Photophysical, Photoelectrochemical, and Photocatalytic Properties of Novel SnWO₄ Oxide Semiconductors with Narrow Band Gaps

Novel SnWO4 visible-light active photocatalysts with two polymorphs (orthorhombic α and cubic β phases) were prepared by a conventional solid-state reaction method, and their optical properties, electronic band structure, and photocatalytic activities were investigated. It was found that the low-temperature phase, α-SnWO4 with corner-shared WO6 octahedra, exhibited a dark-red color and indirect band gap of 1.64 eV, whereas the high-temperature phase, β-SnWO4 with unshared WO4 tetrahedra, exhibited a light-yellow color and direct band gap of 2.68 eV. The Mott−Schottky plots obtained using a thick film electrode in 1 M NaCl electrolyte revealed the n-type semiconductive properties of the SnWO4 polymorphs; i.e., the flat-band potential values of α- and β-SnWO4 were −0.61 and −0.66 V (SCE), respectively. From the electronic band structure calculations performed using density functional theory, the Sn 5p and O 2p orbitals were hybridized to construct the valence band in both SnWO4 polymorphs. However, the constructions of the conduction band were quite different. β-SnWO4 with its shorter W−O bond lengths in the WO4 tetrahedra has a higher conduction-band potential than α-SnWO4 phase, which has larger W−O bond lengths in the WO6 octahedra and, thus, was able to produce H2 from an aqueous methanol solution under visible-light irradiation (>400 nm). Both SnWO4 polymorphs also exhibited good photocatalytic activity for the degradation of rhodamine B dye solution under visible-light irradiation (>420 nm). The photocatalytic activity of these SnWO4 polymorphs was higher than that of other visible-light active photocatalysts with much smaller particle sizes, such as nanosized WO3 (9.72 m2/g) and TiONx (112.13 m2/g). This higher photocatalytic activity of the SnWO4 polymorphs is mainly attributed to their smaller band gaps and unique band structures, resulting from their different bonding nature

Direct Printing Synthesis of Self-Organized Copper Oxide Hollow Spheres on a Substrate Using Copper(II) Complex Ink: Gas Sensing and Photoelectrochemical Properties

The direct printing synthesis of metal oxide hollow spheres in the form of film on a substrate is reported for the first time. This method offers facile, scalable, high-throughput production and device fabrication processes. The printing was carried out via a doctor-blade method using Cu­(II) complex ink with controllable high viscosity based on formate–amine coupling. Following only thermal heating in air, well-defined polycrystalline copper oxide hollow spheres with a submicrometer diameter (≤1 μm) were formed spontaneously while being assembled in the form of a film with good adhesion on the substrate. This spontaneous hollowing mechanism was found to result from the Kirkendall effect during oxidation at elevated temperature. The CuO films with hollow spheres, prepared via direct printing synthesis at 500 °C, led to the creation of a superior p-type gas sensor and photocathode for photoelectrochemical water splitting with completely hollow cores, a rough/porous shell structure, a single phase, high crystallinity, and no organic/polymer residue. As a result, the CuO hollow-sphere films showed high gas responses and permissible response speeds to reducing gases and high photocurrent density compared to conventional CuO powder films and the values previously reported. These results exemplify the successful realization of a high-throughput printing fabrication method for the creation of superior nanostructured devices

Wolframite-type ZnWO₄ Nanorods as New Anodes for Li-Ion Batteries

A divalent wolframite-structure zinc tungstate (ZnWO4) was synthesized using a facile hydrothermal process at 180 °C, with pH adjustment to drive the preferential growth along the [100] direction, resulting in the formation of one-dimensional nanorods. The resulting nanorods were characterized in detail using X-ray diffraction, Raman spectroscopy, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive X-ray analysis, and Brunauer–Emmett–Teller measurements. In addition, the Li electroactivities of the ZnWO4 nanorods were investigated using cyclic voltammetry and galvanostatic cycling. The ZnWO4 nanorods could deliver reversibly sustained high capacities of over 420 mAh g–1 after 150 cycles, which are much higher than those of graphite-based anodes

Tailoring the Morphology and Structure of Nanosized Zn₂SiO₄: Mn²⁺ Phosphors Using the Hydrothermal Method and Their Luminescence Properties

Mn2+-doped Zn2SiO4 phosphors with nanoellipsoid or nanowire morphologies were synthesized at low temperature 2SiO4 phase with a willemite structure was formed at pH 7 and 9, a Zn4Si2O7(OH)2·H2O phase with a hemimorphite structure was formed at pH 11. The as-prepared powders with a willemite structure showed an intense green emission (λ ≈ 525 nm) under 254 nm excitation, whereas the as-prepared powders with a hemimorphite structure did not show any emission. However, all of the powders showed a willemite structure while retaining their original shape after annealing at 900 °C under a reducing atmosphere. The annealed sheaves of willemite with an ellipsoid shape showed a more intense green emission with a longer decay time than the phase-transformed willemite nanowires. These results were discussed in terms of the surface defects and dopant concentration

Facile Hydrothermal Synthesis of SrNb₂O₆ Nanotubes with Rhombic Cross Sections

Novel SrNb2O6 nanotubes with rhombic cross sections were produced via a facile hydrothermal route without any surfactants or templates by controlling the reaction conditions, such as the pH value and temperature. The prepared nanotubular powders were characterized using X-ray diffraction, field-emission electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). From the FESEM and HRTEM analysis, it was found that the morphological evolution of the one-dimensional tubular structure was originated from the stepwise bending growth in the [011] direction around the a-axis and its rhombic cross section was connected to the crystallographic nature

Visible-Light-Induced Photocatalytic Activity in FeNbO₄ Nanoparticles

A novel method was used to synthesize orthorhombic FeNbO4 nanoparticles by a hydrothermal process followed by calcination at 600 °C, and their optical, photoelectrochemical, and photocatalytic properties were investigated. The microstructural and local structural properties were characterized using X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy (TEM), and Raman spectroscopy. The FeNbO4 particles obtained were composed of much smaller nanocrystallines, with an average size of 10−20 nm, compared to particles prepared at 1000 °C through a conventional solid-state reaction method. Moreover, the optical band gap energy of the nanoparticles was estimated to be 1.93 eV from the UV−vis diffuse reflectance, and their flat-band potential in 1 M NaOH was −0.4 V (SCE). The X-ray photoelectron spectroscopy analysis revealed that the nanoparticles had fewer surface defects, such as oxygen vacancies, than the particles prepared by the solid-state reaction method. The FeNbO4 nanoparticles also exhibited a much higher photocatalytic activity for the degradation of rhodamine B dye solution under visible light irradiation (>420 nm). This higher photocatalytic activity of the FeNbO4 nanoparticles was attributed to their higher optical absorption ability and smaller particle size, as well as fewer surface defects

Surfactant-Assisted Shape Evolution of Thermally Synthesized TiO₂ Nanocrystals and Their Applications to Efficient Photoelectrodes

TiO2 nanocrystals were synthesized via a two-phase thermal process, and the shape of the nanocrystals was controlled from nanospheres to nanorods by the ratio of two surfactants. The shape control of nanocrystals was ascribed to the selective adsorption of the two surfactants. The shape of TiO2 nanocrystals influenced the photocatalytic performances of the photoelectrodes through two compromising factors:  the relative surface area and the electron transport. The photoelectrode composed of nanorods showed a slower charge recombination rate, while it showed a smaller specific surface area, compared to nanospheres. As a result, the photoelectrodes showed the optimal photocatalytic performance when the nanospheres and the nanorods were mixed

Indium−Tin−Oxide-Based Transparent Conducting Layers for Highly Efficient Photovoltaic Devices

Additional hydrogen (H2) annealing and subsequent electrochemical treatment are found to make tin-doped indium oxide (ITO)-based photoelectrodes suitable for highly efficient dye sensitized solar cells. The additional H2 annealing process recovered the electrical conductivity of the ITO film the same as its initial high conductivity, which enhanced the charge collecting property. Moreover, the employment of electrochemical oxidation of TiO2/ITO photoelectrode improved the energy conversion efficiency of the ITO-based dye-sensitized solar cells (DSSC), higher than that of a conventional FTO-based DSSC. Electrochemical impedance analysis showed that the H2 annealing process reduced the internal resistance of the cell, i.e., the resistance of the ITO and the Schottky barrier at the TiO2/ITO interface were reduced, and that the electrochemical treatment recovered the diodelike characteristics of the DSSC by retarding back electron transfer from the photoelectrode to the electrolyte. The present work demonstrates that thermally and electrochemically modified ITO-based photoelectrode is another alternative to the conventionally used FTO-based photoelectrode

Revisiting Whitlockite, the Second Most Abundant Biomineral in Bone: Nanocrystal Synthesis in Physiologically Relevant Conditions and Biocompatibility Evaluation

The synthesis of pure whitlockite (WH: Ca₁₈Mg₂(HPO₄)₂(PO₄)₁₂) has remained a challenge even though it is the second most abundant inorganic in living bone. Although a few reports about the precipitation of WH in heterogeneous phases have been published, to date, synthesizing WH without utilizing any effects of a buffer or various other ions remains difficult. Thus, the related research fields have encountered difficulties and have not been fully developed. Here, we developed a large-scale synthesis method for pure WH nanoparticles in a ternary Ca(OH)₂–Mg(OH)₂–H₃PO₄ system based on a systematic approach. We used excess Mg²⁺ to impede the growth of hydroxyapatite (HAP: Ca₁₀(PO₄)₆(OH)₂) and the formation of other kinetically favored calcium phosphate intermediate phases. In addition, we designed and investigated the synthesis conditions of WH under the acidic pH conditions required to dissolve HAP, which is the most thermodynamically stable phase above pH 4.2, and to incorporate the HPO₄^2– group into the chemical structure of WH. We demonstrated that pure WH nanoparticles can be precipitated under Mg²⁺-rich and acidic pH conditions without any intermediate phases. Interestingly, this synthesized nano-WH showed comparable biocompatibility with HAP. Our methodology for determining the synthesis conditions of WH could provide a new platform for investigating other important precipitants in aqueous systems

Anionic Ligand Assisted Synthesis of 3‑D Hollow TiO₂ Architecture with Enhanced Photoelectrochemical Performance

Hollow structured materials have shown great advantages for use in photoelectrochemical devices. However, their poor charge transport limits overall device performance. Here, we report a unique 3-D hollow architecture of TiO₂ that greatly improves charge transport properties. We found that citric acid (CA) plays crucial roles in the formation of the 3-D hollow architecture. First, CA controls the hydrolysis rate of Ti ions and facilitates surface hydrolysis on templates during hydrothermal synthesis. Second, CA suppresses the growth of the carbon template at the initial reaction stage, resulting in the formation of comparatively small hollow fibers. More importantly, a prolonged hydrothermal reaction with CA enables a hollow sphere to grow into entangled hollow fibers via biomimetic swallowing growth. To demonstrate advantages of the 3-D hollow architecture for photoelectrochemical devices, we evaluated its photoelectrochemical performance, specifically the electrolyte diffusion and electron dynamics, by employing dye-sensitized solar cells as a model device. A systemic analysis reveals that the 3-D hollow architecture greatly improves both the electrolyte diffusion and electron transport compared to those of the nanoparticle and hollow sphere due to the elongated porous hollow morphology as well as the densely interconnected nanoparticles at the wall layer