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A hierarchical Cu2+1O nanowire covered ultrathin Sn/Cu6Sn5 alloy composite on copper foam is produced in 90 minutes by a fully electrochemical wet process for use as a stable Li metal anode to prevent the formation of Li dendrite.

The electrolysis of water using renewable energy inputs is a promising sustainable approach to produce clean hydrogen fuel. The conventional water electrolysis, where the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are tightly coupled to satisfy the thermodynamic potential of at least 1.23 V, occasionally encounters gas crossover through the membrane, resulting in the formation of explosive gas mixtures and reactive oxygen species. In this study, an asymmetric cell design of 3 M H2SO4|V(II)/V(III)|1 M KOH equipped with nickel foam electrodes is used for achieving decoupled HER and OER under kinetically favorable conditions by dividing the process into two steps using vanadium ions as redox mediators. The actual overall water splitting at an average cell voltage of 1.3 V and a current density of 10 mA cm−2 is accomplished even in the presence of membranes with outstanding cycling stability. The well-designed system for decoupled water electrolysis can allow the production of clean energy fuel using a low-power input in renewables–to–hydrogen conversion.

In this study, vertically aligned p–n junction TiO2 nanotubes can be formed by anodization of the Ti substrate following the addition of a high electric field in a Pd precursor-containing electrolyte. The bottom region of the TiO2 nanotubes with a high concentration of Pd because of the high electric field which was induced to be p-type. In contrast, the region with a low Pd concentration (top of the TiO2 nanotubes) was determined to be n-type, similar to the pristine TiO2. The concentration profile of the dopant in TiO2 nanotubes was investigated via TOF-SIMS and XPS. Defect formation energies in TiO2 nanotubes were estimated using density-functional theory calculation to understand the p–n junction formation. The p–n junction TiO2 nanotubes showed a high photocatalytic hydrogen production rate of 25.2 μL cm−2 h−1 under solar light irradiation as a result of the enhancement of visible light photoactivity.

Herein, we present a simple and innovative method for preparing Ni–Fe oxide doped with trace amounts of Ru on a stainless steel 304 substrate by single-step anodization. The procedure yields a highly efficient, durable, and flexible bifunctional catalyst for water splitting. The material exhibits outstanding electrocatalytic performance with high stability in both the hydrogen and oxygen evolution reactions, owing to the enhanced activity resulting from the incorporation of a small amount of Ru into anodic Ni–Fe oxide and a change in morphology that facilitates mass transfer. A stable cell voltage of 1.83 V is achieved at a current density of 100 mA cm−2 in a symmetric electrolyzer composed of flexible Ru-doped Ni–Fe oxide electrodes that can be formed into various shapes. The applicability of bifunctional electrodes of various shapes is demonstrated by their excellent stability for 100 h.

A high-purity copper foil was passivated in aqueous solution of 1.0 M NaOH at −200 mV at room temperature. Top-view micrographs were taken after 30, 90, 150 and 600 s of passivation. Nanowires with a diameter of up to 56 nm, consisting of Cu2O and CuO were obtained. The mechanism of the nanowires growth was investigated based on the current density–time curves, relevant micrographs and morphology analyses. It was found that the nuclei is formed at electrolyte-electrode interface, leading to the formation of the first bunches of nanowires. Then, nanowires grow by the expansion and decomposition of the formed oxide bunches in the electrolyte, which can be understood through the current density – time curves.

A SnO2–TiO2 electrode was prepared via anodization and subsequent anodic potential shock for a binder-free anode for lithium-ion battery applications. Perpendicularly oriented TiO2 microcones are formed by anodization; SnO2, originating in a Na2SnO3 precursor, is then deposited in the valleys between the microcones and in their hollow cores by anodic potential shock. This sequence is confirmed by SEM and TEM analyses and EDS element mapping. The SnO2–TiO2 binder-free anode is evaluated for its C-rate performance and long-term cyclability in a half-cell measurement apparatus. The SnO2–TiO2 anode exhibits a higher specific capacity than the one with pristine TiO2 microcones and shows excellent capacity recovery during the rate capability test. The SnO2–TiO2 microcone structure shows no deterioration caused by the breakdown of electrode materials over 300 cycles. The charge/discharge capacity is at least double that of the TiO2 microcone material in a long-term cycling evaluation.

International audience; Stainless steel, a cost-effective material comprising Fe, Ni, and Cr with other impurities, is considered a promising electrode for green electrochemical energy storage and conversion systems. However, the Cr in stainless steel and its passivating property in electrochemical systems hinder the commercial use of stainless steel in the energy conversion and storage industry. Therefore, many studies have revealed the usability of stainless steel by developing various surface treatment techniques to modify the electrode surface to take advantage of the intrinsically active elements in stainless steel. In this review, we present the recent advances and breakthroughs in surface treatment approaches to adjust surface composites and the electrochemical performance and rational design of electrodes in green energy storage and conversion systems, including (photo)electrochemical water splitting, Li-ion batteries, and supercapacitors. © 2022 Elsevier B.V.

This paper presents an investigation of anodic TiO2 nanotube arrays (TNAs), with a Co3 O4 /CuO coating, for lithium-ion batteries (LIBs). The coated TNAs are investigated using various analytical techniques, with the results clearly suggesting that the molar ratio of Co3 O4 /CuO in the TiO2 nanotubes substantially influences its battery performance. In particular, a cobalt/copper molar ratio of 2:1 on the TNAs (Co2 Cu1 @TNAs) features the best LIBs anode performance, exhibiting high reversible capacity and enhanced cycling stability. Noticeably, Co2 Cu1 @TNAs achieve excellent rate capability even after quite a high current density of 20.0 A g-1 (≈25 C, where C corresponds to complete discharge in 1 h) and superior volumetric reversible capacity of ≈3330 mA h-1 cm-3 . This value is approximately seven times higher than those of a graphite-based anode. This outstanding performance is attributed to the synergistic effects of Co2 Cu1 @TNAs: 1) the structural advantage of TNAs, with their large amount of free space to accommodate the large volume expansion during Li+ insertion/extraction and 2) the optimized ratio of Co3 O4 and CuO in the composite for improved capacity. In addition, no binder or conductive agent is used, which is partly responsible for the overall improved volumetric capacity and electrochemical performance.

Reduced graphene oxide (RGO)-coated TiO2 microcones have been synthesized via simple anodization and cyclic voltammetry for use in lithium-ion batteries (LIBs). Microcones had a perpendicularly ori...

We investigated the effects of applied potential and anodizing time on the morphological and structural changes of TiO2 microcones for different electrolytes. We found that the mean diameter of the TiO2 microcones produced in (COOH)2 linearly increased with the applied voltage, whereas in other electrolytes, the mean diameters converged to a constant value. Regardless of the type of electrolyte used, the percentage of area occupied by the TiO2 microcones is estimated at 20% of the entire surface when the microcones are fully-grown and well-dispersed. All the TiO2 microcones are composed of the anatase phase, the intensity of which increases in the XRD spectra as the voltage or anodizing time increases. We identified the formation mechanism of the TiO2 microcones using early stage current density-time transient graphs and SEM images.

A uniformly colored TiO 2 , on which the surface is functionalized with nanotubes to control wettability, was prepared by a two-step anodization; the first anodization was carried out to prepare nanotubes for a super-hydrophilic or -hydrophobic surface and the second anodization was performed to fabricate a thin film barrier oxide to ensure uniform coloring. The effect of the nanotubes on barrier oxide coloring was examined by spectrophotometry and UV-vis-IR spectroscopy. We found four different regimes governing the color changes in terms of anodization voltage, indicating that the color of the duplex TiO 2 was primarily determined by the thickness of the barrier oxide layer formed during the second anodization step. The surface wettability, as confirmed by the water contact angle, revealed that the single barrier TiO 2 yielded 74.6° ± 2.1, whereas the nanotubes on the barrier oxide imparted super-hydrophilic properties as a result of increasing surface roughness as well as imparting a higher hydrophobicity after organic acid treatment.

Die cast, high-Si content ADC12 Al alloy samples were successfully anodized without surface cracks. This was accomplished with a 0.3 M sulfuric acid electrolyte with a high concentration of sodium aluminate. During anodization, the AlO2- anions were attracted to the positively-charged Al substrate and deposited in the cracks formed by un-oxidized Si islands within the ADC12. Anodic films prepared in electrolytes with a high concentration of AlO2- drastically enhanced surface morphology, thickness uniformity, Vickers hardness, and corrosion behavior in comparison with anodic film prepared without AlO2- concentration. The simultaneous sealing mechanism by AlO2- anions during anodization is reported in detail.

A fast and non-toxic sealing method based on the use of NaAlO 2 solution is investigated in detail. AlO 2 sealing is carried out using NaAlO 2 as the sealant, which is neutralized with H 2 SO 4 and optimized to pH 7. Temperature, sealing time, pH is optimized in terms of chemical/mechanical properties, measured by Vickers hardness test, Tafel analysis, and sealing quality test based on ASTM-B680. The Vickers hardness, corrosion resistance, and sealing properties of the samples obtained by the proposed sealing method are found to be superior compared to those of the samples prepared by the conventional sealing methods. XPS data show a high intensity of boehmite (AlOOH) peaks in the sample obtained by the NaAlO 2 -based sealing, similar to the case of the sample obtained by hydrothermal sealing. During the NaAlO 2 sealing process, a hard and dense layer of boehmite, which is formed from the dissipated AlO 2 − present in the sealant solution and the dissolved anodic oxide, is deposited within the pores of the anodic alumina in a short time; this leads to enhanced chemical/mechanical properties of the anodic porous alumina sample sealed using NaAlO 2 .

MnO 2 was homogenously doped into anodic nanotubular TiO 2 by a potential shock method, in which a high potential was imposed on the anodic nanotubes immediately after anodization process. We found that the potential shock process is inapplicable in a high-conductivity aqueous electrolyte, e.g. a MnO 2 precursor solution (herein, 0.006 M KMnO 4 : 852–933 μS/m). To avoid exceeding the output compliance of the current source, the potential shock voltage was optimized in ethylene glycol for the application of water oxidation. We found an optimal potential shock voltage of 140 V, which led to the doping of 0.7 at. % MnO 2 into the high-aspect-ratio nanotubular TiO 2 within 10 s. The TiO 2 nanotubes doped with MnO 2 were successfully employed as electrodes for the non-noble catalysis of water oxidation. Although the doping concentration of Mn was found to be linearly proportional to the applied potential shock voltages, potential shocks greater than 140 V significantly increased the thickness of the barrier oxide layer, which increased the overpotential in the water oxidation process.

In this research, hematite nanoparticles were synthesized by DC thermal plasma process to increase the overall surface area. The effect of binders on hematite electrodes was investigated by changing the type and composition of binders when preparing electrodes. Nitrogen gas was also added to the DC thermal plasma process in order to dope the hematite with N for enhancing photoelectrochemical properties of hematite nanoparticles. The efficiency of water splitting reaction was measured by linear sweep voltammetry (LSV) under solar simulator. In LSV measurements, the onset potential and maximum current density at a fixed voltage were measured. The durability of electrodes was checked by repeating LSV measurements. CMC (carboxymethyl cellulose) binder with 50 : 1 composition exhibits the highest current density of 12 mA/cm 2 and CMC binder

전기화학적 양극산화 기법으로 제조한 타이타늄 나노튜브는 타이타늄 특유의 강한 화학내구성 및 나노튜브의 높은 종횡비로 인하여 넓은 범위에 응용된 소재이다. 전해질의 구성 성분과 종류, pH, 전압, 온도 그리고 양극산화 시간이 타이타늄 나노튜브의 성상을 결정짓는 요소들이며 도핑을 통해 촉매능을 부여할 수 있다. 비금속 및 금속 원소 모두도핑 가능하며 도핑 방법 역시 다양하다. 도핑 방법에는 합금 양극산화, 열처리법, 함침법, 전기도금법 등 다양한 방법들이 이용되며 점차 간단하고 빠른 도핑 방법을 찾는 방향으로 연구가 진행되고 있다. 본 총설에서는 타이타늄 나노튜브의 생성 원리와 상용된 제법들에 관하여 기술하고 도핑과 그 응용 및 최근의 도핑 연구 동향을 다루도록 하겠다.

Tungsten trioxide (WO 3 ) prepared by anodization of a W foil was doped with NH 4 through NH 4 OH treatment at 450 °C. Since aqueous NH 4 OH was used during doping instead of NH 3 gas, the treatment step does not require complicated annealing facilities. Moreover, the state of doped N is a form of NH 3 -W instead of W 2 N, which lowers the bandgap but increases photocorrosion. We found that incorporation of NH 4 into WO 3 leads to reduction of the bandgap from 2.9 eV to 2.2 eV, regardless of the amount of NH 4 OH treatment, lowering the onset potential and increasing the current density at fixed potential for oxygen evolution reaction under illumination. Scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy were employed to investigate the surface morphologies, crystallinities of tungsten oxides and existence of NH 4 doping, respectively. The bandgap energy was determined by UV–Vis spectroscopy to measure the transmittance and refraction. The water splitting performance of each sample was measured by electrochemical linear sweep voltammetry in a 3-electrode configuration under illumination.

9¦ ." b# ,¯#*`, #&*' &# z, 9o#*j3# pits #:¯%E'v5F#&²#," .#- +Þ#:i, 9o*š ~ . i5^*AE#:O:#,ž: .b+Þ#%F+C9o*†# 10 min s)â#*j3+Þ#.^:#9o# J –# b*¾"š#:O, #8o #+ž*j#.Z –# ž C, # › ›# 10 µm*U# 5 µm o#,2#,¯ AE ¶#*j3# pits # š0E9o*š ~ . We describe the preparation of highly-ordered etching pits on the Nb foil through a micromachining. The effects of electro-chemical polishing on the formation of uniformly-patterned protective epoxy layer was investigated. Unlike the previous proc-ess using O

Most of biomaterials come in direct contact with the body, making standardized methods of evaluation and validation of biocompatibility an important aspect to biomaterial development. However, biomaterial validation guidelines have not been fully established, until now. This study was to compare the in vitro behavior of osteoblasts cultured on nanomaterial surfaces to osteoblast behavior on culture plates. Comparisons were also made to cells grown in conditioned media (CM) that creates an environment similar to the in vivo environment. Comparisons were made between the different growth conditions for osteoblast adhesion, proliferation, differentiation, and functionality. We found that the in vivo-like system of growing cells in concentrated CM provided a good validation method for biomaterial development and in vivo implant therapy. The materials were biocompatible, showing similar behavior to that observed in vivo. This study provided valuable information that would aid in the creation of guidelines into standardization and evaluation of biocompatibility in biomaterials.

Studies on niobium anodization in the mixture of 1 M and 1 wt % HF at galvanostatic anodization are described here in detail. Interestingly, duplex niobium oxide consisting of thick barrier oxide and correspondingly thick porous oxide was prepared at a constant current density of higher than 0.3 , whereas simple porous type oxide was formed at a current density of lower than 0.3 . In addition, simple barrier or porous type oxide was obtained by galvanostatic anodization at a single electrolyte of either 1 M or 1 wt % HF, respectively. The formation mechanism of duplex type structures was ascribed to different forming voltages required for moving anions.

The degree of self-assembly and the size variation of nanotubular structures in anodic titanium oxide prepared by the anodization of titanium in ethylene glycol containing 0.25 wt % at 40 V were investigated as a function of anodization time. We found that the degree of self-assembly and the size of the nanotubes were strongly dependent on thickness deviation and thus indirectly on anodization time, as the thickness deviation was caused by the dissolution of the topmost tubular structures at local areas during long anodization. A large deviation in thickness led to a large deviation in the size and number of nanotubes per unit area. The dissolution primarily occurred at the bottoms of the nanotubes () in the initial stage of anodization (up to 6 h), which led to the growth of nanotubes. Dissolution at the tops () was accompanied by after the formed structures contacted the electrolyte after 12 h, generating the thickness deviation. After extremely long anodization (here, 70 h), was the dominant mode due to increase in pH, meaning that there was insufficient driving force to overcome the size distribution of nanotubes at the bottom. Thus, the nanotube array became disorder in this regime.

Electropolishing of Ti was carried out in alcohol-based electrolyte and subsequently the polished Ti was anodized in ethylene glycol containing 0.25 wt% NH4F. Anodization of polished and non-polished Ti, respectively, showed no big difference in the degree of local arrangement of the nanotubes, whereas the surface roughness on a large area was dramatically improved in the anodized polished Ti. Anodization of polished Ti leads to the formation of nanotubular TiO2 structures with more uniform length and relatively time-independent density of nanotubes compared to that prepared with anodization of non-polished Ti, meaning that electropolishing is indispensable for the formation of uniform nanotubular TiO2 structures.

In this work, we investigate anodization of Pb in ethylene glycol containing small amount of NH4F, demonstrating that β-PbF2 particles with octahedral morphology can be prepared by adjusting the applied potential and anodizing time. FE-SEM images and XRD measurements of anodic nanostructures as a function of anodizing time clearly show that PbO is first formed on Pb. Subsequently, a local dissolution of PbO leads to formation of skeleton structure of PbO, releasing Pb 2+ ions in the electrolyte. The lead ions can be precipitated on the walls or intersection of the skeleton walls when the concentration of lead ions is saturated. The method described in this article shows the feasibility of formation of metal fluoride crystal by anodization of metal in a fluoride containing solution.

The nanosurface-confined nucleation and growth processes of Pt and TiO2 were investigated when Pt was sputter deposited and TiO2 was sol-gel coated on a unique surface-designed substrate. The substrate was an anodic aluminum oxide (AAO) film with self-assembled grouped nanopores (SGNPs). The SGNPs gave rise to unique nucleation sites comprising very small-sized boundaries and nanopores. Pt sputter deposition onto the SGNPs showed restricted growth of nanogranules. The TiO2 sol-gel coating onto the SGNPs resulted in unique formations of nanopore and network structures. The unique nucleation phenomena of Pt and TiO2 on the nanometer-sized surfaces are explained by a combination of two effects: confinement of the degree of freedom at the nucleation sites and growth direction. This is different from conventional surface nucleation that yields the growth of islands, layer-by-layer deposition, and epitaxy.

A novel approach to surface modification of hydroxyapatite (HAp) nano-crystals was described based on in-situ synthesis of surface thiol-functionalized HAp (HAp-SH) and subsequent grafting polymerization of ethylene glycol methacrylate phosphate (EGMP). Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses showed that thiol groups were introduced on HAp surfaces by adding 3-mercaptopropionic acid during hydrothermal synthesis of HAp nano-crystals. The radical chain transfer to surface thiol groups generated the sulfur-centered radicals on HAp nano-surfaces, which initiated the surface grafting polymerization of EGMP. Fourier transform infrared (FT-IR) spectroscopy and powder X-ray diffraction (XRD) analyses confirmed the grafting reaction on HAp surfaces. Zeta potentials of control HAp, thiol-functionalized HAp (HAp-SH), and PolyEGMP-grafted HAp in phosphate buffered saline (PBS) solutions (pH 7.4) were negative and decreased with increasing the amount of grafted PolyEGMP. TEM measurements and time-dependent phase monitoring suggested that the colloidal stability of PolyEGMP-grafted HAp over synthesized HAp nano-crystals in water dramatically increased without inter-crystal aggregation.

Monodisperse silver nanowires with high aspect ratio are prepared via electrochemical plating into monodomain porous alumina templates. The nanowires have a length of 30 μm or more, adjustable uniform diameters ranging between 180 and 400 nm, and a monodispersity of about 2%. The templates were fabricated by anodization of imprinted aluminum with an interpore distance of 500 nm. Nearly 100% pore filling was obtained due to a thinning process which guaranteed a barrier layer of homogeneous thickness of a few nanometers. Moreover, hierarchically ordered porous alumina with about 25-nm−40-nm pore diameter on one side and 180-nm pore diameter on the other side was successfully infiltrated by the same method, yielding branched silver nanowires.

Transmission electron microscopy analysis of self-ordered porous alumina obtained by electrochemical anodization shows that self-ordering requires a porosity of 10%, independent of the specific anodization conditions. This corresponds to a volume expansion of alumina to aluminum of about 1.2. We propose that self-ordering of porous alumina with any interpore distance is possible if the applied potential, which mainly determines the interpore distance, and the pH value of the electrolyte, which mainly defines the pore radius, match the 10% porosity rule.

We describe a novel procedure to fabricate WO3@surface-mounted metal-organic framework (SURMOF) hybrid materials by electrodeposition of WO3 nanoparticles into HKUST-1, also termed Cu3(BTC)2 SURMOFs. These materials have been characterized using x-ray diffraction, time-of-flight secondary ion mass spectrometry, scanning electron microscopy, x-ray photoelectron spectroscopy as well as linear sweep voltammetry. The WO3 semiconductor/SURMOF heterostructures were further tested as hybrid electrodes in their performance for hydrogen evolution reaction from water.

Titanium oxide (TiO_2) nanowires were prepared for an electrolytic capacitor application by the automatic dipping technique using a porous alumina template. The automatic dipping technique allows us to exactly control the dipping rate so that we can obtain homogenous infiltration of nanowires in the porous alumina membrane, even though the solution is very acidic. From the TEM, SEM and XRD measurements, we confirmed that anatase phase TiO2 nanowires are highly infiltrated into the porous alumina template. In addition, the electrostatic capacitance of nanowires was measured and compared with a theoretical calculation using an effective thickness (delta e). We found that the effective thickness corresponds to the mean radius of nanowires and the experimental measurements were in good agreement with the calculations.

We describe the preparation of a tungsten pillar nanoimprint stamp without the use of lithography and etching techniques. Structures with heights of 15 nm were prepared on the basis of self-ordered porous alumina templates and this was followed by DC sputtering of tungsten. The stamp was successfully used to prepare an aluminium surface to obtain highly ordered porous anodic alumina films after a single anodization step. The preparation efficiency for highly ordered porous alumina was dramatically improved as compared to the more conventional two-step anodization–strip-anodization method, as a sacrificial layer with a thickness of a few hundred micrometres was not required. In addition, by fractal calculations, we have evaluated the degree of ordering of the asperities on the nanoimprint master stamp.

Herein, we describe a facile method to produce nickel oxalate nanostructures by chemical reaction of oxalic acid and a nickel foil in various organic solvents and water. Grass-like structures consisting of nickel oxalate are produced by the chemical reaction within 30 min for all solvents. Interestingly, nickel oxalate nanowires can be produced by the addition of a small amount of water in certain solvents. Annealing of nickel oxalate structures leads to formation of nickel oxide structures with a slight morphological change. Compositions of the nanostructures are investigated by TEM and FT-IR analyses. In addition, the supercapacitance of the nickel oxalate nanostructures is characterized, and the results show that they are superior to that of nickel oxide nanostructures.

A novel approach to the fabrication of porous scaffolds with surface-immobilized nano-hydroxyapatite (N-HAp) is developed for effective bone tissue engineering. The discrete nano-level anchoring of N-HAp on the pore surface of chitosan scaffolds is achieved using surface-repellent stable colloidal N-HAp with surface phosphate functionality. Field-emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) confirm pronounced exposure of N-HAp on the surfaces of chitosan scaffolds at the nano-level, which can not be accomplished with the conventional polymer/N-HAp composite scaffolds. This rational surface engineering enables surface-anchored N-HAp to express its overall intrinsic bioactivity, since N-HAp is not phase-mixed with the polymers. The porous chitosan scaffolds with surface-immobilized N-HAp provide more favorable environments than conventional bulk phase-mixed chitosan/N-HAp scaffolds in terms of cellular interaction and growth. In vitro biological evaluation using alkaline phosphatase activity assay supports that immobilized N-HAp on pore surfaces of chitosan scaffolds contributed to the more enhanced in vitro osteogenic potential. In addition, scaffolds with surface-exposed N-HAp provide favorable environments for enhanced in vivo bone tissue growth, estimated by characteristic biomarkers of bone formation such as collagen. The results suggest that the newly developed hybrid scaffolds with surface-immobilized N-HAp may serve as useful 3D substrates with pore surfaces featuring excellent bone tissue-regenerative properties.

Without using any additives, cuprous oxide (Cu2O) nanoparticles with a size of less than 20 nm that are hexagonally arrayed on a substrate are prepared by electrochemical methods based on the preferential nucleation of seeds on a nanostructured substrate. The nanosized Cu2O particles develop to become uniform microparticles of Cu2O. We found that the nanostructured substrate leads to the lowering of the overpotential for the nucleation of Cu2O particles and narrows the size distribution of the particles, compared to a polished substrate.

In this paper, a method to prepare anodic porous niobium oxide with a thickness of more than half a micrometre is described in terms of delaying the chemical dissolution of the formed oxide in fluorinated electrolytes either by controlling the anodization temperature or by making a protective oxide. It was revealed that both the growth rate and the dissolution rate in the formation of porous niobium oxide films increase as the anodization temperature increases. X-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS) analyses show that the anodically prepared niobium oxide consists of amorphous Nb2O5. For the strategy to make protective oxide, self-ordered nanoporous niobium oxide with double layers consisting of an outer layer of around 90–130 nm and an inner layer of around 300–400 nm is prepared by anodization–annealing–anodization. We believe that the outer oxide, which undergoes annealing, plays the role of a protective layer for the formation of the inner oxide film grown underneath the outer layer, leading to anodic niobium oxide with a thickness greater than that obtained by single anodization.