Efficient visible light-induced photoelectrochemical oxidation of water was achieved using a tungsten(IV) oxide (WO 3 ) film composed of perpendicularly oriented plate-like crystallites, a flake-wall film, prepared on a transparent conductive substrate by controlling anisotropic crystal growth of tungsten oxide hydrate (WO 3 •H 2 O) followed by calcination.Water splitting using solar light is a key technology for the establishment of a low-carbon society. 1 Because of the positive Gibbs energy change in the water splitting, a direct one-step reaction may not proceed even though compensating energy is injected from outside the system, but it can be achieved by dividing into two parts: reduction of water to hydrogen and oxidation of water. Fujishima and Honda reported, for the first time, that photoirradiated titania and platinum electrodes could be used for the oxidation and reduction parts, respectively. 2 Although such a "photoelectrochemical" cell requires additional external energy as electrical or chemical bias to compensate the potential drop due to resistance of a circuit as well as overpotentials for the redox (electron transfer) reactions, the anode and cathode can be designed and their performance can be optimized separately and individually, which is the most significant merit of photoelectrochemical systems compared with particulate photocatalytic systems. Photoelectrochemical oxidation of water to evolve molecular oxygen is induced on an n-type semiconductor electrode by valence band holes generated by band-gap excitation. 3,4 Significant incident photon-to-current conversion efficiencies (IPCEs) for oxidation of water have been reported on WO 3 films, which are responsive to the blue part of visible light owing to the band gap of ca. 2.6 eV. 5-9 Nanocrystalline WO 3 films prepared by sol-gel techniques possessed porous network structures consisting of well-crystallized nanoparticles exhibiting a large surface area for semiconductor/liquid junctions. Since porous electrodes require a thickness of several micrometers to maximize light absorption, 10,11 the increase in thickness would increase grain boundaries, at which free electrons are scattered and recombination of photogenerated electron-hole pairs occurs, resulting in retardation of electron transfer to a back-contacted conductive substrate. 8,11,12 It has been reported that films composed of perpendicularly oriented crystallites showed higher photoelectrochemical performance than that of films with nanocrystalline particles. 11,13-16 For WO 3 , there have been few works focusing on the photoelectrochemical behavior of perpendicularly oriented crystalline films. 16 Herein, for the first time, WO 3 films consisting of perpendicularly oriented crystalline flakes were successfully fabricated by a wet process using anisotropic crystal growth of WO 3 •H 2 O with a layered crystal structure. Since the crystalline flakes were oriented normal to the substrate like a wall, we named this film a flake-wall film. Figure 1a shows the crystal structure of WO 3...