Nanoporous Oxide Research Articles

Structural Color Control of CoFeB-Coated Nanoporous Thin Films

Unlike color dyes, structural colors only slightly fade during long-term usage. Here, structural colors were controllably achieved by constructing CoFeB photonic crystal layers on the surface of a nanoporous aluminum oxide (AAO) substrate by magnetron sputtering deposition. The resulting material showed a wide visible spectral response and achieved structural color control with a high resolution, high color purity, and saturation. The angle-dependent color changes of CoFeB@AAO films were further investigated by changing the incident light angle. The simulation results of the model are consistent with the experiments, which is significant in practical applications. This strategy may have great potential applications for solid structure color coatings, anti-counterfeiting and security, information storage, and electromagnetic sensors.

Flexible iron oxide supercapacitor electrodes by photonic processing

We report the preparation of flexible and nano-porous iron oxide-reduced graphitic oxide (Fe2O3–rGO) electrodes using a novel photonic processing method. Due to this unique technique, high-temperature thermal processing could be accomplished on inexpensive and low-temperature substrates instantaneously as opposed to longer processing times of conventional thermal processing. The nano-porous morphology of the electrode not only accommodates the volume changes of the electrode but also facilitates the transport of the electrolyte ions into the electrodes. The as-prepared electrode showed excellent electrochemical performance with an initial specific capacitance of 179 F/g at 2 A/g. Moreover, it exhibited excellent specific capacitance retention after 5000 cycles (70%), revealing its superior cyclic stability. Along with having specific capacitance comparable to that of rigid electrodes, the as-prepared electrode is bendable and lightweight, signifying its potential application in foldable and wearable consumer electronic devices which require continuous energy supply while going through physical deformation.Graphic abstract

Features of Electroluminescence of Aluminum and Its Nanoporous Oxide in Esters and Ketones

Features of Electroluminescence of Aluminum and Its Nanoporous Oxide in Esters and Ketones

Laser Irradiation for Enhancing Mass Transfer in the Solid Phase of Metallic Materials

Possibilities of using laser irradiation to enhance mass transfer in the solid phase of metallic materials in order to form structures with improved physical and mechanical properties were reviewed. The features of the diffusion mass transfer in metals and alloys under shock exposure were specified. In this case, the rate of diffusion processes of mass transfer can be significantly increased. The conditions for intensification of mass transfer in metallic materials by pulse-periodic laser irradiation were determined and the synthesis of nanoporous and composite oxide nanomaterials was described. A significant increase of the diffusion coefficient in a metallic material, in comparison to plain exposure to laser beam heating, was identified. It could be attributed to the synergy of heat exposure and laser-induced vibrations, mainly in the range of sound frequencies, as a result of a pulse-periodic laser irradiation. The condition for intensifying mass transfer in the solid phase of selectively oxidable metallic materials was identified as a non-stationary stress-strain state caused by laser-induced sound waves. The exploitation of this synergy effect permitted the implementation of a novel approach for the creation of structures of nanomaterials. At the same time, a targeted influence on mass transfer and the accompanying relaxation processes make it possible to achieve an increase in the efficiency of methods for processing metals and alloys.

Low-cost high- performance NO2 sensor based on nanoporous indium tin oxide (ITO) film

The electrochemical corrosion of commercial indium tin oxide (ITO) glasses enabled the preparation of a low-cost nanoporous ITO film-based sensor exhibiting a high NO2 response. The NO2 sensing characteristics of the nanoporous ITO films were studied at different processing voltages and test temperatures. Under the optimal test condition (10 V, 100 °C), the response of the sensor was 3.1 to 0.1 ppm NO2 and 180–500 ppm NO2. Compared with previous literature, the as-obtained sensor has a low working temperature (100 °C), wide detection range (0.1–500 ppm NO2), and high response (12.34 1 ppm NO2). Moreover, the sensor exhibited excellent repeatability and a high selectivity toward NO2. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller was employed to study the structural changes of the ITO film due to electrochemical corrosion and revealed a nanoporous structure with abundant surface hydroxyl groups. The influences of the nanoporous structure and surface hydroxyl groups on the response are discussed in detail. Finally, it was found that the Langmuir model provided a good fit to the working curve of the sensor, and thus explains the mechanism of the enhanced response based on the nanoporous structure. Overall, this study demonstrates that an electrochemically corroded ITO film is suitable for detecting NO2.

Confined Crystallization and Melting Behaviors of 3-Pentadecylphenol in Anodic Alumina Oxide Nanopores.

To explore the effects of end groups on the confined crystallization of an alkyl chain, 3-pentadecylphenol (PDP) was infiltrated into the anodic aluminum oxide template (AAO) to investigate the melting and crystallization behaviors of PDP in a nanoconfined environment. Wide-angle X-ray diffraction (WAXD) found that the solid–solid phase transition of PDP occurred under confined conditions, and the absence of the (00L) reflections indicated that the stacking of the end groups of the alkyl chain layered structure was seriously disturbed. Thermal analysis (TG) showed that the thermal stability of the confined samples decreased due to the confinement effect, and the introduction of end groups made the confinement effect more obvious. Differential scanning calorimeter (DSC) results well reflected the space–time equivalence in the PDP crystallization processes, i.e., the solid–solid phase transition can be achieved by reducing the cooling rate or confining PDP in the nanometer space. Compared with C15, the introduction of the end groups with a phenol ring led to the disappearance of the solid–solid phase transition of an alkyl chain at high cooling rates. In the confined environment, the introduction of the end groups with a phenol ring caused the melting double peaks of the alkyl chain to become a single melting peak, and it also caused the disappearance of the surface freezing monolayer for alkyl chains. Through the analysis of crystallinity, it was found that AAO-PDP was more sensitive to AAO pore size changes than AAO-C15, the Xc of AAO-PDP had a good linear relationship with the pore size d, but the Xc of the AAO-C15 had a nonlinear relationship with the pore size d. Attenuated total reflection (ATR)-IR proved that in the confined environment, the order of the alkyl chain decreased and the degree of chain distortion increased.

Nanoporous Nickel-Molybdenum Oxide with an Oxygen Vacancy for Electrocatalytic Nitrogen Fixation under Ambient Conditions.

The electrochemical nitrogen reduction reaction (NRR) is regarded as a sustainable method for N2 fixation. N2 adsorption and N≡N cleavage are the main challenges for the NRR. Herein, we propose a potential approach to enhance N2 activation via introducing oxygen vacancies (OVs) into nanoporous NiO/MoO3. Nanoporous NiO/MoO3 with OVs (np-OVs-NiO/MoO3) is prepared by a two-step process of dealloying and solid-state reaction. np-OVs-NiO/MoO3 exhibits a high NH3 yield of 35.4 μg h-1 mgcat-1 and a Faradaic efficiency (FE) of 10.3% in 0.1 M PBS solution. The introduction of OVs enhances the conductivity, N2 adsorption, and catalytic performance of np-NiO/MoO3. The dual-metal sites with OVs have a unique electronic structure in favor of the "π back-donation" behavior, which decreases the energy barrier of protonation steps and improves the whole NRR process. This approach provides new insight into the design of composite transition metal oxides with OVs for the NRR catalyst under ambient conditions.

Performance Comparison between the Nanoporous NiO x Layer and NiO x Thin Film for Inverted Perovskite Solar Cells with Long-Term Stability.

The development of hole-transport layers (HTLs) that elevate charge extraction, improve perovskite crystallinity, and decrease interfacial recombination is extremely important for enhancing the performance of inverted perovskite solar cells (PSCs). In this work, the nanoporous nickel oxide (NiOx) layer as well as NiOx thin film was prepared via chemical bath deposition as the HTL. The sponge-like structure of the nanoporous NiOx helps to grow a pinhole-free perovskite film with a larger grain size compared to the NiOx thin film. The downshifted valence band of the nanoporous NiOx HTL can improve hole extraction from the perovskite absorbing layer. The device based on the nanoporous NiOx layer showed the highest efficiency of 13.43% and negligible hysteresis that was better than the one using the NiOx thin film as the HTL. Moreover, the PSCs sustained 80% of their initial efficiency after 50 days of storage. This study provides a powerful strategy to design PSCs with high efficiency and long-term stability for future production.

Effects of surface nanocrystallization on the anodic oxidation behavior of Aluminum

Gradient microstructures on the surface of metals induced by severe shot peening (SSP) is relatively a new research topic in the field of severe plastic deformation processing. The treated surface is known to show three distinct features: a spectrum of grain size from nanometer to sub-micron to micron regime, a work-hardened surface layer, and a compressive residual stress profile near the surface. The current study is defined to investigate the effects of SSP pre-preprocessing on the anodization behavior of Aluminum. Gradient microstructures were obtained on AA1050 Al substrate using SSP with different coverage levels. SSP was performed with ceramic media to prevent contamination of the treated surface. Then the treated surface was anodized at a constant voltage in a sulfuric acid solution. The current-time responses were recorded to study the reaction kinetics. The average grain size on the surface treated by SSP was evaluated to be 255.6 nm and 181.5 nm for 1000% and 2000% coverage levels, respectively, using X-ray diffraction; while the Vickers microhardness was increased from 10.8 HV for the untreated sample to 19.8 HV and 25.8 HV by SSP with 1000% and 2000% coverage levels, respectively. It was found that the nanocrystalline microstructure of Al substrate enhances the growth rate of anodic oxide. A variable growth rate is detected within the treated layer. The nucleation of anodic oxide nanopores is discussed in terms of underlying oxide formation mechanisms and the extent of surface grain refinement.

Understanding Oxygen Release from Nanoporous Perovskite Oxides and Its Effect on the Catalytic Oxidation of CH4 and CO.

The design of nanoporous perovskite oxides is considered an efficient strategy to develop performing, sustainable catalysts for the conversion of methane. The dependency of nanoporosity on the oxygen defect chemistry and the catalytic activity of perovskite oxides toward CH4 and CO oxidation was studied here. A novel colloidal synthesis route for nanoporous, high-temperature stable SrTi0.65Fe0.35O3-δ with specific surface areas (SSA) ranging from 45 to 80 m2/g and pore sizes from 10 to 100 nm was developed. High-temperature investigations by in situ synchrotron X-ray diffraction (XRD) and TG-MS combined with H2-TPR and Mössbauer spectroscopy showed that the porosity improved the release of surface oxygen and the oxygen diffusion, whereas the release of lattice oxygen depended more on the state of the iron species and strain effects in the materials. Regarding catalysis, light-off tests showed that low-temperature CO oxidation significantly benefitted from the enhancement of the SSA, whereas high-temperature CH4 oxidation is influenced more by the dioxygen release. During isothermal long-term catalysis tests, however, the continuous oxygen release from large SSA materials promoted both CO and CH4 conversion. Hence, if SSA maximization turned out to efficiently improve low-temperature and long-term catalysis applications, the role of both reducible metal center concentration and crystal structure cannot be completely ignored, as they also contribute to the perovskite oxygen release properties.

Utilization of Nanoporous Nickel Oxide as the Hole Injection Layer for Quantum Dot Light-Emitting Diodes.

Nickel oxide (NiOx) has been extensively investigated as the hole injection layer (HIL) for many optoelectronic devices because of its excellent hole mobility, high environmental stability, and low-cost fabrication. In this research, a NiOx thin film and nanoporous layers (NPLs) have been utilized as the HIL for the fabrication of quantum dot light-emitting diodes (QLEDs). The obtained NiOx NPLs have spongelike nanostructures that possess a larger surface area to enhance carrier injection and to lower the turn-on voltage as compared with the NiOx thin film. The energy levels of NiOx were slightly downshifted by incorporating the nanoporous structure. The amount of Ni2O3 species is higher than that of NiO in the NiOx NPL, confirming its good hole transport ability. The best QLED was achieved with a 30 nm thick NiOx NPL, exhibiting a maximum brightness of 68 646 cd m–2, a current efficiency of 7.60 cd A–1, and a low turn-on voltage of 3.4 V. More balanced carrier transport from the NiOx NPL and ZnO NPs/polyethylenimine ethoxylated (PEIE) is responsible for the improved device performance.

An additive-free silicon anode in nanotube morphology as a model lithium ion battery material

Ordered arrays of parallel, cylindrical silicon nanotubes are obtained by aluminothermic reduction of SiO2 nanotubes generated by atomic layer deposition (ALD) on nanoporous aluminum oxide templates. The reduction to amorphous Si (a-Si) is characterized by a combination of X-ray diffraction (XRD), solid-state cross-polarization magic-angle spinning nuclear magnetic resonance (29Si CP-MAS NMR), ultraviolet-visible spectroscopy, attenuated total reflectance infrared spectroscopy (ATR-IR), and X-ray photoelectron spectroscopy (XPS). These a-Si nanotube arrays are electrochemically active in a lithium-ion battery environment when prepared on Cu current collectors without any additives. The absence of the traditional additive carbon black, which is an electrochemically inert conductor, increases the proportion of capacity associated with faradaic reactions (Li incorporation) with respect to the capacitive component. Electrochemical impedance spectroscopy (EIS) and charge-discharge tests demonstrate that the nanotube morphology yields an improved tolerance to fast cycling.

DNA Aptamers for Early Detection of Ebolavirus

Nucleic acid aptamers are emerging as the new generation molecular recognition elements for diagnostics based on their synthetic nature, stability under a wide range of temperatures and amenability to different sensing platforms. Aptamers can further be modified in sequence and chemically to increase their specificities and affinities for their target molecule and to enhance their stabilities in the presence of nucleases. Such characteristics are compatible with the small electrochemical aptasensor that we are developing for point of care diagnosis of infectious virus species. Ebola virus, which kills up to 90% of those it infects, is one of the deadliest known viruses. Being highly contagious, the Ebola virus calls for rapid diagnosis for isolation of infected individuals to prevent viral outbreak. For this purpose, we have selected DNA aptamers with high affinities and specificities for Ebola virus (EBOV) soluble glycoprotein (sGP). The presence of sGP in abundance into the blood stream of infected individuals even during the early stages of infection, makes it an excellent biomarker for diagnosis. The selected aptamers functioned well on a portable nanoporous aluminum oxide (NAAO) sensor to detect sGP in Ebola infected macaque serum samples. This NAAO sensor which allows label free detection of ebola infected serum samples, now being tested against many samples for accuracy of reporting, has the potential to greatly accelerate EBOV diagnosis on site and to automatically relay that information to central authorities, thereby preventing the spread of this highly contagious virus and saving thousands of lives.

Water-assisted crystallization of nanoporous tin oxide formed by anodic oxidation on cold sprayed tin coating

In this study, nanoporous tin oxide films were formed on cold sprayed tin coating via a modified electrochemical anodization process. The amorphous as-anodized was crystallized during a facile process with the assistance of hot water. The morphology and crystallinity of tin oxide films during thermal annealing and annealing by induced of hot water (50, 75, and 100 °C) during the various time were investigated by X-ray diffraction (XRD), field-emission scanning electron microscope (FESEM), and transmission electron microscope (TEM). It was observed that the crystallized tin oxide with the assistance of water was formed by the dissolution-precipitation mechanism. During this process, rutile SnO2 was formed through reorganization of the SnO6 octahedral units. Also, the texture coefficient and the degree of preferred orientation for different conditions were calculated from XRD spectra. It was found that the degree of preferred orientation of tin oxide film remains almost constant during annealing for the sample annealed by water with a temperature of 50 °C. However, for other samples, the degree of preferred orientation decreased during annealing.

Fast electrochemical activation of the broadband saturable absorption of tungsten oxide nanoporous film

The on-demand modulation of defects in materials for the effective modulation of optical nonlinearity is desirable, while it remains a great challenge. In this work, we demonstrate that electrochemical activation is a facile and convenient approach to modulating the broadband third-order nonlinear absorption of nanoporous tungsten oxide (WO3−x) thin film. The film does not exhibit optical nonlinearity at the initial state, while shows a distinct saturable absorption under an applied voltage of −2.5 V with the excitation of 515, 800, and 1,030 nm laser. The nonlinear absorption coefficient (βeff) is −766.38 ± 6.67 cm·GW−1 for 1,030 nm laser, −624.24 ± 17.15 cm·GW−1 for 800 nm laser, and −120.70 ± 11.49 cm·GW−1 for 515 nm laser, and the performance is competitive among inorganic saturable absorbers. The activation is accomplished in 2 min. The performance enhancement is ascribed to the formation of abundant in-gap defect states because of the reduction of the tungsten atoms, and a Pauli-blocking effect occurs during the excitation of in-gap defect states. The small feature size of WO3−x (∼ 12 nm) enables the effective and fast introduction and removal of the defects in porous film, and accordingly the fast and broadband modulation of optical nonlinearity. Our results suggest a controllable, effective, and convenient approach to tuning the nonlinear absorption of materials.

Strong coloration of nanoporous tungsten oxides by in-vacuo lithiation for all-solid-state electrochromic devices

Electrochromic windows allow for an improved management of solar heat gains in buildings and enhanced visual comfort of building users. All-solid-state electrochromic devices can be especially durable. During their fabrication lithium has to be inserted, preferably in-vacuo. For such a dry lithiation process it is essential to determine to which extent sufficiently thick electrochromic layers can be colored and with which efficiency. In this study, we focus on the dry lithiation of sputtered nanoporous tungsten trioxide films and the induced optical response. The porosity of the WO3 films was in the order of 18 %, as determined from variable angle spectroscopic ellipsometry and effective medium approximation. Films with a thickness above 1 µm were dry-lithiated yielding a deep coloration, with a lithiation efficiency LE up to 1.5⋅106 cm2⋅mol−1. Hereby the light transmittance was reduced from 85.7% to 2.0%, with the spectral transmittance approaching 0% in the range from 600 nm to 1500 nm. These results suggest that lithium can diffuse over a large distance in nanoporous WO3. The columnar morphology and nanoporous structure of the thin films deposited at high working pressure is beneficial for the displacement of the lithium ions. All-solid-state devices were prepared using in-vacuo lithiation, tantalum pentoxide as a solid electrolyte and nickel oxide as a counter electrode. During cycling the solar transmittance varied between 8 % and 52 % and the light transmittance from 7.8% to 53.1%, the latter with a coloration efficiency CE up to 63 cm2⋅C−1. Accordingly, switching times of 17 s for coloration and 13 s for bleaching were determined. The gained insights are of high practical relevance for the fabrication of all-solid-state electrochromic devices.

Formation of spinel CoFe2O4 nanoporous oxide layers by electrochemical anodization

In the present letter, we report a simple method for fabricating self-ordered CoFe2O4 nanoporous oxide layers by electrochemical anodization. For that, optimal electrolyte and anodization conditions were applied for anodizing the parent alloy FeCoV sheet, leading to an ordered nanoporous structure of CoFe2O4. The as-synthesized sample are characterized by Scanning Electron Microscopy (SEM), X-ray spectroscopy (EDS) and X-ray Diffraction (XRD). Our SEM findings showed nanopores size constant in the range of 15 nm independent of anodization potential. Also, our XRD measurements, along with the energy dispersive measurements confirm the transformation of the as-anodized nanoporous oxide layers into nanoporous oxide layers CoFe2O4 phase.

Tuning the Photoelectrochemical Properties of Narrow Band Gap Nanoporous Anodic SnOx Films by Simple Soaking in Water.

Nanoporous tin oxide layers obtained via anodic oxidation of metallic tin at the potential of 4 V in the alkaline electrolyte (1 M NaOH) were soaked in distilled water for various durations (from 2 h to 120 h) to verify the influence of water-enabled crystallization on the morphology, composition, and related optical and photoelectrochemical properties of such kind of anodic SnOx. Although water soaking generally contributes to more stoichiometric and crystalline tin oxide, it was confirmed that at the initial stages of the water-induced dissolution–redeposition process, material exhibits enhanced photoelectrochemical performance under simulated sunlight irradiation. However, long-time exposure to water results in a gradual widening of the material’s band gap, shifting of the photoelectrochemical spectra towards higher energies, and almost complete deterioration of the photoelectrochemical activity under sunlight irradiation.

Growth of porous anodic TiO2 in silver nitrate solution without fluoride: Evidence against the field-assisted dissolution reactions of fluoride ions

Al, Ti, Zr and other metals can be anodized to form metal oxides with a porous structure or a nanotube structure. However, the mechanism of formation of oxide nanopores is still controversial. Anodic TiO2 nanotubes are usually obtained in an electrolyte containing fluoride ions, so a field-assisted dissolution reaction involving fluoride ions is considered to be the main reason for the formation of nanopores. In this paper, titanium was anodized at high current in a silver nitrate solution without fluoride, and a porous titanium oxide structure was obtained, which proves that a field-assisted dissolution reaction involving fluoride ions is not essential for the formation of nanopores. The electronic current leads to oxygen evolution, and the oxygen bubble mold effect is the real reason for the formation of nanopores. These interesting results show that the mechanism of formation of anodized TiO2 nanotubes is the same as that of porous oxide structures. Moreover, the growth rate of nanotubes in an aqueous solution of silver nitrate was much higher than that in a glycol solution of ammonium fluoride at the same voltage.

Preparation of Ordered Nanoporous Indium Tin Oxides with Large Crystallites and Individual Control over Their Thermal and Electrical Conductivities.

Metal oxides are considered suitable candidates for thermoelectric materials owing to their high chemical stabilities. The formation of ordered nanopores within these materials, which decreases thermal conductivity (κ), has attracted significant interest. However, the electrical conductivity (σ) of reported nanoporous metal oxides is low, owing to electron scattering at the thin pore walls and many grain boundaries formed by small crystallites. Therefore, a novel synthesis method that can control pore walls while forming relatively large crystallites to reduce κ and retain σ is required. In this study, we used indium tin oxide (ITO), which is a typical example among metal oxides with high σ. Nanoporous ITOs with large crystallite sizes of several hundred nanometers and larger were successfully prepared using indium chloride as a source of indium. The pore sizes were varied using colloidal silica nanoparticles with different particle sizes as templates. The crystal phase and nanoporous structure of ITO were preserved after spark plasma sintering at 723 K and 80 MPa. The κ was significantly lower than that reported for bulk ITO due to the phonon scattering caused by the nanoporous structure and thin pore walls. There was a limited decrease in σ even with high porosity. These findings show that κ and σ are independently controllable through the precise control of the structure. The control of the thickness of the pore walls at tens of nanometers was effective for the selective scattering of phonons, while almost retaining electron mobility. The remarkable preservation of σ was attributed to the large crystallites that maintained paths for electron conduction and decreased electron scattering at the grain boundaries.