Heterogeneous Catalysts. Группа авторов

3 Electrochemical Synthesis of Nanostructured Catalytic Thin Films

       Hoi Ying Chung and Yun Hau Ng

       City University of Hong Kong, School of Energy and Environment, Kowloon, Hong Kong, Special Administrative Region (S.A.R.)

3.1 Introduction

      Catalytically active materials in the physical form of thin film (in the range of nanometer to micrometer) have found wide applications in reactions involving thermal catalysis, electrocatalysis, and photocatalysis [1]. Although depending on the targeted applications, usage of catalytic thin films offers a few advantages from the operational viewpoint over the powder or homogeneous catalyst counterpart. For instance, the elimination of catalyst separation process upon completion of reactions is helpful in simplifying processes. Improved robustness against sintering at elevated operating temperature is another crucial benefits offered by thin films to prolong the stability of catalyst because the heat‐induced sintering always results in the loss of activity. Furthermore, catalytic reactions involving electrical circuit such as electrochemical hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and other electrocatalytic reactions must at least require the active materials to be immobilized on the electrodes. Thin film is one of the most common forms of active electrodes – see, for example, their detailed applications in electrochemical water splitting, polymer electrolyte membrane fuel cells, and photo/electrochemical CO₂ reduction in Chapters 30, 32, and 36, respectively.

These catalytic thin films are prepared either by direct growth of catalytic materials on thin substrates (e.g. glass or metal sheets) or they can be pre‐synthesized as powder materials followed by an immobilization process on the thin films [2, 3]. Traditionally, flat thin films can be prepared using thermal/chemical/physical/vapor deposition, sputtering, spin/ dip/doctor‐blade coating, electroplating, etc. [4–10] Principles used in guiding the formation of thin films are vastly different. For example, in vapor deposition methods, usually low pressures and high temperatures are needed for generating the vapor of precursors. In particle coating techniques, particle size, binder, and viscosity modifier play important roles in ensuring good adhesion with the substrates. Control over the uniformity of thickness, composition of materials, and strength of adhesion are the typical aspects considered in the flat thin film synthesis. Evolved from these flat thin films, catalytic thin films with nanostructure (whether with or without ordered nanostructure; with or without regular pattern) are emerging as a new class of functional materials. Nanostructures of catalytic components on thin films can be generally grouped into 1D (e.g. nanotubes, nanorods), 2D (e.g. nanosheets), and 3D (hierarchical nanostructures, e.g. tetrapods, nanoflowers, sea urchin‐like structures) configurations, with unique properties found in each nanostructure. Modulation of these anisotropic nanostructures, where the shapes of the nanostructures are formed as a result of preferential growth (or leaching) in certain directions, on thin films is another domain that needs to be addressed with precise control. As the conventional flat thin films are typically made of bulk materials (metal, metal oxides, metal‐based semiconductors, polymeric structures, etc.), the introduction of nanostructures offers additional properties (e.g. physical, electronic, and optical) [11–13] to the thin film, in which new applications are found.

      Among all methods capable of fabricating traditional flat catalytic thin film, a handful of existing methods can be adopted with modification to find usefulness in the preparation of nanostructured thin films. In this chapter, electrochemical method, which has been used in both the fabrication of flat single‐component thin films and the emerging complicated nanostructured multicomponent thin films, will be discussed in detail. Upon reading this chapter, the readers will understand the core principles shared within all electrochemical synthetic methods and at the same time keep up with the latest progress in the evolution of these techniques in meeting the renewed requirements in designing functional nanostructured catalytic thin films.

3.2 Principle of Electrochemical Method in Fabricating Thin Film

      Electrochemical processes have been extensively used for preparing thin films with their unique advantages in scalable production and ability to form films with precise control of thickness and its homogeneity [14]. Based on the principle of electrochemical processes, thin films made of metal, simple metal oxides, or polymerized organic film can be formed using anodization, cathodic electrolytic deposition, electrophoretic deposition, electro‐oxidative polymerization, and combinatory methods. All these mentioned methods are operating based on the manipulation of electrons induced by a simple power supply or potentiostat with various functionalities.

In principle, electrochemical method is a versatile technique because it involves only cost‐effective and basic apparatus setup as shown in Figure 3.1. A simple electrochemical setup consists of two electrodes (i.e. anode and cathode, with an optional separate reference electrode), a power supply or potentiostat, and a single or separated electrolyte medium. In a typical electrochemical process, driven by the applying voltage, electrons flow from anode to cathode through an external circuit. Oxidation–reduction (redox) reactions occur when electrons are withdrawn from anodic site (oxidation happens at anode) to reach the cathodic site to reduce substances (reduction happens at cathode). The potential can be manipulated with respect to the reference electrode in the case of a three‐electrode configuration (as shown in Figure 3.1) or simply across the anode and cathode in a two‐electrode configuration to control the extent or vigor of such redox reactions. As it is essential to have both reduction and oxidation to occur simultaneously to complete the electron‐flow circle, a redox reaction can also be called as two half‐reactions, i.e. one representing the oxidation process and the other reduction reaction, respectively. Basically, the foundation of electrochemical fabrication of thin films lies in the manipulation of such redox reactions at either anode or cathode of electrochemical setup. The techniques can be grouped into anodization (when the substrate of interest undergoes oxidation reaction), cathodic electrodeposition (when the substrate of interest undergoes reduction reaction), and electrophoretic deposition (attractions of oppositely