## 2 Strategies and Device Designs for Efficient PEC Water Splitting  
### Absorption of Solar Radiation  
After the first report on photocatalytic water splitting by [PERSON] and [PERSON] [4], TiO\({}_{2}\) had been focused upon as pioneer photocatalyst at the early-stage of development because of its abundance and its good resistance to various chemicals. In the same manner, considering that PEC water splitting is generally conducted using the aqueous solutions containing organic salts, various oxide semiconductors have been used as photoelectrodes, owing to its excellent chemical resistance. However, the large band gap energy of most oxide semiconductors limits the absorption of light to the near-UV region. Thus, the poor photocatalytic performances have been considered as a significant problem. In order to solve the problem, many researchers have endeavored to discover small band gap oxide semiconductors. As a result of such efforts, a few candidates have been introduced and investigated extensively. The promising candidates are BiVO\({}_{4}\)[5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24], Fe\({}_{2}\)O\({}_{3}\)[25; 26; 27; 28; 29; 30; 31; 32], and BiFeO\({}_{3}\)[33; 34; 35]. Figure 2 shows the band gap energies and band edge positions of various semiconductors with respect to the water redox potential. After introduction of those small band gap oxides, the increased light absorption and photocatalytic efficiency were enabled by the utilization of visible light. Another solution for overcoming the limitation of the light absorption is the application of localized surface plasmon resonance (LSPR) in metal nanoparticles (NPs) [36; 37]. LSPR represents the oscillation of electrons in the metal NPs in response to the incident light, or electromagnetic waves. The physical phenomena for the LSPR-induced enhancement effects include: (1) the formation of the localized electric field and (2) the enhanced light absorption by the oscillation of the surface plasmon in the metal, which made is response to the incident light, and thereby, (3) the enhanced charge carrier generation [16]. Many researchers have reported the application of metal NPs into their PEC devices, since a peak resonant wavelength can be controlled in the visible region through adjusting metal elements, size, and shape [9; 16; 17; 23; 36; 37; 38; 39; 40]. This strategy is so called \"plasmonic photocatalysis\". In the field of plasmonic photocatalysis, Au [9; 17; 20; 23; 32; 41; 42] and Ag [16; 43] are the most frequently used metal species. Au and Ag NPs have been introduced to oxide semiconductors such as BiVO\({}_{4}\)[9; 16; 17; 20; 23], TiO\({}_{2}\)[41; 42; 43], and Fe\({}_{2}\)O\({}_{3}\)[32]. In addition to the monometallic NPs, the multi-metallic NPs of PtAuCu alloys have been recently discovered as an effective hydrogen evolution catalyst [44]. Therefore, it is reasonable to expect that the investigation of multi-metallic NPs on the plasmonic photocatalysis will be performed in the near future. Figure 2: Band edge positions of various semiconductors with respect to the water redox potentials. Reprinted with permission from [3]. Copyright 2016, American Chemical Society. Recently, [PERSON] et al. reported the BiVO\({}_{4}\) photoanode with shape-controlled Au NPs [23]. The shape-dependent LSPR effect was observed in the device, as shown in Figure 3a,b. The hemispherical Au NPs were deposited by an e-beam evaporator directly on the BiVO\({}_{4}\) film, whereas the octahedral Au NPs were synthesized using seed-mediated growth method followed by attachment on the BiVO\({}_{4}\). The authors performed a finite-different time-domain (FDTD) simulation to represent the spatial distribution of the LSPR-induced electric fields (LSPR-EFs) near the Au NPs (Figure 3c-h). The results showed that the octahedral Au NP shows higher LSPR-EFs, especially at sharp edges of the Au NP, than that of the hemispherical one.