Electrochemical Anodization Process Research Articles

Enhanced biocompatibility of TiO2 surfaces by highly reactive plasma

In the present study the biological response to various nanotopographic features after gaseous plasma treatment were studied. The usefulness of nanostructured surfaces for implantable materials has already been acknowledged, while less is known on the combined effect of nanostructured plasma modified surfaces. In the present work the influence of oxygen plasma treatment on nanostructured titanium oxide (TiO2) surfaces was studied. Characterization of the TiO2 surface chemical composition and morphological features was analyzed after plasma modification by x-ray photoelectron spectroscopy and by scanning electron microscopy while surface wettability was studied with measuring the water contact angle. Cell adhesion and morphology was assessed from images taken with scanning electron microscopy, whereas cell viability was measured with a calorimetric assay. The obtained results showed that oxygen plasma treatment of TiO2 nanotube surfaces significantly influences the adhesion and morphology of osteoblast-like cells in comparison to untreated nanostructured surfaces. Marked changes in surface composition of plasma treated surfaces were observed, as plasma treatment removed hydrocarbon contamination and removed fluorine impurities, which were present due to the electrochemical anodization process. However no differences in wettability of untreated and plasma treated surfaces were noticed. Treatment with oxygen plasma stimulated osteoblast-like cell adhesion and spreading on the nanostructured surface, suggesting the possible use of oxygen plasma surface treatment to enhance osteoblast-like cell response.

Planar Arrays of Nanoporous Gold Nanowires: When Electrochemical Dealloying Meets Nanopatterning.

Nanoporous materials are of great interest for various technological applications including sensors based on surface-enhanced Raman scattering, catalysis, and biotechnology. Currently, tremendous efforts are dedicated to the development of porous one-dimensional materials to improve the properties of such class of materials. The main drawback of the synthesis approaches reported so far includes (i) the short length of the porous nanowires, which cannot reach the macroscopic scale, and (ii) the poor organization of the nanostructures obtained by the end of the synthesis process. In this work, we report for the first time on a two-step approach allowing creating highly ordered porous gold nanowire arrays with a length up to a few centimeters. This two-step approach consists of the growth of gold/copper alloy nanowires by magnetron cosputtering on a nanograted silicon substrate, serving as a physical template, followed by a selective dissolution of copper by an electrochemical anodic process in diluted sulfuric acid. We demonstrate that the pore size of the nanowires can be tailored between 6 and 21 nm by tuning the dealloying voltage between 0.2 and 0.4 V and the dealloying time within the range of 150-600 s. We further show that the initial gold content (11 to 26 atom %) and the diameter of the gold/copper alloy nanowires (135 to 250 nm) are two important parameters that must carefully be selected to precisely control the porosity of the material.

Photothermal deflection investigation of thermally oxidized mesoporous silicon

In this paper we investigate the thermal oxidation effect on optical and thermal properties of meso-porous silicon (meso-PS) layers using photoluminescence (PL) and photothermal deflection techniques (PDS, PTD). Samples have been successfully prepared by electrochemical anodization process. After a pre-oxidation for 1h 30min at 300°C, layers are thermally oxidized in dry oxygen at different temperatures (800, 1000°C) and durations. PL measurements show that the annealed layers have comparable spectra with a peak position focused on 2.1eV. This behavior indicates the formation of luminescent silicon (Si) nanocrystallites with comparable average sizes. From the effective mass theory, the diameter of those nanocrystallites is estimated to be around 2.2nm. Another estimation of the mean size of the silicon nanocrystallites obtained from the evolution of the thermal conductivity of the meso-PS layers based on photothermal deflection technique (PTD) data was close to the values obtained from the PL results. Photothermal deflection spectroscopy (PDS) measurements show that the thermal oxidation affects the absorption edge of the Si substrate. The optical band gap energy of the started substrates determined from the Tauc's relation is observed to increase with the thermal temperature and duration. The possible origins of this shift are discussed.

Enhanced field emission from titanium dioxide nanotube arrays decorated with graphene sheets and silver nanoparticles

The TiO2 nanotube arrays decorated with graphene sheets (GSs) and Ag nanoparticles were prepared on Ti foils and field emission performances were explored in detail. The conventional electrochemical anodization process was used to synthesize TiO2 nanotube arrays on Ti foils. The GSs and Ag nanoparticles were deposited on as-prepared TiO2 nanotube arrays by electrophoretic deposition process and chemical deposition. The field emission properties of samples were dramatically improved after depositing GSs and Ag nanoparticles on TiO2 nanotube arrays. Compared with pristine TiO2 nanotube arrays, the TiO2 nanotube arrays decorated with GSs and Ag nanoparticles composite film show good field emission stability and a higher field enhancement factor of 3450. The turn-on and threshold fields can be decreased from 6.63 to 2.68 V/μm and from 15.28 to 6.13 V/μm, respectively. Quantitative discussion about the hybrid hetero-nanostructure field emission improvement is presented. The good field emission properties of the decorated TiO2 make it a promising candidate for applying in high-performance emission device.

A Highly Controllable Electrochemical Anodization Process to Fabricate Porous Anodic Aluminum Oxide Membranes.

Due to the broad applications of porous alumina nanostructures, research on fabrication of anodized aluminum oxide (AAO) with nanoporous structure has triggered enormous attention. While fabrication of highly ordered nanoporous AAO with tunable geometric features has been widely reported, it is known that its growth rate can be easily affected by the fluctuation of process conditions such as acid concentration and temperature during electrochemical anodization process. To fabricate AAO with various geometric parameters, particularly, to realize precise control over pore depth for scientific research and commercial applications, a controllable fabrication process is essential. In this work, we revealed a linear correlation between the integrated electric charge flow throughout the circuit in the stable anodization process and the growth thickness of AAO membranes. With this understanding, we developed a facile approach to precisely control the growth process of the membranes. It was found that this approach is applicable in a large voltage range, and it may be extended to anodization of other metal materials such as Ti as well.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1202-y) contains supplementary material, which is available to authorized users.

Anodic TiO2 nanotube layer directly formed on the inner surface of Ti pipe for a tubular photocatalytic reactor

We demonstrated the successful water treatment performance of a continuous-flow tubular TiO2 nanotube reactor made of Ti pipe where a TiO2 nanotube (TNT) layer was directly formed on the inner wall by an electrochemical anodization process. A cylindrical UV lamp was inserted into the pipe to irradiate the inner TNT surface as a light source. To optimize the thickness of TNT layer on Ti pipe, TNT layers with 1, 5 and 10μm thickness were prepared and their photocatalytic activities were tested for the degradation of various organic substrates (acid orange 7 (AO7), dichloroacetate (DCA), and 4-chlorophenol (4-CP)) under UV illumination. The TNT layer with 10μm thickness showed the highest photocatalytic activity, with first-order removal rate constants (k:min−1) of 0.019, 0.039, and 0.025 for AO7, 4-CP, and DCA, respectively. The directly formed TNTs (10μm thickness) on Ti pipes in the continuous-flow tubular TNT reactor, having a strong adhesiveness and a high surface area, enabled the successful degradation of organic compounds and to have kept the activity without decrease during the repeated cycles and long-term experiments. The photocatalytic performance of the tubular TNT reactor was tested with a higher concentration, smaller volume ([4-CP]0=100μM, solution vol.=15mL) and a lower concentration, larger volume ([4-CP]0=20μM, solution vol.=100mL) in the circulation-flow mode (20mLmin−1). A good performance was achieved under the low intensity UV illumination (≈2mWcm−2) with a half-life of ca. 50min and ca. 11h with 80% and 79% removal of TOC for the above two test conditions, respectively. The proposed tubular TNT reactor can be an effective option in the design of continuous flow-type photocatalytic reactors for practical water purification.

Gold nanoparticles-immobilized, hierarchically ordered, porous TiO2 nanotubes for biosensing of glutathione.

Glutathione (GSH) is vital for several functions of our human body such as neutralization of free radicals and reactive oxygen compounds, maintaining the active forms of vitamin C and E, regulation of nitric oxide cycle, iron metabolism, etc. It is also an endogenous antioxidant in most of the biological reactions. Given the importance of GSH, a simple strategy is proposed in this work to develop a biosensor for quantitative detection of GSH. This particular biosensor comprises of gold nanoparticles (Au NPs)-immobilized, hierarchically ordered titanium dioxide (TiO2) porous nanotubes. Hexagonally arranged, honeycomb-like nanoporous tubular TiO2 electrodes are prepared by using a simple electrochemical anodization process by applying a constant potential of 30 V for 24 hours using ethylene glycol consisting of ammonium fluoride as an electrolytic medium. Structural morphology and crystalline nature of such TiO2 nanotubes are analyzed using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Interestingly, nanocomposites of TiO2 with Au NPs is prepared in an effort to alter the intrinsic properties of TiO2, especially tuning of its band gap. Au NPs are prepared by a well-known Brust and Schiffrin method and are immobilized onto TiO2 electrodes which act as a perfect electrochemical sensing platform for GSH detection. Structural characterization and analysis of these modified electrodes are performed using FESEM, XRD, and UV-visible spectroscopic studies. GSH binding events on Au NPs-immobilized porous TiO2 electrodes are monitored by electrochemical techniques, namely, cyclic voltammetry (CV) and chronoamperometry (CA). Several parameters such as sensitivity, selectivity, stability, limit of detection, etc are investigated. In addition, Au NPs dispersed in aqueous medium are also explored for naked-eye detection of GSH using UV-visible spectroscopy in order to compare the performance of the proposed sensor. Our studies clearly indicate that these materials could potentially be used for GSH sensing applications.

A new method for improving the performance of dye sensitized solar cell using macro-porous silicon as photoanode

This paper presents a novel method to improve the function of dye-sensitized solar cell (DSSC). The pro- posed method includes use of macro-porous silicon (PSi) as a photoanode for DSSC. The photoanode, PSi, was fabri- cated by a simple electrochemical anodization process. The effects of the obtained PSi-based photoanode on the DSSC performance are investigated by using photocurrent-volt- age, UV-visible spectroscopy, and reflectance spec- troscopy measurements. The results show that the macro- porous structure in the PSi-based photoanode not only increases internal surface area in the photoanode, but also functions as an anti-reflective/light-trapping layer. Macro- porous structure leads to the both enhancement of pho- toanode dye loading capacity and increasing the optical path length of the incident light inside the photoanode. This way, macro-porous structure plays an important role in the improvement of light harvesting efficiency. The optimal power conversion efficiency of 3.56 and 3.93 % (spin coated and sputtered TiO2 respectively) is obtained from the PSi-based DSSC, with 58 and 75 % improvement in the efficiency as compared to a fabricated conventional DSSC. The measurement results suggest that exploiting PSi in the structure of photoanode raises hopes for low-cost and high- performance production of hybrid Si/dye solar cells in the future.

A Controllable and Widely Applicable Electrochemical Anodization Process to Fabricate Porous Anodic Aluminum Oxide Membrane

Inspired by the versatile applications of porous alumina nanostructures on solar cells, supercapacitors, biosensors, and so forth, research on fabrication of anodized aluminum oxide (AAO) with nanopore structure has achieved substantial advancements. Porous AAO membrane with nanostructures has become one of the most popular template materials for synthesis of a broad range of functional nanostructures such as nanowires, nanopillars, nanotubes, etc. While fabrication of highly ordered nanoporous AAO with tunable geometric features has been widely reported, its growth rate can be easily affected by the fluctuation of parameters such as acid concentration and temperature during the electrochemical anodization process. In order to meet the demand of fabricating AAO with various surface parameters and precise target pore depth control for scientific research and commercial applications, a controllable fabrication process is essential. Here, we developed a method which can implement precise pore depth control of AAO layer during the electrochemical anodization process, via monitoring the integrated charge. This method is applicable in a large voltage range and can be widely applied in fabrication of nanostructure with material such as TiO2and so on.

Electrochemical capacitance of iron oxide nanotube (Fe-NT): effect of annealing atmospheres

The effect of annealing atmosphere on the supercapacitance behavior of iron oxide nanotube (Fe-NT) electrodes has been explored and reported here. Iron oxide nanotubes were synthesized on a pure iron substrate through an electrochemical anodization process in an ethylene glycol solution containing 3% H2O and 0.5 wt.% NH4F. Subsequently, the annealing of the nanotubes was carried out at 500 °C for 2 h in various gas atmospheres such as air, oxygen (O2), nitrogen (N2), and argon (Ar). The morphology and crystal phases evolved after the annealing processes were examined via field emission scanning electron microscopy, x-ray diffraction, Raman spectroscopy, and x-ray photoelectron spectroscopy. The electrochemical capacitance properties of the annealed Fe-NT electrodes were evaluated by conducting cyclic voltammetry (CV), galvanostatic charge–discharge, and electrochemical impedance spectroscopy tests in the Li2SO4 electrolyte. Based on these experiments, it was found that the capacitance of the Fe-NT electrodes annealed in air and O2 atmospheres shows mixed behavior comprising both the electric double layer and pseudocapacitance. However, annealing in N2 and Ar environments resulted in well-defined redox peaks in the CV profiles of the Fe-NT electrodes, which are therefore attributed to the relatively higher pseudonature of the capacitance in these electrodes. Based on the galvanostatic charge–discharge studies, the specific capacitance achieved in the Fe-NT electrode after annealing in Ar was about 300 mF cm−2, which was about twice the value obtained for N2-annealed Fe-NTs and three times higher than those annealed in air and O2. The experiments also demonstrated excellent cycle stability for the Fe-NT electrodes with 83%–85% capacitance retention, even after many charge–discharge cycles, irrespective of the gas atmospheres used during annealing. The increase in the specific capacitance was discussed in terms of increased oxygen vacancies as a result of the enhanced transformation of the hematite (α-Fe2O3) phase to the magnetite (Fe3O4) phase for the electrodes annealed in the N2 and Ar atmospheres.

Fabrication, characterization and photocatalytic performance of Fe-doped TiO2 nanotube composite for efficient degradation of water pollutants

Pure TiO2 nanotube and Fe-doped TiO2 nanotube composite were fabricated by the electrochemical anodic oxidation process and the impregnating-calcination method, respectively. The structural and spectral properties were characterized by scanning electron microscopy, energy dispersive X-ray spectrometer, X-ray diffraction, Fourier transform infrared spectroscopy, photoluminescence, and UV–vis spectrum. These results showed that Fe was effectively deposited on the surface of TiO2 nanotube. It is demonstrated that the Fe-doped TiO2 nanotube composite can inhibit the recombination of electron–hole pairs and enhance the light harvesting ability. The photocatalytic activity of Fe-doped TiO2 nanotube composite under UV irradiation was examined in terms of color and UV–vis absorbance for the degradation of methyl orange. The photocatalytic performance of UV-C irradiation in presence of Fe-doped TiO2 nanotube composite increased obviously for the removal efficiency of color and UV–vis absorbance compared to that of UV-A irradiation, and the reaction rate constant was about 6.8 times faster than that of UV-A irradiation. This can be ascribed to the effective separation of the photo-generated charge carriers.

A method for the formation of Pt metal nanoparticle arrays using nanosecond pulsed laser dewetting

Nanosecond pulsed laser dewetting of Pt thin films, deposited on a dimpled Ta (DT) surface, has been studied here in order to form ordered Pt nanoparticle (NP) arrays. The DT substrate was fabricated via a simple electrochemical anodization process in a highly concentrated H2SO4 and HF solution. Pt thin films (3–5 nm) were sputter coated on DT and then dewetted under vacuum to generate NPs using a 355 nm laser radiation (6–9 ns, 10 Hz). The threshold laser fluence to fully dewet a 3.5 nm thick Pt film was determined to be 300 mJ/cm2. Our experiments have shown that shorter irradiation times (≤60 s) produce smaller nanoparticles with more uniform sizes, while longer times (>60 s) give large nanoparticles with wider size distributions. The optimum laser irradiation time of 1 s (10 pulses) has led to the formation of highly ordered Pt nanoparticle arrays with an average nanoparticle size of 26 ± 3 nm with no substrate deformation. At the optimum condition of 1 s and 500 mJ/cm2, as many as 85% of the dewetted NPs were found neatly in the well-defined dimples. This work has demonstrated that pulsed laser dewetting of Pt thin films on a pre-patterned dimpled substrate is an efficient and powerful technique to produce highly ordered Pt nanoparticle arrays. This method can thus be used to produce arrays of other high-melting-point metal nanoparticles for a range of applications, including electrocatalysis, functionalized nanomaterials, and analytical purposes.

2D XANES–XEOL Spectroscopy Studies of Morphology-Dependent Phase Transformation and Corresponding Luminescence from Hierarchical TiO2 Nanostructures

Hierarchical TiO2 nanostructures with the nanograss at the top and the close-ended nanotube at the bottom were synthesized by an extended electrochemical anodization process. Upon thermal annealing, the hierarchical TiO2 nanostructures exhibit a morphology-dependent phase transformation behavior: the amorphous as-prepared nanotube layer undergoes an amorphous-anatase-rutile phase transformation upon annealing at 550 °C and 850 °C, respectively; the nanograss, on the other hand, remains in the anatase phase with an annealing temperature up to 850 °C and becomes anatase-dominant anatase–rutile-multiphase at 900 °C. In addition, the nanotube and nanograss exhibit intriguing light-emitting properties. A two-dimensional X-ray absorption near-edge structure – X-ray excited optical luminescence (2D XANES–XEOL) spectroscopy technique is used to investigate such phenomenon, which correlates the origin of the luminescence with the local chemical entrainment of the nanostructured TiO2. Implications of these observations are thoroughly discussed. These properties are desirable for the engineering of novel TiO2 nanoheterostructures for photocatalysis as well as optical and photonic devices, especially the fabrication of a stable high temperature anatase phase.

Porous Si Based Al Schottky Structures on p+-Si: A Possible Way for Nano Schottky Fabrication

This paper presents the results of a comparison research between the Al/p-Si/Al, Al/p+-Si/Al and Al/PS/p+-Si/Al devices. The current transport mechanism of an Al/por-Si/p+-Si/Al heterojunction device with a porous silicon layer fabricated by the electrochemical anodization process has been investigated using temperature dependent current–voltage measurements in the temperature range of 10–300K, and these measurements are realized even for Al/p-Si/Al diode. The porosity properties of porous Si layer are investigated by a cross-sectional SEM view and a PL measurement. The good rectification for a Schottky like diode which was formed by Al metallization of porous silicon (PS) on p+-Si substrate was observed and it continues even in 6K temperature, despite Al/p-Si/Al diode freezing out at 20K. Moreover, it is reported that the Al/PS/p+-Si/Al heterojunction device reveals the excellent junction parameters with an ideality factor very close to unity, a high barrier height which is 0.63eV and a small value of series resistance that is 30Ω, at 300K. However, the high serial resistance values for this heterojunction device compared to that of the crystalline Si based on the Al/p-Si/Al diode are related to PS. At high temperatures, it is noted that the current transport mechanism is through the thermoionic-field emission mechanism. However, at low temperatures, the unreliable variation of the Richardson plot is seen, and no distinct Gaussian distribution is seen in the relation of Φb vs. q/2kT; and this behaviour is attributed to the highly doped silicon. Furthermore, in this study, the current transport mechanism is discussed by using the Fowler Nordheim tunnelling theory in PS, and it is proposed that tunnelling via the migration/diffusion is the main mechanism of charge transport in PS.

Visible light activity of sulfur-doped TiO2 nanostructure photoelectrodes prepared by single-step electrochemical anodizing process

S-doped TiO2 nanostructure (STN) photocatalyst films were prepared in an aqueous solution containing NH4F and different concentrations of potassium disulfite as the sulfur source, via the electrochemical oxidation of titanium substrates. The resulting samples were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX). Diffuse reflectance spectra showed a shift toward longer wavelengths relative to pure TiO2. The dependence of photoactivity on sulfur doping on TiO2 was studied for photocatalytic hydrogen generation and photodegradation of rhodamine B (RhB). After S-doping, the removal rate of RhB and the hydrogen photogeneration amount increased under visible light radiation. The doping also led to the remarkable photocurrent enhancement. Also, clear dependence between photocurrent intensity and photoactivity was observed. This work demonstrated a feasible and simple anodization method to fabricate an effective, reproducible, and inexpensive visible-light-driven photocatalyst for environmental applications.

Systematic in vitro nanotoxicity study on anodic alumina nanotubes with engineered aspect ratio: Understanding nanotoxicity by a nanomaterial model

Here, we report a detailed and systematic approach for studying the in vitro nanotoxicity study of high aspect ratio (HAR) nanomaterials using anodic alumina nanotubes (AANTs) as a nanomaterial model. AANTs with bio-inert properties and tailored aspect ratios ranging from 7.8 to 63.3 were synthesized by an electrochemical pulse anodization process. Cytotoxicity studies were conducted with RAW 264.7 mouse macrophage cells and MDA-MB 231-TXSA human breast cancer cells through several toxicity parameters, including cell viability and morphology, pro-inflammatory response, mitochondrial depolarization, lysosomal membrane permeabilization (LMP), induction of autophagy and endoplasmic reticulum (ER) stress. The resulting toxicity patterns were cell-type dependent and strongly related with AANTs dose, length of time, and importantly the AR of AANTs. Long AANTs triggered enhanced cell death, morphological changes, tumor necrosis factor α (TNF-α) release, LMP and ER stress than short AANTs. The toxic AR window of AANTs was determined to be 7.8, which is shorter than that of other previously reported HAR nanomaterials. This toxic AR window provides a promising opportunity to control the nanotoxicity of HAR nanomaterials for their advanced drug delivery application.

Rapid and Sensitive Detection of Lung Cancer Biomarker Using Nanoporous Biosensor Based on Localized Surface Plasmon Resonance Coupled with Interferometry

We propose a nanobiosensor to evaluate a lung cancer‐specific biomarker. The nanobiosensor is based on an anodic aluminum oxide (AAO) chip and functions on the principles of localized surface plasmon resonance (LSPR) and interferometry. The pore‐depth of the fabricated nanoporous AAO chip was 1 µm and was obtained using a two‐step electrochemical anodization process. The sensor chip is sensitive to the refractive index (RI) changes of the surrounding medium and also provides simple and label‐free detection when specific antibodies are immobilized on the gold‐deposited surface of the AAO chip. In order to confirm the effectiveness of the sensor, the antibodies were immobilized on the surface of the AAO chip, and the lung cancer‐specific biomarker was applied atop of the immobilized‐antibody layer using the self‐assembled monolayer method. The nanoporous AAO chip was used as a sensor system to detect serum amyloid A1, which is a lung cancer‐specific biomarker. The specific reaction of the antigen‐antibody contributes to the change in the RI. This in turn causes a shift in the resonance spectrum in the refractive interference pattern. The limit of detection (LOD) was found to be 100 ag/mL and the biosensor had high sensitivity over a wide concentration range.

Microstructural and magnetic characterizations of CoFeCu electrodeposited in self-assembled mesoporous silicon

Abstract Self-assembled mesoporous silicon with quasi-regular pore arrangements has been fabricated by the electrochemical anodization process in hydrofluoric acid solution. CoFeCu was electrodeposited in this structure from a bath containing sodium acetate as a complexing agent with a pH value of 5. The effect of current density on the morphology, the structure and the magnetic properties of CoFeCu deposit was studied by SEM, EDS, DRX and VSM. It has been shown that the morphology and structure of samples were strongly influenced by the current density and etching duration. The micrographs show the vertical and branched nanowires and also a discontinuous growth of wires. Further, the growth of a thick layer from the grain boundaries of released CoFeCu wires is produced. The magnetic hysteresis loops demonstrate that the CoFeCu nanowires exhibit easy magnetic axis perpendicular to the PS channels axis when the current density varied from 3 to 10 mA/cm 2 . Nevertheless, they reveal a no magnetic anisotropy of CoFeCu nanostructures deposited only in the outside of porous silicon, probably due to the vanishing the shape anisotropy.

Optimizing UV detection properties of n-ZnO nanowire/p-Si heterojunction photodetectors by using a porous substrate

In this work, the effect of porous silicon substrates with various structures on response in ultraviolet range of a zinc oxide nanowire based heterojunction ultraviolet detectors was studied. This was realized through modifications made in bandgap of silicon as a result of porosification for the purpose of increasing the sensitivity and selectivity. Different porous substrates were prepared by electrochemical anodic etching process at different anodization times, current densities and acid concentration which is considered an easy and cost-effective fabrication process that makes the proposed heterojunction a suitable choice for ultraviolet sensing applications. The electrical and optical properties of the prepared samples were measured and the sample with highest porosity exhibits the most selective response with the highest sensitivity (\(2\times 10^{4}\) at \(-\)3.4 V) and the lowest dark current (\(1.2\times 10^{-8}\) A at \(-\)3.4 V) among all samples, although the sample with crystalline silicon substrate shows the highest responsivity (1,109 A/W at 356 nm).

Influence of annealing temperature on the morphology and the supercapacitance behavior of iron oxide nanotube (Fe-NT)

The article demonstrates the influence of annealing temperature on the supercapacitance behavior of iron oxide nanotube synthesized on pure iron substrate by electrochemical anodization process. Anodization was performed in an ethylene glycol solution containing 3% H2O and 0.5 wt. % NH4F. The as-anodized nanotubes were annealed in an ambient atmosphere at various temperatures ranging from 200 to 700 °C for a fixed duration of time (2 h). The morphology and crystal phases developed after anodization and subsequent annealing processes were examined using field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and X-ray photospectroscopy (XPS). Cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) experiments were performed in 1 M Li2SO4 to evaluate the electrochemical capacitance properties of the oxide nanotube electrodes. It was found that the electrode annealed at 300 °C exhibited superior electrochemical capacitance compared to the electrodes annealed at other temperatures. The highest specific capacitance achieved after annealing at 300 °C was about 314 mF cm−2. The electrodes annealed at 200, 500, 600 and 700 °C displayed much lower specific capacitance compared to those annealed at 300 and 400 °C. Galvanostatic charge–discharge experiments conducted on some of the annealed electrodes demonstrated excellent cycle stability with more than 80% capacitance retention after 1000 charge–discharge cycles.