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 charged particles onto the substrate) [15–17]. Recent developments also see the adoption of combinatory methods integrating electrochemical methods with other chemical or physical ways in preparing complex material thin film with binary, ternary, and multicomponent structures [18]. Sections below will introduce the working principle of anodization, cathodic electrodeposition (sometimes it is called electroplating), electrophoretic deposition, and some examples of combinatory methods. The resultant catalytic films with properties unique to the method will be discussed.
Figure 3.1 Schematic drawing of a general electrochemical setup with basic components used for electrochemical synthesis of nanostructured thin films. In this specific configuration, the cathode (on which the reduction reaction takes place) is the working electrode, with potential applied with respect to the reference electrode. The anode (on which the oxidation reaction takes place) in this case is the counter electrode. (See online version for color figure).
3.2.1 Anodization
Anodization is usually referred to the electrochemical oxidation of metallic thin films. It is a useful tool for the creation of catalytic oxide films with the focus in recent years on array nanostructures of nanotubes and nanorods. When a metallic foil is used as an anode in an electrolyte containing oxide etching agent, nanostructured metal oxide is systematically formed when anodic voltage is applied between the metallic anode and the cathode (as counter electrode). During anodization, the positive voltage applied from the potentiostat drives the electrons away from the metallic foil results in the oxidation of metallic foil (Eq. 3.1). The formation of this anodic oxide layer of metal, however, faces the competition of chemical/electric‐assisted dissolution of the formed oxide layer. This dissolution of metal oxide is promoted by the presence of oxide etching agent in the electrolyte (Eq. 3.2), in which fluoride ions are the typical etching dissolution agent:
The growth of the anisotropic nanostructures of metal oxide is driven by the competition between anodic oxidation and chemical/electric field‐assisted dissolution [19]. As an example, Figure 3.2 shows the growth mechanism of titanium dioxide (TiO2) nanotube arrays during anodization. When anodization of titanium (Ti) foil starts, dense and thin TiO2 layer are quickly formed (Figure 3.2a). Subsequently, this thin and dense oxide layer will undergo localized dissolution (induced by the oppositely charged fluoride anions in the electrolyte and the weakened outer layer of surface metal–oxygen bond) to initialize the small pores formation (Figure 3.2b). The continuous thinning of barrier oxide layer through the localized dissolution leads to an increasing electric field intensity across the barrier oxide layer to deepen the pores further (Figure
