## 3 Challenging PEC Device Designs toward Practical Solar Water Splitting
### Application of Facet Engineering in PEC Water Splitting
The results obviously indicated that the increased carrier transfer and the transport efficiency contributed to the improved PEC performance of the p-BVO. The electrochemical impedance spectroscopy (EIS) measurement supported the results by showing the smaller bulk and interface resistances of the p-BVO (Figure 5h). However, in order to more deeply understand the intrinsic carrier transport properties depending on the crystallographic orientations and facets of the monoclinic-scheelite BiVO\({}_{4}\), it is necessary to study the establishment of epitaxial single crystalline thin films. In general, the establishment of epitaxial thin film photoelectrode has been widely recognized to maximize the potential of photoelectrode materials in pursuit of a further breakthrough by exploring its fundamental properties. For that reason, recently, various research on the growth of the epitaxial BiVO\({}_{4}\) have been reported, to explore its fundamental properties for PEC water splitting [13; 19; 56; 57; 58; 59; 60; 61; 62; 63]. Based on many theoretical predictions, the research related to the growth of the epitaxial monoclinic BiVO\({}_{4}\) (a = 5.1956 A, b = 5.0935 A, c = 11.6972 A, \(\beta\) = 90.387\({}^{\circ}\)) films have been focused on growth along the \(c\)-axis. The growth of epitaxial BiVO\({}_{4}\) (001) thin films was firstly achieved on a yttria-stabilized zirconia (YSZ, a = 5.145 A) substrate by molecular beam epitaxy [56]. However, the PEC properties for the epitaxial BiVO\({}_{4}\) (001) thin films could have not measured due to the absence of conductive electrode materials. On the other hand, [PERSON] et al. fabricated the epitaxial BiVO\({}_{4}\) (001) thin films by the PLD, and the photocathodic properties for the epitaxial BiVO\({}_{4}\) (001) thin films were observed by introducing SrRuO\({}_{3}\) as the bottom electrode [58]. In addition to epitaxial monolayer thin films, many studies have been reported on epitaxial heterojunction thin films. Our group recently reported that the self-assembled BiVO\({}_{4}\)-WO\({}_{3}\) heterostructured thin films could be grown epitaxially in the \(c\)-axis orientation to investigate the charge interaction between the BiVO\({}_{4}\) and the WO\({}_{3}\)[59]. The growth of epitaxial BiVO\({}_{4}\) (001) thin films also could be achieved on the SrTiO\({}_{3}\) (001) (STO, a = 3.905 A) substrate by the insertion of the WO\({}_{3}\) as template layer, which could reduce the lattice mismatch between the BiVO\({}_{4}\) and the STO [13]. In addition, [PERSON] et al. reported on the improved PEC performance of the epitaxial BiVO\({}_{4}\) (001) thin film by inserting lutetium oxide (Lu\({}_{2}\)O\({}_{3}\)) as a hole blocking layer [60]. Although the above-mentioned studies have well demonstrated on the growth of epitaxial BiVO\({}_{4}\) (001), further identification of the theoretical predictions that the electrical conductivity of the BiVO\({}_{4}\) is greater along the _ab_-plane than along the _c_-axis requires more challenging investigations and techniques [62; 63]. The anisotropic transport property of the BiVO\({}_{4}\) can be explained in terms of the crystalline structure, which is that charge carrier transport along the c-axis is only generated by nearest-neighbor hopping, while along the ab-plane it can be generated by both nearest-neighbor hopping and next-nearest-neighbor hopping. Therefore, the difference causes the enhanced hopping in the _ab_-plane. Most recently, based on the anisotropic electrical conductivity of the BiVO\({}_{4}\), the anisotropic charge transport efficiency of the BiVO\({}_{4}\) has been investigated by fabricating the epitaxial oxide thin films with different crystallographic orientations for the PEC water splitting [19]. In that report, the epitaxial BiVO\({}_{4}\) (001) thin films were fabricated on the YSZ (001) substrate as reported in the previous studies, while the epitaxial BiVO\({}_{4}\) (010) thin films were firstly grown on the STO (001) substrate by the domain match epitaxy (DME).