## 2 Fabrication Strategy for Functional Materials with 3D Periodic Nanostructures
### Infiltration of Functional Inorganics into 3D Nanostructured Polymer Templates via Electroplating
created 3D meso-lattices made of Cu and Ni through electroplating using porous templates formed by DLW (Figure 3b) [81]. An indium tin oxide (ITO)-deposited glass cover slip was used as a conductive substrate for electroplating. A three-electrode electroplating system with a Pt counter electrode and an Ag/AgCl reference electrode was used to ensure the stability of the electroplating. To electroplate Cu and Ni into the templates, a Cu bath containing 125 g/L CuSO4.5H2O and 50 g/L H2 SO4 and a Ni bath containing 240 g/L NiSO4.5H2O and 45 g/L NiCl2.6H2O and 40 g/L H3 BO3.2H2O were used, respectively. The applied potential to the electrochemical cells was 2 V. After electroplating, the polymer template made of positive-tone photoresist (AZ4620, Microchem) was gently removed through a solution process with N-methylpyrrolidone (NMP), yielding metallic porous meso-lattices. [PERSON] et al. reported the successful fabrication of 3D mesoporous Ni-Sn through the combination of MBIL and electroplating (Figure 3c) [53]. To achieve this, a mesoporous Ni scaffold was first prepared by 3D nanostructured template-assisted electroplating and subsequent template removal. Ni-Sn was then conformally grown on the mesoporous Ni scaffold through pulsed electroplating (0.2 s on and 10 s off) at -0.22 V by using an electrolyte composed of 0.06 M NiCl2, 0.2 M SnCl2, 1 M K4P2O7, 0.04 M potassium sodium tartrate, and 0.04 M glycine. The mesoporous MnO2 also can be produced in a similar way by using an electrolyte that contains 0.1 M Na2 SO4, 0.1 M CH3 COONa, and 0.1 M MnSO4. [PERSON] et al. recently reported a large-scale 3D nanostructured Ni produced by template-assisted electroplating (Figure 3d) [82]. An inch-sized polymer template with 3D periodic nanostructure was first prepared by PnP. Then, Ni was bottom-up grown from the seed layer, which was pre-deposited on the substrate, through pulsed electroplating (5 s on and 5 s off) at 2 mA/cm2 to completely fill the 3D nanostructured template with Ni. The template was carefully removed by an ashier to yield porous Ni. In a similar way, [PERSON] et al. reported a 3D nanostructured Au with hierarchically arranged pores (Figure 3e) [83]. To achieve hierarchical porous structures, Au-Ag alloy was grown in the 3D nanostructured template by pulsed electrodeposition (5 s on and 5 s off) at an applied potential of -1.1 V. After template removal and selective etching of Ag through a dealloying procedure, a large-scale mesopore-in-macroproe Au structure was produced. This approach allows not only metals and alloys but also semiconductors to be 3D nanostructured materials. [PERSON] et al. produced a 3D nanostructured titania (TiO2) by combining MBIL and electroplating (Figure 3f) [84]. To electroplate TiO2 on the template with a periodic 3D nanostructure defined by MBIL, electroplating was performed using a three-electrode system with a H2 SO4-based aqueous solution of crystalline Ti powder with high purity (5N). The applied potential was controlled from 1.0 to 1.3 V and the constant current was 10 mA. Through optimization of the calcination process, the generation of thermal-induced defects in the 3D nanostructure during the template removal was significantly suppressed. In a similar way, [PERSON] et al. and [PERSON] et al. successfully developed 3D nanostructured Cu2O with face-centered cubic (FCC)-like and woodpile symmetries (Figure 3g) [22; 23]. The polymer templates were patterned on ITO substrates via MBIL. A three-electrode electroplating system with an aqueous electrolyte containing 0.2 M cupric sulfate and 1.6 M lactic acid was used to deposit Cu2O on the templates. The pH of the solution was adjusted to \(\sim\)9 by adding sodium hydroxide, and the temperature of the solution was kept constant at 65 \({}^{\circ}\)C during electroplating. To ensure complete filling of Cu2O in templates, a potentiostatic condition at \(-\)0.55 V was used. Through the template removal by reactive ion etching (RIE) under O2 and CF4 flows, highly symmetric inverse Cu2O structures were obtained.