Porous Anodic Alumina Substrates Research Articles

High performance microheater-based catalytic hydrogen sensors fabricated on porous anodic alumina substrates

Catalytic combustion-type gas sensors are widely used in industry and everyday life for detecting flammable gases. However, high power consumption is one of the main drawbacks of the catalytic sensors. Replacing the platinum wire heating element with a thin-film microheater can significantly reduce power consumption, but maintaining high sensitivity and long-term stability of such devices is a challenge. Here we developed microheater-type catalytic hydrogen sensors based on porous anodic aluminium oxide served simultaneously as a substrate for platinum microheater and as a carrier for a catalyst. The fabricated sensors have a high sensitivity of 76 mV/vol% hydrogen. The deviation of the sensor response during continuous operation for 14 days is less than 4%. The relative humidity of the ambient atmosphere does not affect the sensor response. The fast sensor response time (0.4 s) makes it possible to use the pulsed power supply mode to reduce the power consumption to 3.2 mW without sacrificing the measurement accuracy. The performance of the developed catalytic hydrogen sensors promotes the high competitiveness in the market and the prospects for industrial applications.

Preparation of ordered nanoporous WO3 thin films and the mechanism of large room-temperature ferromagnetism

Undoped porous WO3 nanofilms with large surface area were fabricated on porous anodic alumina substrates by magnetron sputtering. The nanofilms exhibited significant room-temperature ferromagnetism, at least one order of magnitude stronger than other reported undoped WO3 films. By controlling the oxygen partial pressure and the sputtering time during the deposition process, the saturation magnetization of the samples could be tuned over a large range, from 0.1 to 21.7 emu/cm3. Obvious magnetic anisotropy was found in the porous samples, indicating that the magnetic behavior is related to the regular pore structure perpendicular to the surface. Moreover, the first theoretical evidence that the Curie temperature of the sample is higher than room temperature was provided. When annealed in argon, the WO3 nanofilms turned black and showed a combination of a narrowed bandgap and enhanced ferromagnetism. These interesting phenomena are explained based upon the concept of hydrogen-like impurity states associated with oxygen vacancies.

Electric field manipulation of nonvolatile magnetization in MgO-based memory device with resistive switching effect

In the development of multifunctional data storage devices, controlling magnetic properties by electric field would be highly desirable. This work reported the relationship of magnetic properties of MgO film on porous anodic alumina (PAA) substrates to the different resistance states of Ag/MgO/PAA/Al(Ag/MP/Al) device at room temperature. We suggest that the fully deterministic magnetic switching in this composite film is based on the high sensitivity of oxygen vacancy defect-based conductive filaments to electric field. Our review offers a convenient way to control the magnetic properties using electric field, and shows great promises for developing novel multifunctional material in the future.

A Stable NanoPAA-ZnO/ZnCl2 Composite with Variable 3D Structured Morphology and Sustained Superhydrophilicity.

A ZnO/ZnCl2 composite with stable 3D structural morphologies and long lasting superhydrophilicity was synthesized on the top surface of a nano porous anodic alumina (nanoPAA) substrate. The wettability of a nanoPAA-ZnO/ZnCl2 was systematically characterized and the experimental data indicated that the water contact angle (WCA) of 0° could be achieved as well as maintained over 7 days and still remained at 4.36° after 50 days, and its 3D structural morphology had no clearly observable change during this period. The mechanism for the superhydrophilicity of the composites was interpreted in terms of the inherent hydrophilicity of ZnO/ZnCl2 nanofilm, the three-dimensional structures of wrinkled nanoflakes, the nanogaps between neighbor nanoflakes, the difference of structual morphologies (i.e., size, shape, and upright posture of nanoflakes), and the measured True Volume of voids in the nanocomposite. The structural morphologies were mainly determined by the parameters such as the original concentration of precursor ZnCl2 and the pore diameter of nanoPAA substrate. The study proposes a promising superhydrophilic nanomaterial and a cost-effective synthesis method, which will play a practical role in the fields of biomedical molecular sensors and micro/nanofluidic chips.

Microhotplates based on Pt and Pt-Rh films: The impact of composition, structure, and thermal treatment on functional properties

Recent progress in portable semiconductor and thermocatalytic gas sensors requires the development of the microhotplates with reduced power consumption, increased shock resistance, and low resistance drift. Rh is known as alloying component, which allows one to decrease the diffusion mobility of Pt in bulk resistive heating elements. Here, a comparison study on the stability of microhotplates based on Pt and Pt-Rh thin films is performed. The porous anodic alumina is used as a substrate, which provides high adhesion of the metal film and the thermal expansion coefficient compatible with Pt and Pt-Rh alloys. Morphology and crystal structure of metal films are studied after recrystallization annealing in air at the temperatures 600–810 °C. To simulate thermal effects of the microhotplates, a model based on the finite element method is developed. The perspectives of Pt-11 % Rh alloy as a material for microhotplates operating at the temperatures above 500 °C are discussed. The achieved characteristics of the fabricated Pt-Rh microhotplates on porous anodic alumina substrates allow one to use them as a universal platform for semiconductor and thermocatalytic gas sensors manufacturing.

Template-free Synthesis of Mesoporous and Crystalline Transition Metal Oxide Nanoplates with Abundant Surface Defects

Summary Mesoporous transition metal oxides with crystalline walls show promising applications in catalysis and energy storage. Syntheses of those crystalline mesoporous oxides, however, currently remain as an unmet challenge because of the large volume shrinkage during crystallization in a sol-gel process. Here, we demonstrate a facile and general template-free method to prepare six mesoporous transition metal oxides, e.g., CuO, CoO, and spinel MCo2O4 (M = Co, Cu, Mn, Zn), with highly crystalline walls and continuous mesopores in the forms of two-dimensional nanoplates or nanosheets. The method is based on the thermal crystalline transformation from basic carbonates to mesoporous metal oxides. The crystal interconversion not only endows the crystallinity and mesoporosity of oxides but also generates abundant surface-step defects that show remarkable enhancement of the catalytic activity of porous oxides. Our method likely provides an alternative paradigm for cost-effective and universal synthesis of crystalline mesoporous oxides beyond the traditional templating methods.

Microstructural and optical properties of Sn4Sb6S13 nanocrystals deposited on PAA templates

In this study, the Sn4Sb6S13 (TAS) nanocrystals were grown on the porous anodic alumina (PAA) templates through the vacuum thermal evaporation process at different deposition temperatures. The influence of deposition temperature on the microstructural and optical properties of TAS/PAA was investigated systematically. As the substrate temperature is increased, crystallite size increases while the strain and the dislocation density decrease. An improvement of the crystallinity is deduced from Raman spectra and X-Ray diffraction (XRD) patterns. The homogeneous distribution of TAS nanocrystals on PAA substrates was observed from surface morphology by using atomic force microscopy (AFM) and scanning electron microscope (SEM). The average reflectance of TAS/PAA films decreased significantly from 77 % to 60 % in the visible and near-infrared spectral regions, indicating the large absorbance of the deposited thin film. The optical parameters and thickness of TAS/PAA layers were estimated on the basis of spectroscopic ellipsometry measurements.

Preparation of graphene-coated anodic alumina substrates for selective molecular transport

In this paper, we report graphene composite membranes prepared by transfer of a layer of chemical vapor deposition graphene onto porous anodic alumina (AA) substrates with nominal pore size 20 and 30 nm, referred as 20AA and 30AA. The coated and uncoated substrates were characterized using optical and electron microscopy techniques. The bare substrates exhibited a smooth surfaces with a well-organized array of hexagonal pores, displaying an average pore size of 17 ± 3 (20AA) and 23 ± 3 nm (30AA). The scanning electron microscopy and atomic force microscopy analyses confirmed the successful transfer of graphene layer onto the target substrates. The molecular transport study was performed by introducing 0.5 M potassium chloride (KCl) and deionized water in a Side-bi-Side Franz diffusion cell. The graphene/20AA specimen blocked 66% ions transport, and graphene/30AA membrane about 64%. The ions blockage exceeded 90%, near the characteristics of defect-free graphene when the defects of the transferred graphene were sealed with Nylon 6,6. The results of this study suggest the potential use of graphene on AA substrates for water desalination and gas purification applications. CVD graphene has shown > 90% blockage of KCl molecules when tested on porous alumina substrate (pore size ~ 20 nm).

Voltage-Driven Room-Temperature Resistance and Magnetization Switching in Ceramic TiO2/PAA Nanoporous Composite Films.

Voltage control of room-temperature ferromagnetism has remained a big challenge which will greatly influence the multifunctional memory devices. In this paper, porous TiO2 thin films were deposited by dc-reactive magnetron sputtering onto ordered porous anodic alumina (PAA) substrates. Voltage-driving room-temperature resistance and magnetization switching without external magnetic field are simultaneously found in an Ag/TiO2/PAA/Al (Ag/TP/Al) device. Further analysis indicates that the formation/rupture of oxygen vacancy defect-based conductive filaments would be responsible for the changes of resistivity and magnetization. Our present results suggest that the TP nanoporous composite film material may therefore be used to achieve voltage control of magnetism and resistance switching in the future multifunctional memory devices. The Ag/TP/Al devices can also be used for new spintronic devices, neuromorphic operations, and alternative logic circuits and computing.

Antidot patterned single and bilayer thin films based on ferrimagnetic Tb–Co alloy with perpendicular magnetic anisotropy

Hysteresis properties and magnetization reversal in TbCo(30 nm) and FeNi(10 nm)/TbCo(30 nm) films with nanoscale antidot lattices are investigated to test the effect of nanoholes on the perpendicular anisotropy in the TbCo layer and the induced exchange bias in the FeNi layer. The antidots are introduced by depositing the films on top of hexagonally ordered porous anodic alumina substrates with pore diameter and interpore distance fixed to 75 nm and 105 nm, respectively. The analysis of combined vibrating sample magnetometry, Kerr microscopy and magnetic force microscopy imaging measurements has allowed us to link macroscopic and local magnetization reversal processes. For magnetically hard TbCo films, we demonstrate the tunability of magnetic anisotropy and coercive field (i.e., it increases from 0.2 T for the continuous film to 0.5 T for the antidot film). For the antidot FeNi/TbCo film, magnetization of FeNi is confirmed to be in plane. Although an exchange bias has been locally detected in the FeNi layer, the integrated hysteresis loop has increased coercivity and zero shift along the field axis due to the significantly decreased magnetic anisotropy of TbCo layer.

Synthesis and magnetic properties of ceramic MgO porous film

Ordered porous MgO films with pore diameters in the 15–28nm range have been prepared on porous anodic alumina substrates at 573K in a fixed O2 pressure of 1.4×10−4mbar using DC-reactive magnetron sputtering. X-ray diffraction and scanning probe microscope observations have revealed that homogeneous MgO ceramics with face-centered cubic structure are formed with a highly ordered nanopore array arranged in a close packed hexagonal pattern. The results of magnetic measurements have shown that the porous MgO ceramics possess remarkable room temperature ferromagnetism and the maximum saturation magnetization along the out-of-plane direction was as high as 78emu/cm3. Experimental and theoretical results suggest that oxygen vacancies and the unique porous structure of the films are responsible for the high magnetization. These results provide new insights into magnetic ordering in undoped dilute ferromagnetic semiconductor oxides and may be useful in the development of MgO-based spintronics devices and novel multifunctional materials.

HfO2/porous anodic alumina composite films for multifunctional data storage media materials under electric field control

New materials for achieving direct electric field control of ferromagnetism and resistance behavior are highly desirable in the development of multifunctional data storage devices. In this paper, HfO2 nanoporous films have been fabricated on porous anodic alumina (PAA) substrates by DC-reactive magnetron sputtering. Electrically induced resistive switching (RS) and modulated room temperature ferromagnetism are simultaneously found in a Ag/HfO2/PAA/Al (Ag/HP/Al) heterostructure. The switching mechanism between low resistance state and high resistance state is generally attributed to the formation/rupture of conductive filaments which may consist of oxygen vacancies. The combination of the electric field control of magnetization change and RS makes HP films possible for the multifunctional data storage media materials.

Microstructures and room temperature ferromagnetism of ordered porous ZrO2 thin films sputter deposited onto porous anodic alumina substrates

Ordered porous ZrO2 films with pore diameters in the 5–45nm range have been prepared on porous anodic alumina substrates using DC-reactive magnetron sputtering of pure Zr targets. A saturation magnetization (MS) value as high as 119emu/cm3 was obtained for the ZrO2 film with pores of 45nm in diameter when the external field was perpendicular to the film surface. The significant out-of-plane saturation magnetization is associated with the porous structure of the film. Experimental and theoretical results showed that the origin of the room temperature ferromagnetism (RTFM) is closely related to the ordered porous structure and the single charged oxygen vacancies of the films. These findings suggest that porous ZrO2 films are promising to be applied to the spin electronic devices.

Palladium decorated silicon carbide nanocauliflowers for hydrogen gas sensing application

Silicon carbide (SiC) nanostructures have attracted significant attention for device applications due to their unique structural, electronic and catalytic properties. Here, the efficiency of three dimensional SiC nano-cauliflowers (NCs) for hydrogen gas sensing application has been investigated. We have deposited a vertically aligned, ordered Pd decorated SiC NCs directly on a silver (Ag) coated porous anodic alumina (AAO) substrate by DC magnetron cosputtering and then it used as a gas sensor without additional processing. The sensor shows remarkably high and selective responses to hydrogen gas with low detection range 2–500ppm at relatively high temperature range (30–500°C). Our results demonstrate the potential application of Pd decorated SiC NCs for fabricating highly sensitive and selective hydrogen gas sensors.

Preparation and magnetic properties of DC-sputtered porous HfO2 films

Ordered hafnium oxide films were successfully prepared on porous anodic alumina substrates using DC-reactive magnetron sputtering, and remarkable room-temperature ferromagnetism has been observed in them. A saturation magnetization (MS) value as high as 128emu/cm3 was obtained for 80nm thick HfO2 film with the external field perpendicular to the film surface. Experimental and theoretical results indicate that the single charged oxygen vacancies and the unique porous structure of the films are responsible for the large ferromagnetism. The coupling between single charged oxygen vacancies may be explained either by direct exchange interactions between oxygen vacancies (Vo+-Vo+) or by mediation of conduction electrons (Vo+-e−-Vo+). These observations may be useful in the development of HfO2-based spintronics devices.

Electric field control of magnetization in Cu2O/porous anodic alumina hybrid structures at room temperature

Cu2O nanoporous films are deposited on porous anodic alumina (PAA) substrates by DC-reactive magnetron sputtering. This paper focuses on voltage driven magnetization switching in Cu2O/PAA (CP) composite films prepared by DC-reactive magnetron sputtering. By applying a dc electric field, the magnetization of the CP composite films can be controlled in a reversible and reproducible way and shows an analogous on-off behavior. The magnitude of the change in the magnetization was about 75 emu/cm3 as the electric field was switched on and off. Resistive switching behavior was also observed in as-prepared CP composite films. Further analysis indicated that the formation/rupture of conducting filaments composed of oxygen vacancies is likely responsible for the changes in the magnetization as well as in the resistivity. Such reversible change of magnetization controlled by an electric field at room temperature may have applications in spintronics and power efficient data storage technologies.

Ionic Polymer Microactuator Activated by Photoresponsive Organic Proton Pumps

An ionic polymer microactuator driven by an organic photoelectric proton pump transducer is described in this paper. The light responsive transducer is fabricated by using molecular self-assembly to immobilize oriented bacteriorhodopsin purple membrane (PM) patches on a bio-functionalized porous anodic alumina (PAA) substrate. When exposed to visible light, the PM proton pumps produce a unidirectional flow of ions through the structure’s nano-pores and alter the pH of the working solution in a microfluidic device. The change in pH is sufficient to generate an osmotic pressure difference across a hydroxyethyl methacrylate-acrylic acid (HEMA-AA) actuator shell and induce volume expansion or contraction. Experiments show that the transducer can generate an ionic gradient of 2.5 μM and ionic potential of 25 mV, producing a pH increase of 0.42 in the working solution. The ΔpH is sufficient to increase the volume of the HEMA-AA microactuator by 80%. The volumetric transformation of the hydrogel can be used as a valve to close a fluid transport micro-channel or apply minute force to a mechanically flexible microcantilever beam.

Photo- and cathodoluminescence of strontium titanate xerogel films doped with terbium ions

Terbium-doped strontium titanate thin films are synthesized via the sol–gel method. The xerogel is deposited onto single-crystal silicon and porous anodic alumina substrates formed on single-crystal silicon. The structure and phase composition of the films are studied. The strontium titanate xerogel on single-crystal silicon exhibits a photoluminescence spectrum with a selective terbium emission band that is difficult to identify. The photoand cathodoluminescence spectra of the xerogel in porous matrices contain emission bands due to the electronic transitions of trivalent terbium.

Significant room-temperature ferromagnetism in porous ZnO films: The role of oxygen vacancies

Pure porous ZnO films were prepared by direct current reactive magnetron sputtering on porous anodic alumina substrates. Remarkably large room-temperature ferromagnetism was observed in the films. The highest saturation moment along the out-of-plane direction was about 110 emu/cm3. Experimental and theoretical results suggested that the oxygen vacancies and the unique porous structure of the films are responsible for the large ferromagnetism. There are two modes of coupling between oxygen vacancies in the porous ZnO films: (i) exchange interactions directly between the oxygen vacancies and (ii) with the mediation of conduction electrons. In addition, it was found that the magnetic moment of ZnO films can be changed by tuning the concentration of oxygen vacancies. These observations may be useful in the development of ZnO-based spintronics devices.

Resistive property and ferromagnetism in ZnO/PAA nanoporous composite films

ZnO nanoporous films are deposited on porous anodic alumina (PAA) substrates by DC-reactive magnetron sputtering. A significant ferromagnetism, accompanied by stable bipolar resistive switching behavior, was observed in as-prepared nanoporous composite films. It is demonstrated that the resistive and ferromagnetic behavior strongly depend on the oxygen vacancies. The magnetization of the films into the high resistance state is about three times larger than that in the as-prepared state, and was obtained in the low resistance state. The ferromagnetism of the films can be manipulated not only by preparation conditions, but by applying electric field. This oxide material with ferromagnetism and resistance hysteresis is very promising for research in next generation memory and spintronics devices.