The preparation of metal oxides on a conductive substrate has been an important issue for improving photoelectrochemical water splitting efficiency. In this work, a facile synthetic process is reported for single-crystalline WO3 microplates with variable thicknesses grown directly on a fluorine-doped tin oxide substrate followed by an annealing procedure. The WO3 microplate electrode showed an improved photocurrent under 1 sun irradiation (1.9 mA/cm2 at 0.6 V vs Ag/AgCl, 100 mW/cm2) for water oxidation, including a significant enhancement in the visible light region compared to either a nanoparticle or a bulk film electrode. The enhanced water oxidation activity originates from both the single crystallinity with an optimum thickness and the oxygen-deficient characteristic of the WO3 microplates. To improve the photochemical stability, FeOOH electrocatalysts were deposited on the surfaces of the WO3 microplates. The resulting WO3/FeOOH composite electrode showed enhanced stability for water oxidation reactions.

A well-defined WO 3 /Bi 2 S 3 composite comprised of single-crystalline Bi 2 S 3 nanowire (Bi 2 S 3 NW) layers on top of the WO 3 nanoparticles (WO 3 NP) was synthesized via an in situ hydrothermal reaction. The single-crystalline Bi 2 S 3 nanowires were uniformly grown on the surface of the WO 3 nanoparticle layer. This in situ hydrothermal process is also a general route for the synthesis of well-aligned Bi 2 S 3 nanowires on various metal oxide substrates, such as TiO 2 , BiVO 4 , and ZnO. Compared to the sole Bi 2 S 3 electrode, the resulting WO 3 NP/Bi 2 S 3 NW composite showed enhanced photoelectrochemical activity. The origin of this enhanced activity is mainly attributed to the enhancement of charge separation on the Bi 2 S 3 layer, due to the effective photogenerated electron transfer from the Bi 2 S 3 conduction band to that of WO 3 . Furthermore, the single-crystalline longitudinal structure of the Bi 2 S 3 nanowires can provide a direct electrical pathway through a single domain of nanowires.

A pinhole-free BiVO4 electrode was successfully synthesized using an ultrasonic-assisted synthetic method on a conductive substrate. The pinhole-free BiVO4 electrode showed highly improved photoelectrochemical activity for both sulfite oxidation and water oxidation. The blocking recombination processes were examined to clarify the enhanced photoelectrochemical performances.

The addition of water initiates the phase transition of hexagonal CoO to Co(OH) nanocrystals. Inducing the phase transition of h-CoO on various substrates results in efficient chemical bonding between Co(OH) and the substrate. The efficient deposition of Co(OH) is widely applicable for electrochemical and photoelectrochemical water oxidation reactions.

Noble-metal-decorated metal oxide sensors have shown promising gas-sensing properties. However, the optimal dispersion of metal particles on a semiconductor is still challenging for most sensing materials. In this study, Co3O4 nanocubes (NCs) and Pt-supported Co3O4 NCs were prepared as sensing materials for acetone gas detection. Transmission electron microscopy and X-ray diffraction were used to examine the structure and exposed facets of Co3O4 NCs. As a result, the Co3O4 NCs were identified as the single-crystalline phase of spinel Co3O4, and each surface exposed the {100} plane. To examine the cocatalytic effect of Pt combined with Co3O4 NCs on the sensing performance, Pt nanoparticles were photodeposited on Co3O4 (Pt–Co3O4 NCs). The Pt–Co3O4 NC-based sensor provided a higher p-type response than the Co3O4 NC sensor in the detection of 500 ppb acetone at 200 °C, with the highest response of 3.1 (R g/R a). The enhanced performance of the Pt–Co3O4 NCs is caused by the exposed {100} planes of Co3O4, in addition to the loaded Pt nanoparticles. The sensor with Co3O4 NCs has a larger neck diameter and hole accumulation layer at the interface than that with Co3O4 nanospheres and thus provides a wide channel for charge carriers, resulting in better gas-sensing responses and high selectivity toward acetone over other volatile compounds. Moreover, the Pt nanoparticles stimulate O2 dissociation on the Co3O4 surface, thus increasing the concentration of chemisorbed oxygen species by the spillover effect. Thus, the incorporation of Pt with Co3O4 NCs promotes the sensitivity of the material in the detection of acetone gas and also enhances the selectivity. This study highlights the possibility of the rapid deposition of metal nanoparticles for the improvement of gas sensors.

Conversion reaction-based transition metal oxides have been considered as advanced anode materials for lithium batteries because of their high storage capacities; however, the initial lithiation/delithiation mechanism remains poorly understood. In this study, we synthesized single-crystalline spindle-type mesoporous Fe 2 O 3 (MS-Fe 2 O 3 ), which contained a high fraction of textural porosity that appears as a unique tunnel structure. The MS-Fe 2 O 3 electrode exhibited a remarkably high initial Coulombic efficiency of 85.4% and stable cycling performance with a specific capacity of 1250 mA h g −1 after 100 cycles. During the lithiation process, the initial α-Fe 2 O 3 phase was transformed to nanograin-Fe embedded in the Li 2 O matrix, while subsequent delithiation changed the Fe phase into γ-Fe 2 O 3 . Despite the initial irreversible phase conversion, a reversible electrochemical reaction (Fe 3+ → Fe 0 → Fe 3+ ) was retained in the first cycle, leading to the high ICE and discharge capacity. This study provides crucial information on the lithiation/delithiation mechanism of transition metal oxides and benefits the design of advanced materials for lithium batteries.

The ultrasonic‐assisted synthesis method provides a fast, simple, and large‐scale route for synthesizing desired materials under ambient conditions. In this work, we report on the facile preparation of ZnO‐ZnS core‐shell nanorods on a fluorine‐doped tin oxide (FTO) substrate. The core‐shell nanorods were synthesized by sequential nanoscale reactions involving the preparation of ZnO nanorods and conversion of the ZnO surface into a ZnS shell on the FTO substrate, using an in situ sonochemical method. The ZnO‐ZnS core‐shell nanorods showed improved photocurrents compared with ZnO nanorods for the water oxidation reaction. During the water oxidation reaction, the ZnS shell passivates the surface‐defects of the ZnO, which results in enhanced charge separation in the ZnO nanorods and higher performance.

A BiVO4/Bi2S3 composite comprising Bi2S3 nanowires on top of a BiVO4 film was prepared via hydrothermal reaction. Because additional Bi3+ ions were not delivered during the reaction, BiVO4 served as the Bi3+ ion source for the development of Bi2S3. A detailed growth mechanism of the nanowire was elucidated by an analysis of the concentration gradient of Bi3+ and S2– ions during the reaction. The in situ growth was followed by the etching of BiVO4 to Bi3+ and VO4 3– ions and regrowth to Bi2S3, which resulted in the rapid evolution of nanowires on the BiVO4 substrate. The fabricated BiVO4/Bi2S3NW composite exhibited an improved photoelectrochemical activity compared to other Bi2S3 samples. The improved efficiency was mainly attributed to both improved charge separation and effective adhesion obtained by the in situ growth.