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Although bismuth vanadate (BiVO4) has been promising as photoanode material for photoelectrochemical water splitting, its charge recombination issue by short charge diffusion length has led to various studies about heterostructure photoanodes. As a hole blocking layer of BiVO4, titanium dioxide (TiO2) has been considered unsuitable because of its relatively positive valence band edge and low electrical conductivity. Herein, a crystal facet engineering of TiO2 nanostructures is proposed to control band structures for the hole blocking layer of BiVO4 nanodots. We design two types of TiO2 nanostructures, which are nanorods (NRs) and nanoflowers (NFs) with different (001) and (110) crystal facets, respectively, and fabricate BiVO4/TiO2 heterostructure photoanodes. The BiVO4/TiO2 NFs showed 4.8 times higher photocurrent density than the BiVO4/TiO2 NRs. Transient decay time analysis and time-resolved photoluminescence reveal the enhancement is attributed to the reduced charge recombination, which is originated from the formation of type II band alignment between BiVO4 nanodots and TiO2 NFs. This work provides not only new insights into the interplay between crystal facets and band structures but also important steps for the design of highly efficient photoelectrodes.

The hydrothermal method is a facile route for the synthesis of TiO2 nanostructured materials, but it requires accurate process optimizations and adjustments to circumvent the undesirable morphology products. In this study, systematically controlled experimental studies are carried out under thermodynamic and kinetic considerations to understand the formation of well-aligned TiO2 nanorods during the hydrothermal reaction. In this regard, TiO2 nanorods are synthesized on various types of substrates, including single crystal TiO2 and sapphire, fluorine doped tin oxide, and silicon. Variable growth parameters are classified and investigated for their effects on the morphological evolution of TiO2 nanorods. The preferred morphology of TiO2 nanorods with the {110} facet is confirmed based on the crystallographic results for TiO2 nanorods acquired by extensive transmission electron microscopy studies during the entire growth processes. The presence of the seeds on the substrates is found to be mandatory for the formation, growth, and strong adhesion of TiO2 on the applied foreign substrates. The results elucidate that the growth kinetics of the process is significantly governed by the amount of applied HCl concentration, which is highly influential on the morphology of the synthesized TiO2 nanorods. Accordingly, the growth mechanism for the preferential growth maintaining the rod shape is derived.

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CuO-added (1-x)KNbO 3 -x mol%BaZrO 3 ceramics with 0.0≤x≤7.0 were sintered at 960°C. Large double polarization vs electric field (P-E) and sprout-shaped strain vs electric field (S-E) hysteresis curves were obtained from the specimens with x≤2.0. They exhibited large polarizing electric fields (E P ) owing to the presence of a large number of defect dipoles (P D s) that formed between Cu 2+ ions and oxygen vacancies. Small double P-E hysteresis curves were observed for the specimens with x≥3.0 with reduced E P because of the decreased number of P D s and the presence of a polymorphic phase structure containing both orthorhombic and pseudocubic structures. In particular, the specimen with x=5.0 exhibited a large strain of 0.16% at 8.0 kV/mm with a small E P of 1.2 kV/mm and good fatigue property: this specimen maintained a strain of 0.13% at 6.0 kV/mm after 10 6 cycles of 3.0 kV/mm.