Electrochemical Deposition: An Advanced Approach for Templated

Article Cite This: Acc. Chem. Res. 2018, 51, 1764−1773

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Electrochemical Deposition: An Advanced Approach for Templated Synthesis of Nanoporous Metal Architectures Cuiling Li,† Muhammad Iqbal,† Jianjian Lin,‡ Xiliang Luo,‡ Bo Jiang,† Victor Malgras,† Kevin C.-W. Wu,§ Jeonghun Kim,*,∥ and Yusuke Yamauchi*,∥,⊥

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†

International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China § Department of Chemical Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan ∥ School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia ⊥ Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea CONSPECTUS: Well-constructed porous materials take an essential role in a wide range of applications, including energy conversion and storage systems, electrocatalysis, photocatalysis, and sensing. Although the tailored design of various nanoarchitectures has made substantial progress, simpler preparation methods are compelled to meet large-scale production requirements. Recently, advanced electrochemical deposition techniques have had a significant impact in terms of precise control upon the nanoporous architecture (i.e., pore size, surface area, pore structure, etc.), enabling access to a wide range of compositions. In this Account, we showcase the uniqueness of electrochemical deposition techniques, detail their implementation toward the synthesis of novel nanoporous metals, and finally outline the future research directions. Nanoporous metallic structures are attractive in that they can provide high surface area and large pore volume, easing mass transport of reactants and providing high accessibility to catalytically active metal surface. The great merit of the electrochemical deposition approach does not only lie in its versatility, being applicable to a wide range of compositions, but also in the nanoscale precision it affords when it comes to crystal growth control, which cannot be easily achieved by other bottom-up or top-down approaches. In this Account, we describe the significant progress made in the field of nanoporous metal designed through electrochemical deposition approaches using hard templates (i.e., porous silica, 3D templates of polymer and silica colloids) and soft templates (i.e., lyotropic liquid crystals, polymeric micelles). In addition, we will point out how it accounts for precise control over the crystal growth and describe the unique physical and chemical properties emerging from these novel materials. Up to date, our group has reported the synthesis of several nanoporous metals and alloys (e.g., Cu, Ru, Rh, Pd, Pt, Au, and their corresponding alloys) under various conditions through electrochemical deposition, while investigating their various potential applications. The orientation of the channel structure, the composition, and the nanoporosity can be easily controlled by selecting the appropriate surfactants or block copolymers. The inherent properties of the final product, such as framework crystallinity, catalytic activity, and resistance to oxidation, are depending on both the composition and pore structure, which in turn require suitable electrochemical conditions. This Account is divided into three main sections: (i) a history of electrochemical deposition using hard and soft templates, (ii) a description of the important mechanisms involved in the preparation of nanoporous materials, and (iii) a conclusion and future perspectives. We believe that this Account will promote a deeper understanding of the synthesis of nanoporous metals using electrochemical deposition methods, thus enabling new pathways to control nanoporous architectures and optimize their performance toward promising applications such as catalysis, energy storage, sensors, and so forth.

1. INTRODUCTION Nanoporous structures are attractive architectures for practical

Received: March 15, 2018 Published: July 9, 2018

applications because the resulting open surfaces greatly © 2018 American Chemical Society

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DOI: 10.1021/acs.accounts.8b00119 Acc. Chem. Res. 2018, 51, 1764−1773

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Accounts of Chemical Research

Figure 1. Synthetic methods (hard- and soft-templating methods) to produce electrode materials by using electrochemical depositions.

enhance the accessible surface area for mass transport.1,2 In contrast to the well-developed traditional nanoporous materials (e.g., silica,3,4 titania,5,6 and carbon7,8), nanoporous metals possess the intrinsic catalytic properties of the metals themselves, combined with the unique features inherent from nanoporous structures.9−12 A survey over previous research shows that the preparation of nanoporous/mesoporous metals has been mostly dominated by hard-templating and softtemplating methods.13−16 The reduction of metals is usually driven by chemical and electrochemical reactions. The chemical reduction of metal ions to synthesize nanoporous metals has been extensively investigated using a variety of reducing agents.17−20 When these reducing agents are used in solution, however, they are randomly distributed and the excess inevitably induces complex collateral reactions, thus decreasing the product quality and yield. Mesoscopic longrange ordering and homogeneous distribution of nanopores can both be promoted through the use of simple and nonintrusive electrochemical reduction techniques. Although several reviews on nanoporous metals fabricated by hard- and soft-templating approaches have been reported before,2,9−12,14 none of them has focused on electrochemical plating method for nanoporous metals. Electrochemical techniques can be used not only as efficient analytical approaches for evaluating the performances of

electrode materials, but also as alternative synthetic methods to produce electrode materials.21,22 In a typical deposition process, metal precursors are electrochemically reduced to their solid metal states by directly transferring electrons from the working electrode.23,24 The potential applied on the electrodes directly influences the rate at which electrons are generated, as well as the nucleation and crystal growth. These parameters are critical to control the final morphology of the nanoporous product. In the past two decades, these electrodeposition techniques have played an important role in the development of efficient and reliable fabrication processes yielding nanoporous metals (Figure 1). This Account focuses on the recent advances in the field of nanoporous metals fabricated through electrochemical methods. We emphasize on how these approaches promote a superior control over composition and morphology.

2. HARD-TEMPLATING METHOD BY ELECTROCHEMICAL DEPOSITION The direct negative replication from ordered nanoporous hardtemplates is regarded as a common and effective strategy for preparing nanoporous metals.25 It typically involves four main steps: (i) formation of a sacrificial template with a desired nanoporous structure; (ii) filling of the pores with the target metal precursors; (iii) reduction of the precursors to their 1765


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Figure 2. (a) Schematic illustration of the synthesis of 3D continuous macroscopic metal nanowire networks by hard-templated electrodeposition technique. (b) TEM image of the electrodeposited Pt film with double gyroid structure (white arrow: the [311] projection of the double gyroid structure). Reproduced with permission from ref 26. Copyright 2012 American Chemical Society. SEM images of (c) 2D hexagonally ordered mesoporous silica film and (d) its replicated Pt nanowires. Reproduced with permission from ref 27. Copyright 2006 Royal Chemical Society.

metallic states; and finally (iv) selective removal of the original templates to obtain the nanoporous metal replica (Figure 2a). Electrodeposition can be employed in step (iii) to control the reduction of the metal precursors enclosed inside the hard templates. A three-electrode electrochemical cell, which includes the sacrificial hard-templates as working electrode, a counter electrode, and a reference electrode, is normally used. When three-dimensional nanoporous structures are used, the resulting product usually consists in continuous 3D networks relying on the original constructions of the template, and is retained even after template removal (Figure 2b).26 2D hexagonally ordered mesoporous silica films are typical templates which have been often utilized (Figure 2c,d).27,28 To obtain well-defined nanoporous metals, the reduction behavior of the precursors must be precisely controlled according to the target templates and metal precursors. For instance, by applying a pulsed potential of −2.0 V vs Ag/AgCl for 50 ms, followed by 950 ms at open circuit potential, electrodeposited Pt can perfectly penetrate the voids of a nanoporous silica template and retain the double gyroid morphology (Figure 2b).26 Osaka et al. obtained nanoporous Pt electrode with a perpendicular orientation using titania nanopillars as template under a constant potential of 0.35 V vs. Ag/AgCl.29 Three-dimensional colloidal assembly of nano/microspheres can also be used as a hard-template to form ordered porous metals.30,31 The precursors are driven electrochemically inside the spaces between the spheres, resulting in a product with cage-type pores.32,33 The pore size depends on the diameter of the spheres while the pore ordering relies on the original colloidal assembly. Particular care must be taken when establishing the electrochemical conditions to confine precursors in templates with small apertures, especially for metals with relatively low deposition potential. Accordingly,

nanoporous Ni, Co, Fe, and Cu, could be obtained by using a relatively high reducing power.

3. LIQUID CRYSTAL TEMPLATING APPROACH BY ELECTROCHEMICAL DEPOSITION Amphiphilic molecules, which contain both hydrophobic and hydrophilic moieties, can be driven to form micelles in solvent when a concentration higher than the critical micelle concentration (CMC) is reached. Further increasing the concentration (normally beyond 30 wt %) leads to the formation of lyotropic liquid crystals (LLCs) where the micelles adopt a specific arrangement (i.e., mesophase).34,35 This system can be used as a soft-template to synthesize various nanoporous metals.36,37 LLC-assisted electrodeposition follows three steps: (i) formation of the LLC phase on a conductive substrate, (ii) electrodeposition of the metals through an appropriate electrochemical method, and (iii) removal of the surfactants to obtain the nanoporous metal film. Here, the counter and reference electrodes are directly inserted into the as-prepared LLCs to form the typical three-electrode electrochemical cell. The precursors are reduced in their metal state through the application of an external potential. The electrodeposition of mesoporous metal in LLCs can be dated back to 1997, when Attard et al. first electrodeposited porous Pt film with a hexagonal structure by replicating the hexagonal phase of the LLC.36 Since then, many researchers devoted great efforts to extend both the composition and the porous structure using this concept. On this basis, nanoporous metals including Ni, Ru, Pd, Sn, and Pt, have been achieved by LLCassisted electrodeposition approach.38−43 Nanoporous structures with high quality and monodisperse pores relies directly on the homogeneity of the LLCs. This turns to be difficult to achieve because of the high viscosity of typical LLCs which are required to be handled in a compact electrochemical cell. 1766


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Figure 3. (a) Illustration of the electrodeposition procedure for the preparation of mesoporous PtAu alloy films by LLCs prepared through solvent evaporation. (b−g) SEM images and elemental mapping of Pt and Au for the mesoporous PtAu films with different Pt:Au molar ratios: (b−d) 90:10 and (e−g) 62:38. Reproduced with permission from ref 48. Copyright 2012 American Chemical Society.

Figure 4. Cross-sectional and top surface SEM images of the electrodeposited Pt film with (a, d) cage-type, (b, e) 2D-hexagonal, and (c, f) lamellar mesostructures. Schematic illustration of the relative size between the headgroup and hydrophobic volume is shown at the bottom. Reproduced with permission from ref 50. Copyright 2010 American Chemical Society.

shapes.46,47 The nature of the coordination between the metal ions and liquid-crystalline phase dictates the ordering within the LLC and thus, within the final porous structure. During the preparation of nanoporous Pt−Au alloy films using LLCs,48 the composition ratios between Pt and Au played a critical role in the formation of the nanoporous structure (Figure 3a). Increasing the Au content gradually lowers the pore ordering in the film (Figure 3b−g), and no obvious ordered mesostructure was observed when the Au content is higher than 57%. The collapse of the porous structure in Au-

In order to circumvent issues related to handling highly viscous LLCs, we proposed a three-step method following: (i) the preparation of a diluted solution by mixing the metal precursors, water, a volatile solvent (e.g., ethanol, tetrahydrofuran), and the surfactants (or block copolymers); (ii) the casting of the precursor solutions onto a conductive substrate; and (iii) the formation of the LLCs assisted by the evaporation of the volatile solvent (Figure 3).44,45 This promising approach was further employed by other research groups to prepare various nanoporous metals and alloys with different 1767


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Figure 5. (a) Synthetic concept of mesoporous Pt films by electrochemical micelle assembly approach. (b) Controllable thickness by deposition time. (c, d) Cross-sectional SEM images of the electrodeposited mesoporous Pt film. (e, f, g) TEM and (h, i, j) SEM images of mesoporous Pt films prepared with (e, h) PEO4500-PPO3200-PEO4500, (f, i) F127, and (g, j) Brij 58 as the pore-directing agents, respectively. Reproduced with permission from ref 51. Copyright 2012 American Chemical Society.

In order to faithfully replicate the LLC pattern to the final porous metals, electrochemical deposition under mild conditions takes an essential role. We suggest that the LLCs undergo plastic deformation during the metal reduction reaction. Therefore, an optimized reduction rate is essential to confine the metal reduction inside the LLC. Sometimes a slow reduction rate allows the liquid-crystalline phase to reorganize and allow large crystals to grow.37

rich films is probably due to the direct coordination between Au3+ and ethylene oxide groups. On the other hand, Pt4+ ions coordinate with water molecules to form metal-aqua complex which interact with the ethylene oxide groups through hydrogen bonding, and retard the evaporation of water in the LLCs phase, thus enhancing its stability. A variety of mesophases can be induced by simply controlling the concentration and/or compositions of surfactants (or block copolymers), thus enabling a wide range of nanoporous structures.49,50 The nanostructure in the film can be tailored as lamellar, cage-type or hexagonal by increasing the compositional ratios between the Pt precursors and the block copolymers (Figure 4).50 Indeed the relative size of the hydrophilic headgroup increases with the content of Pt precursors, which thus induces a stronger curvature of the LLC mesophases and, in turn, triggers mesoscopic phase change from lamellar to hexagonal or to cage-type.

4. MICELLE ASSEMBLY APPROACH BY ELECTROCHEMICAL DEPOSITION The viscosity of LLCs not only limits the experimental feasibility in a typical three-electrode cell apparatus, but also decreases the flexibility of the soft-template during the synthesis of nanoporous metals. Electrochemical micelle assembly in a surfactant solution (>CMC) is a novel avenue enabling the synthesis of nanoporous metals with different 1768


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Figure 6. (a) Linear scan voltammetric plots recorded at a scan rate of 10 mV s−1 showing the interaction between different nonionic surfactants and PdCl2. (b) Low-angle XRD patterns from the Pd films electrodeposited with different surfactants. (c) Top-surface SEM image of the mesoporous Pd film electrodeposited by using P123. Reproduced with permission from ref 52. Copyright 2017 American Chemical Society. (d) SEM and (e) HAADF-STEM images of the electrodeposited Pd film with vertically oriented 1D mesochannels. Reproduced with permission from ref 53. Copyright 2015 American Chemical Society.

structures. Micelles formed in solution are sensitive to environmental perturbations. Therefore, maintaining the micellar structure as a sturdy soft-template involves a fair deal of efforts. In 2012, we for the first time reported the electrochemical micelle assembly as an effective strategy to fabricate nanoporous Pt films with pore sizes tunable between 5 and 30 nm, simply controlled through the molecular weight of the surfactants (herein, PEO4500-PPO3200-PEO4500, F127, or Brij 58) (Figure 5).51 In this case, spherical micelles are formed when the nonionic surfactants are dissolved in water with a concentration slightly higher than their CMC values. The dissolved Pt ions form Pt-aqua complexes which coordinate with the external hydrophilic region of the micelles. Under external potential, the surfactant micelles decorated with the Pt species migrate toward the working electrode. When accepting electrons from the working electrode, the reduced Pt atoms located in the hydrophilic region of the micelles agglomerate and gradually coalesce into a mechanically stable framework of thick walls. With the increase of the deposition time, the film thickness can be linearly increased (Figure 5b). From the cross-sectional SEM image, it is revealed that uniformly sized nanopores are formed inside the film (Figure 5c, d). In addition, the pore size is strongly dependent on the type of pore-directing agents used (Figure 5e−j). This highlights the importance of controlling the growth of metal nanocrystals in the presence of micelles when fabricating nanoporous metals. The interaction between the micelles and the dissolved metal ions influences the redox potential of the metal precursors and, consequently, the crystal growth. In the case of Pd electrochemical deposition, we found that typical

nonionic surfactants with similar components show different stabilization tendencies and greatly influence the reduction behavior (Figure 6a).52 Relatively large propylene oxide (PO) units are necessary to stabilize the dissolved Pd species accommodated inside the ethylene oxide (EO) units, which is probably why only P123 can yield nanoporous Pd films (Figure 6b, c). Although strong ion interactions between micelles and metal ions are essential to stabilize the system, they also substantially increase the barrier for the reduction of metal ions.53 For example, the cationic surfactant cetylmethylammonium chloride (CTAC) strongly interacts with Na2PdCl4 to form stable CTA+/[PdCl4]2− intermediates. The electrodeposition must be carried out at higher temperature (50 °C) to drive the reduction of the Pd precursor, thus enabling the formation of nanoporous films. Once the [PdCl4]2− is reduced, the four chloride ions which are released at the electrode surface probably induce a dynamic phase change from spherical to rod-like micelle. By applying a constant potential (0.0 V vs Ag/AgCl), the alignment of the cylindrical micelles in the vicinity of the surface of the electrode forms a soft-template for the formation of Pd films with perpendicularly aligned nanochannels (Figure 6d, e). The electrochemical micelle assembly approach is entirely performed in solution and is flexible, enabling a broad range of compositions (including PtAu,54 PtPd,55 PtCu,56,57 and PtRu58) and shapes.59,60 Another advantage proper to electrodeposition techniques lies in the film thickness which can be accurately controlled by adjusting the deposition time, thus providing a simple way to obtain layer-by-layer films with catalytically active heterointerfaces. We electrodeposited nanoporous Pt/Pd bimetallic 1769


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Figure 7. (a) Typical three-electrode electrochemical cell for polymeric micelle electrodeposition. (b) Schematic illustration of the formation mechanism of mesoporous Au films through polymeric micelle assembly directed electrodeposition. (c) Top-surface SEM image of the as-prepared mesoporous Au film. Reproduced with permission from ref 64. Copyright 2015 Nature Publishing Group. (d, e) Typical TEM images of the polymeric micelles (d) without and (e) with reduced Au nanoparticles. The initially reduced Au nanoparticles are marked by arrows. Reproduced with permission from ref 65. Copyright 2016 Springer.

bonding (Figure 7b). Moreover, the free AuCl4− ions are present both in solution and in the hydrophilic domains of the PS-b-PEO micelles. The micelles/AuCl4− system can be either neutral, negatively or positively charged, depending on the H3O+/AuCl4− ratio in the EO shells. It has been confirmed that the micelle solution, after the addition of aqueous solution of HAuCl4, is slightly positive according to zeta-potential measurements. This suggests that the H3O+-rich micelles are positively charged and then moved to the surface of the working (negative) electrode. Under optimal electrochemical conditions, the growth of Au nanocrystals is confined to the hydrophilic PEO region and the PS core is responsible for the formation of the pores, thus resulting in nanoporous Au films (Figure 7c). The pore-directing effect of PS-b-PEO for the nanoporous Au frameworks could be visually observed under TEM (Figure 7d, e). In addition, both the sizes and number of polymeric micelles can be controlled by continuously tuning the solvent composition,67 which will be useful for the preparation of nanoporous metals. The electrochemical polymeric micelle assembly approach can also be carried out to prepare other nanoporous metals which can hardly be synthesized by other methods.68 Recently, we prepared multilayered metal films with different compositions from a single electrolyte, by precisely programming the electrochemical plating.69

alternating layers by drawing from two different batches of precursor solutions containing Pt and Pd.61

5. POLYMERIC MICELLE ASSEMBLY APPROACH BY ELECTROCHEMICAL DEPOSITION In most cases, the obtained pore size is dictated by the size of the micellar hydrophobic core. It is, therefore, possible to enlarge the pore size by using larger molecules (e.g., block copolymers). Block copolymers with amphiphilicity can be assembled into stable polymeric micelles with their hydrophobic and hydrophilic moieties pointing inward and outward, respectively.62,63 Once the polymeric micelles are steadily formed in aqueous solutions, a typical three-electrode electrochemical setup can be used for electrodepositing the nanoporous metals (Figure 7a). The block copolymers sometimes precipitate even at relatively low water content due to the formation of large-size aggregates. The micelle solution stability (i.e., electrolytes) can be maintained by either adding appropriate solvents or by tuning the hydrophilicity/ hydrophobicity of the block copolymers. Although nanoporous Au has shown great potential toward optical applications, its synthesis via a facile and effective softtemplating approach remains challenging because of the difficulty to control the Au crystal growth. Polymeric micelle templates can become handy in such a case, for instance, by using polystyrene-block-poly(oxyethylene) diblock copolymer (PS-b-PEO) as pore-directing agent.64−66 By regulating the solvent composition, polymeric micelles, with a hydrophobic PS block as the core and hydrophilic PEO block as the shell, can be formed (Figure 7b). The PS-b-PEO micelles effectively interact with the soluble Au species. The H3O+ and AuCl4− ions released after dissolution of HAuCl4 can interact with the EO shell domains of the PS-b-PEO micelles by hydrogen

6. CONCLUSIONS AND OUTLOOK In this Account, we have summarized various electrochemical approaches to fabricate nanoporous metal films with a wide variety of pore sizes, porous structures, and compositions. Well-defined nanoporous architectures can be realized by precisely regulating crystal growth and deposition kinetics by selecting the appropriate electrochemical conditions. Although 1770


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Accounts of Chemical Research these approaches may seem complex, they afford a considerable control over the features of the final product from few simple parameters tuned via an electrochemical workstation. Recently, several efforts have been made to develop other ways for preparing nanoporous materials. The unique property of electrodeposition is that various reactions taking place at the working electrodes can be triggered depending on the external potentials employed. While electrodepositing metals, hydrogen evolution reaction can take place at relatively low potential, and the amount of generated hydrogen bubbles depend on the applied potentials. These bubbles can also act as a template to prepare porous metals films, although it is rather difficult to control the pore sizes and porous architecture.70,71 As for the dealloying method, a bi/trimetallic alloy is generally used as the working electrode. By applying a moderate electrochemical technique, one or two less stable components are dissolved, leaving the more stable component in place as a continuous porous framework.72,73 Unfortunately, this approach seriously limits the range of possible compositions. For the past 30 years, there were significant advances related to the synthesis of new mesoporous materials with emergent functions. New compositions are now extending from traditional metal oxides (e.g., SiO2, TiO2) and carbons to metals and alloys. Upscaling to industrial production remains, however, a major challenge. As we know, electrochemical plating methods are already used for large-scale production, and micelle assembly approach assisted electrodeposition represents a simple and inexpensive upgrade to make nanoporous metals. From a scientific viewpoint, improving our understanding of the catalytic reaction mechanisms taking place at the surface of nanoporous metals will shine the light on these emergent materials with improved catalytic performances for fuel molecules (e.g., formic acid,53,74,75 methanol,57−61,65,69 ethanol52,69) and oxygen reduction reaction (ORR).26,29,60,76 Keeping these goals in mind, future progress in electrochemical technologies will undoubtedly offer increasing opportunities to precisely control both hierarchical porous structures and compositions. We expect that the electrochemical deposition techniques will become a key tool in the development of complex materials designed for specific requirements.

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Muhammad Iqbal received both bachelor and master degrees from the Department of Engineering Physics, Institut Teknologi Bandung, Indonesia. He is currently a Ph.D. student under the supervision of Prof. Yamauchi in Waseda University. Jianjian Lin received her Ph.D degree (2015) in University of Wollongong, Australia. She is currently a full professor in Qingdao University of Science and Technology. Xiliang Luo received his Ph.D. degree from Nanjing University in 2005. He joined the Qingdao University of Science and Technology in 2011. Bo Jiang received his Ph.D. degree (2017) under the supervision of Prof. Yamauchi in Waseda University. He is now a postdoctoral fellow at NIMS. Victor Malgras received his Ph.D. degree in University of Wollongong. His expertise lies between the fabrication of nanomaterials and their electrochemical and optical properties. He is now ICYS researcher at NIMS. Kevin C.-W. Wu obtained his Ph.D. degree in 2005 from The University of Tokyo and worked as a postdoctoral researcher at Waseda University and Iowa State University He started his own research group in the National Taiwan University in 2008. Jeonghun Kim received his Ph.D. degree (2012) in Chemical and Biomolecular Engineering at Yonsei University in Seoul, South Korea. He is currently a research fellow in The University of Queensland (UQ). Yusuke Yamauchi received his Bachelor’s (2003), Master’s (2004), and Ph.D. (2007) degrees from Waseda University, Japan. He is now a full professor in School of Chemical Engineering and a senior group leader in AIBN, UQ. He was selected as one of the Highly-Cited Researchers in Chemistry (Thomson Reuters) in 2016 and 2017.

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ACKNOWLEDGMENTS This work was supported by the Australian Research Council (ARC) Future Fellow (FT150100479), JSPS KAKENHI (17H05393 and 17K19044), and the research fund by the Suzuken Memorial Foundation. The authors would like to thank New Innovative Technology (NIT) for helpful suggestions and discussions. The project was partially supported by the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (16KJB430015), the National Natural Science Foundation (NNSF) of China (61604070), the National Natural Science Foundation of Jiangsu Province (BK20161000), and the Taishan Scholar Program of Shandong Province of China (ts20110829).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

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ORCID

REFERENCES

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Xiliang Luo: 0000-0001-6075-7089 Kevin C.-W. Wu: 0000-0003-0590-1396 Jeonghun Kim: 0000-0001-6325-0507 Yusuke Yamauchi: 0000-0001-7854-927X Notes

The authors declare no competing financial interest. Biographies Cuiling Li received her Ph.D. degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2012. Since then she started postdoctoral work with Prof. Yamauchi supported by JSPS Postdoctoral Fellowship in NIMS. 1771


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