Bifunctional 2D Superlattice Electrocatalysts of Layered Double

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Bifunctional 2D Superlattice Electrocatalysts of Layered Double HydroxideTransition Metal Dichalcogenide Active for Overall Water Splitting Md. Shahinul Islam, Minho Kim, Xiaoyan Jin, Seung Mi Oh, Nam-Suk Lee, Hyungjun Kim, and Seong-Ju Hwang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00134 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Energy Letters

Bifunctional 2D Superlattice Electrocatalysts of Layered Double Hydroxide−Transition Metal Dichalcogenide Active for Overall Water Splitting Md. Shahinul Islam,†,§ Minho Kim,‡,§ Xiaoyan Jin,† Seung Mi Oh,† Nam-Suk Lee,ǁ Hyungjun Kim,‡,┴,* and Seong-Ju Hwang†,* †

Center for Hybrid Interfacial Structure (CICS), Department of Chemistry and Nanoscience,

Ewha Womans University, Seoul 03760, Republic of Korea ‡

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced

Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ǁ

National Institute for Nanomaterials Technology (NINT), Pohang University of Science and

Technology, Pohang 37673, Republic of Korea ┴

Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST),

Daejeon 34141, Republic of Korea AUTHOR INFORMATION, Corresponding Author: *[email protected] (H.K.); *[email protected] (S.-J.H.) §

These authors contributed equally to this work.

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ABSTRACT: Bifunctional 2D superlattice electrocatalysts of alternating layered double hydroxide (LDH)−transition metal dichalcogenide (TMD) heterolayers were synthesized by interstratification of the exfoliated nanosheets. Density functional theory calculations predict an increased interfacial charge transfer between interstratified LDH and TMD nanosheets, which would lead to enhanced electrocatalytic activity. The electrostatically-driven self-assembly of oppositely-charged 2D building blocks, i.e. exfoliated Ni−Al-LDH/Ni−Fe-LDH and MoS2 nanosheets,

yields

mesoporous

heterolayered

Ni−Al-LDH−MoS2/Ni−Fe-LDH−MoS2

superlattices. The synthesized superlattices show improved electrocatalytic activity with enhanced durability for oxygen and hydrogen evolution reactions, and water splitting. The interstratification improves the chemical stability of LDH in acidic media, thus expanding its possible applications. The high electrocatalytic activity of the superlattices may be attributed to an enhanced affinity for OH−/H+, improved electrical conduction and charge transfer, and the increase of active sites. This study indicates that the formation of superlattices via self-assembly of 2D nanosheets provides useful methodology to explore high-performance electrocatalysts with improved stability.

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ACS Energy Letters

Since the first report of graphene, exfoliated 2D nanosheets (NSs) of a wide variety of elements and inorganic compounds have attracted great interest because of their unique physicochemical properties and valuable functionalities, which originate from the unusually high structural and morphological anisotropies.1,2 These 2D NSs present a broad spectrum of physicochemical properties depending on their chemical composition and crystalline structrue.1,2 The formation of highly anisotropic 2D NS leads to a remarkable expansion of the surface area and changes in the electronic and surface structures, significantly improving the performance of transition metal compounds as electrocatalysts, electrodes, photocatalysts, etc.2,3 Recently, the research related to 2D inorganic NSs has focused on a new class of 2D heterolayered hybrids, in which two kinds of NSs are alternately stacked in a layer-by-layer manner.4−6 When using the 2D NSs as building blocks, different 2D hybrid superlattices may be synthesized. For example, a new type of heterolayered hybrid material could be obtained based on the van der Waals interactions between neutral NSs such as graphene, boron nitride, and black phosphorus.7,8 In addition, most of the metal compound NSs were synthesized by soft-chemical exfoliation process such as transition metal dichalcogenide (TMD), transition metal oxide (TMO), and layered double hydroxide (LDH) possess distinct layer charges.9 Thus, materials composed of such charged NSs can be synthesized by electrostatically-driven interstratification between oppositely-charged inorganic NSs.10−14 In the resulting superlattice, the nanometer-level thickness and wide 2D surface of the NSs allow for an unusually strong chemical interaction and efficient electronic coupling between them. These features may lead to the optimization of preexisting functionalities and to the synergistic creation of novel unexpected characteristics. For example, the interstratification between oxygen evolution reaction (OER)-active LDH and hydrogen evolution reaction (HER)-active TMD NSs is expected to yield efficient bifunctional



electrocatalysts for both reactions. Since most electrocatalysts developed are only active for either OER or HER, such hybridization strategy represents a powerful method to explore efficient bifunctional electrocatalysts active for both processes, which would be useful for water splitting technologies. Remarkably, the noble metal-free materials of the LDH and TMD NSs are inexpensive and abundant; thus, the resulting LDH−TMD superlattice provides economical advantage compared to conventional Ir/Pt/Ru based electrocatalysts.15,16 In addition, the strong interfacial interaction and electronic coupling between the interstratified LDH and TMD NSs enable further improvement of the electrocatalytic activity via the optimization of their electronic configurations. There are several reports for LDH-based electrocatalysts17−19 and their noninterstratified TMD nanohybrids that show remarkably improved electrocatalytic activities.20−22 This indicates that the hybridization strategy using exfoliated LDH and TMD NSs would provide the expected results. Most of these works focus on the use of LDH−TMD nanocomposites for either OER or HER, rather than as bifunctional electrocatalysts for water splitting. J. Hu et al. reported the synthesis of vertically aligned MoS2 sheets decorated with Ni−Co-LDH nanocrystals on a carbon fiber paper substrate, which shows excellent HER activity in alkaline solution.21 Although van der Waals superlattices of interstratified NSs have been reported,7,8 we are unaware of any other study related to the synthesis of 2D NS-assembled superlattice nanohybrids with charged LDH and TMD NSs that show bifunctional OER and HER activity. In this work, we report a protocol to synthesize artificial superlattices of strongly coupled LDH−TMD nanohybrids via the self-assembly of 2D building blocks, i.e., cationic LDH and anionic TMD NSs. Two kinds of LDH i.e. Ni−Al-LDH and Ni−Fe-LDH materials were selected as precursors to verify the applicability of this strategy. The crystal structure, morphology, and chemical bonding nature of the resulting Ni−Al-LDH−MoS2 and Ni−Fe-LDH−MoS2


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ACS Energy Letters

nanohybrids were systematically investigated along with their electrocatalytic activity and chemical stability. The hybridization effect on the electronic structure and electrocatalytic activity of the LDH−TMD nanohybrids was analyzed with density functional theory (DFT) calculations and various spectroscopic techniques. The nanohybrids show excellent bifunctional electrocatalytic activity for OER and HER and thus for overall water splitting.

Figure 1. DFT calculated reaction thermodynamics. Free energy diagrams of OER in alkaline media (4OH− → O2 + 2H2O + 4e−), which is catalyzed by the surfaces of (a) Ni−Al-LDH, (b) Ni−Al-LDH−MoS2, (c) Ni−Fe-LDH, and (d) Ni−Fe-LDH−MoS2 NSs. The corresponding DFT optimized structures are shown in Figure S1. (e) The three-dimensional charge difference map at the isosurface level of 1.00∆ρmax (100% of the maximum value of ∆ρ) charge accumulation (yellow) and depletion (cyan), and the corresponding planar averaged charge difference along the surface normal direction (right pane) are shown. Atoms are color coded: brown for Ni, dark purple for Fe, magenta for O, white for H, turquoise for Mo, and yellow for S. (f) Scheme of the band edge alignment of Ni−Fe-LDH and MoS2 before (left) and after (right) hybridization. The dashed line indicates the Fermi level of each system. (g) The density of state (DOS) of Ni−FeLDH (top), MoS2 (middle), and Ni−Fe-LDH−MoS2 (NFM, bottom) nanohybrid; where the x-



axis of the energy was aligned with respect to the vacuum level (zero). Partial contributions of LDH (blue) and MoS2 (orange) NSs to the total DOS of the LDH−MoS2 nanohybrids as well as the Fermi level of each system (vertical dashed line) are shown. Ni−Al-LDH−MoS2 is presented in Figure S2. The effects of interstratification between LDH and MoS2 NSs on the thermodynamics of electrocatalysis and electronic structure were investigated with DFT calculations. Considering the point of zero charge of LDH surface ranging from 8 to 12,23 the active site for the alkaline OER was modeled as deprotonated surface of LDH and the reaction free-energy diagrams in Figures 1a−1d were calculated using the DFT-optimized structures (Figure S1). Among four elementary steps, it is interesting to note that only the fourth oxidation step was calculated to be slightly downhill. In previous DFT studies, this step was reported as uphill24 or downhill25 however, all previous results and our result agree in terms that the absolute magnitude of this fourth step’s reaction energy is as small as

Bifunctional 2D Superlattice Electrocatalysts of Layered Double

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