Self-Assembly of Complex Multimetal Nanostructures from Perforated

Apr 12, 2017 - The morphologies of the BCP thin films and metal nanostructures were .... We have successfully fabricated hexagonally perforated metall...
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Self-Assembly of Complex Multimetal Nanostructures from Perforated Lamellar Block Copolymer Thin Films Seung Keun Cha,† Gil Yong Lee,† Jeong Ho Mun,† Hyeong Min Jin,† Chang Yun Moon,† Jun Soo Kim,† Kwang Ho Kim,*,‡ Seong-Jun Jeong,*,§ and Sang Ouk Kim*,† †

National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly, Department of Material Science and Engineering, KAIST, Daejeon 34141, Republic of Korea ‡ Department of Materials Science and Engineering, Pusan National University, Pusan 46241, Republic of Korea § Device Laboratory, Device & System Research Center, Samsung Advanced Institute of Technology, Suwon 16678, Republic of Korea S Supporting Information *

ABSTRACT: We introduce a facile and effective fabrication of complex multimetallic nanostructures through block copolymer self-assembly. Two- and three-dimensional complex nanostructures, such as “nanomesh,” “double-layered nanomeshes,” and “surface parallel cylinders on nanomesh,” can be fabricated using the self-assembly of perforated lamellar morphology in block copolymer thin films. Simultaneous integration of various metallic elements, including Pt, Au, and Co, into the self-assembled morphologies generates multimetal complex nanostructures with highly interconnected morphology and a large surface. The resultant metal nanostructures with a large surface area, robust electrical connectivity, and well-defined alloy composition demonstrate a high-performance electrochemical catalysis for hydrogen evolution reaction (current density: 6.27 mA/[email protected] V and Tafel slope: 43 mV/dec). KEYWORDS: block copolymer, self-assembly, metal nanostructure, catalyst, hydrogen evolution

1. INTRODUCTION Nanoscale self-assembly offers enormous potential for highly efficient scalable nanofabrication.1−6 Functional nanopattern structures prepared using the self-assembly principle can be useful for a broad spectrum of application fields, including electronics, photonics, sensors, and energy storage/conversion.7−11 Block copolymer (BCP) self-assembly is a wellestablished representative self-assembly principle, which has been widely exploited for nanopatterning applications particularly aiming at semiconductor device fabrication.12−21 It provides dense periodic arrays of self-assembled nanodomains with a typical characteristic dimension of 3−50 nm.13,22−27 In contrast to the sphere, cylinder, or lamellar self-assembled morphology commonly used for line or dot array patterning, perforated lamellar (PL) phase, known to be metastable in bulk phase, can be stabilized in thin films.28−31 Knoll et al. reported that cylinder-forming BCP thin films generated PL structures under specific conditions because of an interplay between the surface field and confinement effects.30 Lyakhova et al. demonstrated that various structures including the PL phase were found in BCP thin films, resulting from the different surface fields of the two interfaces.31 Such a PL morphology may provide densely ordered nanoporous structures, which can be used as a template for the complex nanoporous structure formation, such as membranes.32 © 2017 American Chemical Society

In this work, we demonstrate a simple and effective selfassembly of multimetal complex nanoporous structures with an interconnected morphology using a PL BCP self-assembly. Various nanoporous structures including two-dimensional (2D) nanomesh to three-dimensional (3D) nanocomplex structures can be stabilized in BCP thin films by judicious control of the film thickness and interfacial energy. Subsequently, highly specific incorporation of metallic precursors into self-assembled hydrophilic nanodomains generates multimetal complex nanostructures. Among various resultant metal nanostructures, Pt nanomesh was used as a model catalyst for the electrochemical hydrogen evolution reaction (HER). Owing to the highly interconnected reliable electrical pathway and large enough surface area, Pt nanomesh catalysts exhibit an improved HER performance than the flat Pt catalysts and are at a level similar to the Tafel slope of commercial Pt/C catalysts.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Monolayer Metal Nanomesh Patterns. Polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP; Mn = 18 kg/mol for PS and 9 kg/mol for P2VP) was purchased from Polymer Source, Inc. PS-b-P2VP was dissolved in toluene (Sigma-Aldrich) to prepare Received: March 8, 2017 Accepted: April 12, 2017 Published: April 12, 2017 15727

DOI: 10.1021/acsami.7b03319 ACS Appl. Mater. Interfaces 2017, 9, 15727−15732

Research Article

ACS Applied Materials & Interfaces different concentrations of BCP solutions (from 0.8 to 2 wt %). Thin BCP films (∼35 nm) were spin-cast on a cleaned Si wafer by adjusting the solution concentration and the spinning speed. Thermal annealing of the BCP films in a vacuum oven at 250 °C induces well-ordered monolayer PL nanostructures. The samples were immersed in a weakly acidic aqueous platinum precursor (K2PtCl4, Strem Chemicals) solution to load metal anions selectively into the protonated vinyl groups.17 After rinsing and drying the samples, oxygen (O2) plasma treatment (50 sccm, 50 W, and controlled time) was used to remove the polymer layers and obtain the metal nanomesh patterns. For bimetallic nanomeshes, precursors of platinum and another metal (HAuCl4, K3Co(CN)6, or Na2PdCl4; Strem Chemicals) were simultaneously dissolved in a weakly acidic aqueous solution. 2.2. Multistep Process for 3D Complex Nanostructures. After metal loading into the annealed BCP samples, UV cross-linking was performed to fix the BCP morphology. A new layer of BCP film was spin-cast on the cross-linked films with an appropriate thickness. The same procedures including thermal annealing, metal loading, and plasma treatment were performed again. Upon the metal-loading process into the first and second BCP layers, a variety of multimetallic 3D nanostructures could be synthesized with a combination of metal precursors. 2.3. Single-Step Approach for 3D Complex Nanostructures. Thick BCP films (55−90 nm) were spin-cast on the Si substrates and thermally annealed in the same way as mentioned above. According to the film thickness, various 3D nanoporous structures were selfassembled. After metal ion loading and O2 plasma etching, 3D metallic nanoporous structures were generated. Judicious optimization of the plasma-etching conditions (1 min for double-layered nanomesh and 2 min for nanomesh/nanolines) was required for the successful 3D nanostructure formation without collapse. 2.4. HER Measurements. For the conductive substrate formation for HER measurement, thin films of Cr (∼10 nm) and Au (∼80 nm) were deposited on the Si substrate using e-beam evaporation. Pt nanomesh was fabricated on the Cr/Au-deposited Si substrate with the same process as mentioned above. Pt/C and flat Pt film samples were used as references. Electrochemical reactions were performed using a three-electrode system (Bio-Logic SP-200) with a 5 mV/s scan rate in a 0.5 M H2SO4 solution. Ag/AgCl and graphite electrodes were used as reference and counter electrodes, respectively. 2.5. Characterization. The morphologies of the BCP thin films and metal nanostructures were imaged using a Hitachi S-4800 FESEM. Elemental analysis of multimetallic nanomesh structures was performed using X-ray photoelectron spectroscopy (XPS) (K-Alpha, Thermo VG Scientific).

Figure 1. (a) Schematic illustration of the self-assembly process for integrating the 2D metal nanomesh and 3D metal complex nanostructures. Plane-view SEM images of (b) Pt-precursor-loaded hexagonally perforated lamellar forming PS-b-P2VP BCP thin film, (c) 2D Pt nanomesh after O2 plasma etching for 1 min, (d) double-layered Pt nanomesh, and (e) Pt nanolines on nanomesh complex structures.

3. RESULTS AND DISCUSSION Two-dimensional metal nanomesh structures were produced as follows (Figure 1a): A compositionally asymmetric PS-b-P2VP BCP thin film of ∼35 nm thick was spin-coated on a Si wafer and annealed at 250 °C for self-assembly. The BCP thin film spontaneously self-assembled into a hexagonally perforated lamellar morphology. We note that the PL phase was known to be stabilized in thin films of a specific thickness (35 nm) as a result of surface reconstruction.29−31,33 After self-assembly, the thermally annealed samples were then immersed in a diluted HCl (aq) solution including various metallic anion precursors. Protonated P2VP blocks protruded from the PS surface layer under the acidic condition, which facilitated a metal precursorloading process (Figure 1a, cross-section).20 Finally, O2 plasmaetching process removed all organic components and produced metal nanomesh structures on the Si substrates. Figure 1b,c exhibits the plane-view scanning electron microscopy (SEM) images of the Pt-loaded PL BCP thin films before and after the O2 plasma treatment (see Figure S1a,b for low-magnification large-area SEM images). Note that during immersion in the acidic aqueous solution of the platinum precursor (K2PtCl4),

the negatively charged PtCl42− precursor was complexed with the protonated nitrogens in the P2VP domains by electrostatic interaction. The bright matrix and the dark dots represent Ptloaded P2VP and PS nanodomains, respectively (Figure 1b). After O2 plasma etching (for 1 min), the obtained Pt nanomesh showed hexagonal nanoscale holes of 30 nm period (Figure 1c). Note that in this work, the O2 plasma-etching condition was carefully controlled for optimized nanomesh patterns. As shown in Figure S1c,d, O2 plasma etching for 30 s generates holes of smaller diameter (∼21 nm) and thicker metalinterconnected frames (∼11 nm) because of the polymer residue. By contrast, O2 plasma etching for 3 min results in the hole diameter and the interconnected line width of ∼25 and ∼5 nm, respectively. The carbon element decreases with etching time, but it is hard to completely remove the BCP layer (see Figure S2a,b), presumably because of the blocking effect of the upper metal nanomesh. In addition, the resultant metal nanomesh becomes considerably oxidized during the O2 plasma treatment (Figure S2c). 15728

DOI: 10.1021/acsami.7b03319 ACS Appl. Mater. Interfaces 2017, 9, 15727−15732

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cylinder/PL complex phase” was observed, which may be strongly influenced by the thickness commensurability and surface/interfacial energies. As shown in Figure 2c, the selfassembled BCP nanostructure consisted of a cylindrical bottom layer and a PL top layer, where the Si substrate was preferentially wetted by P2VP blocks. The morphologies shown in Figure 2b,c are 3D Pt complex nanostructures after Pt ion loading and O2 plasma etching. The O2 plasma-etching time was also carefully controlled (for “double-layered Pt nanomeshes” and “nanoline/nanomesh”, O2 plasma-etching times were 1 and 2 min, respectively). A brief etching for less than 1 min was not enough to reveal the bottom nanostructures, and a long etching for over 3 min destroyed the 3D complex nanostructures (Figure S3). The perforated P2VP lamellar phase is an attractive nanotemplate for highly interconnected multimetal nanostructures. Figure 3 shows the plane-view SEM images of bimetallic

For the formation of 3D metal complex nanostructures, such as “double-layered metal nanomeshes” or “nanolines on nanomesh”, a multistep process was used (see the illustration in Figure 1a). First, Pt-loaded monolayer PL-forming PS-bP2VP BCP samples were prepared as described above and cross-linked using UV light to preserve the nanoscale structure during the following overlay process. Subsequently, the second PS-b-P2VP BCP layer was prepared in the same way over the cross-linked first layer. The phase morphology of the second layer was delicately dependent on the film thickness. Typically, the second BCP thin films with thicknesses of 35 and 30 nm were deposited, where PL and surface parallel cylinder phases were formed, respectively. After the second Pt-loading process, the polymer template was completely removed by O2 plasma etching. The resultant 3D Pt nanocomplex structures with a Pt nanomesh bottom layer are shown in Figure 1d,e. Plane-view SEM images reveal that the underlying Pt nanomesh layers are well-maintained and intact. The hole diameter and the interconnected line width of the double-layered Pt nanomesh structure were ∼25 and ∼5 nm (Figure 1d), respectively, and the Pt line width in the Pt nanoline/nanomesh complex was 5 nm (Figure 1e). This multistep approach can be further repeated for obtaining thicker 3D nanoporous structures. Three-dimensional metal complex nanostructures could also be obtained by a single-step deposition of BCP films with a precisely controlled thickness (see the illustration in Figure 2a).

Figure 2. (a) Schematic illustration of the single-step process for 3D metal nanostructures, based on the precise control of BCP film thickness for surface reconstruction. Plane-view SEM images of (b) double-layered Pt nanomesh structures after O2 plasma etching for 1 min and (c) Pt nanomesh on Pt lines after O2 plasma etching for 2 min.

Figure 3. (a) Schematic procedure of bimetallic nanomeshes through simultaneous loading of two different metallic precursors. Plane-view SEM images of metal nanomeshes according to the relative compositions of different metals (Pt−Au, Pt−Pd, or Pt−Co) at 0 mol % Pt (b,e,h), at 25 mol % Pt (c,f,i), and at 50 mol % Pt (d,g,j), respectively.

Compositionally asymmetric BCP thin films enable various nanostructures, including wetting layer (dis), spherical microdomains, a PL, and cylinders (C⊥ or C∥), as a result of the surface reconstruction modulated by interference and confinement effects.30,33 Interestingly, PS-b-P2VP BCP thin films (Mn = 18 kg/mol for PS and 9 kg/mol for P2VP) with the thicknesses of ∼35 and ∼60 nm demonstrated single- and double-layered PL phases, respectively (Figure 2b). When the film thickness was ∼83 nm, a “double-layered surface parallel

nanomeshes produced by the simultaneous incorporation of two different metal precursors (of Pt−Au, Pt−Pd, or Pt−Co). Interestingly, when a single precursor of Au, Pd, or Co was loaded without Pt precursor, the resultant morphologies readily collapsed (Figure 3b,e,h). By contrast, pure Pt nanomesh showed an excellent morphological stability (Figures 1 and 2). This can be attributed to a high loading density of Pt precursor 15729

DOI: 10.1021/acsami.7b03319 ACS Appl. Mater. Interfaces 2017, 9, 15727−15732

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ACS Applied Materials & Interfaces into BCP templates with a strong agglomeration behavior. As shown in Figure S4, Pt nanodots showed densely agglomerated morphologies with small diameters (13−15 nm). By contrast, Pd (15−18 nm) and Au dots (16−20 nm) showed coarsely packed nanocluster morphologies with larger diameters, which indicates that the loaded metal patterns do not exactly follow the BCP nanotemplate. Co anions were not loaded into the BCP template enough, so the resultant Co dot pattern was unclear with many defects. These results demonstrate that Pt is a good metal element to maintain the entire nanopatterns of BCP films, enabling the stable nanomesh morphology during the metal-loading and plasma-etching processes. As a result, when the Pt composition was above 50 mol % in the multimetallic nanostructure formation, the morphological stability was significantly improved (Figure 3d,g,j). The exact chemical compositions of the bimetallic nanomeshes were confirmed using XPS analysis (Figure S5 and Table 1). The fabricated bimetallic nanomeshes were composed of Pt (38 at %)−Au (62 at %), Pt (56 at %)−Pd (44 at %), and Pt (79 at %)−Co (21 at %). Table 1. Composition of Bimetallic Nanomeshes of Pt−Au, Pt−Pd, and Pt−Co Analyzed Using XPSa

a

element

composition of Pt (at %)

composition of second metal (at %)

Pt−Au Pt−Pd Pt−Co

38 56 79

62 44 21

Figure 4. (a) Schematic procedure of the fabrication of 3D multimetal complex nanomeshes through the multistep process. SEM images of 3D multimetallic nanomeshes with the elements of (b) Pt−Pd−Co (bottom Pt−Pd and top Pt−Co) and (c) Pt−Co−Au (bottom Pt−Co and top Pt−Au). (d) XPS spectra of the Pt−Pd−Co multimetallic nanomesh double layer (b).

The molar ratio between the two precursors is 1/1.

Multimetal complex 3D nanostructures could be accomplished by multistep repetition of BCP self-assembly and metallization cycles (see Figure 4a). First, precursors of Pt and another metal were loaded into the first self-assembled PL BCP layers. Subsequently, the samples were illuminated using UV light for morphology fixing by chemical cross-linking. The second BCP layer was overlaid upon the cross-linked BCP layer by spin-coating and thermally annealed for self-assembly. A subsequent metal-loading process with precursors of Pt and a third metal and O2 plasma treatment achieved 3D multimetal complex nanostructures. Figure 4b,c shows the plane-view SEM images of “double-layered nanomeshes” with a Pt−Co top layer and a Pt−Pd bottom layer and with a Pt−Au top layer and a Pt−Co bottom layer, respectively. The chemical composition of the multimetal complex nanostructures was also confirmed using the XPS analysis (Figures 4d and S6). The fabricated trimetallic complex nanostructure were composed of Pt (52.8 at %)−Pd (33.8 at %)−Co (13.4 at %) and Pt (53.7 at %)−Co (26.9 at %)−Au (19.4 at %) (Table S1). To confirm the contamination possibility of the preceding lower layer during the subsequent metal-loading process, we prepared a Ptprecursor-loaded PS-b-P2VP BCP film. After its cross-linking and immersion into a Co precursor solution, its metal nanomesh was carefully analyzed using XPS. Note that its polymer part was sufficiently removed by O2 plasma before XPS analysis. As shown in Figure S7, we have not found the second metal element (Co) other than the first metal element (Pt) in the metal nanomesh within the detection limit. The result clearly demonstrates that the metal-loading processes we used enabled the multimetal complex nanomesh structures without any contaminations between each layer. It may be due to the weak interaction between metal anions and cross-linked P2VP blocks. These results elucidate that our scalable approach

successfully achieved 3D multimetal complex nanostructures while maintaining the different underlying bimetallic nanomeshes. Note that we could also fabricate the multimetal complex nanostructures based on nanolines on nanomesh with a Pt−Co−Pd alloy (see Figure S8). Finally, we recorded the electrochemical HER measurements using a three-electrode setup under an acidic solution condition to characterize the HER catalyst performance of the fabricated metallic nanomesh structures. Hydrogen has been attracting enormous research attention as a promising alternative energy carrier. To date, various catalyst materials have been exploited to maximize the energy conversion efficiency. In this work, a highly interconnected Pt nanomesh with a large surface area was used as the HER catalyst. As a reference for HER performance, a commercial Pt/C catalyst and a flat Pt film were also characterized. Figure 5 shows the typical polarization curves (I−V plot) and the Tafel plots of the three samples. Although the commercial Pt/C catalyst showed the highest HER performance, the improved HER performance of Pt nanomesh was also demonstrated (current density of Pt/C = 17.5 mA/[email protected] V, Ptnanomesh = 6.27 mA/[email protected] V, and Ptflat = 1.04 mA/[email protected] V). The Tafel slope of the Pt nanomesh was at a level similar to that of Pt/C catalyst [Tafel slope (T) of the three samples: T(Pt/C) = 36 mV/dec, T(Pt nanomesh) = 43 mV/dec, and T(flat Pt film) = 85 mV/dec]. These superior HER performances of Pt nanomesh could be attributed to the highly connected robust electrical pathway and the large active catalytic surface area of the nanomesh structure. 15730

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Figure 5. (a) Polarization curves of flat Pt film, Pt/C, and Pt nanomesh and (b) corresponding Tafel plots.

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4. CONCLUSIONS We have successfully fabricated hexagonally perforated metallic nanoscale mesh structures via a PL-forming BCP self-assembly. Such a 2D nanomesh structure formation principle could be further developed for 3D complex nanoporous structures by a reliable repetition of the multistep process. The peculiar characteristic of the P2VP self-assembled nanodomains to specifically attract various anionic metal precursors facilitates diverse multimetallic complex nanostructures with different alloy compositions at each different mesh layer. It was found that a highly specific Pt precursor is crucial for the structural stability of the metal nanostructures. Nonetheless, synergistic integration of other metallic elements to form versatile multimetallic complex 3D nanoscale morphologies can be potentially useful for advanced technologies, including electronics, photonics, and catalysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03319. Additional SEM images of flat Pt nanomeshes and unoptimized complex nanostructures, SEM images and corresponding diameters of diverse metal nanodots, XPS analysis of Pt nanomesh and multimetallic nanomeshes, and SEM images of multimetallic complex nanomesh/ cylinder structures (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.H.K.). *E-mail: [email protected] (S.-J.J.). *E-mail: [email protected] (S.O.K.). ORCID

Sang Ouk Kim: 0000-0003-1513-6042 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly (2015R1A3A2033061) and the Global Frontier Hybrid Interface Materials (GFHIM) of the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2013M3A6B1078874). 15731

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