전기화학촉매 기반 수소 생성용 저차원 MoS2의 합성 및 분석

Abstract

Hydrogen (H2) is an excellent source of clean energy and is perceived as a suitable alternative for hydrocarbon-based fuel. Among the other methods, the H2 production via an electrochemical water-splitting is considered more economical owing to the abundance of water. In electrochemical water splitting, the hydrogen evolution reaction (HER) occurring at the cathode surface can be facilitated by the introducing a catalyst on the electrode surface. The best catalysts known, so far, for the efficient HER, are the noble metal (Pt-based) groups, which are not viable economically. Therefore, developing cost effective materials with comparable or even higher HER performance is highly desirable. Generally, an ideal electrocatalyst should have high density of active sites that simultaneously exhibits high intrinsic catalytic activity. Besides, the additional prerequisites are the high stability and low-cost. In the recent years, transition metals dichalcogenides and oxides (MX2), such as layered MoS2, and/or TiO2 have been widely investigated for HER application, owing to its structural stability, high electrocatalytic activity, and earth abundance. However, the electrocatalytic HER efficiency of these pristine materials is still limited by low active sites and electrochemical conductivity. In this thesis, we aim to synthesize and investigate on MoS2 and TiO2 based materials that can substantially enhance the electrochemical catalytic activities for practical HER application. To obtain this objective, one of the strategies was to scale down bulk MoS2 to atomic thickness scale, namely two-dimensional (2D-MoS2). By scaling the MoS2 down to monolayer (ML-MoS2), the HER performance can be enhanced owning to the minimum charge-transfer resistant between the outermost exposed surface and the electrode. Here the substantial requirement arising firstly is how to achieve the ML-MoS2 in large scale, which is essential in terms of cost-effectiveness. In chapter 3, a method so-called metal-organic chemical vapor deposition (MOCVD) using organic gaseous precursors has been adopted to synthesize large scale of a continuous ML-MoS2 film. In order to enhance the HER performance, the active sites and the exposed surface area of ML-MoS2 film should be essentially maximized. As discussed in the chapter 4, we have employed the N2-plasma to induce more active sites, including the edges sites that are generated at the cracked grain boundary, the S-vacancies that are formed at the basal plane, and N-doped sites. An optimized treatment condition was obtained to attain the highest HER efficiency, with the maximum active sites. The hybridization ML-MoS2 with a high-surface-area material have emerged as an appropriate alternative that tremendously enlarges the exposure surface area of the MoS2 film. Among proper materials, TiO2 has been attracted a great attention owning to its strong chemical stability, nontoxicity, and earth-abundance. In chapter 3, the growth process of the high-surface-area material i.e. TiO2 nanorod array (TNRs) used as support for ML-MoS2, is explained in detail. Interestingly, the as-grown shell−core heterostructure (ML-MoS2@TNRs) shows a significant enhancement in the HER activity with an onset overpotential at -140 mV vs reversible hydrogen electrode and a Tafel slope of ∼80 mV/dec. Based on our experimental results together with first-principle calculations, we attribute the enhanced HER performance of ML-MoS2@TNRs to the synergetic effect of the following characteristics. (i) A large number of active sites owning to high surface to volume ration of TNRs. (ii) A considerable reduction in the charge transfer resistance caused by the direct growth of ML-MoS2 over the TNR array, naturally rendering low electrical loss contacts compared to the conventional transfer process. Moreover, the direction of the built-in electric field in the MoS2/TiO2 heterostructure also facilitates the flow of electrons from the electrode to the electrocatalyst surface, consequently decreasing the charge transfer resistance. (iii) The high intrinsic HER activity of the active sites owing to the low Gibbs free energy of the catalytic surface (ML-MoS2@TNRs). Moreover, by virtue of the high crystalline quality of ML-MoS2, the ML-MoS2@TNRs sample shows excellent stability and working durability. In this thesis, the morphologies, atomic structures, optical properties, and chemical compositional analysis of samples were carefully characterized by: FESEM, HRTEM, PL spectrum, Raman spectrum, XPS, UPS techniques. The electrocatalytic HER activities of samples were examined by the linear sweep voltammetry (LSV) and the corresponding Tafel plot to obtain the onset overpotential and Tafel slope, the cyclic voltammetry (CV) in non-faradic potential range to evaluate the electrochemical active surface area (ECSA), the impedance spectroscopy (EIS) to calculate the electrochemical charge-transfer resistance, turn over frequency (TOF) to estimate the intrinsic catalytic activity per active site.