Kenta Kawashima

The cross-substitution effect of tantalum on the visible-light-driven water oxidation activity of BaNbO2N crystals grown directly by an NH3-assisted flux method

Various perovskite-type transition metal oxynitride photocatalysts have been intensively investigated for photocatalytic water splitting under visible light. The band gap engineering and the formation of solid solution have been demonstrated to be one of the beneficial approaches to achieve a high solar energy conversion efficiency. As a member of the niobium-based oxynitride family, BaNbO2N has a small band gap energy of 1.79 eV and light absorption up to 690 nm. Here, we have investigated the cross-substitution effect of tantalum on the photocatalytic water oxidation activity of BaNbO2N crystals grown directly by an NH3-assisted flux method. It was found that with increasing the amount of tantalum substituted for niobium in BaNb1−xTaxO2N (x = 0, 0.25, 0.50, 0.75, and 1), the average crystal size and the intensity of background absorption gradually reduced and the O2 evolution rate increased monotonically for BaTaO2N crystals due to the shift of the top of the valence band to a more positive side and the lowered densities of mid-gap states associated with defects. The CoOx/flux-grown-BaTaO2N/Ta photoanode exhibited a photocurrent of about 0.85 mA cm−2 at 1.2 VRHE, which decreased gradually by about 35% after 24 h, evidencing that the flux-grown BaTaO2N crystals are highly stable for photoelectrochemical water oxidation. BaTaO2N modified with CoOx (2 wt% Co) as an O2 evolution cocatalyst exhibited an apparent quantum yield (AQY) of 0.24% at 420 nm for an O2 evolution reaction in the presence of Ag+ ions as sacrificial electron acceptors. In addition, the results from the first-principles DFT calculations conform to the experimentally obtained data on the photocatalytic water oxidation activity of the BaNb1−xTaxO2N crystals.

Amount of tungsten dopant influencing the photocatalytic water oxidation activity of LaTiO2N crystals grown directly by an NH3-assisted flux method

Solar water splitting technologies for hydrogen generation using visible-light-active (oxy)nitride photocatalysts have been intensively studied to achieve a greater supply of clean and renewable energy. Here, we have investigated the amount of tungsten dopant influencing the photocatalytic water oxidation activity of LaTiO2N (LTON) crystals grown directly by an NH3-assisted KCl flux method in the absence of a cocatalyst. With the maximum amount of tungsten dopant (x = 0.10), LaWO0.6N2.4 was formed as a minor phase to a W-doped LaTiO2N phase (W-LTON). The optimum W-doped LaTiO2N crystals were grown with the tungsten amount of x = 0.05, showing a nearly three-fold higher O2 evolution rate (51 μmol h−1) compared to pure LaTiO2N crystals (18 μmol h–1). The improvement in the O2 evolution rate of the LaTiO2N crystals was presumably due to the improved conductivity, enhanced light absorption and efficient charge separation stemmed from tungsten doping. According to first-principles density function theory (DFT) calculations, although no significant change in band gap by tungsten doping was observed, slight broadening of valence and conduction bands was noted in W-LTON which may improve the electron/hole conductivity due to reduction of the effective mass of electrons. Also, tungsten doping was beneficial in creating effective defects and in enhancing the photocatalytic water oxidation activity of the LaTiO2N crystals.

NH3-assisted chloride flux-coating method for direct fabrication of visible-light-responsive SrNbO2N crystal layers

Perovskite-type SrNbO2N crystal layers were prepared on niobium substrates by using an NH3-assisted chloride flux-coating method. The optimization of synthesis parameters (holding temperature and strontium source : flux molar ratio) was performed using a NaCl–KCl flux. By choosing the optimal synthesis conditions, platelet SrNbO2N crystals were grown over the entire substrate surface, and each SrNbO2N platelet has a single-crystalline structure in a cubic symmetry. The optimal crystal layer possesses a well-adhered SrNbO2N/NbNx/Nb structure and absorbed photons with wavelengths up to 680 nm. In addition, the optimal SrNbO2N/NbNx photoelectrode was used for photoelectrochemical water oxidation (2H2O → 4H+ + 4e− + O2↑), as one half of the water splitting reaction. A photocurrent density of 113 μA cm−2 at 1.23 VRHE was recorded on the SrNbO2N/NbNx electrode without any additional co-catalyst loading and treatment under simulated sunlight, due to the higher crystallinity of SrNbO2N and higher interface-adhesion of the SrNbO2N/NbNx/Nb structure, which suppress the recombination of photogenerated electrons and holes at the defects and lead to an increase of the photogenerated electron collection efficiency in a niobium substrate, respectively. This study is the first to address the fabrication of quaternary oxynitride crystal layers on a conductive substrate using an NH3-assisted flux-coating method.

Self-Assembled Cu–Sn–S Nanotubes with High (De)Lithiation Performance

Through a gelation-solvothermal method without heteroadditives, Cu-Sn-S composites self-assemble to form nanotubes, sub-nanotubes, and nanoparticles. The nanotubes with a Cu3-4SnS4 core and Cu2SnS3 shell can tolerate long cycles of expansion/contraction upon lithiation/delithiation, retaining a charge capacity of 774 mAh g-1 after 200 cycles with a high initial Coulombic efficiency of 82.5%. The importance of the Cu component for mitigation of the volume expansion and structural evolution upon lithiation is informed by density functional theory calculations. The self-generated template and calculated results can inspire the design of analogous Cu-M-S (M = metal) nanotubes for lithium batteries or other energy storage systems.