Fabrication of self-organized TiO2 nanotube arrays for photocatalytic reduction of CO2

The photocatalytic conversion of CO₂ and H₂O to alcohols was achieved using self-organized TiO₂ nanotube arrays (TNAs), which were prepared by electrochemical anodization of Ti foils in 1 M (NH₄)₂SO₄ electrolyte containing 0.5 wt% NH₄F. Experimental results revealed that the morphology and structure of self-organized TNAs could be strongly influenced by the applied voltage and anodization temperature, and the optimized TNAs were prepared by electrochemical anodization of Ti foils under optimal conditions (i.e., at 20 V for 2 h at 30 °C). The as-prepared TNAs were amorphous and could be transformed to anatase phase during the thermal treatment at 450 °C in air for 3 h. By using the annealed TNAs as a photocatalyst, the photocatalytic reduction of CO₂ to alcohol, predominately methanol and ethanol, was demonstrated under Xenon lamp illumination. Based on the photocatalytic measurements, the production rates of methanol and ethanol were calculated to be ～10 and ～9 nmol cm^?2 h^?1, respectively. In addition, the formation mechanism of methanol and ethanol was also tentatively proposed.     

The Synthesis and Photocatalytic Properties of TiO2 Nanotube Array by Starch‐Modified Anodic Oxidation

In this study, the characterization and photocatalytic activity of TiO₂ nanotube arrays prepared by anodization process with starch addition were investigated in detail. The results suggested that the optimum mass fraction of starch added in anodization process was 0.1%, with which TiO₂ nanotube arrays owning good tubular structure were synthesized. The tube length and average inner diameter of nanotubes were approximately 4 μm and 30 nm, respectively. Through the characterization of TiO₂ nanotube arrays by energy dispersive spectrometer, scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, Fourier Transform Infrared (FTIR) spectroscopy, it was found that the as‐prepared nanotubes possessed well uniformed and higher photodegradation responsive than the pure TiO₂. Moreover, it was expected that the as‐prepared nanotubes exhibited good photocatalytic activity for the degradation of RhB under UV‐light irradiation, which could be ascribed to their good morphology, enhanced UV‐light absorption property and electron transmission ability during the photocatalytic reaction. In addition, the nanotubes were not significantly regenerated during the cycling runs experiment. Overall, this study could provide a principle method to synthesize TiO₂ nanotube arrays with enhanced photocatalytic activity by anodization process with starch addition for environmental purification.     

脉冲沉积制备Cu2O/TiO2纳米管异质结的可见光光催化性能

在用阳极氧化法制备有序排列TiO₂纳米管阵列薄膜的基础上,引入脉冲沉积工艺,成功实现了均匀、弥散分布的Cu₂O纳米颗粒修饰改性TiO₂纳米管阵列,形成Cu₂O/TiO₂ 纳米管异质结复合材料. 利用场发射扫描电镜（FESEM）、场发射透射电镜（FETEM）、X射线衍射（XRD）、X射线光电子能谱（XPS）和紫外-可见漫反射光谱（UV-Vis DRS）对样品进行表征,重点研究了Cu₂O/TiO₂ 纳米管异质结的光电化学特性和对甲基橙（MO）的可见光催化降解性能. 结果表明,Cu₂O纳米颗粒均匀附着在TiO₂纳米管阵列的管口和中部位置,所制备的Cu₂O/TiO₂ 纳米管异质结具有高效的可见光光催化性能;在浓度为0.01 mol·L^-1的CuSO₄溶液中制得的Cu₂O/TiO₂纳米管异质结表现出最好的电化学特性和光催化性能;另外,对Cu₂O纳米颗粒影响光催化活性的机理进行了讨论.     

Highly-ordered dye-sensitized TiO2 nanotube arrays film used for improving photoelectrochemical electrodes

Thin titanium oxide nanotube arrays (TNAs) films were synthesized by anodization of titanium foil in an aqueous dimethyl sulfoxide solution using a platinum foil counter electrode.TNAs up to 6.8 μm in length,120 nm in inner pore diameter,and 20 nm in wall thickness were obtained by 40 V potentials anodization for 24 h.Their microstructures and surface morphologies were characterized by XRD,TEM,SAED and UV-vis spectroscopy.The photoelectrochemical properties of as-prepared unsensitized and dye-sensitized TNAs electrodes were examined under simulated solar light (AM 1.5,100 mW/cm2) illumination.The results showed that the photocurrent of the dye-sensitized TNAs electrodes reached 6.9 mA/cm2,which was 6 times more than that of the dye-sensitized TiO2 nanoparticles (TNPs) electrodes.It implied that the electron transport process and the charge recombination suppression within TNAs electrodes were much more favorable in comparison with that in the TNPs electrodes.Electrodes applying such kind of titania nanotubes will have a potential to further enhance the efficiencies of TNAs-based dye-sensitized solar cells.     

Improvement the wastewater purification by TiO2 nanotube arrays: The effect of etching-step on the photo-generated charge carriers and photocatalytic activity of anodic TiO2 nanotubes

The current study demonstrates how the etching step in anodization process effects on the photocatalytic activity of TiO₂ nanotubes. In this regard, the TiO₂ nanotubes were made by one-step and two-step anodization process on two different substrates Ti and etched-Ti foils, respectively. The results revealed that two-step anodization process is a beneficial way to prepare highly well-organized structure and regular surface. The two-step anodization by an enhancement in the fluoride ions diffusion led to a decrease in nanotubes' porosity and an increment in the nanotubes’ surface area, a factor of roughness, and the ratio of length to diameter, respectively. As a consequence of the improvement in geometrical properties, the two-step TiO₂ nanotubes led to the intensification of photocurrent density (from 0.383 to 0.677 mA cm⁻²⁾ and photoconversion efficiency (from 0.18% to 0.29%) in comparison with the one-step nanotubes, respectively. Further, a synergetic impact of the photoelectrochemical measurement and photocatalytic process was observed. The degradation efficiencies of 2,4-dichlorophenol by two-step nanotubes increased from 47 to 55% under visible light, and from 58 to72% under UV irradiation, which it was attributed to more light harvesting, more photo-generated electrons, higher separation efficiency and improvement in geometrical properties. Furthermore, the kinetic study showed that the reactions follow first-order kinetics and the reaction rate constants by two-step nanotubes are 1.25 and 1.44 times as great as those of one-step nanotubes under visible and UV irradiation, respectively. Moreover, the reusability tests showed that 2-step TiO₂ nanotubes has good stability and is active even up to the Fifth run.     

高度有序的二氧化钛纳米管阵列的制备及其光催化活性的研究

采用电化学阳极氧化法在钛表面构筑了一种结构有序、微米级的TiO₂纳米管阵列膜层. 考察了制备电压、氧化时间、溶液搅拌等实验参数对TiO₂纳米管阵列形貌和尺寸的影响. 应用SEM和XRD对膜层的形貌和晶型进行了分析和表征, 并通过TiO₂纳米管阵列膜对甲基橙的光催化降解, 研究了TiO₂纳米管阵列膜层结构与光催化活性的关系. 结果表明: 阳极电压和溶液搅拌对制备TiO₂纳米管阵列的结构起到关键的作用. 控制20 V电压制备的TiO₂纳米管阵列膜, 管长达2.6～3.3 μm, 经500 ℃热处理后具有最高的光催化活性, 其光催化性能明显优于一般的TiO₂纳米颗粒膜.     

Effect of the anodization voltage on the dimensions and photoactivity of titania nanotubes arrays

This work compares the behaviors of TiO₂ nanotube (TNTs) array obtained by anodization of Ti foils in an ethylene glycol/NH₄F/water electrolyte with different applied voltages during a constant anodization time, and for the same electrolyte composition. The crystal structure and surface morphology of the annealed anodic films are investigated by X‐Ray diffraction system, Raman spectroscopy and scanning electron microscopy, respectively. The TiO₂ nanotubes obtained at potentials of 20–40–60 V show different inner diameters (42–89–124 nm), tube length (1.2–3.3–12.7 µm) and wall thicknesses (12–15–18 nm). The influence of these geometric parameters on the photoelectrochemical properties and the photocatalytic activity were investigated in detail. The results showed that the photocatalytic performances of TNT films are improved when the specific surface, the tube length and the solid fraction are increased, but the increase is slowed down when a limiting thickness of the layer is reached. The surface states which usually show high density in nanostructured layers do not seem to influence significantly the photocatalytic activity of the layers. Copyright © 2013 John Wiley & Sons, Ltd.     

含氯离子电解液中二氧化钛纳米管的阳极氧化形成机理及应用

采用电化学阳极氧化的方法,以氯化铵(NH₄Cl)水溶液为电解液,在纯钛表面制备了二氧化钛(TiO₂)纳米管。考察了制备电压、氧化时间、Cl^-浓度和钛基体的退火处理对阳极氧化过程的影响规律,探讨了在含氯离子电解液中纳米管的形成机理,并基于上述含氯离子电解液中纳米管形成机制,通过两步阳极氧化法得到无支撑纳米管薄膜。     

Preparation and characterization of Zr doped TiO2 nanotube arrays on the titanium sheet and their enhanced photocatalytic activity

This paper described a new method for the preparation of Zr doped TiO₂ nanotube arrays by electrochemical method. TiO₂ nanotube arrays were prepared by anodization with titanium anode and platinum cathode. Afterwards, the formed TiO₂ nanotube arrays and Pt were used as cathode and anode, respectively, for preparation of Zr/TiO₂ nanotube arrays in the electrolyte of 0.1 M Zr(NO₃)₄ with different voltage and post-calcination process. The nanotube arrays were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectra (XPS) and UV-Vis diffusion reflection spectra (DRS). The photocatalytic activities of these nanotubes were investigated with Rhodamine B as the model pollutant and the results demonstrated that the photocatalytic efficiency of Zr doped TiO₂ nanotubes was much better than that of TiO₂ nanotubes under UV irradiation. Zr/TiO₂ nanotube arrays doped at 7 V and calcined at 600 °C (denoted as TiO₂-7 V-600) achieved the best photocatalytic efficiency and the most optimal doping ratio was 0.047 (Zr/Ti). TiO₂-7 V-600 could be reused for more than 20 times and maintained good photocatalytic activities.     

Optimizing TiO2 nanotube morphology for enhanced photocatalytic H2 evolution using single-walled and highly ordered TiO2 nanotubes decorated with dewetted Au nanoparticles

In the present work we introduce the formation of highly ordered single-walled TiO₂ nanotubes based on anodizing nanoimprinted titanium substrates. In a first step, anodization on nanoimprinted substrates leads to the formation of highly ordered TiO₂ nanotubes. These as-formed TiO₂ nanotubes have a double-walled nature. After an optimized chemical treatment in piranha solution, the inner wall is removed and an array of single-walled TiO₂ nanotubes is obtained that has a high degree of hexagonal ordering. These single walled arrangements show an enhanced photocatalytic activity for H₂ evolution in comparison with classic self-ordered TiO₂ nanotubes.     

Highly ordered nanoporous TiO2 and its photocatalytic properties

Hexagonally packed nanoporous TiO₂ has been developed by a self-templating electrochemical anodization method on titanium foil. The template is made by a two-step anodization process, where highly ordered hexagonally packed dimples are formed on titanium surface through self-organization. The template is then subjected to a third anodization process with a fast voltage ramp rate, from which a highly ordered porous structure is obtained. Such a material with an ordered structure shows enhanced photocatalytic activity as compared to titania nanotube.     

Synthesis of Bi-doped TiO2 Nanotubes and Enhanced Photocatalytic Activity for Hydrogen Evolution from Glycerol Solution

Bi‐doped TiO₂ nanotubes with variable Bi/Ti ratios were synthesized by hydrothermal treatment in 10 mol·L^?1 NaOH (aq.) through using Bi‐doped TiO₂ particles derived from conventional sol‐gel method as starting materials. The effects of Bi content on the morphology, textural properties, photo absorption and photocatalytic activity of TiO₂ nanotubes were investigated. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS) observations of the obtained samples revealed the formation of titanate nanotube structure doped with Bi, which exists as a higher oxidation state than Bi³⁺. Bi‐doping TiO₂ nanotubes exhibited an extension of light absorption into the visible region and improved photocatalytic activities for hydrogen production from a glycerol/water mixed solution as compared with pure TiO₂ nanotubes. There was an optimal Bi‐doped content for the photocatalytic hydrogen production, and high content of Bi would retard the phase transition of titanate to anatase and result in morphology change from nanotube to nanobelt, which in turn decreases the photocatlytic activity for hydrogen evolution.     

Formation Mechanistism Study of TiO2 Film Comprising Nanotubes and Nanoparticles

A novel titanium dioxide (TiO₂) film comprising both nanotubes and nanopaticles was fab-ricated by an anodization process of the modified titanium. The local electric field at the anodized surface was simulated and its influence on the morphology of the TiO₂ film was discussed. The results show that the electric field strength is enhanced by the covering. The growth rate of TiO₂ increases with the assist of the local electric field. However, TiO₂ dis-solution is hindered since the local electric field prevents [TiF₆]^6- from diffusing. It means that the balance condition for the formation of nanotubes is broken, and TiO₂ nanoparticles are formed. Moreover, the crystal structure of the TiO₂ film was confirmed using X-ray diffraction and Raman analysis. The anatase is a main phase for the proposed film.     

Photocatalytic Crystalline and Amorphous TiO2 Nanotubes Prepared by Electrospinning and Atomic Layer Deposition

In this work core/shell composite polymer/TiO₂ nanofibers and from those TiO₂ nanotubes were prepared. First, poly(vinyl alcohol) (PVA) and poly(vinylpyrrolidone) (PVP) fibers were synthetized by electrospinning. They were covered with a 100 nm thick amorphous TiO₂ layer by atomic layer deposition at 50 °C. Later the polymer core was removed by two different methods: dissolution and annealing. In the case of dissolution in water, the as-prepared TiO₂ nanotubes remained amorphous, while when annealing was used to remove the polymers, the TiO₂ crystallized in anatase form. Due to this, the properties of amorphous and crystalline TiO₂ nanotubes with exactly the same structure and morphology could be compared. The samples were investigated by SEM-EDX, ATR-IR, UV-Vis, XRD and TG/DTA-MS. Finally, the photocatalytic properties of the TiO₂ nanotubes were studied by decomposing methyl-orange dye under UV light. According to the results, crystalline anatase TiO₂ nanotubes reached the photocatalytic performance of P25, while amorphous TiO₂ nanotubes had observable photocatalytic activity.     

The effect of annealing temperatures on surface properties,hydroxyapatite growth and cell behaviors of TiO2 nanotubes

Well‐ordered TiO₂ nanotubes were prepared by the electrochemical anodization of titanium in an ethylene glycol electrolyte containing 1 wt% NH₄F and 10 wt% H₂O at 20 V for 20 min, followed by annealing. The surface morphology and crystal structure of the samples were examined as a function of the annealing temperature by field emission scanning electron microscopy (FE‐SEM) and X‐ray diffraction (XRD), respectively. Crystallization of the nanotubes to the anatase phase occurred at 450 °C, while rutile formation was observed at 600 °C. Disintegration of the nanotubes was observed at 600 °C and the structure vanished completely at 750 °C. Electrochemical corrosion studies showed that the annealed nanotubes exhibited higher corrosion resistance than the as‐formed nanotubes. The growth of hydroxyapatite on the different TiO₂ nanotubes was also investigated by soaking them in simulated body fluid (SBF). The results indicated that the tubes annealed to a mixture of anatase and rutile was clearly more efficient than that in their amorphous or plain anatase state. The in vitro cell response in terms of cell morphology and proliferation was evaluated using osteoblast cells. The highest cell activity was observed on the TiO₂ nanotubes annealed at 600 °C. Copyright © 2010 John Wiley & Sons, Ltd.     

Formation of anodic TiO2 nanotube or nanosponge morphology determined by the electrolyte hydrodynamic conditions

The present work studies the effect of hydrodynamic conditions on the growth of anodic TiO₂ nanostructures on Ti in a glycerol/water/NH₄F electrolyte. Parameter screening for fluoride content, anodization voltage and rotation rate (Reynolds number) of a rotating anode showed that two distinctly different TiO₂ morphologies could be obtained: the classic ordered nanotubes or a nanoscale sponge layer. We present conditions for TiO₂ sponge formation and growth to several micrometers of thickness, and show that in specific cases sponge layers can be superior to tube morphologies, as illustrated for photoelectrochemical water-splitting under standard AM 1.5 conditions.     

Enhanced electron collection in photoanode based on ultrafine TiO2 nanotubes by a rapid anodization process

The separated and ultrafine TiO₂ nanotubes are fabricated by a modified rapid anodization method, which cannot be achieved through conventional anodization. Then, model dye-sensitized solar cells based on the prepared TiO₂ nanotubes and commercial TiO₂ nanoparticles (P25) are investigated, and a discrepancy is discovered between the light-harvesting capability and the power conversion efficiency. The charge transport and recombination are studied by the electrochemical impedance spectroscopy and the open-circuit voltage decay technique. Results show that the nanotube photoanode owns a longer electron diffusion length and a larger electron lifetime than the nanoparticle one, which can compensate for the loss of light absorption. The enhanced electron collection efficiency observed is attributed to the facilitated charge carrier pathways in the photoanode composed by the separated TiO₂ nanotubes fabricated in this work. Therefore, the TiO₂ nanotubes synthesized by this method are verified to have good electronic properties, which might find applications not only in photovoltaic, but also in catalysis, sensors, and other areas.     

Preparation and Characterization of TiO2 Nanotube Arrays via Anodization of Titanium Films Deposited on FTO Conducting Glass at Room Temperature

Self-organized TiO₂ nanotube arrays with micro-scale length were prepared on fluorine-doped tin oxide (FTO) conducting glass in NH₄F/glycerol electrolyte by electrochemical anodization of pure titanium films deposited by radio frequency magnetron sputtering (RFMS) at room temperature. The samples were characterized by means of field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and photoelectrochemistry methods. The results showed that Ti films prepared at the condition of Ar pressure 0.5 Pa, power 150 W, and 0.5 h at room temperature possessed the zone T model structure with good homogeneity and high denseness. When the anodization time was prolonged from 1 to 3 h at the voltage of 30 V, the pore diameter of TiO₂ nanotubes increased from 50 to 75 nm, and the length increased from 750 to 1100 nm and then gradually decreased to 800 nm, while their wall morphology changed from smooth to rough. Also with increasing the anodization voltage, the pore diameter became larger, and the remaining oxide layer reduced, which could be easily removed by ultrasonic-chemical cleaning in 0.05% (w, mass fraction) diluted HF solution. Moreover, the photocurrent response curves and electrochemical impedance spectroscopy (EIS) results indicated that UV-illumination clearly enhanced the effective separation of the electron-hole pairs and the crystallized electrodes from the annealing treatment of as-anodized electrodes at 450 °C exhibited a better photoelectrochemical performance.     

Effect of electrolysis conditions on photocatalytic activities of the anodized TiO2 films

Photocatalytic activities of anodized TiO₂ films for decomposition of gaseous acetaldehyde were investigated. The anodized TiO₂ films were fabricated by galvanostatic anodization in a mixed electrolyte composed of H₂SO₄, H₃PO₄, and H₂O₂. Pre-nitridation treatment effectively enhanced the photocatalytic activity of the anodized TiO₂ films. The electrolysis parameters such as anodization time, current density, electrolyte temperature, and electrolyte composition significantly affected the photocatalytic activity of the anodized TiO₂ films. The improvement of photocatalytic activity of the anodized films is attributed to increase in surface areas of the anodized specimens.     

Photoelectron Properties and Organic Molecules Photodegradation Activity of Titania Nanotubes with CuxO Nanoparticles Heat Treated in Air and Argon

Titania is very famous photocatalyst for decomposition of organic pollutants. Its photocatalytic properties significantly depend on the morphology and chemical composition of the samples. Herein, the TiO₂ nanotubes/Cu_xO nanoheterostructures have been synthesized and the effect of heat treatment performed in molecular atmospheres of air and argon on their photoelectrochemical and photocatalytic properties has been studied. The prepared samples have a higher reaction rate constant compared to TiO₂ nanotubes in the decomposition reaction of methylene blue molecules. It is established that in argon treated nanoheterostructures, the copper oxide is present in two phases, CuO and Cu₂O, while in air treated ones there is only CuO. In the TiO₂ nanotubes/Cu_xO samples, Cu²⁺ ions and molecular O₂⁻ radicals were detected while in TiO₂ nanotubes only carbon dangling bond defects are present. The dynamics of O₂⁻ radicals under illumination are discussed. It was shown that the TiO₂ nanotubes do not exhibit photocatalytic activity under visible light. The mechanism of the photocatalytic reaction on the surface of the TiO₂ nanotubes/Cu_xO samples was proposed. It is assumed that a photocatalytic decomposition of organic molecules under visible light at the surface of the nanoheterostructures under investigation is realized mainly by the reaction of these molecules with photogenerated O₂⁻ radicals. The results obtained are completely original and indicate the high promise of the prepared photocatalysts.