Dynamic transformation between bilayer islands and dinuclear clusters of Cr oxide on Au(111) through environment and interface effects.

SignificanceFor oxide catalysts, it is important to elucidate and further control their atomic structures. In this work, well-defined CrO2 bilayer islands and Cr2O7 dinuclear clusters have been grown on Au(111) and unambiguously identified by scanning tunneling microscopy and theoretical calculations. Upon cycled redox treatments, the two kinds of oxide nanostructures can be reversibly transformed. It is interesting to note that both Cr oxides do not exist in bulk but need to be stabilized by the metal surface and the specific environment. Our results suggest that both redox atmosphere and interface confinement effects can be used to construct an oxide nanostructure with the specific chemical state and structure.

Mechanistic Insights into Enhanced Hydrogen Evolution of CrOx /Rh Nanoparticles for Photocatalytic Water Splitting.

The hydrogen evolution reaction (HER) of Rh nanoparticles (RhNP) coated with an ultrathin layer of Cr-oxides (CrOx ) was investigated as a model electrode for the Cr2 O3 /Rh-metal core-shell-type cocatalyst system for photocatalytic water splitting. The CrOx layer was electrodeposited over RhNP on a transparent conductive fluorine-doped tin oxide (FTO) substrate. The CrOx layer on RhNP facilitates the electron transfer process at the CrOx /RhNP interface, leading to the increased current density for the HER. Impedance spectroscopic analysis revealed that the CrOx layer transferred protons via the hopping mechanism to the RhNP surface for HER. In addition, CrOx restricted electron transfer from the FTO to the electrolyte and/or RhNP and suppressed the backward reaction by limiting oxygen migration. This study clarifies the crucial role of the ultrathin CrOx layer on nanoparticulate cocatalysts and provides a cocatalyst design strategy for realizing efficient photocatalytic water splitting.

Quantum dots decorated photoanodes in bioelectrochemical fuel cells: Enhanced electricity generation using green algae.

The power performance of the bio-electrochemical fuel cells (BEFCs) depends mainly on the energy harvesting ability of the anode material. The anode materials with low bandgap energy and high electrochemical stability are highly desirable in the BEFCs. To address this issue, a novel anode is designed using indium tin oxide (ITO) modified by chromium oxide quantum dots (CQDs). The CQDs were synthesized using facile and advanced pulsed laser ablation in liquid (PLAL) technique. The combination of ITO and CQDs improved the optical properties of the photoanode by exhibiting a broad range of absorption in the visible to UV region. A systematic study has been performed to optimize the amount of CQDs and green Algae (Alg) film grown using the drop casting method. Chlorophyll (a, b, and total) content of algal cultures (with different concentrations) were optimized to investigate the power generation performance of each cell. The BEFC cell (ITO/Alg10/Cr3//Carbon) with optimized amounts of Alg and CQDs demonstrated enhanced photocurrent generation of 120 mA cm-2 at a photo-generated potential of 24.6 V m-2 . The same device exhibited a maximum power density of 7 W m-2 under continuous light illumination. The device also maintained 98% of its initial performance after 30 repeated cycles of light on-off measurements.

Unmasking the Resolution-Throughput Tradespace of Focused-Ion-Beam Machining.

Focused-ion-beam machining is a powerful process to fabricate complex nanostructures, often through a sacrificial mask that enables milling beyond the resolution limit of the ion beam. However, current understanding of this super-resolution effect is empirical in the spatial domain and nonexistent in the temporal domain. This article reports the primary study of this fundamental tradespace of resolution and throughput. Chromia functions well as a masking material due to its smooth, uniform, and amorphous structure. An efficient method of in-line metrology enables characterization of ion-beam focus by scanning electron microscopy. Fabrication and characterization of complex test structures through chromia and into silica probe the response of the bilayer to a focused beam of gallium cations, demonstrating super-resolution factors of up to 6 ± 2 and improvements to volume throughput of at least factors of 42 ± 2, with uncertainties denoting 95% coverage intervals. Tractable theory models the essential aspects of the super-resolution effect for various nanostructures. Application of the new tradespace increases the volume throughput of machining Fresnel lenses by a factor of 75, enabling the introduction of projection standards for optical microscopy. These results enable paradigm shifts of sacrificial masking from empirical to engineering design and from prototyping to manufacturing.

Diurnal variations and time to reach steady state of external markers used to estimate fecal excretion in sheep.

We executed two studies to investigate time to reach steady state (EXP1) and diurnal variations (EXP 2) of markers, in order to recommend spot fecal sampling in sheep. Eight lambs were used in EXP 1. Each animal has received titanium dioxide (TDOX) and chromium oxide (COX) together during two periods of 15 days. Thirty sheep of the Santa Ines breed were used in EXP 2. Fecal samples were taken with 2-h intervals in the last 5 days of each period, simultaneously to total feces collection. A sine-cosine model was used to evaluate EXP 2, and broken-line model for EXP 1. TDOX could complete its recovery after 2.3 days, and it could accurately promote fecal estimates after 3 days of marker infusion. COX could stabilize after 4.1 days, and it could promote fecal estimates after 5 days. However, estimated fecal excretion became similar to total feces output after 3 days for TDOX and after 6 days for COX. For both markers, a total 6 days of adaptation period could be summed to a 3-day collection period. Spot sampling was similar to total fecal output at 6:00 to 8:00 on morning and between 5:00 and 7:00 on afternoon for TDOX. Also, COX could be sampled at 6:00 to 8:00 on morning and between 5:00 and 7:00 on afternoon. External markers could be evaluated from 3-day fecal spot collection. Fecal collections could be made before the morning feeding, and around 6 p.m. for both markers.

Chromium Oxide Tetrafluoride and Its Reactions with Xenon Hexafluoride; the [XeF5 ]+ and [Xe2 F11 ]+ Salts of the [CrVI OF5 ]- , [CrV OF5 ]2- , [CrV 2 O2 F8 ]2- , and [CrIV F6 ]2- Anions.

Molten mixtures of XeF6 and CrVI OF4 react by means of F2 elimination to form [XeF5 ][Xe2 F11 ][CrV OF5 ]⋅2 CrVI OF4 , [XeF5 ]2 [CrIV F6 ]⋅2 CrVI OF4 , [Xe2 F11 ]2 [CrIV F6 ], and [XeF5 ]2 [CrV 2 O2 F8 ], whereas their reactions in anhydrous hydrogen fluoride (aHF) and CFCl3 /aHF yield [XeF5 ]2 [CrV 2 O2 F8 ]⋅2 HF and [XeF5 ]2 [CrV 2 O2 F8 ]⋅2 XeOF4 . Other than [Xe2 F11 ][MVI OF5 ] and [XeF5 ][MVI 2 O2 F9 ] (M=Mo or W), these salts are the only Group 6 oxyfluoro-anions known to stabilize noble-gas cations. Their reaction pathways involve redox transformations that give [XeF5 ]+ and/or [Xe2 F11 ]+ salts of the known [CrV OF5 ]2- and [CrIV F6 ]2- anions, and the novel [CrV 2 O2 F8 ]2- anion. A low-temperature Raman spectroscopic study of an equimolar mixture of solid XeF6 and CrOF4 revealed that [Xe2 F11 ][CrVI OF5 ] is formed as a reaction intermediate. The salts were structurally characterized by LT single-crystal X-ray diffraction and LT Raman spectroscopy, and provide the first structural characterizations of the [CrV OF5 ]2- and [CrV 2 O2 F8 ]2- anions, where [CrV 2 O2 F8 ]2- represents a new structural motif among the known oxyfluoro-anions of Group 6. The X-ray structures show that [XeF5 ]+ and [Xe2 F11 ]+ form ion pairs with their respective anions by means of Xe- - -F-Cr bridges. Quantum-chemical calculations were carried out to obtain the energy-minimized, gas-phase geometries and the vibrational frequencies of the anions and their ion pairs and to aid in the assignments of their Raman spectra.

Hepatotoxicity and chromosomal abnormalities evaluation due to single and repeated oral exposures of chromium oxide nanoparticles in Wistar rats.

Metal oxide nanoparticles (NPs) have widespread uses ranging from nanoelectronics to nanotherapeutics. Because of their expanding industrial applications, a better understanding of their toxicity is needed. So far, limited reports are available on chromium oxide NPs (Cr2O3 NPs) toxicity. In this work, Cr2O3 NPs were synthesized and characterized in a sequential manner using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and transmission electron microscopy. Dose- and time-dependent toxicity assessment of Cr2O3 NPs was carried out in Wistar rats by examining liver function biomarkers, tissue histopathology, micronuclei (MN) formation, and chromosomal aberrations (CAs) in bone marrow along with sperm abnormalities. The results of this study demonstrated typical XRD and FTIR patterns of Cr2O3 NPs with a size of approximately 23.47 nm. Animals exposed to Cr2O3 NPs, exhibited a significant increase in aspartate transaminase, alanine transaminase, alkaline phosphatase, gamma glutamyltransferase, and total bilirubin, signifying liver injury. Histopathology data also supported the marked alterations in the liver biochemistry of NPs-exposed animals. Further, an increase in the frequency of MN, CA, and sperm abnormalities suggested Cr2O3 NPs-mediated genotoxicity. It is, therefore, suggested that possible safety issues of Cr2O3 NPs should be addressed promptly with limited future use in occupational settings.

An efficient utilization of chromium-containing vanadium tailings: Extraction of chromium by soda roasting-water leaching and preparation of chromium oxide.

Chromium-containing vanadium tailings (CCVT), an industrial waste, were utilized to extract chromium efficiently by soda roasting-water leaching process and for the preparation of highly pure chromium oxide. The effect of extraction of chromium under different roasting and leaching conditions were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The maximum chromium extraction rate of 91.51% was obtained when soda (Na2CO3) and CCVT were mixed in a molar ratio (n (Na2CO3)/n (Cr2O3)) of 8, roasted at 900 °C and maintained for 120 min. Then, the roasted product was leached in water at 60 °C for 60 min with a liquid-solid mass ratio (L/S) of 10. During soda roasting, the chromium-containing phase (Fe0.6Cr0.4)2O3 combines with Na2CO3 to form Na2CrO4, which was then transferred into the leaching liquid, post water leaching. The by-products such as NaFeTiO4, Na2CaSiO4, and Na0.68Fe0.68Si0.32O2 were left in the leaching residue which was called chromium tailings (CT). 87.40% chromium oxide was recovered from the unpurified leaching liquid after reduction and precipitation by adding Na2S, followed by roasting the deposit. This process not only relieved the potential threat of the industrial waste CCVT to the environment but also realized the recovery of the valuable element chromium.

Chromium oxide nanoparticle-induced biochemical and histopathological alterations in the kidneys and brain of Wistar rats.

Chromium oxide nanoparticles (Cr2O3 NPs) have a wide range of applications in industry. They are used as pigments, catalysts, wear-resistant or high-temperature-resistant coating material and are used in liquid crystal displays. In view of ever escalating use of NPs, risk assessment becomes obligatory to ensure the safety of both human health and the ecosystem. The present study was designed and conducted to evaluate biochemical changes and histopathological alterations in kidneys and brain of rats, following exposure to Cr2O3 NPs. Male Wistar rats were divided into low-dose (50 µg/100 g body weight (bwt) groups and high-dose (200 µg/100 g bwt) groups. Each group type received oral administration of Cr2O3 NPs for multiple durations (single dosing, once daily for 7 days and once daily for 14 days, respectively). According to our data, this allotment presented a meaningful picture of NPs behaviour in different scenarios. In the kidneys and brain of Cr2O3 NPs-exposed animals, reactive oxygen species (ROS) production caused a significant increase in malondialdehyde (MDA) concentration along with a significant decrease in superoxide dismutase and glutathione levels, as compared to controls. Histopathological changes in these organs confirmed cellular injury and functional damage due to exposure to Cr2O3 NPs. In this study, we have distinguished pathological alterations consequent to deleterious oxidative stress due to enhanced ROS generation after Cr2O3 NPs exposure.

Effect of Nitrogen and Fluorine Co-substitution on the Structure and Magnetic Properties of Cr2 O3.

First-principles density functional calculations were carried out to determine the structure as well as electronic and magnetic properties of N and F co-substituted Cr2 O3 . The formation of strong CrN bonds upon substitution of oxygen with nitrogen leads to large distortions in the local structure and changes in magnetic moments, which are partly compensated by co-substitution with fluorine. The effects of spin-orbit coupling are relatively weak, but its combination with local structural distortions gives rise to canting of spins and an overall magnetic moment in N, F co-substituted Cr2 O3 . Experimentally, we observe spin canting in N, F co-substituted Cr2 O3 with considerable enhancement in the coercive field at low temperatures.

Chromium oxide nanoparticle-induced genotoxicity and p53-dependent apoptosis in human lung alveolar cells.

Chromium oxide (Cr2 O3 ) nanoparticles (NPs) are being increasingly used as a catalyst for aromatic compound manufacture, abrading agents and as pigments (e.g., Viridian). Owing to increased applications, it is important to study the biological effects of Cr2 O3 NPs on human health. The lung is one of the main exposure routes to nanomaterials; therefore, the present study was designed to determine the genotoxic and apoptotic effect of Cr2 O3 NPs in human lung epithelial cells (A549). The study also elucidated the molecular mechanism of its toxicity. Cr2 O3 NPs led to DNA damage, which was deduced by comet assay and cytokinesis block micronucleus assay. The damage could be mediated by the increased levels of reactive oxygen species. Further, the oxygen species led to a decrease in mitochondrial membrane potential and an increase in the ratio of BAX/Bcl-2 leading to mitochondria-mediated apoptosis induced by Cr2 O3 NPs, which ultimately leads to cell death. Hence, there is a need of regulations to be imposed in NP usage. The study provided insight into the caspase-dependent mechanistic pathway of apoptosis.

Blending Cr2O3 into a NiO-Ni electrocatalyst for sustained water splitting.

The rising H2 economy demands active and durable electrocatalysts based on low-cost, earth-abundant materials for water electrolysis/photolysis. Here we report nanoscale Ni metal cores over-coated by a Cr2 O3 -blended NiO layer synthesized on metallic foam substrates. The Ni@NiO/Cr2 O3 triphase material exhibits superior activity and stability similar to Pt for the hydrogen-evolution reaction in basic solutions. The chemically stable Cr2 O3 is crucial for preventing oxidation of the Ni core, maintaining abundant NiO/Ni interfaces as catalytically active sites in the heterostructure and thus imparting high stability to the hydrogen-evolution catalyst. The highly active and stable electrocatalyst enables an alkaline electrolyzer operating at 20 mA cm(-2) at a voltage lower than 1.5 V, lasting longer than 3 weeks without decay. The non-precious metal catalysts afford a high efficiency of about 15 % for light-driven water splitting using GaAs solar cells.

A study on the growth of Cr2O3 in ordered mesoporous silica and its replication.

A systematic study on the growth of Cr2O3 in three-dimensional cubic ordered mesoporous silica (KIT-6) and its replication through nanocasting is reported. By changing the loading time and amount of precursor, the size and shape of the obtained replica could be controlled to some extent. More interestingly, in contrast to previously published studies, when KIT-6 with an aging temperature of 100 °C, which has a high degree of interconnectivity, was used as a hard template, a cubic ordered mesoporous Cr2O3 replica with an open uncoupled subframework structure and reduced symmetry was obtained. Formation of a replica with different symmetry and uncoupled subframework structure is not only related to the degree of interconnectivity of the parent, but also strongly depends on the type of metal oxide and its growth mechanism in the silica template. Nanocasting of Cr2O3 with a low loading results in a replica with monomodal pore size distribution that has same symmetry as the hard template, whereas increasing the loading amount alters the symmetry of the replica and yields a replica with bimodal distribution.

Understanding the Activation Mechanism of RhCrOx Cocatalysts for Hydrogen Evolution with Nanoparticulate Electrodes.

Mixed oxides of Rh-Cr (RhCrOx), containing Rh3+ and Cr3+ cations, are commonly used as cocatalysts for the hydrogen evolution reaction (HER) on particulate photocatalysts. The precise physicochemical mechanisms of the HER at the catalytic sites of these oxides are not well understood. In this study, model cocatalyst electrodes, composed of nanoparticulate RhCrOx, were fabricated to investigate the physicochemical mechanisms of the HER. Electroanalytical and X-ray photoelectron spectroscopic measurements revealed that nanoparticulate RhCrOx produces reduced Rh (Rh0) species by maintaining an electrode potential more negative than 0.03 V versus the reversible hydrogen electrode (VRHE). This results in significant enhancement of the HER activity. The catalytic activity for the HER stems from the reduced Rh species, and the inclusion of Cr3+ (CrOx) aided in the electron transfer process at the solid/liquid interface, resulting in a higher current density during the HER. To achieve a solar-to-hydrogen efficiency of over 3%, the conduction band minimum of the particulate photocatalyst should be positioned more negatively than -0.10 VRHE. Moreover, the formation of electron trap states at potentials more positive than 0.03 VRHE should be avoided. This study highlights the importance of understanding the catalytic sites on metal oxide cocatalysts. Moreover, it offers a design strategy for enhancing the efficiency of photocatalytic water splitting.

Electrical properties of strained off-stoichiometric Cu-Cr-O delafossite thin films.

Off-stoichiometric Cu-Cr-O delafossite thin films with different thicknesses were grown by metal organic chemical vapor deposition on substrates with different coefficients of thermal expansion. Seebeck thermoelectric coefficient and resistivity measurements were performed on the range of 300-850 K. A qualitative change in the temperature-dependence of the resistivity is observed at the temperature corresponding to the deposition process, where the transition from tensile to compressive strain takes place. Arrhenius plots reveal different slopes in these two thermal ranges. The fact that the shift is more pronounced for the thinner films might indicate the induced strain plays a role in changing electrical behaviour. Furthermore, changes below 0.1% in electrical mobility were measured when the strain is induced by mechanical bending.

Green synthesis, characterization and applications of Phyllanthus emblica fruit extract mediated chromium oxide nanoparticles.

The green synthesis of metallic nanoparticles is attributable towards diverse applications in various fields, recently. In this research, we report simple and eco-friendly synthesis of chromium oxide (Cr2O3) nanoparticles using the fruit extract of Phyllanthus emblica as a reducing and capping agent. The absorbance peaks at 350 nm and 450 nm validated the nanoparticle formation in UV-visible spectrum. FTIR spectrum revealed the nature of functional groups. The crystalline properties of nanoparticles were ascertained by XRD analysis. EDX spectrum corroborated the elemental composition of nanoparticles in which chromium and oxygen constituted 68% of total weight. SEM images demonstrated agglomeration of nanoparticles resulting in the formation of large irregularly shaped flakes. Cr2O3 nanoparticles demonstrated excellent antimicrobial properties against 11 bacterial isolates and 1 fungal isolate. The largest inhibition zone (53 mm) was measured against A. baumannii while the smallest inhibition zone (26 mm) was recorded against S. aureus. Minimum inhibitory concentration (MIC) values were < 1 µg/ml for all microbes. However, the synthesized nanoparticles did not reveal synergism with any of the selected antibiotics (FICI values > 1). Nanoparticles possessed potent anti-biofilm powers with maximum (77%) inhibition of E. coli biofilms and minimum (45%) inhibition of S. enterica biofilms. Photocatalytic activity of Cr2O3 nanoparticles was evaluated to determine their efficacy in environmental bioremediation. Outcomes demonstrated degradation of methyl red (84%) but not of methylene blue dye. Furthermore, the Cr2O3 nanoparticles displayed considerable antioxidant (43%) as well as anti-inflammatory (44%) potentials. Hence, the present study accounts for the versatile applications of P. emblica-mediated Cr2O3 nanoparticles which could be pursued for future biomedical and environmental applications.

Corrosion Resistance in Artificial Perspiration of Cr-Based Decorative Coatings.

We aim at developing hexavalent chromium-free coatings for frequently touched decorative parts. Cr(N,O) and multilayered CrN/CrO coatings were deposited by means of reactive magnetron sputtering. All samples presented good adhesion to the substrates enhanced by an epoxy layer designed to enhance PVD coating adhesion. Similar substrates are found in the automotive industry and can be used in appliances where a metallic finish is desired by the consumer. Corrosion behavior was induced, using artificial sweat to simulate long exposure to human touch for 96 h. In potentiodynamic polarization tests, the coatings were revealed to be nobler than the substrate alone. Cr displayed a non-existent passivation region, while gCrN exhibited a quick passivation of the surface and its respective breakdown and several current fluctuations, indicating the occurrence of pitting, which was confirmed by SEM micrography after the corrosion. Regarding EIS results, all films depicted a diminution of impedance modulus (|Z|) after 96 h, which indicates a diminution of corrosion resistance against artificial sweat. Nitride films exhibited the worst anticorrosive features. On the other hand, Cr and CrO exhibited the highest |Z| values. These results are corroborated by low the corrosion rates of both coatings. The equivalent electrical circuit allows us to confirm oxide formation in the outermost layer of the films due to electrolyte/surface interaction, indicating a self-protecting mechanism. Nitride films showed the lowest values and less corrosion resistance, confirming the results obtained in polarization potentiodynamic tests. The coatings developed in this work, namely Cr and CrO, showed a promising corrosion resistance behavior that could endure a lifetime of frequent human touch in various decorative applications either automotive or general appliances.

Caecal digestibility as an approximation of ileal protein digestibility evaluation in rats.

The rat model can be used to assess ileal protein digestibility rapidly and in first intention, but no standardised method exists. Our objective was to compare methods to assess protein digestibility, depending on collection site (ileum/caecum) and use of a non-absorbable marker. A meal containing either casein, gluten or pea protein and chromium oxide as non-absorbable marker was given to male Wistar rats and the entire digestive content was collected 6 h later. Total chromium recovery was incomplete and variable, depending on protein source. We observed no significant difference in digestibility between the methods for any of the protein sources tested. Although none of the methods tested is optimal, our results suggest that caecal digestibility can be used as a proxy of ileal digestibility in rats without using a non-absorbable marker. This simple method makes it possible to evaluate protein digestibility of new alternative protein sources for human consumption.

Amorphous chromium oxide confined Ni/NiO nanoparticles-assembled nanosheets for highly efficient and stable overall urea splitting.

Applications of urea oxidation reaction (UOR) in various sustainable energy-conversion systems are greatly hindered by its slow kinetics. Herein, we demonstrate an in-situ confined synthesis method that produces amorphous chromium oxide confined Ni/NiO nanoparticles-assembled nanosheets (Ni/NiO@CrOx) with fast reaction kinetics towards UOR. The confinement effect of the in-situ generated CrOx overlay contributes to ultrafine Ni/NiO nanoparticles, bringing about rich Ni/NiO and NiO/CrOx interfaces. In-situ Raman and electrochemical characterization show that both CrOx and metallic Ni can promote the formation of the NiOOH species and the electron transfer, leading to high intrinsic activity and fast reaction kinetics. At 1.40 V vs. reversible hydrogen electrode, the Ni/NiO@CrOx delivers a current density of 275 mA cm-2, which is about 2.6 and 6.1 times as large as those of the NiO@CrOx and NiO, respectively. In addition, the protective effect of the CrOx overlay leads to robust working stability towards UOR. Further, the Ni/NiO@CrOx nanosheets are used as bifunctional catalysts for overall urea splitting, and a small electrolysis cell voltage of 1.44 V is needed to reach the benchmark current density of 10 mA cm-2.

Insights into the Fe3+ Doping Effects on the Structure and Electron Distribution of Cr2O3 Nanoparticles.

Herein, we carefully investigated the Fe3+ doping effects on the structure and electron distribution of Cr2O3 nanoparticles using X-ray diffraction analysis (XRD), maximum entropy method (MEM), and density functional theory (DFT) calculations. We showed that increasing the Fe doping induces an enlargement in the axial ratio of c/a, which is associated with an anisotropic expansion of the unit cell. We found that as Fe3+ replaces Cr in the Cr2O3 lattice, it caused a higher interaction between the metal 3d states and the oxygen 2p states, which led to a slight increase in the Cr/Fe-O1 bond length followed by an opposite effect for the Cr/Fe-O2 bonds. Our results also suggest that the excitations characterize a well-localized bandgap region from occupied Cr d to unoccupied Fe d states. The Cr2O3 and Fe-doped Cr2O3 nanoparticles behave as Mott-Hubbard insulators due to their band gap being in the d-d gap, and Cr 3d orbitals dominate the conduction band. These findings suggest that the magnitude and the character of the electronic density near the O atom bonds in Cr2O3 nanoparticles are modulated by the Cr-Cr distances until its stabilization at the induced quasi-equilibrium of the Cr2O3 lattice when the Fe3+ doping values reaches the saturation level range.