Porous Nanostructured Films Research Articles

Stray Light in 3D Porous Nanostructures of Single‐Crystalline Copper Film

In the design of optical devices and components, geometric structures and optical properties of materials, such as absorption, refraction, reflection, diffraction, scattering, and trapping, have been utilized. Finding the ideal material with certain optical and geometric characteristics is essential for a customized application. Herein, unoxidizable achromatic copper films (ACFs) are fabricated on Al2O3 substrates utilizing an atomic sputtering epitaxy apparatus. ACFs are made up of two regions vertically: a comparatively flat layer region and a 3D porous nanostructured region on top of the flat region. The measured specular reflectance displays low‐pass filter behavior with a sharp cutoff frequency in the infrared spectrum. Furthermore, the measured diffusive reflectance spectra show light‐trapping behavior in the spectral region above the cutoff frequency, where there are no known absorption mechanisms, such as phonons and interband transitions. A focused ion beam scanning electron microscope is utilized to study the thin film's nanostructured region through 3D tomographic analysis in order to comprehend the phenomena that are observed. This work will shed fresh light on the design and optimization of optical filters and light‐trapping employing porous nanostructured metallic thin films.

Nanostructured porous WO3 for photoelectrochemical splitting of seawater

Seawater, due to its salinity, can serve as an electrolyte in photoelectrochemical (PEC) systems, which have the potential to enable solar-driven production of hydrogen and oxidizing species. In this study several micrometer thick and highly porous nanostructured WO3 films were formed and their performance in photoanodic oxidation of chloride and sulfate ions, the most abundant anionic species in seawater, was investigated. Layers were characterized using scanning electron microscopy, X-ray diffraction, diffuse reflectance spectroscopy, and BET surface area measurements. The study demonstrates that efficient PEC performance is achieved through optimal light absorption by sufficiently thick WO3 layers (3–5 μm), a highly porous structure that facilitates electrolyte permeation and ionic transport and provides a large electrochemically active interfacial area, and a nanosheet morphology where the sheet thickness is shorter than the hole diffusion length in WO3. The Faradaic efficiency of ClO- + ClO2- generation in neutral chloride-based electrolytes was found to be close to 100 %, but decreased significantly with photoelectrolysis time, presumably, due to the formation of higher chlorine oxo compounds. In addition, photocurrent stability and dissolution of WO3 photoanodes in acidic sulfate and neutral chloride media was compared. Though dissolution of WO3 in 0.5 M NaCl was found to be up to 3 times higher, photocurrent decay was more significant in 0.5 M H2SO4. The phenomenon was attributed to effective hole scavenging by surface-adsorbed Cl- ions, which prevents passivation of the photoelectrode surface due to formation of W(VI) peroxospecies.

Porous silicon structures passivated with 10-undecenoic acid for possible ethanol sensing

Multifunctional sensing platforms are useful for classifying and quantifying different analytes. This work presents a simple optical sensor based on porous silicon film modified using open-air pyrolysis with 10-undecenoic acid. The fabricated structure was characterized using vis-NIR reflectance spectroscopy and the functional groups were identified using Fourier transform infrared spectroscopy. With respect to the control, the proposed undecenoic acid passivated porous silicon nanostructured film has been tested as reusable ethanol sensor in water.

Chemiresistors with In2O3 Nanostructured Sensitive Films Used for Ozone Detection at Room Temperature.

Detection of greenhouse gases is essential because harmful gases in the air diffuse rapidly over large areas in a short period of time, causing air pollution that will induce climate change with catastrophic consequences over time. Among the materials with favorable morphologies for gas detection (nanofibers, nanorods, nanosheets), large specific surfaces, high sensitivity and low production costs, we chose nanostructured porous films of In2O3 obtained by the sol-gel method, deposited on alumina transducers, with gold (Au) interdigitated electrodes (IDE) and platinum (Pt) heating circuits. Sensitive films contained 10 deposited layers, involving intermediate and final thermal treatments to stabilize the sensitive film. The fabricated sensor was characterized using AFM, SEM, EDX and XRD. The film morphology is complex, containing fibrillar formations and some quasi-spherical conglomerates. The deposited sensitive films are rough, thus favoring gas adsorption. Ozone sensing tests were performed at different temperatures. The highest response of the ozone sensor was recorded at room temperature, considered to be the working temperature for this specific sensor.

Temperature Sensing with Thin Films of Flame-Formed Carbon Nanoparticles

A porous nanostructured film of flame-formed carbon nanoparticles has been produced with a one-step procedure. The morphological and structural characteristics of the film have been characterized by atomic force microscopy and Raman spectroscopy. The electrical resistance as a function of the temperature has been investigated in the range from ambient temperature to 120 °C. A nonmetallic behavior has been observed, with a monotonic decrease of the film resistance as temperature increases. Electrical conduction is explained in terms of charge carriers tunneling and percolation between the carbon grains and is not perfectly described by an Arrhenius behavior. A negative temperature coefficient of resistance (TCR) of the order of −100 × 10−4 K−1 has been measured. The high absolute TCR value, together with the ease of material microfabrication processing and biocompatibility of the carbon material make this film ideal for temperature sensing in many environments. A functional relationship between resistance and temperature, which is necessary for practical applications, has been finally derived. A very good agreement between experimental data and fit is obtained with a fifth order polynomial.

Optical and Laser-Induced Damage Characterization of Porous Structural Silicon Oxide Film with Hexagonal Period by Nanoimprint Lithography

We designed and fabricated a porous nanostructured film with a hexagonal period for a high-power laser system. The proposed nanostructure exhibits polarization-independent, infrared, and antireflective properties. The measured transmittance of the structural film does not drop below 93% between 948 nm and 2500 nm (exceeding 95% from 1411–2177 nm), and this performance is maintained for incident angles ranging from 0–30°. The laser-induced damage threshold (LIDT) of the structural film (17.94 J/cm2) is much higher than that of the single layer of SiO2 film (7.06 J/cm2). These results show that the preparation process is an effective technique to obtain a large-scale structural surface for high-power laser systems.

All‐Printed Green Micro‐Supercapacitors Based on a Natural‐derived Ionic Liquid for Flexible Transient Electronics

AbstractThe fabrication and characterization of green, flexible, and ultra‐thin supercapacitors that are able to operate above 1.5 V is reported, using an all‐printed fabrication process. The devices are produced by aqueous spray casting of a natural‐derived electrolyte ionogel composed by 2‐hydroxyethyl cellulose and by the ionic liquid choline lactate, while the electrodes are composed of highly porous nanostructured carbon films deposited by supersonic cluster beam deposition (SCBD). The obtained supercapacitors (device thickness < 10 μm) are stable to bending and they possess power values up to 120 kW kg–1. The combination of aqueous spray casting and SCBD constitutes a versatile, scalable, and eco‐friendly fabrication process able to directly print interconnected elements suitable for transient electronic systems.

Nucleation and Growth-Controlled Morphology Evolution of Cu Nanostructures During High-Pressure Thermal Evaporation

The formation of porous material structures has been widely investigated for the development of high-performance energy materials, catalysts, and chemical sensing devices. Various nanoporous structure fabrication methods are based on wet-chemical processes, which require precise control of the process parameters. Physical vapor deposition such as thermal evaporation utilizes high vacuum so that the deposition process is relatively simple, free of contamination, and easily reproduced. However, because of the long mean-free-path of the evaporated atoms in high vacuum, heterogeneous nucleation and the growth of adatoms occurs on the substrate surface, which results in the formation of dense and compact thin films. But by changing the working pressure, various morphologies of porous nanostructures can be obtained. As applied to copper, with increasing pressure the thin film evolves from a dense structure to a coral-like nanoporous structure through a porous columnar structure. All of the porous structures consist of nanoparticle aggregates, where copper nanoparticles are connected to each other, and many nano-gaps are found inside the aggregates. A surface plasmonic effect is expected. The porous copper nanostructured films demonstrated high surfaceenhanced Raman spectroscopy activity.

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.

Structural Dependent Eu3+ Luminescence, Photoelectric and Hysteresis Effects in Porous Strontium Titanate.

Eu3+ doped porous nanostructured SrTiO3 films and powder fabricated by sol-gel route without using any precursor template are characterized by different morphology and phase composition. The films and the power show red and yellow luminescence with the most intensive photoluminescence (PL) bands at 612 nm and 588 nm, respectively. Raman, secondary ion mass spectrometry (SIMS), and X-ray diffraction (XRD) analysis of undoped nanostructured porous SrTiO3 films showed the presence of TiO2, SrO, and SrTiO3 phases and their components. The undoped porous SrTiO3 films are photosensitive and demonstrate resistive switching. The capacitance-voltage hysteresis loops with the width of about 6 V in the frequency range of 2 kHz—2 MHz were observed.

Low-temperature highly selective and sensitive NO2 gas sensors using CdTe-functionalized ZnO filled porous Si hybrid hierarchical nanostructured thin films

In this report, we present improved NO2 gas sensing properties based on CdTe-functionalized ZnO filled porous Si (PSi) hybrid hierarchical nanostructured thin films (CdTe/ZnO@PSi). The CdTe and ZnO nanostructured thin films were grown on the PSi substrate using a scalable magnetron sputtering technique. The PSi substrate was prepared using the electrochemical anodization method. The proposed CdTe/ZnO@PSi sensor was characterized in terms of structural, morphological, and compositional properties. The sensing performance and corresponding mechanism of NO2 gas sensor based on CdTe/ZnO@PSi were described comprehensively. In order to compare, the sensing characteristics of both CdTe/ZnO@PSi and pristine ZnO@PSi thin film sensors were tested in different temperature range varied from 30 °C to 180 °C for 1 ppm concentration of NO2 gas. We found that CdTe/ZnO@PSi sensor displays enhanced NO2 sensing characteristics (∼3.5-fold greater sensor response ∼19.82 %, fast response/recovery time ∼13 s/54 s) at the relatively low working temperature of 90 °C towards 1 ppm NO2 gas in comparison to the pristine ZnO@PSi sensor (sensor response ∼5.72 % and response/recovery time ∼41 s/124 s at an operating temperature of 150 °C). Further, the fabricated NO2 gas sensor demonstrated excellent reversibility and good stability, and enabling CdTe/ZnO@PSi a promising candidate for its practical use in IoT sensing devices.

High-throughput characterization of Ag–V–O nanostructured thin-film materials libraries for photoelectrochemical solar water splitting

Ag–V–O thin-film materials libraries, with both composition (Ag22-77V23-78Ox) and thickness (123–714 nm) gradients were fabricated using combinatorial reactive magnetron co-sputtering aiming on establishing relations between composition, structure, and functional properties. As-deposited libraries were annealed in air at 300 °C for 10 h. High-throughput characterization methods of composition, structure and functional properties were used to identify photoelectrochemically active regions. The phases AgV6O15, Ag2V4O11, AgVO3, and Ag4V2O7 were observed throughout the composition gradient. The photoelectrochemical properties of Ag–V–O films are dependent on composition and morphology. An enhanced photocurrent density (~300–554 μA/cm2) was obtained at 30 to 45 at.% Ag along the thickness gradient. Thin films of these compositions show a nanowire morphology, which is an important factor for the enhancement of photoelectrochemical performance. The photoelectrochemically active regions were further investigated by high-throughput synchrotron-X-ray diffraction and transmission electron microscopy (Ag32V68Ox) which confirmed the presence of Ag2V4O11 as the dominating phase along with the minor phases AgV6O15 and AgVO3. This enhanced photoactive region shows bandgap values of ~2.30 eV for the direct and ~1.87 eV for the indirect bandgap energies. The porous nanostructured films improve charge transport and are hence of interest for photoelectrochemical water splitting.

Conversion of low-grade heat via thermal-evaporation-induced electricity generation on nanostructured carbon films

Nowadays, due to increasingly serious energy crisis and environmental pollution problems, conversion of ubiquitous low-grade heat to electricity is attracting increasing worldwide interests. Recently, energy-conversion methods mediated by molecular interactions between water and nanostructured carbon materials has been reported to directly harvest the environmental energy without any auxiliary equipment and energy input. Herein, we propose a method of conversion of low-grade heat by the thermal-evaporation-induced electricity generation on a porous nanostructured carbon films (NCFs). These NCFs are directly fabricated on an Al2O3 substrate and then pre-treated by annealing and UV/Ozone treatments to maintain the spontaneous water imbibition at high temperatures. This sample can unremittingly generate electricity at relative large temperature range (~22–82 °C) via the thermal-evaporation of water. This temperature range is a common temperature of low-grade heat sources. The maximum thermal-evaporation-induced voltages are measured to be about 0.79 ± 0.05 V, and the maximum output power is calculated to be approximately 80 nW with a power density of 150 μW/m2. Furthermore, our experimental results and theoretical analysis indicate that the substrate-property of NCFs can seriously influence the performance of thermal-evaporation-induced electricity generation.

Epitaxial growth of Co3O4 thin films using Co(dpm)3 by MOCVD

We report on the growth of epitaxial Co3O4 (1 1 1) thin films by close injection showerhead (CIS)-MOCVD on sapphire substrates (c-, a- and r- planes). The deposition was carried out by using tri-dipivaloylmethanato-cobalt (Co(dpm)3) as precursor for Co and pure O2 or H2O vapor as oxidant. The effects of growth conditions, including Co(dpm)3 vaporization temperature, substrate temperature, O2/H2O flow rates, growth pressure and substrate orientation on the growth of the Co3O4 thin films were investigated. The crystallinity, phase purity and surface morphology of the films were studied by XRD, Raman and UV-visible spectroscopies, RHEED, SEM, and AFM. The as-grown films were found to be pure (1 1 1) oriented normal spinel type epitaxial Co3O4 thin films. The morphology of the films was strongly dependent on the identity of oxygen source used. Films grown using pure oxygen showed strongly faceted structures while those grown using H2O were dominated by porous interwoven and nanostructured films. The size and density of the faceted surface are strongly dependent on the growth rate of the films, controlled by adjustment of growth parameters. High growth rates resulted in small and dense grains. The growth of Co3O4 (1 1 1) was only successful on the a- and c-plane sapphire substrates, with no indication of nucleation and growth on r-plane sapphire substrates. Under otherwise identical growth conditions, the growth rate of films grown using H2O vapor was found to be 5 times higher than when using pure oxygen. These results demonstrate that Co(dpm)3 is a viable source for Co3O4 thin film growth by MOCVD, with resultant film morphology being highly dependent on the oxygen source and growth parameters.

Nanoinstabilities of Cu2O porous nanostructured films as driven by nanocurvature effect and thermal activation effect

In this work, the instabilities at the nanoscale (i.e. nanoinstabilities) of triangular pyramids-like Cu2O porous nanostructured films (PNFs) are studied by heating treatments under different atmosphere and temperature. It is found that the nanoscale building triangular pyramids turn round preferentially at the sharp angles and/or coalesce with their contacting ones by directional diffusion and plastic flow of atoms, which are driven by the nonuniformly-distributed surface nanocurvature. As a result, the triangular pyramids become quasi-sphere shape and the PNF evolves into a big, dense particles film. It is also observed that the heating or thermal activation effect efficiently promotes the reduction or oxidation of Cu2O pyramids and the crystallization or growth of the as-achieved Cu or CuO grains. The above physical and chemical instabilities or changes at the nanoscale of Cu2O PNFs can be well accounted for by the combined mechanism of nanocurvature effect and thermal activation effect. The nanocurvature effect can lower the energy barrier for the atom diffusion or plastic flow and lower the activation energy for the chemical reactions, while the thermal activation effect can supply the required kinetic energy or activation energy and make the atomic transportations and reactions kinetically possible. The findings reveal the evolution laws of morphology, crystal structure and composition of triangular pyramids-like Cu2O PNF during heating treatments, which can further be extended to other types of Cu2O PNFs. Also, the findings have important implications for the nanoinstabilities of Cu2O PNFs-based devices, especially those working at a high temperature.

Luminescence of Porous Nanostructured Strontium Titanate Films Doped with Eu3+ Ions

Porous nanostructured strontium titanate films doped with europium SrTiO3:Eu[Formula: see text] fabricated on silicon and porous anodic alumina by the sol–gel route are presented. The films with the thicknesses of 430[Formula: see text]nm and 510[Formula: see text]nm after depositions of three and five layers, respectively, have the perovskite structure confirmed by X-ray diffraction. The porous films demonstrate Eu[Formula: see text] photoluminescence peaks at 593 ([Formula: see text]), 615 ([Formula: see text]) and 705 ([Formula: see text])[Formula: see text]nm. The porosity and PL intensity of the fabricated films noticeably reduce with the number of deposited layers.

Cu2O porous nanostructured films fabricated by positive bias sputtering deposition

In this work, the authors fabricated Cu2O porous nanostructured films (PNFs) on glass slide substrates by the newly developed positive bias deposition approach in a balanced magnetron sputtering (MS) system. It was found that the surface morphology, crystal structure and optical property of the as-deposited products were greatly dependent on the applied positive substrate bias. In particular, when the substrate was biased at +50 and +150 V, both of the as-prepared Cu2O PNFs exhibited a unique triangular pyramids-like structure with obvious edges and corners and little gluing, a preferred orientation of (111) and a blue shift of energy band gap at 2.35 eV. Quantitative calculation results indicated that the traditional bombardment effects of electrons and sputtering argon ions were both negligible during the bias deposition in the balanced MS system. Instead, a new model of tip charging effect was further proposed to account for the controllable formation of PNFs by the balanced bias sputtering deposition.

Liquid switchable radial polarization converters made of sculptured thin films

A radial polarization converter is a super-structured optical retarder that converts a conventional linearly polarized light beam into a structured beam with radial or azimuthal polarization. We present a new type of these sophisticated optical elements, which is made of porous nanostructured sculptured single thin films or multilayers prepared by physical vapor deposition at an oblique angle. They are bestowed with an axisymmetric retardation activity (with the fast axis in a radial configuration). In particular, a Bragg microcavity multilayer that exhibits a tunable transmission peak in the visible range with a retardance of up to 0.35 rad has been fabricated using this methodology. Owing to the highly porous structure of this type of thin films and multilayers, their retardance could be switched off by liquid infiltration. These results prove the possibility of developing wavelength dependent (through multilayer optical design) and switchable (through vapor condensation or liquid infiltration within the pore structure) radial polarization converters by means of oblique angle physical vapor deposition.

Enhanced visible-light photoelectrochemical and photoelectrocatalytic activity of nano-TiO2/polyimide/Ni foam photoanode

We report a novel method for fabrication of a nano-TiO2/polyimide (PI)/Ni foam photoanode. Characterization results indicated that porous nanostructured TiO2 films were successfully immobilized and dispersed into the indivisible PI/Ni foam substrate. The prepared photoanode exhibited intense visible-light absorption from 400 to 700 nm, high photoinduced current of 175 μA/cm2, and photoelectrocatalytic (PEC) efficiency of 98.8 % for degradation of methylene blue (MB) within 180 min of simulated solar light irradiation. Moreover, the TiO2/PI/Ni foam photoanode showed obvious visible-light PEC performance, mainly attributed to formation of TiO2–PI charge-transfer (CT) complexes and high separation efficiency of photoinduced charge carriers by the applied bias potential. This study provides a new perspective for preparation of cheap, high-performance visible-light photoelectrocatalytic films.

Electrochromic behavior of WO3 thin films prepared by GLAD

WO3 thin films fabricated by glancing angle deposition (GLAD) are proposed as excellent electrochromic coatings with favorable ion diffusion. A ∼ 500-nm film prepared by GLAD had a relatively large transmittance modulation. The crystallization structure, surface morphology, chemical state, optical and electrochromic properties of WO3 thin films were systematically characterized upon annealing treatment. Compared with annealed WO3 porous nanostructured films, the amorphous as-deposited films exhibited a high coloration efficiency and stable reversibility. Furthermore, the GLAD WO3 films exhibit the tunable angular selectivity under illumination with p-polarized light because of the birefringence, which could extend the application range of nanostructured films in the electrochromic field.