Ternary Oxide CuWO4/BiVO4/FeCoOx Films for Photoelectrochemical Water Oxidation: Insights into the Electronic Structure and Interfacial Band Alignment.

Photoelectrochemical (PEC) water oxidation using ternary oxide systems has been considered a promising approach for investigating the effective utilization of sunlight and the production of green fuel. Herein, we report a ternary-oxide-based CuWO4/BiVO4/FeCoOx film deposited entirely by RF-magnetron sputtering using homemade ceramic targets. Our CuWO4/BiVO4 photoanode exhibits a significant photocurrent density of 0.82 mA cm-2 at 1.23 V vs RHE under AM 1.5G illumination, which is a record 382% increase compared to that of the bare CuWO4 film. To further boost the PEC performance, we deposited an ultrathin layer of amorphous FeCoOx cocatalyst, resulting in a triple CuWO4/BiVO4/FeCoOx heterojunction with a significant reduction in onset potential and a 500% increase in the photocurrent density of bare CuWO4. Experimental and theoretical approaches were used to provide insights into the interfacial band alignment and photoinduced charge carrier pathway across heterojunctions. Our results reveal noticeable interface potential barriers for charge carriers at the CuWO4/BiVO4 heterojunction, potentially limiting its application in tandem systems. Conversely, the deposition of the FeCoOx ultrathin layer over the CuWO4/BiVO4 heterojunction induces a p-n junction on the BiVO4/FeCoOx interface, which, when combined with the abundant FeCoOx oxygen vacancies, results in improved charge separation and transport as well as enhanced photoelectrochemical stability. Our study provides a feasible strategy for producing photocatalytic heterojunction systems and introduces simple tools for investigating interface effects on photoinduced charge carrier pathways for PEC water splitting.

Optimizing Active Sites for High CO Selectivity during CO2 Hydrogenation over Supported Nickel Catalysts.

Controlling the selectivity of CO2 hydrogenation catalysts is a fundamental challenge. In this study, the selectivity of supported Ni catalysts prepared by the traditional impregnation method was found to change after a first CO2 hydrogenation reaction cycle from 100 to 800 °C. The usually high CH4 formation was suppressed leading to full selectivity toward CO. This behavior was also observed after the catalyst was treated under methane or propane atmospheres at elevated temperatures. In situ spectroscopic studies revealed that the accumulation of carbon species on the catalyst surface at high temperatures leads to a nickel carbide-like phase. The catalyst regains its high selectivity to CH4 production after carbon depletion from the surface of the Ni particles by oxidation. However, the selectivity readily shifts back toward CO formation after exposing the catalysts to a new temperature-programmed CO2 hydrogenation cycle. The fraction of weakly adsorbed CO species increases on the carbide-like surface when compared to a clean nickel surface, explaining the higher selectivity to CO. This easy protocol of changing the surface of a common Ni catalyst to gain selectivity represents an important step for the commercial use of CO2 hydrogenation to CO processes toward high-added-value products.

In situ study of copper reduction in SrTi1-xCuxO3 nanoparticles.

Perovskite strontium titanate is a promising functional material for gas sensors and catalysis applications. Herein, we report the preparation of SrTi1-xCuxO3 nanoparticles with Cu doped in the B sites using a modified polymeric precursor method. This study describes in detail the structural and local atomic configurations for the substitution of Cu into the titanium sites and its reducibility using X-ray diffraction (XRD), field emission gun scanning and transmission electron microscopies (FEG-SEM and TEM), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR) analyses. Our results indicate that copper is segregated for x≥ 0.06. After exposing the samples to a hydrogen-rich atmosphere at temperatures over 500 K, copper is reduced from Cu(2+) to metallic Cu. This reduction was attributed to copper atoms that originated primarily from the CuO phase.

Structural, optical and photoelectrochemical characterizations of monoclinic Ta3N5 thin films.

Monoclinic Ta3N5 thin films were synthesized by thermal nitridation of amorphous Ta2O5 films directly sputtered by radio frequency magnetron sputtering. The samples were studied by high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-Vis-NIR spectrophotometry, rietveld refinements, spectroscopic ellipsometry and electrochemical techniques. The surface composition of Ta3N5 thin film was found to be different than the underlying film, affecting the optical properties of the material. Rietveld refinement has confirmed that the nitridation process results in Schottky and oxygen substitutional defects within the crystalline structure of monoclinic Ta3N5 thin film. The optical constants of the film were obtained by spectroscopic ellipsometry within a spectral range of 4.60-0.54 eV, i.e. 270-2300 nm. The suitable parameterization was found to consist of three Tauc-Lorentz and one Lorentz oscillators. The conduction band, valence band and the flat band positions were determined by photoelectrochemical techniques, presenting a strong dependence on pH of the eletrolyte. Improved photocurrent was obtained in alkaline conditions and attributed to the shorter depletion region width measured by Mott-Schottky and the lower recombination life time measured by open circuit potential decay analyses.

Easy Access to Metallic Copper Nanoparticles with High Activity and Stability for CO Oxidation.

Copper catalysts are very promising, affordable alternatives for noble metals in CO oxidation; however, the nature of the active species remains unclear and differs throughout previous reports. Here, we report the preparation of 8 nm copper nanoparticles (Cu NPs), with high metallic content, directly deposited onto the surface of silica nanopowders by magnetron sputtering deposition. The as-prepared Cu/SiO2 contains 85% Cu0 and 15% Cu2+ and was enriched in the Cu0 phase by H2 soft pretreatment (96% Cu0 and 4% Cu2+) or further oxidized after treatment with O2 (33% Cu0 and 67% Cu2+). These catalysts were studied in the catalytic oxidation of CO under dry and humid conditions. Higher activity was observed for the sample previously reduced with H2, suggesting that the presence of Cu-metal species enhances CO oxidation performance. Inversely, a poorer performance was observed for the sample previously oxidized with O2. The presence of water vapor caused only a small increase in the temperature require for the reaction to reach 100% conversion. Under dry conditions, the Cu NP catalyst was able to maintain full conversion for up to 45 h at 350 °C, but it deactivated with time on stream in the presence of water vapor.

Insights into the active surface species formed on Ta2O5 nanotubes in the catalytic oxidation of CO.

Freestanding Ta2O5 nanotubes were prepared by an anodizing method. As-anodized amorphous nanotubes were calcined at high temperature to obtain a crystalline phase. All materials were studied by means of BET analysis, XRD, TEM, SEM, XPS, and FTIR and were evaluated in the catalytic oxidation of CO. An XPS study confirmed the formation of different tantalum surface species after high temperature treatment of amorphous Ta2O5 nanotubes. Calcination at 800 °C generated Ta(4+) while higher temperature (1000 °C) treatment led to the formation of Ta(3+) species. These materials also showed significant differences in catalytic activity. Higher activity was observed for samples calcined at 800 °C than at 1000 °C, suggesting that Ta(4+) species are active sites for CO oxidation.

Photocatalytic hydrogen production of Co(OH)2 nanoparticle-coated α-Fe2O3 nanorings.

The production of hydrogen from water using only a catalyst and solar energy is one of the most challenging and promising outlets for the generation of clean and renewable energy. Semiconductor photocatalysts for solar hydrogen production by water photolysis must employ stable, non-toxic, abundant and inexpensive visible-light absorbers capable of harvesting light photons with adequate potential to reduce water. Here, we show that α-Fe2O3 can meet these requirements by means of using hydrothermally prepared nanorings. These iron oxide nanoring photocatalysts proved capable of producing hydrogen efficiently without application of an external bias. In addition, Co(OH)2 nanoparticles were shown to be efficient co-catalysts on the nanoring surface by improving the efficiency of hydrogen generation. Both nanoparticle-coated and uncoated nanorings displayed superior photocatalytic activity for hydrogen evolution when compared with TiO2 nanoparticles, showing themselves to be promising materials for water-splitting using only solar light.