## 3 Challenging PEC Device Designs toward Practical Solar Water Splitting
### Application of Facet Engineering in PEC Water Splitting  
Many theoretical predictions based on computational methods shows that intrinsic anisotropic material properties such as electronic and optical properties in semiconductors should be understood for better explanation on the enhancement mechanism and realization of practical PEC devices. For instance, the intrinsic carrier transport properties and optical properties, depending on the  
Figure 4: Schematic representation of surface area-controlled BiVO\({}_{4}\) film deposited by pulsed laser deposition. Microstructural changes and charge collection abilities of BiVO\({}_{4}\) film deposited at (**a**,**d**) 230 \({}^{\circ}\)C, (**b**,**e**) 330 \({}^{\circ}\)C, (**c**,**f**) 430 \({}^{\circ}\)C. (**g**) PEC performances, (**h**) UV-vis transmission spectra, and (**i**) incident photon-to-current efficiency (IPCE) of the samples. Reprinted with permission from [10]. Copyright 2017, American Chemical Society. crystallographic orientations of monoclinic-scheelite BiVO\({}_{4}\), have been calculated using density functional theory (DFT) [53; 54]. The calculation results show that the mobility of charge carriers is different depending on the BiVO\({}_{4}\) crystal axis [54]. In detail, the mobility of electrons along [010] is 5 times higher than that of the [110] direction where a = 5.195 A, b = 11.701 A, c = 5.094 A. In the same manner, however, the mobility of holes along [110] is two times higher than that of the [010] direction. Furthermore, it have been also known that the optical properties such as refractive indices and light absorption are different, so that preferentially oriented crystals can have spectral selectivity compared with the corresponding bulk crystal [53]. Therefore, it is implied that the devices with preferentially oriented crystal facets are needed since optimization between the carrier transport properties and light absorption of photoelectrodes can be achieved using such systems. There are some experiments on these considerations. Mostly, investigations have been performed for various semiconductors including BiVO\({}_{4}\)[18; 20], TiO\({}_{2}\)[55], and Fe\({}_{2}\)O\({}_{3}\)[28] in the form of powder. In order to investigate the facet-dependent PEC activity, those investigations showed photoreduction of Au or Ag compound on certain facet of the semiconductors. As a result, distinctive morphological changes, which were owing to the reduction of Au or Ag compound by photoexcited electrons at certain facets, could be observed by SEM images of each sample. However in those experimental circumstances, further understandings on PEC processes such as the carrier separation efficiency and the water redox kinetics, are limited. So far, a few research groups have reported on the fabrication of the preferentially oriented photoelectrode thin films, and have demonstrated the facet engineering that can show the relation between the facets of the semiconductor and the PEC water splitting activity. [PERSON] et al. deposited [001] preferentially oriented polycrystalline BiVO\({}_{4}\) (p-BVO) thin films (where a = 5.1935 A, b = 5.0898 A, c = 11.6972 A) on F-doped SnO\({}_{2}\) (FTO) glass substrates by PLD [15]. For comparison, they also synthesized randomly oriented BiVO\({}_{4}\) (r-BVO) by a sol-gel method. The synthesized films showed that about 41% of the BiVO\({}_{4}\) grains were [001] oriented in the p-BVO while this was only \(\sim\)3% for the r-BVO. Thus, a schematic image for the p-BVO and the r-BVO could be represented, as shown in Figure 5a. The PEC performance of both samples clearly showed that the photocurrent density of the p-BVO reached \(\sim\)3.9 mA/cm\({}^{2}\) at 1.23 V\({}_{\text{FHE}}\) which was about 16 times higher than that of the r-BVO (Figure 5b-d). A higher IPCE of the p-BVO (Figure 5d) could be attributed to the efficient charge separation (Figure 5f) and transport (Figure 5g) of the p-BVO in comparison with the r-BVO, since almost the same light absorption spectra were observed (Figure 5e).