블록공중합체와 금속 전구체 결합을 통한 새로운 금속 나노 구조체 제조

Abstract

Metal nanostructures for catalyst of electrochemical reactions such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), have been widely fabricated by various bottom-up processes, for instance, co-reduction synthesis, hydrothermal synthesis, electrodeposition, de-alloy methods and chemical vapour deposition (CVD). Especially, to achieve at high current densities with low cell voltage in electrochemical reactions, metal catalyst structures are directly fabricated on a metallic electrode by hydrothermal synthesis. However, since the dimension of the most metal structures are larger than hundred nanometers, those show high overpotential (>>300 mV) at high current density (500 mA cm-2). Also the mass activity is too low because the loading is few mgs/cm2. To solve these drawbacks, in this thesis, I prepared a new fabrication method of metal nanostructures for an electrochemical catalyst from poly(2-vinylpyridine)-block-poly(4-vinylpyridne) copolymer (P2VP-b-P4VP; P24VP) thin films. Block copolymers have been widely used to fabricate various nanostructures such as 2D metal nanopatterns, nanoporous structures, photonic and memory devices as well as cell adhesion, metal precursors are only interacting with P2VP (or P4VP) chains. On the other hand, because P24VP has both P2VP and P4VP chains, I could prepare 3D nanoporous metal structures with higher catalytic activity. In chapter 2, I studied the coordination power for metal precursors between P4VP and P2VP chains. To check the coordination power, I studied the phase behavior of P24VP containing gold nanoparticles. Although both blocks of P24VP exhibited attractive interaction to gold precursors, unusual phase behavior was observed depending on the amount of gold nanoparticles. As the amount of gold nanoparticles increased, the order-to-disorder transition temperature (TODT) of P24VP with gold nanoparticles decreased first, then increased, and finally decreased again. To explain this phenomenon, I prepared two block copolymers: polystyrene-block-P2VP copolymer (PS2VP) and polystyrene-block-P4VP copolymer (PS4VP) containing gold nanoparticles. With increasing the amount of gold particles, the TODT of PS2VP increased continuously, whereas that of PS4VP gradually decreased. For PS4VP containing gold nanoparticles, because P4VP chains can interact with gold nanoparticle surface, density fluctuations exist near the gold nanoparticle surfaces, which causes to decrease the TODT. On the other hand, although pyridine ring in P2VP could be associated with gold surface, P2VP chains become stretched due to steric hindrance arising from the ortho position of nitrogen in P2VP. The chain stretching increases the TODT. Thus, the decrease of TODT for P24VP originates from P4VP microdomains containing gold nanoparticles, while the increase of TODT is attributed to the P2VP microdomains containing gold nanoparticles. With increasing the amounts of gold nanoparticles, the contribution of P4VP microdomains containing gold nanoparticles on the TODT becomes dominant, causing to re-decrease the TODT. According to the result from the phase behaviour, P4VP exhibits stronger coordination power than P2VP to metal precursors. In chapter 3, I fabricated 3D nanoporous metal structures from P24VP thin film with vertically oriented lamellar nanodomains by coordinating corresponding metal precursors followed by reduction to metals. Although metal precursors are coordinated with both P2VP and P4VP blocks, the metal coordination power toward P4VP block is much greater than that toward P2VP block. Thus, most of the metal precursors are located in the P4VP block, while a few exist in the P2VP block. After the metal precursors were reduced to corresponding metals by reactive ion etching (RIE), metals located in P4VP regions became continuous main frames. However, metals in P2VP regions could not be continuous because of smaller amounts, resulting in nanoporous structures. Using this 3D nanoporous structures, I measured the electrocatalytic activity for hydrogen evolution reaction. 3D nanoporous platinum (Pt) showed enhanced catalytic activity compared with Pt flat film due to the large surface area. Moreover, I fabricated the 3D nanoporous bimetallic structures from the P24VP thin film to improve catalytic activity with previous 3D nanoporous Pt structures. As controlling the composition of metal precursor solution, I fabricated 3D nanoporous bimetallic platinum-cobalt (Pt-Co; Pt3Co) structure. I confirmed that the final bimetallic structures are consisted with two metal component by high resolution transmission electron microscopy (HRTEM). 3D nanoporous bimetallic Pt3Co structure exhibits better catalytic activity than the 3D nanoporous Pt structure and commercial Pt/C catalyst for HER at high current density, and oxygen reduction reaction (ORR) at high voltage.