Metal Oxides For Applications Research Articles

Templated encapsulation of platinum-based catalysts promotes high-temperature stability to 1,100 °C.

Stable catalysts are essential to address energy and environmental challenges, especially for applications in harsh environments (for example, high temperature, oxidizing atmosphere and steam). In such conditions, supported metal catalysts deactivate due to sintering-a process where initially small nanoparticles grow into larger ones with reduced active surface area-but strategies to stabilize them can lead to decreased performance. Here we report stable catalysts prepared through the encapsulation of platinum nanoparticles inside an alumina framework, which was formed by depositing an alumina precursor within a separately prepared porous organic framework impregnated with platinum nanoparticles. These catalysts do not sinter at 800 °C in the presence of oxygen and steam, conditions in which conventional catalysts sinter to a large extent, while showing similar reaction rates. Extending this approach to Pd-Pt bimetallic catalysts led to the small particle size being maintained at temperatures as high as 1,100 °C in air and 10% steam. This strategy can be broadly applied to other metal and metal oxides for applications where sintering is a major cause of material deactivation.

Microbial reduction of metal-organic frameworks enables synergistic chromium removal

Redox interactions between electroactive bacteria and inorganic materials underpin many emerging technologies, but commonly used materials (e.g., metal oxides) suffer from limited tunability and can be challenging to characterize. In contrast, metal-organic frameworks exhibit well-defined structures, large surface areas, and extensive chemical tunability, but their utility as microbial substrates has not been examined. Here, we report that metal-organic frameworks can support the growth of the metal-respiring bacterium Shewanella oneidensis, specifically through the reduction of Fe(III). In a practical application, we show that cultures containing S. oneidensis and reduced metal-organic frameworks can remediate lethal concentrations of Cr(VI) over multiple cycles, and that pollutant removal exceeds the performance of either component in isolation or bio-reduced iron oxides. Our results demonstrate that frameworks can serve as growth substrates and suggest that they may offer an alternative to metal oxides in applications seeking to combine the advantages of bacterial metabolism and synthetic materials.

The influence of manganese(IV) oxide addition on the dispersion characteristics and structural integrity of MWCNTs in metal oxides

Recent advancement in material science and engineering has fostered the utilization of metal oxides (MOs) in diverse engineering applications. Despite the widespread application of MOs, they have certain limitations such as brittleness and low thermal conductivity. Therefore, the integration of multiwalled carbon nanotubes (MWCNTs) into their structure will augment the properties of MOs since they posses excellent mechanical, thermal and electrical properties. In order to achieve the effective transfer of the unique properties of MWCNTs into MOs, it is paramount to homogenously disperse the nanotubes within the MO matrix. Past works have emphasized that MWCNTs tend to agglomerate during their incorporation into metal matrices. In this study, the homogeneous dispersion of 1 wt% MWCNTs in metal oxides was accomplished by the introduction of manganese oxide (MnO2) of various (5, 10, 15) weight percentages in titanium oxide (TiO2). This was carried out by the adoption of regulated milling parameters: speed 100 rpm, time 6 h, and ball to powder ratio 10:1 using a high-energy ball mill. The dispersion characteristics and structural integrity of the MWCNTs in MOs were evaluated by the adoption of scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray diffraction (XRD) techniques. The results indicated that MWCNTs were homogeneously dispersed in the MOs; however, better dispersion with minimal structural strain to the MWCNTs was achieved at a higher weight percent of MnO2 in the composite powder mixture.

Aldehyde reduced Co3O4 to form oxygen vacancy and enhance the electrochemical performance for oxygen evolution reaction and supercapacitors

Oxygen vacancy is a feasible approach to boost the electrochemical properties for metal oxides. In this work, a Co3O4 with abundant oxygen vacancy is synthesized via aldehyde reduction. After the procedure, the reduced Co3O4 exhibits larger electrochemical active surface areas and better electrical conductivity. These outstanding characteristics can improve its performance of catalytic and energy storage. As for catalyst of oxygen evolution reaction, the reduced Co3O4 delivers a smaller potential of 1.55 V versus the reversible hydrogen electrode to realize a current density of 10 mA cm−2 and a lower Tafel slope of 71 mV dec−1 in alkaline solution, and these values are smaller than those of pristine Co3O4. Especially the reduced Co3O4 possesses superior stability: the measurements of the polarization curves before and after 15h of stability tests basically coincide. In a supercapacitor, the positive electrode of reduced Co3O4 achieves about 1.7 times areal capacitance of pristine Co3O4 at current density of 1 mA cm−2. Significantly, the superior cycling stability is still retained. Also, an aqueous asymmetric supercapacitor is assembled to evaluate the energy storage performance of the R-Co3O4. Moreover, the oxygen vacancy formation strategy for Co3O4 may be generally extended to other metal oxides for application in energy storage and conversion.

A Universal Strategy for Constructing Seamless Graphdiyne on Metal Oxides to Stabilize the Electrochemical Structure and Interface.

The structural and interfacial stabilities of metal oxides (MOs) are key issues while facing the volumetric variation and intensive interfacial polarization in electrochemical applications, including lithium-ion batteries (LIBs), supercapacitors, and catalysts. The growth of a seamless all-carbon interfacial layer on MOs with complex dimensions is not only a scientific problem, but also a practical challenge in these fields. Here, the growth of graphdiyne under ultramild condition is successfully implemented in situ for coating MOs of complex dimensions. The seamless all-carbon interface and conductive network are formed at the same time. This method cleverly avoids the structural degradation of MOs at a high temperature in the presence of traditional carbon materials. Under the protection of the high-quality graphdiyne layer, the samples as LIB anodes deliver high performances in terms of Coulomb efficiency, capacity, long-term retention, and structural and interfacial stabilities. Both experimental achievements and theoretical calculations demonstrate that the graphdiyne is a particular protection layer for MOs and plays a crucial role for preventing the structural and interfacial degradation of the electrode. Furthermore, the universality of this method will promote the potential applications of many promising MOs in other electrochemical fields.

Low-Voltage Reversibly Switchable Wettability through Electrochemical Manipulation of Oxidation State

Various biological organisms in nature exhibit unique surface wettability in order to adapt to their living environment. Diverse range of wettability can be observed, ranging from the low-adhesion superhydrophobic lotus leaf with self-cleaning property, high-adhesion superhydrophobic gecko foot, to the in-air superhydrophilic and underwater superoleophobic fish scale. Inspired by these natural systems with superwetting/antiwetting properties, significant efforts have been devoted to fabricate artificial surfaces with different wettabilities by engineering the surface morphology and chemical composition. Of particular interest is the stimuli-responsive surface which integrates two extreme wetting states of water-attracting and water-repelling properties. External stimuli such as temperature, light, pH, and electrical potential could induce reversible changes in the wetting behavior of the smart surface through transformation in the topological structure and/or surface chemistry. Here we present a novel approach for reversible wettability cycling on dendritic core-shell copper nanostructure surface through electrochemical modulation of the oxidation state. Application of low voltage in the range of regular alkaline battery (<1.5 V) converts the as-prepared copper-based surface from roll-off superhydrophobic, to sticky superhydrophobic, and superhydrophilic wetting state. Precise control over the rate and extent of the wetting switching is achieved by tuning the magnitude and period of the applied voltage. Air drying at room temperature for 1 hour or mild heat drying at 100°C for 30 min reverses the wettability transition to initial superhydrophobic state with low adhesion easy roll-off property. We describe the underlying mechanism for the reversible adhesion and wettability switching from the physical and electrochemical perspectives, as well as practical applicability of this method with specific demonstration for on-demand oil-water separation. The in-situ adhesion and wettability control reported in this work provide a platform for design of oxidation state-mediated wetting transition on different metal oxides for application in remote water filtration, atmospheric water harvesting, droplet manipulation, and microfluidic lab-on-a-chip application.

Metal Oxides for Application in Conductometric Gas Sensors: How to Choose?

This article gives a brief description of metal oxides, acceptable for using in advanced conductometric gas sensors, as well as a consideration of approaches, that can be used to select metal oxides for the manufacture of devices intended to sensor market.

Effective harvesting of UV induced production of excitons from Fe3O4 with proficient rGO-PTh acting as BI-functional redox photocatalyst

We report the synthesis of PTh-rGO-Fe3O4 nanocomposite from a simple combination of graphene and polythiophene (PTh) with metal oxide for application as photocatalyst. The structure and morphology of the nanocomposite were studied by powder XRD, FTIR spectroscopy, Raman analysis, EDAX and TEM. The observed properties of the nanocomposite were fineset for the application in the field of photocatalyst. The photocatalytic effect was studied by utilizing the nanocomposite in water splitting for hydrogen production, degradation of Malachite green and shielding of Rose Bengal from UV degradation. The maximum quantum efficiency for hydrogen production obtained with 0.1% w/v of PTh-rGO-Fe3O4 photocatalyst was 11.48% with a hydrogen production rate of 178.4 μmol/h. Furthermore, the photocatalyst shows 87.9% degradation of Malachite green and acts as an effective UV shielding layer preventing 53.09% degradation of Rose Bengal. This study reveals that our nanocomposite will be a better photocatalytic material.

Ultraviolet Radiation Induced Dopant Loss in a TiO2 Photocatalyst

Doped TiO2 has been studied with intense interest in recent decades because of its ability to utilize visible wavelengths and enhance the efficiency of photocatalytic processes. Thus, as a class of materials, it is of significant interest for use in environmental ambient energy utilization applications. Using a popular and well-studied form of doped TiO2 (nitrogen doped) as an example, we show how 28 days of UVA irradiation which is identical in intensity with solar conditions is sufficient to cause the UV induced surface segregation and eventual loss of nitrogen dopant species in TiO2. This is evidenced by X-ray photoelectron spectroscopy and transient absorption spectroscopy. The loss of interstitial nitrogen dopants correlates with the eventual permanent loss of photocatalytic activity and visible light absorption. The UV induced loss of dopants in a metal oxide is unprecedented and represents a potential problem where the environmental use of doped metal oxides in applications is concerned.

Electrochemical assembly of MnO2 on ionic liquid–graphene films into a hierarchical structure for high rate capability and long cycle stability of pseudocapacitors

Hierarchical nanostructures are of prime importance due to their large surface area, easy accessibility to reaction sites, fast ion and electron transport, and mechanical integrity. Herein, we demonstrate the synthesis of hierarchically structured MnO₂/ionic liquid-reduced graphene oxide (IL-RGO) nanocomposites through the electrochemical self-assembly. The structures of MnO₂/IL-RGO nanocomposites and their formation mechanism are investigated by spectroscopic methods and as a consequence, correlated with the electrochemical behaviours. The specific capacitance (511 F g⁻¹) of conformally MnO₂-deposited IL-RGO composites is significantly higher than 159 F g⁻¹ of pure MnO₂ film. High rate capability (61% retention at 30 A g⁻¹) of the MnO₂/IL-RGO composite is attributed to the facilitated ion diffusion and electron transport, whereas its long cycle life (95% retention after 2000 cycles) is related to the mechanical robustness. These results provide a new insight into the rational design of hierarchical and complex heterostructures consisting of carbon nanomaterials and metal oxides for applications in energy conversion and storage.

Encapsulation of Perchlorate Salts within Metal Oxides for Application as Nanoenergetic Oxidizers

AbstractIn this work, high‐oxygen‐content strong oxidizer perchlorate salts were successfully incorporated into current nanothermite composite formulations. The perchlorates were encapsulated within mild oxidizer particles through a series of thermal decomposition, melting, phase segregation, and recrystallization processes, which occurred within confined aerosol droplets. This approach enables the use of hygroscopic materials by stabilizing them within a matrix. Several samples, including Fe2O3/KClO4, CuO/KClO4 and Fe2O3/NH4ClO4 composite oxidizer particles, have been created. The results show that these composite systems significantly outperform the single metal oxide system in both pressurization rate and peak pressure. The ignition temperatures for these mixtures are significantly lower than those of the metal oxides alone, and time‐resolved mass spectrometry shows that O2 release from the oxidizer also occurs at a lower temperature and with high flux. The results are consistent with O2 release being the controlling factor in determining the ignition temperature. High‐speed imaging clearly shows a much more violent reaction. The results suggest that a strategy of encapsulating a very strong oxidizer, which may not be environmentally compatible, within a more stable weak oxidizer offers the opportunity to both tune reactivity and employ materials that previously could not be considered.

Thermal and electrical properties of magnetite filled polymers

By the addition of metal-oxide particles to plastics the electrical and thermal conductivity of polymers can be increased significantly. Such particle filled polymers can substitute metals and metal oxides in applications like radio frequency interference shielding. Furthermore, particle filled polymers with higher thermal conductivity than unfilled ones become more and more important in applications with decreasing geometric dimensions and increasing output of power, like in computer chips. Therefore, thermal and electrical properties of polypropylene and polyamide with metal-oxide particle filler (magnetite, Fe3O4) are investigated. Different particle sizes of magnetite and types of additives were added in various proportions to a standard polypropylene and polyamide in an extrusion process. Samples were prepared by injection molding to investigate thermal and electrical properties systematically. The thermal conductivity increases from 0.22 to 0.93 W/(m K) for a filler content of 44 vol% of magnetite, whereas the electrical resistivity decreases more than seven orders of magnitude from an insulator (0% of magnetite) to 10 kΩ m (47 vol% of magnetite). The experimental results of thermal and electrical conductivity were correlated to the amount and particle sizes of magnetite filler. Electrical resistivity shows a significant drop at the theoretical percolation threshold (∼0.33) and for filler contents exceeding 33 vol% the magnetite particles have point contacts and are surrounded by the polymer matrix.