Metal Oxide Coatings Research Articles

Fabrication of mesoporous metal oxide coated-nanocarbon hybrid materials via a polyol-mediated self-assembly process.

After clarifying the formation mechanism of a typical metal glycolate precipitate, Ti glycolate, in a polyol-mediated synthesis using acetone as a precipitation medium, we describe a simple template-free approach based on an ethylene glycol-mediated synthesis to fabricate mesoporous metal oxide coated-nanocarbon hybrid materials including TiO₂ coated-carbon nanotube (CNT), SnO₂ coated-CNT, Cu₂O/CuO coated-CNT and TiO₂ coated-graphene sheet (GS). In the approach, metal oxide precursors, metal glycolates, were first deposited on CNTs or GSs, and subsequently transformed to the metal oxide coatings by pyrolysis or hydrolysis. By a comparison between the characterization of two TiO₂-CNT hybrid materials using carboxylated CNTs and pristine CNTs without carboxyl groups, the driving force for initiating the deposition of metal glycolates on the carboxylated CNTs is confirmed to be the hydrogen bonding between the carboxyl groups and the polymer chains in metal glycolate sols. The electrochemical performances of the mesoporous TiO₂ coated-carboxylated CNTs and TiO₂-pristine CNT hybrid materials were investigated. The results show that the mesoporous TiO₂ coated-carboxylated CNT with a uniform core-shell nanostructure exhibits substantial improvement in the rate performance in comparison with its counterpart from 0.5 C to 100 C because of its higher electronic conductivity and shorter diffusion path for the lithium ion. At the extremely high rate of 100 C, the specific capacity of TiO₂ of the former reaches 85 mA h g(-1), twice as high as that of the latter.

Structure and gas permeability of nanoporous metal oxide coatings produced by the alkoxide method

Peculiarities of the preparation of selective nanoporous metal oxide coatings on the surface of macroporous rutile support (pore size 0.1 μm and mixed aluminum and titanium oxides (pore size 0.05 μm) are presented. Gas selective coatings of empirical formula of P x Ti1 − 0.5x O2 ± δ with uniform distribution of nanosized pores have been obtained with the alkoxide method using titanium alkoxide and ettriol phosphite (precursor of the phosphorous-containing component) at their various total concentrations. Surface morphology of the obtained membranes has been investigated and major parameters of gas selective coatings have been determined. It has been shown that the dependences of the He, N2, CO2, and C3H8 gas permeability on temperature and pressure drop are consistent with the molecular regime of gas flow. The reproducible effect of permeability anisotropy of ∼502?60% is observed when the integral vacuum method is used.

Nanocrystalline structure of surface layer of technical-purity titanium subjected to induction-thermal oxidation

Metal-oxide coatings on a smooth technical-purity titanium surface were produced that were characterized by lamellar, needled, and prismatic shapes of crystals. Regularities of variations in nanometer morphology characteristics of the titanium surface subjected to induction-thermal oxidation are determined.

The deposition of crystallized TiO2 coatings by closed field unbalanced magnetron sputter ion plating

Metallic oxide coatings, for example TiO2, are of increasing technological importance in a wide range of industrially relevant functional surfaces on components for renewable energy sector, self-cleaning, antimicrobial surfaces etc.Reactive closed field unbalanced magnetron sputter ion plating (CFUBMSIP) provides an effective, industrially-compatible processing route for the deposition of metallic oxides, with controlled composition, structure, mechanical, electronic and optical properties. The production of crystallized as-deposited TiO2 thin coatings by reactive CFUBMSIP process is described. The arrangement of the deposition system; deposition parameters, including: working gases, deposition pressure, the sputtering power; and the influence of interlayer on the nucleation and the crystalline structure of TiO2 coatings are investigated.

Significant impact on cathode performance of lithium-ion batteries by precisely controlled metal oxide nanocoatings via atomic layer deposition

LiCoO2 in the commercial lithium ion batteries has been suffering from its poor cycling performance at high cutoff voltages. In this study, we employ an atomic layer deposition (ALD) technique to surface-modify a LiCoO2 material with various thickness-controlled metal oxide (TiO2, ZrO2 and Al2O3) coatings to improve its battery performance. The effects of the metal oxide coatings on the electrochemical performance of LiCoO2 electrode are studied in detail. It is demonstrated that a uniform and dense coating via the ALD route on LiCoO2 powder can lower the battery performance due to an obvious decrease in lithium diffusion and electron transport with the coating layers. In contrast, it is revealed that a direct coating on prefabricated LiCoO2 electrodes performs much better than a coating on LiCoO2 powders. It is further disclosed that the improved electrochemical performance of coated LiCoO2 electrode is highly dependent on the coating materials. Of the three coating materials, the Al2O3 coating results in the best cycling stability while the ZrO2 coating contributes to the best rate capability. It is thus suggested that the coating materials are functionally specific, and for the best improvement of a cathode, a particular coating material should be sought.

Large effect of titanium precursor on surface reactivity and mechanical strength of electrospun nanofibers coated with TiO2 by atomic layer deposition

Encapsulating and functionalizing polymer nanofibers can improve the polymers chemical resistance and surface reactivity, enabling new applications including biosensing, flexible electronics, gas filtration, and chemical separations. Polymer fiber functionalization typically involves energy intensive wet chemical treatments and/or plasma exposure. Recent results show low temperature atomic layer deposition (ALD) to be a viable means to coat nanofibers with uniform and conformal inorganic and hybrid organic–inorganic layers. For this article, the authors describe how the mechanical properties of nylon-6 nanofibers are affected by ALD coatings of TiO2 and other metal oxides. They find that the stress–strain behavior of nylon-6 nanofibers depends strongly on the specific precursor chemistry used in the coating process. For ALD TiO2 coatings, titanium tetrachloride tended to embrittle the fibers, whereas titanium isopropoxide had a more subtle effect. Physical characterization shows that the TiCl4 diffused into the nylon-6 and reacted subsurface, whereas the titanium isopropoxide tended to react on the surface producing a more abrupt organic/inorganic interface. Results show that precursor choice is an important factor when designing thin film coating processes on polymeric substrates.

Effect of composition on electrochemical properties of RuO2–TiO2 mixed metal oxide coatings

Mixed metal oxide coatings of RuO2–TiO2 with different compositions were synthesised by thermal decomposition on Ti substrate. The effect of composition on the electrocatalytic activity of anodes for oxygen evolution reaction in acidic solution was investigated. The electrochemical behaviour of coatings was studied using different electrochemical techniques. Surface properties of the mixed metal oxide in solution and related redox reactions were measured by cyclic voltammetry. The reaction mechanisms and electrocatalytic properties of mixed metal oxide coatings were studied systematically by anodic polarisation measurement in 0·5M H2SO4 solution. It was observed that the electrochemically active surface areas reach a maximum around intermediate composition. The different reaction mechanisms were proposed according to different Tafel slopes obtained from the Tafel lines. It was shown the real electrochemical activity is related to electronic factors and synergetic effects.

Nanosecond pulsed laser irradiation of stainless steel 304L: Oxide growth and effects on underlying metal

Nanosecond-pulsed, infrared (1064nm) laser irradiation was used to create metal oxide coatings on the surface of polished stainless steel 304L austenite for application as color markings and unique tags/identifiers. By rastering a Gaussian-shaped, focused laser beam across a specimen in air, continuous metal oxide coatings were grown to thicknesses in the range of 20 to ~500nm. Oxide coating thickness generally increased with laser fluence. However, for large accumulated fluences in excess of ~600–800J/cm2, oxide growth was affected by evaporation and particle ejection resulting in a decreased coating thickness. Transmission electron microscopy and X-ray diffraction revealed that oxide coatings developed a polycrystalline, spinel structure having a lattice constant=8.4Å (consistent with MnCr2O4 and Fe3O4). Pulsed laser irradiation and oxide growth modified the composition of stainless steel substrates by reducing the Cr and Mn concentration within the melt zone. The reflectance and chromaticity of laser-fabricated oxide coatings were characterized using spectrophotometry. These optical properties are described in the context of measured oxide thicknesses.

Multifunctional bulk plasma source based on discharge with electron injection

A bulk plasma source, based on a high-current dc glow discharge with electron injection, is described. Electron injection and some special design features of the plasma arc emitter provide a plasma source with very long periods between maintenance down-times and a long overall lifetime. The source uses a sectioned sputter-electrode array with six individual sputter targets, each of which can be independently biased. This discharge assembly configuration provides multifunctional operation, including plasma generation from different gases (argon, nitrogen, oxygen, acetylene) and deposition of composite metal nitride and oxide coatings.

Application of Nanosols in Textile Industry

Inorganic metal oxide nanoparticles are mainly synthesized by sol–gel process. The most important beneficial advantage of mentioned process is facile and the time-consuming route. The resultant meta-stable synthesized inorganic nanoparticles can easily modify different substrates and alter their performance. The presented review investigates the possible applications of nanosols (especially silica and titanium dioxide sols) in the field of textile industry, including the formation of hydrophobic, bioactive or protective metallic oxide coatings on textiles by physical or chemical modifying of nanosols.

Self-propagating high-temperature synthesis in the production of functional coatings

The direct synthesis of refractory compounds and the reduction of metal oxides by self-propagating high-temperature synthesis are subjected to thermodynamic analysis. The laws of self-propagating high-temperature synthesis are investigated, and the possibility of creating functional metal-oxide coatings of specified composition by that means is considered.

Sol–gel titania coating on unmodified and surface-modified polyimide films

Sol–gel coating of metal oxides on polymer substrates is a useful process to fabricate various organic–inorganic hybrid materials under mild conditions. However, this process is hardly applicable to pristine polyimide (PI) films because their surfaces do not display effective functional groups for metal oxide coatings. In this study, we firstly examined direct sol–gel coating of titania thin layers on unmodified PI film surfaces. The results confirmed homogeneous, ultrathin titania layer coating and showed that the thickness and microscopic morphology of the titania layers were affected by titanium alkoxide concentrations in the spin coating solutions. We next investigated titania layer coating on surface-modified PI films that prepared using alkaline hydrolysis, which generated carboxylic acid groups on the film surfaces. Optimal hydrolysis time was determined using FT-IR spectroscopy and contact angle measurements. After sol–gel titania coating on the hydrolyzed PI film surfaces, the Scotch tape test was conducted to evaluate adhesion strength between the titania layers and PI film surfaces. Morphological observations of the sample surfaces after the tests clearly showed that surface modification of PI films increased titania layer adhesions. Effect of hydrothermal treatments on film formability and adhesion strength between titania-PI film interfaces was also evaluated.

Fluorescence labeling of colloidal core–shell particles with defined isoelectric points for in vitro studies

In the light of in vitro nanotoxicological studies fluorescence labeling has become standard for particle localization within the cell environment. However, fluorescent labeling is also known to significantly alter the particle surface chemistry and therefore potentially affect the outcome of cell studies. Hence, fluorescent labeling is ideally carried out without changing, for example, the isoelectric point. A simple and straightforward method for obtaining fluorescently labeled spherical metal oxide particles with well-defined isoelectric points and a narrow size distribution is presented in this study. Spherical amorphous silica (SiO 2, 161 nm diameter) particles were used as the substrate material and were coated with silica, alumina (Al 2O 3), titania (TiO 2), or zirconia (ZrO 2) using sol–gel chemistry. Fluorescent labeling was achieved by directly embedding rhodamine 6G dye in the coating matrix without affecting the isoelectric point of the metal oxide coatings. The coating quality was confirmed by high resolution transmission electron microscopy, energy filtered transmission electron microscopy and electrochemical characterization. The coatings were proven to be stable for at least 240 h under different pH conditions. The well-defined fluorescent particles can be directly used for biomedical investigations, e.g. elucidation of particle–cell interactions in vitro.

Enhanced Electrochemical Properties of LiFePO<sub>4</sub>/C Cathode Material by Metal Oxide Coating

LiFePO4/C composite cathode material prepared by carbothermal reduction method was coated by metal oxide MnO2, Al2O3, CuO, respectively, by a chemical precipitation method. The effects of metal oxide coating on the structure and electrochemical performance of LiFePO4/C composites were systematically investigated. The structure and morphology of the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and the electrochemical properties were evaluated by constant-current charge/discharge cycling tests. It is found that the coating of metal oxide could greatly improve its high-rate dischargeability and cycling performance. The LiFePO4/C cathode material coated by MnO2 exhibits a specific discharge capacity of 118.5 mAh/g at 3C rate, much higher than the uncoated sample (95.1 mAh/g), with a capacity degradation rate of only 6.3 % after 250 cycles at 3C rate.

Enhanced Electrochemical Performance of Metal Oxide Coatings on Graphite in Combination with Fluorine-Free, Aqueous Processable Binder

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Effectiveness of Low-Pressure Metal–Organic Chemical Vapor Deposition Coatings on Metal Surfaces for the Mitigation of Fouling from Heated Jet Fuel

Thin films of alumina, zirconia, tantalum oxide, and platinum were deposited on AISI304 by metal–organic chemical vapor deposition to investigate the effectiveness of these coatings in inhibiting carbon deposition and sulfide formation from thermal oxidative degradation of jet fuel. Coated AISI304 foils were heated in a laboratory scale flow reactor with a commercial jet fuel (Jet-A) flowing at 1 mL/min at a wall temperature of 350 °C and reactor pressure of 500 psig (3.4 MPa) for 5 h. Under these conditions, both liquid phase autoxidation and thermal decomposition of jet fuel contribute to carbon deposition. The surface composition of the metal oxide coatings was found by X-ray photoelectron spectroscopy. The morphology of the coating and the carbonaceous deposits formed during thermal stressing were examined by field emission scanning electron microscopy. The amount of solid carbonaceous deposits on the coated and uncoated surfaces was measured by temperature-programmed oxidation. The effectiveness of the coatings in mitigating carbon deposition was found to decrease in the following order: platinum > Ta2O5 > alumina from acetyl acetonate > ZrO2 > alumina from aluminum trisecondary butoxide > AISI304. The coatings cover the metal surface by forming a protective layer that inhibits the formation of metal sulfides from the reaction of sulfur compounds in jet fuel with iron and nickel on stainless steel and inconel surfaces, respectively. The variation in the activity of the coatings can be attributed to the interaction of oxygenated intermediates formed by autoxidation during thermal stressing with coating surfaces having different degrees of acidity.

The influence of plasma spraying regime and initial substance injection location on the structure of deposited coatings

Atmospheric pressure non-equilibrium plasma spray technology has been developed, tested and successfully employed for deposition of catalytic and tribological coatings. Aluminum hydroxide based coatings doped with carbon particles up to 50μm of thickness were deposited on the stainless steel substrates.Results of experimental investigation on structural characteristics of plasma sprayed coatings in dependence of prevailing external factors are presented. The effect of plasma source operating regime and injection location of precursor dispersed particles (DP) on the morphology and properties of metal oxide coatings was investigated. It has been determined the relation between microstructure of coatings and arc current in plasma source.Significant differences in size and shape of coating grains were observed during deposition process when metal oxide dispersed particles were placed into the reactor channel and directly into the plasma torch discharge chamber together with the reacting gas. The plasma spray pyrolysis process has occurred in some particular regimes which have been successfully applied in the synthesis of micron- and submicron-sized uniform shaped coatings with narrow size pores and controlled surface morphologies.

Plasma Spray-CVD: A New Thermal Spray Process to Produce Thin Films from Liquid or Gaseous Precursors

New dedicated coating processes which are based on the well-known LPPS (TM) technology but operating at lower work pressure (100 Pa) are being actively developed. These hybrid technologies contribute to improve the efficiencies in the turbine industry such as aero-engines and land-based gas turbines. They also have a great potential in the domain of new energy concepts in applications like Solid Oxide Fuel Cells, membranes, and photovoltaic with the adoption of new ways of producing coatings by thermal spray. Such processes include Plasma Spray-Thin Film (PS-TF) which gives the possibility to coat thin and dense layers from splats through a classical thermal spray approach but at high velocities (400-800 m/s) and enthalpy (8000-15000 kJ/kg). Plasma Spray-PVD (PS-PVD) which allows producing thick columnar-structured Thermal Barrier Coatings (100-300 mu m) from the vapor phase with the employment of the high enthalpy gun and specific powder feedstock material. On the other hand, the Plasma Spray-CVD (PS-CVD) process uses modified conventional thermal spray components operated below 100 Pa which allows producing CVD-like coatings (< 1-10 mu m) at higher deposition rates using liquid or gaseous precursors as feedstock material. The advantages of such thermal spray-enhanced CVD processes are the high ionization degree and high throughput for the deposition of thin layers. In this article, we present an overview of the possibilities and limitations encountered while producing thin film coatings using liquid and gaseous precursors with this new type of low pressure plasma spray equipment and point out the challenges faced to obtain efficient injection and mixing of the precursors in the plasma jet. In particular, SiO (x) thin films from Hexamethyldisiloxane (HMDSO or C6H18OSi2) can be deposited on wafers at deposition rates of up to 35 nm/s at an efficiency of about 50%. The process was also used for producing metal oxide coatings (Al2O3, ZnO, and SnO2) by evaporating different metals in combination with an oxygen gas flow. The effect of process parameters on the deposition rate, coating build up, uniformity, and quality of the coatings are discussed. An overview of different potential applications of this new technology will be also presented.

Electrochemical and time-of-flight secondary ion mass spectrometry analysis of ultra-thin metal oxide (Al 2O 3 and Ta 2O 5) coatings deposited by atomic layer deposition on stainless steel

Ultra-thin (5–50 nm) layers of aluminium and tantalum oxides deposited by atomic layer deposition (ALD) on a stainless steel substrate (316L) for corrosion protection have been investigated by electrochemical methods (linear scan voltammetry, LSV, and electrochemical impedance spectroscopy, EIS) and time-of-flight secondary ion mass spectrometry, ToF-SIMS. The effects of the deposition temperature (250 °C and 160 °C) and coating thickness were addressed. ToF-SIMS elemental depth profiling shows a marked effect of the organic and water precursors used for deposition and of the substrate surface contamination on the level of C and OH trace contamination in the coating, and a beneficial effect of increasing the deposition temperature. The polarization data show a decrease of the current density by up to four orders of magnitude with increasing coating thickness from 5 to 50 nm. The 50 nm films block the pitting corrosion in 0.8 M NaCl. The uncoated surface fraction (quantified from the current density and allowing a ranking of the efficiency of the coating, also confirmed by the capacitance and resistance values extracted from the EIS data) was 0.03% with a 50 nm thick Al 2O 3 film deposited at 250 °C. The correlation between the porosity values of the coatings and the level of C and OH traces observed by ToF-SIMS points to a marked effect of the coating contaminants on the sealing performance of the coatings and on the corrosion resistance of the coated systems.

Effective electron emitters by molybdenum oxide-coated carbon nanotubes core–shell nanostructures

In this article, we showed that simple metal oxide coatings such as MoO3 can be an effective enhancer for carbon nanotubes (CNTs) in field emission (FE) performance. For comparison, the FE properties of the pristine vertically aligned multi-walled CNTs with the metal oxide-coated CNTs were investigated. The metal oxide coating of the pristine CNTs was carried out by metal–organic chemical vapor deposition (MOCVD) method at 400 °C using Mo(CO)6 as the precursor. The core–shell structure of the nanocomposite was studied by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) results showed that the surface of the coating material was mainly MoO3. FE test indicated that the MoO3-coated CNTs film exhibited an enhanced performance than the pristine CNTs with a turn-on field of 1.33 V μm−1 and a field enhancement factor β estimated to be ~7000. Ultraviolet photoelectron spectroscopy (UPS) results confirmed a lower electron emission barrier height for MoO3-coated CNTs than for the pristine CNTs. The mechanism of the enhanced FE performance is discussed based on Schottky barrier effect.