Fast Growth of Highly Ordered TiO2 Nanotube Arrays on Si Substrate under High-Field Anodization

Highly ordered TiO_2 nanotube arrays(NTAs) on Si substrate possess broad applications due to its high surfaceto-volume ratio and novel functionalities, however, there are still some challenges on facile synthesis. Here, we report a simple and cost-effective high-field(90–180V) anodization method to grow highly ordered TiO_2 NTAs on Si substrate,and investigate the effect of anodization time, voltage, and fluoride content on the formation of TiO_2 NTAs. The current density–time curves, recorded during anodization processes, can be used to determine the optimum anodization time. It is found that the growth rate of TiO_2 NTAs is improved significantly under high field, which is nearly 8 times faster than that under low fields(40–60 V). The length and growth rate of the nanotubes are further increased with the increase of fluoride content in the electrolyte.

Simple Method to Fabricate Au Nanoparticle-Decorated TiO2 Nanotube Arrays for Enhanced Visible Light Photocurrent

Au nanoparticle-decorated TiO2 nanotube arrays are prepared by a simple method, which is a thermal annealing thin gold film deposited on anodie oxidized TiO2 nanotube arrays. These electron microscope images present that Au nanoparticles are well dispersed within the wall and on the surface of the XiO2 nanotubes. Meanwhile, the morphologies of Au nanoparticles can be controlled by changing the thickness of the deposited gold film. Associ- ated with the excitation of localized surface plasmon resonances, the prepared Au nanoparticle-decorated TiO2 nanotube arrays could work as visible light responsive photocatalysts to produce a greatly enhanced photocurrent density. By varying the initial gold film thickness, such Au nanoparticle-decorated TiO2 nanotube arrays could be optimized to obtain the highest photocurrent generation efficiency in the visible and UV light regions.

Synthesis and photoelectrical performance of nanoscale PbS and Bi2S3 co-sensitized on TiO2 nanotube arrays

TiO2 films have been widely applied in photo- voltaic conversion techniques. TiO2 nanotube arrays （TiO2 NAs） can be grown directly on the surface of metal Ti by the anodic oxidation method. Bi2S3 and PbS nanoparticles （NPs） were firstly co-sensitized on TiOa NAs （denoted as PbS/Bi2S3（n）/TiO2 NAs） by a two-step process containing hydrothermal and sonication-assisted SILAR method. When the concentration of Bi3＋ is 5 mmol/L, the best photoelectrical performance was obtained under simulated solar irradiation. The short-circuit photocurrent （Jsc） and photoconversion efficiency （η） of PbS/Bi2S3（5）/TiO2 NAs electrode were 4.70 mA/cm and 1.13 %, respectively.

Two-Step Anodization of Maltilayer TiO_2 Nanotube and Its Photocatalytic Activity under UV Light

A double-layer TiO2 nanotube arrays were formed by two-step anodization of Ti foils in different electrolytes. First, Ti in 0.5 wt% HF was anodized to form thin nanotube layer. Afterwards a second anodization was conducted in a formamide based electrolyte, which allowed the second layer of nanotube growing directly underneath the first one. From FESEM investigation we found that the thickness of second layer corresponded to the anodization time, the increasing of which would lead to the excessive etching on the first layer. The first layer protected the lower one from fluoride corrosion during anodization process. The double layer TiO2 nanotube arrays showed no benefit to photodegradation effect in methyl orange degradation experiments.

Synthesis and Characterization of Niobium-doped TiO_2 Nanotube Arrays by Anodization of Ti-20Nb Alloys

Well crystallized niobium-doped TiO; nanotube arrays （TiNbO-NT） were successfully synthesized via the anodization of titanium/niobium alloy sheets, followed with a heat treatment at 550 ℃ for 2 h. Morphology analysis results demonstrated that both the titanium/niobium alloy microstructure and the dissolution strength of electrolyte played major roles in the formation of nanotube structure. A single-phase microstructure was more favorable to the formation of uniform nanotube arrays, while modulating the dissolution strength of electrolyte was required to obtain nanotube arrays from the alloys with multi-phase microstructures. X-ray diffraction （XRD） and X-ray photoelectron （XPS） analysis results clearly demonstrated that niobium dopants （Nb^5＋） were successfully doped into TiO2 anatase lattice by substituting Ti^4＋ in this approach.

Fabrication, modification and environmental applications of TiO2 nanotube arrays （TNTAs） and nanoparticles

Among the semiconductors, titanium dioxide has been identified as an effective photocatalyst due to its abundance, low cost, stability, and superior electronic energy band structure. Highly ordered nanotube arrays of titania were produced by anodization and mild sonication. The band gap energy of the titania nanotube arrays was reduced to 2.6 eV by co-doping with Fe, C, N atoms using an electrolyte solution containing K3Fe（CN）6. The photo- conversion of phenol in a batch photoreactor increased to more than 18% based on the initial concentration of phenol by using a composite nanomaterial consisting of titania nanotube arrays and Pt/ZIF-8 nanoparticles. A layer-by- layer assembly technique for the deposition of titania nanoparticles was developed to fabricate air filters for the degradation of trace amounts of toluene in the air and preparation of superhyrophobic surfaces for oil-water separation and anti-corrosion surfaces.

Fabrication of TiO_2 Nanotube Arrays by Rectified Alternating Current Anodization

Anodization is a popular method of preparing TiO2nanotube array films（TiNTs） by using direct current（DC）power as the driving voltage.In this study,three driving voltage modes,namely,the sine alternating current（sine） mode,the full-wave rectification of sine waves via four diodes（sine-4D,where D means diode） mode,and the DC mode,were used to prepare TiNTs by anodization.At 20 V,TiNTs were formed under sine-4D mode but only irregular porous TiO2films were formed under DC mode.At 50 V,TiNTs formed under both the sine-4D and DC modes.No TiNTs formed in the sine mode anodization at either 20 or 50 V.Compared with the DC mode,the sine-4D mode required a lower oxidation voltage for TiNT formation,which suggests that sine-4D is an economical,convenient,and efficient driving voltage for TiNT preparation by anodization.The morphologies and structures of TiNT samples anodized at 50 V in the sine-4D and DC modes at different oxidation time（1,5,10,30,60,and 120 min） were analyzed.TiNT growth processes were similar between the studied modes.However,the growth rate of the films was faster under the sine-4D mode than the DC mode during the first 30 min of anodization.

Morphology dependence of TiO_2 nanotube arrays on anodization variables and buffer medium

Vertically oriented TiO_2 nanotube arrays were prepared by potentiostatic anodization of Ti foils in HF/acetic acid（HAC） aqueous solution.Anodization variables including anodization electrolyte concentration,anodization voltage, anodization time and buffer medium can be chosen and adjusted to manipulate the nanotube arrays to give the required length and morphology.

Effect of Anodization Parameters on Morphology and Photocatalysis Properties of TiO_2 Nanotube Arrays

Highly ordered TiO2 nanotube arrays were fabricated via electrochemical anodization of high purity Ti foil in fluoride-containing electrolyte. The effects of applied anodization potential, anodization time on the formation of TiO2 nanotube arrays and the photocatalytic degradation of methylene blue （MB） were discussed. The TiO2 nanotube arrays calcined at 500 ℃ for 2 h show pure anatase phase. The pore diameters of TiO2 nanotube arrays can be adjusted from 30 to 90 nm using a different anodization voltage. Anodization time mainly influenced TiO2 tube length, and by increasing the anodization time, the nanotube length became longer gradually. When the anodization potential was 40 V, the average growth rate of TiO2 nanotube was about 4.17 μm/h. Both anodization potential and time had important effects on the photocatalytic efficiency. The TiO2 nanotube arrays obtained at anodization potential of 40 V for I h showed the best photocatalytic degradation ratio of MB.

Vertically oriented TiO_2 nanotube arrays with different anodization times for enhanced boiling heat transfer

Pool boiling of saturated water on a plain Ti surface and surfaces covered with vertically-oriented TiO2 nanotube arrays(NTAs) has been studied.The technique of potentiostatic anodization using non-aqueous electrolytes was adopted to fabricate three types of TiO2 NTAs distinguished by their anodization time.Compared to the bare Ti surface,the incipient boiling wall superheat on the TiO2 NTAs was decreased by 11 K.Both the critical heat flux and heat transfer coefficient of pool boiling on the TiO2 NTAs were higher than those from boiling on a bare Ti surface.The measured maximum critical heat flux and heat transfer coefficient values were 186.7 W/cm2 and 6.22 W/cm2K,respectively.Different performances for the enhancement of heat transfer by the three types of TiO2 NTAs were attributed to the different degrees of deformation in the nanostructure during boiling.Long-term performance of the nanomaterial-coated surfaces for enhanced pool boiling showed degradation of the TiO2 NTAs prepared with an anodization time of 3 hours.

Enhanced CH4 yield by photocatalytic CO2 reduction using TiO2 nanotube arrays grafted with Au, Ru, and ZnPd nanoparticles

Metal nanoparticle （NP） co-catalysts on metal oxide semiconductor supports are attracting attention as photocatalysts for a variety of chemical reactions. Related efforts seek to make and use Pt-free catalysts. In this regard, we report here enhanced CH4 formation rates of 25 and 60 μmol·g^-1·h^-1 by photocatalytic CO2 reduction using hitherto unused ZnPd NPs as well as Au and Ru NPs. The NPs are formed by colloidal synthesis and grafted onto short n-type anatase TiO2 nanotube arrays （TNAs）, grown anodically on transparent glass substrates. The interfacial electric fields in the NP-grafted TiO2 nanotubes were probed by ultraviolet photoelectron spectroscopy （UPS）. Au NP-grafted TiO2 nanotubes （Au-TNAs） showed no band bending, but a depletion region was detected in Ru NP-grafted TNAs （Ru-TNAs） and an accumulation layer was observed in ZnPd NP-grafted TNAs （ZnPd-TNAs）. Temperature programmed desorption （TPD） experiments showed significantly greater CO2 adsorption on NP-grafted TNAs. TNAs with grafted NPs exhibit broader and more intense UV-visible absorption bands than bare TNAs. We found that CO2 photoreduction by nanoparticle-grafted TNAs was driven not only by ultraviolet photons with energies greater than the TiO2 band gap, but also by blue photons close to and below the anatase band edge. The enhanced rate of CO2 reduction is attributed to superior use of blue photons in the solar spectrum, excellent reactant adsorption, efficient charge transfer to adsorbates, and low recombination losses.

Unveiling the controversial mechanism of reversible Na storage in TiO2 nanotube arrays： Amorphous versus anatase TiO2

Due to their inherent safety, low cost, and structural stability, TiO2 nanostructures represent a suitable choice as anode materials in sodiumion batteries. In the recent years, various hypotheses have been proposed regarding the actual mechanism of the reversible insertion of sodium ions in the TiO2 structure, and previous reports are often controversial in this respect. Interestingly, when tested as binder- and conducting additive-free electrodes in laboratory-scale sodium cells, amorphous and crystalline （anatase） TiO2 nanotubular arrays obtained by simple anodic oxidation exhibit peculiar and intrinsically different electrochemical responses. In particular, after the initial electrochemical activation, anatase TiO2 shows excellent rate capability and very stable long-term cycling performance with larger specific capacities, and thus a clearly superior response compared with the amorphous counterpart. To obtain deeper insight, the present materials are thoroughly characterized by scanning electron microscopy and ex situ X-ray diffraction, and the insertion of sodium ions in the TiO2 bulk phases is systematically modeled by density functional theory calculations. The present results may contribute to the development of more systematic screening approaches to identify suitable active materials for highly efficient sodium-based energy storage systems.

Formation Mechanistism Study of TiO2 Film Comprising Nanotubes and Nanoparticles

A novel titanium dioxide （TiO2） film comprising both nanotubes and nanopaticles was fabricated 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 dissolution 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.

Fabrication and Photocatalytic Activity of Ag3PO4-TiO2 Heterostructural Nanotube Arrays

To extend the absorption capability of TiO_2 into visible light region and inhibit the recombination of photogenerated electrons and holes,we put forward an effective strategy of the coupling of TiO_2 with a suitable semiconductor that possesses a narrow band gap.Meanwhile,Ag_3PO_4-TiO_2 heterostructuralnanotube arrays were prepared by the two-step anodic oxidation to obtain the TiO_2 nanotube arrays and then by a deposition-precipitation method to load Ag_3PO_4.The samples were characterized by field emission scanning electron microscopy（FESEM）,energy dispersive spectrometry（EDS）,X-ray diffraction（XRD）,and UV-vis diffuse reflectance spectroscopy（UV-vis DRS）.The experimentalresults showed that Ag_3PO_4 nanoparticles were uniformly dispersed on the highly ordered TiO_2 nanotube arrays,which increased the visible-light absorption of TiO_2 photocatalyst.The photocurrent density and photocatalytic degradation of methylorange indicated that the performance of Ag_3PO_4-TiO_2 heterostructuralnanotube arrays was better than that of the TiO_2 nanotube arrays,which could be attributed to the effective electron-hole separation and the improved utilization of visible light.

Study of Cell Behaviors on Anodized TiO_2 Nanotube Arrays with Coexisting Multi-Size Diameters

It has been revealed that the different morphologies of anodized TiO_2 nanotubes, especially nanotube diameters, triggered different cell behaviors. However, the influence of TiO_2 nanotubes with coexisting multi-size diameters on cell behaviors is seldom reported. In this work, coexisting four-diameter TiO_2 nanotube samples, namely,one single substrate with the integration of four different nanotube diameters(60, 150, 250, and 350 nm), were prepared by repeated anodization. The boundaries between two different diameter regions show well-organized structure without obvious difference in height. The adhesion behaviors of MC3T3-E1 cells on the coexisting fourdiameter TiO_2 nanotube arrays were investigated. The results exhibit a significant difference of cell density between smaller diameters(60 and 150 nm) and larger diameters(250 and 350 nm) within 24 h incubation with the coexistence of different diameters, which is totally different from that on the single-diameter TiO_2 nanotube arrays. The coexistence of four different diameters does not change greatly the cell morphologies compared with the singlediameter nanotubes. The findings in this work are expected to offer further understanding of the interaction between cells and materials.

Electrochemical Preparation and Photoelectric Properties of Cu_2O-loaded TiO_2 Nanotube Arrays

TiO2 nanotube （TNT） arrays were fabricated by anodic oxidation of titanium foil in a fluoride- based solution, on which Cu20 particles were loaded via galvanostatic pulse electrodeposition in cupric acetate solutions in the absence of any other additives. The structure and optical properties of Cu2O-loaded TiO2 nanotube arrays （Cu2O-TNTs） were analyzed by scanning electron microscopy （SEM）, X-ray diffraction （XRD） and UV-Vis absorption, and the photoelectrochemical performance was measured using an electrochemical work station with a three-electrode configuration. The results show that the Cu2O particles distribute uniformly on the highly ordered anatase TiO2 nanotube arrays. The morphologies of Cu2O crystals change from branched, truncated octahedrons to dispersive single octahedrons with increasing deposition current densities. The Cu2O- TNTs exhibited remarkable visible light responses with obvious visible light absorption and greatly enhanced visible light photoelectrochemical performance. The I-V characteristics under visible light irradiation show a distinct plateau in the region between approximately -0.3 and 0 V, resulting in higher open-circuit voltages and larger short-circuit currents with increased Cu2O deposition.

Nitrogen-doped carbon encapsulated in mesoporous TiO2 nanotubes for fast capacitive sodium storage

Controllable synthesis of insertion-type anode materials with beneficial micro-and nanostructures is a promising approach for the synthesis of sodium-ion storage devices with high-reactivity and excellent electrochemical performance.In this study,we developed a sacrificial-templating route to synthesize TiO_(2)@N-doped carbon nanotubes(TiO_(2)@NC-NTs)with excellent electrochemical performance.The asprepared mesoporous TiO_(2)@NC-NTs with tiny nanocrystals of anatase TiO_(2) wrapped in N-doped carbon layers showed a well-defined tube structure with a large specific surface area of 198 m^(2) g^(-1) and a large pore size of~5 nm.The TiO_(2)@NC-NTs delivered high reversible capacities of 158 m A h g^(-1) at 2 C(1 C=335 m A g^(-1))for 2200 cycles and 146 m A h g^(-1) at 5 C for 4000 cycles,as well as an ultrahigh rate capability of up to 40 C with a capacity of 98 m A h g^(-1).Even at a high current density of 10 C,a capacity of 138 m A h g^(-1) could be delivered over 10,000 cycles.Thus,the synthesis of mesoporous TiO_(2)@NC-NTs was demonstrated to be an efficient approach for developing electrode materials with high sodium storage and long cycle life.

Photocatalytic decompositions of gaseous HCHO and methylene blue with highly ordered TiO_2 nanotube arrays

The highly ordered TiO2 nanotubes (NTs) were fabricated by the anodic oxidation method. Their morphology, structure and crystalline phase were characterized by scanning electron microscopy (SEM) and X-ray diffractometer (XRD). The effects of morphology, specific surface area, pore structures and photo catalytic activity of the TiO2 NTs were investigated. UV-vis spectra analysis showed that its light absorption had been extended to the visible light range. The photocatalytic activity of the as-prepared samples was evaluated by photocatalytic oxidation of gaseous HCHO and MB aqueous solution. The samples had better adhesion strength in the dark and showed a higher photocatalytic activity than nanoparticles. Especially, with ultraviolet light pretreatment, the nanotubes exhibited more stable active for photocatalytic decomposition and the photodecomposition rate remained at high level after 3 cycles of the photocatalysis experiment. Thus, how the number of surface active group center dot OH increased and the mechanism for the great improvement for the photocatalytic activity are discussed. (C) 2016 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.

Preparation and Optical Properties of V_2O_5 Nanotube Arrays

V2 O5 nanotube arrays in porous atomic alumina （ PAA ） template were obtained from V2O5 sols prepared by melt quenching method. X-ray powder diffraction and selected area electron diffraction investigations demonstrate that V2O5 nanotubes are orthorhombic. Results by scanning electron mieroscopy and transmission electron microscopy results shove that V2O5 nanotubes with a uniform diameter form highly ordered arrays. The diameter and length of the nanotubes depend on the pore diameter and the thickness of the PAA template used. It is proved that the sol-gel template process is a cost-saving , simple and readily-controlled method to prepare metal oxides nanomaterials .Owing to the quantum size effect. the optical absorption edge of V2O5 nanotubes in PAA ehibits a significant blue shift with respect to that of bulk V2O5.

Analysis of the factors controlling performances of Au-modified TiO2 nanotube array based photoanode in photo-electrocatalytic(PECa)cells

The efficiency of photo-electrocatalytic（PECa） devices for the production of solar fuels depends on several limiting factors such as light harvesting, charge recombination and mass transport diffusion. We analyse here how they influence the performances in PECa cells having a photo-anode based on Au-modified TiOnanotube（TNT） arrays, with the aim of developing design criteria to optimize the photo-anode and the PECa cell configuration for water photo-electrolysis（splitting） and ethanol photo-reforming processes.The TNT samples were prepared by controlled anodic oxidation of Ti foils and then decorated with gold nanoparticles using different techniques to enhance the visible light response through heterojunction and plasmonic effects. The activity tests were made in a gas-phase reactor, as well as in a PECa cell without applied bias. Results were analysed in terms of photo-generated current, Hproduction rate and photoconversion efficiency. Particularly, a solar-to-hydrogen efficiency of 0.83% and a Faradaic efficiency of 91%were obtained without adding sacrificial reagents.