Hierarchical top-porous/bottom-tubular TiO2 nanostructures decorated with Pd nanoparticles for efficient Photoelectrocatalytic decomposition of synergistic pollutants

In this paper, top-porous and bottom-tubular TiO(2) nanotubes (TiO(2) NTs) loaded with palladium nanoparticles (Pd/TiO(2) NTs) were fabricated as an electrode for an enhanced photoelectrocatalytic (PEC) activity toward organic dye decomposition. TiO(2) NTs with a unique hierarchical top-porous and bottom-tubular structure were prepared by a facile two-step anodization method and Pd nanoparticles were decorated onto the TiO(2) NTs via a photoreduction process. The PEC activity of Pd/TiO(2) NTs was investigated by decomposition of methylene blue (MB) and Rhodamine B (RhB). Because of formation Schottky junctions between TiO(2) and Pd, which significantly promoted the electron transfer and reduced the recombination of photogenerated electrons and holes, the Pd/TiO(2) NT electrode showed significantly higher PEC activities than TiO(2) NTs. Interestingly, an obvious synergy between two dyes was observed and corresponding mechanism based on facilitated transfer of electrons and holes as a result of a suitable energy level alignment was suggested. The findings of this work provide a fundamental insight not only into the fabrication but also utility of Schottky junctions for enhanced environmental remediation processes.     

In situ growth of single-crystal TiO2 nanorod arrays on Ti substrate: Controllable synthesis and photoelectrochemical water splitting

Despite one-dimensional (1D) semiconductor nanostructure arrays attracting increasing attention due to their many advantages,highly ordered TiO2 nanorod arrays (TiO2 NR) are rarely grown in situ on Ti substrates.Herein,a feasible method to fabricate TiO2 NRs on Ti substrates by using a through-mask anodization process is reported.Self-ordered anodic aluminum oxide (AAO) overlaid on Ti substrate was used as a nanotemplate to induce the growth of TiO2 NRs.The NR length and diameter could be controlled by adjusting anodization parameters such as electrochemical anodization voltage,anodization time and temperature,and electrolyte composition.Furthermore,according to the proposed NR formation mechanism,the anodized Ti ions migrate and deposit in the AAO nanochannels to form Ti(OH)4 or amorphous TiO2 NRs under electric field,owing to the confinement effect of the template.Photoelectrochemical tests indicated that,after hydrogenation,the TiO2 NRs presented higher photocurrent density under simulated sunlight and visible light illuminations,suggesting their potential use in photoelectrochemical water splitting,photocatalysis,solar cells,and sensors.     

Electrochemical fabrication of titania nanotube arrays with tunning nature of dimethyl sulfoxide and its application for hydrogen sensing

In the present work, tuning effects of dimethyl sulphoxide (DMSO) on the length, wall thickness, dimension and morphology of titania nanotube arrays fabricated by anodization was investigated. DMSO presented in both ethylene glycol and glycerol electrolytes provided excellent tunability in length, wall thickness and diameter of the produced TiO2 nanotube arrays by systematically varying the DMSO concentration. At the DMSO concentration of 12 wt% in ethylene glycol, the nanotube length could be up to 13.0 microm at 20 V for 8 h anodization. The TiO2 nanotube arrays produced in DMSO containing ethylene glycol or glycerol electrolytes displayed a high sensitivity to hydrogen at room temperature compared to the absence of DMSO. The further enhancement of resistance response could be achieved by coating a Pt and Pd layer on the surface of TiO2 nanotube-arrays. The presented work provided a simple way to control formation of TiO2 nanotube arrays through the tuning effects of DMSO in ethylene glycol or glycerol electrolytes.     

Fabrication of highly ordered TiO2 nanotube arrays via anodization of Ti-6Al-4V alloy sheet

Uniform and highly ordered TiO2 nanotube arrays were fabricated by the electrochemical anodic oxidation on Ti-6Al-4V surface, using graphite plate as cathode and ethylene glycol (EG) with addition of a certain amount of H2O and NH4F as electrolyte, and the anodization voltage went up to a presetting voltage by stepwise increment. The morphology, structure and composition of TiO2 nanotube arrays were characterized by SEM, EDS, XRD and XPS. The formation process of TiO2 nanotubes was introduced in brief. The experiments were arranged by an orthogonal experiment method and the experimental results showed that the formation of TiO2 nanotube arrays was influenced by not only each factor (F- content, H2O content, external voltage and duration), but also cross correlation among the four factors. The optimal condition was F- content 0.2 wt%, H2O content 4 vol%, external voltage 40 V and duration 1 h in the studied electrochemical system, and the length of obtained nanotubes was 1.5 microm, the outer diameter was approximately 100 nm and the aspect ratio was 15. As-formed nanotube arrays were amorphous and changed to anatase TiO2 after annealed at 500 degrees C for 2 h in air ambience. XPS survey spectra revealed the surface of as-formed nanotube arrays containing Ti, O, C, F and N. The nanotube arrays on Ti-6Al-4V surface with better thermo-stability and crystallinity would have great potential in biomaterials.     

离子液体中微波辅助合成纳米TiO2／PMMA及其光催化活性

微波加热法制备了[Bmim]BF4、[Bmim]PF6、[Amim]BF4三种离子液体,并分别以该离子液体为反应介质,在微波辐射条件下合成了纳米TiO2／PMMA复合材料,在高压汞灯照射下用甲基橙溶液对其光催化降解活性进行了测试,并用XRD、IR和TG对该复合材料的结构进行了测试和表征,结果表明,分别以这三种离子液体作为反应的介质,采用微波辐射加热能够制备出纳米TiO2／PMMA复合材料。该复合材料不需要经过高温煅烧就表现出较高的光催化活性,离子液体的存在均能够显著提高复合材料的光催化活性,但三种离子液体中又以[Bmim]PF6作介质所制得的复合材料催化活性为高。     

Enhancement of ionic conductivity of PEO based polymer electrolyte by the addition of nanosize ceramic powders

The ionic conductivity of polyethylene oxide (PEO) based solid polymer electrolytes (SPEs) has been improved by the addition of nanosize ceramic powders (TiO2 and AL2O3). The PEO based solid polymer electrolytes were prepared by the solution-casting method. Electrochemical measurement shows that the 10 wt% TiO2 PEO-LiClO4 polymer electrolyte has the best ionic conductivity (about 10(-4) S cm(-1) at 40-60 degrees C). The lithium transference number of the 10 wt% TiO2 PEO-LiClO4 polymer electrolyte was measured to be 0.47, which is much higher than that of bare PEO polymer electrolyte. Ac impedance testing shows that the interface resistance of ceramic-added PEO polymer electrolyte is stable. Linear sweep voltammetry measurement shows that the PEO polymer electrolytes are electrochemically stable in the voltage range of 2.0-5.0 V versus a Li/Li+ reference electrode.     

Influence of structure parameters and crystalline phase on the photocatalytic activity of TiO2 nanotube arrays

Titanium oxide nanotube arrays (TiO2-NTAs) with different diameters and lengths are prepared by anodization of titanium foils in a water/ethylene glycol solution (5:95 V/V) containing 0.3 wt% NH4F. The effects of the diameters, lengths and crystalline phases of the NTAs on the photocatalytic (PC) activity are systematically evaluated. Larger pore diameter results in higher PC activity. The PC activity increases initially and then decreases with lengths for TiO2-NTAs and the optimal length that yields the highest PC activity is observed to be 6.2 microm. The crystalline phase and corresponding PC activity depend on the calcination temperature and their relationship is also investigated. The amorphous-to-anatase and anatase-to-rutile phase transitions initially occur at 300 and 500 degrees C, respectively. The PC activity of TiO2-NTAs initially increases with calcination temperature from 250 to 500 degrees C and then decreases at higher calcination temperature. The enhanced PC activity observed from the samples annealed at 250-450 degrees C is attributed to the better anatase crystalline structure at higher calcination temperature. The highest PC activity with regard to photodecomposition of methyl orange is observed from TiO2-NTAs calcined at 500 degrees C, which coincides with the anatse-to-rutile phase transformation. The synergistic effect of the anatase TiO2-NTAs and rutile barrier layers facilitate interfacial electron transfer consequently enhancing the PC activity. Further elevation of the calcination temperatures to 550 and 600 degrees C exhibits diminished PC activity because the NTs become shorter due to conversion of the bottom of anatase NTs into rutile film.     

Preparation and photoelectrocatalytic activity of ZnO nanorods embedded in highly ordered TiO(2) nanotube arrays electrode for azo dye degradation

In this article, the ZnO nanorods embedded in highly ordered TiO(2) nanotube arrays (ZnO/TiO(2) NR/Ts) electrodes were fabricated through two steps: (1) electrosynthesis of TiO(2) nanotube arrays (TiO(2) NTs) in HF solution by anodization method; (2) followed by cathodic electrodeposition of ZnO embedded in the TiO(2) nanotube arrays. The morphological characteristics and structures of ZnO/TiO(2) NR/Ts electrodes were examined by field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD) analysis, and UV-vis spectra. The linear-sweep photovoltammetry response on the ZnO/TiO(2) NR/Ts electrode was presented and the photocurrent was dramatically enhanced on the ZnO/TiO(2) NR/Ts electrode, comparing with that on bare TiO(2) NTs electrode. The photocatalytic and photoelectrocatalytic activity of ZnO/TiO(2) NR/Ts electrode was evaluated in degradation of methyl orange (MO) in aqueous solution.     

Pattern formation in nanoporous titania templates

We have carried out a systematic investigation into the formation of nanoscaled patterns in titania (TiO2) templates under dc anodization of Ti in HF acid. At lower acid concentrations (around 0.5 wt% HF) either pores or tubes form at the surface of anodized titanium foil. The pores or nanotubes are separated from the bottom Ti layer by a thin barrier layer of TiO2. The critical voltage where the transition from pores to tubes occurs has been determined. It is observed that the transition voltage shift towards higher voltages as acid concentration is increased, with pore formation disappearing altogether at high acid concentrations. We have also carried out a systematic investigation into the dependence of pore and tube parameters on the applied dc anodization voltage. Our results indicate that the barrier layer thickness, pore and tube length increase as a function of applied voltage.     

Sodium fluoride-assisted modulation of anodized TiO₂ nanotube for dye-sensitized solar cells application

This work reports the use of sodium fluoride (in ethylene glycol electrolyte) as the replacement of hydrofluoric acid and ammonium fluoride to fabricate long and perpendicularly well-aligned TiO? nanotube (TNT) (up to 21 μm) using anodization. Anodizing duration, applied voltage and electrolyte composition influenced the geometry and surface morphologies of TNT. The growth mechanism of TNT is interpreted by analyzing the current transient profile and the total charge density generated during anodization. The system with low water content (2 wt %) yielded a membrane-like mesoporous TiO? film, whereas high anodizing voltage (70 V) resulted in the unstable film of TNT arrays. An optimized condition using 5 wt % water content and 60 V of anodizing voltage gave a stable array of nanotube with controllable length and pore diameter. Upon photoexcitation, TNTs synthesized under this condition exhibited a slower charge recombination rate as nanotube length increased. When made into cis-diisothiocyanato-bis(2,2?-bipyridyl-4,4?-dicarboxylato) ruthenium(II) bis (tetrabutyl-ammonium)(N719) dye-sensitized solar cells, good device efficiency at 3.33 % based on the optimized TNT arrays was achieved with longer electron time compared with most mesoporous TiO? films.     

Vertically Oriented Titania Nanotubes Prepared by Anodic Oxidation on Si Substrates

Vertically oriented titania nanotube arrays were fabricated by anodization of titanium film deposited on silicon substrates under different processing conditions. The anodic formation of nanoporous titania on silicon substrate was investigated in aqueous solutions mixed with highly corrosive Na₂SO₄/NaF/citric acid. In the result of the anodization of titanium film deposited at room temperature, a very thin layer of ~70 nm having a worm-like structure was grown on the top of the porous layer. But, in the case of titanium film deposited at 500deg, vertically oriented TiO₂ nanotube arrays were formed. The average tube outer diameter of the nanotube was 74 nm to 100 nm. The longest nanotube of 681 mum was obtained at 15 V and 30 min. The current density transient curve recorded during anodization under a constant voltage showed a typical behavior for self-organized pore formation.     

A Two-step anodization to grow high-aspect-ratio TiO2 nanotubes

High-aspect-ratio TiO₂ nanotubes with small diameter are favored in dye-sensitized solar cells for large dye loading provided by high surface areas. However, long TiO₂ nanotubes with small diameter are difficult to grow under usual anodizing conditions due to unavoidable chemical dissolution of the top portion of the as-fashioned tubes. In the present work, two kinds of double-layered TiO₂ nanotube arrays were prepared by changing voltage from high to low (i.e., from 30 V to 15 V) or from low to high (i.e., from 15 V to 30 V). It is found that the top layer can serve as a sacrificial layer to protect the continuous growth of the bottom layer from chemical dissolution. Accordingly, the two-step anodization from high voltage to low voltage is proposed to produce high-aspect-ratio TiO₂ nanotubes with small diameter underneath a sacrificial top layer.     

An innovative approach to synthesize highly-ordered TiO2 nanotubes

An innovative route to prepare highly-ordered and dimensionally controlled TiO2 nanotubes has been proposed using a mild sonication method. The nanotube arrays were prepared by the anodization of titanium in an electrolyte containing 3% NH4F and 5% H2O in glycerol. It is demonstrated that the TiO2 nanostructures has two layers: the top layer is TiO2 nanowire and underneath is well-ordered TiO2 nanotubes. The top layer can easily fall off and form nanowires bundles by implementing a mild sonication after a short annealing time. We found that the dimensions of the TiO2 nanotubes were only dependent on the anodizing condition. The proposed technique may be extended to fabricate reproducible well-ordered TiO2 nanotubes with large area on other metals.     

Fabrication of free standing anodic titanium oxide membranes with clean surface using recycling process

Large area of self-organized, free standing anodic titanium oxide (ATO) nanotube membranes with clean surfaces were facilely prepared to desired lengths via electrochemical anodization of highly pure Ti sheets in an ethylene glycol electrolyte, with a small amount of NH4F and H2O at 50 V, followed by self-detachment of the ATO membrane from the Ti substrate using recycling processes. In the first anodization step, the nanowire oxide layer existed over the well-arranged ATO nanotube. After sufficiently rinsing with water, the whole ATO layer was removed from the Ti sheet by high pressure N2 gas, and a well-patterned dimple layer with a thickness of about 30 nm existed on the Ti substrate. By using these naturally formed nano-scale pits as templates, in the second and third anodization process, highly ordered, vertically aligned, and free standing ATO membranes with the anodic aluminum oxide (AAO)-like clean surface were obtained. The inter-pore distance and diameter was 154 +/- 2 nm and 91+/- 2 nm, the tube arrays lengths for 25 and 46 hours were 44 and 70 microm, respectively. The present study demonstrates a simple approach to producing high quality, length controllable, large area TiO2 membrane.     

TiO2 nanotubes/MWCNTs nanocomposite photocatalysts: synthesis, characterization and photocatalytic hydrogen evolution under UV-vis light illumination

Nanocomposite of TiO2 nanotubes (TiO2NTs) and multiwalled carbon nanotubes (MWCNTs) has been synthesized by a hydrothermal method and firstly used in photocatalytic hydrogen production. The obtained TiO2 NTs/MWCNTs composites were characterized by X-ray diffraction, transmission electron microscopy, Raman spectrum and ultraviolet-visible diffuse reflectance spectroscopy. The experimental results revealed that the MWCNTs were decorated with well dispersed anatase TiO2 nanotubes with a diameter of 8-15 nm. A slight blue shift and weak symmetry was observed for the strongest Raman peak which resulted from strain gradients originating from interface integration between TiO2 nanotubes and MWCNTs. The photocatalytic activity of the as-prepared samples was evaluated by hydrogen evolution from water splitting using Na2S and Na2SO3 as sacrificial reagents under UV-vis light irradiation. Enhanced photocatalytic activity compared with P25 has been observed for the resulted samples. The nanocomposite with optimized MWCNTs content of 1% displayed a hydrogen production rate of 161 micromol x h(-1) x g(-1). Good photocatalytic stability of the as-synthesized samples was observed as well.     

Fast fabrication of long TiO2 nanotube array with high photoelectrochemical property on flexible stainless steel

Oriented highly ordered long TiO2 nanotube array films with nanopore structure and high photoelectrochemical property were fabricated on flexible stainless steel substrate (50 microm) by anodization treatment of titanium thin films in a short time. The samples were characterized by means of field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and photoelectrochemical methods, respectively. The results showed that Ti films deposited at the condition of 0.7 Pa Ar pressure and 96 W sputtering power at room temperature was uniform and dense with good homogeneity and high crystallinity. The voltage and the anodization time both played significant roles in the formation of TiO2 nanopore-nanotube array film. The optimal voltage was 60 V and the anodization time is less than 30 min by anodizing Ti films in ethylene glycerol containing 0.5% (w) NH4F and 3% (w) H2O. The growth rate of TiO2 nanotube array was as high as 340 nm/min. Moreover, the photocurrent-potential curves, photocurrent response curves and electrochemical impedance spectra results indicated that the TiO2 nanotube array film with the nanoporous structure exhibited a better photo-response ability and photoelectrochemical performance than the ordinary TiO2 nanotube array film. The reason is that the nanoporous structure on the surface of the nanotube array can separate the photo electron-hole pairs more efficiently and completely than the tubular structure.     

乙醇和磁搅拌对高压制备阳极氧化铝膜的影响

硬质阳极氧化(HA)过程中添加乙醇和使用磁搅拌能够有效抑制烧穿现象的发生。将0.3mol/L草酸电解液的溶剂设置为体积比分别为1:1、2:1、3:1和4:1的去离子水与乙醇混合溶液,同时使用磁搅拌进行二次高压阳极氧化。通过分析电流时间曲线和原子力显微镜(AFM)表征的表面形貌,研究了乙醇添加量和磁搅拌对高压阳极氧化的影响。结果表明:溶剂中水与乙醇体积比的大小对制备出模板的孔径和孔间距几乎没有影响;氧化过程中及时改变磁搅拌速度可以有效抑制烧穿现象,使用变速磁搅拌也可制备出高有序度的阳极氧化铝模板。     

Different TiO2 nanotubes for back illuminated dye sensitized solar cell: fabrication,characterization and electrochemical impedance properties of DSSCs

In the present work we reported the fabrication of different TiO₂ nanotube arrays (TiO₂ NTs) by anodization method. When used in dye-sensitized solar cells, the TiO₂ NTs prepared in the two-step anodization process (2-step TiO₂ NTs) showed better efficiency than those of TiO₂ NTs prepared in one step anodization process (1-step TiO₂ NTs). The 2-step TiO₂ NTs show a remarkable efficiency of 1.56 %. This is higher than those of TiO₂ NTs prepared in one step anodization process. Electrochemical impedance spectroscopy has been performed for qualitative analysis of charge transport process in dye-sensitized solar cells.     

In-situ fabrication of AAO template without oxide barrier layer and its applications

Porous anodic aluminum oxide (AAO) film is the most widely used template in combination with electrodeposition (ED) method to fabricate one-dimensional nanostructures such as nanowires, nanotubes and nanorods. However, the existing oxide barrier layer after the anodization blocks the application of AAO template in synthesis of nanostructures via direct electrodeposition. In this paper, AAO template without oxide barrier layer was successfully fabricated by stepwise voltage decrement; influence of two types of stepwise voltage decrement on the removal of oxide barrier layer was introduced. Field Emission Scanning Electron Microscopy (FESEM) images indicated that stepwise voltage decrement could make the oxide layer thin effectively. Meanwhile, highly ordered gold nanowire arrays were fabricated by using direct electrodeposition method based on AAO template with the second anodization process with stepwise voltage decrement of 1 V/min, FESEM image showed that as-prepared gold nanowires are uniform in diameter and the diameter is in accordance with the diameter of AAO template pores. XRD pattern revealed that gold nanowires were indexed as face-centered cubic phase.     

TiO2负载工艺对膨胀石墨吸附原油性能的影响

将异丙醇钛盐与水洗后的可膨胀石墨，或者与水洗并干燥后的可膨胀石墨混合并加热，分别制备了负载TiO2的两种膨胀石墨吸附材料-膨胀石墨／TiO2-1（记为EG／TiO2-1）和膨胀石墨／TiO2-2（记为EG／TiO2-2）。结果表明：异丙醇钛盐与水洗后的可膨胀石墨混合后，部分TiO2溶胶存在于石墨层间；而与水洗并干燥后的可膨胀石墨混合后，绝大部分TiO2溶胶分布在石墨层表面及边缘。EG／TiO2-1和EG／TiO2-2对原油的最大吸附量分别为57g／g和55g／g。