Revealing the Beneficial Effects of FeVO₄ Nanoshell Layer on the BiVO₄ Inverse Opal Core Layer for Photoelectrochemical Water Oxidation

In this paper we developed a template-assisted three-dimensionally ordered BiVO₄ inverse opal (IO) film by sandwich-type infiltration through self-assembled colloidal polystyrene (PS) opal beads with a diameter of 410 nm (±20 nm) for photoelectrochemical hydrogen production. Herein, the ordered BiVO₄ inverse opal structure possessed a pore diameter of ∼340 nm and wall thickness of ∼20 nm, providing a large surface area. Their photoelectrochemical behavior were assessed under 1 sun illumination (100 mW/cm² with AM 1.5 filter) in 0.5 M Na₂SO₄ (pH 7) which displayed a photocurrent density (<i>J</i>_sc) of 0.8 mA/cm² at 1.23 V vs a normal hydrogen electrode (NHE). Low photocurrents of BiVO₄ IO photoelectrodes are due to their limited photoelectrochemical ability to split water under light irradiation and their intrinsically low electronic conductivities. To overcome these problems, BiVO₄ IO film was modified to deposit a nanolayer of n-type FeVO₄ having a narrow band gap (<i>E</i>_g = 2.06 eV). The bilayered BiVO₄/FeVO₄ core–shell film has efficient photoelectrochemical (PEC) properties compared to unmodified BiVO₄, showing a photocurrent density of 2.5 mA/cm² at 1.23 V vs NHE, probably resulting from a favorable charge transfer/transport phenomenon under beneficial band alignment as well as visible light absorption by the FeVO₄ layer

Structural Modification of Self-Organized Nanoporous Niobium Oxide via Hydrogen Treatment

Niobium pentoxide (Nb₂O₅) is an interesting material with applications in Li battery and hybrid capacitor electrodes. The main limitation of this material is its low electronic conductivity. In this study, H₂ treatment is introduced to address this issue. Self-ordered Nb₂O₅ films were prepared by anodizing Nb foils and subsequently treating them in a H₂ atmosphere. Electron microscopy revealed that the Nb₂O₅ film had a hierarchical porous microstructure consisting of macropores and mesopores. X-ray diffraction analysis showed that the crystal structure could be changed by the H₂ treatment compared to the air treatment. Oxygen deficiencies in the Nb₂O₅ film were induced by the treatment, as confirmed by X-ray photoelectron spectroscopy. Mott–Schottky analysis was performed and indicated that the electronic conductivity of the material was significantly improved by the oxygen deficiencies. Thus, the electrochemical Li storage kinetics in porous Nb₂O₅ films can be greatly enhanced by H₂ treatment

Hollow Nanostructured Metal Silicates with Tunable Properties for Lithium Ion Battery Anodes

Hollow nanostructured materials have attracted considerable interest as lithium ion battery electrodes because of their good electrochemical properties. In this study, we developed a general procedure for the synthesis of hollow nanostructured metal silicates via a hydrothermal process using silica nanoparticles as templates. The morphology and composition of hollow nanostructured metal silicates could be controlled by changing the metal precursor. The as-prepared hierarchical hollow nanostructures with diameters of ∼100–200 nm were composed of variously shaped primary particles such as hollow nanospheres, solid nanoparticles, and thin nanosheets. Furthermore, different primary nanoparticles could be combined to form hybrid hierarchical hollow nanostructures. When hollow nanostructured metal silicates were applied as anode materials for lithium ion batteries, all samples exhibited good cyclic stability during 300 cycles, as well as tunable electrochemical properties

ZnWO₄/WO₃ Composite for Improving Photoelectrochemical Water Oxidation

A rapid screening technique utilizing a modified scanning electrochemical microscope has been used to screen photocatalysts and determine how metal doping affects its photoelectrochemical (PEC) properties. We now extend this rapid screening to the examination of photocatalyst (semiconductor/semiconductor) composites: by examining a variety of ZnWO₄/WO₃ composites, a 9% Zn/W ratio produced an increased photocurrent over pristine WO₃ with both UV and visible irradiation on a spot array electrode. With bulk films of various thickness formed by a drop-casting technique of mixed precursors and a one-step annealing process, the 9 atomic % ZnWO₄/WO₃ resulted in a 2.5-fold increase in the photocurrent compared to pristine WO₃ for both sulfite and water oxidation at +0.7 V vs Ag/AgCl. Thickness optimization of the bulk-film electrodes showed that the optimum oxide thickness was ∼1 μm for both the WO₃ and ZnWO₄/WO₃ electrodes. X-ray diffraction showed the composite nature of the WO₃ and ZnWO₄ mixtures. The UV/vis absorbance and PEC action spectra demonstrated that WO₃ has a smaller band gap than ZnWO₄, while Mott–Schottky analysis determined that ZnWO₄ has a more negative flat-band potential than WO₃. A composite band diagram was created, showing the possibility of greater electron/hole separation in the composite material. Investigations on layered structures showed that the higher photocurrent was only observed when the ZnWO₄/WO₃ composite was formed in a single annealing step

Enhanced Solar Water Oxidation Performance of TiO₂ via Band Edge Engineering: A Tale of Sulfur Doping and Earth-Abundant CZTS Nanoparticles Sensitization

We report the rational design and fabrication of earth-abundant, visible-light-absorbing Cu₂ZnSnS₄ (CZTS) nanoparticle (NP) in situ sensitized S doped TiO₂ nanoarchitectures for high-efficiency solar water splitting. Our systematic studies reveal that these nanoarchitectures significantly enhance the visible-light photoactivity in comparison to that of TiO₂, S doped TiO₂, and CZTS NP sensitized TiO₂. Detailed photoelectrochemical (PEC) studies demonstrate an unprecedented enhancement in the photocurrent density and incident photon to electron conversion efficiency (IPCE). This enhancement is attributed to the significantly improved visible-light absorption and more efficient charge separation and transfer/transport, resulting from the synergistic influence of CZTS NP sensitization and S doping, which were confirmed by electrochemical impedance spectroscopy (EIS). Moreover, density functional theory (DFT) calculations supported by the experimental evidence revealed that the gradient S dopant concentration along the depth direction of TiO₂ nanorods led to the band gap grading from ∼2.3 to 2.7 eV. This S gradient doping introduced a terraced band structure via upshift of the valence band (VB), which provides channels for easy hole transport from the VB of S-doped TiO₂ to the VB of CZTS and thereby enhances the charge transport properties of the CZTS/S-TNR photoanode. This work demonstrates the rational design and fabrication of nanoarchitectures via band edge engineering to improve the PEC performance using simultaneous earth-abundant CZTS NP sensitization and S doping. This work also provides useful insight into the further development of different nanoarchitectures using similar combinations for energy-harvesting-related applications