Pure Metal Oxides Research Articles

Buffer Layer Assisted Chemistry over Amorphous Solid Water: Oxide Thin Film or Metallic Nanoparticles Formation.

Novel procedures to grow pure thin metal oxide films are always welcome in view of their wide range of applications including photocatalysis, solar cells, sensors, and more. In this paper we present a unique way to grow pure nanofilms of metal oxides in vacuo at the temperature range 110-170 K. The reactive layer assisted deposition (RLAD) procedure for thin oxide films growth is based on the evaporation of a reactive metal element on top of a condensed layer of amorphous solid water (D2O-ASW). When applied to metals that do not react with the water layer, the process yields metal nanoclusters on the substrate. We observed that metal oxide films are formed if the redox potential is of -1.0 V or less, leading to deuterium molecules ejection to the gas phase (e.g., Ti and Al) while metals such as Zn, Fe, and Ag, with redox potentials more than -1.0 V, transform into nanoclusters, as revealed by SEM studies. We conclude that the redox potential ia a parameter that enables one to predict the nature and outcome of the ASW buffer layer assisted chemistry.

Structural Changes of Highly Active Pd/MeOx (Me = Fe, Co, Ni) during Catalytic Methane Combustion

Fe2O3, Co3O4 and NiO nanoparticles were prepared via a citrate method and further functionalized with Pd by impregnation. The pure oxides as well as Pd/Fe2O3, Pd/Co3O4, and Pd/NiO (1, 5 and 10 wt % Pd) were employed for catalytic methane combustion under methane lean (1 vol %)/oxygen rich (18 vol %, balanced with nitrogen) conditions. Already, the pure metal oxides showed a high catalytic activity leading to complete conversion temperature of T100 ≤ 500 °C. H2-TPR (Temperature-programmed reduction) experiments revealed that Pd-functionalized metal oxides exhibited enhanced redox activity compared to the pure oxides leading to improved catalytic combustion activity at lower temperatures. At a loading of 1 wt % Pd, 1Pd/Co3O4 (T100 = 360 °C) outperforms 1Pd/Fe2O3 (T100 = 410 °C) as well as 1Pd/NiO (T100 = 380 °C). At a loading of 10 wt % Pd, T100 could only be slightly reduced in all cases. 1Pd/Co3O4 and 1Pd/NiO show reasonable stability over 70 h on stream at T100. XPS (X-ray photoelectron spectroscopy) and STEM (Scanning transmission electron microscopy) investigations revealed strong interactions between Pd and NiO as well as Co3O4, respectively, leading to dynamic transformations and reoxidation of Pd due to solid state reactions, which leads to the high long-term stability.

Recovery of Antimony: A Laboratory Study on the Thermal Decomposition and Carbothermal Reduction of Sb(III), Bi(III), Zn(II) Oxides, and Antimony Compounds from Metal Oxide Varistors

As antimony is typically present in industrial and commercial products only in small amounts, the concentration of antimony in waste types is low and a limited amount of antimony is currently recycled. One product relatively rich in antimony is the metal oxide varistor (MOV) used for overvoltage protection in electric circuits. To increase the antimony concentration, the MOV was pulverized (< 65 μm) and leached, resulting in an insoluble MOV residue containing 186 ± 2 mg/g of antimony. This work investigates the thermal decomposition and carbothermal reduction of pure metal oxides (Sb2O3, Bi2O3, and ZnO) and MOV residue. Thermogravimetric (TG) analysis was used in order to propose a temperature range in which it is possible to separate antimony oxide from the MOV residue. TG results indicate that during thermal decomposition of pure metal oxides, sublimated antimony oxide can be recovered at 650 °C, leaving Bi2O3 and ZnO unreacted. The addition of carbon caused mainly volatilization, with some reduction, of Sb2O3 and reduction of Bi2O3 to occur at nearly the same temperature, approximately 600 °C. However, volatilization of Bi was not troublesome below 800 °C due to slow kinetics. Thermal decomposition of antimony from the MOV residue was not possible in the temperature range studied (< 1000 °C), while carbothermal reduction to the MOV residue revealed antimony volatilization occurred near 800 °C.

Light-Activated Metal Oxide Gas Sensors: A Review.

Conductometric gas sensors facilitated by photons have been investigated for decades. Light illumination may enhance device attributes including operational temperature, sensing sensitivity and selectivity. This paper aims to provide an overview on the progress of light-activated gas sensors, with a specific focus on sensors based on metal oxides. The material systems that have been studied include pure metal oxides, heterostructures of semiconductor-metal oxides and metal-metal oxides, and metal oxides with dopant. Other reported works on the use of different nanostructures such as one-dimensional and porous nanostructures, study of sensing mechanisms and the interplay between various factors are also summarized. Possible directions for further improvement of sensing properties, through optimizing the size of nanomaterials, film thickness, light intensity and wavelength are discussed. Finally, we point out that the main challenge faced by light-activated gas sensors is their low optical response, and we have analyzed the feasibility of using localized surface plasmon resonance to solve this drawback. This article should offer readers some key and instructive insights into the current and future development of light-activated gas sensors.

In2O3-decorated ordered mesoporous NiO for enhanced NO2 sensing at room temperature

In this work, ordered mesoporous structures of In2O3-decorated NiO were prepared by a two-step process, comprising of the synthesis of ordered mesoporous NiO followed by injection of In3+ into their pores. The pore size distribution of the as prepared samples was between 4.1 and 21.1 nm. Furthermore, their sensing performances toward NO2 were tested systematically. The results showed the highest response about 3 towards 15 ppm NO2 sensing at room temperature for 5.0 at.% In2O3-decorated NiO compared to other decorated and pure samples. Moreover, the sensor displayed excellent selectivity towards NO2 in the presence of other interfering gases, such as carbon monoxide, ammonia, ethanol, methanol, formaldehyde, toluene, acetone. The exceptional NO2 sensing performance of the In2O3-decorated mesoporous NiO may be attributed to their high specific surface area and the formation of p–n junction with modified carrier concentration caused by In3+ doping. This method can act as an effective strategy for enhancement of gas-sensing properties of pure metal oxides.

Reaction of SO2 with Group IV and VI transition metal oxide clusters

The addition of SO2 to the Group IV (MO2)n and VI (MO3)n (n = 1, 2, 3) nanoclusters was studied using density functional theory (DFT) and coupled cluster theory (CCSD(T)). Structures and ligand binding energies were predicted for the Lewis acid-base addition (physisorption), or SO32− or SO42− formation (chemisorption) of SO2 to these clusters. Physisorption is predicted to be thermodynamically allowed for the Mo and W clusters at 298K, and for Cr only at lower temperatures. Chemisorption by SO42− formation is thermodynamically allowed only for Cr clusters due to the higher reducibility of the metal center. Correlations were made between the ligand binding energies (LBEs) of the (MO3)nSO2 clusters and the chemical properties of the parent (MO3)n clusters (Lewis acidity, reducibility, and MO and MO bond strengths). Physisorption and chemisorption by SO32− formation is predicted to be thermodynamically possible for Group IV clusters, and SO4−2 formation is predicted not to occur, suggesting that sulfate formation does not proceed by the direct reaction of a pure metal oxide surface and SO2.

“One-Pot” Solar Fuels

Chemists have always taken great pride in simplifying a complex multistep chemical synthesis into a single step process, in just one reactor. The motivation for doing so, apart from demonstrating synthetic competency and elegance, is the pursuit of green and sustainable solutions to chemical problems, as single step strategies have the inherent advantage of being atomically, energetically and economically efficient, as well as environmentally friendly and responsible.

Recent developments in electrochemical sensors based on nanomaterials for determining glucose and its byproduct H2O2

The development in glucose and H2O2 electrochemical sensors has significantly progressed using some original nanomaterials, such as nanoparticles and nanowires in metal, metal oxide, or carbon nanomaterials. In this review, we discussed and analyzed the mechanism, performance, and characteristics of the enzyme/nonenzyme glucose and H2O2 electrochemical sensors based on some pure metal (Au, Pd, Ni, Pt, and Cu), metal oxide (ZnO, NiO, CuO x , TiO2, and Co3O4), and carbon (nanotubes and graphene) nanomaterials. Although the introduction of nanomaterials can effectively improve the sensitivity of enzyme glucose/H2O2 sensors by enhancing the activity of protein enzyme, the enzymes are sensitive to the biochemical environment. Meanwhile, the sensing performance of nonenzyme glucose/H2O2 sensors significantly depends on the morphology, uniformity, and distribution of nanomaterials.

High-temperature thermochemical energy storage based on redox reactions using Co-Fe and Mn-Fe mixed metal oxides

Metal oxides are potential materials for thermochemical heat storage via reversible endothermal/exothermal redox reactions, and among them, cobalt oxide and manganese oxide are attracting attention. The synthesis of mixed oxides is considered as a way to answer the drawbacks of pure metal oxides, such as slow reaction kinetics, loss-in-capacity over cycles or sintering issues, and the materials potential for thermochemical heat storage application needs to be assessed. This work proposes a study combining thermodynamic calculations and experimental measurements by simultaneous thermogravimetric analysis and calorimetry, in order to identify the impact of iron oxide addition to Co and Mn-based oxides. Fe addition decreased the redox activity and energy storage capacity of Co3O4/CoO, whereas the reaction rate, reversibility and cycling stability of Mn2O3/Mn3O4 was significantly enhanced with added Fe amounts above ~15mol%, and the energy storage capacity was slightly improved. The formation of a reactive cubic spinel explained the improved re-oxidation yield of Mn-based oxides that could be cycled between bixbyite and cubic spinel phases, whereas a low reactive tetragonal spinel phase showing poor re-oxidation was formed below 15mol% Fe. Thermodynamic equilibrium calculations predict accurately the behavior of both systems. The possibility to identify other suitable mixed oxides becomes conceivable, by enabling the selection of transition metal additives for tuning the redox properties of mixed metal oxides destined for thermochemical energy storage applications.

Useful X-ray Photoelectron Spectroscopy-Based Chemical Tool: Differential Charging Studies of Complex Composite Materials

Traditional X-ray photoelectron spectroscopy can inappropriately indicate the presence of two or more oxidation states for metal(s) in certain polycrystalline phase pure metal-oxides. We attribute this behavior to differential charging of samples. By employing an external bias, DC or AC, to the sample holder during spectral acquisition, we were able to minimize charging effects and determine properties dependent on grain/grain and substrate/grain interactions of complex composite polycrystalline materials.

Improvement of reduction and oxidation performance of MMgOx (M = Fe, Co, Ni, and Cu) particles for chemical looping combustion

The objective of this study was to use Mg to improve the performance in chemical looping combustion. Oxygen carrier particles were synthesized in which Mg was doped into the M-sites with Fe, Co, Ni, and Cu to produce FeaMgbOx, CoaMgbOx, NiaMgbOx, and CuaMgbOx, respectively, with M/Mg ratios ranging from 1.5 to 3.0. The structure and properties of the prepared samples were characterized using XRD, SEM, CH4-/CO-TPD, XPS, and TGA. The XRD results explore that the Fe2Mg1Ox sample has a pure cubic structure of Fe2MgO4, whereas the Co2Mg1Ox and Ni2Mg1Ox samples exist as a solid solution. Nevertheless, the mixed compound formation of orthorhombic structure of Cu2MgO3 and cubic structure of MgO has been observed in Cu2Mg1Ox. The CH4 absorption is the highest on the Cu2Mg1Ox sample according to the CH4-TPD results. The CO-TPD and H2–TPR results showed that the adsorption of the Cu and Mg complex metal oxide is higher than that of the pure metal oxide. The oxygen carrier capacity of the samples was studied by isothermal H2 (FR)/air (AR) and CH4 (FR)/air (AR) redox cycle experiments at 850°C using TGA. The promising results obtained from chemical looping combustion indicate that the CuaMgbOx samples are potentially useful for CLC in H2 and CH4 atmospheres.

Mixed CuFe and ZnFe (hydr)oxides as reactive adsorbents of chemical warfare agent surrogates

Two sets of zinc-iron and copper-iron mixed (hydr)oxides were prepared by a simple co-precipitation method. Either nitrate or chloride was a source of the metals. The decontamination ability of the materials was tested in closed vials saturated with vapors of 2-chloroethyl ethyl sulfide (CEES) or dimethyl chlorophosphate (DMCP), a blister agent and a nerve agent surrogate, respectively. In both cases, the weight uptakes on the mixed oxides were superior to the ones reported for the pure metal oxides or hydroxides. When exposed to CEES for 5days, zinc-iron (hydr)oxides show much higher activity than the copper-iron ones. The products of reactions in the vessel headspace were investigated by GC/MS and on the surface by FTIR. Ethyl vinylsulfide and chloromethane are the main products of the reactive adsorption of CEES and DMCP, respectively. This indicates that CEES is mainly degraded by dehydrochlorination and DMCP- by hydrolysis.

Mineralogical characterization and design of a treatment process for Yunnan nickel laterite ore, China

The use of laterite ore as a major nickel source is gradually increasing because of the depletion of sulphide ores. However, in contrast to high-grade nickel sulphide ore, low-grade nickel laterite ore cannot be as easily upgraded by current technologies due to its complicated mineral structure and nickel distribution. This study investigated the mineralogical characteristics, occurrence state and dissemination characteristics of nickel within Yunnan nickel laterite ore in China. Nickel laterite ore had a complex internal structure and mainly consisted of limonite (10wt.%) and serpentine (87wt.%). Sulphides were not a significant part of this laterite ore and were rarely observed. Furthermore, the nickel was mainly hosted in serpentine by substitution for Mg or by adsorption. Serpentine had a mean grade of approximately 0.77wt.% Ni and represented approximately 81.49wt.% of the total laterite resource. A portion of the laterite ore had a pore structure that was formed by destruction of the original protolith during and after its formation, with a specific surface area of 70.042m2/g. The actual temperature for the reduction of metal oxides within the ore was analysed by H2-TPR (temperature-programmed reduction), which indicated that it was higher than the reduction temperature of the pure metal oxides. Finally, based on the above analyses, a non-melting reduction treatment process was proposed for efficient and reasonable utilization of Yunnan laterite ore.

Top-down synthesis of zinc oxide based inks for inkjet printing

A top-down approach for synthesizing pure and doped inkjet printable metal oxide inks.

Pure and multi metal oxide nanoparticles: synthesis, antibacterial and cytotoxic properties.

Th antibacterial activity of metal oxide nanoparticles has received marked global attention as they can be specifically synthesized to exhibit significant toxicity to bacteria. The importance of their application as antibacterial agents is evident keeping in mind the limited range and effectiveness of antibiotics, on one hand, and the plethora of metal oxides, on the other, along with the propensity of nanoparticles to induce resistance being much lower than that of antibiotics. Effective inhibition against a wide range of bacteria is well known for several nano oxides consisting of one metal (Fe3O4, TiO2, CuO, ZnO), whereas, research in the field of multi-metal oxides still demands extensive exploration. This is understandable given that the relationship between physicochemical properties and biological activity seems to be complex and difficult to generalize even for metal oxide nanoparticles consisting of only one metal component. Also, despite the broad scope that metal oxide nanoparticles have as antibacterial agents, there arise problems in practical applications taking into account the cytotoxic effects. In this respect, the consideration of polymetallic oxides for biological applications becomes even greater since these can provide synergetic effects and unify the best physicochemical properties of their components. For instance, strong antibacterial efficiency specific of one metal oxide can be complemented by non-cytotoxicity of another. This review presents the main methods and technological advances in fabrication of nanostructured metal oxides with a particular emphasis to multi-metal oxide nanoparticles, their antibacterial effects and cytotoxicity.

Gas-Solid Reaction Route toward the Production of Intermetallics from Their Corresponding Oxide Mixtures

Near-net shape forming of metallic components from metallic powders produced in situ from reduction of corresponding pure metal oxides has not been explored to a large extent. Such a process can be probably termed in short as the “Reduction-Sintering” process. This methodology can be especially effective in producing components containing refractory metals. Additionally, in situ production of metallic powder from complex oxides containing more than one metallic element may result in in situ alloying during reduction, possibly at lower temperatures. With this motivation, in situ reduction of complex oxides mixtures containing more than one metallic element has been investigated intensively over a period of years in the department of materials science, KTH, Sweden. This review highlights the most important features of that investigation. The investigation includes not only synthesis of intermetallics and refractory metals using the gas solid reaction route but also study the reaction kinetics and mechanism. Environmentally friendly gases like H2, CH4 and N2 were used for simultaneous reduction, carburization and nitridation, respectively. Different techniques have been utilized. A thermogravimetric analyzer was used to accurately control the process conditions and obtain reaction kinetics. The fluidized bed technique has been utilized to study the possibility of bulk production of intermetallics compared to milligrams in TGA. Carburization and nitridation of nascent formed intermetallics were successfully carried out. A novel method based on material thermal property was explored to track the reaction progress and estimate the reaction kinetics. This method implies the dynamic measure of thermal diffusivity using laser flash method. These efforts end up with a successful preparation of nanograined intermetallics like Fe-Mo and Ni-W. In addition, it ends up with simultaneous reduction and synthesis of Ni-WN and Ni-WC from their oxide mixtures in single step.

Angular optical characteristics of light scattered by double-layer metal–oxide nanoparticles

Indicatrices of the light scattering on homogeneous nanoparticles with radii of r0 = 50, 75, and 100 nm and on double-layer nanoparticles with core radii of r0 = 40, 65, and 90 nm and a shell thickness of Δr = 10 nm are calculated numerically for light wavelengths of λ = 300, 560, and 1000 nm. Metals Ti, Ni, and Zn and their oxides, TiO2, NiO, and ZnO, were used as materials for homogeneous nanoparticles. The same metals and their oxides were used, respectively, as materials for the core and the shell of the double-layer nanoparticles. It is shown that, in comparison with the pure metal or pure oxide dielectric nanoparticles, the presence of the oxide shell on the titanium and nickel cores leads to a significant decrease in the backscattering for λ = 300 nm.

Superaerophobic Electrode with Metal@Metal‐Oxide Powder Catalyst for Oxygen Evolution Reaction

A stable and highly active oxygen evolution reaction (OER) electrode is the key for fast and robust O2 production, which is one of the essential points for various kinds of energy conversion systems, such as water splitting, lithium‐O2 battery and artificial photosynthesis. Here, superaerophobic electrodes with metal@metal‐oxide powder catalysts are shown, which demonstrate high and stable OER activity. The active‐site‐density of metal@metal‐oxide catalysts is increased over one order of magnitude than those of pure metal oxides due to the large enhancement of electrical conductivity, revealing the substantial enhancement of electrochemical OER kinetics. Furthermore, the superaerophobic property of electrodes is favorable for fast O2 desorption, which improves electrochemical active surface area (EASA) during OER. The superaerophobic electrode with metal@metal‐oxide powder catalysts provides the new insight for increase of active‐site‐density and EASA simultaneously, which are the key factors to determine the activity of OER electrode.

Co3V2O8 Sponge Network Morphology Derived from Metal-Organic Framework As an Excellent Lithium Storage Anode Material

Metal-organic framework (MOF) based synthesis of battery electrodes has presently become a hot topic of significant research interest. Considering the complications to prepare Co3V2O8 due to the criticality of its stoichiometric composition, we report here by a simple MOF formation by solvothermal technique followed by air annealing to synthesize Co3V2O8 for use as potential anodes for lithium battery applications. Structrual investigations by X-ray diffraction, X-ray photoelectron spectroscopy, high resolution electron microscopy, and surface area studies revealed that the phase pure Co3V2O8 nanoparticles are inter connected to form a sponge-like morphology with porous properties. Electrochemical measurements exposed the excellent lithium storage (~ 1000 mAhg-1 at 200 mAg-1) and retention properties (500 mAhg-1 at 1000 mAg-1 after 700 cycles) of the prepared Co3V2O8 electrode. A notable rate performance of 430 mAhg-1 at 3200 mAg-1 was also observed and ex-situ analysis confirmed the morphological and structural stability of this material after long cycles performance. These results validate that the unique nanostructured morphology arising from the use of the ordered array of MOF networks is favorable for improving the cyclability and rate capability in battery electrodes. The synthetic strategy presented herein may provide solutions to develop phase pure mixed metal oxides for high performance electrodes for useful energy storage applications. Figure 1

Novel strategy for the preparation of graphene-encapsulated mesoporous metal oxides with enhanced lithium storage

As potential anode materials for lithium-ion batteries, mesoporous metal oxides show high reversible capacities but relatively poor cycle stability due to the structural collapse during cycles. Graphene-encapsulated mesoporous metal oxides may increase the electronic conductivity of the composite as well as stabilize the mesostructure of metal oxides, thereby enhancing the electrochemical performance of mesoporous metal oxides. Herein we describe a novel strategy for the preparation of graphene-encapsulated mesoporous metal oxides (SnO2, Mn3O4), which exhibit superior electrochemical performance compared to pure mesoporous metal oxides. Moreover, some mesoporous metal oxides may be further reduced to low-valence metal oxides when calcined in presence of graphene. Mesoporous metal oxides with high isoelectric points are not essential for this synthesis method since metal oxides are connected with graphene through mesoporous silica template, thus expanding the types of graphene-encapsulated mesoporous metal oxides.