Nanostructured Oxide Films Research Articles

Spatially Ordered Matrix of Nanostructured Tin-Tungsten Oxides Nanocomposites Formed by Ionic Layer Deposition for Gas Sensing.

The process of layer-by-layer ionic deposition of tin-tungsten oxide films on smooth silicon substrates and nanoporous anodic alumina matrices has been studied. To achieve the film deposition, solutions containing cationic SnF2 or SnCl2 and anionic Na2WO4 or (NH4)2O·WO3 precursors have been used. The effect of the solution compositions on the films deposition rates, morphology, composition, and properties was investigated. Possible mechanisms of tin-tungsten oxide films deposition into the pores and on the surface of anodic alumina are discussed. The electro-physical and gas-sensitive properties of nanostructured SnxWyOz films have been investigated. The prepared nanocomposites exhibit stable semiconductor properties characterized by high resistance and low temperature coefficient of electrical resistance of about 1.6 × 10−3 K−1. The sensitivity of the SnxWyOz films to 2 and 10 ppm concentrations of ammonia at 523 K was 0.35 and 1.17, respectively. At concentrations of 1 and 2 ppm of nitrogen dioxide, the sensitivity was 0.48 and 1.4, respectively, at a temperature of 473 K. At the temperature of 573 K, the sensitivity of 1.3 was obtained for 100 ppm of ethanol. The prepared nanostructured tin-tungsten oxide films showed promising gas-sensitivity, which makes them a good candidate for the manufacturing of gas sensors with high sensitivity and low power consumption.

Influence of temperature and nickel catalyst on the structural and optical properties of indium oxide nanostructured films synthesized by chemical vapor deposition technique

This work reports the synthesis of indium oxide pyramids, octahedrons, and faceted nanostructures on silicon and nickel-coated silicon substrates via carbothermal reduction of In2O3 powder assisted by a 1% hydrogen/balance nitrogen mixed gas as forming and carrier gas. The influence of deposition temperature on the morphology, structural, and optical properties of the produced films was investigated by varying the source temperature from 850 to 1000 °C. Characterization of the In2O3 nanostructures was performed using FESEM, EDX, XRD, and UV–vis spectrophotometry. Material growth, film thickness, crystallites size, and crystallinity were observed to be enhanced with increased deposition temperature. The use of Ni as a catalyst resulted in the production of high yield films even at the low growth temperature. XRD revealed polycrystalline films with preferential growth in the (222) plane. Films synthesized on Si substrate showed more regular reflection as compared to those grown on Ni/Si. The optical band gap was found to decrease from 3.4 to 2.89 eV with increased deposition temperature. Bandgap narrowing was attributed to the increase in grain size and film thickness.

Atmospheric pressure chemical vapor deposition of indium oxide nanostructured films for photoelectrochemical application

In this work, indium oxide nanostructured films were synthesized by hydrogen assisted atmospheric pressure chemical vapor deposition technique at various deposition temperatures. The surface morphology, crystallinity, and optical properties of the synthesized films were investigated by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and UV–vis-NIR measurement, respectively. Photoelectrochemical (PEC) activity of the fabricated films was investigated in a 0.5 M aqueous solution of KOH under simulated solar illumination (150 W Xenon arc lamp). The result of the indium oxide photoanodes' PEC performance shows that the sample grown at 950°C exhibited optimal performance. It generates an overall photocurrent density of 2.56 mA/cm2, with an incident photon to current conversion efficiency (IPCE) of 37% around λ=390 nm of the incident spectrum. The sample also recorded a significant applied bias to photon conversion efficiency (ABPE) of 0.60%. The superiority of the PEC performance of the sample could be attributed to the film composition, crystallinity and bandgap reduction, as well as low flat band potential and enhanced charge carrier density.

Low-Cost Nanostructured Coating of Anodic Aluminium Oxide Synthesized in Sulphuric Acid as Electrolyte

The anodic oxidation of aluminium is an electrochemical technique that allows obtaining nanostructures with easily adjustable morphology depending on the synthesis variables, for its application in medicine, engineering, biotechnology, electronics, etc. In this work, low-cost aluminium oxide nanostructured films were synthesized and morphologically characterized using two anodization steps in sulphuric acid, varying the concentration and temperature of the electrolyte and anodization voltage. The order of the porous matrix, pore diameter, interpore distance, pore density, thickness, and porosity were measured and statistically analyzed. The results showed that under the proposed conditions it is possible to synthesize low-cost nanoporous aluminium oxide films, with a short-range ordering, being the best ordering conditions 10 °C and 0.3 M sulphuric acid at 20 V and 5 °C and 2 M sulphuric acid at 15 V. Furthermore, it was determined that the pore diameter and the interpore distance vary proportionally with the voltage, that the pore density decreases with the voltage and increases with the concentration of the electrolyte, and that the thickness of the oxide film increases with electrolyte concentration, temperature, and anodization voltage.

Metalizadora Construída em Laboratório: Deposição de Filme de Alumínio sobre Substrato de Vidro e Posterior Anodização para Obtenção de Óxido de Alumínio

In this work, the assembly of a low-cost metallizer is presented, based on physical vapor deposition, to obtain metallic aluminum film deposited over a glass substrate. Later, the Al film is anodized and converted into a porous nanostructured oxide film. The metallic film thickness was measured by profilometry, and the sample position inside the chamber was evaluated. Samples positioned at the center of the sample holder showed considerable thickness and best homogeneity compared to those samples positioned at the edges of the sample holder. A thick metallic film of Al (6 to 7 µm) was deposited over the glass substrate for subsequent anodizing treatment. Mild anodization allowed the total conversion of metallic Al after 7000 s, producing a transparent anodic aluminum oxide (AAO) film adhered to the glass substrate. The AAO morphology was investigated by SEM. Some factors have been listed to describe the low regularity and homogeneity of nanopores in the outer layer of AAO, such as internal defects, compaction of deposited Al film layers and the glass substrate roughness.

Constructing nanostructured functional film on EH40 steel surface for anti-adhesion of Pseudomonas aeruginosa

Biofilm formation is an important cause of microbiologically influenced corrosion (MIC), and inhibition of microbial adhesion is one of the most effective ways to control biofilm formation. In this work, nanostructured functional film of EH40 steel is prepared by one-step anodization in NaOH solution to inhibit adhesion of Pseudomonas aeruginosa (P. aeruginosa) without and with light. Results show that the functional films fabricated on EH40 steel surface in a series of NaOH solutions inhibit the adhesion of P. aeruginosa, and its anti-adhesion performance first increases and then decreases with varying concentrations of NaOH solution. The optimal anti-adhesion performance under dark condition, i.e., anti-adhesion rate of 92.7% compared with bare steel, is obtained for the nanostructured oxide film fabricated in 12 M NaOH solution under anodization potential of 1.0 V for 2 h. Under visible light irradiation, the best anti-adhesion rate increases to 99.9% due to hydroxyl radicals (OH) produced by the nanostructured oxide film with photocatalytic activity.

(Invited) Anodizing of Metals for Fabrication of Functional Nanostructured Surfaces

Anodizing of metals, including aluminum, titanium, and iron, is a simple and const-effective process to form various nanostructured oxide films, such as nanoporous, nanotubular, and nanowire films. Such nanostructured anodic films have attracted much attention for many potential functional applications as well as fundamental interests on the mechanism of self-organized nanostructure formation. Here, our recent studies on the formation of nanostructured anodic films on aluminum, copper, and zinc and their application to controlled wettability will be outlined.Anodizing of zinc in KHCO3 electrolyte produces a nanowire-type anodic film on entire metal surface. The formation of the nanowire film involves the initial pitting corrosion; pits are developed on the zinc metal surface, and embryos of nanowires are formed within the pits. When the pits are covered with a thin air-formed or electropolishing film, the nanowire formation is promoted as a consequence of the increased concentration of dissolved zinc species within the pits. Then, the nanowires grow toward the outside of the pits and cover the entire metal surfaces. Because of this growth process, the nanowires grow radial direction before covering the entire surface. The nanowire surface is one of the promising geometry for superhydrophobicity. In fact, anodic nanowire film surface becomes superhydrophobic after fluoroalkylsilane monolayer coating.The anodic nanowire film is also developed on copper by anodizing in KOH electrolyte. The copper surface can be completely covered with the nanowires only for 30 s at a constant current density of 10 mA cm–2. The nanowires consist of a single crystalline Cu(OH)2 phase, and the nanowires on copper are formed uniformly without forming pits. The nanowires are reduced readily to copper metal with the nanowire morphology remaining almost unchanged, although the copper metal nanowires are polycrystalline. The copper metal nanowires are also superhydrophobic after fluoroalkylthiol monolayer coating. The copper substrate with the superhydrophobic metal nanowires maintain the high thermal conductivity and effectively removes the condensed water droplet, being very suitable for heat exchanger applications with high efficiency.Aluminum is the most suitable metal to control the surface morphology by wet process. The author’s group successfully fabricated hierarchically micro-/nano-porous aluminum surface by a combination of chemical etching and anodizing in acid electrolytes. Micropits are developed by chemical etching containing HCl solution and the HCl concentration control the size of pits. Nanopores are generated by anodizing in sulfuric acid or oxalic acid, the pore widening process can be applied to control the pore size and porosity. The hierarchically porous aluminum surface is used to form super-repellency even for low surface tension liquids, including rapeseed oil (surface tension of 35 mN m–1), hexadecane (28 mN m–1), octane (22 mN m–1), and even hexane (18 mN m–1). The nanopores can be used to infiltrate the self-healing agent of organic coating, so that self-healing super-liquid-repellency occurs repeatedly even if the thin organic coating is damaged. The porous aluminum surface is also suitable for fabricating the slippery liquid infused porous surface (SLIPS) by impregnating a lubricant liquid into the pores. The SLIPS on aluminum show excellent liquid slipping property, anti-snow-sticking property, and high corrosion resistance.References T. Inoue, A. Koyama, D. Kowalski, C. Zhu, Y. Aoki and H. Habazaki, physica status solidi (a), 1900836 (2020). K. Nakayama, A. Koyama, C. Zhu, Y. Aoki and H. Habazaki, Advanced Materials Interfaces, 5, 1800566 (2018). K. Nakayama, E. Tsuji, Y. Aoki, S.-G. Park and H. Habazaki, The Journal of Physical Chemistry C, 120, 15684 (2016).

Characterization Studies of Nickel Oxide Nanostructure films Prepared by Electrolysis Method for Photo Detectors Applications

Characterization Studies of Nickel Oxide Nanostructure films Prepared by Electrolysis Method for Photo Detectors Applications

Interplay among work function, electronic structure and stoichiometry in nanostructured VOx films.

The work function is the parameter of greatest interest in many technological applications involving charge exchange mechanisms at the surface. The possibility to produce samples with a controlled work function is then particularly interesting, albeit challenging. We synthetized nanostructured vanadium oxide films by a room temperature supersonic cluster beam deposition method, obtaining samples with tunable stoichiometry and work function (3.7-7 eV). We present an investigation of the electronic structure of several vanadium oxide films as a function of the oxygen content via in situ Auger, valence-band photoemission spectroscopy and work function measurements. The experiments probed the partial 3d density of states, highlighting the presence of strong V 3d-O 2p and V 3d-V 4s hybridizations which influence 3d occupation. We show how controlling the stoichiometry of the sample implies control over work function, and that the access to nanoscale quantum confinement can be exploited to increase the work function of the sample relative to the bulk analogue. In general, the knowledge of the interplay among work function, electronic structure, and stoichiometry is strategic to match nanostructured oxides to their target applications.

Generation of molybdenum hydride species via addition of molecular hydrogen across metal-oxygen bond at monolayer oxide/metal composite interface

Generation of molybdenum hydride species on monolayer oxide/metal composite via addition of molecular hydrogen across metal-oxygen bond is investigated for the first time utilizing periodic Van der Waals density-functional calculations. Lewis acid-base pair constructed by the interfacially defected oxide film and the metal support provides novel active sites for activating H2. The produced heterolytic dissociative state exhibits negative dissociative adsorption energy of −0.315 eV which thermodynamically facilitate the dissociation process of H2 on insulating oxide films. The penitential energy pathways are calculated to reveal the dynamics and reaction processes for H2 splitting at the oxide-metal interface. The differential charge density contour, electronic density plots, particular occupied orbitals, work function and electron localization function of H2 dissociation are interpreted to better understand the electronic properties of the unique dissociation behavior of H2 at interfacially defected magnesia. It is anticipated that the results here could help understand the mechanism of hydrogenation reactions on nanostructured oxide film and provide useful clue for enhancing the reactivity of insulating oxide toward activating H2.

(Invited) Formation, Characterization and Applications of Nanowire Films Formed By Anodizing of Metals

Anodizing of metals and alloys is a simple electrochemical process to form various nano-structured oxide films, including self-ordered nanoporous and nanotubular films. Typical examples are nanoporous alumina films with honeycomb morphology and nanotubular array of anodic titanium oxide. Recently, several nanowire-type anodic films have been prepared on metals, including aluminum, copper and zinc. The formation mechanism of of the nanowire films may be different from those of the nanoporous and nanotubular anodic films. Here, our recent studies on the formation, characterization and applications of the anodic nanowire films on metals are presented. When zinc plate was anodized at a constant voltage in KHCO3 aqueous electrolytes, nanowires were covered on the entire zinc surface. From the SEM observations, it was found that pits were initially formed on zinc surface. The low-voltage SEM observations disclosed that some small pits were covered with a thin passive film, and in such pits nanowires were preferentially nucleated probably because of the increased concentration of dissolved Zn2+ ions in the pits. Then, the nanowires were grown radially over the pits and covered the entire zinc surface. The nanowires were not composed of ZnO. XPS and FT-IR analyses of the nanowire films indicated the presence of carbonate and hydroxide species; the nanowire consisted of basic zinc carbonate. TEM observations confirmed the single crystalline nature of the as-formed nanowires. The basic zinc carbonate was converted to ZnO without changing the nanowire morphology by heat treatment in air, although single crystalline nanowires changed to polycrystalline one. In addition, slit-type nanopores were introduced into the nanowires so that the specific surface area of the nanowire films was highly enhanced by the heat treatment. The nanowire films exhibits unique wettability to liquids. The as formed nanowire films showed superhydrophilicity with a water contact angle close to 0o and after organic coating the surface changed to superhydrophobic with a water contact angle greater than 150o. Such surface may have many potential applications.

Microwave-assisted hierarchical bismuth oxide worm-like nanostructured films as room-temperature hydrogen gas sensors

With a plausible reaction mechanism, a simple and rapid, microwave-assisted chemical synthesis method has been proposed to obtain hierarchical worm-like bismuth oxide (Bi2O3) nanostructured films as an efficient and effective hydrogen (H2) gas sensors. The structural elucidation demonstrates a diffraction peak at 27.94° for [201] directional growth with a lattice fringe distance of 0.31 nm. The Raman shift and photoelectron spectroscopy measurements, additionally, support the formation of the phase pure Bi2O3. Estimated 14.88 m2g-1 specific surface area and 10–20 nm pore-size of as-obtained Bi2O3 evidences its mesoporous character. Among various gases tested, H2 gas endows 50% sensing performance for hierarchical Bi2O3 worm-like film sensors with a considerable response of 42 s and recovery of 83 s for 100 ppm H2 gas concentration at room-temperature, suggesting an importance of proposed method in obtaining the phase pure Bi2O3 film sensors. The H2 gas sensing mechanism has been proposed on X-ray photoelectron spectroscopy results. Finally, an influence of a relative humidity on the Bi2O3 film sensor has demonstrated 32% response at 20% RH with response/recovery time of just 7/10 s, owing to its unique surface architecture, high surface area and mesoporous nature.

Electrochemical activity of the polycrystalline cerium oxide films for hydrogen peroxide detection

Abstract Polycrystalline cerium oxide thin films (15 nm) deposited on a glassy carbon substrate were used as an electrode in a mediator-free, non-enzymatic electrochemical sensor for hydrogen peroxide. The electrode surface was characterized by X-ray photoelectron spectroscopy, resonant photoelectron spectroscopy, scanning electron microscopy and atomic force microscopy. The electrode sensitivity, detection limit and pH range of sensor stability were determined by applying electrochemical techniques: cyclic voltammetry and chronoamperometry. It was found that the sensor reactivity to H2O2 is directly related to the presence of electroactive cerium centres of 3+ character on the electrode surface. The Michaelis–Menten mechanism of catalase-like activity of ceria film is suggested as an explanation of the data and discussed. The results confirmed the sensing abilities of technologically well-accessible nanostructured cerium oxide films for hydrogen peroxide detection without using a mediator, i.e. the enzymatic properties of CeO2/GC electrode.

CeO2/CuOx Nanostructured Films for CO Oxidation and CO Oxidation in Hydrogen-Rich Streams Using a Micro-Structured Reactor

The CO oxidation reaction has a strong implication in environmental catalysis; besides a promising performance of CeO2/CuOx systems in inverse configuration has been demonstrated, in which ceria is deposited in larger copper oxide particles that act as support. In this work, in situ growths of nanostructured copper oxide films (CuOx) modified with ceria nanoparticles (Nps) were obtained, which were studied in the CO oxidation and the CO oxidation in hydrogen-rich streams (COProx). There was an effective stabilization of ceria Nps on the nanometric crystals of CuOx growths and the CeO2/CuOx films exhibited an enhanced activity in the oxidation of CO and COProx due to new active sites developed at the contact interface between both oxide nanostuctures. The microreactors were stable for 24 h in reaction at high conversion levels without deactivating, thus preserving the high synergy between CeO2 and CuOx. In COProx, the selectivity at high temperature was also improved by reducing the amount of oxygen in the reagents stream that lowered the contribution of the H2 oxidation undesirable reaction. Micro-structured reactors with inverse CeO2/CuOx phase have potential as a simple low cost alternative with high performance in the oxidation of CO in both air and hydrogen-rich streams.

Anodization of titanium alloys for orthopedic applications

In recent years, nanostructured oxide films on titanium alloy surfaces have gained significant interest due to their electrical, catalytic and biological properties. In literature, there is variety of different approaches to fabricate nanostructured oxide films. Among these methods, anodization technique, which allows fine-tuning of oxide film thickness, feature size, topography and chemistry, is one of the most popular approaches to fabricate nanostructured oxide films on titanium alloys, and it has been widely investigated for orthopedic applications. Briefly, anodization is the growth of a controlled oxide film on a metallic component attached to the anode of an electrochemical cell. This review provides an overview of the anodization technique to grow nanostructured oxide films on titanium and titanium alloys and summarizes the interactions between anodized titanium alloy surfaces with cells in terms of cellular adhesion, proliferation and differentiation. It will start with summarizing the mechanism of nanofeatured oxide fabrication on titanium alloys and then switch its focus on the latest findings for anodization of titanium alloys, including the use of fluoride free electrolytes and anodization of 3D titanium foams. The review will also highlight areas requiring further research to successfully translate anodized titanium alloys to clinics for orthopedic applications.

Vanadium oxide nanostructured thin films prepared by Aerosol Spray Pyrolysis for gas sensing and thermochromic applications

Vanadium oxide (VxOy) nanostructured films were prepared by Aerosol Spray Pyrolysis technique with different concentrations of precursor solution (0.005 M, 0.01 M, 0.02 M), using ammonium metavanadate (NH4VO3) as precursor. The deposition was carried out onto glass substrates (Corning glass) at two different substrate temperatures (300 °C and 400 °C). Furthermore, oxalic acid was added to the precursor solution in order to grow the thermochromic VO2(M) monoclinic phase via reduction of the Vanadium oxides in which Vanadium is in a higher oxidizing state. All films were characterized by X-Ray Diffraction technique and UV/Vis/NIR spectroscopy to identify their structural and optical properties, respectively. V2O5 films were tested against ozone, having a sensitivity of 29% in ultra-low concentration of 5ppb ozone, at room temperature. Moreover, nanostructured VO2 films appeared to be thermochromic, having an IR-switching of about 5% at λ = 2500 nm, upon heating.

Synthesis of Bi2O3/TiO2 nanostructured films for photocatalytic applications

Dendritic growth of bismuth oxide nanostructured films was accomplished by reactive magnetron sputtering. The deposition of the Bi2O3 template layers was adapted to abide a vapour-liquid-solid mechanism in order to develop a 3D growth morphology with high surface area templates for photocatalytic applications. TiO2 photocatalytic thin films were deposited at a later stage onto Bi2O3 layers. The obtained heterostructured films were characterized by scanning electron microscopy, X-ray diffraction and atomic force microscopy. Additionally, the photocatalytic efficiency was assessed by conducting an assay using methylene blue dye as testing pollutant under a UV-A illumination. The photocatalytic tests revealed that the Bi2O3 layers functionalized with TiO2 thin films are more efficient at degrading the pollutant, by a factor of 6, when compared with the individual layered films.

Structural, morphological and optical studies of nanostructured cadmium oxide films: the role of pH

In this work, nanocrystalline cadmium oxide (CdO) films were deposited on glass substrates for various pH values ranging from 11.3 to 12.5 by successive ionic layer adsorption and reaction (SILAR) technique. To obtain good film stoichiometry, the annealing of films in static air at a temperature of 623 K for 1 h was carried out. The comparison of structural, morphological and optical properties of CdO films at different pH of the solution has been studied. The effects of pH on the crystal structures, the orientation of crystallization and crystallite sizes i.e., structural properties of the films were analyzed with X-ray diffraction (XRD). XRD analysis revealed the polycrystalline nature of the CdO films with cubic structure and show preferential orientation along (111) and (200) planes. The surface morphology of the prepared films having different pH values was investigated by scanning electron microscope (SEM). The optical band gap of nanocrystalline CdO films at different pH has been measured using Ultraviolet–Visible (UV–Vis) spectroscopy. The optical properties showed that a significant reduction in the optical band gap is observed from 3.26 to 2.35 eV by increasing the pH value of the solutions from 11.3 to 12.5, respectively.

Exploiting the dodecane and ozone sensing capabilities of nanostructured tungsten oxide films

Tungsten oxide thin films are grown by DC reactive magnetron sputtering combining glancing angle deposition (GLAD) and reactive gas pulsing process (RGPP). Inclined, zigzag and spiral columnar architectures are produced with various oxygen injections during the growth. The dodecane and ozone sensing properties of these tungsten oxide nanostructured active layers are comparatively studied with that of films deposited by conventional sputtering process. Microstructure, morphologies and electrical behaviors are characterized as a function of the growing conditions. As-deposited films systematically exhibit an amorphous crystal structure with a porous microstructure, which depends on the GLAD architecture. Annealing at 300 °C in air for 12 h leads to a polycrystalline structure keeping voided networks. Dodecane and ozone sensing performances are the most significant for inclined columnar architectures but they also depend on the RGPP sputtering conditions. As a result, the GLAD + RGPP combination proves to be a relevant strategy for the fabricating tungsten oxide nanostructured films as active layers with an attractive potential for gas sensors.

Field emission study of urchin like nanostructured cobalt oxide films prepared by pulsed laser deposition

Cobalt oxide (Co3O4) urchin structures have been grown via Pulsed Laser Deposition (PLD) technique followed by thermal treatment in air. Morphological and structural analysis of as synthesized nanostructures has been carried out using Field Emission Scanning Electron Microscopy (FESEM) and X-ray Diffraction (XRD) technique. Chemical nature of the films have been revealed using X-ray Photoelectron Spectroscopy (XPS). Field emission measurements were carried out at a base pressure of 1 × 10−8 mbar. Turn-on field corresponding to an emission current density of 10 μA/cm2 was found to be 3 V/μm and a high field enhancement factor of ∼3900 was estimated, both of which can be attributed to radially grown nanowires of long length with sharp tips. Overall good field emission current stability was observed for Co3O4 urchin structured films. Dependence of FE properties such as, current density and current stability on morphology and density of emitting sites has been observed and analyzed systematically. The observed FE results demonstrate Co3O4 urchin structures as potential candidate for applications in various vacuum micro/nanoelectronic devices.