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 configuration will continue. Electrochemical synthesis of nanostructured catalytic thin film can also offer the opportunities in exploring new catalytic materials beyond the currently dominant simple oxide (or other simple composition). Introduction of secondary component in the making of composite materials and the transformation of simple oxide into ternary/quaternary counterparts may become an apparent alternative in the near future to prepare complex functional catalytic thin films. The matrix of composition afforded by electrochemical synthesis of thin films might also be helpful in discovering new catalytic reactions.
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