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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.)

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 CO 2reduction 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.