Letter Cite This: Nano Lett. 2019, 19, 4518−4526
pubs.acs.org/NanoLett
Interface Modulation of Two-Dimensional Superlattices for Efficient Overall Water Splitting Pan Xiong,†,∥ Xiuyun Zhang,§,∥ Hao Wan,‡,∥,⊥ Shijian Wang,† Yufei Zhao,† Jinqiang Zhang,† Dong Zhou,† Weicheng Gao,§ Renzhi Ma,*,‡ Takayoshi Sasaki,*,‡ and Guoxiu Wang*,†
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†
Centre for Clean Energy Technology, School of Mathematical and Physical Sciences, University of Technology Sydney, Sydney, NSW 2007, Australia ‡ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan § College of Physical Science and Technology, Yangzhou University, Yangzhou 225002, China S Supporting Information *
ABSTRACT: Molecular-scale modulation of interfaces between different unilamellar nanosheets in superlattices is promising for efficient catalytic activities. Here, three kinds of superlattices from alternate restacking of any two of the three unilamellar nanosheets of MoS2, NiFe-layered double hydroxide (NiFe-LDH), and graphene are systematically investigated for electrocatalytic water splitting. The MoS2/NiFe-LDH superlattice exhibits a low overpotential of 210 and 110 mV at 10 mA cm−2 for oxygen evolution reaction (OER) and alkaline hydrogen evolution reaction (HER), respectively, superior than MoS2/ graphene and NiFe-LDH/graphene superlattices. High activity and stability toward the overall water splitting are also demonstrated on the MoS2/NiFe-LDH superlattice bifunctional electrocatalyst, outperforming the commercial Pt/C-RuO2 couple. This outstanding performance can be attributed to optimal adsorption energies of both HER and OER intermediates on the MoS2/NiFe-LDH superlattice, which originates from a strong electronic coupling effect at the heterointerfaces. These results herald the interface modulation of superlattices providing a promising approach for designing advanced electrocatalysts. KEYWORDS: Interface modulation, superlattices, unilamellar nanosheets, overall water splitting
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been explored and exhibited improved OER activities as well as water splitting performance.12−14 The heterostructured composites not only inherit the intrinsic properties of both components but also generate some completely novel and significantly improved activities, which are primarily attributed to the synergetic effects associated with the heterointerfaces.7,15−18 In particular, 2D heterostructures of vertically stacked, atomically thin layers with an impressive ratio of heterointerfaces are expected to exhibit more striking synergetic effects.4,19,20 So far, many 2D heterostructures based on ultrathin nanosheets have been developed.21−24 For instance, Jia et al. reported a heterostructure by stacking exfoliated layered double hydroxide nanosheets onto defective
wo-dimensional (2D) materials, especially 2D unilamellar crystals with a typical thickness that is atomically thin, that is, elementary building blocks of the parent layered compounds, have gained enormous attention in electrocatalysis by virtue of their unique structural and electronic properties.1,2 Recently, the construction of tailored heterostructures based on these nanosheets has provided new possibilities to control electronic states for further improved performance.3,4 For example, molybdenum disulfide (MoS2) is considered as a promising noble metal-free catalyst with high hydrogen evolution reaction (HER) activity in acidic media.5 However, its sluggish HER kinetics in alkaline electrolytes still remains a big challenge.6 In this regard, heterostructures of 2D MoS2 nanosheets have been developed with efficient HER activity in basic media.7−11 Similarly, heterostructures based on 2D layered double hydroxide nanosheets, a kind of promising nonprecious-metal electrocatalyst for oxygen evolution reaction (OER), have also © 2019 American Chemical Society
Received: March 31, 2019 Revised: May 26, 2019 Published: June 3, 2019 4518
DOI: 10.1021/acs.nanolett.9b01329 Nano Lett. 2019, 19, 4518−4526
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Figure 1. (A) Schematic of design and synthesis of superlattice by solution-phase assembly of different kinds of oppositely charged unilamellar nanosheets. Schematic of (B) LDH/G superlattice for OER, (C) MoS2/G superlattice for HER, and (D) MoS2/LDH superlattice for overall water splitting.
only active materials for electrocatalytic hydrogen evolution but also as a conductive matrix to improve the conductivity of the entire heterostructures.31−33 Although these unilamellar nanosheets are similar in terms of highly anisotropic 2D morphology, they are significantly different in crystal and electronic structures, which have profound effects on the coupling interactions of the resulting superlattices. Therefore, it is highly demanded to systematically investigate the interfacial hybridization effects between different unilamellar nanosheets and determine the best combinations for ultimately efficient energy electrocatalysis. However, to the best of our knowledge the molecular-scale manipulation of interfaces between adjacent unilamellar nanosheets in superlattices has rarely been systematically investigated. Herein, three kinds of superlattices, MoS2/NiFe-LDH (MoS2/LDH), MoS2/graphene (MoS2/G), and NiFe-LDH/ graphene (LDH/G), were investigated as catalysts for electrochemical water splitting (Figure 1). The MoS2/LDH superlattice composed of alternately restacked unilamellar metallic MoS2 and LDH nanosheets showed much improved electrocatalytic activities than MoS2/G and LDH/G superlattices. A low overpotential of 210 and 110 mV was obtained at 10 mA cm−2 for OER and alkaline HER, respectively. First-principles calculations revealed a much stronger coupling effect between MoS2 and LDH nanosheets in the MoS2/LDH superlattice than those in MoS2/G and LDH/G superlattices, which provides optimal adsorption energies for intermediates of both OER and HER. As a result, a possible mechanism for water splitting reactions on the MoS2/LDH superlattice bifunctional catalyst
graphene as a bifunctional catalyst for water splitting. The enhanced activities can be ascribed to direct and strong coupling interactions between the defect sites of graphene and transition metal atoms on exfoliated layered double hydroxide nanolayers.24 Recently, superlattice-like structures using 2D unilamellar nanosheets as building blocks has been developed for full utilization of the advantages of different unilamellar nanosheets for efficient electrocatalysis.4,25−27 The 2D superlattices are composed of alternately restacked 2D unilamellar nanosheets on top of each other. Such a periodic structure of alternate stacking is very different from traditional heterostructures in which each monolayer can simultaneously act as an active component and as an interface. Because of the ultimately thin thickness of the unilamellar nanosheets, it is also possible that enhanced interfacial charge separation and transfer between layers are both available.20,28 As the types of known and available 2D unilamellar nanosheets are increasing, the selective combination of these unilamellar nanosheets for a superlattice allows vast possibilities. Generally, these superlattices as energy-functional materials are based on at least one conductive component for rapid charge transfer. The molecular-scale integration of conductive graphene nanosheets with unilamellar layered double hydroxide and transition metal dichalcogenide (TMD) nanosheets has demonstrated great potential for efficient electrocatalysis.29−31 As alternative candidates for graphene, other conductive nanosheets such as metallic TMDs have attracted intensive attention due to their high conductivity and rich surface chemistries.11,32 Li-intercalated and exfoliated MoS2 nanosheets with a high concentration of metallic 1T phase can act as not 4519
DOI: 10.1021/acs.nanolett.9b01329 Nano Lett. 2019, 19, 4518−4526
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Figure 2. Typical AFM images of (A) MoS2 nanosheets and (B) LDH nanosheets. (C) TEM image, (D) XRD pattern, and (E) HRTEM image of MoS2/LDH superlattice. (F) SEM image and (G) corresponding elemental mapping of MoS2/LDH superlattice.
Figure 2C shows a TEM image of the flocculated MoS2/LDH product in which the morphology of restacked thin sheets was observed. Figure 2D illustrates a typical X-ray diffraction (XRD) pattern of the as-obtained MoS2/LDH. Two broad peaks at small angles corresponding to a basal spacing of 0.85−0.95 nm were observed. The other four peaks were ascribed to the inplane 10 and 11 reflections of these two unilamellar nanosheets. The measured basal spacing is close to half of the sum of thicknesses of MoS2 (∼1 nm) and LDH (∼0.8 nm) nanosheets, which suggests that the observed broad peak may be identified as the second-order peak of the MoS2/LDH superlattice.31,35−37 Besides, the XRD simulation result based on the superlattice-like structure indicates a much stronger intensity of the second-order diffraction peak than that of the first-order peak, which is consistent with the measured XRD pattern of the as-prepared MoS2/LDH (Figure S4). A high-resolution TEM image (Figure 2E) shows a distinct multilayered structure with a repeating periodicity of approximately 1.8 nm, which is close to twice the observed spacing in the XRD pattern. This further confirms the formation of the superlattice. Scanning electron microscopy (SEM) images (Figure 2F and Figure S5) show a 3D architecture with restacked thin nanosheets. The corresponding elemental mapping results shown in Figure 2G verified a homogeneous heteroassembly of MoS2 and LDH nanosheets. Figure S6 presents the high-resolution X-ray photoelectron spectroscopy (XPS) spectra of Mo 3d in the exfoliated MoS2 nanosheets and MoS2/LDH superlattice. A high concentration of metallic 1T phase was observed in the chemically exfoliated MoS2 nanosheets, which is similar to the previous reports.38
was proposed, yielding a superior overall water splitting activity superior than that of the commercial Pt/C−RuO2 catalysts. The MoS2/LDH superlattice was synthesized via a solutionphase assembly of negatively charged MoS2 and positively charged LDH nanosheets (Figure 1). These two oppositely charged unilamellar nanosheets were electrostatically attracted to each other and assembled into a superlattice-like structure. The stable suspensions of MoS2 and LDH nanosheets were prepared through a Li intercalation−exfoliation method and an ion exchange−exfoliation approach, respectively.29,31 Atomic force microscopy (AFM) images clearly showed a sheetlike morphology of these two exfoliated nanosheets. Moreover, the thickness of the MoS2 (Figure 2A) and LDH (Figure 2B) nanosheets were measured to be approximately 1.0 and 0.8 nm, respectively, confirming the monolayer geometry. Transmission electron microscopy (TEM) images further demonstrated the ultrathin thickness of the MoS2 (Figure S1) and LDH (Figure S2) nanosheets. The zeta-potential measurements showed an average value of +(43 ± 3) mV for LDH nanosheets in formamide and −(38 ± 5) mV for MoS2 aqueous suspension. The oppositely charged nature of these two kinds of nanosheets not only ensures their colloidal suspensions with a long-term stability but also suggests a possible spontaneous electrostatic heterostacking when they are mixed. In addition, a hypothesized area matching model in the lateral dimensions of both nanosheets was considered for a full face-to-face alignment.34−37 The mass ratio between MoS2 and LDH nanosheets was estimated to be approximately 0.8 (Figure S3). 4520
DOI: 10.1021/acs.nanolett.9b01329 Nano Lett. 2019, 19, 4518−4526
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Figure 3. (A) OER linear sweeping voltammetry curves of LDH and MoS2 nanosheets, LDH/G and MoS2/LDH superlattices, and commercial RuO2 in 1 M KOH solution. Comparison of (B) overpotential at 10 mA cm−2 and (C) Tafel slopes of LDH and MoS2 nanosheets, and LDH/G and MoS2/ LDH superlattices. (D) HER linear sweeping voltammetry curves of LDH and MoS2 nanosheets, MoS2/G and MoS2/LDH superlattices, and commercial Pt/C in 1 M KOH solution. Comparison of (E) overpotential at 10 mA cm−2 and (F) Tafel slopes of LDH and MoS2 nanosheets, and MoS2/G and MoS2/LDH superlattices. (G) Chronopotentiometry curves of MoS2/LDH superlattice at a constant current density of 10 mA cm−2 for OER and HER. (H) OER and (I) HER linear sweeping voltammetry curves of MoS2/LDH superlattice before and after 10 h test.
After flocculation with LDH nanosheets, the metallic 1T phase was preserved in the MoS2/LDH superlattice. The Ni 2p and Fe 2p peaks from MoS2/LDH superlattice are apparently shifted to a higher binding energy with respect to exfoliated LDH nanosheets (Figure S7). Meanwhile, the Mo 3d peaks of MoS2/LDH superlattice are shifted to lower binding energy compared with exfoliated MoS2 nanosheets (Figure S7). These suggest a strong interaction with spontaneous electron transfer from the LDH to MoS2 in the MoS2/LDH superlattices. The MoS2/G and LDH/G superlattices were also prepared by the same method using the corresponding two kinds of unilamellar nanosheets according to our previous reports.29,31 The electrocatalytic OER performance of the MoS2/LDH superlattice was first investigated in 1 M KOH solution. LDH nanosheets have been regarded as a promising nonprecious OER electrocatalyst. As shown in Figure 3A, the LDH nanosheets showed a comparable performance to that of the RuO2 benchmark. After hybridization with graphene or MoS2 nanosheets at a molecular scale, both the LDH/G and MoS2/ LDH superlattices exhibited improved OER activities. Moreover, the MoS2/LDH superlattices displayed the best activity with a low overpotential of 0.21 V at a current density of 10 mA cm−2 (Figure 3B) and a small Tafel slope of 46 mV dec−1 (Figure S8 and Figure 3C). The significantly lower Tafel slope value of the MoS 2/LDH superlattice indicates a superior OER kinetics,12,15 which was further confirmed by the electrochemical impedance spectroscopy (EIS) measurements (Figure
S9). Among the studied catalysts, the smallest charge transfer resistance was observed for MoS2/LDH superlattice, suggesting the most favorable charge transfer between the MoS2 and LDH nanosheets. The HER activities of the MoS2/LDH superlattice in a basic solution of 1 M KOH were also investigated. In our previous report, a MoS2/G superlattice composed of alternately restacked metallic MoS2 and modified graphene nanosheets was designed for enhancing the HER performance in acidic solution.31 In the present report, the HER activity of this superlattice in basic solution was also tested. Interestingly, both the MoS2/G and MoS2/LDH superlattices exhibited an impressive improvement in the alkaline HER activities compared with the exfoliated MoS2 nanosheets (Figure 3D). Furthermore, the MoS2/LDH superlattice exhibited the lowest overpotential of 0.11 V at 10 mA cm−2 (Figure 3E) and the smallest Tafel slope of 77 mV dec−1 (Figure 3F and Figure S10). This clearly indicates a favorable alkaline HER kinetics, which is further supported by the Nyquist plots (Figure S11).9,11 The stability of the MoS2/LDH superlattice for both OER and HER in basic media was examined (Figure 3G). No noticeable change of overpotential was observed during the chronopotentiometric measurements at a current density of 10 mA cm−2 for 10 h. As can be seen from Figure 3H,I, the LSV curves of MoS2/LDH superlattices for OER and HER remained almost unchanged after 10 h of continuous operation. The MoS2/LDH superlattice preserved their original multilayered 4521
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Figure 4. (A) Side-view of differential charge density by Bader charge analysis of MoS2/G, LDH/G, and MoS2/LDH bilayer structures with an isosurface value of 0.003 e Å−3. The red arrow shows the electron transfer from one layer to the other. (B) Free energy diagram of OER on LDH, LDH/ G, and MoS2/LDH. (C) Energy barrier of water dissociation process on MoS2, MoS2/G, and MoS2/LDH. (D) Free energy diagram of HER on MoS2, MoS2/G, and MoS2/LDH.
calculations on the MoS2/G, LDH/G, and MoS2/LDH superlattices. The charge distributions in these bilayers were first investigated. Figure S14 shows the top view of differential charge density in three fully relaxed bilayer structures. The Bader charge analysis shows a substantial charge distribution in the form of electron accumulation at the interface and electron transfer from one layer to the adjacent one in all three kinds of superlattices, as shown with the red arrows in Figure 4A. A significant electron transfer of 0.45 Å−2 from LDH to MoS2 was confirmed for MoS2/LDH, which is consistent with the XPS results (Figure S7). This value is much greater than that of 0.15 Å−2 for LDH/G and 0.0042 Å−2 for MoS2/G. The electron redistribution results in a strong interaction between the adjacent layers, which provides a fast electron transfer during electrochemical reactions. More importantly, it can modulate the electronic structure and thus the adsorption abilities of each layer for intermediates during the electrocatalytic reactions.14,15 In particular, the localized electron accumulation at the MoS2 edges is supposed to enhance the HER performance due to the improvement of H+ affinity, whereas hole accumulation on the LDH nanosheets originating from the electron transfer is favorable for OER because of the enhanced affinity for the OH− reactants.24 Therefore, it is concluded that the improved HER and OER activities of all three superlattices over those of the corresponding exfoliated nanosheets can be attributed to the electron transfer at the interfaces of superlattices. Furthermore, the rational heteroassembly of MoS2 and LDH nanosheets provides accumulated electrons on MoS2 and holes on LDH simultaneously. As a result, the MoS2/LDH superlattice
structure after the OER and HER durability tests (Figure S12). Figure S13 shows the XPS spectra of Ni 2p, Fe 2p, and Mo 3d in MoS2/LDH superlattice before and after the durability tests. The elemental atomic ratio of the MoS2/LDH superlattice was compared before and after the electrochemical measurements (Table S1). No obvious changes were observed, indicating the high electrochemical stability. The electrochemical results suggest that the design of a superlattice structure is a useful strategy to improve electrocatalytic activities of exfoliated nanosheets.4 The excellent OER and HER activities should be ascribed to the unique molecular-scale integrated superlattice structure, in which every unilamellar nanosheet is separated and stabilized with each other and thus simultaneously acts as an active component during electrocatalytic reactions. In addition, the charge transfer from LDH to MoS2 (Figure S7) enables a strong coupling interaction at the heterointerfaces between these two kinds of nanosheets, which leads to the smallest charge transfer resistance (Figures S9 and S11) and consequently synergistically increases the electrocatalytic activities of the MoS2/LDH superlattice.4,24 In order to further improve the electrocatalytic activities, preparation of ternary MoS2/LDH/G could be a promising approach. However, it is difficult to control the alternate stacking of three different kinds of unilamellar nanosheets based on the solution-phase heteroassembly strategy. First-principles density functional theory (DFT) calculations were performed to achieve fundamental understanding of the enhanced electrocatalytic activities of MoS2/LDH. The corresponding bilayer models were used for the DFT 4522
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Figure 5. (A) Schematic illustration of the probable electrocatalytic mechanism of the alkaline overall water splitting on the interface of MoS2/LDH superlattice. (B) Linear sweeping voltammetry curves of MoS2/LDH superlattice and commercial RuO2−Pt/C couple for overall water splitting in 1 M KOH. (C) Comparison of the overall water splitting performance of MoS2/LDH superlattice with some reported bifunctional catalysts. (D) Chronopotentiometry curves of MoS2/LDH superlattice at a constant current density of 10 mA cm−2 for overall water splitting. The inset photo shows the H2 and O2 bubbles.
chemical results. For the HER in a basic solution, the H has to be discharged from H2O instead of from hydronium ions in acidic media. In addition to the binding free energy of H* intermediates, the energy barrier for initial water dissociation is also very important.9−11 The DFT calculations for alkaline HER kinetics were based on both the initial water dissociation process via a transition state (TS) and the subsequent chemisorption of intermediates (*OH and H*) (Figure S16). Figure 4C compares the energy barrier (Ew) of the water dissociation process, namely the cleavage of OH bonds of water. As shown in Figure 4D, the binding free energy of H* intermediates (ΔGH*) of all three catalysts is much lower than that of products from water dissociation (*OH + *H). This indicates that the water dissociation is the rate-limiting process of alkaline HER for all three superlattice catalysts.24 Although, a small ΔGH* of 0.35 eV was estimated for the MoS2 (Figure 4D), the highest Ew significantly hinders the dissociation of water to *OH and H* intermediates (Figure 4C) and leads to sluggish alkaline HER kinetics. After hybridization with graphene, the Ew of MoS2/G was reduced 1.0 eV (Figure 4C). Notably, the
exhibited significantly improved activities for both OER and HER. Besides this, the more pronounced charge distribution in the MoS2/LDH superlattice than that in the MoS2/G and LDH/ G superlattices provides a more significant electronic coupling effect, resulting in the highest enhanced activities of MoS2/LDH among all three superlattices.9,15,21 The energy profiles of several critical OER and HER intermediates on these superlattices were calculated. A fourstep OER mechanism that proceeds through *OH, *O, and *OOH (the * denotes the adsorption site) was considered (Figure S15). As shown in Figure 4B, a potential-determining step (pds) of adsorption of OOH on the LDH layer was calculated with a large ΔGpds of ∼7.8 eV. After hybridization with graphene, the pds was changed to the oxidation of *OOH to O2 with a much lower ΔGpds of ∼2.2 eV. The MoS2/LDH showed the smallest ΔGpds of 1.8 eV with the formation of *OOH as the pds. The DFT results suggest that the OER activity of LDH is substantially enhanced by strongly coupling with graphene or MoS2, and the MoS2/LDH shows the most favorable energetics, which are in agreement with the electro4523
DOI: 10.1021/acs.nanolett.9b01329 Nano Lett. 2019, 19, 4518−4526
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In summary, we have systematically investigated three kinds of superlattices, MoS2/LDH, MoS2/G, and LDH/G, as catalysts for electrochemical water splitting. The combination of metallic MoS2 and LDH nanosheets provides a significant electronic coupling effect at the interface of the MoS2/LDH superlattice, which synergistically favors the adsorption of both OER and HER intermediates and thus effectively accelerates the overall water splitting reactions. As a result, the MoS2/LDH superlattice achieved an improved activity and superior stability for both OER and HER in alkaline media. This work demonstrates a promising molecular-scale modulation of heterointerfaces between different 2D unilamellar nanosheets in superlattices for advanced electrocatalysis and could be extended to fabricate other heterostructures for energy-related applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b01329.
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Details of experimental sections, characterizations, and theoretical calculations, electrochemical measurements of all studied catalysts, and comparison of electrochemical performance (PDF) Simultaneous evolution of both H2 and O2 bubbles (MP4)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (R.M.). *E-mail:
[email protected] (T.S.). *E-mail:
[email protected] (G.W.). ORCID
Pan Xiong: 0000-0001-9483-6535 Xiuyun Zhang: 0000-0001-9977-4273 Jinqiang Zhang: 0000-0001-5476-0134 Renzhi Ma: 0000-0001-7126-2006 Takayoshi Sasaki: 0000-0002-2872-0427 Guoxiu Wang: 0000-0003-4295-8578 Present Address
⊥ School of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan 410083, China.
Author Contributions
P.X. conceived the idea and designed the experiment. G.W. supervised the project. P.X. and H.W. carried out the sample synthesis, characterization, and electrochemical measurement. X.Z. and W.G. carried out the theoretical calculations. The manuscript was written through contributions of all authors. Author Contributions ∥
P.X., X.Z., and H.W. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Australian Research Council through the ARC Discovery projects (DP160104340 and DP170100436). This work is partly supported by the World Premier International Research Center Initiative on Materials Nanoarchitectonics (WPI-MANA), MEXT, Japan. X.Z. ac4524
DOI: 10.1021/acs.nanolett.9b01329 Nano Lett. 2019, 19, 4518−4526
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Nano Letters
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knowledges the supports from NSFC (11574262) and the Qinglan Project of Jiangsu Province.
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