Oxidation state of Mo affects dissolution and visible-light photocatalytic activity of MoO3 nanostructures

Influence of Proteins and Building Direction on the Corrosion and Tribocorrosion of CoCrMo Fabricated by Laser Powder Bed Fusion

Cobalt-chromium-molybdenum (CoCrMo) alloys are common wear-exposed biomedical alloys and are manufactured in multiple ways, increasingly using additive manufacturing processes such as laser powder bed fusion (LPBF). Here, we investigate the effect of proteins and the manufacturing process (wrought vs LPBF) and building orientation (LPBF-XY and XZ) on the corrosion, metal release, tribocorrosion, and surface oxide composition by means of electrochemical, mechanical, microscopic, diffractive, and spectroscopic methods. The study was conducted at pH 7.3 in 5 g/L NaCl and 5 mM 2-(N-morpholino) ethanesulfonic acid (MES) buffer, which was found to be necessary to avoid metal phosphate and metal-protein aggregate precipitation. The effect of 10 g/L bovine serum albumin (BSA) and 2.5 g/L fibrinogen (Fbn) was studied. BSA and Fbn strongly enhanced the release of Co, Cr, and Mo and slightly enhanced the corrosion (still in the passive domain) for all CoCrMo alloys and most for LPBF-XZ, followed by LPBF-XY and the wrought CoCrMo. BSA and Fbn, most pronounced when combined, significantly decreased the coefficient of friction due to lubrication, the wear track width and severity of the wear mechanism, and the tribocorrosion for all alloys, with no clear effect of the manufacturing type. The wear track area was significantly more oxidized than the area outside of the wear track. In the reference solution without proteins, a strong Mo oxidation in the wear track surface oxide was indicative of a pH decrease and cell separation of the anodic and cathodic areas. This effect was absent in the presence of the proteins.

Peculiarities of bioaccumulation and toxic effects produced by nanoparticles of molybdenum (VI) oxide under multiple oral exposure of rats: examination and comparative assessment

INTRODUCTION

Molybdenum (VI) oxide nanoparticles (MoO3 NPs) are widely used in various economic activities. This creates elevated risks of exposure to this nanomaterial for workers and population in general and, consequently, there can be an increased number of developing pathological changes caused by exposure to MoO3 NPs.

OBJECTIVE

To examine and comparatively assess peculiarities of bioaccumulation and toxic effects produced by MoO NPs under multiple oral introductions.

METHODS

We evaluated sizes of analyzed particles by scanning electronic microscopy; specific surface area was calculated by the method of Brunauer, Emmett and Taylor; the total pore volume, by Barrett, Joyner and Halenda. Rats were exposed as per the scheme introduced by Lim with colleagues. We examined biochemical and hematological blood indicators, molybdenum concentrations and pathomorphological changes in tissues of various organs 24 hours after the last exposure. The study involved comparison with effects produced by MoO3 microparticles.

RESULTS

The tested MoO3 sample was established to be a nanomaterial as per the whole set of its physical properties. 50% of animals in the exposed group died on the 16th day in the experiment after the total exposure dose of MoO3 NPs reached 6500 mg/kg of body weight. Having analyzed blood plasma, we determined the following. There was a growth in quantity of leukocytes and a share of segmented neutrophils and monocytes, which were by 1.76-3.50 times higher than in the control group. Activity of alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, gamma-glutamyl transpeptidase, alpha-amylase, and lactate dehydrogenase, and concentrations of urea, crude and direct bilirubin were higher by 1.61-22.86 times. Decrease in the number of platelets, plateletcrit, the relative number of lymphocytes, the number and proportion of large platelets by 1.31-2.71 times. We detected elevated molybdenum concentrations in the lungs, heart, liver, kidneys, brain and blood under exposure to MoO3 NPs in an amount exceeding the control values by 12.10-361.75 times. Rats exposed to MoO3 NPs had liver parenchymal steatosis, inflammatory changes, hemorrhagic infarctions and hyperplasia in the lungs.

CONCLUSION

MoO3 NPs have a more apparent ability to bioaccumulate and produce toxic effects in comparison with their microdispersed analogue under multiple oral introductions into the body.

Oxidation Protection of High-Temperature Oxidation-Resistant Coatings on the Surface of Mo-Based Alloys—A Review

Molybdenum and its alloys, with high melting points, excellent corrosion resistance and high temperature creep resistance, are a vital high-temperature structural material. However, the poor oxidation resistance at high temperatures is a major barrier to their application. This work provides a summary of surface modification techniques for Mo and its alloys under high-temperature aerobic conditions of nearly half a century, including slurry sintering technology, plasma spraying technology, chemical vapor deposition technology, and liquid phase deposition technology. The microstructure and oxidation behavior of various coatings were analyzed. The advantages and disadvantages of various processes were compared, and the key measures to improve oxidation resistance of coatings were also outlined. The future research direction in this field is set out.

Microstructure and Oxidation Behavior of Anti-Oxidation Coatings on Mo-Based Alloys through HAPC Process: A Review

Mo and Mo-based alloys are important aerospace materials with excellent high temperature mechanical properties. However, their oxidation resistance is very poor at high temperature, and the formation of volatile MoO3 will lead to catastrophic oxidation failure of molybdenum alloy components. Extensive research on the poor oxidation problem has indicated that the halide activated pack cementation (HAPC) technology is an ideal method to solve the problem. In this work, the microstructure, oxide growth mechanism, oxidation characteristics, and oxidation mechanism of the HAPC coatings were summarized and analyzed. In addition, the merits and demerits of HPAC techniques are critically examined and the future scope of research in the domain is outlined.

Z-scheme Au decorated carbon nitride/cobalt tetroxide plasmonic heterojunction photocatalyst for catalytic reduction of hexavalent chromium and oxidation of Bisphenol A

Au/g-C₃N₄/Co₃O₄ plasmonic heterojunction photocatalyst was successfully prepared by in-situ forming Co₃O₄ nanocubes on the Au/g-C₃N₄ nanosheets. The catalytic activities of the photocatalysts were systematically studied through the catalytic reduction of hexavalent chromium (Cr⁶⁺) and oxidation of Bisphenol A (BPA) under visible light irradiation, while according to the degradation products determined by GC-MS, the catalytic degradation pathway of BPA was proposed. 4Au/g-C₃N₄/Co₃O₄ sample exhibits the most efficient catalytic activities, and the photocatalytic reduction and photocatalytic oxidation efficiencies can obtain 85.6% and 90.3%, respectively. The main reasons of the enhancing catalytic performance are the high absorption capability to visible light generated by localized surface plasmon resonance and the effective interface charge separation. Finally, we speculated that the Au/g-C₃N₄/Co₃O₄ sample followed Z-scheme charge transfer mechanism in this study, which is verified by the analysis of experiment and theoretical calculation results.

Chemically synthesized Sb2S3 hollow-spheres for significantly fast and reliable visible light driven dye photodegradation

Sb₂S₃ hollow-spheres in powder form were synthesized through a facile chemical route. The synthesized material was found to have notably high specific surface area. After annealing it showed broadband absorption of light within the visible region. The valance band and conduction band of the synthesized semiconductor were also positioned appropriately (w.r.t NHE) so that the required redox reactions with water in presence of the photogenerated excitons are facilitated. These factors make it a suitable candidate for photocatalytic applications towards the degradation of dye based water pollutants. The synthesized material was established through systematic structural, compositional and optical characterizations. The photocatalytic efficacy toward the degradation of cationic, anionic and neutral dyes has been studied and the best degradation efficiency of 99.72% within 20 min has been achieved at a rate of 0.2920/min, which is significantly higher than many previous reports. Reusability, one of the major factors for the practical application of a catalyst, has also been studied in detail by investigating the probable changes in structural properties as well as in performance after several cycles of photodegradation. The reliability studies yielded encouraging results even after 50th cycle of photodegradation. The effect of catalyst loading on the photodegradation efficacy has also been studied.

Study on the Photocatalysis Mechanism of the Z-Scheme Cobalt Oxide Nanocubes/Carbon Nitride Nanosheets Heterojunction Photocatalyst with High Photocatalytic Performances

An efficient Z-scheme Co₃O₄/g-C₃N₄ heterojunction photocatalyst was developed via in-situ forming Co₃O₄ nanocubes on the g-C₃N₄ nanosheet in the hydrothermal process. The obtained photocatalyst exhibited high photocatalytic activity for the visible-light-driven catalytic reduction of Cr(VI) and catalytic oxidation of tetracycline (TC). Among the as-synthesized catalysts, Co₃O₄/g-C₃N₄-0.04 (the mass ratio of g-C₃N₄ to Co₃O₄ is 0.04) sample exhibits the most efficient catalytic activities. The photocatalytic reduction and photocatalytic oxidation efficiencies of Co₃O₄/g-C₃N₄-0.04 can obtain 81.3 and 92.6 %, respectively. Moreover, the TC is mineralized in the course of photocatalytic degradation, 72.2% of TOC is removed from the reaction system. In addition, the apparent quantum efficiency for the removal of Cr(VI) was also obtained and the the Co₃O₄/g-C₃N₄-0.04 could achieve the highest apparent quantum efficiency among the samples. The enhancing photocatalytic activities originated from the efficient interfacial charge migration and separation obtained in Co₃O₄/g-C₃N₄-0.04, which is preliminarily confirmed by the photoluminescence spectra, time-resolved photoluminescence spectra and the photoelectrochemical characterizations. Finally, we speculate that the Co₃O₄/g-C₃N₄ heterostructures follow a more reasonable Z-scheme charge transfer in this study, which is confirmed by analyzing the results of electron paramagnetic resonance, radical scavenging experiments, and theoretical calculations.

Insights into the photocatalysis mechanism of the novel 2D/3D Z-Scheme g-C3N4/SnS2 heterojunction photocatalysts with excellent photocatalytic performances

A novel 2D/3D Z-scheme g-C₃N₄/SnS₂ photocatalyst was successfully fabricated via self-assembly forming 3D flower-like SnS₂ microspheres on the surface of the 2D g-C₃N₄ nanosheets. The photocatalytic performances of the samples were systematically explored through catalytic reduction of Cr⁶⁺ and oxidation of Bisphenol S (BPS) under the illumination of visible light, and the photocatalytic degradation pathway of BPS was also proposed based on the degradation products confirmed by GCMS. Among the as-prepared samples, 0.4-g-C₃N₄/SnS₂ exhibited the most efficient photocatalytic performances, and the apparent quantum efficiency (QE) for the removal of Cr⁶⁺ could achieve 30.3 %, which is 2.8 times higher than that of the SnS₂. The enhancing photocatalytic activities originated from the efficient interfacial charge migration and separation obtained in g-C₃N₄/SnS₂, which was firstly verified via the photoluminescence spectra, time-resolved photoluminescence spectra and photoelectrochemical characterizations. Importantly, the DFT calculated shows that the band distribution of the g-C₃N₄/SnS₂ sample is staggered near the forbidden, which can facilitate the efficient interfacial charge migration and separation as well as result in the improvement of the catalytic activity. Finally, we put forward a more reasonable Z-scheme charge transfer mechanism, it was verified by analysing the results of free radical scavenging tests, EPR experiments and theoretical calculations.

An Investigation on the Synthesis of Molybdenum Oxide and Its Silica Nanoparticle Composites for Dye Degradation

The molybdenum oxide (MoO₃) and MoO₃@SiO₂ nanoparticles were successfully prepared using the chemical bath deposition (CBD) method. The photocatalytic activities of molybdenum oxide (MoO₃), SiO₂, and MoO₃@SiO₂ nanoparticles composite have shown a synergistic photocatalytic effect of SiO₂ combined with MoO₃. The first-order degradation rate constants for MoO₃, SiO₂, and MoO₃@SiO₂ nanocomposite were 10.3 × 10⁻³ min⁻¹, 15.1 × 10⁻³ min⁻¹, and 16.3 × 10⁻³ min⁻¹, respectively. The MoO₃@SiO₂ composite showed degradation efficiencies in the methylene blue solution close to 100% after 60 min of UV irradiation. The X-ray diffraction (XRD) showed that the MoO₃ powder has a hexagonal crystal structure and the silica is the tridymite type of SiO₂. The crystallite size was about 94 nm, 32 nm, and 125 nm for MoO₃, silica, and MoO₃@SiO₂, respectively, as calculated by the Scherrer equation. The scanning electron microscopy (SEM) images revealed that the MoO₃ powder consisted of a uniform hexagonal structure; the silica showed a rod-like micro-flake morphology and the MoO₃@SiO₂ composite had the appearance of coral-like structures.

A pH-Sensing Fluorescent Metal-Organic Framework: pH-Triggered Fluorescence Transition and Detection of Mycotoxin

Due to its great relevance to environmental, biological, and chemical processes, the precise detection of pH or acidic/basic species is an ongoing and imperative need. In this context, pH-sensitive luminescent systems are highly desired. We reported a three-dimensional Zn(II) MOF synthesized from a bipyridyl-tetracarboxylic ligand and composed of 4-fold interpenetrated diamond frameworks. Because the steric hindrance in the ligand prevents metal coordination with the pyridyl group, the MOF features free basic N sites accessible to the small H⁺ ions, which renders pH responsivity. The aqueous dispersion exhibits an abrupt, high-contrast, and reversible on-off fluorescence transition in the narrow pH range of 5.4-6.2. The sensitive bistable system can be used for the precise monitoring of pH within the range and for use as a pH-triggered optical switch. The responsive mechanism through pyridyl protonation is collaboratively supported by data fitting, absorption spectra, and molecular orbital calculations. In particular, spectral and theoretical analyses reveal the destruction of n → π* transitions and the appearance of intramolecular charge-transfer transitions upon pyridyl protonation. Moreover, by virtue of the pH-responsive fluorescence, the MOF shows appealing sensing performance for the detection of 3-nitropropionic acid, a major mycotoxin in moldy sugar cane.

New Electrospun ZnO:MoO3 Nanostructures: Preparation, Characterization and Photocatalytic Performance

New molybdenum trioxide-incorporated ZnO materials were prepared through the electrospinning method and then calcination at 500 °C, for 2 h. The obtained electrospun ZnO:MoO3 hybrid materials were characterized by X-ray diffraction, scanning and transmission electron microscopies, ultraviolet (UV)-diffuse reflectance, UV-visible (UV-vis) absorption, and photoluminescence techniques. It was observed that the presence of MoO3 as loading material in pure ZnO matrix induces a small blue shift in the absorption band maxima (from 382 to 371 nm) and the emission peaks are shifted to shorter wavelengths, as compared to pure ZnO. Also, a slight decrease in the optical band gap energy of ZnO:MoO3 was registered after MoO3 incorporation. The photocatalytic performance of pure ZnO and ZnO:MoO3 was assessed in the degradation of rhodamine B (RhB) dye with an initial concentration of 5 mg/L, under visible light irradiation. A doubling of the degradation efficiency of the ZnO:MoO3 sample (3.26% of the atomic molar ratio of Mo/Zn) as compared to pure ZnO was obtained. The values of the reaction rate constants were found to be 0.0480 h-1 for ZnO, and 0.1072 h-1 for ZnO:MoO3, respectively.

Phase Transformations and Photocatalytic Activity of Nanostructured Y2O3/TiO2-Y2TiO5 Ceramic Such as Doped with Carbon Nanotubes

This work is devoted to the study of phase transition processes in nanostructured ceramics of the Y₂O₃/TiO₂-Y₂TiO₅ type doped with carbon nanotubes as a result of thermal annealing, as well as to the assessment of the prospects of the effect of phase composition on photocatalytic activity. By the method of X-ray phase analysis, it was found that an increase in the annealing temperature leads to the formation of the orthorhombic phase Y₂TiO₅, as well as structural ordering. Based on the obtained UV spectra, the band gap was calculated, which varies from 2.9 eV (initial sample) to 2.1 eV (annealed at a temperature of 1000 °C). During photocatalytic tests, it was established that the synthesized nanostructured ceramics Y₂O₃/TiO₂-Y₂TiO₅ doped CNTs show a fairly good photocatalytic activity in the range of 60–90% decomposition of methyl orange.