Facile Preparation of Hierarchical TiO₂ Nano Structures: Growth Mechanism and Enhanced Photocatalytic H₂ Production from Water Splitting Using Methanol as a Sacrificial Reagent

Owing to unique features, hierarchical nanostructure of TiO₂ has superior photocatalytic activity. In this work a facile hydrothermal route has been explored to prepare 3D hierarchical TiO₂ (3D-HTiO₂), 1D/3D hybrid hierarchical TiO₂ composite (HHC), and 3D hierarchical protonated titanate microspheres H₂Ti₂O₅·H₂O (3DHPTMS) at the expense of free-standing titania nanotube membrane (TiO₂-Memb). It proceeded through the formation of peroxotitanium complex, a water-soluble Ti complex as an intermediate. Mechanism of formation, role of membrane crystallinity, and reaction parameters giving fine control on tuning morphology and crystal structure have been investigated systematically. Photocatalytic activities were determined by measuring the amount hydrogen generated from water splitting under UV irradiation in the presence of methanol as a sacrificial reagent. Self-assembled hierarchical titania nanostructures exhibited much superior photocatalytic activity compared to that of starting material, i.e., TiO₂-Memb. Enhanced photocatalytic activity is due to characteristic morphology, increased surface area, and enhanced production of photogenerated charge carriers

Artificial Photosynthesis for Formaldehyde Production with 85% of Faradaic Efficiency by Tuning the Reduction Potential

In the present study, artificial photosynthesis by CO₂ reduction in NaCl is studied for solar fuel production. C1 fuels are evolved from various photoelectrochemical reactions at different reduction potentials depending on the multistep electron-transfer processes to CO₂. The (040) facet engineered BiVO₄ (040-BVO)|NaCl|Cu system is illuminated with solar light, and the external bias potential is tuned from 0.7 V to 1.5 (vs RHE) for the CO₂ reduction reaction. Integrating the applied external bias potential into the conduction band minimum of 040-BVO enables the CO₂ molecules to be converted into valuable chemical fuels. This thermodynamic control leads to product selectivity and increased faradaic efficiency. We observed that the selectivity and yield of the products depend on the magnitude of the CO₂ reduction potential. Its products were obtained by tuning the appropriate reduction potential, leading to faradaic efficiencies of 62% for formic acid, 85% for formaldehyde, 8% for MeOH, and 6% for EtOH at different bias in NaCl electrolyte. The correlation between the production of solar chemical fuels and CO₂ reduction potential tuning was studied by applying an external bias potential on 040-BVO|NaCl|Cu. The Cu–C bond distance change during photoelectrochemical CO₂ reduction reaction was investigated with in situ synchrotron X-ray. We suggest that the present results represent the most viable strategy to selectively and efficiently produce solar fuels via artificial photosynthesis

Crystalline Matrix of Mesoporous TiO₂ Framework for Dye-Sensitized Solar Cell Application

In the present study, a well-ordered columnar porous TiO₂ matrix is designed via inverted triblock copolymers self-assembly and introduced as a photoanode for dye-sensitized solar cells (DSSCs). The inverted triblock copolymer, polystyrene-<i>b</i>-poly­(ethylene oxide)-<i>b</i>-polystyrene, with the hydrophobic polystyrene segments at both ends of a hydrophilic poly­(ethylene oxide) chain is synthesized by atom transfer radical polymerization. These reverse-featured triblock copolymers allow facile stacking to 3-dimensional (D) columnar porous matrix from 2-D porous film via hydrophobic–hydrophilic interaction. A 3-D matrix with well-ordered cylindrical pores is favorable to current flow by providing a direct electron pathway. DSSCs with a 3-D matrix of 2 μm thickness show an enhanced photocurrent density of 8.1 mA cm^–2 and higher photoconversion efficiency of 4.23% compared with those of TiO₂ nanoparticle photoelectrode under the illumination of 1 sun (AM 1.5 G 100 mW cm^–2). For the first time, we address that a 3-D metal oxide electrode with columnar pore is demonstrated via reverse-featured triblock copolymer and analyzed with relationships between their structural features and impedance spectroscopy for DSSCs

Ferromagnetism of Single-Crystalline Cu₂O Induced through Poly(<i>N</i>‑vinyl-2-pyrrolidone) Interaction Triggering d‑Orbital Alteration

Ferromagnetic-like properties of cuprous oxide (Cu₂O) are induced through its interaction with chemisorbed surfactant poly­(<i>N</i>-vinyl-2-pyrrolidone) (PVP), which alters the intrinsic d¹⁰ configuration of Cu ions. Structural and magnetism-related properties of intact Cu₂O crystals (i-Cu₂O) and those capped with PVP (c-Cu₂O) were examined using various analytical instruments. SEM, TEM (corresponding selected area electron diffraction (SAED)), and XRD of i-Cu₂O and c-Cu₂O showed cubic and hexagonal shapes of single crystallinity with facets of {200} and {111}, respectively, resulting from the differential growth rates of the original identical crystals along the facets over time. Bulk magnetic susceptibility (χ) of i-Cu₂O and c-Cu₂O at room temperature in field-dependent magnetization and the difference in their magnetic moment in temperature-dependent magnetization showed diamagnetic and ferromagnetic properties, respectively. The difference in the fluorescence mode of X-ray absorption near edge structure (XANES) spectra between i-Cu₂O and c-Cu₂O, showing no quadruple pre-edge peak for the transition 1s → 3d in Cu­(II) ions with d⁹ electronic configuration, indicates an orbital alteration on the surface of c-Cu₂O caused by an interaction with PVP. Two peaks for c-Cu₂O at higher binding energies in O 1s X-ray photoelectron spectroscopy may be indicative of the ligand-to-metal charge transfer (LMCT) from O atoms of PVP to Cu ions of Cu₂O, generating a chemical interaction through coordination bonding. Large hyperfine splitting constants in electron paramagnetic resonance (EPR) spectra of c-Cu₂O support this interpretation, with septet hyperfine splitting suggestive of Cu–Cu interactions on the surface of c-Cu₂O via the interaction with O atoms of PVP. These results demonstrate that PVP capping of Cu₂O crystal (c-Cu₂O) induces ferromagnetism of Cu­(I) ions through coordination with O atoms of chemically adsorbed PVP. This may induce LMCT and Cu–Cu interactions that lead to changes in electronic configurations, deriving the ferromagnetic moments of c-Cu₂O

Facile Fabrication of WO₃ Nanoplates Thin Films with Dominant Crystal Facet of (002) for Water Splitting

Single crystalline orthorhombic phase tungsten trioxide monohydrate (<i>O</i>-WO₃·H₂O, space group: <i>Pmnb</i>) nanoplates with a clear morphology and uniform size distribution have been synthesized by the hydrothermal method and fabricated on the surface of fluorine doped tin oxide (FTO) coated glass substrates with selective exposure of the crystal facet by the finger rubbing method. The rubbing method can easily arrange the <i>O</i>-WO₃·H₂O nanoplates along the (020) facet on the FTO substrate. The <i>O</i>-WO₃·H₂O nanoplate can be converted to monoclinic phase WO₃ (γ-WO₃, space group: <i>P</i>21/<i>n</i>) with dominant crystal facet of (002) without destroying the plate structure. Crystal morphologies, structures, and components of the powders and films have been determined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, etc. The band gap energies of the <i>O</i>-WO₃·H₂O and γ-WO₃ nanoplates were determined as ca. 2.26 and 2.49 eV, respectively. Photoelectrochemical properties of the films with (002) dominant crystal facet have also been checked for discussion of further application in water oxidation. The advantage of (002) facet dominant film was investigated by comparing to one spin-coated γ-WO₃ thin film with the same thickness via photoelectrochemical characterizations such as photocurrent, incident photon to current efficiency, and electrochemical impedance spectroscopy