High-performance photocatalytic reduction of Cr(VI) using a retrievable Fe-doped WO3/SiO2 heterostructure

The enhancement of the photocatalytic performance of pristine WO3 was systematically adjusted due to its fast recombination rate and low reduction potential. A designed heterostructure photocatalyst was necessarily synthesised by Fe3+ metal ions doping into WO3 structure with and composition modification. In this study, we synthesised a retrievable Fe-doped WO3/SiO2 heterostructure using a surfactant-assisted hydrothermal method. This heterostructure was then employed as an effective photocatalyst for the removal of Cr(VI) under visible light irradiation. Enlarged photocatalytic reduction was observed over a synergetic 7.5 mol% Fe-doped WO3/SiO2-20 nanocomposite, resulting in dramatically increased activity compared with undoped WO3 and SiO2 nanomaterials under visible light illumination within 90 min. The presence of 7.5 mol% Fe3+ ion dopant in WO3 optimised electron-hole recombination, consequently reducing WO3 photocorrosion. After adding SiO2 nanoparticles, the binary WO3-SiO2 nanocomposite played roles as both adsorbent and photocatalyst to increase specific surface area. Thus, the 7.5 mol% Fe-doped WO3/SiO2-20 nanocomposite catalyst had more active sites on the surface of catalyst, and enhanced photocatalytic reduction was significantly achieved. The results showed 91.1% photocatalytic reduction over the optimum photocatalyst, with a photoreduction kinetic rate of 21.1 × 10-3 min-1, which was approximately four times faster than pristine WO3. Therefore, the superior optimal photocatalyst demonstrated reusability, with activities decreasing by only 9.8% after five cycles. The high photocatalytic performance and excellent stability of our photocatalyst indicate great potential for water pollution treatments.

Z-Scheme Photocarrier Transfer Realized in Tungsten Oxide-Based Photocatalysts by Combining with Bismuth Vanadate Quantum Dots

Multicomponent photocatalysts with a Z-scheme charge transfer are promising in converting solar to hydrogen fuel because of their significantly improved light absorption and restrained photocarrier recombination while keeping their redox capacity. In this work, a composite photocatalyst of BiVO₄ quantum dot-decorated WO₃ nanosheet arrays was synthesized and investigated. The existence of the Z-scheme charge transfer behavior was confirmed by the redox probe technique. Such a Z-scheme charge transfer makes it possible to generate hydrogen without bias. An optimized photocatalyst produces a hydrogen generation rate of 0.75 μmol/h without bias and a photocurrent of 1.91 mA/cm² at 1.23 V versus RHE, which is about 70% higher than that of pure WO₃. We attributed these improvements to the enhanced light absorption, extended conduction band level of BiVO₄, as well as the unique charge transfer behavior in the Z-scheme structure. This work presents a generalizable method to improve the redox capacity of a variety of semiconductors through rationally selecting the building material blocks in view of energy levels.

Photochromic Behavior of ZnO/MoO3 Interfaces

ZnO/MoO₃ powder mixture exhibits a huge photochromic effect in comparison with the corresponding single oxides. The coloring efficiency of such combined material after UV-light irradiation was studied in terms of intensity, kinetics, and ZnO/MoO₃ powder ratio. Additionally, the incidence of the pretreatment step of the ZnO and MoO₃ powders under different atmospheres (air, Ar or Ar/H₂ flow) was analyzed. The huge photochromic effect discovered herein was interpreted as the creation of "self-closed Schottky barrier" at the solid/solid interfaces between the two oxides, associated with the full redox reaction which can be pictured by the equation ZnO_1-ε + MoO₃ → ZnO + MoO_3-ε. Remarkable optical contrast between virgin and color states as well as self-bleaching in dark allowing the reversibility of the photochromism is emphasized. From this first discovery, deeper characterization of the self-bleaching process shows that the photochromic mechanism is complex with a bleaching efficiency (possibility to come back to the virgin material optical properties without any deterioration) and a bleaching kinetics, which are both dependent on the coloring irradiation time. This demonstrates that the oxygen exchange through the Schottky interface proceeds in at least two convoluted steps: an anionic surface exchange allowing a reversibility of the redox reaction followed by bulk diffusion of the exchanged anions which are then definitively trapped. An emergent "negative photochromism effect" (i.e., photochromism associated with a self-bleaching instead of a darkening under irradiation) is observed after a long irradiation time.

ZnO-embedded S-doped g-C3N4 heterojunction: mediator-free Z-scheme mechanism for enhanced charge separation and photocatalytic degradation

The design of UV-visible light active photocatalysts for organic pollutant removal is a challenging task. Herein, we have developed an LED light active ZnO-embedded S-doped g-C₃N₄ (SCN) heterojunction by a facile sol–gel assisted calcination method. The heterojunction between ZnO and SCN nanoparticles generates a Z-scheme photocatalyst, which helps to separate the photo-induced charge carriers in the opposite direction, and is beneficial for more visible light absorption for photocatalytic dye degradation. The composite heterojunction shows better photocatalytic redox in comparison with pristine nanomaterials. The enhanced degradation efficiency is attributed to the high production rate of ˙OH (hydroxyl) radicals during the photocatalysis process, which is analyzed by the TA test and elemental trapping experiment. Hence, we hope that this Z-scheme heterojunction provides a new way to develop UV-visible light active photocatalysts for environmental remediation applications.

We have developed the LED light active ZnO-embedded S-doped g-C₃N₄ (SCN) mediator free Z-scheme heterojunction photocatalyst for effective organic pollutant degradation.

Efficient tri-metallic oxides NiCo2O4/CuO for the oxygen evolution reaction

In this study, a simple approach was used to produce nonprecious, earth abundant, stable and environmentally friendly NiCo2O4/CuO composites for the oxygen evolution reaction (OER) in alkaline media. The nanocomposites were prepared by a low temperature aqueous chemical growth method. The morphology of the nanostructures was changed from nanowires to porous structures with the addition of CuO. The NiCo2O4/CuO composite was loaded onto a glassy carbon electrode by the drop casting method. The addition of CuO into NiCo2O4 led to reduction in the onset potential of the OER. Among the composites, 0.5 grams of CuO anchored with NiCo2O4 (sample 2) demonstrated a low onset potential of 1.46 V vs. a reversible hydrogen electrode (RHE). A current density of 10 mA cm-2 was achieved at an over-potential of 230 mV and sample 2 was found to be durable for 35 hours in alkaline media. Electrochemical impedance spectroscopy (EIS) indicated a small charge transfer resistance of 77.46 ohms for sample 2, which further strengthened the OER polarization curves and indicates the favorable OER kinetics. All of the obtained results could encourage the application of sample 2 in water splitting batteries and other energy related applications.

An advanced and efficient Co3O4/C nanocomposite for the oxygen evolution reaction in alkaline media

The design of efficient nonprecious catalysts for the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER) is a necessary, but very challenging task to uplift the water-based economy. In this study, we developed a facile approach to produce porous carbon from the dehydration of sucrose and use it for the preparation of nanocomposites with cobalt oxide (Co3O4). The nanocomposites were studied by the powder X-ray diffraction and scanning electron microscopy techniques, and they exhibited the cubic phase of cobalt oxide and porous structure of carbon. The nanocomposites showed significant OER activity in alkaline media, and the current densities of 10 and 20 mA cm-2 could be obtained at 1.49 and 1.51 V versus reversible hydrogen electrode (RHE), respectively. The impedance study confirms favorable OER activity on the surface of the prepared nanocomposites. The nanocomposite is cost-effective and can be capitalized in various energy storage technologies.

Conformal BiVO4-Layer/WO3-Nanoplate-Array Heterojunction Photoanode Modified with Cobalt Phosphate Cocatalyst for Significantly Enhanced Photoelectrochemical Performances

Constructing semiconductor heterojunctions via surface/interface engineering is an effective way to enhance the charge carrier separation/transport ability and thus the photoelectrochemical (PEC) properties of a photoelectrode. Herein, we report a conformal BiVO₄-layer/WO₃-nanoplate-array heterojunction photoanode modified with cobalt phosphate (Co-Pi) as oxygen evolution cocatalyst (OEC) for significant enhancement in PEC performances. The BiVO₄/WO₃ nanocomposite is fabricated by coating a thin conformal BiVO₄ layer on the surface of presynthesized WO₃ nanoplate arrays (NPAs) via stepwise spin-coating, and the decoration of Co-Pi OEC is realized by photoassisted electrodeposition method. The optimized Co-Pi@BiVO₄/WO₃ heterojunction photoanode shows a maximum photocurrent of 1.8 mA/cm² at 1.23 V vs RHE in a phosphate buffer electrolyte under an AM1.5G solar simulator, which is 5 and 12 times higher than those of bare WO₃ and BiVO₄ photoanode, respectively. Measurements of UV-vis absorption spectra, electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra reveal that the enhanced PEC performances can be attributed to the increased charge carrier separation/transport benefited from the type II nature of BiVO₄/WO₃ heterojunction and the promoted water oxidation kinetics and photostability owing to the decoration of Co-Pi cocatalyst.

The chemically reduced CuO–Co3O4 composite as a highly efficient electrocatalyst for oxygen evolution reaction in alkaline media

The fabrication of efficient, alkaline-stable and nonprecious electrocatalysts for the oxygen evolution reaction is highly needed; however, it is a challenging task.

WO3-decorated ZnO nanostructures for light-activated applications

Calyx-like ZnO nanostructures decorated by WO₃ nanoparticles are investigated for NO_X degradation and self-cleaning end-uses.

In situ synthesis and photocatalytic performance of WO3/ZnWO4 composite powders

The WO₃/ZnWO₄ composite powders were synthesized through an in situ reaction process with tunnel structure K₁₀W₁₂O₄₁·11H₂O filiform crystallites used as a precursor.

Photoresponsive and wetting performances of sheet-like nanostructures of tungsten trioxide thin films grown on wood surfaces

Tungsten trioxide films with sheet-like nanostructures coated on wood substrates possessed photoresponsive behavior and superhydrophobic performance after OTS treatment.

Preparation and enhanced photoelectrochemical performance of a p–n heterojunction CuFe2O4/WO3 nanocomposite film

CuFe₂O₄ was loaded on the surface of a WO₃ film to form a p–n heterojunction photoanode with a better performance than WO₃.