A centimeter scale self-standing two-dimensional ultra-thin mesoporous platinum nanosheet

Yunqi Liab, Yuwei Liuab, Jing Liab, Danping Xiongab, Xiran Chenab, Mingtao Liuab, Zheng Zhongab, Victor Malgrasc, Yoshio Bandoc, Yusuke Yamauchi*d and Jun Xu*ef
aDepartment of Automotive Engineering, School of Transportation Science and Engineering, Beihang University, Beijing, 100191, China
bVehicle Energy & Safety Laboratory (VESL), Beihang University, Beijing, 100191, China
cWPI Center for Materials Nanoarchitectonics (MANA) and International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS), Tsukuba, 3050044, Japan
dSchool of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, Queensland 4072, Australia. E-mail: Yamauchi.Yusuke@nims.go.jp
eDepartment of Mechanical Engineering and Engineering Science, The University of North Carolina at Charlotte, Charlotte, NC 28223, USA. E-mail: jun.xu@uncc.edu
fVehicle Energy & Safety Laboratory (VESL), North Carolina Motorsports and Automotive Research Center, The University of North Carolina at Charlotte, Charlotte, NC 28223, USA

Received 22nd July 2019 , Accepted 24th September 2019

First published on 24th September 2019


Here, we report the unprecedented synthesis of centimeter scale self-standing mesoporous Pt nanosheets. Due to solvent evaporation, spherical micelles made of poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-P2VP-PEO) self-assemble in a well-ordered fashion, over the entire surface of the substrate, and confined within an ultra-thin layer. Metal deposition around the micelles is carried out through a two-step reduction process using two different reducing agents. After removing the micelles and substrate, a continuous self-standing ultra-thin mesoporous Pt nanosheet can be obtained (∼15 nm thick). Due to several unique porous structural features, including abundant active sites, high surface area, and long-range orthogonally ordered pores, the obtained material is expected to deliver high performance in a wide range of electrochemical applications. This novel synthetic strategy opens a new route to design and control novel 2D mesoporous metal nanosheets with various functions.



New concepts

We report unprecedented synthesis of centimeter scale self-standing mesoporous Pt nanosheets due to the micelle self-assembly of poly(styrene-b-2-vinyl pyridine-b-ethylene oxide) (PS-P2VP-PEO) in a well-ordered and monolayered fashion over the entire surface of the substrate. Although mesoporous metallic materials have been extensively studied, we have failed to synthesize self-standing ultra-thin mesoporous metal films. This manuscript demonstrates the first example of a ultra-thin mesoporous metal film, which is a big breakthrough in the mesoporous community as well as materials science community. The surface coverage and the ordering of the micelles can be controlled by the micelle concentration in the starting precursor solution. A two-step reduction method using both DMAB vapor (for nucleation) and AA (for further growth) enables complete Pt deposition and determines the preparation of continuously ordered mesoporous nanosheets. Such 2D mesoporous metallic nanosheets with an ultra-thin thickness of ca. 15 nm can be used as building blocks to construct flexible self-standing electrodes containing regulated nanochannels and interconnected networks. Due to several unique porous structural features, including abundant active sites, high surface area, and long-range-ordered pores orthogonal and continuous throughout the thickness, such a material is expected to deliver high performance in a wide range of electrochemical applications.

Various inorganic nanomaterials have been designed through different methods including sol–gel, electrochemical/chemical reduction, hydrothermal reactions, etc.1 The dimensionality of these nanomaterials can be classified as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D).2 In contrast to 0D and 1D nanomaterials, 2D nanomaterials have recently attracted great interest in next generation technologies, because of their unusual electronic and thermal transport properties.3 Such 2D materials, however, often suffer from post-synthetic self-stacking/assembly,3,4 which seriously reduces their active surface areas and impairs their unique low-dimensionality properties, thus devaluing their potential. In particular, engineering nanospace in 2D materials (i.e., adding nano- or meso-pores in the 2D crystal planes) has attracted much attention in recent years.5,6 Recently, a graphene nanomesh with a high density of nanoscale pores has been reported.7 A graphene nanomesh retains the inherent properties of graphene, exhibiting a direct band gap, combined with an open porous structure with a high surface area. It is important, however, to expand this type of structure to other compositions. Therefore, new synthetic concepts are necessary to obtain inorganic 2D materials (metals, metal oxides, dichalcogenides) with porous architectures in order to increase both their surface area and the availability of active sites.

Identifying the requisite parameters for strictly controlling the direction of micelle assembly in lateral directions is primordial to enable further studies on different compositions.8 Zhao and coworkers reported 2D mesoporous carbon materials synthesized via hydrothermal reaction-induced co-assembly of surfactants and the carbon source on the surface of oxide substrates, followed by carbonization and removal of the substrates via chemical etching.9,10 2D mesoporous conducting polymers were also synthesized by redox polymerization of pyrrole/aniline on single-layer graphene oxide or at the interface with 2D lipid bilayers derived from an aliphatic amine or perfluorocarboxylic acids.11,12 Unfortunately, these methods are not suitable for the synthesis of metallic compositions which can be utilized for a wide range of electrochemical applications.

We have been recently witnessing a continuous improvement in the efficiency and durability of Pt catalysts through nanostructuring;13 therefore, the target of our study is to develop a robust technique to fabricate 2D ultra-thin mesoporous Pt nanosheets. There are several examples evidencing the benefits of low-dimensionality Pt catalysts. For instance, 1D Pt nanomaterials with high aspect ratios can improve electron transport. Ultrafine jagged Pt nanowires were fabricated via an electrochemical dealloying approach, leading to a high electrochemically active surface area as well as high ORR activity, nearly 50 times higher than the commercial Pt/C catalyst.14 2D (e.g., nanoplates15) and 3D (e.g., nanocages16) Pt nanomaterials also exhibit unusual catalytic properties. The exposed atoms on the surface of nanomaterials can easily escape from the lattice, resulting in the formation of vacancy-type defects. Accompanied by structural disorder, such vacancy defects can reduce the coordination number of surface atoms, exhibiting superior catalytic activities.17

In this study, we report for the first time ultra-thin mesoporous Pt nanosheets by combining a highly ordered arrangement of micelles and a suitably designed chemical reduction. The ca. 15 nm thickness of the ultra-thin nanosheets was confirmed by using scanning transmission microscopy (SEM). Such 2D mesoporous metallic nanosheets can be used as building blocks to construct flexible self-standing electrodes containing nanochannels and interconnected networks. Their unique porous structures can include significantly more space for the transportation or storage of electrons/ions, gas and liquid. As a result, they can be ideal electrode materials for supercapacitors, fuel cells, and batteries, and ought to be capable of delivering high energy densities.

The experimental procedure is illustrated in Fig. 1a. Initially, a core–shell–corona-type micelle solution made of PS-P2VP-PEO in methanol is prepared through a dialysis method.18 The protonated P2VP+ blocks in acidic media (pH < 2.3) can interact with the negatively charged PtCl62− ions and serve as primary accommodation sites.19 The resulting PS-P2VP-PEO/PtCl62 composite micelles are then formed and well-dispersed in the solution, because the PEO block corona acts as a micelle stabilizer through steric repulsive forces.20 This neutralization reaction is further confirmed by a decrease in the hydrodynamic diameter (Dh) and zeta potential.21–23 The obtained micelle solution is then spin-coated on a silicon substrate. To enhance adhesion between the micelles and the substrate, the silicon substrate is first treated to form a thin layer of hydrophilic oxide (∼200 nm). Upon solvent evaporation, the self-assembly occurs spontaneously, inducing an alignment of the micelles over the entire surface. The silicon substrates were placed in a closed vessel with a small amount of dimethylamine borane (DMAB) powder and kept at 28 °C for a day. For further reduction, the silicon substrates were immersed in 0.1 M aqueous solution of ascorbic acid for 10 min. After depositing Pt on the micelles, the PS-P2VP-PEO triblock copolymer is calcined during thermal treatment at 350 °C (Fig. S1, ESI). Finally, after dissolving the thin oxide layer in a NaOH solution, centimeter scale self-standing mesoporous Pt nanosheets can be detached from the substrate. The lateral size of the mesoporous Pt nanosheets can be easily controlled as it directly depends on the size of the substrate (the photographs with a width of 15 mm, 25 mm and 40 mm are displayed in Fig. 1a).


image file: c9mh01142k-f1.tif
Fig. 1 (a) Illustration describing the synthesis of the meso-Pt NS. Due to solvent evaporation, micelles self-assemble to form an ordered structure on a silicon substrate (SiOx/Si). Pt is deposited on the substrate with the assistance of a dual DMAB and AA reducing agent. When removing the template, the resulting meso-Pt NS exhibits a spatially ordered mesoporous structure. After being separated from the substrate, it can form a thin catalyst film. (b) Cross-section SEM, (c) top-view SEM, (d) TEM and (e) HRTEM images of the meso-Pt NS. The inset in (d) shows the SAED pattern.

The porous structure and morphology were initially characterized by scanning electron microscopy (SEM). Looking at the cross-section SEM image, the thin layer is uniformly distributed on the surface of the silicon substrate and the thickness is measured to be ∼15 nm (Fig. 1b). The thickness of the ultra-thin nanosheets can, however, be easily increased by adding successive spin-coating and drying steps. As shown in Fig. 1c, the top-view image shows a hexagonal packing of uniform mesopores with an average size of 32 nm. This pore size coincides with the PS core of the starting micelle sizes (Fig. S2, ESI). The molecular weight of the hydrophobic PS blocks determines the diameter of the mesopore, which will be discussed later. The uniform distribution of the mesoporous architecture throughout the nanosheets can also be observed by transmission electron microscopy (TEM) (Fig. 1d). The selected-area electron diffraction (ED) pattern comprises the representative diffraction rings of (111), (200), (220) and (311), assigned to the Pt face-centered cubic (fcc) crystal structure. The high-resolution TEM (HRTEM) image indicates that the pore walls have lattice fringes corresponding to the (111) planes, with a d-spacing and dihedral angle of 0.23 nm and ∼70°, respectively (Fig. 1e).

The morphology of the micelles on the substrates before Pt deposition was observed by AFM. The surface coverage and the ordering of the micelles can be controlled by the micelle concentration in the starting precursor solution. At a concentration of 0.1 mg mL−1, each micelle is separated from their neighbors, yielding a low coverage (Fig. 2a). Such a disordered arrangement is caused by the loosely packed micelles which interact poorly. After increasing the micelle concentration to 0.2 mg mL−1, although short-range hexagonal ordering can be observed locally, the micelles remain relatively loosely packed (Fig. 2b). At 0.4 mg mL−1, the micelles strongly interact with their neighbors due to reduced inter-micelle distance and improved spatial repulsion (Fig. 2c). With further increasing the concentration to an excess (up to 0.8 mg mL−1), the micelles become randomly aggregated and therefore, lose their long-range ordering24 (Fig. 2d). The resulting strong steric interactions also affect the shape/size of several micelles which become smaller to accommodate their high density. Based on the above results, we selected an optimal concentration of 0.4 mg mL−1 to carry on and prepare the ordered mesoporous Pt nanosheets, as shown in Fig. 1b–e. The micelle size (∼40 nm), including both the hydrophilic and hydrophobic blocks, is expected to be larger than the pore size in the Pt nanosheets, formed only from the hydrophobic PS block (∼32 nm) (Fig. S2, ESI). Studies on the influence of micelle concentration upon the mesoporosity in the Pt nanosheets reveal that, at low concentration, a discrete mesoporous network takes place (Fig. 3a), while at a higher concentration (0.8 and 1.5 mg mL−1), a continuous but disordered Pt network is formed (Fig. 3b and c). We also investigated the modification of the Pt surface morphology through the tuning of the PtCl62− concentration. At a constant micelle concentration of 0.4 mg mL−1, the P2VP+/PtCl62− molar ratio was varied from 1[thin space (1/6-em)]:[thin space (1/6-em)]10 to 1[thin space (1/6-em)]:[thin space (1/6-em)]40, resulting in the gradual expansion of the wall thickness from 12 to 25 nm as additional Pt crystallizes on both the P2VP and PEO blocks (Fig. 3d–f).


image file: c9mh01142k-f2.tif
Fig. 2 AFM topographies of micelles spin-coated on SiOx/Si. The PS-P2VP-PEO micelle concentrations are (a) 0.1, (b) 0.2, (c) 0.4 and (d) 0.8 mg mL−1.

image file: c9mh01142k-f3.tif
Fig. 3 SEM images of Pt nanosheets prepared at micelle concentrations of (a) 0.1 mg mL−1, (b) 0.8 mg mL−1 and (c) 1.5 mg mL−1, while keeping the molar ratio of P2VP+/PtCl62− at 1[thin space (1/6-em)]:[thin space (1/6-em)]20. SEM images of Pt nanosheets prepared at a molar ratio of P2VP+/PtCl62− of (d) 1[thin space (1/6-em)]:[thin space (1/6-em)]10, (e) 1[thin space (1/6-em)]:[thin space (1/6-em)]20 and (f) 1[thin space (1/6-em)]:[thin space (1/6-em)]40, while keeping the concentration of micelles constant at 0.4 mg mL−1.

Selecting an appropriate reducing agent is also a key factor for the preparation of continuously ordered mesoporous nanosheets. In the present experiment, the Pt deposition was initially carried out in DMAB vapor at 27 °C, leading to a stable porous skeleton and uniformly sized Pt nuclei. Due to relatively low DMAB vapor concentration, the Pt species cannot be entirely reduced (i.e., the deposited Pt area is discontinuous, Fig. S3, ESI). Ascorbic acid (AA) cannot be used for this pre-treatment stage, because of its low volatility (i.e., no Pt reduction). On the other hand, when we immersed the substrate with micelles in the reducing agent solution without pre-treatment, the micelles detach from the substrate, resulting in the formation of Pt precipitates in the solution. Therefore, a two-step reduction method using both DMAB vapor (for nucleation) and AA (for further growth) can enable a complete reaction. If further deposition is carried out by DMAB solution (the same concentration as the AA solution) following the DMAB vapor pre-treatment, an ordered mesoporous structure cannot be obtained. Because the reduction rate in DMAB solution is much higher than in AA solution, the Pt deposition takes place randomly outside the template.25

The unique approach reported here can be extended to other metals and alloys. For instance, we prepared mesoporous bimetallic PtNi nanosheets via the same method. Alloying Pt with Ni is important to reduce the content of scarce and costly Pt catalyst. High-angle annular dark-field (HAADF) STEM clearly shows that the as-prepared PtNi sample possesses mesopores. The corresponding elemental mapping reveals that both Pt and Ni atoms are uniformly distributed in the pore walls of the nanosheet (Fig. S4, ESI). The crystallinity of the mesoporous Pt and PtNi nanosheets (abbreviated as meso-Pt NS and meso-PtNi NS) was investigated by wide-angle X-ray diffraction (XRD), which is consistent with the standard diffraction patterns of pure Pt (JCPDS no. 65-2868). The diffraction peaks of the meso-PtNi NS have a slight offset towards higher angles, due to the presence of Ni atoms (Fig. S5a, ESI).26

XPS characterization was carried out to investigate the surface composition and electronic structures of the meso-Pt NS and meso-PtNi NS. On the high-resolution XPS spectrum centered at Pt 4f of the meso-Pt NS, the most intense doublet located at 71.29 eV and 74.59 eV can be assigned to the Pt 4f7/2 and Pt 4f5/2 of metallic Pt, respectively (Fig. S5b, ESI). The weaker peaks at 72.29 eV and 75.69 eV can be attributed to Pt2+ species from Pt oxides.27 Meanwhile, the Pt 4f peaks of the meso-PtNi NS are slightly positively shifted (0.2 eV) compared to that of meso-Pt NS, due to the contribution of the electronic orbitals from Ni.28 Metallic Ni 2p3/2 and Ni 2p1/2 from meso-PtNi NS (shown in Fig. S5c, ESI) appear at 852 eV and 868 eV, respectively, while the oxidized species show a signature at 856 and 874 eV, respectively.29 The Ni atomic and mass concentration are calculated to be 15.1% and 5.13%, respectively.

This novel design of porous 2D structure benefits from enlarged surface area and creates more electrochemical active sites.30 Electrochemical surface area (ECSA) can be calculated from the charge transferred during hydrogen desorption in the potential range of −0.2–0.2 V in 0.5 M H2SO4. The ECSA of meso-Pt NS (22.06 m2 g−1) is twice that of commercially available Pt black (9.13 m2 g−1) and superior to simple Pt mesoporous materials19,31,32 (Fig. S6a, ESI). In fuel cell applications, Pt can be used as both cathodic and anodic catalysts. The electrochemical performance towards methanol oxygen reduction (MOR) taking place at the anode is investigated in 0.5 M CH3OH (H2SO4 solution). Two typical anodic peaks can be observed during the forward and backward sweeps at 0.65 and 0.43 V, respectively (Fig. S6b, ESI). When the activity is normalized by the ECSA value, the forward peak current density of meso-Pt NS exhibits a superior current density (1.45 mA cm−2) to that of other published materials, such as mesoporous nanowires (0.98 mA cm−2),32 mesoporous nanospheres (1.24 mA cm−2)19 and mesoporous nanorods (0.45 mA cm−2) (Table S1, ESI)31 It is necessary to point out the superior utilization efficiency of the 2D mesoporous structure providing sufficient accessibility to all active sites, which can interact with the targeted molecules in the solution. During the chronoamperometric stability measurement, the meso-Pt NS exhibits high initial current density and superior activity retention throughout the whole 2000 s cycle at a loading of 0.6 V (Fig. S6c, ESI).

The oxygen reduction reaction (ORR) is the cathodic half-cell reaction, which determines the working efficiency of a full cell. The performance of three catalysts, 20% Pt/C, meso-Pt NS and meso-PtNi NS, was investigated in O2-saturated 0.1 M HClO4. As shown in the typical polarization plots, the meso-PtNi NS and meso-Pt NS exhibit a higher onset potential than 20% Pt/C, indicating that the 2D nanosheet decorated with a porous structure promotes ORR activity. In addition, the presence of Ni can further increase the ORR activity. This contribution is explained by DFT analysis in the following paragraph. In comparison, the half-wave potential of the meso-Pt NS, meso-PtNi NS and 20% commercial Pt/C catalyst are 0.89, 0.91 and 0.85 V, respectively. Owing to the presence of Ni atoms, the meso-PtNi NS exhibits superior electrocatalytic activity towards the ORR (Fig. 4a). Besides, the stability of the catalysts is another critical parameter which determines the lifetime of a fuel cell. After voltage-sweeping in O2-saturated 0.1 M HClO4 for 5000 cycles, the polarization plot was collected. The negative shift of the half-wave potential of the 20% Pt/C (37 mV) is twice as large as that of the meso-PtNi NS (18 mV) (Fig. 4b). We can then conclude that the structure of the 2D porous PtNi nanosheets promotes stability. From the above results, because of the synergistic catalytic activity of Pt and Ni atoms and the optimal architecture of these nanosheets, meso-PtNi NS showcases improved ORR performance and stability. Compared with the previously reported PtNi based catalyst, the obtained meso-Pt NS and meso-PtNi NS in this study exhibit a comparable half-wave potential (Table S2, ESI).


image file: c9mh01142k-f4.tif
Fig. 4 (a) ORR polarization curves of the catalysts in an O2-saturated 0.1 M HClO4 solution with a scan rate of 10 mV s−1 and a rotation speed of 1600 rpm. (b) LSV curves of meso-PtNi NS and 20 wt% Pt/C before and after 5000 cycles. (c) Modeled Pt-Ni systems: (I) Pt(111), (II) PtNi(111)300, (III) PtNi(111)210, (IV) PtNi(111)120, (V) PtNi(111)111, (VI) Ni(111). The structure is composed of a five-layer slab and a 20 Å vacuum. Blue and white spheres represent the Pt and Ni atoms, respectively. (d) DFT-calculated adsorption energy of Pt and each Pt–Ni system presented in (c).

Through DFT calculations, the PtNi alloys were modelled with a 2 × 2 × 5 supercell consisting of 17 Pt atoms and 3 Ni atoms (15 at% of Ni). Two subsequent slabs were separated with a vacuum gap of about 20 Å. In order to verify the enhancement of PtNi alloy nanosheets for the ORR, as shown in Fig. 4c, five alloy systems are adopted to study the adsorption properties of O and OH on the (II) PtNi(111)300, (III) PtNi(111)030, (IV) PtNi(111)210, (V) PtNi(111)120 and (VI) PtNi(111)111 (subscripts represent the number of Ni atoms on the top, second and third layer, chronologically). The surface adsorption properties of (I) Pt(111) and (VII) Ni(111) were calculated for comparison. The geometric properties of Pt(111) can be modified by the replacement of Ni. The calculated distance between the surface layer and the second layer, d(M1M2), for the Ni(111), PtNi(111)300, PtNi(111)030, PtNi(111)210, PtNi(111)120, PtNi(111)111 and Pt(111) are 2.01 Å, 2.09 Å,2.15 Å, 2.09Å, 2.21 Å, 2.32 Å and 2.34 Å, respectively, which is consistent with published calculations.33 Consequently, there are four types of surface adsorption sites: top site, bridge site and hollow site (fcc and hcp).34 The specific adsorption sites are illustrated in Fig. S7 (ESI). Then, the adsorption of typical ORR chemical species (O and OH) is investigated. With the presence of transition metal Ni, the width of the surface d-band is altered by the strain and ligand effects, which leads to the up or down shift of the energy to maintain the band filling, respectively.35 As shown in Fig. 4d, the adsorption strengths are enhanced through the doping with Ni atoms. The PtNi(111)300 shows the strongest adsorption strength both in O and OH adsorption energies. In addition, as the Ni atoms are distributed from the surface to the inside, the adsorption energy is gradually weakened. For the adsorption strength of both O and OH, there is an order of PtNi(111)030 < PtNi(111)111 < PtNi(111)120 < PtNi(111)210 < PtNi(111)300. This result shows that the doping of Ni atoms on the surface of the catalyst can enhance the O and OH adsorption strength and promote the ORR, which coincides very well with the results of ref. 34. In reality, it is difficult to precisely locate Ni atoms only on the surface.

Conclusions

In conclusion, we have prepared novel ultra-thin mesoporous Pt nanosheets through controlled self-assembly of micelles made of PS-b-P2VP-b-PEO triblock copolymer. Traditional 2D nanomaterials generally contain galleries between restacked nanosheets, which serve as major channels. This introduces a fatal weakness such that cross transport in the vertical direction is limited in volume and rate. In contrast, even when assembling our 2D porous nanomaterials, it will bring a substantial advantage in increasing the cross flux due to the existence of in-plane pores. The highly interconnected nanochannel network will provide an interesting system for fundamental investigation of high-speed ionic transport in solids, which will be a core component for future electrochemical energy conversion and storage.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Y. Li would thank the financial support by the National Key Research and Development Program of China (2018YFB0105400) and National New Energy Vehicle Technology Innovation Center. Y. Yamauchi would like to thank the support from Australian Research Council (ARC) Future Fellowship (FT150100479). Y. Yamauchi also thank the performance of part of this work at the Queensland node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for the researchers of Australia.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9mh01142k

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