ZrO2 Nanotubes Research Articles

A three-layer zirconia structure composed of nanotubes, dense layer and nanotubes: Evidence against the FADT

Porous anodic oxides have become a research hotspot, but their formation mechanisms are still controversial. The classical field-assisted dissolution theory (FADT) is a theory of top-down dissolution to form nanotubes. However, certain key problems such as the three-stage current-time curve in anodizing process cannot be explained by this theory. The oxygen bubble mold (OBM) is a new mechanism of bottom-up growth of nanotubes, which can explain the phenomenon that cannot be explained by the FADT. Compared to the anodic TiO2 nanotubes, ZrO2 nanotubes have a more complex anodizing current-time curve which is not studied sufficiently. Under certain conditions, there are double complete three-stage current-time curves and two-layer nanotubes. In this study, to explore the formation mechanism of anodic ZrO2 nanotubes, the current-time curve and the interesting morphology were analyzed, after zirconium anodizing and SEM characterizing. An interesting three-layer structure composed of upper nanotubes, dense layer and lower nanotubes formed by zirconium anodizing is reported for the first time. The lower nanotubes and the dense layer are tightly bonding, which strongly refutes the FADT. The OBM was used to explain this phenomenon, which promoted the development of the formation mechanism of anodic ZrO2 nanotubes.

One-step synthesis of freestanding and translucent ZrO2 nanotube membranes by direct electrochemical anodization

One-step synthesis of freestanding zirconium oxide nanotube (ZrO2-NT) membranes was obtained by direct electrochemical anodization of zirconium (Zr) substrates in aqueous electrolytes (mixture of 1 M (NH4)2SO4 and 0.75 M NH4F). The maximum membrane area was 9 cm2 and showed translucency from 400 to 700 nm. The ZrO2-NTs have an external diameter of ∼80 nm, an internal diameter of ∼55 nm, and a length of ∼22.5 μm. Moreover, the ZrO2-NTs showed a cubic crystal structure after anodization. The results are highly promising for applications requiring independent ZrO2 nanotube membranes.

Effect of water-based electrolyte on surface, mechanical and tribological properties of ZrO2 nanotube arrays produced on zirconium

In this work, highly ordered ZrO2 nanotube arrays were fabricated on commercial pure Zr substrates through anodic oxidation in the water-based electrolyte at various voltages (30 V, 40 V and 50 V) for 1 h. The monoclinic- and tetragonal-ZrO2 phases were obtained on ZrO2 nanotubes through anodic oxidation. 13 vibration modes have been observed for the samples grown at low voltages (30 V and 40 V), which are assigned to monoclinic symmetry (7Ag + 6Bg), while—with the increasing growth voltage, the dominant phonon peak intensities associated with the monoclinic symmetry 6 times are decreased, and Eg (268 and 645 cm − 1) mode corresponding to tetragonal symmetry is observed. The nanotube array surfaces exhibited hydrophilic and super-hydrophilic behavior compared to the bare Zr surface. The elastic modulus values of ZrO2 nanotube surfaces (14.41 GPa) were highly similar to those of bone structure (10–30 GPa) compared to bare Zr substrate (120.5 GPa). Moreover, hardness values of ZrO2 nanotube surfaces were measured between ∼76.1 MPa and ∼ 283.0 MPa. The critical load values required to separate the nanotubes from the metal surface were measured between ∼1.6 N and ∼26.3 N. The wear resistance of the ZrO2 nanotube arrays was improved compared to that of plain Zr substrate.

Anodic oxidation of Zr-5Fe alloy in ethylene glycol/fluoride electrolyte for Cr(VI) removal under sunlight

In this work, Fe-doped ZrO2 nanotubes (Fe-ZNTs) were synthesized by anodization of Zr-5 wt% Fe alloy in fluoride-containing ethylene glycol (EG)/8 vol% H2O electrolyte at varying ammonium fluoride (NH4F) concentrations (0.1, 0.3 and 0.5 wt%) at 40 V for 3 h. The morphology and crystal structure of as-anodized Fe-ZNTs were analyzed by Field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The as-anodized Fe-ZNTs were annealed at 500 °C for 3 h in a tube furnace for crystallization of the oxide film. The crystal structure, surface functional groups, and surface chemistry of annealed Fe-ZNTs were analyzed by XRD, Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS), respectively. From the FESEM results, large inner diameter (79 nm) with short nanotubes (1.9 µm) were grown in 0.5 wt% NH4F due to enhanced chemical dissolution of Fe-ZNTs in high fluoride content. XRD result of as-anodized Fe-ZNTs indicate the enhanced intensity of tetragonal-ZrO2 peaks with increasing fluoride content from 0.1 to 0.3 wt% NH4F. The cubic-ZrO2 started to emerge at 0.5 wt% NH4F. For annealed Fe-ZNTs, pure monoclinic-ZrO2 was observed in 0.1 wt% NH4F, while tetragonal-ZrO2 + monoclinic-ZrO2, and cubic-ZrO2 + monoclinic-ZrO2 were co-existed in 0.3 wt% NH4F and 0.5 wt% NH4F, respectively. Cr(VI) photoreduction under sunlight indicates 93 % Cr(VI) removal efficiency over annealed Fe-ZNTs synthesized in 0.3 wt% NH4F which ascribed to high Cr(VI) adsorption and high photocatalytic activity of tetragonal-ZrO2.

Formation of Fe-doped ZrO2 nanotube arrays in glycerol/formamide at varying fluoride content for Cr(VI) photoreduction

Fe-doped ZrO2 nanotubes (Fe-ZNTs) were produced by anodizing a Zr-5 wt% Fe (Zr-5Fe) alloy in a mixture of fluoride-containing glycerol/formamide (FA) (1:1 ratio)/1 vol% H2O at 50 V for 3 h in different concentrations of ammonium fluoride (NH4F) (0.3, 0.5, and 1.0 wt%). These materials were synthesized to serve as photocatalysts for the removal of hexavalent chromium, Cr(VI) ions by photoreduction. To crystallize the oxide film, the as-anodized Fe-ZNTs were subjected to annealing at 500 ℃ for 3 h in a tube furnace. The morphology and crystal structure of the annealed Fe-ZNTs were analyzed using Field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The surface functional groups and surface chemical of the samples were analyzed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), respectively. The formation of hollow structure of the Fe-ZNT was validated by High-resolution transmission electron microscopy (HRTEM). The dimensions of the Fe-ZNTs were measured using Image J. UV–Visible spectroscopy was used to determine the Cr(VI) concentration remaining in the sample after photoreduction. From the FESEM micrographs, the diameter of the Fe-ZNTs was enlarged with higher fluoride content, while the length abruptly decreased due to excessive etching of the oxide film at oxide|electrolyte interface. The XRD results indicate the high intensity of tetragonal-ZrO2 in high fluoride content after annealing. From the photocatalytic results of 30 ppm Cr(VI) under sunlight, the Fe-ZNTs synthesized in 1.0 wt% NH4F exhibited the highest Cr(VI) removal efficiency with 100 % after 5 h due to enhanced Cr(VI) adsorption and high photoactivity of tetragonal-ZrO2.

Preparation and photoluminescence properties of Tb3+ and rhodamine B composite films supported by ZrO2 nanotube arrays

ZrO2 can be used as both the matrix for rare earth ions and the supporting material for organic fluorescent molecules. In this work, using ZrO2 nanotube array films (ZNA) as the supporting material, Tb3+ composite films (Tb-ZrO2/ZNA) were obtained by depositing Tb3+-doped ZrO2 nanoparticles into ZNA through ionic layer adsorption deposition method. Then, rhodamine B (RB) was immobilized on Tb-ZrO2/ZNA to obtain RB@Tb-ZrO2/ZNA composite films. The photoluminescence (PL) properties, fluorescence resonance energy transfer (FRET) and ultraviolet (UV) display performances of the composite films were studied. Results show that the Tb3+ ions in RB@Tb-ZrO2/ZNA can significantly enhance the PL emission of RB with a maximum enhancement multiple of up to 3.93-fold. The composite films can display the UV intensity directly through brightness and color changes, among which RB@Tb-ZrO2/ZNA is the brightest and most sensitive to UV light. This work can provide ideas and methods for the preparation and application of organic-inorganic hybrid fluorescent materials.

In-situ spectroscopic ellipsometry for deeply understanding the interface evolution dynamics of anodic ZrO2 nanotubes

The interface evolution dynamics within the initial anodization of Zr remains ambiguous while it occupies a pivotal role in understanding the growth kinetics and preparing desirable ZrO2 nanotubes. Herein, we conducted an in-situ study on the Zr electrode/electrolyte interface evolution with highly sensitive spectroscopic ellipsometry. Based on the characteristic parameters of the interface layer derived from in-situ ellipsometric spectra, the dynamic evolution of the interface structure was visualized. In addition, the effects of temperature, voltage and F− concentration in the anodic solution on the interface evolution and growth kinetics of ZrO2 nanotubes were revealed. Without F−, ZrO2 grows with a self-limited mode and forms an ultrathin layer. In the presence of F−, the ZrO2 layer dissolves and forms nanotubes under the electric field-assisted dissolution (FAD) effect, and the interface evolution mainly includes three stages of ZrO2 layer formation, void initiation and nanotube generation. The interface parameters derived from in-situ ellipsometric spectra indicate that the preferred F− concentration, voltage and temperature are 0.15 M, 20 V and 293 K, respectively. Under the optimal condition, ZrO2 nanotubes grow linearly (25.4 nm s−1) with a porosity of ∼ 50%. Moreover, the apparent activation energy of ZrO2 formation is approximately 13.5 kJ mol−1 (fitted by the Arrhenius formula). This fundamental work on interface research provides practical guidance for the precise and controllable preparation of ZrO2 nanotubes via anodization.

Highly sensitive and selective fluorescence sensor based on zirconia nanotube array films for Cu2+ detection

ZrO2 nanotube array films (ZNA) were prepared by anodisation method, and ZNA was modified with γ-aminopropyltriethoxysilane (APTES) to prepare amino-functionalized ZNA (ZNA-NH2). Through immobilizing Cu2+ ions and RB on ZNA and ZNA-NH2, the detection of Cu2+ ions could be realized by measuring the change of fluorescence intensity of RB, and the effects of APTES and RB concentrations on the detection performances were studied. The results show that the fluorescence detection performances of ZNA-NH2 carrier are significantly better than those of ZNA. The best detection performances for Cu2+ ions are achieved with the RB concentration of 10 mg/L and the APTES concentration of 0.4 M. The minimum detection limit is 0.68 nM (S/N = 3), and the linear range is from 1 × 10−3 to 3 × 102 μM under the optimal experimental conditions. The carriers can be regenerated and reused after being soaked in acetic acid solution. This work can be applied to the detection of Cu2+ ions in real seawater.

Impact of Annealing on ZrO2 Nanotubes for Photocatalytic Application

This work aims to study the structural, optical, and photocatalytic properties of ZrO2 nanotubes (NTs) that have been synthesized using the electrochemical anodization method. The structural and morphological characteristics of unannealed and annealed (400 °C, 500 °C, and 700 °C) ZrO2 NTs were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Details of the structural and morphological results are depicted to clarify the effect of annealing temperature on the NTs. Furthermore, the reflectivity and photoluminescence of ZrO2 NTs were found to depend on the annealing temperature. The resulting bandgap values were 3.1 eV for samples annealed at 400 °C and 3.4 eV for samples annealed at 550 and 700 °C. Thus, amorphous and annealed ZrO2 NTs were tested in terms of their photocatalytic degradation of Black Amido (BA) dye. Samples annealed at 400 °C exhibited 85.4% BA degradation within 270 min compared to 77.5% for samples annealed at 550 °C and 70.2% for samples annealed at 700 °C. The anodized ZrO2 NTs that were annealed at 400 °C showed the coexistence of tetragonal and monoclinic crystalline phases and exhibited the fastest photocatalytic performance against the BA dye. This photocatalytic behavior was correlated to the crystalline phase transformation and the structural defects seen in anodized ZrO2.

Thermally stimulated luminescence of oxygen-deficient zirconia nanotubes

ZrO2 nanotubular arrays with intrinsic defects are a promising solid-state basis for the development of devices for detecting, storing, and converting energy. Layers of the self-ordered zirconia nanotubes of 5 μm length and 30 nm diameter, containing oxygen vacancies and their complexes, have been synthesized by anodic oxidation. The spectrally resolved TSL (thermally stimulated luminescence) above room temperature for the samples exposed by UV-irradiation with an energy of 4.1 eV have been studied. Two emission bands with maxima near 2.5 and 2.8 eV, associated with radiative relaxation of T and F+ centers, respectively, have been found. An analysis of the measured glow curves within the framework of general order kinetics established the presence of four TSL peaks caused by charge carriers traps with activation energies of 0.7–0.8 eV. A band diagram is proposed to explain the role of optically active centers based on electron (vacancies in positions of three- and four-coordinated oxygen) and hole (interstitial oxygen ions) traps in observed TSL processes during the irradiation and the followed heating of anion-deficient ZrO2 nanotubes.

Preparation and formation mechanism of fast-growing ZrO2 nanotubes and slow-growing TiO2 nanotubes

The preparation and application of porous anodic oxides have been widely studied, but their formation mechanism remains controversial. In this paper, TiO2 nanotubes and ZrO2 nanotubes were prepared in the same electrolyte at different voltages. The results show that the average growth rate of TiO2 nanotubes is slow, the length of nanotubes is short, and the length to diameter ratio is only 19. Under the same conditions, the average growth rate of the ZrO2 nanotubes is very fast, the length of the nanotubes is very long, the length to diameter ratio is up to 356. In addition, ZrO2 nanotubes are all burst nanotubes, while TiO2 nanotubes are mostly unburst nanotubes in SEM photos. All these interesting phenomena cannot be explained using the traditional field-assisted dissolution theory. In this paper, the oxygen bubble mold and the theory of electronic current were used to analyze the current-time curve of anodization. The total current was divided into ionic current and electronic current, and the growth mechanism and average growth rate of the two kinds of nanotubes were explained because of the difference of ionic current and electronic current.

Ordered zirconium dioxide nanotubes covered with an evaporated gold layer as reversible, chemically inert and very efficient substrates for surface-enhanced Raman scattering (SERS) measurement

The deposition of a layer of plasmonic metal on a surface of highly ordered nanostructured oxide is one of the important methods of preparation of substrates for surface-enhanced Raman scattering (SERS) measurements. In this contribution we describe formation of SERS substrates by the deposition of a gold layer on ordered ZrO2 nanotubes. The influence of various experimental parameters on the structure of formed composites and the achievable SERS enhancement factor has been analysed. Like commonly used SERS substrates formed by the deposition of plasmonic metals on TiO2 nanotubes, gold-covered ZrO2 nanotubes also could be used as reversible SERS platform after water rinsing: there is no any significant decrease in the SERS activity of the substrate even after 20 radiation-induced cleaning cycles. Moreover, SERS substrates formed on ZrO2 nanotubes are significantly more stable in strongly acidic media than the previously developed SERS substrates based on ordered TiO2 nanotubes.

Synthesis of bi-component ZrO2/Ag nanotube for heavy metal removal

This study was conducted to synthesize bi-component ZrO2/Ag nanotubes through anodization and photoreduction methods. The synthesized nanotubes were characterized and adsorption tests were carried out to evaluate its performance in removing heavy metal, lead (II). ZrO2 nanotubes were synthesized by anodizing zirconium foil in an electrolyte composed of glycerol, ammonium fluoride, formamide, and distilled water. The effect of anodizing time and the annealing process on the morphology of synthesized nanotubes were studied. Bi-component ZrO2/Ag nanotubes were prepared through photochemical reduction which silver precursor solution undergoes Ultraviolet (UV) irradiation in the presence of the active reducing agent. Larger pore diameter and longer length of synthesized nanotubes were formed at the longer anodizing time and the walls of nanotubes were smoother without annealing. The effect of the initial heavy metal concentration and contact time on the adsorption efficiency of synthesized nanotubes was evaluated using lead (II) as the heavy metal ions. Overall, the percentage removal of lead (II) increased with longer adsorption time and higher initial concentration of the lead (II) ions.

Unraveling the six stages of the current-time curve and the bilayer nanotubes obtained by one-step anodization of Zr.

The application and growth mechanism of anodic TiO2 nanotubes have been a hot topic in the last ten years, but the formation mechanism of anodic ZrO2 nanotubes has rarely been studied. In one-step constant voltage anodization of Al and Ti, the typical current–time curve has three stages. Moreover, the current–time curves of the three stages can last for 10 min or even 10 hours, resulting in a single layer of nanotubes with the same diameter due to the constant voltage in one-step anodization. However, in this paper, it was found for the first time that the three stages of the current–time curve appeared twice in succession during one-step constant voltage anodization of Zr for only 900 seconds, and bilayer nanotubes with increased diameter were obtained. This six-stage current–time curve cannot be explained by classical field-assisted dissolution and field-assisted flow or stress-driven mechanisms. Here, the formation mechanism and growth kinetics of bilayer ZrO2 nanotubes have been clarified rationally by the theories of ionic current, electronic current and oxygen bubble mold. The interesting results presented in this paper are of great significance for revealing the anodizing process of various metals and the formation mechanism of porous structures.

Formation process of zirconia nanotubes and porous structures and model of oxygen bubble growth

Preparation and growth mechanism of anodization of Ti and Al has been widely concerned for two decades, but the research on anodic ZrO2 is relatively lacking. In this paper, anodic TiO2 and ZrO2 nanotubes were prepared in glycerol electrolyte containing 0.35 M NH4F and 4 vol% H2O under different anodizing voltages. We had successfully prepared the anodic ZrO2 nanotubes (AZNTs) with a complete top and a “bulb” at the bottom under 60 V, and with the increase of the applied anodizing voltage, the “bulb” cavity also increased. However, under the same anodizing conditions, the surface of anodic TiO2 nanotubes (ATNTs) is a cluster of nano-tip morphology, and the bottom of the ATNTs is a conventional hemisphere shape. In addition, both AZNTs and porous anodic zirconia (PAZ) were found to coexist in the anodic ZrO2 layer prepared at 60 V. Here, we used the oxygen bubble model and ionic current and electronic current theories to analyze the reason of the special morphology. It is confirmed that the porous anodic oxides are actually evolved from nanotubes. In other words, the structure is essentially the same.

The growth rate of nanotubes and the quantity of charge during anodization

Porous anodic oxides have attracted great attention because of their unique morphology and porous structure. At the same time, the study of the mechanism has become a hot topic. Here, in order to study the formation mechanism, the growth of porous oxides of Ti and Zr at different voltages has been studied. The results show that the growth rate of ZrO2 nanotube is much higher than that of TiO2 nanotube, and the length of ZrO2 nanotube is much longer than that of TiO2 nanotube. By comparing the current–time curves, it is found that the difference of growth rate of nanotube is related to the quantity of charge, that is, integral of current–time curve (Q=∫I dt). At the same time, it is also found that the growth rate of ZrO2 nanotubes is higher than that of TiO2 nanotubes. For example, the average growth rate of ZrO2 nanotubes is 0.422 μm/min, while the growth rate of TiO2 nanotubes is only 0.131 μm/min at 20 V. These interesting findings negate the idea of fluoride ion dissolution in the classical field-assisted dissolution theory. The oxygen bubble model and electronic current theory can well explain this conclusion.

Effect of zirconia nanotube coating on the hydrophilicity and mechanochemical behavior of zirconium for biomedical applications

Zirconium has attracted considerable attention in the biomedical field owing to its biocompatibility and desirable tribological and mechanical properties. In this study, we anodized pure zirconium in an ammonium fluoride and ethylene glycol electrolyte, which produced a coating of ZrO2 nanotubes (NTs). The ZrO2 coated samples were annealed at different temperatures, and the morphology and structure of the coated substrates were studied using XPS, SEM, TEM, EDS, and SAED. The micro/nanomechanical properties and corrosion resistance of the samples were evaluated. Wear tests performed on bare and coated substrates revealed that the coated samples annealed at 400 °C had a significantly lower average coefficient of friction than the other substrates. The corrosion test was performed on different substrates, and the results showed that the corrosion resistance of the coated sample annealed at 400 °C was considerably higher than that of the other substrates. According to the nanoindentation tests, the elastic modulus of the Zr sample decreased from 74.3 to 31.7 GPa after anodization and the creation of ZrO2 NTs. Biocompatibility tests revealed that cell attachment to the surface of the ZrO2 NTs decreased due to the presence of F−; however, the cell viability increased after the ZrO2 NT-coated samples were annealed at 200 and 400 °C.

Comparative study on the anodizing process of Ti and Zr and oxide morphology

Metal anodizations have attracted much attention, but the formation mechanisms of the porous anodic oxides are still unclear. Compared with the Ti anodization, the anodization of Zr is relatively less studied. In order to compare the anodizing processes of Ti and Zr, a comparative study of Ti and Zr was carried out under the different voltages of anodizing process in the same mixed electrolyte of ammonium fluoride and phosphate acid. It was found that the structure and morphology of their nanotubes were very different. The anodizing curve of Ti showed a state of constant current. However, the anodizing curve of Zr did not show the state of constant current and ZrO2 nanotubes were still formed. This indicates that there is no equilibrium between oxide growth and dissolution in the anodization. TiO2 nanotubes show an orderly array of nanotubes, but ZrO2 nanotubes have three kinds of composite structures: nanoflower, nanotube and dense films. By comparing the great difference of current-time curves between the two anodizing processes, the composite structure of ZrO2 nanotubes are interpreted reasonably by combining electronic current theory and oxygen bubble mold.

Structure, Phase Composition, and Mechanical Properties of Composites Based on ZrO2 and Multi-Walled Carbon Nanotubes

Structure, Phase Composition, and Mechanical Properties of Composites Based on ZrO2 and Multi-Walled Carbon Nanotubes

Photoluminescence properties of Eu3+ doped ZrO2 with different morphologies and crystal structures

Eu3+-doped ZrO2 nanotube arrays and Eu3+-doped ZrO2 nanoparticles were prepared by anodisation of the Zr–Eu alloy and by chemical co-precipitation method, respectively. The effects of anodisation conditions and annealing temperature on the morphology, crystalline phase and properties of Eu3+-doped ZrO2 nanotube arrays were studied. The photoluminescence (PL) excitation and emission spectra of different samples were compared and the related reasons were discussed. Results show that the crystal structures of ZrO2 nanotube arrays prepared in organic (Eu-ZNTA-o) or water (Eu-ZNTA-w) solution and the Eu3+-doped ZrO2 nanoparticles (Eu-ZP) are much different. When annealed at 600 °C, the ZrO2 nanotube arrays contain both tetragonal and monoclinic phases (with only 5.9% of tetragonal phase for Eu-ZNTA-o and 64.1% tetragonal ZrO2 for Eu-ZNTA-w), while the Eu3+-doped ZrO2 nanoparticles are pure tetragonal phase. The PL emission spectra of Eu-ZNTA-o and Eu-ZP are significantly different. The PL emission spectra of Eu-ZNTA-w approximate the superposition of the former two. Compared with the tetragonal phase, the monoclinic phase matrix is more favorable to the absorption and transfer of energy. In addition, the morphology of the sample can lead to lattice distortion, which can also have an effect on the photoluminescence properties.