# Atomic Layer Deposition of Inorganic Thin Films on 3D Polymer Nanonetworks
## 4 Application of 3D Inorganic Nanonetworks
### Highly Porous Hollow Inorganics  
If the original polymer template is removed through calcination or an O\({}_{2}\) plasma etching process, a highly porous inorganic material with a hollow 3D nanonetwork structure can be obtained. The hollow structure is very suitable for applications such as photocatalysts, which require a high surface area owing to surface reaction. Recently, a series of studies have reported on the fabrication of a periodic 3D nanonetwork monolith made of pure TiO\({}_{2}\) or N-doped TiO\({}_{2}\) through the combination of PnP and ALD, which is applied as a reusable photocatalytic film [21; 22]. In the case of pure TiO\({}_{2}\), amorphous TiO\({}_{2}\) was deposited on the polymer template through thermal ALD at room temperature, which subsequently changed to anatase-phase TiO\({}_{2}\) with excellent photocatalytic efficiency in the calcination process to remove the template. For N-doped TiO\({}_{2}\) with enhanced visible light activity, an ultrathin (\(\sim\)10 nm) TiN layer was additionally deposited on the 3D hollow TiO\({}_{2}\) nanonetwork through ALD at 350 \({}^{\circ}\)C. The TiN layer is then converted to an N-doped TiO\({}_{2}\) layer by subsequent annealing at 550 \({}^{\circ}\)C in an atmospheric environment (Figure 7a-c). These monolithic films can be reused at least five times while maintaining good photocatalytic performance comparable to commercial TiO\({}_{2}\) nanoparticles (P25). Figure 7: Application of highly porous inorganic materials with a hollow 3D nanonetwork structure: (**a**) digital image of a large-area (625 mm\({}^{2}\)), 3D nanonetwork-structured, N-doped TiO\({}_{2}\) film produced by PnP and ALD; (**b**) cross-sectional SEM image of (**a**); (**c**) photocatalytic degradation of methylene blue solution under solar irradiation (reprinted with permission from [22]. Copyright 2018 The Royal Society of Chemistry); (**d**) optical microscope image of a 3D nanonetwork-structured ZnO film transferred on a microfabricated thermoelectric measurement platform (MTMP); (**e**) cross-sectional SEM image of (**d**); (**f**) digital image of a thermoelectric device prototype based on 3D nanonetwork-structured ZnO stripes (reprinted with permission from [27]. Copyright 2018 The Royal Society of Chemistry); (**g**) SEM image of an abnormally resilient 3D nanonetwork-structured TiN film produced by DLW and ALD (reprinted with permission from [15]. Copyright 2013 Nature Materials); (**h**) cross-sectional SEM image of a 3D nanonetwork-structured Al\({}_{2}\)O\({}_{3}\) film with a thickness of 6 nm; (**i**) digital image of a highly transparent, 3D nanonetwork-structured Al\({}_{2}\)O\({}_{3}\) film coated on a glass substrate (reprinted with permission from [29]. Copyright 2015 John Wiley and Sons). One of the recently emerged applications of hollow inorganic nanonetworks is nanostructured thermoelectric materials [93; 94; 95; 96; 97]. A thermoelectric material is defined as a material with the ability to create a current or electric potential generated by a temperature difference between the two ends of the material, called the Seebeck effect. Thermoelectric devices have recently been reilluminated as a wearable energy-harvesting system because they can generate a weak current from the temperature difference between humans and their surroundings [98; 99; 100]. A representative approach for increasing the dimensionless figure-of-merit (ZT), which represents the performance of a thermoelectric material, is to reduce the thermal conductivity selectively while maintaining the Seebeck coefficient and electrical conductivity. This can be achieved by structuring the thermoelectric material into a hollow 3D nanonetwork. [PERSON] et al. reported a nanostructured ZnO monolith with a low thermal conductivity (~3.6 W m\({}^{-1}\) K\({}^{-1}\) at 333 K), which is approximately 1/10 of that of the bulk ZnO through periodic 3D hollow nanostructuring of ZnO (Figure 7d-f) [27].