federal University, Ekaterinburg, Russia Institute of Solid State Chemistry, UB RAS, Ekaterinburg, Russia Peter Grünberg Institut 5 and Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany.
ABSTRACT: Designing and realizing functional nanostructures with specific properties is a demanding challenge. It often requires, beyond the knowledge of the atomic arrangement, assembling a puzzle in order to decipher both the growth mechanism and to explain the measured macroscopic properties. Here, we use the hybrid system of Cu2-xS-MoS2 to present one case study where the structural features, growth mechanism, optical properties and catalytic performance, are all woven into one complete picture. We show a facile synthesis of Cu2-xS-MoS2 hybrids, where Cu2-xS serves as a template for a MoS2 layer. The MoS2 phase is altered and modified, structurally and electronically, as a result of the contact with Cu2-xS, according to both experimental HRTEM data and DFT calculations. The electronic reconstruction around the interface activates the outermost S atoms within MoS2 by tuning of the hydrogen adsorption free energy to a more moderate value, thereby promoting the hydrogen evolution reaction (HER). This concept provides a new design strategy to optimize functional MoS2 by pairing a 2D layer with a suitable substrate.
The rational design of functional hybrid materials such as photocatalysts is based on the in-depth understanding of the structure-properties relationship. Although appealing, rational design is limited by extending the materials’ bulk properties to the atomic scale. However, in a realistic world, atomic scale reconstruction at interfaces and even the placement of single atoms affects the overall properties of the hybrid.1 Nevertheless, such reconstructions are hard to characterize, let alone adequately control. Nanostructured MoS2, which has emerged as an efficient catalyst for the hydrogen evolution reaction (HER), is one such example where atomic-scale characterization has contributed to rational design. Triggering the catalytic activity of its basal plane was achieved by doping with single atoms of various transition metals, significantly improving its overall catalytic activity.2 An alternative route is to create an interface of MoS2 with a substrate as means to activate the basal plane of the MoS2, which is relatively inert. Optically active junctions of CdS or Cu-S with MoS2 or WS2 were previously reported.3 Wrapping Cu-S nanoparticles with MoS2 has shown (photo)catalytic activity towards HER4-6, especially with the addition of a metal co-catalyst such as Pt6 or Au.4 However, the design of these materials was based on their bulk known properties, without the fundamental knowledge of the structural and electronic
reconstruction at the interface and the synergistic effect on the catalytic activity. Here we show a one-pot synthesis to prepare octahedral cores of Cu2-xS embedded within a MoS2 cage of 1-2 layers. We present an in-depth understanding of the formation mechanism of the hybrid structures using the atomic-scale structure and DFT calculations, and provide insight to the origin of the improved catalytic activity for HER. We show that mobile Cu atoms promote the crystallographic phase transfer to the metallic-like 1T-MoS2. More importantly, we show that the contact of the Cu2-xS template and the MoS2 layer brings an electronic reconstruction that eventually improves the catalytic activity of the S atoms within the 2H-MoS2 basal plane. The Cu2-xS lattice structure is an anionic framework, where the S2- anions are the skeleton, and the smaller Cu(I) cations fill it with different filling ratios. The deficiency of Cu(I) results in vacancy self-doping, where the charge carrier density depends on the stoichiometry, but is high enough to support localized surface plasmons (LSPR).7-8 The Cu deficiency provides band gaps between 1.2 to 2.0 eV.9-10 The Cu(I) cations within the Cu2-xS are mobile, to the extent that they can leach out and change the charge carri-
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er density and the electro-optical properties of the nanoparticles.11 We developed a one-pot process to synthesize the Cu2xS-MoS2 hybrids: a complex of CuCl2 with octadecene, oleylamine and oleic acid was used as the precursor for the Cu2-xS, and (NH4)2MoS4 in oleylamine as the precursor for MoS2 (full synthetic protocols provided in the supporting information). The nanoparticles were octahedral, 15-20 nm in diagonal and 7-12 nm in edge length (figure 1A). Visible single- or bi- layer MoS2 exist on the octahedra facets (See elemental maps in Figure S2). The yield of the hybrid structures is close to 100%, with mostly single overlayers of MoS2.
monic features of the Cu2-xS system, exhibiting the typical plasmonic wide band in the IR. The very broad plasmon bands may be attributed to the relative size inhomogeneity and to the octahedral shape of the nanoparticles that supports several close plasmon modes. In the different solvents, the plasmon peak red-shifts ⁓130 nm (Figure 2), a signature behavior of LSPR.7 Using the Drude model we calculated the charge carrier density in the hybrids as 4.5∙1020 holes/cm3, or app. 90 holes/particle, which is correlated with Cu vacancies (full calculation in the supporting information).
Figure 2. Optical properties of Cu2-xS-MoS2 nano-octahedra. Dependence of the LSPR maxima on the varying refractive index of the solvents.
Figure 1. TEM images of Cu2-xS-MoS2 nano-octahedra (A) Balland-stick model of the Cu2-xS-MoS2 nano-octahedra (B) and a cross section of the model showing the Cu2-xS core. S in yellow, Mo in purple and Cu in blue.
To understand the growth mechanism of the nanostructures, we followed the change in the UV-Vis absorbance of aliquots taken from the reaction mixture. The typical Cu(0) plasmon at 560 nm12 was not observed during the reaction (figure S3), indicating that the reaction mechanism did not involve two-electron reduction to Cu(0) and then a subsequent sulfidization. Instead, the Cu(II) from the CuCl2 produces a Cu(I) complex,13 seen by the change of color of the reaction mixture from deep blue to green and then to yellow, accompanied by the dampening of the Cu(II) absorption band at 660 nm. Above 2100C, another color transition occurs from yellow to orange, and is attributed to the formation of active monomer species.14-15 The decomposition of (NH4)2MoS4 produces H2S and S that serve as the source for the sulfidization of the active Cu monomer species. Xray photoelectron spectroscopy (XPS) confirmed that the final product contained Cu(I) and Cu(0) was not detected (figure S4). MoS2 was not detected by XRD, probably due to the small fraction of the phase within the structures. The absorbance spectra of the Cu2-xS-MoS2 nanooctahedra were measured in solvents of different refractive indices (figure S5). The hybrids preserved the plas-
HRTEM was used to resolve the atomic structure of MoS2.16-17 Figure 3 shows a high-resolution phase image, after focal series reconstruction, of the atomic structure of the MoS2||Cu2-xS interface. The upper MoS2 facet is in the 2H configuration (usually n-type semiconductor). The lower one is in the thermodynamically unstable 1T configuration, which is metallic. The curved MoS2 layers, especially in the frame marked by (2), potentially indicate trapped atoms within the interface. We did not see a preference for the 2H or 1T formation with respect to specific Cu2-xS facets. To characterize the Cu2S||MoS2 interface, density functional theory (DFT) was used to estimate the stability of a variety of models (details are available in the supporting information). When the interface included a stoichiometric Cu2S phase, only models with the thermodynamically stable 2H-MoS2 were successfully optimized or they had the lowest total energy, which could not explain the occurrence of 1T-MoS2 in the experimental data. In contrast, the calculations confirmed that Cu vacancies, produced by the leaching out of the mobile Cu ions, stabilize the Digenite lattice (Figure S7). Thereafter, the expelled Cu ions are free to migrate to the Cu2-xS||MoS2 interface. The octahedral sites within the vdW gap possess the lowest intercalation energies, 0.3-0.4 eV/Cu-atom for the 1T phase and 0.8-1.2 eV/Cu-atom for the 2H phase. The relative stability of intercalated Cu at 1T-MoS2 is appreciably higher than for Cu intercalated next to 2H-MoS2 in almost all the range of Cu
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Chemistry of Materials occupancy (Figure S9), leading to the conclusion that layers of 2H-MoS2 will inevitably transform into 1T-MoS2 when at a forced contact with an intercalated Cu plane.
Figure 3. Atomic-resolution TEM phase image of MoS2 on the facets of a Cu2-xS octahedron (one-pot synthesis). The phase image reveals the atomic structure of (1) the cubic Cu2-xS phase in 110 viewing, (2) the upper facet terminated by a 2H MoS2 layer and (3) the lower facet with 1T-MoS2 facing a copper terminated Cu2-xS surface. Structure models are superimposed in the magnified parts (Yellow: S, Blue: Cu, Red: Mo).
We considered further only the two interfaces: Cu2S||2HMoS2 and Cu2-xS||1T-MoS2 (Cu2-xS in the Digenite phase). The DFT results unambiguously confirm that Cu2-xS||1TMoS2 is more stable than Cu2S||2H-MoS2, by 1.2 eV/MoS2. Using the calculation of the densities of electronic states (DOS), it is evident that the 2H-MoS2 and the 1T-MoS2 interact differently with the Cu2S phase: The 2H-MoS2 serves as a single electron donor, while the 1T-MoS2 is a single electron acceptor. Therefore, electron density around the Mo atoms of the 2H-MoS2 phase is diminished while it is enhanced in 1T, although in both cases the Mo-S bond remains polar (Figure 4). The redistribution of electron density involves the S atoms, placing a significant negative charge on the outermost S atomic layer in the 1T-MoS2 phase. The latter may explain the preference to form a single layer of MoS2 shell: the precursor MoS42- anions could be repelled electrostatically by the negatively charged external S atoms, potentially preventing further decomposition of the precursor and hindering the formation of a second MoS2 layer. In addition, according to the calculations, the contact of MoS2 with Cu2-xS lowers the hydrogen adsorption free energy (∆GH*) values of the outermost S atom of the MoS2 shell to 0.20 eV for the 2H-MoS2 (Figure 4c). It is already known, from both theoretical and experimental perspectives, that the criterion for a good HER catalyst is a Gibbs free energy of adsorbed H which is close to the
thermo-neutral (i.e., ∆GH≈0). Therefore, the change in the adsorption free energy of the basal S atoms of 2H-MoS2 potentially activates them for catalytic HER.
Figure 4. Charge density redistribution after the formation of a Cu2S||MoS2 interface between: (a) 2H-MoS2 layer in contact with non-polar Cu2S slab; (b) 1T-MoS2 layer in contact with polar Cu2S slab. In both variants, MoS2 is in contact to Cuterminated Cu2S surface and, formally, the models may be attributed to the Cu1.75S||(Cu1.0@MoS2) interfaces. (c) The calculated free-energy (∆GH*) diagram of HER for various possible catalytic sites on the basal planes of MoS2. In red, the calculated energy for the outermost S atom in a basal plane of a single MoS2 layer wrapped over Cu2S template, showing ∆GH=0.20 eV. Other values are presented in Table S1. DFT calculations.
We further explored the activity of the nanohybrids towards hydrogen evolution (HER) in 0.5 M H2SO4 using a three-electrode electrochemical cell. The electrochemical polarization curves are plotted in Figure 5 and confirm that the hybrid materials are improved HER catalysts compared with MoS2 or Cu2S. At a current density of 10 mA/cm2, the Cu2-xS-MoS2 required a substantial lower over-potential of 320 mV compared to the 472 mV (for MoS2) and 592 mV (for Cu2S).
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Cu2S -2
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E (V vs. RHE) Figure 5. The HER polarization curves obtained with Cu2-xSMoS2 hybrid, MoS2, Cu2S and commercial Pt/C in 0.5 M H2SO4. (Tafel slopes are presented in Figure S11 (supporting information; scan rate of the polarization curves was 10 mV·sec-1).
The HER kinetics were also estimated using the corresponding Tafel plots (Figure S11). MoS2 or Cu2S alone show inferior catalytic properties compared with the hybrid for both the overpotential and the Tafel slope. The Tafel slope values for Cu2-xS-MoS2 is ~60 mV dec-1, much smaller than that of MoS2 (~ 150 mV dec-1), indicating a more rapid HER rate for the hybrid. The excellent Tafel slope value is better than many recently reported MoS2based catalysts reported.2, 18-19 The templating of the MoS2 shell using the Cu2-xS core brought about a synergistic effect: First, the electronic reconstruction around the interface results in improved ∆GH* values for the S atoms within the basal plane, thus creating new, additional, catalytic sites. Moreover, the nano-octahedra structure, naturally fitting on the Cu2-xS, was already predicted as a potential catalyst due to the atomic-scale construction of its corners.20-21 The Cu ions at the MoS2 interface further promote the formation of the 1T-MoS2 phase, by itself of benefit for catalysis due to its higher conductivity. Last, the Cu ions may induce lattice defects or even Cu dopants into the MoS2. Recent DFT calculations reveal that the catalytic activity of the basal plane of 1T-MoS2 phase can be significantly improved by Cu substituents.22 In a control experiment, Cu2-xS-MoS2 hybrids were synthesized in a two-stage process. The prepared Cu2S particles (see Figure 5) were wrapped with MoS2 and characterized (See the supporting information for full description). The Cu2S particles were close to stoichiometric as evident by the absence of the plasmonic peak in the optical spectrum, but the deposition of the MoS2 was accompanied by the emergence of the plasmonic peak that indicates the deviation from stoichiometry (Figure S13). The crystallographic phase of the Cu2S was hexagonal, as opposed to the particles formed by the one-pot synthesis, and the phase was retained after the MoS2 deposition (Figure S14). Although the Cu2-xS||MoS2 interface in the two-stage particles is very different from the one of the one-pot synthesis,
both hybrid particles perform remarkably similar as HER catalysts (Figure S15), confirming that we describe a general phenomenon. In summary, we showed the in-depth understanding of the growth mechanism of Cu2-xS-MoS2 grown in a one-pot synthesis using colloidal chemistry. The obtained structures are stabilized by leaching of Cu ions to the interface with the MoS2, thus creating an improved MoS2 surface for hydrogen evolution. In addition, the formation of the 1T phase is accompanied by a substantial difference in the electronic structure, which is a possible factor in the growth mechanism and the arrested growth of additional MoS2 layers. The atomic-scale structural analysis provides the indispensable insights into the growth process and the origin of the macroscopic properties, which were supported by the DFT calculations. This approach opens a new design strategy to optimize functional MoS2 by pairing a 2D layer with a suitable substrate. Moreover, this phenomenon is an important parameter in designing devices based on single layered MoS2.
ASSOCIATED CONTENT Supporting Information. Synthetic Protocols, XRD, XPS, UVVis, TEM and DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Corresponding Author *
[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Funding Sources This research project was funded by ISF grant 808/16 and supported by Act 211 Government of the Russian Federation, contract No.02.A03.21.0006.
ACKNOWLEDGMENT We appreciate support from Dr. Vladimir Ezersky and Oren E. Meiron.
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