Crystalline Facet-Directed Generation Engineering of Ultrathin

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Crystalline Facet-Directed Generation Engineering of Ultrathin Platinum Nanodendrites Dongdong Xu, Hao Lv, Haibao Jin, Yanhang Ma, Ying Liu, Min Han, Jianchun Bao, and Ben Liu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03861 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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The Journal of Physical Chemistry Letters

Crystalline Facet-Directed Generation Engineering of Ultrathin Platinum Nanodendrites Dongdong Xu,a Hao Lv,a Haibao Jin,b Ying Liu,a Yanhang Ma,c Min Han,a Jianchun Bao,a,* and Ben Liua,* aJiangsu

Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of

Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, Jiangsu 210023, China. bDepartment cSchool

of Chemistry, University of North Carolina at Chapel Hill, North Carolina 27599, USA.

of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China.

Abstract: In this work, we successfully prepare two-dimensional ultrathin single-crystalline platinum nanodendrites (PtNDs) with precisely controlled generation (size) through a surfactant-directing solution-phase synthesis. Amphiphilic surfactant of C22H45-N+(CH3)2CH2COOH (Br-) acts as the structure-directing template and facet-capping agent simultaneously to kinetically engineer in-theplane epitaxial growth of Pt nanocrystals along selectively exposed {111} facets into ultrathin PtNDs. A novel formation mechanism defined as crystalline facet-directed step-by-step in-the-plane epitaxial growth, similar to the synthesis of organic dendrimers, is proposed based on the nanostructure and crystalline evolution of PtNDs. The generation growth process is readily extended to precisely engineer the generations of PtNDs (from 0 to 25), and can also be utilized to grow other noble metal NDs (e.g., PdNDs and AuNDs) and core-shell Pt-Pd NDs. Thanks to the structural advantages, ultrathin PtNDs exhibit enhanced electrocatalytic performance toward hydrogen evolution reaction.

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Dendrimers are developed as the fourth-category of well-defined organic polymers, whose symmetric

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topological structure and terminal functionality render them particularly useful in catalysis,

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biochemicals and pharmchemicals, and host-guest chemistries.1-4 Generally, dendrimers are composed

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of a multifunctional central core (denoted as generation 0 (Gen 0)) surrounded by highly branched and

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fractal repeating functional units (Gen n which exactly depends on the number of repeating units).5-6

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Generation synthesis of a highly symmetric dendrimer requires the repetitive step-by-step

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polymerization process from polyfunctional units of the central core, where the organic functional

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groups (e.g., amino, carboxyl) play the role of reactive sites. Recently, inorganic dendrimer-like

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nanomaterials have attracted increasing attention due to more catalytical active sites and kinetically

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faster mass/electron transfer ability.7-13 However, generation-controlled growth/engineering and

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formation mechanism of inorganic dendrimers remain largely unresolved.14-15

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Precise control in sizes, structures and compositions of inorganic nanocrystals (NCs) is essential in

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tailoring their chemical and physical features.16-19 Of multiple nanostructures available, two-

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dimensional (2D) ultrathin noble metal NCs have recently received more attention in the

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(electro)catalytic applications.20-29 Many synthetic strategies based on surfactants and special reducing

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agents have been well developed to construct 2D noble metal NCs.20-21, 30-36 Breaking intact 2D NCs

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into thermodynamically unfavourable dendritic and anisotropic nanostructures with hierarchically

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branched fractals enlarges the surface area and exposes more catalytical active sites.37-38 However, the

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synthesis of such 2D ultrathin noble metal nanodendrites (NDs) is still technically challenged.

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Especially, 2D metal NDs generally lack of the controllability in generation numbers (behave in sizes),

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due to the difficulty in the balance between nanoconfined in-the-plane crystalline growth and

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epitaxially branched fractals. Engineering surface diffusion and epitaxial growth on selectively

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exposed crystalline facets has recently been utilized as a powerful method to tailor the nanostructures

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and uniformity of anisotropic noble metal NCs.39-41 For example, assisted by some special facet-

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capping species, selectively exposed facets (e.g., {111}) actually act as the functional reaction sites or

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“groups” to break the symmetry of NCs and direct the growth of anisotropic nanorods and

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nanowires.42-45 In the anisotropic synthetic systems of NCs, however, the selectively exposed facet-

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directed growth mostly focuses on nanorods and nanowires, and polyhedrons.46-47 Therefore, there is

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an interesting yet intriguing challenge: Can we further extend the crystalline facet-directed growth to 2 ACS Paragon Plus Environment

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engineer 2D ultrathin nanostructures of NDs, and Can we precisely control the generation growth of

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NDs as organic dendrimers?

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Herein, we present a facile and precisely controlled crystalline facet-directed step-by-step in-the-

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plane epitaxial growth of 2D ultrathin single-crystalline noble metal NDs through a surfactant-

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directing solution-phase synthesis under ambient conditions. The key in our synthesis is the utilization

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of the long-chain amphiphilic surfactant with quaternary ammonium and carboxyl functional heads,

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and halide counter ion of Br- as the structure-directing template and facet-capping agent to kinetically

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control in-the-plane epitaxial growth of metal NCs along selectively exposed crystalline facets. By

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systematically tuning the concentrations and feed ratios of surfactant template and metal precursor as

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well as reduction kinetics, the step-by-step generation growth of inorganic metal NDs (taking PtNDs

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as the example) is precisely controlled (Figure 1). The generation of ultrathin PtNDs is readily tuned

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in the range from Gen 0 (core, 2 nm) to ~Gen 25 (~ 500 nm) and even to > Gen 50 (> 1000 nm).

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Additionally, our approach is also suitable for the preparation of other inorganic metal NDs and core-

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shell bimetallic NDs. Owing to 2D single-crystalline ND nanostructures that expose more catalytical

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active sites and kinetically facilitate the mass/electron transfers during the catalysis, as-resulted PtNDs

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exhibit enhanced electrocatalytic performance in hydrogen evolution reaction (HER).

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Figure 1. Schematics and corresponded TEM images of 2D inorganic PtNDs with different

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generations. 3 ACS Paragon Plus Environment


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Ultrathin single-crystalline PtNDs are successfully synthesized by a facile one-step bottom-up

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approach in an aqueous solution. In our protocol, the long-chain amphiphilic surfactant of C22H45-

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N+(CH3)2CH2COOH (Br-) (denoted as C22N-COOH (Br-), Figure S1), H2PtCl6, and ascorbic acid (AA)

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are used as the structure-directing template and facet-capping agent, the Pt precursor, and the reducing

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agent, respectively (see Experimental section for details). Generation synthesis of 2D ultrathin PtNDs

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relies on the in-the-plane step-by-step epitaxial growth within nanoconfined lamellar mesophase

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formed by the co-assembly of C22N-COOH and H2PtCl6. This synthetic target greatly depends on

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designed functional surfactant of C22N-COOH (Br-) along with the appropriate concentration of

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reactants, reduction rate and reaction temperature, in which the kinetic control of step-by-step epitaxial

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crystalline growth along selectively exposed facets is the key factor for the formation of ultrathin

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single-crystalline PtNDs (discussed hereafter).

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Figure 2. Structural characterizations of ultrathin 2D single-crystalline Gen-7 PtNDs. (a) Low-

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magnification TEM image of PtNDs, (b) schematic diagram, (c) high-magnification TEM image, and

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(d) corresponded SAED pattern of an individual Gen-7 PtND. (e) HRTEM images of crystalline

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domains labelled in (c) (the insert is the scheme indicating the exposed facet). (f) HRTEM image taken

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from the cross section of a PtND which stands vertically on the TEM grid assisted by CNTs (the insert

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in the bottom left), and the insert in the top right is corresponded Fourier diffractogram. (g) HAADF4 ACS Paragon Plus Environment

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STEM image and corresponded elemental mapping of a Gen-7 PtND. (h) Wide-angle XRD pattern

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and (i) high-resolution Pt 4f XPS spectrum of PtNDs.

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As a model, ultrathin PtNDs with an average generation number of seven (Gen-7 PtNDs) are firstly

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characterized by various techniques. As shown in Figure 2a, typical low-magnification transmission

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electron microscopy (TEM) observation displays a regularly sheet-like and disk-shaped nanostructure

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with a good dispersibility and uniformity, confirming the formation of ultrathin 2D NDs. The average

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diameter of Gen-7 PtNDs is about 150 nm (see Figure S2 for more TEM images). High-magnification

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TEM image of an individual PtND further exhibits the dendritic nanostructure with highly branched

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and fractal subunits along the planar direction (Figure 2c). The fractal branches of PtNDs are very thin

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with an average diameter of 2.5 nm, and the interspace among the branches is in the range of 2-4 nm.

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We further reconstitute the structural configuration and carefully ascertain the generation number.

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Here, we define the concept of one generation in inorganic nanostructures from the point of one new

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crystalline branch to the next new branch. As shown in Figure 2b, the generation number of ultrathin

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PtNDs with the diameter of 150 nm is counted to be seven, as highlighted in seven different colours.

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The average length of one generation is about 20 nm, which has also been verified by the PtNDs with

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the same or different diameters (Figure 1).

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High-resolution TEM (HRTEM) image is carefully acquired at the different domains of ND in

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Figure 2c, to reveal the crystalline feature of PtNDs. The randomly selected four domains reveal totally

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same lattice fringes and directions, in which the d-spacings of 1.96 and 2.25 Å are ascribed to (200)

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and (111) planes, respectively, of face-centered cubic (fcc) Pt (Figure 2e). These results definitely

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demonstrate that the PtND possesses the single-crystalline feature enclosed with {110}-exposed

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surface facet. The selected-area electron diffraction (SAED) pattern taken from this PtND also displays

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a series of spots (Figure 2d), corresponding to the [110] zone axis of fcc Pt. The diffraction spots are

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slightly diffused, possibly due to the shift of lattice orientation induced by the flexible feature

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compared to the bulk counterparts, especially with a larger size. To measure the thickness of PtNDs,

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as-resulted PtNDs are firstly mixed and fixed with commercial carbon nanotubes (CNTs). As shown

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in Figure 1f, the thickness of a typical PtND is measured to be ~2.3 nm based on TEM image taken

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from the section perpendicular to the plane of PtNDs (see Figure S3 for more TEM images). The

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corresponding Fourier diffractogram (FD) demonstrates the [111] zone diffraction, further indicating 5 ACS Paragon Plus Environment


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the {110}-exposed surface facet on PtNDs. High-angle annular dark-field scanning TEM (HAADF-

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STEM) and corresponded elemental mapping (Figure 2g) as well as wide-angle X-ray diffraction

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(XRD, Figure 2h) further confirm the crystallographic structure of ultrathin 2D PtNDs. High-

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resolution X-ray photoelectron spectroscopy (XPS) of Pt 4f shows two asymmetric peaks at 71.2 and

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74.6 eV (Figure 2i), exactly corresponding to metallic Pt. All of above-mentioned morphological and

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structural characterizations suggest the successful synthesis of ultrathin single-crystalline Gen-7

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PtNDs with selectively {110}-exposed facet.

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Figure 3. Structure and crystalline evolution of Gen-7 PtNDs. (a-d) Low-magnification TEM and (e-

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g) corresponded HRTEM images of Pt nanostructure obtained at the reaction period of (a, e) 10 min,

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(b, f) 30 min, (c) 2h and (d, g) 4 h. The insert in top right corner and bottom left corner in (g) are

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corresponded FD and low-magnification TEM image, respectively. (h) Proposed formation

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mechanism for crystalline facet-directed step-by-step in-the-plane epitaxial growth of Gen-7 PtNDs

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selectively along crystallographic orientation. 6 ACS Paragon Plus Environment

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The nanostructure and crystalline evolution of Gen-7 PtNDs have been thoroughly investigated by

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the time-dependent TEM observations, to explore their formation mechanism (Figure 3). Low-

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magnification TEM images show an in-the-plane step-by-step epitaxial growth process of Gen-7

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PtNDs. At the first 10 min of the initial reaction stage, some primary NCs or seeds with a very small

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size in the range of 2-5 nm are clearly seen (Figure 3a). Increasing the reaction time to 30 min, the

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NDs with branched small fractals (~Gen 1) along the planar orientation are observed (Figure 3b). 2-4

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h later, the NDs grow continuously along branched fractals (Figure 3c, d). Finally, the reaction stops

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at 12 h, with a diameter of 150 nm (Figure 2). The crystalline evolution of PtNDs is further

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characterized by detailed HRTEM observations at different reaction times. As shown in Figure 3e,

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HRTEM image exhibits that small NCs are nearly hexagonal in the nanostructure, in which in-the-

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plane exposed {110} crystalline facets of primary NC seeds are enclosed by 4 × {111} and 2 × {200}

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facets (see enlarged HRTEM in Figure S4). Subsequently, the small NCs act as the core seeds for

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epitaxial growth of highly branched Pt NCs. As shown in Figure 3f, the step-by-step formation of Pt

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NCs with branched fractals is epitaxially crystallized and grown only along the direction of

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these NC seeds with a longer reaction period of 30 min. Crystalline facet-directed epitaxial growth

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along the direction is clearly confirmed in the PtNDs with the reaction time of 4 h (Figure 3g),

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where totally same lattice fringes and orientations with a single-crystalline feature are seen in each Pt

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branched fractal. These results indicate that single-crystalline ultrathin PtNDs are formed by facet-

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directed step-by-step in-the-plane epitaxial growth along direction of Pt seeds.

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What drive the step-by-step in-the-plane epitaxial growth of ultrathin PtNDs with controlled

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branched fractals and selectively {110}-exposed facet, instead of 2D nanosheets, 3D NDs or bulk

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nanoparticles? To elaborate the formation mechanism, the key factor focuses on chemically structural

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features of C22N-COOH (Br-) used in our work. Unlike the conventional surfactants for metal NCs,48-

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50

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structure-directing template and facet-capping agent simultaneously. C22N-COOH (Br-) highlights

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three important features for the synthesis of PtNDs, including hydrophobic alkyl tail, hydrophilic

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quaternary ammonium and carboxyl functional head, and halide ion (Figure 3h). First, the long-chain

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hydrophobic alkyl tail (C22) drives the co-assembly between the ionic amphiphilic surfactant and metal

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precursor (PtCl62-) into stabilized lamellar mesophase. Thus, the surfactant as the structure-directing

the lab-synthesized amphiphilic surfactant of C22N-COOH (Br-) plays the crucial roles as the

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template causes in situ in-the-plane reduction of PtCl62- into ultrathin 2D Pt NCs within nanoconfined

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lamellar nanostructure, instead of 3D nanostructures or 0D bulk nanoparticles. The lamellar hybrid

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nanostructures of C22N-COOH/PtCl62- were confirmed by small-angle XRD and SEM observation, and

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a scheme was also utilized to illustrate the formation process of PtNDs (Figure S5). Second, functional

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heads of quaternary ammonium and carboxyl groups cooperatively stabilize the lamellar mesosphase

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and more importantly selectively bind with {110} facet of fcc Pt NCs.49, 51-52 Capping the surfactant

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on {110} facet inhibits the NC growth along {110} direction and thus exposes {110} crystalline facet

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in the resultant ultrathin PtNDs. {110}-exposed facet is definitely indicated by TEM images of PtNDs

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with different reaction times and different generations. Third, the halide ion of Br- would also

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selectively adsorb on {100} facet of fcc Pt NCs and block the epitaxial growth of Pt NCs along {100}

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facet.53-54 Although there are abundant Cl- in the synthesis solution (from PtCl62- precursor), no EDS

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signal assigned to Cl in final PtNDs further confirmed the important role of Br- for crystalline facet-

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directed growth (Figure S6). There are three kind of the low-index crystalline facets of {100}, {110}

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and {111} in fcc Pt NCs. As observed in NC seeds and Gen-0 PtNDs (Figure S4), exposed facets in Pt

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NCs include 2 × {100}, 2 × {110} and 4 × {111} facets. Among them, {110} facets are capped by

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functional quaternary ammonium and carboxyl groups, while {100} facets are blocked by Br-.

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Therefore, the step-by-step epitaxial growth of Pt NCs is only achieved along un-capped and

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completely exposed {111} facets (Figure 3h). Step-by-step in-the-plane epitaxial growth of PtNDs

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along selectively exposed {111} facets thus prefers the formation of ultrathin and highly branched

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PtNDs, rather than un-fractal nanosheets. Based on the TEM observations and structural features of

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the surfactant, we propose that the formation of ultrathin PtNDs with {111}-exposed facet is ascribed

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to crystalline facet-directed step-by-step epitaxial growth process using C22N-COOH (Br-) as the

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structure-directing surfactant and facet-capping agent.

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Crystalline facet-directed step-by-step epitaxial growth of PtNDs is exactly similar to the synthesis

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of organic dendrimers. Pt NCs seeds formed in the first 10 min play the role of dendritic core, whose

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specific {111}-exposed crystalline facets act as the functional reactive sites for step-by-step epitaxial

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growth of the larger NDs with highly uniform branched fractals. To further confirm our hypothesis of

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C22N-COOH (Br-) on formation mechanism of PtNDs, more control experiments are carried out. First,

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the surfactants with the shorter hydrophobic alkyl chains (C20N-COOH (Br-), C18N-COOH (Br-), 8 ACS Paragon Plus Environment

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C16N-COOH (Br-), and C14N-COOH (Br-)) are used as the structure-directing template and facet-

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capping agent. Non-uniform NDs and even nanoparticle aggregates are formed (Figure S7), indicating

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the importance of nanoconfinement of the long-chain hydrophilic tail (e.g., C22) in in-the-plane

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formation of ultrathin 2D nanostructures. Second, when the surfactants are C22N (Br-) or C22-py (Br-)

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without carboxyl functional group (see Figure S1 for the chemically molecular configuration) and

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conventional surfactants cetyltrimethylammonium bromide (CTAB) and Pluronic P123, only porous

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3D Pt nanostructures can be obtained in the same synthetic conditions (Figure S8). Additionally, the

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proper concentrations of reactants and synthetic temperatures are also important to obtain 2D PtNDs

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by means of the control of micellar nanostructures (Figure S9 and 10). Third, C22N-COOH (Cl-) is also

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used to take the place of C22N-COOH (Br-). Due to the weak adsorption affinity of Cl- on Pt {100},

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the step-by-step epitaxial growth of Pt NCs will not be solely limited along the directions. The

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epitaxial directions along both and facets are seen simultaneously, and the resultant Pt

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NCs thus lack of the controllability in uniformity and generation (Figure S11). This further indicates

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the capping effect of Br- for epitaxial growth of Pt NCs by blocking {100} orientation. These results

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demonstrate that both functional tails and halide ion of Br- are critical as the facet-capping agent to

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direct the epitaxial growth of highly branched PtNDs along selectively exposed {111} crystalline

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facets. We also investigate the effect on reducing agents by using hydrazine hydrate and sodium

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borohydride instead of AA. Only branched spheres or nanoparticles are seen (Figure S12), indicating

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the importance of reduction kinetics for the formation of PtNDs.55

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We correlate that {111} facet-directed step-by-step epitaxial growth mechanism is also different

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from two kinds of conventional mechanisms (nanoparticles self-aggregation and oriented attachment).

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On the one hand, due to strong nanoconfined effect inside lamellar C22N-COOH micelles, the crystals

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movement could be prevented more or less, thus avoiding the self-aggregation of Pt NCs. On the other

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hand, if oriented attachment process happened in our synthesis, there should be abundant twin

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boundary at the attachment interfaces.56-58 However, although we had taken sufficient TEM

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observations on final PtNDs or Pt nanostructures at different crystalline period, no such twin

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boundaries could be observed. It indicated the rationality for the proposed mechanism of {111} facet-

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directed step-by-step epitaxial growth of PtNDs in our synthesis system again.

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Figure 4. Epitaxial growth of PdNDs along the periphery of PtNDs. (a) TEM, (b) HRTEM images and

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(c) corresponded FDs, (d) HAADF-STEM image and corresponded elemental mappings of 2D core-

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shell Pt-Pd NDs.

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Step-by-step in-the-plane epitaxial growth of noble metal NDs can be further extended to form

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secondary metal NDs (taking PdNDs as the example) on Gen-7 PtNDs. The synthesis is achieved by

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injecting a Pd precursor solution (containing C22N-COOH (Br-), H2PdCl4 and H2O) into freshly

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prepared PtND solution. The reduction reaction into PdNDs is immediately driven by excessive AA

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for PtNDs. In the synthesis, ultrathin PtNDs behave as the core seeds to grow PdND shell in situ. As

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shown in Figure 4a, the obtained sample has a 2D ultrathin core-shell ND nanostructure (see more

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TEM images in Figure S13). That is, Pd NCs are epitaxially grown only along the branched fractals of

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PtNDs. The epitaxial growth of Pd shell has also been observed from the HAADF-STEM image and

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corresponded elemental mappings (Figure 4d). More interestingly, HRTEM observations indicate that

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epitaxially grown PdNDs have the similar lattice fringes and same crystallographic orientations in

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discrete Pt and Pd components (Figure 4b and c), although metallic Pd and Pt have a tiny lattice

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mismatch.59 These ambiguously suggest that PdND shell on PtND core also undergoes a crystalline 10 ACS Paragon Plus Environment

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facet-directed step-by-step epitaxial growth process along the direction. We also carefully

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check the core-shell Pt-Pd NDs obtained at different growth periods (Figure S14), further confirming

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the epitaxial growth mechanism.

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Figure 5. Growth diagrams between Gen numbers of PtNDs and concentrations of C22N-COOH (Br-)

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and H2PtCl6.

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Ultrathin PtNDs with highly ordered branched fractals are synthesized by crystalline facet-directed

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step-by-step in-the-plane epitaxial growth. Therefore, the generation numbers (sizes) of ultrathin

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PtNDs can be precisely controlled by carefully tailoring the synthetic parameters of concentrations and

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feed ratios of the surfactant and Pt precursor, as the controlled synthesis of different generation organic

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dendrimers. The relationship between Gen number and concentration of C22N-COOH (Br-) and

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H2PtCl6 is displayed in Figure 5 and corresponding TEM images of some typical PtNDs are provided

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in Figure S15. For the PtNDs below Gen-7, the Gen number (and size) of PtNDs is strongly related to

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the concentration of H2PtCl6 (see TEM images of PtNDs with Gen number of 1, 3, 5, 7 in Figure S15b-

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e). A linear relationship between Gen number and concentration of H2PtCl6 definitely indicates the

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good controllability in PtNDs with the unchanged concentration of C22N-COOH (Br-) (0.7 mM). By

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contrast, to obtain the PtNDs with the higher Gen numbers, the concentrations of C22N-COOH (Br-)

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and H2PtCl6 should be increased simultaneously, although the surfactant of C22N-COOH (Br-) is

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excessive. This may be because the higher concentration of surfactants can facilitate the formation of

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the larger lamellar mesophase.60 As shown in Figure 1, PtNDs with adjustable Gen numbers in the 11 ACS Paragon Plus Environment


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range of 10 and 20 are obtained (also see more TEM images for Gen 16 and 20 in Figure S15f and g).

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Unfortunately, our synthetic method is very difficult to grow highly uniform PtNDs with Gen numbers

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higher than 20, which is also similar with the difficulty in the controlled synthesis of the high-

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generation organic dendrimers. A broad Gen distribution for Gen-25 PtNDs as a typical example

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indicates that in-the-plane epitaxial growth may be interrupted based on the poor uniformity of the

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larger lamellar mesophase or the distinct crystal growth kinetics of PtNDs.

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Ultrathin PtNDs render them with diverse structural advantages, including anisotropic 2D

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architecture, dendrite-like nanostructure, and single-crystalline feature, all of which are beneficial to

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enhanced electrocatalytic performance. On the one hand, dendritic nanostructure exposes more

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catalytically active sites (more step/corner atoms) in both front and side parts of branched fractals.

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Besides, ultrathin NDs with the thickness of ~2.3 nm also increase the utilization of precious metals,

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compared to its NP counterpart. On the other hand, 2D single-crystalline nanostructure contributes to

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the faster electron and mass transfers on ultrathin PtNDs. Both more active sites and faster

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mass/electron transfer ability synergistically boost the electrocatalytic activity of ultrathin PtNDs.

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Meanwhile, anisotropic 2D ND nanostructure would suppress the intrinsic dissolution and Ostwald

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ripening of metal NDs,61-62 and thus enhance the self-stability of ultrathin PtNDs during the

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electrocatalysis.


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Figure 6. Electrocatalytic HER activity and stability of ultrathin Gen-7 PtNDs in 0.5 M H2SO4

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electrolyte. (a) CV and (b) LSV curves of ultrathin PtNDs and commercial Pt/C. (c) Summarized HER

4

activities of ultrathin PtNDs and Pt/C at the current density of 10, 25 and 50 mA cm-2. (d) Time-

5

dependent i-t curves of ultrathin PtNDs and Pt/C.

6

Electrochemical HER is performed as a proof-of-concept catalytic reaction to evaluate the activity

7

and stability of ultrathin single-crystalline PtNDs (Gen 7). Commercial Pt carbon (Pt/C, Sigma-

8

Aldrich) is used as a control in our experiments. HER evaluations of two samples are performed in a

9

three-electrodes system in 0.5 M H2SO4 at 25 oC. Figure 6a shows cyclic voltammogram (CV) curves

10

of ultrathin PtNDs and commercial Pt/C collected in N2-saturated 0.5 M H2SO4 solution (50 mV s-1).

11

Well-defined two pairs of the peaks below 0.3 V (vs reversible hydrogen electrode (RHE)) from both

12

ultrathin Gen-7 PtNDs and commercial Pt/C are observed, indicating the hydrogen desorption-

13

adsorption reactions happened on Pt-based nanocatalysts.63 Electrocatalytic active surface area

14

(ECSA) of ultrathin PtNDs is calculated to be 51.4 m2 g-1, higher than that of Pt/C (46.4 m2 g-1). A

15

slight higher ECSA for ultrathin PtNDs is also seen from CO stripping experiments (Figure S16),

16

further demonstrating the structural advantages of 2D ultrathin single-crystalline PtNDs. Linear sweep

17

voltammetry (LSV) curves obtained at a scan rate of 5 mV s-1 are provided in Figure 6b, to reveal 13 ACS Paragon Plus Environment


Page 14 of 22

1

electrocatalytic HER activity (also see LSV curves normalized to ECSAs and mass in Figure S17). As

2

expected, ultrathin PtNDs exhibit a smaller overpotential of < 0.01 V at the current density of 50 mA

3

cm-2, compared to the-state-of-art commercial Pt/C. The result shows that ultrathin single-crystalline

4

PtNDs are electrocatalytically more active than Pt/C for HER. Higher electrocatalytic activity of

5

PtNDs is also confirmed by comparing the overpotentials at different current densities (Figure 6c). The

6

remarkably enhanced HER activity is partially ascribed to the surfactant on the surface of PtNDs also,

7

in which quaternary ammonium and COOH groups may facilitate the adsorption of local H+

8

concentration.13, 64 Besides, Tafel plots are also fitted, and the linear curve with a smaller value of 32.7

9

mV dec-1 for ultrathin PtNDs further evidences the quicker electrocatalytic kinetics (Figure S18).

10

Furthermore, electrocatalytic stability of ultrathin single-crystalline PtNDs is assessed using current-

11

time (i-t) chronoamperometric response (Figure 6d). Current retention for PtNDs retains 88.3% for 13

12

h (higher than 76.7 % for Pt/C), confirming the good stability of ultrathin PtNDs under acidic

13

environment. We emphasize that the electrocatalytic performance of Pt nanostructures could be further

14

improved by means of anchoring Pt nanocrystals in different supports, creating atomic Pt or alloying

15

Pt with other metals.65-69 In current manuscript, enhanced HER performance of PtNDs is only

16

employed to confirm the structural advantages of 2D single-crystalline dendrites compared to common

17

Pt nanocrystals.

18

In summary, inspired from the generation-controlled synthesis of organic dendrimers, we

19

successfully prepare 2D ultrathin inorganic single-crystalline noble metal NDs in an aqueous solution

20

at ambient conditions. The anisotropic growth of ultrathin PtNDs, as an example, is a consequence of

21

both rationally designed surfactant of C22N-COOH (Br-) and reduction kinetics of Pt precursor. The

22

formation mechanism indicates that the surfactant acts as the structure-directing template and facet-

23

capping agent simultaneously to direct in-the-plane step-by-step epitaxial growth of ultrathin single-

24

crystalline PtNDs along selectively exposed {111} crystalline facet. To the best of our knowledge,

25

crystalline facet-directed synthesis of 2D ultrathin noble metal NDs has not been elaborated before.

26

Our approach is also suitable for epitaxially growing core-shell Pt-Pd NDs, precisely controlling the

27

generation of the PtNDs and also preparing other noble metal NDs (for example, PdNDs and AuNDs

28

in Figure S19). Our work demonstrates that ultrathin single-crystalline PtNDs possess multiple

29

structural advantages and thus exhibit remarkably enhanced electrocatalytic HER performance. We 14 ACS Paragon Plus Environment

Page 15 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

hope the findings presented here could provide the in-depth insight and understanding for the

2

formation mechanism of ultrathin single-crystalline NDs and develop a new avenue for rationally

3

designing and synthesizing unique noble metal frameworks with enhanced (electro)catalytic

4

performance.

5

ASSOCIATED CONTENT

6

Supporting Information

7

The Supporting Information is available free of charge on the ACS Publications website at DOI:

8

10.1021/acs.jpclett. jz-2018-03861j.

9

Experiment details, additional electron microscopic images, schemes for the formation of 2D PtNDs,

10

and electrocatalytic tests (PDF)

11

AUTHOR INFORMATION

12

Corresponding Author

13

*E-mails: [email protected]; [email protected]

14

Notes

15

The authors declare no competing financial interest.

16

ACKNOWLEDGMENT

17

The authors acknowledge the financial supports from Jiangsu Specially Appointed Professor Plan,

18

National Natural Science Foundation of China (No. 21501095, 21471081, 21533012, and 21671106),

19

Natural Science Foundation of Jiangsu Province (No. BK20180723). This work is also supported by

20

Priority Academic Program Development of Jiangsu Higher Education Institutions, National and Local

21

Joint Engineering Research Center of Biomedical Functional Materials.

22

REFERENCES

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