Heterogeneous Catalysts. Группа авторов
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 3.2c). The barrier oxide layer serves as the resistance to the flow of reactive ions that are needed to be transported through the anodic oxide layer to sustain the oxidation. Formation of anodic oxide layer ceases when the resistance is increased too high (accompanied by thick barrier oxide layer). However, this issue can be overcome by increasing the applied potentials for a higher electric field or by using a higher concentration of fluoride ions in the electrolyte to allow the creation of internal channels. These channels facilitate the uneven internal resistance to maintain the ongoing oxidation process. The thickness of the barrier oxide layers underneath the pores is kept in an equilibrated state between the two competitive reactions, i.e. the thickness is reduced by dissolution but simultaneously regenerated by oxidation (Figure 3.2d). The electric field distribution at the pores boundaries causes anisotropic widening and deepening of pores. As the pores formed at deeper region, the electric field closer to the underlayer metallic regions increases, thus promoting the anodic oxide growth. Afterward, the formation of anisotropic nanostructures of oxide continues to grow in equilibrium. The growth of such nanostructure will continue until the oxidation rate at the metal–oxide interface equals the chemical dissolution rate at the oxide/electrolyte interface.
Figure 3.2 Schematic diagram of the growth mechanism for anodized metal foil: (a) growth of thin and compact oxide layer, (b) the initial formation of pores, (c) formation of patterned pores within oxide layer, (d, e) continue growth of the pores to form nanostructures.
As the schematic formation of nanostructure during anodization is explained in Figure 3.2, the presence of those processes can be observed in the measurement of current transient pattern during anodization. Figure 3.3 shows a typical transient current profile measured during the reaction under constant applied anodization voltage. The transient current consists of three main stages. In the first stage, the current decays rapidly during the first few minutes of the anodization. At the start of the reaction, electrolyte is in direct contact with the conductive metallic surface, and thus the starting current level is usually high. Under the applied anodization, the oxidation of metallic foil occurs almost instantaneously to form a compact thin oxide surface layer. As the initial oxide layer increases in thickness, so does the electrical resistance of the substrate. As a result, the oxidation of metallic foil slows down, which is reflected by the rapid drop of current density. Sequentially, chemical dissolution of oxide layer by fluoride ion in the electrolyte starts to take place and render more underlayer metal to oxidation; a temporary bounce‐back in current is therefore observed in the second stage. The third stage is the equilibrium stage in which the formation and dissolution of oxide happen at the same competitive rates, reflected by the steady‐state current generation. As the growth of anodic nanostructure is regulated by the formation and dissolution of oxides, the readers can logically expect the important synthetic conditions to be related to the magnitude of applied voltage (strength of oxidation process) and the type or concentration of etching agent (e.g. fluoride ions, which directly influence the strength of dissolution process). Indeed, most of the morphological controls over the anodic oxide thin films are a result of optimal modulation of these two parameters (voltage and electrolyte composition).
Figure 3.3 Typical current profile under a constant applied anodization voltage