Heterogeneous Catalysts. Группа авторов
mean that large particles could not be deposited. Colloidal stability of large particle can be improved by having a very strong surface charge, and the electrophoresis movement can be increased with larger electrical double layer region.
Selection of liquid medium is another factor governing the quality of electrophoretic deposition. Dielectric constant of the liquid medium is highly correlated with the deposition performance. Deposition fails in the liquid medium with too low dielectric constant because of weak dissociative power, while electrophoretic mobility is significantly reduced when a medium with high dielectric constant is used due to the reduced size of double layer region. Consequently, liquids of low dielectric constant (but not too low) are a favorable condition. As mentioned earlier, stabilization of suspension, i.e. colloidal stability, is critical in facilitating smooth deposition, and this stability can be achieved by having high and uniform surface charge on the particle. The ζ potential of particle becomes one of the key factors in the electrophoretic deposition. The magnitude of the ζ potential determines the stability of the suspension (repulsive interaction between particles), while the polarity of the ζ potential (positively or negatively charge) decides the direction and migration velocity of the particles during electrophoretic process. The probability of coagulation of the suspended particle must be minimized by modulating the two forces with opposite effects: electrostatic and van der Waals forces. High electrostatic repulsion attributed to the high ζ potential (high particle charge) is desired to suppress agglomeration. Conveniently, the ζ potential can be tuned by carefully selecting pH and additives to the suspension.
3.2.4 Combinatory Methods Involving Electrochemical Process
Although the electrochemical principles discussed in Section 3.2 are sufficient to guide a variety of thin film synthesis, recently there are more examples adopting a combinatory approach to prepare new functional thin films. The combinatory approaches can integrate two electrochemical methods or combine an electrochemical method with other technique to synthesize thin film with unique properties. As discussed in the earlier section, anodization is an effective method in producing nanostructured thin films made of simple metal oxide. A wide variety of simple metal oxides such as Al2O3, TiO2, WO3, SiO2, MoO3, and NiO fabricated by anodization process has been reported. Relatively rare is the fabrication of ternary or complex oxide using anodization. Composites of metal oxide decorated with other functional materials are also generally not being reported using anodization process alone. In this section, the design of metal oxide‐based composite and the preparation of ternary oxide thin film using a combinatory electrochemical method will be discussed.
3.2.4.1 Combined Electrophoretic Deposition–Anodization (CEPDA) Approach
Combined electrophoretic deposition–anodization (CEPDA) is a combined electrochemical process that developed to allow the simultaneous anodic growth of metal oxide and the electrophoretic‐driven deposition of reduced graphene oxide (RGO) on the resulting anodic oxide [18]. Metallic foil is employed as the anode in a typical anodization setup as shown in Figure 3.7. Formation of TiO2 nanotube (TNT) arrays following the typical anodization route is expected. However, the organic electrolyte of this anodizing setup is suspended with particles/flakes of RGO with negative surface charge. Within the anodization duration, electrophoretic forces drive the negatively charged RGO to the TiO2 anode to form TiO2–RGO composite that find uses in photocatalytic reactions. The RGO deposited on TNTs using CEPDA method has a stronger physical interaction between host and guest compared with the typical coating techniques such as spin, drop, and dip coating. This is a demonstration of preparing composite materials by combining two electrochemical working principles in a single experimental process. Otherwise, this kind of composite thin film can be prepared by multistep process. In this combinatory method, both the growth of oxide and its integration with secondary component can take place simultaneously.
Figure 3.7 Schematic illustration of the apparatus for combined electrophoretic deposition–anodization (CEPDA) approach in the preparation of TiO2 nanotube (TNT)–reduced graphene oxide (RGO) composite.
Source: Yun et al. 2012 [18]. Reproduced with permission of Royal Society of Chemistry.
(See online version for color figure).
Although anodization is, in general, limited to the synthesis of simple oxide thin film, by combining the as‐anodized thin film (before rigid crystallization) with hydrothermal/solvothermal treatment, transformation of the simple metal oxide into complex ternary oxide is possible [18, 33, 34]. Depending on the metallic substrate, bare anodization process usually yields the metal oxide with amorphous phase or partially crystalline structure. Subsequent annealing at elevated temperature is needed to transform the as‐anodized films into well‐crystallized metal oxide that find applications in various reactions. This intermediate state of as‐anodized film offers the possibility to further engineering their composition for final product. For example, upon anodization of tungsten foil, the formed layer is hydrated WO3 (WO3·2H2O) with loosely packed structure. Subjecting this as‐anodized WO3·2H2O film into a hydrothermal treatment containing bismuth species (Bi2O2)2+ forces the interstitial water molecules to be replaced by (Bi2O2)2+. Crystallization process takes place during the hydrothermal treatment, and bismuth tungstate (Bi2WO6) thin film is obtained. With proper control, the rate and the depth for the insertion of Bi cations into host material (example is WO3) can be tuned. As a result, catalytic thin film with two Bi as dopant is made. More interestingly, the concentration of dopant (not limited to Bi) indicates gradient across interfaces. This gradient concentration has important and constructive impact on charge transfer. The combination of anodization with hydrothermal treatment has been proven applicable to prepare different types of ternary oxide films. However, detailed procedures should be strictly considered in this combinatory method. The successful transformation of simple metal oxide into ternary or complex metal oxide is based on the relatively amorphous nature of the as‐anodized metal oxide film. Such transformation is not likely when the highly rigid or crystalline anodized films are employed due to the limited rearrangement of the lattice structure.
3.3 Conclusions and Perspective
Electrochemical route is probably one of the most cost‐effective synthesis methods of functional and nanostructured thin films because it does not require expensive or complicated apparatus. It also offers great versatility in fabricating thin film with various desired properties by simply modulating a few operating parameters. All electrochemical methods discussed in this chapter are based on the same principle of electrochemistry that the deposition is a result of redox reactions and the migration behavior of charged elements given an applied voltage or current (i.e. anodization, cathodic electrodeposition, and electrophoretic deposition). Changing the ways of voltage/current being applied to the system (e.g. through the introduction of pulse) can also have great influence in the quality of the deposition process.
Current status of electrochemical thin film synthesis has seen the domination of catalytic materials with relatively simple composition. In many cases, metal oxides in the form of binary oxides are produced. Modulation of their morphology using electrochemical methods (as described in this chapter) and the subsequent investigation on the impact of such morphological control offer new opportunity in improving the overall reaction kinetics. Recent trend in nanostructuring the catalytic materials in thin film